Constructing CdS-Based Electron Transporting Layers With Efficient Electron Extraction for Perovskite Solar Cells

The electron transporting layer (ETL) is a critical component in perovskite solar cells (PSCs) and plays an important role in extracting and transporting electrons. However, the commonly used wide-bandgap metal oxides (TiO2, ZnO, SnO2, etc.) often involve undesirable photocatalytic activity towards perovskite materials under UV light, which would undermine the long-term stability of PSCs. In this article, nonoxide CdS ultra-thin films were deposited onto conducting substrates through chemical bath deposition to serve as ETLs. It is revealed that the subsequent CdCl2 treatment followed by annealing is not essential to CdS ETLs for PSCs; this fact is quite different from the scenario demonstrated for conventional thin-film solar cells. Moreover, a compact TiO2 (c-TiO2) blocking layer was introduced between the conducting substrate and the CdS layer to further enhance the electron extraction capability of ETLs and boost the photovoltaic performance of PSCs. Through careful morphology, optical and electrical characterizations, it is found that the presence of the c-TiO2 underlayer avoids the partially exposure of substrate bumps, enlarges the subsequently deposited perovskite crystals, forms cascade energy level alignments, and accelerates the electron extraction from perovskite to ETLs. Therefore, the shunt current leakage and the charge recombination at the ETL/perovskite interface are significantly suppressed. Eventually, the CH3NH3PbI3 PSC based on the bilayer c-TiO2/CdS ETL yields a promising power conversion efficiency of 15.11%, which is much higher than that of the single-layer CdS-based device. This work delivers one of the few CdS ETL-based PSCs that exhibit efficiencies over 15%.