SnO_x Atomic Layer Deposition on Bare Perovskite—An Investigation of Initial Growth Dynamics, Interface Chemistry, and Solar Cell Performance

High-end organic-inorganic lead halide perovskite semitransparent p-i-n solar cells for tandem applications use a phenyl-C₆₁-butyric acid methyl ester (PCBM)/atomic layer deposition (ALD)-SnO _x electron transport layer stack. Omitting the PCBM would be preferred for manufacturing, but has in previous studies on (FA,MA)Pb(Br,I)₃ and (Cs,FA)Pb(Br,I)₃ and in this study on Cs_0.05FA_0.79MA_0.16PbBr_0.51I_2.49 (perovskite) led to poor solar cell performance because of a bias-dependent light-generated current. A direct ALD-SnO _x exposure was therefore suggested to form a nonideal perovskite/SnO _x interface that acts as a transport barrier for the light-generated current. To further investigate the interface formation during the initial ALD SnO _x growth on the perovskite, the mass dynamics of monitor crystals coated by partial p-i-n solar cell stacks were recorded in situ prior to and during the ALD using a quartz crystal microbalance. Two major finds were made. A mass loss was observed prior to ALD for growth temperatures above 60 °C, suggesting the decomposition of the perovskite. In addition, a mostly irreversible mass gain was observed during the first exposure to the Sn precursor tetrakis(dimethylamino)tin(IV) that is independent of growth temperature and that disrupts the mass gain of the following 20-50 ALD cycles. The chemical environments of the buried interface were analyzed by soft and hard X-ray photoelectron spectroscopy for a sample with 50 ALD cycles of SnO _x on the perovskite. Although measurements on the perovskite bulk below and the SnO _x film above did not show chemical changes, additional chemical states for Pb, Br, and N as well as a decrease in the amount of I were observed in the interfacial region. From the analysis, these states and not the heating of the perovskite were concluded to be the cause of the barrier. This strongly suggests that the detrimental effects can be avoided by controlling the interfacial design.