Study and fabrication of perovskite solar cells using TiO2/SnO2 and TiO2/WO3 bilayers as the alternative electron transport layers

In planar perovskite solar cells, it is vital to engineer the extraction and recombination of electron–hole pairs at the electron transport layer/perovskite interface for obtaining high performance. This study reports a novel bilayer structures with different Fermi energy levels by combing a compact layer of TiO2 with 50 thickness deposited by two different methods, spin coating and RF sputtering, and an ultra-thin layer (UTL) of WO3 or SnO2 with less than 10 nm thickness deposited by RF sputtering method. Then, the structural and electrical characteristics of the cells were tested by XRD pattern, AFM and FE-SEM images, J-V characterization, electrochemical impedance spectroscopy (EIS), Photoluminesence (pL) and UV-vis spectroscopy in order to study the effect of inserting the above mentioned UTLs in different ETL structures on the operation of the cells and to reach to a better understanding of the electron transport mechanism in PCSs. It was revealed that TiO2/SnO2 or TiO2/WO3 electron transport bilayers combines the advantages in high electron extraction and low interfacial recombination together mainly based on more effective energy level alignment. With these bilayer structures, the front surface recombinations in the cells are significantly suppressed, such that a high efficiency exceeding 12% has been also achieved for planar perovskite solar cells. These results suggest a promising and simple approach to further design the photovoltaic devices from the aspect of charge transport and recombination.