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We report a systematic study of the efficiency limitations of non-fullerene organic solar cells that exhibit a small energy loss (E-loss) between the polymer donor and the non-fullerene acceptor. To clarify the impact of Eloss on the performance of the solar cells, three thieno[3,4-c] pyrrole-4,6-dione-based conjugated polymers (PTPD3T, PTPD2T, and PTPDBDT) are employed as the electron donor, which all have complementary absorption spectra compared with the ITIC acceptor. The corresponding photovoltaic devices show that low Eloss (0.54 eV) in PTPDBDT: ITIC leads to a high open-circuit voltage (Voc) of 1.05 V, but also to a small quantum efficiency, and in turn photocurrent. The high Voc or small energy loss in the PTPDBDT-based solar cells is a consequence of less non-radiative recombination, whereas the low quantum efficiency is attributed to the unfavorable micro-phase separation, as confirmed by the steady-state and time-resolved photoluminescence experiments, grazing-incidence wide-angle X-ray scattering, and resonant soft X-ray scattering (R-SoXS) measurements. We conclude that to achieve high performance non-fullerene solar cells, it is essential to realize a large Voc with small Eloss while simultaneously maintaining a high quantum efficiency by manipulating the molecular interaction in the bulk-heterojunction. ; Funding Agencies|Ministry of science and technology [2016YFA0200700]; Recruitment Program of Global Youth Experts of China; National Natural Science Foundation of China [21574138, 91233205, 91633301]; Strategic Priority Research Program of the Chinese Academy of Sciences [XDB12030200]; Swedish Research Council [VR621-2013-5561]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [200900971]; Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy [DE-AC02-05CH11231]; Alexander-von-Humboldt foundation; King Abdullah University of Science and Technology (KAUST)

Stability is now a critical factor in the commercialization of organic photovoltaic (OPV) devices. Both extrinsic stability to oxygen and water and intrinsic stability to light and heat in inert conditions must be achieved. Triplet states are known to be problematic in both cases, leading to singlet oxygen production or fullerene dimerization. The latter is thought to proceed from unquenched singlet excitons that have undergone intersystem crossing (ISC). Instead, we show that in bulk heterojunction (BHJ) solar cells the photo-degradation of C-60 via photo-oligomerization occurs primarily via back-hole transfer (BHT) from a charge-transfer state to a C-60 excited triplet state. We demonstrate this to be the principal pathway from a combination of steady-state optoelectronic measurements, time-resolved electron paramagnetic resonance, and temperature-dependent transient absorption spectroscopy on model systems. BHT is a much more serious concern than ISC because it cannot be mitigated by improved exciton quenching, obtained for example by a finer BHJ morphology. As BHT is not specific to fullerenes, our results suggest that the role of electron and hole back transfer in the degradation of BHJs should also be carefully considered when designing stable OPV devices. The commercialisation of organic photovoltaic technology calls for research on material degradation mechanisms. Ramirez et al. show that triplet excitons produced by back charge transfer can significantly impact the photo-stability of fullerene-based devices even in the absence of water and oxygen. ; We thank Dr. Olaf Zeika for TPDP synthesis, Dr. Josue Martinez-Hardigree for his insights on morphology, and Professor Natalie Banerji for her valuable advice with TA analysis. A.P. thanks Dr. William Myers and the Centre for Advanced ESR (CAESR) located in the Department of Chemistry of the University of Oxford (supported by EPSRC EP/L011972/1). I.R. thanks TU Dresden technicians for help with sample production and Dr. Frederik Nehm for help with device degradation. This work was supported by European Union's Horizon 2020 research and innovation program under Marie Sklodowska Curie Grant agreement number 722651 (SEPOMO) and by the COST Action MP1307 (StableNextSol). M.R. acknowledges funding from an EU FP7 Marie Curie Career Integration Grant (number PCIG14-GA-2013-630864) and STFC Challenge Led Applied Systems Programe (CLASP, Grant number ST/L006294/1). This publication is based upon work supported by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under award number OSR-2018-CARF/CCF-3079. ; Ramirez, I (corresponding author), Heliatek GmbH, Treidlerstr 3, D-01139 Dresden, Germany. Riede, M (corresponding author), Univ Oxford, Dept Phys, Clarendon Lab, Parks Rd, Oxford OX1 3PU, England. ivan.ramirez@heliatek.com; moritz.riede@physics.ox.ac.uk

Polymorphism of organic semiconducting materials exerts critical effects on their physical properties such as optical absorption, emission and electrical conductivity, and provides an excellent platform for investigating structure-property relations. It is, however, challenging to efficiently tune the polymorphism of conjugated polymers in aggregated, semi-crystalline phases due to their conformational freedom and anisotropic nature. Here, two distinctly different semi-crystalline polymorphs (beta(1) and beta(2)) of a low-bandgap diketopyrrolopyrrole polymer are formed through controlling the solvent quality, as evidenced by spectroscopic, structural, thermal and charge transport studies. Compared to beta(1), the beta(2) polymorph exhibits a lower optical band gap, an enhanced photoluminescence, a reduced pi-stacking distance, a higher hole mobility in field-effect transistors and improved photocurrent generation in polymer solar cells. The beta(1) and beta(2) polymorphs provide insights into the control of polymer self-organization for plastic electronics and hold potential for developing programmable ink formulations for next-generation electronic devices. ; This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 747422, from the Netherlands Organisation for Scientific Research(016.Veni.192.106), from the Chinese Academy of Sciences (Grant No. XDB12020200), and from King Abdullah University of Science and Technology (KAUST). The research also received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) (Grant Agreement no. 607585, project OSNIRO), the European Research Council (ERC Grant Agreement No. 33903), and the Ministry of Education, Culture, and Science (Gravity program 024.001.035). T.M. acknowledges the Foundation for Polish Science co-financed by the European Union under the European Regional Development Fund (First TEAM/2017-3/26). The authors acknowledge Beamline 9 of the DELTA electron storage ring in Dortmund for GIWAXS measurements. The authors thank Mateusz Brzezinski for the discussion about GIWAXS analysis, and Christ Weijtens for UPS measurements.