@article{seyitliyev_qin_jana_janke_zhong_you_mitzi_blum_gundogdu_2023, title={Coherent Phonon-Induced Modulation of Charge Transfer in 2D Hybrid Perovskites}, volume={3}, ISSN={["1616-3028"]}, url={https://doi.org/10.1002/adfm.202213021}, DOI={10.1002/adfm.202213021}, abstractNote={Electron–phonon interactions play an essential role in charge transport and transfer processes in semiconductors. For most structures, tailoring electron–phonon interactions for specific functionality remains elusive. Here, it is shown that, in hybrid perovskites, coherent phonon modes can be used to manipulate charge transfer. In the 2D double perovskite, (AE2T)2AgBiI8 (AE2T: 5,5“-diylbis(amino-ethyl)-(2,2”-(2)thiophene)), the valence band maximum derived from the [Ag0.5Bi0.5I4]2– framework lies in close proximity to the AE2T-derived HOMO level, thereby forming a type-II heterostructure. During transient absorption spectroscopy, pulsed excitation creates sustained coherent phonon modes, which periodically modulate the associated electronic levels. Thus, the energy offset at the organic–inorganic interface also oscillates periodically, providing a unique opportunity for modulation of interfacial charge transfer. Density-functional theory corroborates the mechanism and identifies specific phonon modes as likely drivers of the coherent charge transfer. These observations are a striking example of how electron–phonon interactions can be used to manipulate fundamentally important charge and energy transfer processes in hybrid perovskites.}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Seyitliyev, Dovletgeldi and Qin, Xixi and Jana, Manoj K. and Janke, Svenja M. and Zhong, Xiaowei and You, Wei and Mitzi, David B. and Blum, Volker and Gundogdu, Kenan}, year={2023}, month={Mar} }
 @article{moon_mehta_gundogdu_so_gu_2023, title={Metal-Halide Perovskite Lasers: Cavity Formation and Emission Characteristics}, volume={7}, ISSN={["1521-4095"]}, DOI={10.1002/adma.202211284}, abstractNote={Hybrid metal-halide perovskites (MHPs) have shown remarkable optoelectronic properties as well as facile and cost-effective processability. With the success of MHP solar cells and light-emitting diodes, MHPs have also exhibited great potential as gain media for on-chip lasers. However, to date, stable operation of optically pumped MHP lasers and electrically driven MHP lasers-an essential requirement for MHP laser's insertion into chip-scale photonic integrated circuits-is not yet demonstrated. The main obstacles include the instability of MHPs in the atmosphere, rudimentary MHP laser cavity patterning methods, and insufficient understanding of emission mechanisms in MHP materials and cavities. This review aims to provide a detailed overview of different strategies to improve the intrinsic properties of MHPs in the atmosphere and to establish an optimal MHP cavity patterning method. In addition, this review discusses different emission mechanisms in MHP materials and cavities and how to distinguish them.}, journal={ADVANCED MATERIALS}, author={Moon, Jiyoung and Mehta, Yash and Gundogdu, Kenan and So, Franky and Gu, Qing}, year={2023}, month={Jul} }
 @article{biliroglu_findik_mendes_seyitliyev_lei_dong_mehta_temnov_so_gundogdu_2022, title={Author Correction: Room-temperature superfluorescence in hybrid perovskites and its origins}, volume={4}, url={https://doi.org/10.1038/s41566-022-00997-x}, DOI={10.1038/s41566-022-00997-x}, journal={Nature Photonics}, publisher={Springer Science and Business Media LLC}, author={Biliroglu, Melike and Findik, Gamze and Mendes, Juliana and Seyitliyev, Dovletgeldi and Lei, Lei and Dong, Qi and Mehta, Yash and Temnov, Vasily V. and So, Franky and Gundogdu, Kenan}, year={2022}, month={May} }
 @article{dong_fu_seyitliyev_darabi_mendes_lei_chen_chang_amassian_gundogdu_et al._2022, title={Cavity Engineering of Perovskite Distributed Feedback Lasers}, volume={9}, ISSN={2330-4022 2330-4022}, url={http://dx.doi.org/10.1021/acsphotonics.2c00917}, DOI={10.1021/acsphotonics.2c00917}, number={9}, journal={ACS Photonics}, publisher={American Chemical Society (ACS)}, author={Dong, Qi and Fu, Xiangyu and Seyitliyev, Dovletgeldi and Darabi, Kasra and Mendes, Juliana and Lei, Lei and Chen, Yi-An and Chang, Chih-Hao and Amassian, Aram and Gundogdu, Kenan and et al.}, year={2022}, month={Aug}, pages={3124–3133} }
 @article{ho_pei_qin_zhang_peng_angunawela_jones_yin_iqbal_reynolds_et al._2022, title={Importance of Electric-Field-Independent Mobilities in Thick-Film Organic Solar Cells}, volume={10}, ISSN={["1944-8252"]}, url={http://dx.doi.org/10.1021/acsami.2c11265}, DOI={10.1021/acsami.2c11265}, abstractNote={In organic solar cells (OSCs), a thick active layer usually yields a higher photocurrent with broader optical absorption than a thin active layer. In fact, a ∼300 nm thick active layer is more compatible with large-area processing methods and theoretically should be a better spot for efficiency optimization. However, the bottleneck of developing high-efficiency thick-film OSCs is the loss in fill factor (FF). The origin of the FF loss is not clearly understood, and there a direct method to identify photoactive materials for high-efficiency thick-film OSCs is lacking. Here, we demonstrate that the mobility field-dependent coefficient is an important parameter directly determining the FF loss in thick-film OSCs. Simulation results based on the drift-diffusion model reveal that a mobility field-dependent coefficient smaller than 10-3 (V/cm)-1/2 is required to maintain a good FF in thick-film devices. To confirm our simulation results, we studied the performance of two ternary bulk heterojunction (BHJ) blends, PTQ10:N3:PC71BM and PM6:N3:PC71BM. We found that the PTQ10 blend film has weaker field-dependent mobilities, giving rise to a more balanced electron-hole transport at low fields. While both the PM6 blend and PTQ10 blend yield good performance in thin-film devices (∼100 nm), only the PTQ10 blend can retain a FF = 74% with an active layer thickness of up to 300 nm. Combining the benefits of a higher JSC in thick-film devices, we achieved a PCE of 16.8% in a 300 nm thick PTQ10:N3:PC71BM OSC. Such a high FF in the thick-film PTQ10 blend is also consistent with the observation of lower charge recombination from light-intensity-dependent measurements and lower energetic disorder observed in photothermal deflection spectroscopy.}, journal={ACS APPLIED MATERIALS & INTERFACES}, publisher={American Chemical Society (ACS)}, author={Ho, Carr Hoi Yi and Pei, Yusen and Qin, Yunpeng and Zhang, Chujun and Peng, Zhengxing and Angunawela, Indunil and Jones, Austin L. and Yin, Hang and Iqbal, Hamna F. and Reynolds, John R. and et al.}, year={2022}, month={Oct} }
 @article{biliroglu_findik_mendes_seyitliyev_lei_dong_mehta_temnov_so_gundogdu_2022, title={Room-temperature superfluorescence in hybrid perovskites and its origins}, volume={16}, ISSN={["1749-4893"]}, url={https://doi.org/10.1038/s41566-022-00974-4}, DOI={10.1038/s41566-022-00974-4}, number={4}, journal={NATURE PHOTONICS}, publisher={Springer Science and Business Media LLC}, author={Biliroglu, Melike and Findik, Gamze and Mendes, Juliana and Seyitliyev, Dovletgeldi and Lei, Lei and Dong, Qi and Mehta, Yash and Temnov, Vasily V. and So, Franky and Gundogdu, Kenan}, year={2022}, month={Apr}, pages={324-+} }
 @article{comstock_biliroglu_seyitliyev_mcconnell_vetter_reddy_kirste_szymanski_sitar_collazo_et al._2022, title={Spintronic Terahertz Emission in Ultrawide Bandgap Semiconductor/Ferromagnet Heterostructures}, volume={10}, ISSN={["2195-1071"]}, DOI={10.1002/adom.202201535}, abstractNote={Recent successful integration of semiconductors into spintronic THz emitters has demonstrated a new pathway of control over terahertz (THz) radiation through ultrafast demagnetization dynamics. Here, the spintronic THz emission from different ultrawide bandgap (UWBG) semiconductors interfaced with ferromagnets is studied. The authors show that the Schottky barrier in the UWBG semiconductor AlN acts as a spin filter that increases the polarization of the spin current injected from the ferromagnet. Furthermore, the authors show that the two-dimensional electron gas at the interface between Al0.25Ga0.75N and GaN enhances the magnitude of the emitted radiation due to the high spin-to-charge conversion efficiency induced by the Rashba effect that results in a hallmark asymmetry in emission amplitude. The results provide a framework for future engineering of semiconducting/ferromagnet heterostructures for ultrafast communications technologies beyond 5G.}, journal={ADVANCED OPTICAL MATERIALS}, author={Comstock, Andrew and Biliroglu, Melike and Seyitliyev, Dovletgeldi and McConnell, Aeron and Vetter, Eric and Reddy, Pramod and Kirste, Ronny and Szymanski, Dennis and Sitar, Zlatko and Collazo, Ramon and et al.}, year={2022}, month={Oct} }
 @article{ardekani_wilmington_vutukuru_chen_brandt_swan_gundogdu_2021, title={Broadband micro-transient absorption spectroscopy enabled by improved lock-in amplification}, volume={92}, ISSN={["1089-7623"]}, url={https://doi.org/10.1063/5.0060244}, DOI={10.1063/5.0060244}, abstractNote={Recent breakthroughs in material development have increased the demand for characterization methods capable of probing nanoscale features on ultrafast time scales. As the sample reduces to atomically thin levels, an extremely low-level signal limits the feasibility of many experiments. Here, we present an affordable and easy-to-implement solution to expand the maximum sensitivity of lock-in detection systems used in transient absorption spectroscopy by multiple orders of magnitude. By implementation of a tuned RC circuit to the output of an avalanche photodiode, electric pulse shaping allows for vastly improved lock-in detection. Furthermore, a carefully designed "peak detector" circuit provides additional pulse shaping benefits, resulting in even more lock-in detection signal enhancement. We demonstrate the improvement of lock-in detection with each of these schemes by performing benchmark measurements of a white-light continuum signal and micro-transient absorption spectroscopy on a few-layer transition metal dichalcogenide sample. Our results show the practicality of ultrafast pump-probe spectroscopy for many high-sensitivity experimental schemes.}, number={10}, journal={REVIEW OF SCIENTIFIC INSTRUMENTS}, author={Ardekani, Hossein and Wilmington, Ryan L. and Vutukuru, Mounika and Chen, Zhuofa and Brandt, Ryan and Swan, Anna K. and Gundogdu, Kenan}, year={2021}, month={Oct} }
 @article{vutukuru_ardekani_chen_wilmington_gundogdu_swan_2021, title={Enhanced Dielectric Screening and Photoluminescence from Nanopillar-Strained MoS2 Nanosheets: Implications for Strain Funneling in Optoelectronic Applications}, volume={4}, ISSN={["2574-0970"]}, url={https://doi.org/10.1021/acsanm.1c01368}, DOI={10.1021/acsanm.1c01368}, abstractNote={Nonuniform strain on multilayer transition-metal dichalcogenide (TMDC) nanosheets is an exciting path toward practical optoelectronic devices, as it combines the advantages of localized control of optical and electronic properties with ease of fabrication. However, the weaker photoluminescence (PL) due to their indirect nature poses a challenge to their application. Here, we demonstrate extraordinary enhancement of PL from multilayer MoS2 nanosheets under nonuniform strain generated by nanopillars. We observe charge and exciton funneling to the pillar strain apex. The screening from the increased exciton and charge density lowers the exciton binding energy and renormalizes the band gap. Hence, we attribute the dramatic increase in PL to dissociation of bound excitons to free electron–hole pairs, showing that nonuniform strain on TMDC nanosheets can effectively manipulate the nature of light–matter interaction in these atomically thin materials for application in novel strain-engineered optoelectronics.}, number={8}, journal={ACS APPLIED NANO MATERIALS}, publisher={American Chemical Society (ACS)}, author={Vutukuru, Mounika and Ardekani, Hossein and Chen, Zhuofa and Wilmington, Ryan L. and Gundogdu, Kenan and Swan, Anna K.}, year={2021}, month={Aug}, pages={8101–8107} }
 @article{wilmington_ardekani_rustagi_bataller_kemper_younts_gundogdu_2021, title={Fermi liquid theory sheds light on hot electron-hole liquid in 1L-MoS2}, volume={103}, ISSN={["2469-9969"]}, url={https://doi.org/10.1103/PhysRevB.103.075416}, DOI={10.1103/PhysRevB.103.075416}, abstractNote={Room-temperature electron-hole liquid has recently been experimentally identified in low-dimensional transition metal dichalcogenides. Here, the authors demonstrate that a first-principles Fermi liquid model effectively predicts the photoluminescence response of this phenomenon. Using density functional theory, in conjunction with previous Raman and photoluminescence spectroscopy results, they present a consistent quantitative picture of the electron-hole liquid phase transition in suspended, heat-strained 1$L$-MoS${}_{2}$ monolayers. They show a 23-fold increase in photoluminescence per unit of direct gap carrier density and 9:1 indirect-direct hole population ratio at high strain.}, number={7}, journal={PHYSICAL REVIEW B}, author={Wilmington, R. L. and Ardekani, H. and Rustagi, A. and Bataller, A. and Kemper, A. F. and Younts, R. A. and Gundogdu, K.}, year={2021}, month={Feb} }
 @article{findik_biliroglu_seyitliyev_mendes_barrette_ardekani_lei_dong_so_gundogdu_2021, title={High-temperature superfluorescence in methyl ammonium lead iodide}, volume={15}, ISSN={1749-4885 1749-4893}, url={http://dx.doi.org/10.1038/s41566-021-00830-x}, DOI={10.1038/s41566-021-00830-x}, abstractNote={Light–matter interactions can create and manipulate collective many-body phases in solids1–3, which are promising for the realization of emerging quantum applications. However, in most cases, these collective quantum states are fragile, with a short decoherence and dephasing time, limiting their existence to precision tailored structures under delicate conditions such as cryogenic temperatures and/or high magnetic fields. In this work, we discovered that the archetypal hybrid perovskite, MAPbI3 thin film, exhibits such a collective coherent quantum many-body phase, namely superfluorescence, at 78 K and above. Pulsed laser excitation first creates a population of high-energy electron–hole pairs, which quickly relax to lower energy domains and then develop a macroscopic quantum coherence through spontaneous synchronization. The excitation fluence dependence of the spectroscopic features and the population kinetics in such films unambiguously confirm all the well-known characteristics of superfluorescence. These results show that the creation and manipulation of collective coherent states in hybrid perovskites can be used as the basic building blocks for quantum applications4,5. A collective coherent quantum many-body phase, namely superfluorescence, is observed in CH3NH3PbI3 at 78 K. The excitation fluence dependence of the spectroscopic features and the population kinetics confirm all its well-known characteristics.}, number={9}, journal={Nature Photonics}, publisher={Springer Science and Business Media LLC}, author={Findik, Gamze and Biliroglu, Melike and Seyitliyev, Dovletgeldi and Mendes, Juliana and Barrette, Andrew and Ardekani, Hossein and Lei, Lei and Dong, Qi and So, Franky and Gundogdu, Kenan}, year={2021}, month={Jun}, pages={676–680} }
 @misc{lei_dong_gundogdu_so_2021, title={Metal Halide Perovskites for Laser Applications}, volume={31}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.202010144}, abstractNote={Metal halide perovskites have drawn tremendous attention in optoelectronic applications owing to the rapid development in photovoltaic and light-emitting diode devices. More recently, these materials are demonstrated as excellent gain media for laser applications due to their large absorption coefficient, low defect density, high charge carrier mobility, long carrier diffusion length, high photoluminescence quantum yield, and low Auger recombination rate. Despite the great progress in laser applications, the development of perovskite lasers is still in its infancy and the realization of electrically pumped lasers has not yet been demonstrated. To accelerate the development of perovskite-based lasers, it is important to understand the fundamental photophysical characteristics of perovskite gain materials. Here, the structure and gain behavior in various perovskite materials are discussed. Then, the effects of charge carrier dynamics and electron–phonon interaction on population inversion in different types of perovskite materials are analyzed. Further, recent advances in perovskite-based lasers are also highlighted. Finally, a perspective on perovskite material design is presented and the remaining challenges of perovskite lasers are discussed.}, number={16}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Lei, Lei and Dong, Qi and Gundogdu, Kenan and So, Franky}, year={2021}, month={Apr} }
