Search

Organic solar cells (OSCs) based on donor-acceptor blends have shown a rapid improvement in power conversion efficiency (PCE) now approaching, for small cells, those of the state-of-the art commercial solar modules. However, performance degradation remains one of the most critical impediments for OSC technology commercialization. Ternary solar cells where a third component, for instance an acceptor, is added to a non-fullerene acceptor-polymer donor blend are an effective approach for improving both OSC efficiency and long-term stability. Here, we study the role of two fullerene acceptors, ET18 and PCBM, as the third component in P-D:Y6 blends. These fullerene derivatives significantly enhance the cell stability, which retained > 90% of their initial PCEs (13-14%) even after storage in air for 6 months, compared to only similar to 20% retention for the binary devices. GIWAXS, AFM, in situ impedance spectroscopy and femtosecond transient absorption spectroscopy measurements reveal that the enhanced stability of the ternary devices results from a more robust blend morphology reducing charge recombination in the ternary devices during aging., Funding Agencies|Northwestern University MRSEC (NSF) [DMR-1720139]; AFOSR [FA9550-18-1-0320, 70NANB19H005]; U.S. Department of Commerce, National Institute of Standards and Technology as part of the Center for Hierarchical Materials Design; Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF) [ECCS-1542205]; Department of Energy; DOE Office of Science by Argonne National Laboratory [DE-AC02-06CH11357]; National Science Foundation Materials Research Science and Engineering Center (MRSEC) at Northwestern University [DE-FG02-99ER14999]; U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [ARS01_00334]; University of Palermo acknowledges the Ministry of University and Research (MUR) and its program PON RI [22209169]; National Natural Science Foundation of China

Achieving efficient polymer solar cells (PSCs) requires a structurally optimal donor-acceptor heterojunction morphology. Here we report the combined experimental and theoretical characterization of a benzodithiophene-benzo-thiadiazole donor polymer series (PBTZF4-R; R = alkyl substituent) blended with the non-fullerene acceptor ITIC-Th and analyze the effects of substituent dimensions on blend morphology, charge transport, carrier dynamics, and PSC metrics. Varying substituent dimensions has a pronounced effect on the blend morphology with a direct link between domain purity, to some extent domain dimensions, and charge generation and collection. The polymer with the smallest alkyl substituent yields the highest PSC power conversion efficiency (PCE, 11%), reflecting relatively small, high-purity domains and possibly benefiting from "matched" donor polymer-small molecule acceptor orientations. The distinctive morphologies arising from the substituents are investigated using molecular dynamics (MD) simulations which reveal that substituent dimensions dictate a well-defined set of polymer conformations, in turn driving chain aggregation and, ultimately, the various film morphologies and mixing with acceptor small molecules. A straightforward energetic parameter explains the experimental polymer domain morphological trends, hence PCE, and suggests strategies for substituent selection to optimize PSC materials morphologies., Funding Agencies|Center for Light Energy Activated Redox Processes (LEAP); Energy Frontier Research Center - U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-SC0001059]; U.S. Department of Commerce, National Institute of Standards and Technology as part of the Center for Hierarchical Materials Design (CHiMaD) [70NANB14H012]; U.S. DOE [DE-ACO2-06CH11357]; Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy [DE-ACO2-05CH11231]; Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource [NSF NNCI-1542205]; MRSEC program at the Materials Research Center [NSF DMR-1121262]; International Institute for Nanotechnology (IIN); Keck Foundation; State of Illinois; NSF; VINNOVA [2015-04859]; Swedish Research Council [2016-03979]