Improving the Efficiency of Layer-by-Layer Organic Photovoltaics to Exceed 19% by Establishing Effective...

The power conversion efficiency of layer-by-layer organic solar cells (LOSCs) has reached an impressive level by utilizing sequential processing (SqP) for the individual deposition and regulation of both donor and acceptor materials. However, the fundamental understanding of phase separation in LOSCs remains contentious, hindering the rational design of LOSCs due to the ambiguous contribution of stratification or the beneficial vertical segregation morphology. Here, we systematically investigate the utility of solvent effects on drying kinetics to understand how the interaction between the upper and bottom layers affects the formation of the donor/acceptor (D/A) interface and its impact on the performance of LOSCs. Particularly emphasizing the substantial impact of the upper layer solvent on the establishment of the effective D/A interface rather than on the formation of significant stratification in LOSCs, this understanding facilitates the utilization of blend casting in the SqP, introducing an adequate D/A interface, which contributes to a superior performance of 19.05%. Ultimately, we provide three design rules for enhancing the performance in LOSCs: (1) appropriate selection of solvents for the acceptor material to ensure a desired crystalline orientation, (2) utilization of strongly polar and volatile solvents in the upper layer capable of dissolving the bottom layer to form effective D/A interfacial interaction, and (3) establishment of sufficient D/A interfaces.