Improving water-resistance of inverted flexible perovskite solar cells via tailoring the top electron-selective layers

Inverted flexible perovskite solar cells (FPSCs) offer a promising route towards commercialization by using undoped inorganic hole transport layers and thermally-stable electron transport layers (ETLs), which deliver good environmental stability such as water resistance. But the power-conversion-efficiencies (PCEs) of inverted flexible perovskite cells are still far below those of FPSCs in regular configuration so far. One main challenge for achieving high-performance FPSCs lies in optimizing the ETLs and their adjacent buffering layers. Herein, we modify the fullerene derivatives (Phenyl-C61-butyric acid methyl ester, PCBM) ETLs by adding 3-aminopropyl triethoxysilane (APTS) molecules into the PCBM solutions. The addition of APTS results in smoother and denser PCBM layers via a physically steric-hindrance effect mechanism, which could benefit the electron collection between the PCBM layer and the perovskite layer. More importantly, APTS can offer nucleating sites, such as amine groups or hydroxy groups by partially hydrolyzing, to grow conformal tin oxide (SnOx) layers by atomic layer deposition (ALD), which acts as robust buffer layers to replace traditional bathocuproine (BCP) layer. Finally, high-performance inverted FPSCs based on the optimized PCBM/SnOx electron-selective layers are fabricated, delivering a best PCE of 18.62%. The unencapsulated FPSC showed a slow decay and remained above 90% of its initial PCE after being stored for more than 600 h in a damp environment with a humidity of more than 85% R.H. The strategy is demonstrated to be applicable in the future large-scale production of stable FPSCs.