Recent Advances of Solid Additives Used in Organic Solar Cells: Toward Efficient and Stable Solar Cells

Bulk heterojunction (BHJ) organic photovoltaics (OPVs) are considered to be the most promising generation of green energy technology due to their outstanding device performance and simple processing technology. The morphology of the active layer of OPVs is the key to its device performance and stability. So for, various strategies have been proposed to optimize the morphology, such as thermal annealing, solvent annealing, adding solvent additives and so on. However, the posting annealing is not compatible with large-area printing. In addition, solvent additives are easy to remain in active layer, which continuously deteriorate the morphology and performance of OPVs. Recently, the addition of solid additives has drawn significant attentions due to its unique characteristics, including improved device stability, precise morphology control as well as easy accessibility. Hence, it gradually becomes a universal and popular strategy for optimizing the morphology of OPVs with high power conversion efficiency (PCE). However, the relationship between properties of solid additives and morphologies of active layers is still ambiguous, which inhibit the further development of solid additive and its application in new emerging OPVs systems. To this end, we summarized the recently reported solid additives according to their volatility, i.e., whether solid additives remain in final active layer or not. Furthermore, the different mechanisms that solid additives optimize the morphology of active layer are intensively discussed, including the free volume and diffusivity, intermolecular adsorption energy, interaction among donor and acceptor, and crystal nucleation and growth, which may give predictions for selecting proper solid additives in new emerging blends. Finally, the challenges and future development of solid additives in OPVs are briefly prospected.