 @article{nevola_bataller_kumar_sridhar_frick_o'donnell_ade_maggard_kemper_gundogdu_et al._2021, title={Timescales of excited state relaxation in alpha-RuCl3 observed by time-resolved two-photon photoemission spectroscopy}, volume={103}, ISSN={["2469-9969"]}, url={https://doi.org/10.1103/PhysRevB.103.245105}, DOI={10.1103/PhysRevB.103.245105}, abstractNote={The nonequilibrium properties of strongly correlated materials present a target in the search for new phases of matter. It is important to observe the types of excitations that exist in these materials and their associated relaxation dynamics. We have studied the photoexcitations in a spin-orbit assisted Mott insulator $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Ru}{\mathrm{Cl}}_{3}$ using time-resolved two-photon photoemission spectroscopy and transient reflection spectroscopy. We find that photoexcited carriers (doublons) in the upper Hubbard band rapidly relax to Mott-Hubbard excitons on a timescale of less than 200 fs. Subsequently, further relaxation of these lower-energy quasiparticles occurs with an energy-dependent time constant of that ranges from 370 to 600 fs due to exciton cooling. The population of Mott-Hubbard excitons persists for timescales up to several microseconds.}, number={24}, journal={PHYSICAL REVIEW B}, author={Nevola, Dan and Bataller, Alexander and Kumar, Ankit and Sridhar, Samanvitha and Frick, Jordan and O'Donnell, Shaun and Ade, Harald and Maggard, Paul A. and Kemper, Alexander F. and Gundogdu, Kenan and et al.}, year={2021}, month={Jun} }
 @article{schrickx_sen_booth_altaqui_burleson_rech_lee_biliroglu_gundogdu_kim_et al._2021, title={Ultra-High Alignment of Polymer Semiconductor Blends Enabling Photodetectors with Exceptional Polarization Sensitivity}, volume={10}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.202105820}, abstractNote={Photodetectors that can sense not only light intensity but also light's polarization state add valuable information that is beneficial in a wide array of applications. Polymer semiconductors are an attractive material system to achieve intrinsic polarization sensitivity due to their anisotropic optoelectronic properties. In this report, the thermomechanical properties of the polymer semiconductors PBnDT-FTAZ and P(NDI2OD-T2) are leveraged to realize bulk heterojunction (BHJ) films with record in-plane alignment. Two polymer blends with distinct weight average molar masses (Mw) are considered and either a strain- or rub-alignment process is applied to align the polymer blend films. Optimized processing yields films with dichroic ratios (DR) of over 11 for the high Mw system and nearly 17 for the low Mw system. Incorporating the aligned films into photodetectors results in a polarized photocurrent ratio of 15.25 with corresponding anisotropy ratio of 0.88 at a wavelength of 530 nm, representing the highest reported photocurrent ratio for photodiodes that can operate in a self-powered regime. The demonstrated performance showcases the ability of polymer semiconductors to achieve BHJ films with exceptional in-plane polymer alignment, enabling high performance polarization sensitive photodetectors for incorporation into novel device architectures.}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Schrickx, Harry M. and Sen, Pratik and Booth, Ronald E. and Altaqui, Ali and Burleson, Jacob and Rech, Jeromy J. and Lee, Jin-Woo and Biliroglu, Melike and Gundogdu, Kenan and Kim, Bumjoon J. and et al.}, year={2021}, month={Oct} }
 @article{yi_peng_xu_seyitliyev_ho_danilov_kim_reynolds_amassian_gundogdu_et al._2020, title={Critical Role of Polymer Aggregation and Miscibility in Nonfullerene-Based Organic Photovoltaics}, volume={10}, ISSN={["1614-6840"]}, url={http://dx.doi.org/10.1002/aenm.201902430}, DOI={10.1002/aenm.201902430}, abstractNote={Understanding the correlation between polymer aggregation, miscibility, and device performance is important to establish a set of chemistry design rules for donor polymers with nonfullerene acceptors (NFAs). Employing a donor polymer with strong temperature-dependent aggregation, namely PffBT4T-2OD [poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3″′-di(2-octyldodecyl)-2,2′;5′,2″;5″,2″′-quaterthiophen-5,5-diyl)], also known as PCE-11 as a base polymer, five copolymer derivatives having a different thiophene linker composition are blended with the common NFA O-IDTBR to investigate their photovoltaic performance. While the donor polymers have similar optoelectronic properties, it is found that the device power conversion efficiency changes drastically from 1.8% to 8.7% as a function of thiophene content in the donor polymer. Results of structural characterization show that polymer aggregation and miscibility with O-IDTBR are a strong function of the chemical composition, leading to different donor–acceptor blend morphology. Polymers having a strong tendency to aggregate are found to undergo fast aggregation prior to liquid–liquid phase separation and have a higher miscibility with NFA. These properties result in smaller mixed donor–acceptor domains, stronger PL quenching, and more efficient exciton dissociation in the resulting cells. This work indicates the importance of both polymer aggregation and donor–acceptor interaction on the formation of bulk heterojunctions in polymer:NFA blends.}, number={8}, journal={ADVANCED ENERGY MATERIALS}, author={Yi, Xueping and Peng, Zhengxing and Xu, Bing and Seyitliyev, Dovletgeldi and Ho, Carr Hoi Yi and Danilov, Evgeny O. and Kim, Taesoo and Reynolds, John R. and Amassian, Aram and Gundogdu, Kenan and et al.}, year={2020}, month={Feb} }
 @article{lei_seyitliyev_stuard_mendes_dong_fu_chen_he_yi_zhu_et al._2020, title={Efficient Energy Funneling in Quasi-2D Perovskites: From Light Emission to Lasing}, volume={32}, ISSN={["1521-4095"]}, DOI={10.1002/adma.201906571}, abstractNote={Quasi-2D Ruddlesden–Popper halide perovskites with a large exciton binding energy, self-assembled quantum wells, and high quantum yield draw attention for optoelectronic device applications. Thin films of these quasi-2D perovskites consist of a mixture of domains having different dimensionality, allowing energy funneling from lower-dimensional nanosheets (high-bandgap domains) to 3D nanocrystals (low-bandgap domains). High-quality quasi-2D perovskite (PEA)2(FA)3Pb4Br13 films are fabricated by solution engineering. Grazing-incidence wide-angle X-ray scattering measurements are conducted to study the crystal orientation, and transient absorption spectroscopy measurements are conducted to study the charge-carrier dynamics. These data show that highly oriented 2D crystal films have a faster energy transfer from the high-bandgap domains to the low-bandgap domains (<0.5 ps) compared to the randomly oriented films. High-performance light-emitting diodes can be realized with these highly oriented 2D films. Finally, amplified spontaneous emission with a low threshold 4.16 µJ cm−2 is achieved and distributed feedback lasers are also demonstrated. These results show that it is important to control the morphology of the quasi-2D films to achieve efficient energy transfer, which is a critical requirement for light-emitting devices.}, number={16}, journal={ADVANCED MATERIALS}, author={Lei, Lei and Seyitliyev, Dovletgeldi and Stuard, Samuel and Mendes, Juliana and Dong, Qi and Fu, Xiangyu and Chen, Yi-An and He, Siliang and Yi, Xueping and Zhu, Liping and et al.}, year={2020}, month={Apr} }
 @article{dong_fu_cao_amoah_gundogdu_li_so_2020, title={Multi-mode Organic Light-Emitting Diode to Suppress the Viewing Angle Dependence}, volume={12}, ISSN={["1944-8252"]}, DOI={10.1021/acsami.0c05825}, abstractNote={A typical top-emitting organic light-emitting diode (OLED) has a strong microcavity effect because of the two reflective electrodes. The cavity effect causes a serious color shift with the viewing angles and restricts the organic layer thickness. To overcome these drawbacks, we design a multi-mode OLED structure with dual-dielectric spacer layers, which extend the cavity length by more than 10 times. This design completely eliminates the intrinsic cavity effect caused by the top and bottom boundaries and provides freedom for the organic layer thickness. We demonstrate these effects in a white multi-mode OLED using a white emitter, which shows a negligible angular chromaticity shift of Δuv = 0.006 from 0 to 70° and a Lambertian emission profile. The simple design and the perfect angular color profiles make the multi-mode OLED structure promising in large-area displays and solid-state lighting applications.}, number={28}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Dong, Chen and Fu, Xiangyu and Cao, Linyu and Amoah, Stephen and Gundogdu, Kenan and Li, Jian and So, Franky}, year={2020}, month={Jul}, pages={31667–31676} }
 @article{wang_peng_xiao_seyitliyev_gundogdu_ding_ade_2020, title={Thermodynamic Properties and Molecular Packing Explain Performance and Processing Procedures of Three D18:NFA Organic Solar Cells}, volume={32}, ISSN={["1521-4095"]}, DOI={10.1002/adma.202005386}, abstractNote={Organic solar cells (OSCs) based on D18:Y6 have recently exhibited a record power conversion efficiency of over 18%. The initial work is extended and the device performance of D18-based OSCs is compared with three non-fullerene acceptors, Y6, IT-4F, and IEICO-4Cl, and their molecular packing characteristics and miscibility are studied. The D18 polymer shows unusually strong chain extension and excellent backbone ordering in all films, which likely contributes to the excellent hole-transporting properties. Thermodynamic characterization indicates a room-temperature miscibility for D18:Y6 and D18:IT-4F near the percolation threshold. This corresponds to an ideal quench depth and explains the use of solvent vapor annealing rather than thermal annealing. In contrast, D18:IEICO-4Cl is a low-miscibility system with a deep quench depth during casting and poor morphology control and low performance. A failure of ternary blends with PC71BM is likely due to the near-ideal miscibility of Y6 to begin with and indicates that strategies for developing successful ternary or quaternary solar cells are likely very different for D18 than for other high-performing donors. This work reveals several unique property–performance relations of D18-based photovoltaic devices and helps guide design or fabrication of yet higher efficiency OSCs.}, number={49}, journal={ADVANCED MATERIALS}, author={Wang, Zhen and Peng, Zhengxing and Xiao, Zuo and Seyitliyev, Dovletgeldi and Gundogdu, Kenan and Ding, Liming and Ade, Harald}, year={2020}, month={Dec} }
 @article{zhang_chung_huang_vetter_seyitliyev_sun_gundogdu_castellano_danilov_yang_2020, title={Towards radiation detection using Cs2AgBiBr6 double perovskite single crystals}, volume={269}, ISSN={0167-577X}, url={http://dx.doi.org/10.1016/j.matlet.2020.127667}, DOI={10.1016/j.matlet.2020.127667}, abstractNote={In this work, we studied the optical- and electrical- properties of emerging Cs2AgBiBr6 double perovskite single crystals and demonstrated their potential for detecting ionizing radiation. We prepared Cs2AgBiBr6 double perovskite single crystals from a saturated aqueous solution. Low-temperature photoluminescence (PL) was employed to determine the bandgap energies of Cs2AgBiBr6, which are 2.00 eV (indirect) and 2.26 eV (direct) respectively. Using the space charge limited current method, we estimated the density of trap states and mobility of charge carriers as 1.44 × 1010 cm−3 and 7.02 cm2/V-s respectively. A lower bound value of the mobility-lifetime (μ-τ) product of 2.48 × 10−3 cm2/V was determined using 450 nm laser excitation, which was sufficient for ensuring a long drift distance of charge carriers for several radiation detector applications. Furthermore, we tested the direct response of Cs2AgBiBr6 single crystals to X-ray radiation. Our Cs2AgBiBr6 single crystal device with gold electrodes deposited on the two parallel surfaces exhibited excellent linear response to low energy X-rays.}, journal={Materials Letters}, publisher={Elsevier BV}, author={Zhang, Zheng and Chung, Ching-Chang and Huang, Zhengjie and Vetter, Eric and Seyitliyev, Dovletgeldi and Sun, Dali and Gundogdu, Kenan and Castellano, Felix N. and Danilov, Evgeny O. and Yang, Ge}, year={2020}, month={Jun}, pages={127667} }
 @article{dong_mendes_lei_seyitliyev_zhu_he_gundogdu_so_2020, title={Understanding the Role of Ion Migration in the Operation of Perovskite Light-Emitting Diodes by Transient Measurements}, volume={12}, ISSN={["1944-8252"]}, DOI={10.1021/acsami.0c14269}, abstractNote={Perovskite light-emitting diodes have been gaining attention in recent years due to their high efficiencies. Despite of the recent progress made in device efficiency, the operation mechanisms of these devices are still not well understood, especially the effects of ion migration. In this work, the role of ion migration is investigated by measuring the transient electroluminescence and current responses, with both the current and efficiency showing a slow response in a time scale of tens of milliseconds. The results of the charge injection dynamics show that the slow response of the current is attributed to the migration and accumulation of halide ions at the anode interface, facilitating hole injection and leading to a strong charge imbalance. Further, the results of the charge recombination dynamics show that the slow response of the efficiency is attributed to enhanced charge injection facilitated by ion migration, which leads to an increased carrier density favoring bimolecular radiative recombination. Through a combined analysis of both charge injection and recombination dynamics, we finally present a comprehensive picture of the role of ion migration in device operation.}, number={43}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Dong, Qi and Mendes, Juliana and Lei, Lei and Seyitliyev, Dovletgeldi and Zhu, Liping and He, Siliang and Gundogdu, Kenan and So, Franky}, year={2020}, month={Oct}, pages={48845–48853} }
 @article{bataller_younts_rustagi_yu_ardekani_kemper_cao_gundogdu_2019, title={Correction to Dense Electron–Hole Plasma Formation and Ultralong Charge Lifetime in Monolayer MoS2 via Material Tuning}, volume={19}, url={https://doi.org/10.1021/acs.nanolett.9b02578}, DOI={10.1021/acs.nanolett.9b02578}, abstractNote={ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionNEXTORIGINAL ARTICLEThis notice is a correctionCorrection to Dense Electron–Hole Plasma Formation and Ultralong Charge Lifetime in Monolayer MoS2 via Material TuningAlexander W. BatallerAlexander W. BatallerMore by Alexander W. Bataller, Robert A. YountsRobert A. YountsMore by Robert A. Younts, Avinash RustagiAvinash RustagiMore by Avinash Rustagihttp://orcid.org/0000-0001-6776-9496, Yiling YuYiling YuMore by Yiling Yu, Hossein ArdekaniHossein ArdekaniMore by Hossein Ardekani, Alexander KemperAlexander KemperMore by Alexander Kemperhttp://orcid.org/0000-0002-5426-5181, Linyou CaoLinyou CaoMore by Linyou Caohttp://orcid.org/0000-0002-7834-8336, and Kenan Gundogdu*Kenan GundogduMore by Kenan Gundogduhttp://orcid.org/0000-0001-7149-5766Cite this: Nano Lett. 2019, 19, 7, 4816Publication Date (Web):June 27, 2019Publication History Published online27 June 2019Published inissue 10 July 2019https://doi.org/10.1021/acs.nanolett.9b02578Copyright © 2019 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views1241Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (233 KB) Get e-Alerts Get e-Alerts}, number={7}, journal={Nano Letters}, publisher={American Chemical Society (ACS)}, author={Bataller, Alexander W. and Younts, Robert A. and Rustagi, Avinash and Yu, Yiling and Ardekani, Hossein and Kemper, Alexander and Cao, Linyou and Gundogdu, Kenan}, year={2019}, month={Jul}, pages={4816–4816} }
 @article{bataller_younts_rustagi_yu_ardekani_kemper_cao_gundogdu_2019, title={Dense Electron–Hole Plasma Formation and Ultralong Charge Lifetime in Monolayer MoS2 via Material Tuning}, volume={19}, ISSN={1530-6984 1530-6992}, url={http://dx.doi.org/10.1021/acs.nanolett.8b04408}, DOI={10.1021/acs.nanolett.8b04408}, abstractNote={Many-body interactions in photoexcited semiconductors can bring about strongly interacting electronic states, culminating in the fully ionized matter of electron–hole plasma (EHP) and electron–hole liquid (EHL). These exotic phases exhibit unique electronic properties, such as metallic conductivity and metastable high photoexcitation density, which can be the basis for future transformative applications. However, the cryogenic condition required for its formation has limited the study of dense plasma phases to a purely academic pursuit in a restricted parameter space. This paradigm can potentially change with the recent experimental observation of these phases in atomically thin MoS2 and MoTe2 at room temperature. A fundamental understanding of EHP and EHL dynamics is critical for developing novel applications on this versatile layered platform. In this work, we studied the formation and dissipation of EHP in monolayer MoS2. Unlike previous results in bulk semiconductors, our results reveal that electromechanical material changes in monolayer MoS2 during photoexcitation play a significant role in dense EHP formation. Within the free-standing geometry, photoexcitation is accompanied by an unconstrained thermal expansion, resulting in a direct-to-indirect gap electronic transition at a critical lattice spacing and fluence. This dramatic altering of the material’s energetic landscape extends carrier lifetimes by 2 orders of magnitude and allows the density required for EHP formation. The result is a stable dense plasma state that is sustained with modest optical photoexcitation. Our findings pave the way for novel applications based on dense plasma states in two-dimensional semiconductors.}, number={2}, journal={Nano Letters}, publisher={American Chemical Society (ACS)}, author={Bataller, Alexander W. and Younts, Robert A. and Rustagi, Avinash and Yu, Yiling and Ardekani, Hossein and Kemper, Alexander and Cao, Linyou and Gundogdu, Kenan}, year={2019}, month={Jan}, pages={1104–1111} }
