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 Jsc 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.