Open-Shell Donors and Closed-Shell Acceptors in Organic Solar Cells

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 wellknown 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.