@article{gann_collins_tang_tumbleston_mukherjee_ade_2016, title={Origins of polarization-dependent anisotropic X-ray scattering from organic thin films}, volume={23}, ISSN={["1600-5775"]}, DOI={10.1107/s1600577515019074}, abstractNote={Organic thin films that have no overall in-plane directional ordering often nonetheless produce anisotropic scattering patterns that rotate with the polarization of incident resonant X-rays. Isotropic symmetry is broken by local correlations between molecular orientation and domain structure. Such examples of molecular alignment at domain interfaces and within the bulk of domains, which are both critical to fields such as organic electronics, are simulated and compared with experimental scattering. Anisotropic scattering patterns are found to allow unambiguous identification of the mechanism of local molecular orientation correlations and, as such, promise to be both distinct and complementary to isotropic scattering intensity as a general measure of thin film microstructure.}, journal={JOURNAL OF SYNCHROTRON RADIATION}, author={Gann, Eliot and Collins, Brian A. and Tang, Maolong and Tumbleston, John R. and Mukherjee, Subrangsu and Ade, Harald}, year={2016}, month={Jan}, pages={219–227} }
 @article{mukherjee_proctor_tumbleston_bazan_nguyen_ade_2015, title={Importance of Domain Purity and Molecular Packing in Efficient Solution-Processed Small-Molecule Solar Cells}, volume={27}, ISSN={["1521-4095"]}, DOI={10.1002/adma.201404388}, abstractNote={Connections are delineated between solar-cell performance, charge-carrier mobilities, and morphology in a highperformance molecular solar cell. The observations show that maximizing the relative phase purity and structural order while simultaneously limiting the domain size may be essential for achieving optimal solar-cell performances in solution-processed small-molecule solar cells .}, number={6}, journal={ADVANCED MATERIALS}, author={Mukherjee, Subhrangsu and Proctor, Christopher M. and Tumbleston, John R. and Bazan, Guillermo C. and Nguyen, Thuc-Quyen and Ade, Harald}, year={2015}, month={Feb}, pages={1105–1111} }
 @article{buchaca-domingo_ferguson_jamieson_mccarthy-ward_shoaee_tumbleston_reid_yu_madec_pfannmöller_et al._2014, title={Additive-assisted supramolecular manipulation of polymer:fullerene blend phase morphologies and its influence on photophysical processes}, volume={1}, ISSN={2051-6347 2051-6355}, url={http://dx.doi.org/10.1039/C3MH00125C}, DOI={10.1039/c3mh00125c}, abstractNote={The role of intermixed phases in organic solar cell blends is evaluated through manipulation of their number of phases.}, number={2}, journal={Mater. Horiz.}, publisher={Royal Society of Chemistry (RSC)}, author={Buchaca-Domingo, E. and Ferguson, A. J. and Jamieson, F. C. and McCarthy-Ward, T. and Shoaee, S. and Tumbleston, J. R. and Reid, O. G. and Yu, L. and Madec, M.-B. and Pfannmöller, M. and et al.}, year={2014}, pages={270–279} }
 @article{li_yang_tumbleston_yan_ade_you_2014, title={Controlling Molecular Weight of a High Efficiency Donor-Acceptor Conjugated Polymer and Understanding Its Significant Impact on Photovoltaic Properties}, volume={26}, ISSN={["1521-4095"]}, DOI={10.1002/adma.201305251}, abstractNote={The molecular weight (MW) of PBnDT-FTAZ can be precisely controlled by adjusting the stoichiometric ratio of the two monomers, following the Carothers equation. The study of a set of PBnDT-FTAZ polymers with different MWs reveals that the MW significantly influences the morphology and structural order of PBnDTFTAZ in its bulk heterojunction solar cells, with the highest efficiency (over 7%) achieved with the use of a MW of 40 000 g mol(-1) .}, number={26}, journal={ADVANCED MATERIALS}, author={Li, Wentao and Yang, Liqiang and Tumbleston, John R. and Yan, Liang and Ade, Harald and You, Wei}, year={2014}, month={Jul}, pages={4456-+} }
