Two-Step Close-Space Vapor Transport of MAPbI₃ Solar Cells: Effects of Electron Transport Layers and Residual PbI₂

The effect of the electron transport layer (ETL) on the growth of methylammonium lead iodide (MAPbI₃) thin films by two-step close-space vapor transport (CSVT) is reported. Nanocrystalline CdS, as well as amorphous SnO₂ and C₆₀, were selected as ETLs on indium tin oxide-coated glass substrates prior to two-step CSVT. The ETL affected the PbI₂ growth, leading to different morphological and crystallographic properties. These differences carried over through the methylammonium iodide reaction to the MAPbI₃ phase, but compact films with a reasonable morphology could be made on each ETL. The ETL also affected the PbI₂-to-MAPbI₃ reaction rate. Solar cells processed on each ETL showed a low level of residual PbI₂ was important for good photovoltaic conversion efficiency (PCE). The PCEs were similar on average, but trade-offs in J–V parameters depended on the ETL selection. When films on each ETL were reacted beyond an optimal PbI₂ residual content, solar cells had lower performance driven by decreases in different J–V parameters. The ETL also had practical effects on J–V performance, namely, hysteresis and air stability. The hysteresis of solar cells on C₆₀ was much less than on SnO₂ and CdS. However, the solar cells with C₆₀ ETLs were not stable in air, exhibiting FF and J_sc losses in as little as 15 min of air exposure, while those made on the other ETLs were stable for hours. Thus, the choice of ETL for two-step CSVT affects the growth of PbI₂ and its reaction to MAPbI₃, but interfacial chemistry considerations and effects on current and atmospheric stability appear to be more important for device performance and yield.