Mixed Fullerene Electron Transport Layers with Fluorocarbon Chains Assembling on the Surface: A Moisture-Resistant Coverage for Perovskite Solar Cells

In p-i-n structure perovskite solar cells (PSCs), the most prevalent electron transport layer (ETL), [6, 6]-phenyl-C 61 -butyric acid methyl ester (PC 61 BM), acts as both electron extractor and protective coverage to the underlayer perovskite. Notably, multifunctional mixed fullerene ETLs show great potential in further improving both the power conversion efficiency (PCE) and stability of PSCs compared to the single PC 61 BM ETL. In this work, we reported the mixed fullerene ETLs comprising of PC 61 BM and its two analogs with different length of fluorocarbon chains, [6, 6]-phenyl-C 61 -buryric acid 1H,1H-trifluoro-1-ethyl ester (abbreviated, CF 3 –PC 61 BM) and [6, 6]-phenyl-C 61 -buryric acid 1H, 1H-tridecafluoro-1-heptyl ester (abbreviated, C 6 F 13 –PC 61 BM). We obtained excellent PCEs of 18.37% and 17.71% for 1 wt % CF 3 –PC 61 BM- and C 6 F 13 –PC 61 BM-based PSCs (1 wt % addition of PC 61 BM) with CH 3 NH 3 PbI 3 (MAPbI 3 ) perovskites, respectively. Moreover, champion PCEs of ∼19% were obtained based on the CsFAMAPbIBr perovskites. Subsequent experiments demonstrated that the fluorocarbon chains of CF 3 –PC 61 BM and C 6 F 13 –PC 61 BM assembled at the surfaces of ETLs with the formation of thin-layer moisture-resistant protective coverage above perovskite. Results show that it significantly retarded water penetrating down to perovskite layers and led to optimal humidity stability under ambient atmosphere.