@article{peng_ade_2023, title={Unveiling re-entrant phase behavior and crystalline-amorphous interactions in semi-conducting polymer:small molecule blends}, volume={4}, ISSN={["2051-6355"]}, DOI={10.1039/d3mh00034f}, abstractNote={It has been reported recently that conjugated polymer:small molecule systems might exhibit complex, re-entrant phase behavior with hourglass or closed-loop miscibility gaps due to an 'apparent' lower critical solution temperature branch. However, the study did not firmly establish if the observations were reflecting equilibrium or not. To assure that the observed shapes of the binodals via a mixing experiment represent local near-equilibrium conditions that capture complex molecular interactions or equation-of-state effects, we present here the liquidus and the binodal for the exact same systems, i.e., PTB7-Th:PC61BM, PffBT4T-C9C13:PC71BM and PTB7-Th:EH-IDTBR, with the liquidus measured via a demixing experiment with long annealing time of days to weeks. We observe that the binodal displayed consistent trends with the liquidus, revealing an underlying thermodynamic and not microstructural or kinetic cause behind the complex phase behavior. Our results highlight the need for a novel, sufficiently complex physical model for understanding these non-trivial phase diagrams of semi-conducting materials. We also discover that the composition difference (Δϕ) between liquidus and binodal reflects the crystalline-amorphous interaction, exhibiting a linear relationship with the binodal composition (ϕb,polymer), i.e., Δϕ increases as χaa decreases. This possibly provides a new approach for obtaining the crystalline-amorphous interaction parameter χca(T) beyond the commonly used melting point depression method, which estimates χca near the melting temperature Tm of the crystalline component. The capability of obtaining χca(T) over a more extended temperature range may encourage more extensive studies and facilitate the understanding of χca in general, but particularly for all the novel non-fullerene acceptors that are able to crystallize.}, journal={MATERIALS HORIZONS}, author={Peng, Zhengxing and Ade, Harald}, year={2023}, month={Apr} }
 @article{fu_peng_fan_lin_qi_ran_wu_fan_jiang_woo_et al._2022, title={A Top-Down Strategy to Engineer ActiveLayer Morphology for Highly Efficient and Stable All-Polymer Solar Cells}, volume={7}, ISSN={["1521-4095"]}, DOI={10.1002/adma.202202608}, abstractNote={A major challenge hindering the further development of all-polymer solar cells (all-PSCs) employing polymerized small-molecule acceptors is the relatively low fill factor (FF) due to the difficulty in controlling active layer morphology. The issues typically arise from oversized phase separation resulting from the thermodynamically unfavorable mixing between two macromolecular species, and disordered molecular orientation/packing of highly anisotropic polymer chains. Herein, a facile top-down controlling strategy to engineer the morphology of all-polymer blends is developed by leveraging the layer-by-layer (LBL) deposition. Optimal intermixing of polymer components could be achieved in the two-step process by tuning the bottom-layer polymer swelling during top-layer deposition. As a result, both the molecular orientation/packing of bottom layer and the crystallinity of top layer can be optimized with a suitable processing solvent for the top layer. Consequently, a favorable morphology with gradient vertical composition distribution for efficient charge transport and extraction is realized, affording a high all-PSC efficiency of 17.0% with a remarkable FF of 76.1%. The derived devices also possess excellent long-term thermal stability which can retain >90% of their initial efficiencies after being annealed at 65 °C for 1300 h. These results validate that the distinct advantages of employing such LBL processing protocol to fabricate high-performance all-PSCs. This article is protected by copyright. All rights reserved}, journal={ADVANCED MATERIALS}, author={Fu, Huiting and Peng, Zhengxing and Fan, Qunping and Lin, Francis R. and Qi, Feng and Ran, Yixin and Wu, Ziang and Fan, Baobing and Jiang, Kui and Woo, Han Young and et al.}, year={2022}, month={Jul} }
 @article{gao_qi_peng_lin_jiang_zhong_kaminsky_guan_lee_marks_et al._2022, title={Achieving 19% Power Conversion Efficiency in Planar-Mixed Heterojunction Organic Solar Cells Using a Pseudosymmetric Electron Acceptor}, volume={7}, ISSN={["1521-4095"]}, DOI={10.1002/adma.202202089}, abstractNote={A record power conversion efficiency (PCE) of over 19% is realized in planar-mixed heterojunction (PMHJ) organic solar cells (OSCs) by adopting the asymmetric selenium substitution strategy in making a pseudosymmetric electron acceptor, BS3TSe-4F. The combined molecular asymmetry with more polarizable selenium substitution increases the dielectric constant of the D18/BS3TSe-4F blend, helping lower the exciton binding energy. On the other hand, dimer packing in BS3TSe-4F is facilitated to enable free charge generation, helping more efficient exciton dissociation and lowering the radiative recombination loss (ΔE2 ) of OSCs. As a result, PMHJ OSCs based on D18/BS3TSe-4F achieve a PCE of 18.48%. By incorporating another mid-bandgap acceptor Y6-O into D18/BS3TSe-4F to form a ternary PMHJ, a higher open-circuit voltage (VOC ) can be achieved to realize an impressive PCE of 19.03%. The findings of using pseudosymmetric electron acceptors in enhancing device efficiency provides an effective way to develop highly efficient acceptor materials for OSCs.}, journal={ADVANCED MATERIALS}, author={Gao, Wei and Qi, Feng and Peng, Zhengxing and Lin, Francis R. and Jiang, Kui and Zhong, Cheng and Kaminsky, Werner and Guan, Zhiqiang and Lee, Chun-Sing and Marks, Tobin J. and et al.}, year={2022}, month={Jul} }
 @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{jiang_zhang_zhong_lin_qi_li_peng_kaminsky_jang_yu_et al._2022, title={Suppressed recombination loss in organic photovoltaics adopting a planar-mixed heterojunction architecture}, volume={7}, ISSN={["2058-7546"]}, DOI={10.1038/s41560-022-01138-y}, number={11}, journal={NATURE ENERGY}, author={Jiang, Kui and Zhang, Jie and Zhong, Cheng and Lin, Francis R. and Qi, Feng and Li, Qian and Peng, Zhengxing and Kaminsky, Werner and Jang, Sei-Hum and Yu, Jianwei and et al.}, year={2022}, month={Nov}, pages={1076–1086} }
