Highly efficient fullerene/perovskite planar heterojunction solar cells via cathode modification with an amino-functionalized polymer interlayer

A new amino-functionalized polymer, PN₄N, was developed and applied as an efficient interlayer to improve the cathode interface of fullerene/perovskite (CH₃NH₃PbI_xCl_3-x) planar heterojunction solar cells. The PN₄N polymer is soluble in IPA and n-BuOH, which are orthogonal solvents to the metallohalide perovskite films, and therefore they can be spuncast on the heterojunction layer before the deposition of the metal cathode. This simple modification of the cathode interface showed a remarkable enhancement of power conversion efficiency (PCE) from 12.4% to 15.0% and also reduced the hysteresis of photocurrent. We also found that conventional water-methanol-soluble polymer interlayer, such as PFN, was incompatible with the perovskite films because of the small molecular size of aprotic solvent such as MeOH, which could decompose the perovskite films to PbI₂, resulting in considerably lower solar cell performance. This study provides new design guidelines for efficient interfacial materials and also demonstrates that interface engineering could be a key strategy to improve perovskite solar cells.