Enhanced device performance and stability of perovskite solar cells with low-temperature ZnO/TiO2 bilayered electron transport layers

The instability of perovskite films is a major issue for perovskite solar cells based on ZnO electron transport layers (ETLs). Here, ZnO nanoparticle (NP)- and ZnO sol-gel layers capped with low-temperature processed TiO2, namely ZnO/TiO2 bilayered films, have been successfully employed as ETLs in highly efficient MAPbI(3)-based perovskite solar cells. It is demonstrated that these ZnO/TiO2 bilayered ETLs are not only capable of enhancing photovoltaic performance, but also capable of improving device stability. The best device based on the ZnO/TiO2 bilayered ETL exhibits an efficiency of approximate to 15% under standard test conditions and can retain nearly 100% of its initial efficiency after 30 days of atmosphere storage, showing much higher device performance and stability compared to those devices based on ZnO single-layer ETLs. Moreover, it is found that perovskite films and devices prepared on the single ZnO sol-gel ETLs are much superior to those deposited on the single ZnO NP-ETLs in both stability and performance, which can be ascribed to fewer surface hydroxyl groups and much smoother surface morphology of the ZnO sol-gel films. The results pave the way for ZnO to be used as an effective ETL of low-temperature processed, efficient and stable PSCs compatible with flexible substrates.