 @article{jana_janke_dirkes_dovletgeldi_liu_qin_gundogdu_you_blum_mitzi_et al._2019, title={Direct-Bandgap 2D Silver-Bismuth Iodide Double Perovskite: The Structure-Directing Influence of an Oligothiophene Spacer Cation}, volume={141}, ISSN={["1520-5126"]}, DOI={10.1021/jacs.9b02909}, abstractNote={Three-dimensional (3D) hybrid organic–inorganic lead halide perovskites (HOIPs) feature remarkable optoelectronic properties for solar energy conversion but suffer from long-standing issues of environmental stability and lead toxicity. Associated two-dimensional (2D) analogues are garnering increasing interest due to superior chemical stability, structural diversity, and broader property tunability. Toward lead-free 2D HOIPs, double perovskites (DPs) with mixed-valent dual metals are attractive. Translation of mixed-metal DPs to iodides, with their prospectively lower bandgaps, represents an important target for semiconducting halide perovskites, but has so far proven inaccessible using traditional spacer cations due to either intrinsic instability or formation of competing non-perovskite phases. Here, we demonstrate the first example of a 2D Ag–Bi iodide DP with a direct bandgap of 2.00(2) eV, templated by a layer of bifunctionalized oligothiophene cations, i.e., (bis-aminoethyl)bithiophene, through a collective influence of aromatic interactions, hydrogen bonding, bidentate tethering, and structural rigidity. Hybrid density functional theory calculations for the new material reveal a direct bandgap, consistent with the experimental value, and relatively flat band edges derived principally from Ag-d/I-p (valence band) and Bi-p/I-p (conduction band) states. This work opens up new avenues for exploring specifically designed organic cations to stabilize otherwise inaccessible 2D HOIPs with potential applications for optoelectronics.}, number={19}, journal={JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, author={Jana, Manoj K. and Janke, Svenja M. and Dirkes, David J. and Dovletgeldi, Seyitliyev and Liu, Chi and Qin and gundogdu and You, Wei and Blum, Volker and Mitzi, David B. and et al.}, year={2019}, month={May}, pages={7955–7964} }
 @article{hany_yang_zhou_sun_gundogdu_seyitliyev_danilov_castellano_sun_vetter_et al._2019, title={Low temperature cathodoluminescence study of Fe-doped β-Ga2O3}, volume={257}, ISSN={0167-577X}, url={http://dx.doi.org/10.1016/j.matlet.2019.126744}, DOI={10.1016/j.matlet.2019.126744}, abstractNote={Optical and electrical properties along the b-axis of Fe-doped β-Ga2O3 were studied using low temperature cathodoluminescence (CL) spectroscopy, optical absorption spectroscopy and current-voltage (IV) measurements. The optical absorption spectroscopy showed an absorption edge without near edge shoulder and the corresponding optical bandgap was calculated to be 4.45 eV using direct band gap treatment. The temperature dependent CL measurements exhibited a strong blue to ultraviolet (UV) band composed of multiple low intensity peaks in the blue range, a main blue peak, a main UV peak, and a weak UV band from the as-grown Fe-doped β-Ga2O3. After a controlled annealing in air, the emissions changed to a red to near infrared (R-NIR) band with two sharp peaks and an UV band that is resolved at room temperature to three UV broad peaks. The R-NIR sharp peaks from the air-annealed sample were ascribed to incorporation of nitrogen during air annealing.}, journal={Materials Letters}, publisher={Elsevier BV}, author={Hany, Ibrahim and Yang, Ge and Zhou, Chuanzhen Elaine and Sun, Cheng and Gundogdu, Kenan and Seyitliyev, Dovletgeldi and Danilov, Evgeny O. and Castellano, Felix N. and Sun, Dali and Vetter, Eric and et al.}, year={2019}, month={Dec}, pages={126744} }
 @article{younts_bataller_ardekani_yu_cao_gundogdu_2019, title={Near Band‐Edge Optical Excitation Leading to Catastrophic Ionization and Electron–Hole Liquid in Room‐Temperature Monolayer MoS2}, url={https://doi.org/10.1002/pssb.201900223}, DOI={10.1002/pssb.201900223}, abstractNote={Atomically thin materials exhibit exotic electronic and optical properties. Strong many‐body interactions from the reduced dielectric environment lead to electronic phases that drastically change conductivity and optical response. For example, these many‐body interactions can give rise to the formation of collective states such as Mott metal–insulator transitions, electron–hole liquids and plasmas, and excitonic condensates, which typically occur at cryogenic temperatures and high excitation densities. Herein, it is demonstrated that in monolayer MoS 2 at room temperature, a low‐density (10 10 cm −2 ) excitonic gas is formed with continuous wave (CW) below‐gap optical excitation. A slight increase in the excitation fluence triggers a nanosecond phase transition into a dense electron–hole liquid state with three orders of magnitude higher carrier density. This investigation suggests that while the material is in equilibrium with the CW excitation at the threshold fluence, thermomechanical expansion combined with continuous renormalization of the band gap leads to a sudden increase of optical absorption, which initiates runaway exciton ionization and the formation of a high density electron–hole plasma. Such abrupt changes in the excitation density and carrier population can be the basis of unprecedented applications based on 2D materials.}, journal={physica status solidi (b)}, author={Younts, Robert and Bataller, Alexander and Ardekani, Hossein and Yu, Yiling and Cao, Linyou and Gundogdu, Kenan}, year={2019}, month={Nov} }
 @article{li_zeidell_findik_dunlap-shohl_euvrard_gundogdu_jurchescu_mitzi_2019, title={Phase-Pure Hybrid Layered Lead Iodide Perovskite Films Based on a Two-Step Melt-Processing Approach}, volume={31}, ISSN={["1520-5002"]}, DOI={10.1021/acs.chemmater.9b01265}, abstractNote={Layered lead halide perovskites have recently been heavily investigated due to their versatile structures, tunable electronic properties, and better stability compared with 3D perovskites and have also been effectively incorporated into photovoltaic and light-emitting devices. They are often prepared into thin film form by solution methods and typically contain a mixture of phases with different inorganic layer thicknesses (denoted by “n”). In addition, melt-processing has recently been introduced as an option for film deposition of n = 1 lead iodide-based perovskites. Here, we study the thermal properties of higher n (n > 1) layered perovskites in the family (β-Me-PEA)2MAn–1PbnI3n+1, with n = 1, 2, and 3 and where β-Me-PEA = β-methylphenethylammonium and MA = methylammonium, and reveal that they do not melt congruently. However, they can still be melt-processed in air by using a two-step process that includes a lower temperature postannealing step after the initial brief melting step. While typically higher n films contain a mixture of the different n phases, the resulting two-step melt-processed films are highly crystalline and phase pure. Optical and electrical properties of these films were further characterized by time-resolved photoluminescence and dark/illuminated transport measurements, showing the same order of magnitude single-exciton recombination rates compared to previous single crystal results and >2 orders of magnitude higher conductivity compared to conventional spin-coated films. These results offer new pathways to study the layered perovskites and to integrate them into electronic and optoelectronic devices.}, number={11}, journal={CHEMISTRY OF MATERIALS}, author={Li, Tianyang and Zeidell, Andrew M. and Findik, Gamze and Dunlap-Shohl, Wiley A. and Euvrard, Julie and Gundogdu, Kenan and Jurchescu, Oana D. and Mitzi, David B.}, year={2019}, month={Jun}, pages={4267–4274} }
 @article{ardekani_younts_yu_cao_gundogdu_2019, title={Reversible Photoluminescence Tuning by Defect Passivation via Laser Irradiation on Aged Monolayer MoS2}, volume={11}, url={https://doi.org/10.1021/acsami.9b10688}, DOI={10.1021/acsami.9b10688}, abstractNote={Atomically thin (1L)-MoS2 emerged as a direct band gap semiconductor with potential optical applications. The photoluminescence (PL) of 1L-MoS2 degrades due to aging-related defect formation. The passivation of these defects leads to substantial improvement in optical properties. Here, we report the enhancement of PL on aged 1L-MoS2 by laser treatment. Using photoluminescence and Raman spectroscopy in a gas-controlled environment, we show that the enhancement is associated with efficient adsorption of oxygen on existing sulfur vacancies preceded by removal of adsorbates from the sample’s surface. Oxygen adsorption depletes negative charges, resulting in suppression of trions and improved neutral exciton recombination. The result is a 6- to 8-fold increase in PL emission. The laser treatment in this work does not cause any measurable damage to the sample as verified by Raman spectroscopy, which is important for practical applications. Surprisingly, the observed PL enhancement is reversible by both vacuum and ultrafast femtosecond excitation. While the former approach allows switching a designed micropattern on the sample ON and OFF, the latter provides a controllable mean for accurate PL tuning, which is highly desirable for optoelectronic and gas sensing applications.}, number={41}, journal={ACS Applied Materials & Interfaces}, publisher={American Chemical Society (ACS)}, author={Ardekani, Hossein and Younts, Robert and Yu, Yiling and Cao, Linyou and Gundogdu, Kenan}, year={2019}, month={Oct}, pages={38240–38246} }
 @article{nevola_hoffman_bataller_ade_gundogdu_dougherty_2019, title={Rigid valence band shift due to molecular surface counter-doping of MoS2}, volume={679}, ISSN={["1879-2758"]}, DOI={10.1016/j.susc.2018.09.016}, abstractNote={Adsorption of the acceptor material tetracyanoquinodimethane can control optoelectronic properties of MoS2 by accepting defect generated excess negative charge from the surface that would otherwise interfere with radiative decay processes. Angle Resolved Photoelectron Spectroscopy measurements show that the MoS2 band structure near the Γ point shifts rigidly upward by ∼0.2 eV for a complete surface coverage of acceptor species as expected for an upward Fermi level shift due to charge transfer to the TCNQ. The molecular adsorbate orbitals visible in photoemission are indicative of an anionic species, consistent with interfacial charge transfer but without evidence for hybrid states arising from covalent adsorbate-surface interactions. Thus, our interface studies support the notion that molecular adsorbates are a useful tool for controlling optoelectronic functionality in 2D materials without fundamentally modifying their favorable band structures.}, journal={SURFACE SCIENCE}, author={Nevola, D. and Hoffman, B. C. and Bataller, A. and Ade, H. and Gundogdu, K. and Dougherty, D. B.}, year={2019}, month={Jan}, pages={254–258} }
 @article{yu_bataller_younts_yu_li_puretzky_geohegan_gundogdu_cao_2019, title={Room-Temperature Electron-Hole Liquid in Monolayer MoS2}, volume={13}, ISSN={["1936-086X"]}, DOI={10.1021/acsnano.9b04124}, abstractNote={Excitons in semiconductors are usually non interacting and behave like an ideal gas, but may condense to a strongly correlated liquid like state, i.e. electron hole liquid (EHL), at high density and appropriate temperature. EHL is a macroscopic quantum state with exotic properties and represents the ultimate attainable charge excitation density in steady states. It bears great promise for a variety of fields such as ultrahigh power photonics and quantum science and technology. However, the condensation of gas like excitons to EHL has often been restricted to cryogenic temperatures, which significantly limits the prospect of EHL for use in practical applications. Herein we demonstrate the formation of EHL at room temperature in monolayer MoS2 by taking advantage of the monolayer's extraordinarily strong exciton binding energy. This work demonstrates the potential for the liquid like state of charge excitations to be a useful platform for the studies of macroscopic quantum phenomena and the development of optoelectronic devices.}, number={9}, journal={ACS NANO}, author={Yu, Yiling and Bataller, Alexander W. and Younts, Robert and Yu, Yifei and Li, Guoqing and Puretzky, Alexander A. and Geohegan, David B. and Gundogdu, Kenan and Cao, Linyou}, year={2019}, month={Sep}, pages={10351–10358} }
 @article{dunlap-shohl_barraza_barrette_dovletgeldi_findik_dirkes_liu_jana_blum_you_et al._2019, title={Tunable internal quantum well alignment in rationally designed oligomer-based perovskite films deposited by resonant infrared matrix-assisted pulsed laser evaporation}, volume={6}, ISSN={["2051-6355"]}, DOI={10.1039/c9mh00366e}, abstractNote={RIR-MAPLE enables thin-film deposition of organic–inorganic materials with tunable synergistic photophysics.}, number={8}, journal={MATERIALS HORIZONS}, author={Dunlap-Shohl, Wiley A. and Barraza, E. Tomas and Barrette, Andrew and Dovletgeldi, Seyitliyev and Findik, Gamze and Dirkes, David J. and Liu, Chi and Jana, Manoj K. and Blum, Volker and You, Wei and et al.}, year={2019}, month={Oct}, pages={1707–1716} }
 @article{kim_choi_lee_kim_kim_nguyen_gautam_gundogdu_woo_kim_et al._2018, title={Aqueous Soluble Fullerene Acceptors for Efficient Eco-Friendly Polymer Solar Cells Processed from Benign Ethanol/Water Mixtures}, volume={30}, ISSN={["1520-5002"]}, url={https://doi.org/10.1021/acs.chemmater.8b02086}, DOI={10.1021/acs.chemmater.8b02086}, abstractNote={We present a new series of fullerene derivatives that exhibit solubility in ethanol/water solvent mixtures and implement these materials to fabricate polymer solar cells (PSCs) using environmentally benign solvents. In order to simultaneously optimize the processability of the fullerenes in ethanol/water solvent mixtures and device performance, different fullerene mono-adducts were designed by introducing oligoethylene glycol side chains with different lengths and number of branches. As a result, we achieved power conversion efficiencies up to 1.4% for PSCs processed from benign ethanol/water mixtures in air. Significantly, the new alcohol/water-soluble fullerene derivatives displayed electron mobilities up to 1.30 × 10–4 cm2 V–1 s–1, 150 times higher than those of a previously reported alcohol-soluble fullerene bis-adduct, owing to efficient packing of the fullerenes. Femtosecond transient absorption spectroscopy revealed the acceptor side chain to markedly impact geminate and/or nongeminate charge recombination in the PSCs. In addition, side chain optimization of these fullerenes produced well-intermixed morphologies with high domain purity when blended with p-type polymer to provide hole and electron transport pathways. Our results provide important guidelines for the design of electroactive materials for safe and environmentally benign fabrication of PSCs and other organic electronic devices.}, number={16}, journal={CHEMISTRY OF MATERIALS}, publisher={American Chemical Society (ACS)}, author={Kim, Youngkwon and Choi, Joonhyeong and Lee, Changyeon and Kim, Youngwoong and Kim, Changkyun and Nguyen, Thanh Luan and Gautam, Bhoj and gundogdu and Woo, Han Young and Kim, Bumjoon J. and et al.}, year={2018}, month={Aug}, pages={5663–5672} }
 @article{younts_danilov_gundogdu_gautam_2018, title={Charge generation dynamics in polymer nonfullerene solar cells with low energy loss}, volume={8}, ISSN={["1947-7988"]}, DOI={10.1117/1.JPE.8.032209}, abstractNote={Polymer nonfullerene solar cells are emerging as an alternative of polymer fullerene solar cells. However, maximizing the short-circuit density and open-circuit voltage is a critical issue in these solar cells. Here, using ultrafast spectroscopy, we measured exciton relaxation and charge separation dynamics in polymer nonfullerene blend with low driving force. Our study indicates high polaron yield despite the energy loss of as low as 0.59 eV. This suggests that the small driving force has minimum detrimental effect in realizing high performance in polymer nonfullerene solar cells.}, number={3}, journal={JOURNAL OF PHOTONICS FOR ENERGY}, author={Younts, Robert and Danilov, Evgeny and Gundogdu, Kenan and Gautam, Bhoj}, year={2018} }