 @article{li_abrecht_yang_roland_tumbleston_mcafee_yan_kelly_ade_neher_et al._2014, title={Mobility-Controlled Performance of Thick Solar Cells Based on Fluorinated Copolymers}, volume={136}, ISSN={["0002-7863"]}, DOI={10.1021/ja5067724}, abstractNote={Developing novel materials and device architectures to further enhance the efficiency of polymer solar cells requires a fundamental understanding of the impact of chemical structures on photovoltaic properties. Given that device characteristics depend on many parameters, deriving structure-property relationships has been very challenging. Here we report that a single parameter, hole mobility, determines the fill factor of several hundred nanometer thick bulk heterojunction photovoltaic devices based on a series of copolymers with varying amount of fluorine substitution. We attribute the steady increase of hole mobility with fluorine content to changes in polymer molecular ordering. Importantly, all other parameters, including the efficiency of free charge generation and the coefficient of nongeminate recombination, are nearly identical. Our work emphasizes the need to achieve high mobility in combination with strongly suppressed charge recombination for the thick devices required by mass production technologies.}, number={44}, journal={JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, author={Li, Wentao and Abrecht, Steve and Yang, Liqiang and Roland, Steffen and Tumbleston, John R. and McAfee, Terry and Yan, Liang and Kelly, Mary Allison and Ade, Harald and Neher, Dieter and et al.}, year={2014}, month={Nov}, pages={15566–15576} }
 @article{tumbleston_yang_you_ade_2014, title={Morphology linked to miscibility in highly amorphous semi-conducting polymer/fullerene blends}, volume={55}, ISSN={["1873-2291"]}, DOI={10.1016/j.polymer.2014.07.051}, abstractNote={Molecular miscibility is a defining property of mixtures of electron-donating polymers and electron-accepting fullerenes used in the photoactive layer of bulk heterojunction (BHJ) organic solar cells. Even though miscibility of fullerene in the polymer is a commonly observed property, quantitative measurements have unknown connections with other morphological properties such as domain size and domain purity. By varying the amount of fullerene loading in BHJ thin films, we show that morphological properties are related to miscibility via straightforward relationships. In particular, miscibility influences the sensitivity of domain spacing to fullerene loading and determines the domain volume fractions and the residual fullerene content in the mixed polymer-rich phase in devices. Not only does this work show the relationships between miscibility and morphology, but it also highlights the general importance in determining miscibility levels in BHJ systems in order to target optimum morphologies and domain purities.}, number={19}, journal={POLYMER}, author={Tumbleston, John R. and Yang, Liqiang and You, Wei and Ade, Harald}, year={2014}, month={Sep}, pages={4884–4889} }
 @article{ma_tumbleston_ye_wang_hou_ade_2014, title={Quantification of Nano- and Mesoscale Phase Separation and Relation to Donor and Acceptor Quantum Efficiency, J(SC), and FF in Polymer:Fullerene Solar Cells}, volume={26}, ISSN={["1521-4095"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84903724646&partnerID=MN8TOARS}, DOI={10.1002/adma.201400216}, abstractNote={Dr. W. Ma, Dr. J. R. Tumbleston, Prof. H. Ade Department of Physics North Carolina State University Raleigh , NC 27695 , USA E-mail: harald_ade@ncsu.edu L. Ye, Prof. J. Hou State Key Laboratory of Polymer Physics and Chemistry Beijing National Laboratory for Molecular Sciences Institute of Chemistry Chinese Academy of Sciences Beijing 100190 , China E-mail: hjhzlz@iccas.ac.cn L. Ye University of Chinese Academy of Sciences Beijing 100049, China Dr. C. Wang Advanced Light Source Laurence Berkeley National Laboratory Berkeley , California 94720 , USA}, number={25}, journal={ADVANCED MATERIALS}, author={Ma, Wei and Tumbleston, John R. and Ye, Long and Wang, Cheng and Hou, Jianhui and Ade, Harald}, year={2014}, month={Jul}, pages={4234–4241} }
