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

Zinc oxide nanoparticles (NPs) are very promising in replacing the phenyl-C-butyric acid methyl ester (PCBM) 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/PCBM, 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 (MAPbI) to ZnO NPs and significant interface trap-states lead to the poor performance and severe hysteresis of PSC with MAPbI/ZnO NPs heterojunction. When PCBM/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)/MAPbI/PCBM/ZnO NPs/Al, which can combine the advantages of efficient charge transfer from MAPbI to PCBM 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.