 @article{lo_gautam_selter_zheng_oosterhout_constantinou_knitsch_wolfe_yi_bredas_et al._2018, title={Every Atom Counts: Elucidating the Fundamental Impact of Structural Change in Conjugated Polymers for Organic Photovoltaics}, volume={30}, ISSN={["1520-5002"]}, DOI={10.1021/acs.chemmater.8b00590}, abstractNote={As many conjugated polymer-based organic photovoltaic (OPV) materials provide substantial solar power conversion efficiencies (as high as 13%), it is important to develop a deeper understanding of how the primary repeat unit structures impact device performance. In this work, we have varied the group 14 atom (C, Si, Ge) at the center of a bithiophene fused ring to elucidate the impact of a minimal repeat unit structure change on the optical, transport, and morphological properties, which ultimately control device performance. Careful polymerization and polymer purification produced three “one-atom change” donor–acceptor conjugated alternating copolymers with similar molecular weights and dispersities. DFT calculation, absorption spectroscopy, and high-temperature solution 1H nuclear magnetic resonance (NMR) results indicate that poly(dithienosilole-alt-thienopyrrolodione), P(DTS-TPD), and poly(dithienogermole-alt-thienopyrrolodione), P(DTG-TPD) exhibit different rotational conformations when compared to poly(cyclopentadithiophene-alt-thienopyrrolodione), P(DTC-TPD). Solid-state 1H MAS NMR experiments reveal that the greater probability of the anticonformation in P(DTS-TPD) and P(DTG-TPD) prevail in the solid phase. The conformational variation seen in solution and solid-state NMR in turn affects the polymer stacking and intermolecular interaction. Two-dimension 1H-1H DQ-SQ NMR correlation spectra shows aromatic–aromatic correlations for P(DTS-TPD) and P(DTG-TPD), which on the other hand is absent for P(DTC-TPD). In a thin-film interchain packing study using grazing incidence wide-angle X-ray scattering (GIWAXS), we observe the π-face of the conjugated backbones of P(DTC-TPD) aligned edge-on to the substrate, whereas in contrast the π-faces of P(DTS-TPD) and P(DTG-TPD) align parallel to the surface. These differences in polymer conformations and backbone orientations lead to variations in the OPV performance of blends with the fullerene PC71BM, with the device containing P(DTC-TPD):PCBM having a lower fill factor and a lower power conversion efficiency. Ultrafast transient absorption spectroscopy shows the P(DTC-TPD):PCBM blend to have a more pronounced triplet formation from bimolecular recombination of initially separated charges. With a combination of sub-bandgap external quantum efficiency measurements and DFT calculations, we present evidence that the greater charge recombination loss is the result of a lower lying triplet energy level for P(DTC-TPD), leading to a higher rate of recombination and lower OPV device performance. Importantly, this study ties ultimate photovoltaic performance to morphological features in the active films that are induced from the processing solution and are a result of minimal one-atom differences in polymer repeat unit structure.}, number={9}, journal={CHEMISTRY OF MATERIALS}, author={Lo, Chi Kin and Gautam, Bhoj R. and Selter, Philipp and Zheng, Zilong and Oosterhout, Stefan D. and Constantinou, Iordania and Knitsch, Robert and Wolfe, Rylan M. W. and Yi, Xueping and Bredas, Jean-Luc and et al.}, year={2018}, month={May}, pages={2995–3009} }
 @article{gautam_klump_yi_constantinou_shewmon_salehi_lo_zheng_brédas_gundogdu_et al._2018, title={Increased Exciton Delocalization of Polymer upon Blending with Fullerene}, volume={30}, ISSN={0935-9648}, url={http://dx.doi.org/10.1002/ADMA.201801392}, DOI={10.1002/ADMA.201801392}, abstractNote={Interfaces between donor and acceptor in a polymer solar cell play a crucial role in exciton dissociation and charge photogeneration. While the importance of charge transfer (CT) excitons for free carrier generation is intensively studied, the effect of blending on the nature of the polymer excitons in relation to the blend nanomorphology remains largely unexplored. In this work, electroabsorption (EA) spectroscopy is used to study the excited-state polarizability of polymer excitons in several polymer:fullerene blend systems, and it is found that excited-state polarizability of polymer excitons in the blends is a strong function of blend nanomorphology. The increase in excited-state polarizability with decreased domain size indicates that intermixing of states at the interface between the donor polymers and fullerene increases the exciton delocalization, resulting in an increase in exciton dissociation efficiency. This conclusion is further supported by transient absorption spectroscopy and time-resolved photoluminescence measurements, along with the results from time-dependent density functional theory calculations. These findings indicate that polymer excited-state polarizability is a key parameter for efficient free carrier generation and should be considered in the design and development of high-performance polymer solar cells.}, number={30}, journal={Advanced Materials}, publisher={Wiley}, author={Gautam, Bhoj and Klump, Erik and Yi, Xueping and Constantinou, Iordania and Shewmon, Nathan and Salehi, Amin and Lo, Chi Kin and Zheng, Zilong and Brédas, Jean-Luc and Gundogdu, Kenan and et al.}, year={2018}, month={Jun}, pages={1801392} }
 @article{dunlap-shohl_barraza_barrette_gundogdu_stiff-roberts_mitzi_2018, title={MAPbI(3) Solar Cells with Absorber Deposited by Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation}, volume={3}, ISSN={["2380-8195"]}, DOI={10.1021/acsenergylett.7b01144}, abstractNote={Resonant infrared, matrix-assisted pulsed laser evaporation (RIR-MAPLE) is a gentle thin-film deposition technique that combines the facile chemical control of solution processing with the growth control of vapor-phase deposition, yet one that has not been widely applied to crystalline organic–inorganic hybrid materials. In this work, we investigate the optoelectronic quality of RIR-MAPLE-deposited CH3NH3PbI3 (MAPbI3) perovskite films and report on the fabrication of perovskite solar cells in which the absorber is deposited by RIR-MAPLE. We find the composition, morphology, and optical properties of these perovskite films to be comparable to those produced by more conventional methods, such as spin coating. The champion device reaches a stabilized power conversion efficiency of over 12%, a high value for perovskite solar cells deposited by a laser ablation process, highlighting the ability of this new technique to produce device-quality films.}, number={2}, journal={ACS ENERGY LETTERS}, author={Dunlap-Shohl, Wiley A. and Barraza, E. Tomas and Barrette, Andrew and Gundogdu, Kenan and Stiff-Roberts, Adrienne D. and Mitzi, David B.}, year={2018}, month={Feb}, pages={270–275} }
 @article{awartani_gautam_zhao_younts_hou_gundogdu_ade_2018, title={Polymer non-fullerene solar cells of vastly different efficiencies for minor side-chain modification: impact of charge transfer, carrier lifetime, morphology and mobility}, volume={6}, ISSN={["2050-7496"]}, url={https://doi.org/10.1039/C7TA01746D}, DOI={10.1039/c7ta01746d}, abstractNote={The performance of the 11.25% efficient PBDB-T : ITIC system degraded to 4.35% after a minor side-chain modification in PBDB-O : ITIC. In this study, the underlying reasons behind this vast difference in efficiencies are investigated.}, number={26}, journal={JOURNAL OF MATERIALS CHEMISTRY A}, publisher={Royal Society of Chemistry (RSC)}, author={Awartani, Omar M. and Gautam, Bhoj and Zhao, Wenchao and Younts, Robert and Hou, Jianhui and Gundogdu, Kenan and Ade, Harald}, year={2018}, month={Jul}, pages={12484–12492} }
 @article{gautam_younts_carpenter_ade_gundogdu_2018, title={The Role of FRET in Non-Fullerene Organic Solar Cells: Implications for Molecular Design}, volume={122}, ISSN={["1089-5639"]}, url={https://doi.org/10.1021/acs.jpca.7b12807}, DOI={10.1021/acs.jpca.7b12807}, abstractNote={Non-fullerene acceptors (NFAs) have been demonstrated to be promising candidates for highly efficient organic photovoltaic (OPV) devices. The tunability of absorption characteristics of NFAs can be used to make OPVs with complementary donor–acceptor absorption to cover a broad range of the solar spectrum. However, both charge transfer from donor to acceptor moieties and energy (energy) transfer from high-bandgap to low-bandgap materials are possible in such structures. Here, we show that when charge transfer and exciton transfer processes are both present, the coexistence of excitons in both domains can cause a loss mechanism. Charge separation of excitons in a low-bandgap material is hindered due to exciton population in the larger bandgap acceptor domains. Our results further show that excitons in low-bandgap material should have a relatively long lifetime compared to the transfer time of excitons from higher bandgap material in order to contribute to the charge separation. These observations provide significant guidance for design and development of new materials in OPV applications.}, number={15}, journal={JOURNAL OF PHYSICAL CHEMISTRY A}, publisher={American Chemical Society (ACS)}, author={Gautam, Bhoj R. and Younts, Robert and Carpenter, Joshua and Ade, Harald and Gundogdu, Kenan}, year={2018}, month={Apr}, pages={3764–3771} }
 @article{li_zhong_gautam_bin_lin_wu_zhang_jiang_zhang_gundogdu_et al._2017, title={A near-infrared non-fullerene electron acceptor for high performance polymer solar cells}, volume={10}, ISSN={["1754-5706"]}, DOI={10.1039/c7ee00844a}, abstractNote={Low-bandgap polymers/molecules are an interesting family of semiconductor materials, and have enabled many recent exciting breakthroughs in the field of organic electronics, especially for organic photovoltaics (OPVs).}, number={7}, journal={ENERGY & ENVIRONMENTAL SCIENCE}, author={Li, Yongxi and Zhong, Lian and Gautam, Bhoj and Bin, Hai-Jun and Lin, Jiu-Dong and Wu, Fu-Peng and Zhang, Zhanjun and Jiang, Zuo-Quan and Zhang, Zhi-Guo and Gundogdu, Kenan and et al.}, year={2017}, month={Jul}, pages={1610–1620} }
 @article{younts_duan_gautam_saparov_liu_mongin_castellano_mitzi_gundogdu_2017, title={Efficient generation of long-lived triplet excitons in 2D hybrid perovskite}, volume={29}, number={9}, journal={Advanced Materials}, author={Younts, R. and Duan, H. S. and Gautam, B. and Saparov, B. and Liu, J. and Mongin, C. and Castellano, F. N. and Mitzi, D. B. and Gundogdu, K.}, year={2017} }
 @article{yu_li_huang_barrette_cai_yu_gundogdu_zhang_cao_2017, title={Enhancing Multifunctionalities of Transition-Metal Dichalcogenide Monolayers via Cation Intercalation}, volume={11}, ISSN={1936-0851 1936-086X}, url={http://dx.doi.org/10.1021/ACSNANO.7B04880}, DOI={10.1021/ACSNANO.7B04880}, abstractNote={We have demonstrated that multiple functionalities of transition-metal dichalcogenide (TMDC) monolayers may be substantially improved by the intercalation of small cations (H+ or Li+) between the monolayers and underlying substrates. The functionalities include photoluminescence (PL) efficiency and catalytic activity. The improvement in PL efficiency may be up to orders of magnitude and can be mainly ascribed to two effects of the intercalated cations: p-doping to the monolayers and reducing the influence of substrates, but more studies are necessary to better understand the mechanism for the improvement in the catalytic functionality. The cation intercalation may be achieved by simply immersing substrate-supported monolayers into the solution of certain acids or salts. It is more difficult to intercalate under the monolayers interacting with substrates stronger, such as as-grown monolayers or the monolayers on 2D material substrates. This result presents a versatile strategy to simultaneously optimize multiple functionalities of TMDC monolayers.}, number={9}, journal={ACS Nano}, publisher={American Chemical Society (ACS)}, author={Yu, Yifei and Li, Guoqing and Huang, Lujun and Barrette, Andrew and Cai, Yong-Qing and Yu, Yiling and Gundogdu, Kenan and Zhang, Yong-Wei and Cao, Linyou}, year={2017}, month={Sep}, pages={9390–9396} }
 @article{cho_han_younts_lee_gautam_lee_lee_kim_kim_gundogdu_et al._2017, title={Impact of highly crystalline, isoindigo-based small-molecular additives for enhancing the performance of all-polymer solar cells}, volume={5}, ISSN={["2050-7496"]}, url={https://doi.org/10.1039/C7TA06939A}, DOI={10.1039/c7ta06939a}, abstractNote={We have developed a simple yet versatile approach for enhancing the performance of all-polymer solar cells (all-PSCs) using a highly crystalline small-molecular additive, 6,6′-dithiopheneisoindigo (DTI).}, number={40}, journal={JOURNAL OF MATERIALS CHEMISTRY A}, publisher={Royal Society of Chemistry (RSC)}, author={Cho, Han-Hee and Han, Gibok and Younts, Robert and Lee, Wonho and Gautam, Bhoj R. and Lee, Seungjin and Lee, Changyeon and Kim, Taesu and Kim, Felix Sunjoo and Gundogdu, Kenan and et al.}, year={2017}, month={Oct}, pages={21291–21299} }
 @article{kim_younts_lee_lee_gundogdu_kim_2017, title={Impact of the photo-induced degradation of electron acceptors on the photophysics, charge transport and device performance of all-polymer and fullerene-polymer solar cells}, volume={5}, ISSN={["2050-7496"]}, url={https://doi.org/10.1039/C7TA07535A}, DOI={10.1039/c7ta07535a}, abstractNote={We report a comparative study of the photo-stabilities of all-polymer and fullerene-polymer solar cells based on the same polymer donor.}, number={42}, journal={JOURNAL OF MATERIALS CHEMISTRY A}, publisher={Royal Society of Chemistry (RSC)}, author={Kim, Taesu and Younts, Robert and Lee, Wonho and Lee, Seungjin and Gundogdu, Kenan and Kim, Bumjoon J.}, year={2017}, month={Nov}, pages={22170–22179} }
 @article{popescu_younts_hoffman_mcafee_dougherty_ade_gundogdu_bondarev_2017, title={Monitoring Charge Separation Processes in Quasi-One-Dimensional Organic Crystalline Structures}, volume={17}, ISSN={1530-6984 1530-6992}, url={http://dx.doi.org/10.1021/ACS.NANOLETT.7B02471}, DOI={10.1021/ACS.NANOLETT.7B02471}, abstractNote={We perform the transient absorption spectroscopy experiments to investigate the dynamics of the low-energy collective electron-hole excitations in α-copper phthalocyanine thin films. The results are interpreted in terms of the third-order nonlinear polarization response function. It is found that, initially excited in the molecular plane, the intramolecular Frenkel exciton polarization reorients with time to align along the molecular chain direction to form coupled Frenkel-charge-transfer exciton states, the eigenstates of the one-dimensional periodic molecular lattice. The process pinpoints the direction of the charge separation in α-copper phthalocyanine and similar organic molecular structures. Being able to observe and monitor such processes is important both for understanding the physical principles of organic thin film solar energy conversion device operation and for the development of organic optoelectronics in general.}, number={10}, journal={Nano Letters}, publisher={American Chemical Society (ACS)}, author={Popescu, Adrian and Younts, Robert A. and Hoffman, Benjamin and McAfee, Terry and Dougherty, Daniel B. and Ade, Harald W. and Gundogdu, Kenan and Bondarev, Igor V.}, year={2017}, month={Sep}, pages={6056–6061} }
 @article{nguyen_lee_gautam_park_gundogdu_kim_woo_2017, title={Single Component Organic Solar Cells Based on Oligothiophene-Fullerene Conjugate}, volume={27}, ISSN={1616-301X}, url={http://dx.doi.org/10.1002/ADFM.201702474}, DOI={10.1002/ADFM.201702474}, abstractNote={A new donor (D)–acceptor (A) conjugate, benzodithiophene-rhodanine–[6,6]-phenyl-C61 butyric acid methyl ester (BDTRh–PCBM) comprising three covalently linked blocks, one of p-type oligothiophene containing BDTRh moieties and two of n-type PCBM, is designed and synthesized. A single component organic solar cell (SCOSC) fabricated from BDTRh–PCBM exhibits the power conversion efficiency (PCE) of 2.44% and maximum external quantum efficiency of 46%, which are the highest among the reported efficiencies so far. The SCOSC device shows efficient charge transfer (CT, ≈300 fs) and smaller CT energy loss, resulting in the higher open-circuit voltage of 0.97 V, compared to the binary blend (BDTRh:PCBM). Because of the integration of the donor and acceptor in a single molecule, BDTRh-PCBM has a specific D–A arrangement with less energetic disorder and reorganization energy than blend systems. In addition, the SCOSC device shows excellent device and morphological stabilities, showing no degradation of PCE at 80 °C for 100 h. The SCOSC approach may suggest a great way to suppress the large phase segregation of donor and acceptor domains with better morphological stability compared to the blend device.}, number={39}, journal={Advanced Functional Materials}, publisher={Wiley}, author={Nguyen, Thanh Luan and Lee, Tack Ho and Gautam, Bhoj and Park, Song Yi and Gundogdu, Kenan and Kim, Jin Young and Woo, Han Young}, year={2017}, month={Aug}, pages={1702474} }
