Asymmetric side-chain substitution enables a 3D network acceptor with hydrogen bond assisted crystal packing and enhanced electronic coupling for efficient organic solar cells

Abstract

Side chain modification on small-molecule acceptors (SMAs) is an effective method to realize high device efficiencies for organic solar cells (OSCs), among which the asymmetric side-chain strategy is a promising one. However, the underlying mechanism of this tactic has not been clearly understood from the aspect of material's eigen-properties, especially the single crystal structure. In this work, for the first time this gap is filled by focusing on parent molecules Y6 and BTP-PhC6, together with the corresponding asymmetric molecule BTP-PhC6-C11 (originally synthesized here). These three acceptors present similar optical and electrochemical properties. The crystallographic analysis and theoretical calculation results demonstrate that asymmetric BTP-PhC6-C11 shows stronger pi center dot center dot center dot pi interactions between two terminal accepting units, larger electronic couplings in 3D charge transport networks due to the synergistic effect of hydrogen bonding interactions and small steric hindrance, and comparable internal reorganization energies as compared with symmetric Y6 and BTP-PhC6. Upon pairing these SMAs with polymer donor PM1, the BTP-PhC6-C11-based device realizes a highest PCE of 18.33% as compared with the devices based on Y6 (17.06%) and BTP-PhC6 (17.43%). The best PCE achieved for the PM1:BTP-PhC6-C11 device is mainly attributed to the larger and more symmetric charge mobility, longer carrier lifetime, enhanced molecular packing along the conjugated backbones of BTP-PhC6-C11, and more suitable phase separation. Overall, our systematic study reveals that asymmetric side-chain substitution is a simple and feasible method to enhance pi-pi stacking, increase electronic couplings, and thereby promote photovoltaic efficiency.