“…[28,29] In planar PSCs, a compact thin ETL (HTL) in the range of several tens of nanometers (nm) is deposited on a transparent conducting oxide (TCO) substrate (such as fluorinated tin oxide, FTO, or indium tin oxide, ITO), followed by the deposition of the perovskite absorber layer, HTL (ETL), and a metal back contact (usually silver, Ag, or gold, Au). [30][31][32][33][34][35] This thin layer can be electron or hole selective, which determines the types of PSCs to be n-i-p or p-i-n, respectively (Figure 2a,c). These planar devices can be processed at temperatures below 150 C and therefore are advantageous for mass production.…”