Retarding the Crystallization of a Nonfullerene Electron Acceptor for High-Performance Polymer Solar Cells

Developing a fundamental understanding of the molecular order within the photoactive layer, and the influence therein of solution casting conditions, is a key factor in obtaining high power conversation efficiency (PCE) polymer solar cells. Herein, the molecular order in PBDB-T:INPIC-4F nonfullerene solar cells is tuned by control of the molecular organization time during film casting, and the crucial role of retarding the crystallization of INPIC-4F in achieving high performance is demonstrated. When PBDB-T:INPIC-4F is cast with the presence of solvent vapor to prolong the organization time, INPIC-4F molecules form spherulites with a polycrystalline structure, resulting in large phase separation and device efficiency below 10%. On the contrary, casting the film on a hot substrate is effective in suppressing the formation of the polycrystalline structure, and encourages face-on pi-pi stacking of INPIC-4F. This molecular transformation of INPIC-4F significantly enhances the absorption ability of INPIC-4F at long wavelengths and facilitates a fine phase separation to support efficient exciton dissociation and balanced charge transport, leading to the achievement of a maximum PCE of 13.1%. This work provides a rational guide for optimizing nonfullerene polymer solar cells consisting of highly crystallizable small molecular electron acceptors.