Black Phosphorus Quantum Dot-Engineered Tin Oxide Electron Transport Layer for Highly Stable Perovskite Solar Cells with Negligible Hysteresis

An effective combination of smart materials plays an important role in charge transfer and separation for high photoelectric conversion efficiency (PCE) and stable solar cells. Black phosphorus quantum dots (BPQDs) have been revealed as a direct band gap semiconductor with ultrahigh conductivity, which have been explored in the present work as an additive component to a precursor solution of SnO2 nanoparticles that can effectively improve the performance of SnO2 electron transport layer (ETL)-based perovskite solar cells. Such a device can yield a high PCE of 21% with the SnO2/BPQDs mixed ETL, which is higher than those of perovskite solar cells based on SnO2 single layer (18.2%), BPQDs/SnO2 bilayer (19.5%), and SnO2/BPQDs bilayer (20.5%) samples. The mixed samples still possess good stability of more than 90% efficiency after 1000 h under AM 1.5G lamp irradiation and negligible hysteresis. It is found that the strong interaction of BPQDs with SnO2 can not only modify the defects inherent to the SnO2 layer but also inhibit the oxidation of BPQDs. This work provides a promising functional material for SnO2 ETL-based perovskite solar cells and proves that the BPQD-based modification strategy is useful for designing other solar cells with high performance.