 @article{yu_luo_sun_angunawela_qi_peng_zhou_han_wei_pan_et al._2021, title={A Difluoro-Monobromo End Group Enables High-Performance Polymer Acceptor and Efficient All-Polymer Solar Cells Processable with Green Solvent under Ambient Condition}, volume={31}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.202100791}, abstractNote={In this paper, a difluoro-monobromo end group is designed and synthesized, which is then used to construct a novel polymer acceptor (named PY2F-T) yielding high-performance all-polymer solar cells with 15.22% efficiency. The fluorination strategy can increase the intramolecular charge transfer and interchain packing of the previous PY-T based acceptor, and significantly improve photon harvesting and charge mobility of the resulting polymer acceptor. In addition, detailed morphology investigations reveal that the PY2F-T-based blend shows smaller domain spacing and higher domain purity, which significantly suppress charge recombination as supported by time-resolved techniques. These polymer properties enable simultaneously enhanced JSC and FF of the PY2F-T-based devices, eventually delivering device efficiencies of over 15%, significantly outperforming that of the devices based on the non-fluorinated PY-T polymer (13%). More importantly, the PY2F-T-based active layers can be processed under ambient conditions and still achieve a 14.37% efficiency. They can also be processed using non-halogenated solvent o-xylene (no additive) and yield a decent performance of 13.05%. This work demonstrates the success of the fluorination strategy in the design of high-performance polymer acceptors, which provide guidelines for developing new all-PSCs with better efficiencies and stabilities for commercial applications.}, number={25}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Yu, Han and Luo, Siwei and Sun, Rui and Angunawela, Indunil and Qi, Zhenyu and Peng, Zhengxing and Zhou, Wentao and Han, Han and Wei, Rong and Pan, Mingao and et al.}, year={2021}, month={Jun} }
 @article{ghasemi_balar_peng_hu_qin_kim_rech_bidwell_mask_mcculloch_et al._2021, title={A molecular interaction-diffusion framework for predicting organic solar cell stability}, volume={20}, ISSN={["1476-4660"]}, DOI={10.1038/s41563-020-00872-6}, number={4}, journal={NATURE MATERIALS}, author={Ghasemi, Masoud and Balar, Nrup and Peng, Zhengxing and Hu, Huawei and Qin, Yunpeng and Kim, Taesoo and Rech, Jeromy J. and Bidwell, Matthew and Mask, Walker and McCulloch, Iain and et al.}, year={2021}, month={Apr}, pages={525-+} }
 @article{chen_bai_peng_zhu_zhang_zou_qin_kim_yuan_ma_et al._2021, title={Asymmetric Alkoxy and Alkyl Substitution on Nonfullerene Acceptors Enabling High-Performance Organic Solar Cells}, volume={11}, ISSN={["1614-6840"]}, DOI={10.1002/aenm.202003141}, number={3}, journal={ADVANCED ENERGY MATERIALS}, author={Chen, Yuzhong and Bai, Fujin and Peng, Zhengxing and Zhu, Lei and Zhang, Jianquan and Zou, Xinhui and Qin, Yunpeng and Kim, Ha Kyung and Yuan, Jun and Ma, Lik-Kuen and et al.}, year={2021}, month={Jan} }
 @article{zheng_sun_zhang_chen_peng_wu_yuan_yu_wang_wu_et al._2021, title={Baseplate Temperature-Dependent Vertical Composition Gradient in Pseudo-Bilayer Films for Printing Non-Fullerene Organic Solar Cells}, volume={10}, ISSN={["1614-6840"]}, DOI={10.1002/aenm.202102135}, abstractNote={Numerous previous reports on the sequential deposition (SD) technique have demonstrated that this approach can achieve a p-i-n active layer architecture with an ideal vertical composition gradient, which is one of the critical factors that can influence the physical processes that determine the photovoltaic performance of organic solar cells. Herein, a commonly used photovoltaic system comprised of PM6 as a donor and Y6 as an acceptor is investigated with respect to sequential blade-processing deposition to comprehensively explore the morphology characteristics as a function of baseplate temperature. A systematic study of the temperature-dependent blend morphology elucidates the SD-processed configuration merits and device physics behind temperature-controlled degree of vertical composition gradient, and constructs the temperature-microstructure-property relationship for the corresponding photovoltaic parameters. The result shows, as the temperature increases, the morphology of the active layer has undergone a distinct evolution from the pseudo-bulk heterojunction to a pseudo-planar heterojunction and then to a pseudo-planar bilayer, leading to a non-monotonic correlation between baseplate temperature and device performance. This investigation not only reveals the importance of precisely controlling baseplate temperature for gaining vertical morphology control, but also provides a path toward rational optimization of device performance in the lab-to-fab transition.}, journal={ADVANCED ENERGY MATERIALS}, author={Zheng, Yina and Sun, Rui and Zhang, Meng and Chen, Zhihao and Peng, Zhengxing and Wu, Qiang and Yuan, Xinxin and Yu, Yue and Wang, Tao and Wu, Yao and et al.}, year={2021}, month={Oct} }
 @article{schopp_brus_lee_dixon_karki_liu_peng_graham_ade_bazan_et al._2021, title={Effect of Palladium-Tetrakis(Triphenylphosphine) Catalyst Traces on Charge Recombination and Extraction in Non-Fullerene-based Organic Solar Cells}, volume={31}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.202009363}, abstractNote={The effect of the cross-coupling catalyst tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) on the performance of a model organic bulk-heterojunction solar cell composed of a blend of poly([2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene]{3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7-Th) donor and 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (IOTIC-4F) non-fullerene acceptor is investigated. The effect of intentional addition of different amounts of Pd(PPh3)4 on morphology, free charge carrier generation, non-geminate bulk trap- and surface trap-assisted recombination as well as bimolecular recombination and charge extraction is quantified. This work shows that free charge carrier generation is affected significantly, while the impact of Pd(PPh3)4 on non-geminate recombination processes is limited because the catalyst does not facilitate efficient trap-assisted recombination. The studied system shows substantial robustness towards the addition of Pd(PPh3)4 in small amounts.}, number={15}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Schopp, Nora and Brus, Viktor V. and Lee, Jaewon and Dixon, Alana and Karki, Akchheta and Liu, Tuo and Peng, Zhengxing and Graham, Kenneth R. and Ade, Harald and Bazan, Guillermo C. and et al.}, year={2021}, month={Apr} }
 @article{du_mainville_vollbrecht_dixon_schopp_schrock_peng_huang_chae_ade_et al._2021, title={Insights into Bulk-Heterojunction Organic Solar Cells Processed from Green Solvent}, volume={5}, ISSN={["2367-198X"]}, url={https://doi.org/10.1002/solr.202100213}, DOI={10.1002/solr.202100213}, abstractNote={The environmental impact of solution processed organic solar cells (OSCs) can be mitigated by introducing so-called green solvents during the fabrication processes. However, the effects of such green solvents on the molecular-level structures and optoelectronic properties lack in-depth characterization. Here, insights into the structure–processing–property correlation of a PPDT2FBT:PC61BM bulk-heterojunction (BHJ) system processed from a green solvent, ortho-xylene (o-XY), is investigated in comparison with the same blend processed from a traditional halogenated solvent, chlorobenzene (CB). The BHJ blends are characterized with various techniques probing at difference length scales, and an increased donor:acceptor (D:A) interfacial area as well as well-mixed features in the bulk morphologies of the active layer are observed for the o-XY processed BHJ blend. Furthermore, molecular-level differences in the D–A intermolecular interactions at the BHJ interfaces in o-XY and CB cast films are elucidated by 2-dimensional solid-state nuclear magnetic resonance (ssNMR) measurements and analysis. These results are consistent with the device properties, suggesting that the green-solvent-processed devices have longer charge carrier lifetimes and faster charge carrier extraction. The optimized PPDT2FBT:PC61BM devices processed from o-XY can achieve a noteworthy higher power conversion efficiency (PCE) owing to a higher short-circuit current density and fill factor.}, journal={SOLAR RRL}, publisher={Wiley}, author={Du, Zhifang and Mainville, Mathieu and Vollbrecht, Joachim and Dixon, Alana L. and Schopp, Nora and Schrock, Max and Peng, Zhengxing and Huang, Jianfei and Chae, Sangmin and Ade, Harald and et al.}, year={2021}, month={Jun} }