 @article{albrecht_tumbleston_janietz_dumsch_allard_scherf_ade_neher_2014, title={Quantifying Charge Extraction in Organic Solar Cells: The Case of Fluorinated PCPDTBT}, volume={5}, ISSN={["1948-7185"]}, DOI={10.1021/jz500457b}, abstractNote={We introduce a new and simple method to quantify the effective extraction mobility in organic solar cells at low electric fields and charge carrier densities comparable to operation conditions under one sun illumination. By comparing steady-state carrier densities at constant illumination intensity and under open-circuit conditions, the gradient of the quasi-Fermi potential driving the current is estimated as a function of external bias and charge density. These properties are then related to the respective steady-state current to determine the effective extraction mobility. The new technique is applied to different derivatives of the well-known low-band-gap polymer PCPDTBT blended with PC70BM. We show that the slower average extraction due to lower mobility accounts for the moderate fill factor when solar cells are fabricated with mono- or difluorinated PCPDTBT. This lower extraction competes with improved generation and reduced nongeminate recombination, rendering the monofluorinated derivative the most efficient donor polymer.}, number={7}, journal={JOURNAL OF PHYSICAL CHEMISTRY LETTERS}, author={Albrecht, Steve and Tumbleston, John R. and Janietz, Silvia and Dumsch, Ines and Allard, Sybille and Scherf, Ullrich and Ade, Harald and Neher, Dieter}, year={2014}, month={Apr}, pages={1131–1138} }
 @article{tumbleston_collins_yang_stuart_gann_ma_you_ade_2014, title={The influence of molecular orientation on organic bulk heterojunction solar cells}, volume={8}, ISSN={["1749-4893"]}, DOI={10.1038/nphoton.2014.55}, number={5}, journal={NATURE PHOTONICS}, author={Tumbleston, John R. and Collins, Brian A. and Yang, Liqiang and Stuart, Andrew C. and Gann, Eliot and Ma, Wei and You, Wei and Ade, Harald}, year={2014}, month={May}, pages={385–391} }
 @article{gann_watson_tumbleston_cochran_yan_wang_seok_chabinyc_ade_2014, title={Topographic measurement of buried thin-film interfaces using a grazing resonant soft x-ray scattering technique}, volume={90}, ISSN={["1550-235X"]}, DOI={10.1103/physrevb.90.245421}, abstractNote={The internal structures of thin films, particularly interfaces between different materials, are critical to system properties and performance across many disciplines, but characterization of buried interface topography is often unfeasible. In this work, we demonstrate that grazing resonant soft x-ray scattering (GRSoXS), a technique measuring diffusely scattered soft x rays from grazing incidence, can reveal the statistical topography of buried thin-film interfaces. By controlling and predicting the x-ray electric field intensity throughout the depth of the film and simultaneously the scattering contrast between materials, we are able to unambiguously identify the microstructure at different interfaces of a model polymer bilayer system. We additionally demonstrate the use of GRSoXS to selectively measure the topography of the surface and buried polymer-polymer interface in an organic thin-film transistor, revealing different microstructure and markedly differing evolution upon annealing. In such systems, where only indirect control of interface topography is possible, accurate measurement of the structure of interfaces for feedback is critically important. While we demonstrate the method here using organic materials, we also show that the technique is readily extendable to any thin-film system with elemental or chemical contrasts exploitable at absorption edges.}, number={24}, journal={PHYSICAL REVIEW B}, author={Gann, Eliot and Watson, Anne and Tumbleston, John R. and Cochran, Justin and Yan, Hongping and Wang, Cheng and Seok, Jaewook and Chabinyc, Michael and Ade, Harald}, year={2014}, month={Dec} }