 @article{kim_gadisa_schaefer_yao_gautam_balar_ghasemi_constantinou_so_o'connor_et al._2017, title={Strong polymer molecular weight-dependent material interactions: impact on the formation of the polymer/fullerene bulk heterojunction morphology}, volume={5}, ISSN={2050-7488 2050-7496}, url={http://dx.doi.org/10.1039/C7TA03052E}, DOI={10.1039/c7ta03052e}, abstractNote={The morphological evolution is initiated by L–L or L–S phase separation (left) and further developed by molecular mobility, governed by polymer–solvent interactions which determine the final domain size of the BHJ layer (right).}, number={25}, journal={Journal of Materials Chemistry A}, publisher={Royal Society of Chemistry (RSC)}, author={Kim, Joo-Hyun and Gadisa, Abay and Schaefer, Charley and Yao, Huifeng and Gautam, Bhoj R. and Balar, Nrup and Ghasemi, Masoud and Constantinou, Iordania and So, Franky and O'Connor, Brendan T. and et al.}, year={2017}, pages={13176–13188} }
 @article{kranthiraja_kim_lee_gunasekar_sree_gautam_gundogdu_jin_kim_2017, title={The Impact of Sequential Fluorination of π-Conjugated Polymers on Charge Generation in All-Polymer Solar Cells}, volume={27}, ISSN={1616-301X}, url={http://dx.doi.org/10.1002/ADFM.201701256}, DOI={10.1002/ADFM.201701256}, abstractNote={The performance of all-polymer solar cells (all-PSCs) is often limited by the poor exciton dissociation process. Here, the design of a series of polymer donors (P1–P3) with different numbers of fluorine atoms on their backbone is presented and the influence of fluorination on charge generation in all-PSCs is investigated. Sequential fluorination of the polymer backbones increases the dipole moment difference between the ground and excited states (Δµge) from P1 (18.40 D) to P2 (25.11 D) and to P3 (28.47 D). The large Δµge of P3 leads to efficient exciton dissociation with greatly suppressed charge recombination in P3-based all-PSCs. Additionally, the fluorination lowers the highest occupied molecular orbital energy level of P3 and P2, leading to higher open-circuit voltage (VOC). The power conversion efficiency of the P3-based all-PSCs (6.42%) outperforms those of the P2 and P1 (5.00% and 2.65%)-based devices. The reduced charge recombination and the enhanced polymer exciton lifetime in P3-based all-PSCs are confirmed by the measurements of light-intensity dependent short-circuit current density (JSC) and VOC, and time-resolved photoluminescence. The results provide reciprocal understanding of the charge generation process associated with Δµge in all-PSCs and suggest an effective strategy for designing π-conjugated polymers for high performance all-PSCs.}, number={29}, journal={Advanced Functional Materials}, publisher={Wiley}, author={Kranthiraja, Kakaraparthi and Kim, Seonha and Lee, Changyeon and Gunasekar, Kumarasamy and Sree, Vijaya Gopalan and Gautam, Bhoj and Gundogdu, Kenan and Jin, Sung-Ho and Kim, Bumjoon J.}, year={2017}, month={May}, pages={1701256} }
 @article{kranthiraja_kim_lee_gunasekar_sree_gautam_gundogdu_jin_kim_2017, title={The impact of sequential fluorination of pi-conjugated polymers on charge generation in all-polymer solar cells}, volume={27}, number={29}, journal={Advanced Functional Materials}, author={Kranthiraja, K. and Kim, S. and Lee, C. and Gunasekar, K. and Sree, V. G. and Gautam, B. and Gundogdu, K. and Jin, S. H. and Kim, B. J.}, year={2017} }
 @article{zhu_xue_he_jiang_hu_bai_zhang_xiao_gundogdu_gautam_et al._2016, title={A PCBM Electron Transport Layer Containing Small Amounts of Dual Polymer Additives that Enables Enhanced Perovskite Solar Cell Performance}, volume={3}, ISSN={["2198-3844"]}, DOI={10.1002/advs.201500353}, abstractNote={A polymer/PCBM hybrid electron transport layer is reported that enables high-performance perovskite solar cells with a high power conversion efficiency of 16.2% and with negligible hysteresis. Unlike previous approaches of reducing hysteresis by thermal annealing or fullerene passivation, the success of our approach can be mainly attributed to the doping of the PCBM layer using an insulating polymer (polystyrene) and an amine-containing polymeric semiconductor named PFNOX. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. The field of organic–inorganic metal halide perovskite solar cells (PKSCs) has seen impressive progresses in recent years.1 The rapidly increasing power conversion efficiency (PCE) of PKSCs has attracted much attention of both the academic and industrial research communities and makes PKSCs one of the most promising candidates for the next generation of large scale cost-competitive photovoltaic technology.2 The attractive features of PKSCs include high absorption characteristics, appropriate direct band gaps, high carrier mobility, long charge carrier diffusion length, low cost, and easy fabrication processes.3 These merits had pushed the PCE of PKSCs to a record value of 20.1% recently.4 The conventional PKSC structure5 consists of an electron transport layer (ETL) (e.g., TiO2) with the perovskite layer on top and then capped with a hole transport layer (HTL, such as spiro-OMeTAD). In the past two years, however, inverted structure PKSCs (with a typical structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/perovskite layer/phenyl-C61-butyric acid methyl ester (PCBM)/top electrode) have developed rapidly and can now achieve comparable PCEs with the conventional structure PKSCs.6 One of the main challenges for PKSCs is that typical PKSCs exhibit large hysteresis in their J–V characteristics, which makes PKSCs difficult to characterize or to produce in a reproducible and repeatable manner.[2],7 The hysteresis problem is typically less severe for conventional structure PKSCs based on the mesoporous TiO2. This is because the mesoscopic TiO2 offers a large surface area and the charges generated in the perovskite layer can be transported to the TiO2 mesoporous film or via the self-connected perovskite network.8 In contrast, it is more challenging to optimize the interfaces in planar-structure inverted PKSCs and to achieve a perfectly balanced electron and hole currents in the device. Thus far, several reports have shown hysteresis-less PKSCs by long-time thermal annealing, filling with PCBM,9 and by balancing the electron and hole transports in the perovskite cell.10 However, the majority of the reports on inverted PKSCs are still troubled with various extents of the hysteresis problem and there are limited reports of high-efficiency inverted PKSCs with completely eliminated hysteresis.11 Indicated by previous reports,[6],[11] the hysteresis problem could be related to the hole and electron transport layers (e.g., need to achieve balanced charge transport)[6] as well as the perovskite active layer (e.g., need to grow large crystals and eliminate traps in the perovskite active layer).[11] Therefore, the complete elimination of hysteresis in PKSCs may require the combination of different approaches that address different components of the perovskite cells. As most previous work has been focused more on the perovskite active layer than the PCBM ETL, we hope to improve the PCBM ETL to provide a simple and effective approach to complement previous ones. In our previous work, we have developed a series of polyfluorene-based semiconducting polymers that contain alkylamine side chains.[11],12 One of the classical materials in the family is PFNOX, which has been widely used as interfacial layers for organic light emitting diodes, field effect transistors, and polymer solar cells.13 In our recent work on polymer solar cells, it was discovered that PFNOX can electrically dope PCBM and thus enables efficient electron transfer between the active layer and the interlayer of the polymer solar cell devices.12, 14 The excellent electron transfer characteristics and effective electrical doping of PFNOX to PCBM for polymer solar cells inspired us to use PFNOX as a polymer additive to electrically dope the PCBM ETL for PKSCs. It is expected that improved electron transport property of the PCBM upon PFNOX doping should benefit the performance of PKSCs. In this paper, we report an improved PCBM ETL that contains ≈95% of PCBM and two polymer additives, namely, PFNOX and polystyrene (PS), at very small weight percentages (1%–3%). As been demonstrated in our previous report,15 the function of the polystyrene (with a high molecular weight of 10 m kDa) additive is mainly to increase the viscosity of the ETL solution and thus improving the quality of the ETL films. The high film quality of the ETL also helps to suppress undesirable electron recombination caused by the defects in the ETL, which contributes to the increased Voc of the cell. In this paper, discussions will be mainly focused on the dramatic impacts of using PFNOX as the electrical dopant of the PCBM ETL. When neat PCBM or PCBM/PS are used as the ETL, the PKSCs exhibit large hysteresis and can only achieve PCE of ≈11%. With the additional PFNOX dopant, however, the PKSCs based on PCBM/PFNOX or PCBM/PFNOX/PS as the ETL exhibit dramatic improvements on cell performance with their PCEs increased to 14% and 16.2%, respectively. Most importantly and surprisingly, the cells based on ETL containing the PFNOX dopant exhibits dramatically reduced hysteresis. Our best cell with PCBM/(PFNOX&PS) ETL shows higher efficiency and negligible hysteresis, compared to the devices-based neat PCBM ETL. Combining the study of the transient photoluminescence (PL) and electrochemical impedance spectroscopy (EIS), it was found that the PFNOX doped PCBM ETL facilitates the electron injection from halide perovskite into the ETL and decreases the recombination rate of charge carriers. Our hybrid polymer/PCBM ETL offers a simple yet highly effective approach to improve the performance and reduce the hysteresis of PKSCs. The fundamental studies on the doped ETL also offer important insights on understanding the hysteresis problem and charge transfer and transport processes in PKSCs. The representative perovskite solar cell device with the structure of ITO/PEDOT:PSS/perovskite/PCBM:PFNOX/Ag is shown in Figure 1A. The chemical structure of PFNOX and PCBM are shown in Figure 1B. PFNOX is one of the best-performance example of the alkyl-amine-containing polyfluorene conjugated polymer family. In a typical PFNOX/PCBM hybrid ETL, there is only a small amount of (1.25 wt%) PFNOX in the PCBM layer. A PFNOX solution (5 mg mL−1) was added into a 20 mg mL−1 PCBM solution at a volume ratio of 1:20 and stirred overnight at 60 °C. The energy levels of the PCBM:PFNOX hybrid ETL are similar to those of the neat PCBM films, which are confirmed by the similar peaks in the UV–vis absorption spectra (Figure S1, Supporting Information) and cyclic voltammograms (Figure S2, Supporting Information) of the corresponding films. Figure 1C presents the typical cross-sectional scanning electron microscopy (SEM) image of a high performance photovoltaic device. A well-crystallized mixed halide perovskite (CH3NH3PbCl3−xIx) layer with a thickness of 450 nm is sandwiched between the PEDOT-PSS HTL and the PCBM: PFNOX ETL. Thus, a total thickness of the cell is ≈1 μm including a 200–300 nm ITO layer. To prove that the chlorine is homogeneously distributed in the perovskite layer, we show the SEM mapping, the enerdy dispersive X-ray spectroscopy (EDS) picture and element distribution (Figure S4 and Table S3, Supporting Information) of the perovskite layer. The data show that the chlorine ions are homogeneous and the chlorine only has small percentage in the MAPbCl3−xIx, which is in agreement with previous reports.15 The photocurrent density–voltage (J–V) characteristics measured under AM 1.5G irradiation at a one sun intensity of 100 mW cm−2 for cells with and without PFNOX additives are shown in Figure 2. The detailed performance parameters of these devices are summarized in Table 1. A typical device using the pure PCBM as the ETL can only achieve a PCE of ≈11.4%, with a short-circuit photocurrent (Jsc) 18.6 mA cm−2, an open-circuit voltage (Voc) of 0.952 and a fill factor (FF) of 0.668. However, when loading the PFNOX additive into the PCBM layer, the best performance has increased 14.0% with the Voc increasing to 0.94 V, Jsc increasing to 20.4 mA cm−2, and FF increasing to 0.729. This efficiency increase can be mainly attributed to the increases of the Jsc and FF. The incident photon-to-electron conversion efficiency (IPCE) spectrum is displayed in Figure 2B showing a remarkable increase of the integrated IPCE area upon PFNOX doping. In order to improve the uniformity of the PCBM ETL and further enhance the PCE performance, a second polymer additive, polystyrene (Mw ≈ 10 m), was included into the PCBM/PFNOX ETL. As shown in our previous report, the PS additive can increase the viscosity of the solution, thus improve the uniformity of the PCBM ETL.15, 16 As also shown in Figure 2A, the PS additive further increased the PCE of the PKSCs to an impressive value of 16.2%, with a Voc of 1.01 V, a Jsc of 20.8 mA cm−2, and a FF of 0.769. To check whether the high PCE of our devices is reliable, the stabilized photocurrent was measured at a photovoltage near the maximum power point of the optimal device. The photocurrent was then monitored and plotted as a function of time. As shown in Figure 2C, we obtained a steady-state PCE of 13.5% for PCBM:PFNOX-based device and 15.6% for that based on PCBM:(PFNOX&PS), thereby which is in agreement with the average PCE of the related devices. However, the PCBM only based device has a low PCE of 9.4%. For comparison, we also fabricated devices using the standard structure (FTO/TiO2/MAPbCl3−xIx/Spiro-OMeTAD/Ag) and obtained an efficiency of 14.5% (Table S5, Supporting Information). Hysteresis has been one of major problems for PKSCs, which could be attributed to grain boundaries and defects that cause strong recombination between the perovskite and charge transport layer.17 Several groups previously reported less-hysteresis effect for inverted planar heterojunction PKSCs with a structure of PEDOT:PSS/perovskite/PCBM.9, 18 One of the proposed reasons for the reduced hysteresis was the thermal passivation of the perovskite layer after long-term high temperature annealing, during which small molecular PCBM deposited onto the trap states of perovskite surface and eliminate the photocurrent hysteresis.1 Surface passivation is another approach to decrease the surface defects and reduce the recombination in the perovskite film.