TY - JOUR A1 - Samson, Stephanie A1 - Rech, Jeromy A1 - Perdigon-Toro, Lorena A1 - Peng, Zhengxing A1 - Shoaee, Safa A1 - Ade, Harald A1 - Neher, Dieter A1 - Stolterfoht, Martin A1 - You, Wei T1 - Organic solar cells with large insensitivity to donor polymer molar mass across all acceptor classes JF - ACS applied polymer materials N2 - Donor polymer number-average molar mass (M-n) has long been known to influence organic photovoltaic (OPV) performance via changes in both the polymer properties and the resulting bulk heterojunction morphology. The exact nature of these M-n effects varies from system to system, although there is generally some intermediate M-n that results in optimal performance. Interestingly, our earlier work with the difluorobenzotriazole (FTAZ)-based donor polymer, paired with either N2200 (polymer acceptor) or PC61BM (fullerene acceptor), PcBm demonstrated <10% variation in power conversion efficiency and a consistent morphology over a large span of M-n (30 kg/mol to over 100 kg/mol). Would such insensitivity to polymer M-n still hold true when prevailing small molecular acceptors were used with FTAZ? To answer this question, we explored the impact of FTAZ on OPVs with ITIC, a high-performance small-molecule fused-ring electron acceptor (FREA). By probing the photovoltaic characteristics of the resulting OPVs, we show that a similar FTAZ mn insensitivity is also found in the FTAZ:ITIC system. This study highlights a single-donor polymer which, when paired with an archetypal fullerene, polymer, and FREA, results in systems that are largely insensitive to donor M. Our results may have implications in polymer batch-to-batch reproducibility, in particular, relaxing the need for tight M-n control during synthesis. KW - polymer solar cells KW - conjugated polymers KW - fullerenes KW - fluorination KW - molecular weight KW - non-fullerene acceptors KW - power conversion efficiency Y1 - 2020 U6 - https://doi.org/10.1021/acsapm.0c01041 SN - 2637-6105 VL - 2 IS - 11 SP - 5300 EP - 5308 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Kelly, Mary Allison A1 - Roland, Steffen A1 - Zhang, Qianqian A1 - Lee, Youngmin A1 - Kabius, Bernd A1 - Wang, Qing A1 - Gomez, Enrique D. A1 - Neher, Dieter A1 - You, Wei T1 - Incorporating Fluorine Substitution into Conjugated Polymers for Solar Cells BT - three Different Means, Same Results JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - Fluorinating conjugated polymers is a proven strategy for creating high performance materials in polymer solar cells, yet few studies have investigated the importance of the fluorination method. We compare the performance of three fluorinated systems: a poly(benzodithieno-dithienyltriazole) (PBnDT-XTAZ) random copolymer where 50% of the acceptor units are difluorinated, PBnDT-mFTAZ where every acceptor unit is monofluorinated, and a 1:1 physical blend of the difluorinated and nonfluorinated polymer. All systems have the same degree of fluorination (50%) yet via different methods (chemically vs physically, random vs regular). We show that these three systems have equivalent photovoltaic behavior:,similar to 5.2% efficiency with a short-circuit current (J(sc)) at,similar to 11 mA cm(-2), an open-circuit voltage (v(oc)) at 0.77 V, and a fill factor (FF) of similar to 60%. Further investigation of these three systems demonstrates that the charge generation, charge extraction, and charge transfer state are essentially identical for the three studied systems. Transmission electron microscopy shows no significant differences in the morphologies. All these data illustrate that it is possible to improve performance not only via regular or random fluorination but also by physical addition via a ternary blend. Thus, our results demonstrate the versatility of incorporating fluorine in the active layer of polymer solar cells to enhance device performance. Y1 - 2017 U6 - https://doi.org/10.1021/acs.jpcc.6b10993 SN - 1932-7447 VL - 121 IS - 4 SP - 2059 EP - 2068 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Roland, Steffen A1 - Yan, Liang A1 - Zhang, Qianqian A1 - Jiao, Xuechen A1 - Hunt, Adrian A1 - Ghasemi, Masoud A1 - Ade, Harald A1 - You, Wei A1 - Neher, Dieter T1 - Charge Generation and Mobility-Limited Performance of Bulk Heterojunction Solar Cells with a Higher Adduct Fullerene JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - Alternative electron acceptors are being actively explored in order to advance the development of bulk-heterojunction (BHJ) organic solar cells (OSCs). The indene-C-60 bisadduct (ICBA) has been regarded as a promising