 @article{liu_zhao_tumbleston_wang_gu_wang_briseno_ade_russell_2014, title={Understanding the Morphology of PTB7: PCBM Blends in Organic Photovoltaics}, volume={4}, ISSN={["1614-6840"]}, DOI={10.1002/aenm.201301377}, abstractNote={The structure–property relationships of PTB7‐phenyl‐C61‐butyric acid methyl ester (PCBM)‐based organic photovoltaics are investigated. The morphology is investigated in an active layer setting where a multi‐length‐scale morphology is observed using a solvent additive‐assisted film processing. This multi‐length‐scale structure consists of a phase separated morphology with a characteristic length scale of ≈30 nm, which is critical for producing large currents in devices; a second length scale of ≈130 nm, arises from face‐on PTB7 crystalline aggregates. This latter morphological feature is also observed in films prepared without the use of an additive. By observing the structure formation in situ during solvent evaporation for blade coated thin films, the additive is found to promote the formation of ordered domains of the PTB7 at an earlier stage during the solvent evaporation, which is critical in the development of the final morphology. In studies on PTB7/PCBM bilayers, PCBM is found to diffuse into the PTB7 layer. However, the performance of devices prepared in this manner is low. This diffusion leads to a swelling of the PTB7 and a reduction in the crystallinity of the PTB7, reflecting the strong miscibility of PCBM with PTB7. The morphology resulting from the interdiffusion is single‐length‐scale with slightly large phase separation. This leads to devices with poor performance.}, number={5}, journal={ADVANCED ENERGY MATERIALS}, author={Liu, Feng and Zhao, Wei and Tumbleston, John R. and Wang, Cheng and Gu, Yu and Wang, Dong and Briseno, Alejandro L. and Ade, Harald and Russell, Thomas P.}, year={2014}, month={Apr} }
 @article{yang_tumbleston_zhou_ade_you_2013, title={Disentangling the impact of side chains and fluorine substituents of conjugated donor polymers on the performance of photovoltaic blends}, volume={6}, ISSN={["1754-5706"]}, DOI={10.1039/c2ee23235a}, abstractNote={Side chains and fluorine (F) substituents on conjugated polymers have shown significant impact on the photovoltaic properties of polymer-based bulk heterojunction (BHJ) solar cells, but their respective impact is largely studied independently. In order to disentangle the effect of side chains and F substituents, we comprehensively investigate a series of conjugated polymers with an identical backbone (PNDT–DTBT) but different combinations of side chains and F substituents. Surprisingly, these seemingly marginal changes to the polymer backbone strongly influence the morphology and structure in BHJ thin films (e.g., domain size/purity and the relative orientation of polymer crystallites), as manifested by resonant soft X-ray scattering (R-SoXS) and grazing-incidence wide-angle X-ray scattering (GI-WAXS), thereby exerting significant impact on the photovoltaic properties of these conjugated polymer-based BHJ cells. Devices based on the polymer with long bulky side chains and F substituents (C8,4-C6,2F) simultaneously exhibit large open circuit voltage (Voc), high short circuit current (Jsc) and good fill factor (FF), with an efficiency as high as 5.6% for this series of PNDT–DTBT polymers.}, number={1}, journal={ENERGY & ENVIRONMENTAL SCIENCE}, author={Yang, Liqiang and Tumbleston, John R. and Zhou, Huaxing and Ade, Harald and You, Wei}, year={2013}, month={Jan}, pages={316–326} }
 @article{ma_tumbleston_wang_gann_huang_ade_2013, title={Domain Purity, Miscibility, and Molecular Orientation at Donor/Acceptor Interfaces in High Performance Organic Solar Cells: Paths to Further Improvement}, volume={3}, ISSN={["1614-6840"]}, DOI={10.1002/aenm.201200912}, abstractNote={Abstract}, number={7}, journal={ADVANCED ENERGY MATERIALS}, author={Ma, Wei and Tumbleston, John R. and Wang, Ming and Gann, Eliot and Huang, Fei and Ade, Harald}, year={2013}, month={Jul}, pages={864–872} }
 @article{tumbleston_stuart_gann_you_ade_2013, title={Fluorinated Polymer Yields High Organic Solar Cell Performance for a Wide Range of Morphologies}, volume={23}, ISSN={1616-301X}, url={http://dx.doi.org/10.1002/adfm.201300093}, DOI={10.1002/adfm.201300093}, abstractNote={Abstract}, number={27}, journal={Advanced Functional Materials}, publisher={Wiley}, author={Tumbleston, John R. and Stuart, Andrew C. and Gann, Eliot and You, Wei and Ade, Harald}, year={2013}, month={Feb}, pages={3463–3470} }
