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-C₆₁-butyric acid methyl ester (PC₆₁BM), 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 PC₆₁BM ETL. In this work, we reported the mixed fullerene ETLs comprising of PC₆₁BM and its two analogs with different length of fluorocarbon chains, [6, 6]-phenyl-C₆₁-buryric acid 1H,1H-trifluoro-1-ethyl ester (abbreviated, CF₃–PC₆₁BM) and [6, 6]-phenyl-C₆₁-buryric acid 1H, 1H-tridecafluoro-1-heptyl ester (abbreviated, C₆F₁₃–PC₆₁BM). We obtained excellent PCEs of 18.37% and 17.71% for 1 wt % CF₃–PC₆₁BM- and C₆F₁₃–PC₆₁BM-based PSCs (1 wt % addition of PC₆₁BM) with CH₃NH₃PbI₃ (MAPbI₃) perovskites, respectively. Moreover, champion PCEs of ∼19% were obtained based on the CsFAMAPbIBr perovskites. Subsequent experiments demonstrated that the fluorocarbon chains of CF₃–PC₆₁BM and C₆F₁₃–PC₆₁BM 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.