 @article{jiang_zhang_peng_lin_wu_li_chen_yan_ade_zhu_et al._2021, title={Pseudo-bilayer architecture enables high-performance organic solar cells with enhanced exciton diffusion length}, volume={12}, ISSN={["2041-1723"]}, url={https://doi.org/10.1038/s41467-020-20791-z}, DOI={10.1038/s41467-020-20791-z}, abstractNote={Abstract Solution-processed organic solar cells (OSCs) are a promising candidate for next-generation photovoltaic technologies. However, the short exciton diffusion length of the bulk heterojunction active layer in OSCs strongly hampers the full potential to be realized in these bulk heterojunction OSCs. Herein, we report high-performance OSCs with a pseudo-bilayer architecture, which possesses longer exciton diffusion length benefited from higher film crystallinity. This feature ensures the synergistic advantages of efficient exciton dissociation and charge transport in OSCs with pseudo-bilayer architecture, enabling a higher power conversion efficiency (17.42%) to be achieved compared to those with bulk heterojunction architecture (16.44%) due to higher short-circuit current density and fill factor. A certified efficiency of 16.31% is also achieved for the ternary OSC with a pseudo-bilayer active layer. Our results demonstrate the excellent potential for pseudo-bilayer architecture to be used for future OSC applications.}, number={1}, journal={NATURE COMMUNICATIONS}, publisher={Springer Science and Business Media LLC}, author={Jiang, Kui and Zhang, Jie and Peng, Zhengxing and Lin, Francis and Wu, Shengfan and Li, Zhen and Chen, Yuzhong and Yan, He and Ade, Harald and Zhu, Zonglong and et al.}, year={2021}, month={Jan} }
 @article{peng_ye_ade_2021, title={Understanding, quantifying, and controlling the molecular ordering of semiconducting polymers: from novices to experts and amorphous to perfect crystals}, volume={9}, ISSN={["2051-6355"]}, url={https://doi.org/10.1039/D0MH00837K}, DOI={10.1039/d0mh00837k}, abstractNote={Molecular packing and texture of semiconducting polymers are often critical to the performance of devices using these materials. Although frameworks exist to quantify the ordering, interpretations are often just qualitative, resulting in imprecise use of terminology. Here, we reemphasize the significance of quantifying molecular ordering in terms of degree of crystallinity (volume fractions that are ordered) and quality of ordering and their relation to the size scale of an ordered region. We are motivated in part by our own imprecise and inconsistent use of terminology in the past, as well as the need to have a primer or tutorial reference to teach new group members. We strive to develop and use consistent terminology with regards to crystallinity, semicrystallinity, paracrystallinity, and related characteristics. To account for vastly different quality of ordering along different directions, we classify paracrystals into 2D and 3D paracrystals and use paracrystallite to describe the spatial extent of molecular ordering in 1-10 nm. We show that a deeper understanding of molecular ordering can be achieved by combining grazing-incidence wide-angle X-ray scattering and differential scanning calorimetry, even though not all aspects of these measurements are consistent, and some classification appears to be method dependent. We classify a broad range of representative polymers under common processing conditions into five categories based on the quantitative analysis of the paracrystalline disorder parameter (g) and thermal transitions. A small database is presented for 13 representative conjugated and insulating polymers ranging from amorphous to semi-paracrystalline. Finally, we outline the challenges to rationally design more perfect polymer crystals and propose a new molecular design approach that envisions conceptual molecular grafting that is akin to strained and unstrained hetero-epitaxy in classic (compound) semiconductors thin film growth.}, number={2}, journal={MATERIALS HORIZONS}, publisher={Royal Society of Chemistry (RSC)}, author={Peng, Zhengxing and Ye, Long and Ade, Harald}, year={2021}, month={Dec} }
 @article{peng_balar_ghasemi_ade_2021, title={Upper and Apparent Lower Critical Solution Temperature Branches in the Phase Diagram of Polymer:Small Molecule Semiconducting Systems}, volume={12}, ISSN={["1948-7185"]}, DOI={10.1021/acs.jpclett.1c02848}, abstractNote={Solution-processable semiconducting materials are complex materials with a wide range of applications. Despite their extensive study and utility, their molecular interactions as manifested, for example, in phase behavior are poorly understood. Here, we aim to understand the phase behavior of conjugated systems by determining phase diagrams spanning extensive temperature ranges for various combinations of the highly disordered semiconducting polymer (PTB7-Th) with crystallizable (IT-M and PC61BM) and noncrystallizable (di-PDI) small molecule acceptors (SMAs), with polystyrene as an amorphous control, a nonsemiconducting commodity polymer. We discover that the apparent binodal of the studied blends frequently consists of an upper critical solution temperature (UCST) and lower critical solution temperature (LCST) branch, exhibiting a sharp kink where the branches join. Our work suggests that phase diagrams might be a probe in combination with sophisticated models to understand the complexity of semiconducting materials, including microstructure and molecular interactions.}, number={44}, journal={JOURNAL OF PHYSICAL CHEMISTRY LETTERS}, author={Peng, Zhengxing and Balar, Nrup and Ghasemi, Masoud and Ade, Harald}, year={2021}, month={Nov}, pages={10845–10853} }
 @article{li_guo_peng_qu_yan_ade_zhang_forrest_2020, title={Color-neutral, semitransparent organic photovoltaics for power window applications}, volume={117}, ISSN={["0027-8424"]}, DOI={10.1073/pnas.2007799117}, abstractNote={Significance We demonstrate a semitransparent organic photovoltaic cell that achieves a power conversion efficiency of 10.8% and visible transparency of ∼50% using a nonfullerene acceptor (NFA) featuring strong near-infrared (NIR) absorption and simple synthesis. Contrary to expectations, stronger NIR absorption and closer molecular packing are obtained by employing an additive in these partially, instead of fully fused, rigid NFAs. By combining NIR-absorbing material sets with an optical outcoupling structure as well as transparent electrode, we overcome the trade-offs between efficiency, transparency, and device appearance. These results surpass other semitransparent solar cell technologies based on organic and other thin-film materials systems, showing a promising future for ST-OPVs as power-generating windows and other solar energy harvesting applications.}, number={35}, journal={PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA}, author={Li, Yongxi and Guo, Xia and Peng, Zhengxing and Qu, Boning and Yan, Hongping and Ade, Harald and Zhang, Maojie and Forrest, Stephen R.}, year={2020}, month={Sep}, pages={21147–21154} }
