Can Isotope Effects Enable Organic Solar Cells to Achieve Smaller Non-Radiative Energy Losses and Why?
Fangfang Huang, Tengfei He, Mingpeng Li, Lingxian Meng, Wanying Feng, Huazhe Liang, Yecheng Zhou, Xiangjian Wan, Chenxi Li, Guankui Long, Zhaoyang Yao, Yongsheng Chen
Abstract
It has been proposed that isotope effects could effectively downshift intramolecular vibrational frequencies of light-harvesting materials, thereby reducing the non-radiative recombination from the charge-transfer (CT) state to the ground state (GS) and achieving a smaller non-radiative energy loss (ΔElossnon-rad) theoretically in organic solar cells (OSCs). However, there are no systematic experimental studies to address such a crucial issue: can isotope effects enable OSCs to achieve a smaller ΔElossnon-rad and why? Herein, we constructed 29 non-fullerene acceptors (NFAs) by isotope substitution on different functional groups based on four high-performance NFA systems and further investigated their photovoltaic performance systematically. Large-scale statistical experimental and theoretical analyses indicate no significant difference of PCE and ΔElossnon-rad due to the intrinsically very weak electron-vibration coupling between the CT state and GS (EVCCT-GS) and largely unimpacted coupling strength (tCT-LE) between the CT and local exciton states. Also based on theoretical results from the Huang–Rhys factor, although different vibration modes could have different influences on the strength of EVCCT-GS, all are quite small. Both experimental and theoretical results suggest that an isotope strategy may not be a feasible way to significantly improve PCEs of high-performance OSCs by reducing ΔElossnon-rad at the current stage.