Effects of the rigid and sterically bulky structure of non-fused nonfullerene acceptors on transient photon-to-current dynamics

Non-fused electron-accepting pi-conjugated compounds have been investigated recently for application to nonfullerene acceptors (NFAs) in organic solar cells (OSCs). However, the establishment of rational molecular design for non-fused NFAs is still lagging because the influence of flexible non-fused structures on the dynamics of electron-hole pairs in OSCs is not entirely understood. In this study, we utilized cyclopentene-annelated thiophene with spiro-substituted 2,7-bis(2-ethylhexyl)fluorene (FT) as a rigid and sterically bulky linker unit and developed a non-fused NFA (TT-FT-DCI) containing FT units. Photophysical measurements indicated that the introduction of the FT unit leads to the formation of rigid molecular structure. OSCs based on donor polymer (PBDB-T) and TT-FT-DCI showed an improved power conversion efficiency of 7.13% due to the increase in the short-circuit current density and fill factor. Time-resolved optical and microwave spectroscopies showed that the FT unit contributes to the long lifetimes of excited state and charge-separated state in the PBDBT:TT-FT-DCI blend films. Time-resolved electron paramagnetic resonance measurements showed that the distant charge-separated states of the face-to-face PBDB-T:TT-FT-DCI structure, which is derived by avoiding over-crystallization by the steric bulkiness of TT-FT-DCI, can interact with the cathodes for preferential electron injection following charge generations. This study highlights that by using the rigid pi-conjugated framework and suppressed self-aggregation of the non-fused acceptor, effective molecular design for the appropriate dynamics of photocurrent generation is possible.