Effect of Solvent on the Interfacial Crystallinity in Sequentially Processed Organic Solar Cells

It is important to specify and control factors that significantly affect the performance and stability of organic solar cells (OSCs). Bulk heterojunctions (BHJs) prepared by spin-coating donor/acceptor mixtures form vertically and laterally complex nanostructures, making them difficult to specify and control. Herein, various solvent-dissolved PTB7-th/PC₇₀BM-based sequentially processed OSCs are demonstrated and their thin-film properties in terms of interfacial crystallinity are compared. The crystallinity of the donor/acceptor interfaces and PC₇₀BM over-layers is effectively controlled by varying the boiling points of the PTB7-th solvents in sequential processes. It is found that the structures of the PTB7-th layers formed by solvents with lower boiling points, as well as the PC₇₀BM over-layers, have a higher degree of crystallinity, consequently improving the performance to a degree resembling that of BHJ cells. In addition, sequentially processed samples show much higher thermal stability than BHJ cells, which constitute a nano-blend of donor and acceptor materials. When compared with BHJ cells, whose power conversion efficiency deteriorates within the initial 5 h of thermal treatment, all sequentially processed devices deposited by solvents with different boiling points show significant thermal stability. This work provides comprehensive insight into the interfacial crystallinity of sequentially processed OSCs in terms of efficiency and stability.