 @article{stuart_tumbleston_zhou_li_liu_ade_you_2013, title={Fluorine Substituents Reduce Charge Recombination and Drive Structure and Morphology Development in Polymer Solar Cells}, volume={135}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/ja309289u}, DOI={10.1021/ja309289u}, abstractNote={Three structurally identical polymers, except for the number of fluorine substitutions (0, 1, or 2) on the repeat unit (BnDT-DTBT), are investigated in detail, to further understand the impact of these fluorine atoms on open circuit voltage (V(oc)), short circuit current (J(sc)), and fill factor (FF) of related solar cells. While the enhanced V(oc) can be ascribed to a lower HOMO level of the polymer by adding more fluorine substituents, the improvement in J(sc) and FF are likely due to suppressed charge recombination. While the reduced bimolecular recombination with raising fluorine concentration is confirmed by variable light intensity studies, a plausibly suppressed geminate recombination is implied by the significantly increased change of dipole moment between the ground and excited states (Δμ(ge)) for these polymers as the number of fluorine substituents increases. Moreover, the 2F polymer (PBnDT-DTffBT) exhibits significantly more scattering in the in-plane lamellar stacking and out-of-plane π-π stacking directions, observed with GIWAXS. This indicates that the addition of fluorine leads to a more face-on polymer crystallite orientation with respect to the substrate, which could contribute to the suppressed charge recombination. R-SoXS also reveals that PBnDT-DTffBT has larger and purer polymer/fullerene domains. The higher domain purity is correlated with an observed decrease in PCBM miscibility in polymer, which drops from 21% (PBnDT-DTBT) to 12% (PBnDT-DTffBT). The disclosed "fluorine" impact not only explains the efficiency increase from 4% of PBnDT-DTBT (0F) to 7% with PBnDT-DTffBT (2F) but also suggests fluorine substitution should be generally considered in the future design of new polymers.}, number={5}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Stuart, Andrew C. and Tumbleston, John R. and Zhou, Huaxing and Li, Wentao and Liu, Shubin and Ade, Harald and You, Wei}, year={2013}, month={Jan}, pages={1806–1815} }
 @article{tumbleston_gadisa_liu_collins_samulski_lopez_ade_2013, title={Modifications in Morphology Resulting from Nanoimprinting Bulk Heterojunction Blends for Light Trapping Organic Solar Cell Designs}, volume={5}, ISSN={1944-8244 1944-8252}, url={http://dx.doi.org/10.1021/AM402363R}, DOI={10.1021/am402363r}, abstractNote={Nanoimprinting the photoactive layer of bulk heterojunction (BHJ) organic solar cells is a promising technique for enhancing device performance via improved light absorption. Here, we demonstrate that imprinting poly(3-hexylthiophene) (P3HT) and fullerene BHJ blends leads to adverse morphological changes within the photoactive nanopattern which have been previously overlooked. In particular, nanoimprinting induces a factor of 2 difference in polymer:fullerene composition between the nanopattern posts and interconnecting flash layer that inadvertently moves the composition outside the range for optimal performance. This occurs because of the strong tendency of regioregular P3HT to crystallize since imprinting blends based on amorphous regiorandom P3HT have uniform nanopattern composition. Based on these results, we outline promising design strategies, such as nanoimprinting amorphous polymers, to serve as guidelines for fabricating high-performance nanopatterned BHJ solar cells capable of maximized light absorption.}, number={16}, journal={ACS Applied Materials & Interfaces}, publisher={American Chemical Society (ACS)}, author={Tumbleston, John R. and Gadisa, Abay and Liu, Yingchi and Collins, Brian A. and Samulski, Edward T. and Lopez, Rene and Ade, Harald}, year={2013}, month={Aug}, pages={8225–8230} }