 @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{chai_chang_peng_jia_zou_yu_yu_chen_chow_wong_et al._2020, title={Enhanced hindrance from phenyl outer side chains on nonfullerene acceptor enables unprecedented simultaneous enhancement in organic solar cell performances with 16.7% efficiency}, volume={76}, ISSN={["2211-3282"]}, DOI={10.1016/j.nanoen.2020.105087}, abstractNote={Abstract Inner side-chain engineering on Y6 has been proven successful in improving short-circuit current density (JSC) through fine-tuning aggregated structures of acceptors. However, it fails in tuning the lowest unoccupied molecular orbital level (LUMO) and open-circuit voltage (VOC). In this paper, we turn to focus on engineering the outer side chains on the flanking thienothiophene units with 4-hexylphenyl (PhC6) and 6-phenylhexyl (C6Ph) chains. Use of PhC6 enhances the steric effect between the attached phenyl and the ending group, which in combination with the additional conjugation effect provided by the linking phenyl leads to upshifted energy levels and increased VOC as a result. Again, substitution with the bulkier PhC6 unprecedentedly improves film-morphology with reduced paracrystalline disorder and long period and increased root-mean-square composition variations as well, leading to increased electron and hole mobilities and suppressed monomolecular recombination with JSC and fill-factor (FF) simultaneously enhanced. The PM6:BTP-PhC6-based devices yield a higher efficiency value of 16.7% than the PM6:BTP-C6Ph-based one (15.5%). Therefore, this study shows a conceptual advance in materials design towards reducing the conflict between VOC and JSC in binary blended organic solar cells, which can be achieved by introducing bulkier chains to twist the backbone and simultaneously enhance the packing order.}, journal={NANO ENERGY}, author={Chai, Gaoda and Chang, Yuan and Peng, Zhengxing and Jia, Yanyan and Zou, Xinhui and Yu, Dian and Yu, Han and Chen, Yuzhong and Chow, Philip C. Y. and Wong, Kam Sing and et al.}, year={2020}, month={Oct} }
 @article{qin_xu_peng_hou_ade_2020, title={Low Temperature Aggregation Transitions in N3 and Y6 Acceptors Enable Double-Annealing Method That Yields Hierarchical Morphology and Superior Efficiency in Nonfullerene Organic Solar Cells}, volume={30}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.202005011}, abstractNote={Thermal transition of organic solar cells (OSCs) constituent materials are often insufficiently researched, resulting in trial-and-error rather than rational approaches to annealing strategies to improve domain purity to enhance the power conversion efficiency. Despite the potential utility, little is known about the thermal transitions of the modern high-performance acceptors Y6 and N3. Here, by using an optical method, it is discovered that the acceptor N3 has a clear solid-state aggregation transition at 82 °C. This unusually low transition not only explains prior optimization protocols, but the transition informs and enables a double-annealing method that can fine-tune aggregation and the device morphology. Compared with 16.6% efficiency for PM6:N3:PC71BM control devices, higher efficiency of 17.6% is obtained through the improved protocol. Morphology characterization with x-ray scattering methods reveals the formation of a multilength scale morphology. Moreover, the double-annealing method is illustrated and easily transferred and validated with Y6-based devices, using the transition of Y6 at 102 °C. As a result, the PCE improved from 16.0% to 16.8%. Design of high-performance acceptors with yet lower aggregation transitions might be required for OSCs to successfully transition to low thermal budget industrial processing methods where annealing temperatures on plastic substrates have to be kept low.}, number={46}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Qin, Yunpeng and Xu, Ye and Peng, Zhengxing and Hou, Jianhui and Ade, Harald}, year={2020}, month={Nov} }
 @article{zhang_li_peng_bai_ma_ade_li_yan_2020, title={Near-infrared electron acceptors with fused nonacyclic molecular backbones for nonfullerene organic solar cells}, volume={4}, ISSN={["2052-1537"]}, DOI={10.1039/c9qm00754g}, abstractNote={Two strong electron-donating moieties were fused into the molecular backbone of small molecular acceptors showing dramatically extended absorption beyond 900 nm.}, number={6}, journal={MATERIALS CHEMISTRY FRONTIERS}, author={Zhang, Jianquan and Li, Yunke and Peng, Zhengxing and Bai, Fujin and Ma, Lik-Kuen and Ade, Harald and Li, Zhengke and Yan, He}, year={2020}, month={Jun}, pages={1729–1738} }
 @article{samson_rech_perdigon-toro_peng_shoaee_ade_neher_stolterfoht_you_2020, title={Organic Solar Cells with Large Insensitivity to Donor Polymer Molar Mass across All Acceptor Classes}, volume={2}, ISSN={["2637-6105"]}, DOI={10.1021/acsapm.0c01041}, abstractNote={Donor polymer number-average molar mass (Mn) has long been known to influence organic photovoltaic (OPV) performance via changes in both the polymer properties and the resulting bulk heterojunction...}, number={11}, journal={ACS APPLIED POLYMER MATERIALS}, author={Samson, Stephanie and Rech, Jeromy and Perdigon-Toro, Lorena and Peng, Zhengxing and Shoaee, Safa and Ade, Harald and Neher, Dieter and Stolterfoht, Martin and You, Wei}, year={2020}, month={Nov}, pages={5300–5308} }
 @article{liang_pan_chai_peng_zhang_luo_han_chen_shang_bai_et al._2020, title={Random Polymerization Strategy Leads to a Family of Donor Polymers Enabling Well-Controlled Morphology and Multiple Cases of High-Performance Organic Solar Cells}, volume={32}, ISSN={["1521-4095"]}, DOI={10.1002/adma.202003500}, abstractNote={Developing high-performance donor polymers is important for nonfullerene organic solar cells (NF-OSCs), as state-of-the-art nonfullerene acceptors can only perform well if they are coupled with a matching donor with suitable energy levels. However, there are very limited choices of donor polymers for NF-OSCs, and the most commonly used ones are polymers named PM6 and PM7, which suffer from several problems. First, the performance of these polymers (particularly PM7) relies on precise control of their molecular weights. Also, their optimal morphology is extremely sensitive to any structural modification. In this work, a family of donor polymers is developed based on a random polymerization strategy. These polymers can achieve well-controlled morphology and high-performance with a variety of chemical structures and molecular weights. The polymer donors are D-A1-D-A2-type random copolymers in which the D and A1 units are monomers originating from PM6 or PM7, while the A2 unit comprises an electron-deficient core flanked by two thiophene rings with branched alkyl chains. Consequently, multiple cases of highly efficient NF-OSCs are achieved with efficiencies between 16.0% and 17.1%. As the electron-deficient cores can be changed to many other structural units, the strategy can easily expand the choices of high-performance donor polymers for NF-OSCs.}, number={52}, journal={ADVANCED MATERIALS}, author={Liang, Jiaen and Pan, Mingao and Chai, Gaoda and Peng, Zhengxing and Zhang, Jianquan and Luo, Siwei and Han, Qi and Chen, Yuzhong and Shang, Ao and Bai, Fujin and et al.}, year={2020}, month={Dec} }