[18] In previous studies,[8],19 the fundamental causes of hysteresis have been attributed to several possible factors including grain boundaries in the perovskite films and charge-trapping defects at the perovskite and transport layer interfaces. Previous approaches were mainly focusing on improving the quality of perovskite layer by removing defects or growing large crystals. In contrast, our study focused on improving the PCBM electron transport layer. Figure 2D–F presents the three I–V curves for pure PCBM, PCBM:PFNOX, and PCBM: (PFNOX&PS) ETLs-based photovoltaic devices. For the PFNOX and PS codoped PCBM devices, they both show J–V curves with negligible hysteresis, with PCEs of 16.2% and 15.0% for the reverse and forward scans, respectively. We also performed the forward and reverse J–V scans at different rates and show that the performance is quite insensitive to the scan rates (Figure S5 and Table S4, Supporting Information). The negligible hysteresis is mainly attributed to PFNOX, as PCBM: PS devices (shown in Figure S3, Supporting Information) exhibit large hysteresis but PCBM: PFNOX shows negligible hysteresis. The pure PCBM-based solar devices also exhibit large hysteresis, which is due to the fast recombination by the surface trap states of perovskite films. As shown in a recent report, it is important to achieve balanced electron and hole transports in PKSCs to reduce hysteresis, our results clearly demonstrated that the electron transport ability of PCBM was a limiting factor in our PKSCs. By doping and improving the electron transport ability of the PCBM ETL, the performance of PKSCs can be dramatically enhanced. It is also reasonably to expect that our approach of improving the PCBM ETL can be combined with previous methods on improving the perovskite active layer, as faster electron transport should always be a desirable change for PKSCs. As PFNOX has been demonstrated to dope PCBM, it is reasonable to expect that the PFNOX doped PCBM can facilitate the electron transfer from the perovskite layer to the ETL layer and thus improving the cell performance. To prove this hypothesis, steady state, and transient PL spectroscopy were employed to study the charge carriers extraction process from the perovskite absorber to the PCBM layer.20 The steady state PL spectra of pure halide CH3NH3PbCl3−xIx perovskite and perovskite/ETL films are shown in Figure 3A. All of the samples exhibit a PL peak at 760 nm originated from the perovskite CH3NH3PbCl3−xIx. The PL intensity was largely quenched after the PCBM electron accepting layer coating. However, the extent of PL quenching is more significant for the perovskite sample with PCBM:PFNOX. The PL intensity of the PFNOX-doped PCBM sample decreased by 21 times compared to that of the perovskite sample, while the PL intensity decrease was ≈15 times for the nondoped PCBM sample. This observation indicates that PFNOX doping enables better charge separation from perovskite absorber to the ETL. We tested the electron mobility based on electron-only devices with the structure of ITO/ZnO/electron transport layer/Ca/Al. The J–V curves are shown in Figure 3B and the electrical parameters are summarized in Table S1 (Supporting Information). The result indicates that the electron transport ability of the PCBM:PFNOX is improved because PFNOX can effectively dope the PCBM ETL and thus enabling a greater electron transport ability. Also, our cyclic voltammetry and UV–vis measurements (Figures S1 and S2, Supporting Information) confirmed that the PFNOX-doped PCBM ETL has similar energy levels to the pure PCBM ETL. It is important to note that our study is distinctively different from previously reported hybrid perovskite/polymer solar cells,[6] which used a light-absorbing polymer:fullerene blend (≈50%:50% weight ratio) that contribute to the photo-current generation of the integrated cell. In our approach, the two polymers only account for 1–4 wt% in weight for the blend. In addition, the PFNOX polymer and polystyrene used in our study have very weak light-absorption properties (Figure S1, Supporting Information) in nature. They only function as low-concentration dopants to the PCBM ETL and do not contribute to the photo-current of the PKSCs. In this sense, the cells described in the previous report[6] are truly hybrid perovskite/polymer bulk-heterojunction (BHJ) solar cells, in which both the perovskite part and the polymer:fullerene BHJ film contribute to the photocurrent. However, our cells with PCBM:(PS&PFNOX) is still the typical inverted PKSCs that utilize a PCBM ETL with improved electron transport ability upon doping with two photo-inactive polymers. To examine the transfer process of free charges in these devices, we measured time-resolved PL spectroscopy directly on these solar cell devices for PCBM and PCBM:PFNOX-coated perovskite films. Figure 3C,D shows the decay traces of the emission peaks at 760 nm (1.63 eV) for the two devices with or without PFNOX at short-circuit and open-circuit conditions. More details for the measurement can be found in the Supporting Information and the fitting decay time is shown in Table S2 (Supporting Information). From these data, it is clear to conclude that the electron transfer from the perovskite to the ETL is faster for the PFNOX-doped PCBM-based than that for the neat PCBM-based devices, where the electron transfer rate of the PFNOX-based devices was ≈0.3 ns while that of the devices with neat PCBM was only ≈0.7 ns, suggesting PFNOX doping improves the electron transfer property. To further study the charge transfer process, especially the back recombination from ETL to perovskite layer, we measured devices under operating condition by EIS, which is often used to measure charge transfer and recombination parameters in solar cells, such as chemical capacitance, recombination resistance, charge conductivity, etc.21, 22Figure 4 presents Nyquist plots of the solar cells with PCBM: PFNOX (Figure 4A) and PCBM (Figure 4B) based devices, which were recorded at different applied voltages under 1-sun illumination. The frequency range for all of the EIS measurements is from 0.5 Hz to 1 MHz. All devices were applied a bias voltage ranging from 0 to 0.9 V as transport in the high frequency region would meet very small resistance, making it difficult to model the exact transport resistance value. The Nyquist plots typically can be demarked into two main regions or arcs at high and low frequencies. We fitted our data based on a commonly used equivalent circuit model shown in Figure 4E. Here, in the high frequency region, a transport resistance (Rtrans) with parallel chemical capacitance (Ctrans) subcircuit was applied. Then the arc in the low frequency region is assigned to the recombination resistance Rrec and the parallel chemical capacitance Crec. We introduce a recombination lifetime (Figure 4C) which combines the surface and bulk charge recombination lifetime. Here with the PFNOX doping, the recombinaion life time is much larger, almost one order of magnitude, than without PFNOX doping devices, which proves the recombination routes from PCBM back to perovskite is suppressed. We also calculated the transport mobility from high frequencies arc of the Nyquist plots in Figure 4D. Considering that the only variable factor is PFNOX, we can conclude that the improved tranport mobility is due to the PFNOX doping, as it has been demonstrated before that PFNOX can effectively dope PCBM12 and the doped PCBM apparently should exhibit a better charge transport ability than the nondoped counterpart.(also supported by our electron-only device results in Figure 3B). In summary, we have successfully developed a new polymer:PCBM hybrid ETL that is very simple yet appears to have dramatic impacts in improving the PCE and reducing the hysteresis of PKSCs. The PFNOX-doped devices exhibit a significant enhancement of short-circuit photocurrent (JSC) and PCE of 14.0% compared the PCE of 11.4% for devices without PFNOX. Furthermore, a second additive (polystyrene) was introduced to further increase the efficiency to 16.2% with negligible hysteresis. While the function of the polystyrene additive is to increase the film quality of the ETL, the reduced hysteresis is mainly attributed to the electrical doping of PFNOX to PCBM. Through time-resolved PL and EIS study, the improved PCE and reduced hysteresis are attributed to the faster electron transfer and reduced electron recombination for the PFNOX doped devices. Our work offers a simple and effective approach to addressing hysteresis problem for PKSCs. By combining our new ETL with existing approaches in the PKSC field, further improvement in PCE and hysteresis may be possible. This work was supported by the HK-RGC General Research Funds (HKUST 606511,605710, T23-407/13 N, N_HKUST623/13, and 606012), the National Basic Research Program of China (973 Program; 2013CB834701), the Ministry of Science and Technology (Grant Nos. 2014CB643505 and 2014CB643501), HK-ITF (ITS/004/14), HKUST president's office through SSTSP scheme (Project Ref No.: EP201), and the Natural Science Foundation of China (51323003, 51573057, 51361165301, and #21374090). K.S.W. would like to acknowledge the financial support of AoE/P-02/12 and CUHK1/CRF/12G from the Research Grants Council of Hong Kong. H.A. and B.R.G. would like to acknowledge the financial support of ONR grant N000141410531. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.}, number={9}, journal={ADVANCED SCIENCE}, author={Zhu, Zonglong and Xue, Qifan and He, Hexiang and Jiang, Kui and Hu, Zhicheng and Bai, Yang and Zhang, Teng and Xiao, Shuang and Gundogdu, Kenan and Gautam, Bhoj Raj and et al.}, year={2016}, month={Sep} }
 @article{gautam_younts_li_yan_danilov_klump_constantinou_so_you_ade_et al._2016, title={Charge photogeneration in organic photovoltaics: Role of hot versus cold charge-transfer excitons}, volume={6}, DOI={10.1002/aenm.201670002}, number={1}, journal={Advanced Energy Materials}, author={Gautam, B. R. and Younts, R. and Li, W. T. and Yan, L. and Danilov, E. and Klump, E. and Constantinou, I. and So, Franky and You, W. and Ade, H. and et al.}, year={2016} }
 @article{uddin_kim_younts_lee_gautam_choi_wang_gundogdu_kim_woo_et al._2016, title={Controlling Energy Levels and Blend Morphology for All-Polymer Solar Cells via Fluorination of a Naphthalene Diimide-Based Copolymer Acceptor}, volume={49}, ISSN={["1520-5835"]}, DOI={10.1021/acs.macromol.6b01414}, abstractNote={We investigate the photovoltaic properties and charge dynamics of all polymer solar cells (all-PSCs) based on poly[(N,N′-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl)-alt-5,5′-(2,2′-bithiophene)] (P(NDI2OD-T2)) and its fluorinated analogue, poly[(N,N′-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl)-alt-5,5′-(3,3′-difluoro-2,2′-bithiophene)] (P(NDI2OD-T2F)), as the acceptor polymer and poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-alt-5-octyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione] (PBDTTTPD) as the donor polymer. The PBDTTTPD:P(NDI2OD-T2)-based device has a high open-circuit voltage (VOC) of 1.03 V but suffers from low power conversion efficiency (PCE) of 2.02% with a short-circuit current density (JSC) and fill factor (FF) of 4.45 mA cm–2 and 0.44, respectively. In a stark contrast, the PCE of PBDTTTPD:P(NDI2OD-T2F)-based PSC dramatically increases to 6.09% (VOC = 1.00 V, JSC = 11.68 mA cm–2, and FF = 0.52). These results are attributed...}, number={17}, journal={MACROMOLECULES}, author={Uddin, M. A. and Kim, Y. and Younts, R. and Lee, W. and Gautam, B. and Choi, J. and Wang, C. and gundogdu and Kim, B. J. and Woo, H. Y. and et al.}, year={2016}, month={Sep}, pages={6374–6383} }
 @article{cheema_younts_gautam_gundogdu_el-shafei_2016, title={Design and synthesis of BODIPY sensitizers with long alkyl chains tethered to N-carbazole and their application for dye sensitized solar cells}, volume={184}, ISSN={["1879-3312"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84994504448&partnerID=MN8TOARS}, DOI={10.1016/j.matchemphys.2016.09.024}, abstractNote={In this study, three boron dipyrromethenes (BODIPY) dyes with extended conjugation and electron donating carbazole groups with different alkyl chain lengths tethered to N-carbazole were synthesized and characterized for dye-sensitized solar cells. The goal was to study the effect of different alkyl chain lengths on dye aggregation at TiO2 surface. The proposed molecular strategy resulted in BODIPY dyes which showed interesting electronic absorption and fluorescence properties. It was observed that intramolecular energy transfer decreases with the increase in alkyl chain length possibly due to induced changes in molecular geometry caused by long alkyl chains. Additionally, interface analysis by impedance spectroscopy in comparison to N719 sensitized TiO2 solar cell showed significant charge transport related losses (Nyquist plot) most likely due to impedance resulted from aggregated BODIPY dye on TiO2 surface. Femtosecond transient absorption studies showed the loss of excited electrons by recombination with oxidized ground state of the sensitizers.}, journal={MATERIALS CHEMISTRY AND PHYSICS}, author={Cheema, Hammad and Younts, Robert and Gautam, Bhoj and Gundogdu, Kenan and El-Shafei, Ahmed}, year={2016}, month={Dec}, pages={57–63} }
 @article{dunlap-shohl_younts_gautam_gundogdu_mitzi_2016, title={Effects of Cd Diffusion and Doping in High-Performance Perovskite Solar Cells Using CdS as Electron Transport Layer}, volume={120}, ISSN={["1932-7455"]}, DOI={10.1021/acs.jpcc.6b05406}, abstractNote={Perovskite solar cells with stabilized power conversion efficiency exceeding 15% have been achieved, using a methylammonium lead iodide (MAPbI3) absorber and CdS as the electron transport layer. X-ray photoelectron spectroscopy reveals a small presence of Cd at the surface of most perovskite films fabricated on CdS. Perovskite films were deliberately doped with Cd to understand the possible impacts of Cd diffusion into the perovskite absorber layer. Doping substantially increases the grain size of the perovskite films but also reduces device performance through the formation of an electrical barrier, as inferred by the S-shape of their J–V curves. Time-resolved photoluminescence measurements of the doped films do not indicate substantial nonradiative recombination due to bulk defects, but a secondary phase is evident in these films, which experiments have revealed to be the organic–inorganic hybrid material methylammonium cadmium iodide, (CH3NH3)2CdI4. It is further demonstrated that this compound can for...}, number={30}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Dunlap-Shohl, Wiley A. and Younts, Robert and Gautam, Bhoj and Gundogdu, Kenan and Mitzi, David B.}, year={2016}, month={Aug}, pages={16437–16445} }
 @article{zheng_awartani_gautam_liu_qin_li_bataller_gundogdu_ade_hou_et al._2016, title={Efficient Charge Transfer and Fine-Tuned Energy Level Alignment in a THF-Processed Fullerene-Free Organic Solar Cell with 11.3% Efficiency}, volume={29}, ISSN={0935-9648}, url={http://dx.doi.org/10.1002/adma.201604241}, DOI={10.1002/adma.201604241}, abstractNote={Fullerene-free organic solar cells show over 11% power conversion efficiency, processed by low toxic solvents. The applied donor and acceptor in the bulk heterojunction exhibit almost the same highest occupied molecular orbital level, yet exhibit very efficient charge creation.}, number={5}, journal={Advanced Materials}, publisher={Wiley}, author={Zheng, Z. and Awartani, O. M. and Gautam, B. and Liu, D. L. and Qin, Y. P. and Li, W. N. and Bataller, Alexander and gundogdu and Ade, H. and Hou, J. H. and et al.}, year={2016}, month={Nov}, pages={1604241} }
 @article{younts_duan_gautam_saparov_liu_mongin_castellano_mitzi_gundogdu_2016, title={Efficient Generation of Long-Lived Triplet Excitons in 2D Hybrid Perovskite}, volume={29}, ISSN={0935-9648}, url={http://dx.doi.org/10.1002/ADMA.201604278}, DOI={10.1002/ADMA.201604278}, abstractNote={Triplet excitons form in quasi-2D hybrid inorganic–organic perovskites and diffuse over 100 nm before radiating with >11% photoluminescence quantum efficiency (PLQE) at low temperatures.}, number={9}, journal={Advanced Materials}, publisher={Wiley}, author={Younts, Robert and Duan, Hsin-Sheng and Gautam, Bhoj and Saparov, Bayrammurad and Liu, Jie and Mongin, Cedric and Castellano, Felix N. and Mitzi, David B. and Gundogdu, Kenan}, year={2016}, month={Dec}, pages={1604278} }
 @article{yu_yu_xu_cai_su_zhang_zhang_gundogdu_cao_2016, title={Engineering Substrate Interactions for High Luminescence Efficiency of Transition-Metal Dichalcogenide Monolayers}, volume={26}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.201600418}, abstractNote={It is demonstrated that the luminescence efficiency of monolayers composed of MoS2, WS2, and WSe2 is significantly limited by the substrate and can be improved by orders of magnitude through substrate engineering. The substrate affects the efficiency mainly through doping the monolayers and facilitating defect-assisted nonradiative exciton recombinations, while the other substrate effects including straining and dielectric screening play minor roles. The doping may come from the substrate and substrate-borne water moisture, the latter of which is much stronger than the former for MoS2 and WS2 but negligible for WSe2. Using proper substrates such as mica or hexagonal boron nitride can substantially mitigate the doping effect. The defect-assisted recombination depends on the interaction between the defect in the monolayer and the substrate. Suspended monolayers, in which the substrate effects are eliminated, may have efficiency up to 40% at room temperatures. The result provides useful guidance for the rational design of atomic-scale light emission devices.}, number={26}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Yu, Yifei and Yu, Yiling and Xu, Chao and Cai, Yong-Qing and Su, Liqin and Zhang, Yong and Zhang, Yong-Wei and Gundogdu, Kenan and Cao, Linyou}, year={2016}, month={Jul}, pages={4733–4739} }
 @article{liu_chen_qian_gautam_yang_zhao_bergqvist_zhang_ma_ade_et al._2016, title={Fast charge separation in a non-fullerene organic solar cell with a small driving force}, volume={1}, ISSN={2058-7546}, url={http://dx.doi.org/10.1038/NENERGY.2016.89}, DOI={10.1038/NENERGY.2016.89}, number={7}, journal={Nature Energy}, publisher={Springer Science and Business Media LLC}, author={Liu, Jing and Chen, Shangshang and Qian, Deping and Gautam, Bhoj and Yang, Guofang and Zhao, Jingbo and Bergqvist, Jonas and Zhang, Fengling and Ma, Wei and Ade, Harald and et al.}, year={2016}, month={Jun} }
 @article{yu_yu_xu_barrette_gundogdu_cao_2016, title={Fundamental limits of exciton-exciton annihilation for light emission in transition metal dichalcogenide monolayers}, volume={93}, DOI={10.1103/physrevb.93.201111}, abstractNote={We quantitatively illustrate the fundamental limit that exciton-exciton annihilation (EEA) may impose to the light emission of monolayer transition metal dichalcogenide (TMDC) materials. The EEA in TMDC monolayers shows dependence on the interaction with substrates as its rate increases from 0.1 cm2/s (0.05 cm2/s) to 0.3 cm2/s (0.1 cm2/s) with the substrates removed for WS2 (MoS2) monolayers. It turns to be the major pathway of exciton decay and dominates the luminescence efficiency when the exciton density is beyond 1010 cm-2 in suspended monolayers or 1011 cm-2 in supported monolayers. This sets an upper limit on the density of injected charges in light emission devices for the realization of optimal luminescence efficiency. The strong EEA rate also dictates the pumping threshold for population inversion in the monolayers to be 12-18 MW/cm2 (optically) or 2.5-4x105 A/cm2 (electrically).}, number={20}, journal={Physical Review B}, author={Yu, Y. L. and Yu, Y. F. and Xu, C. and Barrette, A. and gundogdu and Cao, L. Y.}, year={2016} }
