Comparative Study on TiO₂ and C₆₀ Electron Transport Layers for Efficient Perovskite Solar Cells

Electron transport layers (ETLs) in perovskite solar cells (PSCs) play a crucial role in determining their device performance, which form electron-selective contacts with perovskites, and boost the extraction rate of photoinduced electrons and tremendously affect the nucleation and subsequent growth of highly crystallized perovskite films. Herein, four different types of ETLs, i.e., compact titanium dioxide (c-TiO2), c-TiO2 and mesoporous titanium dioxide (m-TiO2) composite layers, fullerene C60, and c-TiO2 and C60 composite layers, are fabricated to compare their effects on the growth of perovskite absorbers and device photovoltaic performance. It is found that a single layer such as c-TiO2 and C60 shows a uniform, dense, and well-defined morphology with minimum roughness, and its film quality is much better than that of c-TiO2/m-TiO2 and c-TiO2/C60 composite layers. Meanwhile, a Cs+/Cl– codoped strategy is adopted and the optimized Cs0.1MA0.9PbI2.95Cl0.05 perovskites are synthesized and coated as light absorbers. Consequently, the c-TiO2-based PSCs show a relatively high efficiency of 20.15%; while the champion devices using the C60 ETL achieve a very high efficiency of 17.15%. To our knowledge, this is a rather high efficiency among pure C60-based PSCs with a regular n-i-p architecture. Moreover, the plastic photovoltaic device via a low-temperature C60 evaporation process is also successfully realized, and it provides a possibility for the application of flexible PSCs.