In bulk heterojunction (BHJ) organic solar cells (OSCs) both the electron affinity (EA) and ionization energy (IE) offsets at the donor–acceptor interface should equally control exciton dissociation. Here, we demonstrate that in low-bandgap non-fullerene acceptor (NFA) BHJs ultrafast donor-to-acceptor energy transfer precedes hole transfer from the acceptor to the donor and thus renders the EA offset virtually unimportant. Moreover, sizeable bulk IE offsets of about 0.5 eV are needed for efficient charge transfer and high internal quantum efficiencies, since energy level bending at the donor–NFA interface caused by the acceptors’ quadrupole moments prevents efficient exciton-to-charge-transfer state conversion at low IE offsets. The same bending, however, is the origin of the barrier-less charge transfer state to free charge conversion. Our results provide a comprehensive picture of the photophysics of NFA-based blends, and show that sizeable bulk IE offsets are essential to design efficient BHJ OSCs based on low-bandgap NFAs.

在体异质结 (BHJ) 有机太阳能电池 (OSC) 中，施主 - 受主界面处的电子亲和能 (EA) 和电离能 (IE) 偏移应同样控制激子解离。在这里，我们证明了在低带隙非富勒烯受体 (NFA) 中，BHJ 的超快供体到受体的能量转移先于从受体到供体的空穴转移，因此使得 EA 偏移几乎不重要。此外，有效的电荷转移和高内部量子效率需要大约 0.5 eV 的相当大的体 IE 偏移，因为受主四极矩引起的施主-NFA 界面的能级弯曲阻止了有效的激子到电荷转移状态的转换在低 IE 偏移量。然而，同样的弯曲是无障碍电荷转移状态到自由电荷转换的起源。