Open-Shell Donors and Closed-Shell Acceptors in Organic Solar Cells
Weiya Zhu, Zhongxin Chen, Jiaxing Huang, Weixuan Liang, Chenghui Liao, Jingwen Wang, Tian Du, Yunfeng Deng, Gongqiang Li, Runfeng Chen, Xiaobin Peng, Jianhui Hou, Yuan Li
Abstract
The active materials of organic solar cells are recognized to show a singlet ground state, and their photo-excited states have been widely investigated in previous work. Herein, we disclose the inherent open-shell singlet ground state of all the donors with obvious electron spin resonance signals. In contrast, the well-known highly efficient non-fullerene acceptors are almost electron spin resonance-silent. These results are carefully studied and confirmed via the combination of variable-temperature nuclear magnetic resonance, electron spin resonance, superconducting quantum interference device, and theoretical calculation. More interestingly, the ground state can be readily tunable between open-shell and closed-shell by changing the quinoidal character of their conjugated backbone. This work provides a new perspective to understand the intrinsic molecular structure of active materials in organic solar cells.