Vacuum-Assisted Growth of Low-Bandgap Thin Films (FA0.8MA0.2Sn0.5Pb0.5I3) for All-Perovskite Tandem Solar Cells

All-perovskite multijunction photovoltaics, combining a wide-bandgap (WBG) perovskite top solar cell (E-G approximate to 1.6-1.8 eV) with a low-bandgap (LBG) perovskite bottom solar cell (E-G < 1.3 eV), promise power conversion efficiencies (PCEs) >33%. While the research on WBG perovskite solar cells has advanced rapidly over the past decade, LBG perovskite solar cells lack PCE as well as stability. In this work, vacuum-assisted growth control (VAGC) of solution-processed LBG perovskite thin films based on mixed Sn-Pb perovskite compositions is reported. The reported perovskite thin films processed by VAGC exhibit large columnar crystals. Compared to the well-established processing of LBG perovskites via antisolvent deposition, the VAGC approach results in a significantly enhanced charge-carrier lifetime. The improved optoelectronic characteristics enable high-performance LBG perovskite solar cells (1.27 eV) with PCEs up to 18.2% as well as very efficient four-terminal all-perovskite tandem solar cells with PCEs up to 23%. Moreover, VAGC leads to promising reproducibility and potential in the fabrication of larger active-area solar cells up to 1 cm(2).