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 π⋯π 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 π-π stacking, increase electronic couplings, and thereby promote photovoltaic efficiency.
Bibliographical noteFunding Information:
Z. Luo, Y. Gao and H. Lai contributed equally to this work. The work described in this paper was supported by the Shenzhen Science and Technology Program (ZDSYS20210623091813040). Y. L. acknowledges the support from the Science and Technology Program of Shanxi Province (2022JM-229). H. W. is thankful for the financial support from the National Research Foundation (NRF) of Korea (2016M1A2A2940911, 2020M3H4A3081814).
© 2022 The Royal Society of Chemistry.
ASJC Scopus subject areas
- Environmental Chemistry
- Renewable Energy, Sustainability and the Environment
- Nuclear Energy and Engineering