 @article{westacott_tumbleston_shoaee_fearn_bannock_gilchrist_heutz_demello_heeney_ade_et al._2013, title={On the role of intermixed phases in organic photovoltaic blends}, volume={6}, ISSN={1754-5692 1754-5706}, url={http://dx.doi.org/10.1039/C3EE41821A}, DOI={10.1039/c3ee41821a}, abstractNote={Recently, an intermixed phase has been identified within organic photovoltaic (OPV) bulk heterojunction (BHJ) systems that can exist in addition to relatively phase-pure regions, highlighting the need for a refined picture of the solid-state microstructure of donor–acceptor blends and for gaining further understanding of the exact nature and role such intermixed phases play in such devices. Here we manipulate the microstructure of polymer–fullerene systems via processing means and the selection of the molecular weight of the donor polymer. This manipulation is used as a tool to vary the fraction of intermixed phase present and its effects on the structure and subsequently the opto-electronic processes. We find clear relationships between the state of mixing and amount of exciton quenching and number of polarons generated per absorbed photon. Furthermore, we observe that blend systems incorporating higher molecular weight polymer result in a greater yield of dissociated polarons, likely due to the increase of the intermixed fraction.}, number={9}, journal={Energy & Environmental Science}, publisher={Royal Society of Chemistry (RSC)}, author={Westacott, Paul and Tumbleston, John R. and Shoaee, Safa and Fearn, Sarah and Bannock, James H. and Gilchrist, James B. and Heutz, Sandrine and deMello, John and Heeney, Martin and Ade, Harald and et al.}, year={2013}, pages={2756} }
 @article{cochran_amir_sivanandan_ku_seo_collins_tumbleston_toney_ade_hawker_et al._2013, title={Synthesis, solid-state, and charge-transport properties of conjugated polythiophene-S,S-dioxides}, volume={51}, ISSN={["1099-0488"]}, DOI={10.1002/polb.23167}, abstractNote={Abstract}, number={1}, journal={JOURNAL OF POLYMER SCIENCE PART B-POLYMER PHYSICS}, author={Cochran, Justin E. and Amir, Elizabeth and Sivanandan, Kulandaivelu and Ku, Sung-Yu and Seo, Jung Hwa and Collins, Brian A. and Tumbleston, John R. and Toney, Michael F. and Ade, Harald and Hawker, Craig J. and et al.}, year={2013}, month={Jan}, pages={48–56} }
 @article{bartelt_beiley_hoke_mateker_douglas_collins_tumbleston_graham_amassian_ade_et al._2013, title={The Importance of Fullerene Percolation in the Mixed Regions of Polymer-Fullerene Bulk Heterojunction Solar Cells}, volume={3}, ISSN={["1614-6840"]}, DOI={10.1002/aenm.201200637}, abstractNote={Abstract}, number={3}, journal={ADVANCED ENERGY MATERIALS}, author={Bartelt, Jonathan A. and Beiley, Zach M. and Hoke, Eric T. and Mateker, William R. and Douglas, Jessica D. and Collins, Brian A. and Tumbleston, John R. and Graham, Kenneth R. and Amassian, Aram and Ade, Harald and et al.}, year={2013}, month={Mar}, pages={364–374} }
 @article{collins_tumbleston_ade_2011, title={Miscibility, Crystallinity, and Phase Development in P3HT/PCBM Solar Cells: Toward an Enlightened Understanding of Device Morphology and Stability}, volume={2}, ISSN={["1948-7185"]}, DOI={10.1021/jz2014902}, abstractNote={Rapid improvements in organic solar cell efficiency have brought these devices into the spotlight as a potential source of abundant electricity. Despite much empirical progress, fundamental understanding is still lacking. Due to the required three-dimentional nanoscale morphology, determining structure–performance relationships has been a major challenge, and a convergent understanding has yet to emerge. We discuss recent major advances in delineating the characteristics of the most common organic solar cells. The large variations in device performance reported, the contradictory morphologies observed, and the determination of the underlying driving forces need to be resolved. Deeper understanding that can provide a roadmap to improved devices will only occur through refined consideration of material characteristics and fabrication procedures in conjunction with increased use of advanced characterization. We additionally highlight the recently discovered partial miscibility of the component materials, its...}, number={24}, journal={JOURNAL OF PHYSICAL CHEMISTRY LETTERS}, author={Collins, Brian A. and Tumbleston, John R. and Ade, Harald}, year={2011}, month={Dec}, pages={3135–3145} }