Organic solar cell (OSC) technology has recently achieved over 13% efficiency through the synthesis of novel non-fullerene small molecule acceptors (NFAs), which can be processed from benign solvents as low-cost third generation photovoltaics[1,2]. Of critical importance to OSCs is understanding the morphological and thermal stability of the active layers governed by thermodynamics and kinetics as an intrinsic stability process which cannot be controlled by encapsulation[3,4]. Here we highlight the importance of ductility of donor polymers on nucleation and growth of micro-size small molecule crystals which leads to the catastrophic failure of the solar cells in the long-term operating condition We consider three high performance polymers P3HT, PBnDBT-FTAZ, and PffBT4T-C9C13 blended with EH-IDTBR as the model systems to investigate the thermal stability of state of the art non-fullerene OSCs, where elevated temperatures were used to accelerate the crystal formation and imitate the long-term operation conditions of OCSs. We also propose an easy accessible method using differential scanning calorimetry (DSC) to investigate the thermal behavior of NFA in the blends. Although non-fullerene solar cells have shown to have better overall morphological stability compared to their fullerene counterparts, our results suggest that catastrophic failure due to micro-size crystal formation in non-fullerene systems can happen at a rate similar to fullerene systems unless the right donor polymer is chosen to suppress the crystallization of small molecule. It is also shown and argued that the growth rate of small molecule crystals can be reduced upon mixing of NFAs with semi-crystalline polymers, such as P3HT with a higher overall density compared to amorphous donor polymers, i.e. PBnDT-FTAZ. Our findings may pave a way to understand and predict the morphological stability of non-fullerene OSCs.
 L. Ye, Y. Xiong, Q. Zhang, S. Li, C. Wang, Z. Jiang, J. Hou, W. You, H. Ade, Adv. Mater. 2017, DOI: 10.1002/adma.201705485.
 S. Holliday, R. S. Ashraf, A. Wadsworth, D. Baran, S. A. Yousaf, C. B. Nielsen, C.-H. Tan, S. D. Dimitrov, Z. Shang, N. Gasparini, M. Alamoudi, F. Laquai, C. J. Brabec, A. Salleo, J. R. Durrant, I. McCulloch, Nat. Commun. 2016, 7, 11585.
 M. Ghasemi, L. Ye, Q. Zhang, L. Yan, J. H. Kim, O. Awartani, W. You, A. Gadisa, H. Ade, Adv. Mater. 2017, 29, 1604603.
 L. Ye, H. Hu, M. Ghasemi, T. Wang, B. A. Collins, J.-H. Kim, K. Jiang, J. Carpenter, H. Li, Z. Li, T. McAfee, J. Zhao, X. K. Chen, J. Y. L. Lai, T. Ma, J.-L. Bredas, H. Yan, H. Ade, Nat. Mater. 2018, DOI: 10.1038/s41563-017-0005-1.
Owing to the recently developed nonfullerene small molecule acceptors, the best power conversion efficiency (PCE) of solution-processable organic solar cells (OSCs) has been boosted up to over 12%, which makes this technology an economically viable contender for commercialization. Along with the steady progress in PCE achieved by spin-coating photovoltaic materials with chlorinated solvents in protective atmosphere, a central issue in the development of OSCs is pursuing a greener and simpler manufacturing protocol, which particularly allows for large-area processing in ambient air. Particularly, it is still a great challenge to replace halogenated solvents with halogen-free, low-toxicity solvents to achieve high-efficiency nonfullerene OSCs.
Here we show that ~11.6% efficiency is achieved in nonfullerene OSC device based on PBDB-T:IT-M by using a non-halogenated solvent combination. Moreover, the device parameters were correlated to the morphology investigated by synchrotron radiation grazing-incidence wide-angle X-ray scattering (GIWAXS), resonant soft X-ray scattering (R-SoXS), and differential scanning calorimetry (DSC). We observed a monotonic correlation between the average composition variations and photovoltaic device characteristics across all processing protocols in this record-efficiency material system. This correlation is indeed universal for OSC, irrespective of acceptor materials used (fullerenes, nonfullerene molecular acceptor, or conjugated polymers) and fabrication methods used (spin-coating or blade-coating).[1-5] We believe this nonhazardous solvent approach will be also applicable in the large area roll-to-roll coating and industrial scale printing of high-efficiency OSCs in air.
 Li, S.; Ye, L.; Zhao, W.; Zhang, S.; Mukherjee, S.; Ade, H.; Hou, J., Energy-Level Modulation of Small-Molecule Electron Acceptors to Achieve over 12% Efficiency in Polymer Solar Cells. Adv. Mater. 2016, 28 (42), 9423-9429.
 Ye, L.; Xiong, Y.; Yao, H.; Gadisa, A.; Zhang, H.; Li, S.; Ghasemi, M.; Balar, N.; Hunt, A.; O’Connor, B. T.; Hou, J.; Ade, H., High Performance Organic Solar Cells Processed by Blade Coating in Air from a Benign Food Additive Solution. Chem. Mater. 2016, 28 (20), 7451-7458.
 Ye, L.; Zhao, W.; Li, S.; Mukherjee, S.; Carpenter, J. H.; Awartani, O.; Jiao, X.; Hou, J.; Ade, H., High-Efficiency Nonfullerene Organic Solar Cells: Critical Factors that Affect Complex Multi-length Scale Morphology and Device Performance. Adv. Energy Mater. 2017, 7, 1602000.
 Ye, L.; Jiao, X.; Zhang, S.; Yao, H.; Qin, Y.; Ade, H.; Hou, J., Control of Mesoscale Morphology and Photovoltaic Performance in Diketopyrrolopyrrole-Based Small Band Gap Terpolymers. Adv. Energy Mater. 2017, 7, 1601138.
 Ye, L.; Jiao, X.; Zhao, W.; Zhang, S.; Yao, H.; Li, S.; Ade, H.; Hou, J., Manipulation of Domain Purity and Orientational Ordering in High Performance All-Polymer Solar Cells. Chem. Mater. 2016, 28 (17), 6178-6185.