 @article{balar_siddika_kashani_peng_rech_ye_you_ade_brendan t. o'conner_2020, title={Role of Secondary Thermal Relaxations in Conjugated Polymer Film Toughness}, volume={32}, ISSN={["1520-5002"]}, url={https://publons.com/wos-op/publon/35208553/}, DOI={10.1021/acs.chemmater.0c01910}, abstractNote={Conjugated polymers have proven to be an important class of materials for flexible and stretchable electronics. To ensure long-term thermal and mechanical stability of associated devices, there is ...}, number={15}, journal={CHEMISTRY OF MATERIALS}, author={Balar, Nrup and Siddika, Salma and Kashani, Somayeh and Peng, Zhengxing and Rech, Jeromy James and Ye, Long and You, Wei and Ade, Harald and Brendan T. O'Conner}, year={2020}, month={Aug}, pages={6540–6549} }
 @article{arunagiri_peng_zou_yu_zhang_wang_lai_zhang_zheng_cui_et al._2020, title={Selective Hole and Electron Transport in Efficient Quaternary Blend Organic Solar Cells}, volume={4}, ISSN={["2542-4351"]}, DOI={10.1016/j.joule.2020.06.014}, abstractNote={Summary Multi-component organic solar cells (OSCs) comprising more than two donor and acceptor materials have attracted significant research attention, as they can offer broader and better absorption, hence increasing solar cell performance. However, the morphology of multi-component OSCs is exceptionally complicated and challenging to control. Here, we develop a highly efficient (near 17.7%) quaternary OSC (q-OSC) using two polymer donors (namely PM6 and PTQ10) along with a fullerene (PC71BM) and a non-fullerene acceptor (N3). Our quaternary system demonstrates a new type of “rivers and streams” functional hierarchical (multi-length scale) morphology, where small domains of PTQ10 and PC71BM act as separators that spatially separate PM6 and N3, which effectively suppressed charge recombination, enhanced hole transport, and balanced charge transportation. These improvements in the quaternary system contribute to the increased internal quantum efficiency (IQE) and, thus, lead to an excellent JSC and device performance, which surpass their respective binary and ternary OSCs.}, number={8}, journal={JOULE}, author={Arunagiri, Lingeswaran and Peng, Zhengxing and Zou, Xinhui and Yu, Han and Zhang, Guangye and Wang, Zhen and Lai, Joshua Yuk Lin and Zhang, Jianquan and Zheng, Yan and Cui, Chaohua and et al.}, year={2020}, month={Aug}, pages={1790–1805} }
 @article{hu_ghasemi_peng_zhang_rech_you_yan_ade_2020, title={The Role of Demixing and Crystallization Kinetics on the Stability of Non-Fullerene Organic Solar Cells}, volume={32}, ISSN={["1521-4095"]}, url={https://doi.org/10.1002/adma.202005348}, DOI={10.1002/adma.202005348}, abstractNote={With power conversion efficiency now over 17%, a long operational lifetime is essential for the successful application of organic solar cells. However, most non-fullerene acceptors can crystallize and destroy devices, yet the fundamental underlying thermodynamic and kinetic aspects of acceptor crystallization have received limited attention. Here, room-temperature (RT) diffusion coefficients of 3.4 × 10−23 and 2.0 × 10−22 are measured for ITIC-2Cl and ITIC-2F, two state-of-the-art non-fullerene acceptors. The low coefficients are enough to provide for kinetic stabilization of the morphology against demixing at RT. Additionally profound differences in crystallization characteristics are discovered between ITIC-2F and ITIC-2Cl. The differences as observed by secondary-ion mass spectrometry, differential scanning calorimetry (DSC), grazing-incidence wide-angle X-ray scattering, and microscopy can be related directly to device degradation and are attributed to the significantly different nucleation and growth rates, with a difference in the growth rate of a factor of 12 at RT. ITIC-4F and ITIC-4Cl exhibit similar characteristics. The results reveal the importance of diffusion coefficients and melting enthalpies in controlling the growth rates, and that differences in halogenation can drastically change crystallization kinetics and device stability. It is furthermore delineated how low nucleation density and large growth rates can be inferred from DSC and microscopy experiments which could be used to guide molecular design for stability.}, number={49}, journal={ADVANCED MATERIALS}, publisher={Wiley}, author={Hu, Huawei and Ghasemi, Masoud and Peng, Zhengxing and Zhang, Jianquan and Rech, Jeromy James and You, Wei and Yan, He and Ade, Harald}, year={2020}, month={Dec} }
 @article{karki_vollbrecht_gillett_xiao_yang_peng_schopp_dixon_yoon_schrock_et al._2020, title={The role of bulk and interfacial morphology in charge generation, recombination, and extraction in non-fullerene acceptor organic solar cells}, volume={13}, ISSN={["1754-5706"]}, DOI={10.1039/d0ee01896a}, abstractNote={This study provides insight into the detailed bulk and interfacial morphological features critical in achieving high PCEs in polymer:NFA OSCs.}, number={10}, journal={ENERGY & ENVIRONMENTAL SCIENCE}, author={Karki, Akchheta and Vollbrecht, Joachim and Gillett, Alexander J. and Xiao, Steven Shuyong and Yang, Yali and Peng, Zhengxing and Schopp, Nora and Dixon, Alana L. and Yoon, Sangcheol and Schrock, Max and et al.}, year={2020}, month={Oct}, pages={3679–3692} }
 @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 PC71 BM 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{karki_vollbrecht_gillett_selter_lee_peng_schopp_dixon_schrock_nadazdy_et al._2020, title={Unifying Charge Generation, Recombination, and Extraction in Low-Offset Non-Fullerene Acceptor Organic Solar Cells}, volume={10}, ISSN={["1614-6840"]}, DOI={10.1002/aenm.202001203}, abstractNote={Even though significant breakthroughs with over 18% power conversion efficiencies (PCEs) in polymer:non-fullerene acceptor (NFA) bulk heterojunction organic solar cells (OSCs) have been achieved, not many studies have focused on acquiring a comprehensive understanding of the underlying mechanisms governing these systems. This is because it can be challenging to delineate device photophysics in polymer:NFA blends comprehensively, and even more complicated to trace the origins of the differences in device photophysics to the subtle differences in energetics and morphology. Here, a systematic study of a series of polymer:NFA blends is conducted to unify and correlate the cumulative effects of i) voltage losses, ii) charge generation efficiencies, iii) non-geminate recombination and extraction dynamics, and iv) nuanced morphological differences with device performances. Most importantly, a deconvolution of the major loss processes in polymer:NFA blends and their connections to the complex BHJ morphology and energetics are established. An extension to advanced morphological techniques, such as solid-state NMR (for atomic level insights on the local ordering and donor:acceptor ππ interactions) and resonant soft X-ray scattering (for donor and acceptor interfacial area and domain spacings), provide detailed insights on how efficient charge generation, transport, and extraction processes can outweigh increased voltage losses to yield high PCEs.}, number={29}, journal={ADVANCED ENERGY MATERIALS}, author={Karki, Akchheta and Vollbrecht, Joachim and Gillett, Alexander J. and Selter, Philipp and Lee, Jaewon and Peng, Zhengxing and Schopp, Nora and Dixon, Alana L. and Schrock, Max and Nadazdy, Vojtech and et al.}, year={2020}, month={Aug} }
