@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{yi_ho_gautam_lei_chowdhury_bahrami_qiao_so_2020, title={Effects of polymer crystallinity on non-fullerene acceptor based organic solar cell photostability}, volume={8}, ISSN={["2050-7534"]}, DOI={10.1039/d0tc03969a}, abstractNote={While there has been rapid progress made in the performance of organic photovoltaic (OPV) cells in recent years, the device stability remains a major bottleneck for commercialization. In this work, we blended a stable acceptor (O-IDTBR) with two photostable donors (PTB7-Th and PffBT4T-2OD) having different polymer crystallinity, and the resulting devices show a significant difference in the OPV degradation rate. The OPV devices employing a highly crystalline polymer PffBT4T-2OD as an active layer show a good resistance against light soaking, maintaining 80% of the initial power conversion efficiency (PCE) up to 100 hours, while the devices employing an amorphous polymer PTB7-Th as an active layer show a significant PCE loss in the initial 20 hours mainly due to a rapid loss of the fill factor. By carrying out a comprehensive analysis of the device degradation mechanisms, we conclude that the origin for the PTB7-Th:O-IDTBR device degradation is the formation of mid-gap states under continuous sunlight illumination, leading to a significant drop in electron mobility. Device simulation revealed that deep traps act as charge recombination centers and increase the trap-assisted recombination rate, lowering the FF and Jsc.}, number={45}, journal={JOURNAL OF MATERIALS CHEMISTRY C}, author={Yi, Xueping and Ho, Carr Hoi Yi and Gautam, Bhoj and Lei, Lei and Chowdhury, Ashraful Haider and Bahrami, Behzad and Qiao, Qiquan and So, Franky}, year={2020}, month={Dec}, pages={16092–16099} }
 @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{ho_kim_xiong_firdaus_yi_dong_rech_gadisa_booth_brendan t. o'connor_et al._2020, title={High-Performance Tandem Organic Solar Cells Using HSolar as the Interconnecting Layer}, volume={10}, ISSN={["1614-6840"]}, url={https://doi.org/10.1002/aenm.202000823}, DOI={10.1002/aenm.202000823}, abstractNote={Tandem structure provides a practical way to realize high efficiency organic photovoltaic cells, it can be used to extend the wavelength coverage for light harvesting. The interconnecting layer (ICL) between subcells plays a critical role in the reproducibility and performance of tandem solar cells, yet the processability of the ICL has been a challenge. In this work the fabrication of highly reproducible and efficient tandem solar cells by employing a commercially available material, PEDOT:PSS HTL Solar (HSolar), as the hole transporting material used for the ICL is reported. Comparing with the conventional PEDOT:PSS Al 4083 (c‐PEDOT), HSolar offers a better wettability on the underlying nonfullerene photoactive layers, resulting in better charge extraction properties of the ICL. When FTAZ:IT‐M and PTB7‐Th:IEICO‐4F are used as the subcells, a power conversion efficiency (PCE) of 14.7% is achieved in the tandem solar cell. To validate the processability of these tandem solar cells, three other research groups have successfully fabricated tandem devices using the same recipe and the highest PCE obtained is 16.1%. With further development of donor polymers and device optimization, the device simulation results show that a PCE > 22% can be realized in tandem cells in the near future.}, number={25}, journal={ADVANCED ENERGY MATERIALS}, publisher={Wiley}, author={Ho, Carr Hoi Yi and Kim, Taesoo and Xiong, Yuan and Firdaus, Yuliar and Yi, Xueping and Dong, Qi and Rech, Jeromy J. and Gadisa, Abay and Booth, Ronald and Brendan T. O'Connor and et al.}, year={2020}, month={Jul} }