candidate, as it provides high open-circuit voltage in BHJ solar cells; however, the photovoltaic performance of such ICBA-based devices is often inferior when compared to cells with the omnipresent PCBM electron acceptor. Here, by pairing the high performance polymer (FTAZ) as the donor with either PCBM or ICBA as the acceptor, we explore the physical mechanism behind the reduced performance of the ICBA-based device. Time delayed collection field (TDCF) experiments reveal reduced, yet field-independent free charge generation in the FTAZ:ICBA system, explaining the overall lower photocurrent in its cells. Through the analysis of the photoluminescence, photogeneration, and electroluminescence, we find that the lower generation efficiency is neither caused by inefficient exciton splitting, nor do we find evidence for significant energy back-transfer from the CT state to singlet excitons. In fact, the increase in open circuit voltage when replacing PCBM by ICBA is entirely caused by the increase in the CT energy, related to the shift in the LUMO energy, while changes in the radiative and nonradiative recombination losses are nearly absent. On the other hand, space charge limited current (SCLC) and bias-assisted charge extraction (BACE) measurements consistently reveal a severely lower electron mobilitiy in the FTAZ:ICBA blend. Studies of the blends with resonant soft X-ray scattering (R-SoXS), grazing incident wide-angle X-ray scattering (GIWAXS), and scanning transmission X-ray microscopy (STXM) reveal very little differences in the mesoscopic morphology but significantly less nanoscale molecular ordering of the fullerene domains in the ICBA based blends, which we propose as the main cause for the lower generation efficiency and smaller electron mobility. Calculations of the JV curves with an analytical model, using measured values, show good agreement with the experimentally determined JV characteristics, proving that these devices suffer from slow carrier extraction, resulting in significant bimolecular recombination losses. Therefore, this study highlights the importance of high charge carrier mobility for newly synthesized acceptor materials, in addition to having suitable energy levels. Y1 - 2017 U6 - https://doi.org/10.1021/acs.jpcc.7b02288 SN - 1932-7447 VL - 121 SP - 10305 EP - 10316 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Würfel, Uli A1 - Perdigon-Toro, Lorena A1 - Kurpiers, Jona A1 - Wolff, Christian Michael A1 - Caprioglio, Pietro A1 - Rech, Jeromy James A1 - Zhu, Jingshuai A1 - Zhan, Xiaowei A1 - You, Wei A1 - Shoaee, Safa A1 - Neher, Dieter A1 - Stolterfoht, Martin T1 - Recombination between Photogenerated and Electrode-Induced Charges Dominates the Fill Factor Losses in Optimized Organic Solar Cells JF - The journal of physical chemistry letters N2 - Charge extraction in organic solar cells (OSCs) is commonly believed to be limited by bimolecular recombination of photogenerated charges. However, the fill factor of OSCs is usually almost entirely governed by recombination processes that scale with the first order of the light intensity. This linear loss was often interpreted to be a consequence of geminate or trap-assisted recombination. Numerical simulations show that this linear dependence is a direct consequence of the large amount of excess dark charge near the contact. The first-order losses increase with decreasing mobility of minority carriers, and we discuss the impact of several material and device parameters on this loss mechanism. This work highlights that OSCs are especially vulnerable to injected charges as a result of their poor charge transport properties. This implies that dark charges need to be better accounted for when interpreting electro-optical measurements and charge collection based on simple figures of merit. Y1 - 2019 U6 - https://doi.org/10.1021/acs.jpclett.9b01175 SN - 1948-7185 VL - 10 IS - 12 SP - 3473 EP - 3480 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Li, Wentao A1 - Abrecht, Steve A1 - Yang, Liqiang A1 - Roland, Steffen A1 - Tumbleston, John R. A1 - McAfee, Terry A1 - Yan, Liang A1 - Kelly, Mary Allison A1 - Ade, Harald W. A1 - Neher, Dieter A1 - You, Wei T1 - Mobility-controlled performance of thick solar cells based on fluorinated copolymers JF - Journal of the American Chemical Society N2 - 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 structureproperty 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. Y1 - 2014 U6 - https://doi.org/10.1021/ja5067724 SN - 0002-7863 VL - 136 IS - 44 SP - 15566 EP - 15576 PB - American Chemical Society CY - Washington ER -