 @article{jiang_wei_lai_peng_kim_yuan_ye_ade_zou_yan_2019, title={Alkyl Chain Tuning of Small Molecule Acceptors for Efficient Organic Solar Cells}, volume={3}, ISSN={["2542-4351"]}, url={https://publons.com/wos-op/publon/27161014/}, DOI={10.1016/j.joule.2019.09.010}, abstractNote={Summary The field of organic solar cells has seen rapid developments after the report of a high-efficiency (15.7%) small molecule acceptor (SMA) named Y6. In this paper, we design and synthesize a family of SMAs with an aromatic backbone identical to that of Y6 but with different alkyl chains to investigate the influence of alkyl chains on the properties and performance of the SMAs. First, we show that it is beneficial to use branched alkyl chains on the nitrogen atoms of the pyrrole motif of the Y6. In addition, the branching position of the alkyl chains also has a major influence on material and device properties. The SMA with 3rd-position branched alkyl chains (named N3) exhibits optimal solubility and electronic and morphological properties, thus yielding the best performance. Further device optimization using a ternary strategy allows us to achieve a high efficiency of 16.74% (and a certified efficiency of 16.42%).}, number={12}, journal={JOULE}, author={Jiang, Kui and Wei, Qingya and Lai, Joshua Yuk Lin and Peng, Zhengxing and Kim, Ha Kyung and Yuan, Jun and Ye, Long and Ade, Harald and Zou, Yingping and Yan, He}, year={2019}, month={Dec}, pages={3020–3033} }
 @article{ghasemi_hu_peng_rech_angunawela_carpenter_stuard_wadsworth_mcculloch_you_et al._2019, title={Delineation of Thermodynamic and Kinetic Factors that Control Stability in Non-fullerene Organic Solar Cells}, volume={3}, ISSN={["2542-4351"]}, DOI={10.1016/j.joule.2019.03.020}, abstractNote={Summary Although non-fullerene small molecular acceptors (NF-SMAs) are dominating current research in organic solar cells (OSCs), measurements of thermodynamics drivers and kinetic factors determining their morphological stability are lacking. Here, we delineate and measure such factors in crystallizable NF-SMA blends and discuss four model systems with respect to their meta-stability and degree of vitrification. We determine for the first time the amorphous-amorphous phase diagram in an NF-SMA system and show that its deep quench depth can result in severe burn-in degradation. We estimate the relative phase behavior of four other materials systems. Additionally, we derive room-temperature diffusion coefficients and conclude that the morphology needs to be stabilized by vitrification corresponding to diffusion constants below 10−22 cm2/s. Our results show that to achieve stability via rational molecular design, the thermodynamics, glass transition temperature, diffusion properties, and related structure-function relations need to be more extensively studied and understood.}, number={5}, journal={JOULE}, author={Ghasemi, Masoud and Hu, Huawei and Peng, Zhengxing and Rech, Jeromy James and Angunawela, Indunil and Carpenter, Joshua H. and Stuard, Samuel J. and Wadsworth, Andrew and McCulloch, Iain and You, Wei and et al.}, year={2019}, month={May}, pages={1328–1348} }
 @article{zhang_rech_yan_liang_peng_ade_wu_you_2019, title={Effect of Cyano Substitution on Conjugated Polymers for Bulk Heterojunction Solar Cells}, volume={1}, ISSN={["2637-6105"]}, DOI={10.1021/acsapm.9b00767}, abstractNote={The design of polymer structures has played a vital role in improving the efficiency of organic solar cells (OSCs). A common approach to increase solar cell efficiency is to add a specific substitu...}, number={12}, journal={ACS APPLIED POLYMER MATERIALS}, author={Zhang, Qianqian and Rech, Jeromy James and Yan, Liang and Liang, Quanbin and Peng, Zhengxing and Ade, Harald and Wu, Hongbin and You, Wei}, year={2019}, month={Dec}, pages={3313–3322} }
 @article{duan_peng_colberts_pang_ye_awartani_hendriks_ade_wienk_janssen_et al._2019, title={Efficient Thick-Film Polymer Solar Cells with Enhanced Fill Factors via Increased Fullerene Loading}, volume={11}, ISSN={["1944-8252"]}, url={https://doi.org/10.1021/acsami.9b00337}, DOI={10.1021/acsami.9b00337}, abstractNote={Developing effective methods to make efficient bulk-heterojunction polymer solar cells at roll-to-roll relevant active layer thickness is of significant importance. We investigate the effect of fullerene content in polymer:fullerene blends on the fill factor (FF) and on the performance of thick-film solar cells for four different donor polymers PTB7-Th, PDPP-TPT, BDT-FBT-2T, and poly[5,5′-bis(2-butyloctyl)-(2,2′-bithiophene)-4,4′-dicarboxylate-alt-5,5′-2,2′-bithiophene] (PDCBT). At a few hundreds of nanometers thickness, increased FFs are observed in all cases and improved overall device performances are obtained except for PDCBT upon increasing fullerene content in blend films. This fullerene content effect was studied in more detail by electrical and morphological characterization. The results suggest enhanced electron mobility and suppressed bimolecular recombination upon increasing fullerene content in thick polymer:fullerene blend films, which are the result of larger fullerene aggregates and improved interconnectivity of the fullerene phases that provide continuous percolating pathways for electron transport in thick films. These findings are important because an effective and straightforward method that enables fabricating efficient thick-film polymer solar cells is desirable for large-scale manufacturing via roll-to-roll processing and for multijunction devices.}, number={11}, journal={ACS APPLIED MATERIALS & INTERFACES}, publisher={American Chemical Society (ACS)}, author={Duan, Chunhui and Peng, Zhengxing and Colberts, Fallon J. M. and Pang, Shuting and Ye, Long and Awartani, Omar M. and Hendriks, Koen H. and Ade, Harald and Wienk, Martijn M. and Janssen, Rene A. J. and et al.}, year={2019}, month={Mar}, pages={10794–10800} }
 @article{zhu_gadisa_peng_ghasemi_ye_xu_zhao_ade_2019, title={Rational Strategy to Stabilize an Unstable High-Efficiency Binary Nonfullerene Organic Solar Cells with a Third Component}, volume={9}, ISSN={["1614-6840"]}, url={https://doi.org/10.1002/aenm.201900376}, DOI={10.1002/aenm.201900376}, abstractNote={Long device lifetime is still a missing key requirement in the commercialization of nonfullerene acceptor (NFA) organic solar cell technology. Understanding thermodynamic factors driving morphology degradation or stabilization is correspondingly lacking. In this report, thermodynamics is combined with morphology to elucidate the instability of highly efficient PTB7-Th:IEICO-4F binary solar cells and to rationally use PC71BM in ternary solar cells to reduce the loss in the power conversion efficiency from ≈35% to <10% after storage for 90 days and at the same time improve performance. The hypomiscibility observed for IEICO-4F in PTB7-Th (below the percolation threshold) leads to overpurification of the mixed domains. By contrast, the hypermiscibility of PC71BM in PTB7-Th of 48 vol% is well above the percolation threshold. At the same time, PC71BM is partly miscible in IEICO-4F suppressing crystallization of IEICO-4F. This work systematically illustrates the origin of the intrinsic degradation of PTB7-Th:IEICO-4F binary solar cells, demonstrates the structure–function relations among thermodynamics, morphology, and photovoltaic performance, and finally carries out a rational strategy to suppress the degradation: the third component needs to have a miscibility in the donor polymer at or above the percolation threshold, yet also needs to be partly miscible with the crystallizable acceptor.}, number={20}, journal={ADVANCED ENERGY MATERIALS}, publisher={Wiley}, author={Zhu, Youqin and Gadisa, Abay and Peng, Zhengxing and Ghasemi, Masoud and Ye, Long and Xu, Zheng and Zhao, Suling and Ade, Harald}, year={2019}, month={May} }