 @article{dong_liu_barange_lee_pardue_yi_yin_so_2019, title={Long-Wavelength Lead Sulfide Quantum Dots Sensing up to 2600 nm for Short-Wavelength Infrared Photodetectors}, volume={11}, ISSN={["1944-8252"]}, DOI={10.1021/acsami.9b16539}, abstractNote={Lead sulfide nanoparticles (PbS NPs) are used in the short wavelength infrared (SWIR) photodetectors because of their excellent photosensitivity, bandgap tunability, and solution processability. It has been a challenge to synthesize high quality PbS NPs with an absorption peak beyond 2000 nm. In this work, using PbS seed crystals with an absorption peak at 1960 nm, we report a successful synthesis of very large mono-dispersed PbS NPs having a diameter up to 16 nm by multiple injections. The resulting NPs have an absorption peak over 2500 nm with a small full-width-at-half-maximum (FWHM) of 24 meV. To demonstrate the applications of such large QDs, broadband heterojunction photodetectors are fabricated with the large PbS QDs of an absorption peak at 2100 nm. The resulting devices have an EQE of 25% (over 50% IQE) at 2100 nm corresponding to a responsivity of 0.385 A/W, and an EQE ~60% in the visible range.}, number={47}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Dong, Chen and Liu, Shuyi and Barange, Nilesh and Lee, Jaewoong and Pardue, Tyler and Yi, Xueping and Yin, Shichen and So, Franky}, year={2019}, month={Nov}, pages={44451–44457} }
 @article{xu_yi_huang_zheng_zhang_salehi_coropceanu_ho_marder_toney_et al._2018, title={Donor Conjugated Polymers with Polar Side Chain Groups: The Role of Dielectric Constant and Energetic Disorder on Photovoltaic Performance}, volume={28}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.201803418}, abstractNote={To better understand the correlation of the dielectric properties with the photovoltaic response in conjugated polymer:fullerene bulk heterojunction materials, the concept of introducing minimal structural change is employed to increase the polymer dielectric constant via polar cyano groups added to the end of butyl or octyl side chains in the poly(dithienosilole‐thienopyrrolodione) system. Density functional theory calculations confirm that the polar groups do not affect the polymer electronic structure but can lead to an increase in overall dipole moment depending on the polymer chain conformation. Despite the increased dielectric constant (from 2.7 to 4.3 for cyano‐octyl side chains and from 2.7 to 3.2 for the cyano‐butyl analogues), the device characteristics employing the cyano‐containing polymers are inferior to those of the devices made with unfunctionalized alkyl chains. It is found that the hole mobilities for the cyano‐containing polymers are two orders of magnitude lower compared to those for the parent polymers and suggest this is due to an increase in energetic disorder caused by the strong local permanent dipoles associated with the cyano groups. The study highlights the complexity in the relationship between the dielectric constant of organic materials, the morphologies that are induced, and their photovoltaic performance.}, number={46}, journal={Advanced Functional Materials}, author={Xu, B. and Yi, X. and Huang, T. and Zheng, Z. and Zhang, J. and Salehi, A. and Coropceanu, V. and Ho, C.H.Y. and Marder, S.R. and Toney, M.F. and et al.}, year={2018}, pages={1803418} }
 @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 p...}, 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{lo_wang_oosterhout_zheng_yi_fuentes-hernandez_so_coropceanu_bredas_toney_et al._2018, title={Langmuir-Blodgett Thin Films of Diketopyrrolopyrrole-Based Amphiphiles}, volume={10}, ISSN={["1944-8252"]}, DOI={10.1021/acsami.7b18239}, abstractNote={We report on two π-conjugated donor-acceptor-donor (D-A-D) molecules of amphiphilic nature, aiming to promote intermolecular ordering and carrier mobility in organic electronic devices. Diketopyrrolopyrrole was selected as the acceptor moiety that was disubstituted with nonpolar and polar functional groups, thereby providing the amphiphilic structures. This structural design resulted in materials with a strong intermolecular order in the solid state, which was confirmed by differential scanning calorimetry and polarized optical microscopy. Langmuir-Blodgett (LB) films of ordered mono- and multilayers were transferred onto glass and silicon substrates, with layer quality, coverage, and intermolecular order controlled by layer compression pressure on the LB trough. Organic field-effect transistors and organic photovoltaics devices with active layers consisting of the amphiphilic conjugated D-A-D-type molecules were constructed to demonstrate that the LB technique is an effective layer-by-layer deposition approach to fabricate self-assembled, ordered thin films.}, number={14}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Lo, Chi Kin and Wang, Cheng-Yin and Oosterhout, Stefan D. and Zheng, Zilong and Yi, Xueping and Fuentes-Hernandez, Canek and So, Franky and Coropceanu, Veaceslav and Bredas, Jean-Luc and Toney, Michael F. and et al.}, year={2018}, month={Apr}, pages={11995–12004} }