 @article{bondarev_popescu_younts_hoffman_mcafee_dougherty_gundogdu_ade_2016, title={Lowest energy Frenkel and charge transfer exciton intermixing in one-dimensional copper phthalocyanine molecular lattice}, volume={109}, ISSN={["1077-3118"]}, DOI={10.1063/1.4968821}, abstractNote={We report the results of the combined experimental and theoretical studies of the low-lying exciton states in crystalline copper phthalocyanine. We derive the eigen energy spectrum for the two lowest intramolecular Frenkel excitons coupled to the intermolecular charge transfer exciton state and compare it with temperature dependent optical absorption spectra measured experimentally, to obtain the parameters of the Frenkel-charge-transfer exciton intermixing. The two Frenkel exciton states are spaced apart by 0.26 eV, and the charge transfer exciton state is 50 meV above the lowest Frenkel exciton. Both Frenkel excitons are strongly mixed with the charge transfer exciton, showing the coupling constant 0.17 eV which agrees with earlier experimental measurements. These results can be used for the proper interpretation of the physical properties of crystalline phthalocyanines.}, number={21}, journal={APPLIED PHYSICS LETTERS}, author={Bondarev, I. V. and Popescu, A. and Younts, R. A. and Hoffman, B. and McAfee, T. and Dougherty, D. B. and Gundogdu, K. and Ade, H. W.}, year={2016}, month={Nov} }
 @article{cheema_younts_ogbose_gautam_gundogdu_el-shafei_2015, title={A femtosecond study of the anomaly in electron injection for dye-sensitized solar cells: the influence of isomerization employing Ru(II) sensitizers with anthracene and phenanthrene ancillary ligands}, volume={17}, ISSN={["1463-9084"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84949117754&partnerID=MN8TOARS}, DOI={10.1039/c4cp04741a}, abstractNote={HD-7 is prone to ISC and shows a continuous increase in the triplet TA signal, whereas HD-8 shows enhanced singlet injection, followed by decay in the TA signal.}, number={4}, journal={PHYSICAL CHEMISTRY CHEMICAL PHYSICS}, author={Cheema, Hammad and Younts, Robert and Ogbose, Louis and Gautam, Bhoj and Gundogdu, Kenan and El-Shafei, Ahmed}, year={2015}, month={Jan}, pages={2750–2756} }
 @article{gautam_lee_younts_lee_danilov_kim_gundogdu_2015, title={Charge Generation Dynamics in Efficient All-Polymer Solar Cells: Influence of Polymer Packing and Morphology}, volume={7}, ISSN={["1944-8244"]}, DOI={10.1021/acsami.5b08531}, abstractNote={All-polymer solar cells exhibit rapid progress in power conversion efficiency (PCE) from 2 to 7.7% over the past few years. While this improvement is primarily attributed to efficient charge transport and balanced mobility between the carriers, not much is known about the charge generation dynamics in these systems. Here we measured exciton relaxation and charge separation dynamics using ultrafast spectroscopy in polymer/polymer blends with different molecular packing and morphology. These measurements indicate that preferential face-on configuration with intermixed nanomorphology increases the charge generation efficiency. In fact, there is a direct quantitative correlation between the free charge population in the ultrafast time scales and the external quantum efficiency, suggesting not only the transport but also charge generation is key for the design of high performance all polymer solar cells.}, number={50}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Gautam, Bhoj R. and Lee, Changyeon and Younts, Robert and Lee, Wonho and Danilov, Evgeny and Kim, Bumjoon J. and Gundogdu, Kenan}, year={2015}, month={Dec}, pages={27586–27591} }
 @article{gautam_younts_li_yan_danilov_klump_constantinou_so_you_ade_et al._2015, title={Charge Photogeneration in Organic Photovoltaics: Role of Hot versus Cold Charge-Transfer Excitons}, volume={6}, ISSN={1614-6832}, url={http://dx.doi.org/10.1002/AENM.201501032}, DOI={10.1002/AENM.201501032}, abstractNote={The role of excess excitation energy on long-range charge separation in organic donor/acceptor bulk heterojunctions (BHJs) continues to be unclear. While ultrafast spectroscopy results argue for efficient charge separation through high-energy charge-transfer (CT) states within the first picosecond (ps) of excitation, charge collection measurements suggest excess photon energy does not increase the current density in BHJ devices. Here, the population dynamics of charge-separated polarons upon excitation of high-energy polymer states and low-energy interfacial CT states in two polymer/fullerene blends from ps to nanosecond time scales are studied. It is observed that the charge-separation dynamics do not show significant dependence on excitation energy. These results confirm that excess exciton energy is not necessary for the effective generation of charges.}, number={1}, journal={Advanced Energy Materials}, publisher={Wiley}, author={Gautam, Bhoj R. and Younts, Robert and Li, Wentao and Yan, Liang and Danilov, Evgeny and Klump, Erik and Constantinou, Iordania and So, Franky and You, Wei and Ade, Harald and et al.}, year={2015}, month={Oct}, pages={1301032} }
 @article{gautam_barrette_mai_yan_zhang_danilov_you_ade_gundogdu_2015, title={Direct Optical Observation of Stimulated Emission from Hot Charge Transfer Excitons in Bulk Heterojunction Polymer Solar Cells}, volume={119}, ISSN={["1932-7447"]}, DOI={10.1021/acs.jpcc.5b06557}, abstractNote={Charge transfer excitons (CTEs) play an important role in semiconducting polymer-based optoelectronic applications. In organic photovoltaics, they are an intermediate step between tightly bound excitons and free charges. Although CT state energies at the interface of bulk heterojunction organic solar cells have been reported using quantum chemical calculations and by sensitive external quantum efficiency (EQE) measurements, direct optical observation of CT states was limited to relaxed, low energy, CT levels. Here we used polarization anisotropy transient absorption experiments to measure emission from high-energy CT levels. These experimental methods provide means to study high energy CT state dynamics in BHJs with controlled molecular orientations and complement theoretical calculations of interfacial CT state energies.}, number={34}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Gautam, Bhoj R. and Barrette, Andy and Mai, Cong and Yan, Liang and Zhang, Qianqian and Danilov, Eygeny and You, Wei and Ade, Harald and Gundogdu, Kenan}, year={2015}, month={Aug}, pages={19697–19702} }
 @article{mai_semenov_barrette_yu_jin_cao_kim_gundogdu_2014, title={Exciton valley relaxation in a single layer ofWS2measured by ultrafast spectroscopy}, volume={90}, ISSN={1098-0121 1550-235X}, url={http://dx.doi.org/10.1103/PhysRevB.90.041414}, DOI={10.1103/physrevb.90.041414}, abstractNote={We measured the lifetime of optically created valley polarization in single-layer ${\mathrm{WS}}_{2}$ using transient absorption spectroscopy. The electron valley relaxation is very short ($<1$ ps). However, the hole valley lifetime is at least two orders of magnitude longer and exhibits a temperature dependence that cannot be explained by single-carrier spin/valley relaxation mechanisms. Our theoretical analysis suggests that a collective contribution of two potential processes may explain the valley relaxation in single-layer ${\mathrm{WS}}_{2}$. One process involves direct scattering of excitons from $K$ to ${K}^{\ensuremath{'}}$ valleys with a spin flip-flop interaction. The other mechanism involves scattering through the spin-degenerate $\ensuremath{\Gamma}$ valley. This second process is thermally activated with an Arrhenius behavior due to the energy barrier between the $\ensuremath{\Gamma}$ and $K$ valleys.}, number={4}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Mai, Cong and Semenov, Yuriy G. and Barrette, Andrew and Yu, Yifei and Jin, Zhenghe and Cao, Linyou and Kim, Ki Wook and Gundogdu, Kenan}, year={2014}, month={Jul} }
 @article{cheema_islam_han_gautam_younts_gundogdu_el-shafei_2014, title={Influence of mono versus bis-electron-donor ancillary ligands in heteroleptic Ru(II) bipyridyl complexes on electron injection from the first excited singlet and triplet states in dye-sensitized solar cells}, volume={2}, ISSN={["2050-7496"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84905571530&partnerID=MN8TOARS}, DOI={10.1039/c4ta01942c}, abstractNote={A novel Ru(<sc>ii</sc>) bipyridyl complex was designed for DSCs.}, number={34}, journal={JOURNAL OF MATERIALS CHEMISTRY A}, author={Cheema, Hammad and Islam, Ashraful and Han, Liyuan and Gautam, Bhoj and Younts, Robert and Gundogdu, Kenan and El-Shafei, Ahmed}, year={2014}, pages={14228–14235} }
 @article{xu_hewitt_wang_guan_boltersdorf_maggard_dougherty_gundogdu_2014, title={Intrinsic and extrinsic effects on the electrostatic field at the surface of Bi2Se3}, volume={116}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.4891719}, DOI={10.1063/1.4891719}, abstractNote={The time evolution of electrostatic fields near a Bi2Se3 surface after a mechanical cleave was observed using Second Harmonic Generation. By comparing samples with different bulk doping levels and samples cleaved in different gas environments, these observations indicate multiple contributions to electric field evolution. These include the intrinsic process of Se vacancy diffusion as well as extrinsic processes due to both reactive and nonreactive surface adsorbates.}, number={4}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Xu, Chao and Hewitt, Andy and Wang, Jingying and Guan, Tianshuai and Boltersdorf, Jonathan and Maggard, Paul A. and Dougherty, Daniel B. and Gundogdu, Kenan}, year={2014}, month={Jul}, pages={043519} }
 @article{cheema_islam_younts_gautam_bedja_gupta_han_gundogdu_el-shafei_2014, title={More stable and more efficient alternatives of Z-907: carbazole-based amphiphilic Ru(II) sensitizers for dye-sensitized solar cells}, volume={16}, ISSN={["1463-9084"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84911879802&partnerID=MN8TOARS}, DOI={10.1039/c4cp04120h}, abstractNote={Here we report two novel amphiphilic Ru(<sc>ii</sc>) heteroleptic bipyridyl complexes, HD-14 and HD-15, compared to previously reported NCSU-10.}, number={48}, journal={PHYSICAL CHEMISTRY CHEMICAL PHYSICS}, author={Cheema, Hammad and Islam, Ashraful and Younts, Robert and Gautam, Bhoj and Bedja, Idriss and Gupta, Ravindra Kumar and Han, Liyuan and Gundogdu, Kenan and El-Shafei, Ahmed}, year={2014}, pages={27078–27087} }
 @article{maity_wu_xu_tracy_gundogdu_bochinski_clarke_2014, title={Spatial temperature mapping within polymer nanocomposites undergoing ultrafast photothermal heating via gold nanorods}, volume={6}, ISSN={["2040-3372"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000345458200080&KeyUID=WOS:000345458200080}, DOI={10.1039/c4nr05179c}, abstractNote={Polarized fluorescence temperature measurements combined with direct detection of nanorod rotation within the polymer melt regions reveal the steady-state temperature gradient on the nanoscale.}, number={24}, journal={NANOSCALE}, author={Maity, Somsubhra and Wu, Wei-Chen and Xu, Chao and Tracy, Joseph B. and Gundogdu, Kenan and Bochinski, Jason R. and Clarke, Laura I.}, year={2014}, pages={15236–15247} }
 @inproceedings{barrette_mai_yu_semenov_jin_kim_cao_gundogdu_2014, title={Ultrafast valley relaxation dynamics in single layer semiconductors}, volume={9198}, booktitle={Ultrafast nonlinear imaging and spectroscopy ii}, author={Barrette, A. and Mai, C. and Yu, Y. F. and Semenov, Y. and Jin, Z. H. and Kim, K. W. and Cao, L. Y. and Gundogdu, K.}, year={2014} }
 @article{mai_barrette_yu_semenov_kim_cao_gundogdu_2013, title={Many-Body Effects in Valleytronics: Direct Measurement of Valley Lifetimes in Single-Layer MoS2}, volume={14}, ISSN={1530-6984 1530-6992}, url={http://dx.doi.org/10.1021/nl403742j}, DOI={10.1021/nl403742j}, abstractNote={Single layer MoS2 is an ideal material for the emerging field of “valleytronics” in which charge carrier momentum can be finely controlled by optical excitation. This system is also known to exhibit strong many-body interactions as observed by tightly bound excitons and trions. Here we report direct measurements of valley relaxation dynamics in single layer MoS2, by using ultrafast transient absorption spectroscopy. Our results show that strong Coulomb interactions significantly impact valley population dynamics. Initial excitation by circularly polarized light creates electron–hole pairs within the K-valley. These excitons coherently couple to dark intervalley excitonic states, which facilitate fast electron valley depolarization. Hole valley relaxation is delayed up to about 10 ps due to nondegeneracy of the valence band spin states. Intervalley biexciton formation reveals the hole valley relaxation dynamics. We observe that biexcitons form with more than an order of magnitude larger binding energy compared to conventional semiconductors. These measurements provide significant insight into valley specific processes in 2D semiconductors. Hence they could be used to suggest routes to design semiconducting materials that enable control of valley polarization.}, number={1}, journal={Nano Letters}, publisher={American Chemical Society (ACS)}, author={Mai, Cong and Barrette, Andrew and Yu, Yifei and Semenov, Yuriy G. and Kim, Ki Wook and Cao, Linyou and Gundogdu, Kenan}, year={2013}, month={Dec}, pages={202–206} }
 @article{gokce_gundogdu_aspnes_2012, title={Control of the oxidation kinetics of H-terminated (111)Si by using the carrier concentration and the strain: a second-harmonic-generation investigation}, volume={60}, ISSN={["1976-8524"]}, DOI={10.3938/jkps.60.1685}, number={10}, journal={JOURNAL OF THE KOREAN PHYSICAL SOCIETY}, author={Gokce, B. and Gundogdu, K. and Aspnes, D. E.}, year={2012}, month={May}, pages={1685–1689} }
 @article{gokce_dougherty_gundogdu_2012, title={Effect of p-type doping on the oxidation of H-Si(111) studied by second-harmonic generation}, volume={30}, number={4}, journal={Journal of Vacuum Science & Technology. A, Vacuum, Surfaces, and Films}, author={Gokce, B. and Dougherty, D. B. and Gundogdu, K.}, year={2012} }
 @article{gokce_gundogdu_adles_aspnes_2011, title={Back-reflection Second-harmonic Generation of (111)Si: Theory and Experiment}, volume={58}, ISSN={["1976-8524"]}, DOI={10.3938/jkps.58.1237}, number={5}, journal={JOURNAL OF THE KOREAN PHYSICAL SOCIETY}, author={Gokce, B. and Gundogdu, K. and Adles, E. J. and Aspnes, D. E.}, year={2011}, month={May}, pages={1237–1243} }
 @article{gokce_aspnes_lucovsky_gundogdu_2011, title={Bond-specific reaction kinetics during the oxidation of (111) Si: Effect of n-type doping}, volume={98}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.3537809}, DOI={10.1063/1.3537809}, abstractNote={It is known that a higher concentration of free carriers leads to a higher oxide growth rate in the thermal oxidation of silicon. However, the role of electrons and holes in oxidation chemistry is not clear. Here, we report real-time second-harmonic-generation data on the oxidation of H-terminated (111)Si that reveal that high concentrations of electrons increase the chemical reactivity of the outer-layer Si-Si back bonds relative to the Si-H up bonds. However, the thicknesses of the natural oxides of all samples stabilize near 1 nm at room temperature, regardless of the chemical kinetics of the different bonds.}, number={2}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Gokce, B. and Aspnes, D. E. and Lucovsky, G. and Gundogdu, K.}, year={2011}, month={Jan}, pages={021904} }
 @article{gokce_aspnes_gundogdu_2011, title={Effect of strain on bond-specific reaction kinetics during the oxidation of H-terminated (111) Si}, volume={98}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.3567528}, DOI={10.1063/1.3567528}, abstractNote={Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation Bilal Gokce, David E. Aspnes, Kenan Gundogdu; Effect of strain on bond-specific reaction kinetics during the oxidation of H-terminated (111) Si. Appl. Phys. Lett. 21 March 2011; 98 (12): 121912. https://doi.org/10.1063/1.3567528 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioApplied Physics Letters Search Advanced Search |Citation Search}, number={12}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Gokce, Bilal and Aspnes, David E. and Gundogdu, Kenan}, year={2011}, month={Mar}, pages={121912} }