 @article{bauer_zhang_rech_dai_peng_ade_wang_zhan_you_2019, title={The impact of fluorination on both donor polymer and non-fullerene acceptor: The more fluorine, the merrier}, volume={12}, ISSN={["1998-0000"]}, DOI={10.1007/s12274-019-2362-3}, number={9}, journal={NANO RESEARCH}, author={Bauer, Nicole and Zhang, Qianqian and Rech, Jeromy James and Dai, Shuixing and Peng, Zhengxing and Ade, Harald and Wang, Jiayu and Zhan, Xiaowei and You, Wei}, year={2019}, month={Sep}, pages={2400–2405} }
 @article{rech_yan_peng_dai_zhan_ade_you_2019, title={Utilizing Difluorinated Thiophene Units To Improve the Performance of Polymer Solar Cells}, volume={52}, ISSN={["1520-5835"]}, DOI={10.1021/acs.macromol.9b01168}, abstractNote={While there are numerous approaches to functionalize conjugated polymers for organic solar cells (OSCs), one widely adopted approach is fluorination. Of the many different locations for fluorination, one of the least studied is the conjugated linker which connects the donor and acceptor moieties; further, all existing reports primarily explore monofluorinated thiophene units. Herein, we synthesize and compare two conjugated polymers, HTAZ and dFT-HTAZ, which have different thiophene linkers. In HTAZ, a bare thiophene unit connects the donor and acceptor moieties, while dFT-HTAZ utilizes difluorinated thiophene (dFT) linkers. These polymers serve as the model system to explore the impact of dFT units in OSCs; additionally, this is the first publication to investigate polymers containing dFT units paired with non-fullerene acceptors. Compared to HTAZ, the incorporation of the dFT units maintained the optical properties while lowering the energy levels by ∼0.4 eV, which allowed for a much improved Voc value of ∼1 V. Importantly, when compared with the appropriate non-fullerene acceptor, dFT-HTAZ:ITIC-Th1 blends reached an efficiency of ∼10%, which is nearly 3× that of the nonfluorinated HTAZ. As most OSC polymers have thiophene linkers, using dFT units could serve as a proficient method to increase OSC performance in many polymer systems, especially those that do not have locations for functionalization on the acceptor moiety.}, number={17}, journal={MACROMOLECULES}, author={Rech, Jeromy J. and Yan, Liang and Peng, Zhengxing and Dai, Shuixing and Zhan, Xiaowei and Ade, Harald and You, Wei}, year={2019}, month={Sep}, pages={6523–6532} }
 @article{hu_li_zhang_peng_ma_xin_huang_ma_jiang_zhang_et al._2018, title={Effect of Ring-Fusion on Miscibility and Domain Purity: Key Factors Determining the Performance of PDI-Based Nonfullerene Organic Solar Cells}, volume={8}, ISSN={["1614-6840"]}, DOI={10.1002/aenm.201800234}, abstractNote={Compared to the rapid development of nonfullerene organic solar cells (OSCs) based on the state-of-the-art indacenodithiophene (IDT)-based small molecule acceptors (SMAs), the progress for perylene diimide (PDI)-based electron acceptors has lagged behind owing to the lack of understanding on the structure–morphology–performance relationship of PDI SMAs. Given the ease of synthesis for PDIs and their high intrinsic electron mobility, it is crucial to identify key material parameters that influence the polymer:PDI blend morphology and to develop rational approaches for molecular design toward high-performance PDI-based SMAs. In this study, three pairs of PDI-based SMAs with and without ring-fusion are investigated and it is found that ring-fusion and domain purity are the key structural and morphological factors determining the fill factors (FFs) and efficiencies of PDI-based nonfullerene OSCs. This data shows that nonfullerene OSCs based on the ring-fused PDI-based SMAs exhibit much higher average domain purity and thus increased charge mobilities, which lead to enhanced FFs compared to those solar cells based on nonfused PDIs. This is explained by higher Florry Huggins interaction parameters as observed by melting point depression measurements. This study suggests that increasing repulsive molecular interactions to lower the miscibility between the polymer donor and PDI acceptor is the key to improve the FF and performance of PDI-based devices.}, number={26}, journal={ADVANCED ENERGY MATERIALS}, author={Hu, Huawei and Li, Yunke and Zhang, Jianquan and Peng, Zhengxing and Ma, Lik-kuen and Xin, Jingming and Huang, Jiachen and Ma, Tingxuan and Jiang, Kui and Zhang, Guangye and et al.}, year={2018}, month={Sep} }
 @article{huang_bin_peng_qiu_sun_liebman-pelaez_zhang_zhu_ade_zhang_et al._2018, title={Effect of Side-Chain Engineering of Bithienylbenzodithiophene-alt-fluorobenzotriazole-Based Copolymers on the Thermal Stability and Photovoltaic Performance of Polymer Solar Cells}, volume={51}, ISSN={["1520-5835"]}, DOI={10.1021/acs.macromol.8b01036}, abstractNote={Side-chain engineering of conjugated polymer donor materials is an important way for improving photovoltaic performances of polymer solar cells (PSCs). On the basis of the polymer J61 synthesized in our group, here, we design and synthesize three new 2D-conjugated polymers J62, J63, and J64 with different types of side chains to further investigate the effect of side chain on their physicochemical and photovoltaic properties. With the narrow bandgap n-type organic semiconductor (n-OS) ITIC as acceptor, the optimized PSCs based on polymer donor of J62 with linear octyl, J63 with linear unsaturated hexylene, and J64 with cyclohexane side chains display power conversion efficiency (PCE) of 10.81%, 8.13%, and 8.59%, respectively. After thermal treatment at 200 °C for 2 h on the active layer,the PCE of the PSC based on J63 still keeps 92% of the original value, which verifies that the cross-linking of the polymer can improve the thermal stability of PSCs. Morphological studies show that the active layer based on J63 displays strong lamellar packing with RMS 1.26, and the active layer based on J64 shows little phase separation with RMS 0.65. The RMS of the active layer based on J62 is 0.900, and the size of phase separation is between that of J63 and J64, which indicates the excessive high lamellar packing or low phase separation is harmful to the performance of PSCs. These results indicate that the side-chain engineering is an effective way to adjust the aggregation of polymers and the morphology of blend films, which are key factors to influence the performance of PSCs.}, number={15}, journal={MACROMOLECULES}, author={Huang, He and Bin, Haijun and Peng, Zhengxing and Qiu, Beibei and Sun, Chenkai and Liebman-Pelaez, Alex and Zhang, Zhi-Guo and Zhu, Chenhui and Ade, Harald and Zhang, Zhanjun and et al.}, year={2018}, month={Aug}, pages={6028–6036} }