 @article{xu_pelse_agarkar_ito_zhang_yi_chujo_marder_so_reynolds_et al._2018, title={Randomly Distributed Conjugated Polymer Repeat Units for High-Efficiency Photovoltaic Materials with Enhanced Solubility and Processability}, volume={10}, ISSN={["1944-8244"]}, DOI={10.1021/acsami.8b15522}, abstractNote={Three structurally disordered terpolymer derivatives of PffBT4T-2OD (PCE11), prepared by replacing a varied amount of bithiophene linkers with single thiophenes, were found to exhibit reduced aggregation in solution with increasing thiophene content, while important redox and optoelectronic properties remained similar to those of PffBT4T-2OD. Solar cells based on random terpolymer-PC71BM blends exhibited average power conversion efficiencies of over 9.5% when processed with preheated substrates, with fill factors above 70%, exceeding those from PffBT4T-2OD. Thanks to increased solubility, random terpolymer devices were able to be fabricated on room-temperature substrates, reaching virtually identical performance among all three polymers despite remarkable thicknesses of ∼400 nm. Thus, we show that the random terpolymer approach is successful in improving processability while maintaining device performance.}, number={51}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Xu, Bing and Pelse, Ian and Agarkar, Shruti and Ito, Shunichiro and Zhang, Junxiang and Yi, Xueping and Chujo, Yoshiki and Marder, Seth and So, Franky and Reynolds, John R. and et al.}, year={2018}, month={Dec}, pages={44583–44588} }
 @article{xu_ma_cheng_xie_yi_gautam_chen_li_lee_so_et al._2017, title={Ultraviolet-ozone surface modification for non-wetting hole transport materials based inverted planar perovskite solar cells with efficiency exceeding 18%}, volume={360}, ISSN={["1873-2755"]}, DOI={10.1016/j.jpowsour.2017.06.013}, abstractNote={Non-wetting hole transport materials (HTMs) have great potential in facilitating large-sized perovskite crystal growth and enhancing device stability by opposing moisture ingress, However, the severe non-wetting issue limits the wide application of these materials in low-temperature solution-processed inverted planar perovskite solar cells (PVSCs), and corresponding devices are rarely reported. Here, a facile ultraviolet-ozone (UVO) modification method is demonstrated to overcome this issue. By carefully controlling the UVO modification time, the surface wettability of poly-TPD can be tuned without affecting the bulk properties of the film, hence perovskite films with desired grain size and excellent coverage can be deposited via a one-step spin-coating method. Benefiting from the high-quality perovskite, well-matched energy level alignment and hydrophobic property of poly-TPD, the resulting PVSCs show a champion power conversion efficiency of 18.19% with significantly enhanced stability as compared to the PEDOT:PSS counterparts. Moreover, the UVO modification approach also demonstrates its validity when being extended to other hydrophobic HTMs. This work not only provides a general strategy to broaden the selection pool of HTMs for solution-processed inverted planar PVSCs, but also may triggers the exploration of more advanced strategies to make non-wetting HTMs applicable in solution-processed inverted planar PVSCs.}, journal={JOURNAL OF POWER SOURCES}, author={Xu, Xiuwen and Ma, Chunqing and Cheng, Yuanhang and Xie, Yue-Min and Yi, Xueping and Gautam, Bhoj and Chen, Shengmei and Li, Ho-Wa and Lee, Chun-Sing and So, Franky and et al.}, year={2017}, month={Aug}, pages={157–165} }
 @article{constantinou_yi_shewmon_klump_peng_garakyaraghi_lo_reynolds_castellano_so_2017, title={effect of polymer-fullerene interaction on the dielectric properties of the blend}, volume={7}, number={13}, journal={Advanced Energy Materials}, author={Constantinou, I. and Yi, X. P. and Shewmon, N. T. and Klump, E. D. and Peng, C. and Garakyaraghi, S. and Lo, C. K. and Reynolds, J. R. and Castellano, F. N. and So, F.}, year={2017} }