Improved performance and stability of perovskite solar cells with bilayer electron-transporting layers

Zinc oxide nanoparticles (NPs) are very promising in replacing the phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) as electron-transporting materials due to the high carrier mobilities, superior stability, low cost and solution processability at low temperatures. The perovskite/ZnO NPs heterojunction has also demonstrated much better stability than perovskite/PC₆₁BM, however it shows lower power conversion efficiency (PCE) compared to the state-of-art devices based on perovskite/PCBM heterojunction. Here, we demonstrated that the insufficient charge transfer from methylammonium lead iodide (MAPbI₃) to ZnO NPs and significant interface trap-states lead to the poor performance and severe hysteresis of PSC with MAPbI₃/ZnO NPs heterojunction. When PC₆₁BM/ZnO NPs bilayer electron transporting layers (ETLs) were used with a device structure of ITO/poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) (PTAA)/MAPbI₃/PC₆₁BM/ZnO NPs/Al, which can combine the advantages of efficient charge transfer from MAPbI₃ to PC₆₁BM and excellent blocking ability of ZnO NPs against oxygen, water and electrodes, highly efficient PSCs with PCE as high as 17.2% can be achieved with decent stability.