 @article{li_huang_peng_sun_yang_zhou_liebman-pelaez_zhu_zhang_zhang_et al._2018, title={Effects of fused-ring regiochemistry on the properties and photovoltaic performance of n-type organic semiconductor acceptors}, volume={6}, ISSN={["2050-7496"]}, DOI={10.1039/c8ta05920a}, abstractNote={The effects of fused-ring regiochemistry on the physicochemical and photovoltaic properties of n-type organic semiconductor (n-OS) acceptors are investigated.}, number={33}, journal={JOURNAL OF MATERIALS CHEMISTRY A}, author={Li, Xiaojun and Huang, He and Peng, Zhengxing and Sun, Chenkai and Yang, Dengchen and Zhou, Jiadong and Liebman-Pelaez, Alex and Zhu, Chenhui and Zhang, Zhi-Guo and Zhang, Zhanjun and et al.}, year={2018}, month={Sep}, pages={15933–15941} }
 @article{peng_jiao_ye_li_rech_you_hou_ade_2018, title={Measuring Temperature-Dependent Miscibility for Polymer Solar Cell Blends: An Easily Accessible Optical Method Reveals Complex Behavior}, volume={30}, ISSN={["1520-5002"]}, url={https://doi.org/10.1021/acs.chemmater.8b00889}, DOI={10.1021/acs.chemmater.8b00889}, abstractNote={In bulk-heterojunction polymer solar cells (PSC), the molecular-level mixing between conjugated polymer donors and small-molecule acceptors plays a crucial role in obtaining a desirable morphology and good device stability. It has been recently shown that the thermodynamic limit of this mixing can be quantified by the liquidus miscibility, the composition of the small-molecule acceptor in amorphous phases in the presence of small-molecule crystals, and then converted to the Flory–Huggins interaction parameter χ. This conversion maps out the amorphous miscibility. Moreover, the quantitative relations between χ and the fill factor of PSC devices were established recently. However, the commonly used measurement of this liquidus miscibility, scanning transmission X-ray microscopy, is not easily and readily accessible. Here, we delineate a method based on common visible light microscopy and ultraviolet–visible absorption spectroscopy to replace the X-ray measurements. To demonstrate the feasibility of this technique and methodology, a variety of conjugated polymers (PffBT4T-C9C13, PDPP3T PBDT-TS1, PTB7-Th, and FTAZ) and their miscibility with fullerenes or nonfullerene small molecules (PC71BM, PC61BM, and EH-IDTBR) are characterized. The establishment of this methodology will pave the way to a wider use of the liquidus miscibility and the critical miscibility-function relations to optimize the device performance and obtain good stability in PSCs and other devices.}, number={12}, journal={CHEMISTRY OF MATERIALS}, publisher={American Chemical Society (ACS)}, author={Peng, Zhengxing and Jiao, Xuechen and Ye, Long and Li, Sunsun and Rech, Jeromy James and You, Wei and Hou, Jianhui and Ade, Harald}, year={2018}, month={Jun}, pages={3943–3951} }
 @article{kelly_zhang_peng_noman_zhu_ade_you_2018, title={The finale of a trilogy: comparing terpolymers and ternary blends with structurally similar backbones for use in organic bulk heterojunction solar cells}, volume={6}, ISSN={["2050-7496"]}, DOI={10.1039/c8ta05132a}, abstractNote={Comparing the efficiency of terpolymers <italic>vs.</italic> ternary blends, this study focuses on two polymers with structurally similar backbones (monoCNTAZ and FTAZ) yet markedly different open circuit voltages.}, number={39}, journal={JOURNAL OF MATERIALS CHEMISTRY A}, author={Kelly, Mary Allison and Zhang, Qianqian and Peng, Zhengxing and Noman, Victoria and Zhu, Chenhui and Ade, Harald and You, Wei}, year={2018}, month={Oct}, pages={19190–19200} }
 @article{bauer_zhang_zhu_peng_yan_zhu_ade_zhan_you_2017, title={Donor polymer fluorination doubles the efficiency in non-fullerene organic photovoltaics}, volume={5}, ISSN={["2050-7496"]}, DOI={10.1039/c7ta07882j}, abstractNote={Donor polymer fluorination in ITIC-based device led to a large increase in the current and an efficiency twice that of the non-fluorinated polymer based device.}, number={43}, journal={JOURNAL OF MATERIALS CHEMISTRY A}, author={Bauer, Nicole and Zhang, Qianqian and Zhu, Jingshuai and Peng, Zhengxing and Yan, Long and Zhu, Chenhui and Ade, Harald and Zhan, Xiaowei and You, Wei}, year={2017}, month={Nov}, pages={22536–22541} }
 @article{zhang_yan_jiao_peng_liu_rech_klump_ade_so_you_2017, title={Fluorinated Thiophene Units Improve Photovoltaic Device Performance of Donor–Acceptor Copolymers}, volume={29}, ISSN={0897-4756 1520-5002}, url={http://dx.doi.org/10.1021/ACS.CHEMMATER.7B01683}, DOI={10.1021/acs.chemmater.7b01683}, abstractNote={Fluorinated conjugated polymers leading to enhanced photovoltaic device performance has been widely observed in a variety of donor–acceptor copolymers; however, almost all these polymers have fluorine substituents on the acceptor unit. Building upon our previously reported PBnDT-FTAZ, a fluorinated donor–acceptor conjugated polymer with impressive device performance, we set this study to explore the effect of adding the fluorine substituents onto the flanking thiophene units between the donor unit (BnDT) and the acceptor unit (TAZ). We developed new synthetic approaches to control the position of the fluorination (3′ or 4′) on the thiophene unit, and synthesized four additional PBnDT-TAZ polymers incorporating the fluorine-substituted-thiophene (FT) units, 3′-FT-HTAZ, 4′-FT-HTAZ, 3′-FT-FTAZ, and 4′-FT-FTAZ. We discover that relocating the fluorine substituents from the acceptor to the flanking thiophene units have a negligible impact on the device characteristics (short circuit current, open circuit volta...}, number={14}, journal={Chemistry of Materials}, publisher={American Chemical Society (ACS)}, author={Zhang, Qianqian and Yan, Liang and Jiao, Xuechen and Peng, Zhengxing and Liu, Shubin and Rech, Jeromy James and Klump, Erik and Ade, Harald and So, Franky and You, Wei}, year={2017}, month={Jul}, pages={5990–6002} }
 @article{li_huang_bin_peng_zhu_xue_zhang_zhang_ade_li_2017, title={Synthesis and Photovoltaic Properties of a Series of Narrow Bandgap Organic Semiconductor Acceptors with Their Absorption Edge Reaching 900 nm}, volume={29}, ISSN={["1520-5002"]}, DOI={10.1021/acs.chemmater.7b03928}, abstractNote={Three n-OS acceptors with Eg values of <1.4 eV were synthesized by introducing double-bond π-bridges into ITIC (ITVIC) and ITIC with monofluorine (ITVfIC) or bifluorine (ITVffIC) substituents on its end groups, and the structure–property relationships of the acceptors were systematically studied. The three n-OS films show broad absorption covering the wavelength range of 550–900 nm with narrow Eg values of 1.40 eV for ITVIC, 1.37 eV for ITVfIC, and 1.35 eV for ITVffIC. Additionally, the fluorine substitution downshifted the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the compounds. The photovoltaic properties of the n-OS acceptors were investigated by using a medium bandgap conjugated polymer J71 as a donor. The optimized polymer solar cells (PSCs) based on J71:ITVffIC demonstrated a power conversion efficiency (PCE) of 10.54% with a high Jsc of 20.60 mA cm–2 and a Voc of 0.81 V, and the highest Jsc reached 22.83 mA cm–2. The high Jsc values of...}, number={23}, journal={CHEMISTRY OF MATERIALS}, author={Li, Xiaojun and Huang, He and Bin, Haijun and Peng, Zhengxing and Zhu, Chenhui and Xue, Lingwei and Zhang, Zhi-Guo and Zhang, Zhanjun and Ade, Harald and Li, Yongfang}, year={2017}, month={Dec}, pages={10130–10138} }