 @article{turner_stone_gundogdu_nelson_2011, title={Invited Article: The coherent optical laser beam recombination technique (COLBERT) spectrometer: Coherent multidimensional spectroscopy made easier}, volume={82}, ISSN={0034-6748 1089-7623}, url={http://dx.doi.org/10.1063/1.3624752}, DOI={10.1063/1.3624752}, abstractNote={We have developed an efficient spectrometer capable of performing a wide variety of coherent multidimensional measurements at optical wavelengths. The two major components of the largely automated device are a spatial beam shaper which controls the beam geometry and a spatiotemporal pulse shaper which controls the temporal waveform of the femtosecond pulse in each beam. We describe how to construct, calibrate, and operate the device, and we discuss its limitations. We use the exciton states of a semiconductor nanostructure as a working example. A series of complex multidimensional spectra—displayed in amplitude and real parts—reveals increasingly intricate correlations among the excitons.}, number={8}, journal={Review of Scientific Instruments}, publisher={AIP Publishing}, author={Turner, Daniel B. and Stone, Katherine W. and Gundogdu, Kenan and Nelson, Keith A.}, year={2011}, month={Aug}, pages={081301} }
 @article{gokce_adles_aspnes_gundogdu_2011, title={Measurement and Control of In-Plane Surface Chemistry During Oxidation of H-Terminated (111)Si}, volume={1399}, ISSN={["0094-243X"]}, DOI={10.1063/1.3666321}, abstractNote={We demonstrate both directional control and measurement of the oxidation of H‐terminated (111)Si. Control is achieved through externally applied strain, with strained back bonds oxidizing faster than unstrained ones. Real‐time measurement is achieved by second‐harmonic generation (SHG), with SHG anisotropy data analyzed with the anisotropic bond‐charge model of nonlinear optics. Anisotropic oxidation also results in structural changes, which appear as rotations of the average orientations of the back bonds from their unperturbed directions.}, journal={PHYSICS OF SEMICONDUCTORS: 30TH INTERNATIONAL CONFERENCE ON THE PHYSICS OF SEMICONDUCTORS}, author={Gokce, Bilal and Adles, Eric J. and Aspnes, David E. and Gundogdu, Kenan}, year={2011} }
 @article{gokce_adles_aspnes_gundogdu_2010, title={Measurement and control of in-plane surface chemistry during the oxidation of H-terminated (111) Si}, volume={107}, ISSN={0027-8424 1091-6490}, url={http://dx.doi.org/10.1073/pnas.1011295107}, DOI={10.1073/pnas.1011295107}, abstractNote={In-plane directional control of surface chemistry during interface formation can lead to new opportunities regarding device structures and applications. Control of this type requires techniques that can probe and hence provide feedback on the chemical reactivity of bonds not only in specific directions but also in real time. Here, we demonstrate both control and measurement of the oxidation of H-terminated (111) Si. Control is achieved by externally applying uniaxial strain, and measurement by second-harmonic generation (SHG) together with the anisotropic-bond model of nonlinear optics. In this system anisotropy results because bonds in the strain direction oxidize faster than those perpendicular to it, leading in addition to transient structural changes that can also be detected at the bond level by SHG.}, number={41}, journal={Proceedings of the National Academy of Sciences}, publisher={Proceedings of the National Academy of Sciences}, author={Gokce, Bilal and Adles, Eric J. and Aspnes, David E. and Gundogdu, Kenan}, year={2010}, month={Sep}, pages={17503–17508} }
 @article{gundogdu_lucovsky_chung_kim_nordlund_2009, title={Application of non-linear optical second harmonic generation and X-ray absorption and spectroscopies to defect related properties of Hf silicate and Hf Si oxynitride gate dielectrics}, volume={86}, ISSN={["1873-5568"]}, DOI={10.1016/j.mee.2009.03.004}, abstractNote={Three different Hf oxide based dielectrics have emerged as viable candidates for applications in advanced ULSI devices. This article focuses on two of these: (i) phase separated Hf silicates with (i) 70-85% nano-crystalline HfO2 with a nano-grain size <2nm, and 15-30% ~2nm non-crystalline SiO2 inclusions, and (ii) Hf Si oxynitride alloys, the most promising of which has a composition, (HfO2)0.3(SiO2)0.3(Si3N4)0.4 designated as 3/3/4 Hf SiON. X-ray absorption spectroscopy has been applied to identification of defect associated with vacancy structures in phase separated silicates, and network disruption defects in the Hf Si oxynitrides. Optical second harmonic generation is introduced in this article for the first time as a non-invasive approach for detecting macroscopic strain, that is shown to be absent in these low defect density dielectrics, the phase separated Hf silicates, and Hf Si oxynitrides, but present in HfO2 films, and Hf silicates with lower HfO2 content, e.g., the 40% HfO2 film of this article.}, number={7-9}, journal={MICROELECTRONIC ENGINEERING}, author={Gundogdu, K. and Lucovsky, G. and Chung, K-B. and Kim, J. -W. and Nordlund, D.}, year={2009}, pages={1654–1657} }
 @article{zimmermann jörg_gundogdu_cremeens_bandaria_hwang_thielges_cheatum_romesberg_2009, title={Efforts toward Developing Probes of Protein Dynamics: Vibrational Dephasing and Relaxation of Carbon–Deuterium Stretching Modes in Deuterated Leucine}, volume={113}, ISSN={1520-6106 1520-5207}, url={http://dx.doi.org/10.1021/jp900516c}, DOI={10.1021/jp900516c}, abstractNote={The spectral position of C-D stretching absorptions in the so-called "transparent window" of protein absorption (1800-2300 cm(-1)) makes them well suited as probes of protein dynamics with high temporal and structural resolution. We have previously incorporated single deuterated amino acids into proteins to site-selectively follow protein folding and ligand binding by steady-state FT IR spectroscopy. Ultimately, our goal is to use C-D bonds as probes in time-resolved IR spectroscopy to study dynamics and intramolecular vibrational energy redistribution (IVR) in proteins. As a step toward this goal, we now present the first time-resolved experiments characterizing the population and dephasing dynamics of selectively excited C-D bonds in a deuterated amino acid. Three differently deuterated, Boc-protected leucines were selected to systematically alter the number of additional C-D bonds that may mediate IVR out of the initially populated bright C-D stretching mode. Three-pulse photon echo experiments show that the steady-state C-D absorption linewidths are broadened by both homogeneous and inhomogeneous effects, and transient grating experiments reveal that IVR occurs on a subpicosecond time scale and is nonstatistical. The results have important implications for the interpretation of steady-state C-D spectra and demonstrate the potential utility of C-D bonds as probes of dynamics and IVR within a protein.}, number={23}, journal={The Journal of Physical Chemistry B}, publisher={American Chemical Society (ACS)}, author={Zimmermann Jörg and Gundogdu, Kenan and Cremeens, Matthew E. and Bandaria, Jigar N. and Hwang, Gil Tae and Thielges, Megan C. and Cheatum, Christopher M. and Romesberg, Floyd E.}, year={2009}, month={Jun}, pages={7991–7994} }
 @article{stone_turner_gundogdu_cundiff_nelson_2009, title={Exciton−Exciton Correlations Revealed by Two-Quantum, Two-Dimensional Fourier Transform Optical Spectroscopy}, volume={42}, ISSN={0001-4842 1520-4898}, url={http://dx.doi.org/10.1021/ar900122k}, DOI={10.1021/ar900122k}, abstractNote={The Coulomb correlations between photoexcited charged particles in materials such as photosynthetic complexes, conjugated polymer systems, J-aggregates, and bulk or nanostructured semiconductors produce a hierarchy of collective electronic excitations, for example, excitons, and biexcitons, which may be harnessed for applications in quantum optics, light-harvesting, or quantum information technologies. These excitations represent correlations among successively greater numbers of electrons and holes, and their associated multiple-quantum coherences could reveal detailed information about complex many-body interactions and dynamics. However, unlike single-quantum coherences involving excitons, multiple-quantum coherences do not radiate; consequently, they have largely eluded direct observation and characterization. In this Account, we present a novel optical technique, two-quantum, two-dimensional Fourier transform optical spectroscopy (2Q 2D FTOPT), which allows direct observation of the dynamics of multiple exciton states that reflect the correlations of their constituent electrons and holes. The approach is based on closely analogous methods in NMR, in which multiple phase-coherent fields are used to drive successive transitions such that multiple-quantum coherences can be accessed and probed. In 2Q 2D FTOPT, a spatiotemporal femtosecond pulse-shaping technique has been used to overcome the challenge of control over multiple, noncollinear, phase-coherent optical fields in experimental geometries used to isolate selected signal contributions through wavevector matching. We present results from a prototype GaAs quantum well system, which reveal distinct coherences of biexcitons that are formed from two identical excitons or from two excitons that have holes in different spin sublevels ("heavy-hole" and "light-hole" excitons). The biexciton binding energies and dephasing dynamics are determined, and changes in the dephasing rates as a function of the excitation density are observed, revealing still higher order correlations due to exciton-biexciton interactions. Two-quantum coherences due to four-particle correlations that do not involve bound biexciton states but that influence the exciton properties are also observed and characterized. The 2Q 2D FTOPT technique allows many-body interactions that cannot be treated with a mean-field approximation to be studied in detail; the pulse-shaping approach simplifies greatly what would have otherwise been daunting measurements. This spectroscopic tool might soon offer insight into specific applications, for example, in detailing the interactions that affect how electronic energy moves within the strata of organic photovoltaic cells.}, number={9}, journal={Accounts of Chemical Research}, publisher={American Chemical Society (ACS)}, author={Stone, Katherine W. and Turner, Daniel B. and Gundogdu, Kenan and Cundiff, Steven T. and Nelson, Keith A.}, year={2009}, month={Sep}, pages={1452–1461} }
 @inbook{turner_stone_gundogdu_nelson_2009, title={Three-Dimensional Electronic Four Wave-Mixing Spectroscopy in GaAs Quantum Wells}, ISBN={9783540959458 9783540959465}, ISSN={0172-6218}, url={http://dx.doi.org/10.1007/978-3-540-95946-5_93}, DOI={10.1007/978-3-540-95946-5_93}, abstractNote={Three-dimensional electronic Fourier transform four wave-mixing spectroscopy of GaAs quantum wells is demonstrated for the first time using wave vector beam shaping and femtosecond spatiotemporal pulse shaping to create four fully phase-coherent, non-collinear optical fields. This technique is an optical analogue of multidimensional and multi-quantum NMR. From a study of GaAs quantum wells, an internally calibrated, yet previously unmeasured two-dimensional projection correlating events between the first two time periods is presented.}, booktitle={Springer Series in Chemical Physics}, publisher={Springer Berlin Heidelberg}, author={Turner, Daniel B. and Stone, Katherine W. and Gundogdu, Kenan and Nelson, Keith A.}, year={2009}, pages={286–288} }
 @article{turner_stone_gundogdu_nelson_2009, title={Three-dimensional electronic spectroscopy of excitons in GaAs quantum wells}, volume={131}, ISSN={0021-9606 1089-7690}, url={http://dx.doi.org/10.1063/1.3245964}, DOI={10.1063/1.3245964}, abstractNote={We demonstrate three-dimensional (3D) electronic Fourier transform spectroscopy of GaAs quantum wells using four fully phase-coherent, noncollinear optical fields. Since the full complex signal field is measured as a function of all three time intervals, nearly every peak in the resulting 3D spectral solid arises from a distinguishable sequence of transitions represented by a single Feynman pathway. We use the 3D spectral peaks to separate two pathways involving weakly bound mixed biexcitons generated in different time orders. In the process, we reveal a peak that was previously obscured by a correlated but unbound exciton pair coherence. We also demonstrate a calibration procedure for the carrier frequency which yields biexciton binding energy values with high accuracy.}, number={14}, journal={The Journal of Chemical Physics}, publisher={AIP Publishing}, author={Turner, Daniel B. and Stone, Katherine W. and Gundogdu, Kenan and Nelson, Keith A.}, year={2009}, month={Oct}, pages={144510} }
 @inbook{stone_gundogdu_turner_li_cundiff_nelson_2009, title={Two-quantum Two-dimensional Fourier Transform Electronic Spectroscopy of Biexcitons in GaAs Quantum Wells}, ISBN={9783540959458 9783540959465}, ISSN={0172-6218}, url={http://dx.doi.org/10.1007/978-3-540-95946-5_81}, DOI={10.1007/978-3-540-95946-5_81}, booktitle={Springer Series in Chemical Physics}, publisher={Springer Berlin Heidelberg}, author={Stone, Katherine W. and Gundogdu, Kenan and Turner, Daniel B. and Li, Xiaoqin and Cundiff, Steven T. and Nelson, Keith A.}, year={2009}, pages={250–252} }
 @article{gundogdu_stone_turner_nelson_2007, title={Multidimensional coherent spectroscopy made easy}, volume={341}, ISSN={0301-0104}, url={http://dx.doi.org/10.1016/j.chemphys.2007.06.027}, DOI={10.1016/j.chemphys.2007.06.027}, abstractNote={We have demonstrated a highly efficient fully coherent 2D spectrometer based on 2D pulse shaping and Fourier beam shaping. The versatility of the design allows one to measure different 2D spectral surfaces consecutively. Easy alignment, inherent phase stability, rotating wave frame detection, and arbitrary waveform generation in all of the beams are important features of this design. We have demonstrated the functionality of the 2D spectrometer by measuring a 2D spectral surface of a GaAs quantum well.}, number={1-3}, journal={Chemical Physics}, publisher={Elsevier BV}, author={Gundogdu, Kenan and Stone, Katherine W. and Turner, Daniel B. and Nelson, Keith A.}, year={2007}, month={Nov}, pages={89–94} }
 @article{sun_macaluso_calvez_valentine_burns_dawson_gundogdu_hall_boggess_jouhti_et al._2004, title={Effects of rapid thermal annealing on the optical properties of low-loss 1.3μm GaInNAs∕GaAs saturable Bragg reflectors}, volume={96}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.1767612}, DOI={10.1063/1.1767612}, abstractNote={We report studies of the effect of rapid thermal annealing (RTA) on the optical properties of a low-loss 1.3μm saturable Bragg reflector (SBR), consisting of a GaInNAs∕GaAs single quantum well embedded in an AlAs∕GaAs Bragg reflector grown monolithically on a GaAs substrate. RTA gives rise to a blueshift of the photoluminescence (PL) peak (and therefore of the excitonic absorption peak) and an enhancement of PL intensity, while the reflectivity properties including peak reflectivity and bandwidth are not degraded. Temperature dependent photoluminescence measurements show that the RTA-induced blueshift of photoluminescence consists of two components: one originating from the increase of optical transition energies and another from the reduction of carrier localization. Time-resolved photoluminescence results at room temperature provide information about the recombination dynamics of carriers directly relevant to the application of the SBR in laser mode locking.}, number={3}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Sun, H. D. and Macaluso, R. and Calvez, S. and Valentine, G. J. and Burns, D. and Dawson, M. D. and Gundogdu, K. and Hall, K. C. and Boggess, T. F. and Jouhti, T. and et al.}, year={2004}, month={Aug}, pages={1418–1424} }
 @article{calvez_hopkins_smith_clark_macaluso_sun_dawson_jouhti_pessa_gundogdu_et al._2004, title={GaInNAs/GaAs Bragg-mirror-based structures for novel 1.3μm device applications}, volume={268}, ISSN={0022-0248}, url={http://dx.doi.org/10.1016/j.jcrysgro.2004.04.072}, DOI={10.1016/j.jcrysgro.2004.04.072}, abstractNote={We report the use of GaInNAs/GaAs material system for a range of 1.3 μm vertical-cavity devices namely VCSELs, VCSOAs, VECSELs and SESAMs. Using optical pumping, we demonstrate that up to 4 mW of 1290 nm output power can be fibre-coupled from a VCSEL. We also show that tayloring the VCSEL structure allows to produce a monolithic long-wavelength VCSOA with up to 16 dB of gain. We also report the first demonstration of a 1.3 μm VECSEL with more than 0.5 W of CW ouptut power. Finally, annealing effects on the properties of a GaInNAs SBR and modelocking of two Nd:doped solid state lasers using this element are described.}, number={3-4}, journal={Journal of Crystal Growth}, publisher={Elsevier BV}, author={Calvez, S. and Hopkins, J.-M. and Smith, S.A. and Clark, A.H. and Macaluso, R. and Sun, H.D. and Dawson, M.D. and Jouhti, T. and Pessa, M. and Gundogdu, K. and et al.}, year={2004}, month={Aug}, pages={457–465} }