Molecular and Energetic Order Dominate the Photocurrent Generation Process in Organic Solar Cells with Small Energetic Offsets

Minimizing the energetic offset between the donor (D) and acceptor (A) in organic solar cells (OSCs) is pivotal for reducing the charge-transfer (CT) loss and improving the open-circuit voltage (V-oc). This nevertheless leads to a topic of debate regarding the driving force for the charge separation in OSCs with small energetic offsets. The molecular packing geometries in the active layer determine the energetic levels and trap density, but their relationship with the driving force is seldom considered. Limited by the complicated demixing morphology and inaccurate measurements of energy levels in the prototypical bulk-heterojunction (BHJ) devices, we thereby demonstrate a concise and robust planar-heterojunction model of PM7/N2200 to investigate the origin of driving force for charge generation. It is surprising to note that the device with smaller energy offset shows higher efficiency. Further analysis reveals that a bilayer device with short-range packing PM7 exhibits smaller energetic offsets along with fewer morphological defects and traps compared to its long-range packing counterparts. This molecular packing characteristic diminishes the energetic disorder at the D/A interfaces and inhibits the trap-assisted charge recombination, contributing to the increased short-circuit current (J(SC)) and V-OC. Our results suggest that the energetic offset actually has limited influence on charge separation, while the synergetic control of molecular and energetic order is vital to the photocurrent generation and energy loss reduction in OSCs.