Coumarin‐Based Volatile and Non‐Volatile Solid Additives Enable Organic Solar Cells with 20.32% Efficiency

Solid additives, including volatile solid (VS) and non-volatile solid (non-VS) additives, have drawn great attention due to their crucial function in improving photovoltaic performance of organic solar cells (OSCs), while the post-treatment of non-VS additives is more convenient. However, compared with VS additives, non-VS additives often fall-short in optimizing device morphology, and their systematic comparisons in working mechanisms are still lacking. Herein, we design and synthesize two coumarin-derived additives, including volatile 7-(diethylamino)-2H-chromen-2-one (C5) and nonvolatile 3-(benzo[d]thiazol-2-yl)-7-(diethylamino)-2H-chromen-2-one (C6), to assess their various impacts on device performance. Both the C5- and C6-treated OSCs obtain optimized morphology, improved charge transfer, and less energy loss than the additive-free device, while C6 plays an additional charge transport channel to promote the exciton utilization through Föster-Reasonable energy transfer (FRET) between C6 and the donor. As a result, the OSC based on C6 treated D18:L8-BO obtained an impressive efficiency of 20.32%, as the highest value among the reported small molecule non-VS additive-treated devices, which is higher than those of additive-free (18.85%) and C5-treated (19.55%) ones. Our work provides a deep insight into the different mechanisms by which non-VS and VS additives boost photovoltaic performance of the resulting OSCs.