Interfacial Chemistry between Formamidinium Lead Trihalide Perovskites and Atomic-Layer-Deposited Tin Oxide

Tin oxide (SnO x ) by atomic-layer deposition (ALD), in combination with fullerene, is widely employed as an electron transport layer in p-i-n perovskite solar cells. This study investigates the direct deposition of ALD SnO x on top of formamidinium (FA)-based perovskites, as a step toward the elimination of the fullerene interlayer and its poor effect on solar cell's long-term stability. The interfacial chemistry between FA-based perovskites (FAPbI3 and FAPbBr3) and ALD SnO x was studied using soft and hard X-ray photoelectron spectroscopy (SOXPES and HAXPES) with a focus on investigating the separate roles FA and different halides play during interface formation. FAPbI3 and FAPbBr3 solar cell structures solely containing ALD SnO x resulted in s-shaped current-voltage characteristics, indicating the formation of a transport barrier at the interface. Both SOXPES and HAXPES measurements revealed the emergence of additional nitrogen states at the interface during the ALD SnO x deposition on FAPbI3 and FAPbBr3, where these states are linked to the decomposition of FA+. The FAPbI3/ALD SnO x interface also showed the presence of lead iodide (PbI2) through additional lead states other than that from FAPbI3 by using SOXPES measurements. Concerning the FAPbBr3/ALD SnO x interface, no additional lead states were observed; however, measurements instead revealed the formation of Sn-Br bonds at the interface along with the migration of bromine ions into the bulk of the ALD SnO x . Thus, FAPbI3 and FAPbBr3 undergo distinct reaction pathways upon direct deposition of ALD SnO x on top of them. We reason that the decomposition of FA+ in both perovskites and the formation of PbI2 at the FAPbI3/ALD SnO x interface and the incorporation of Br in SnO x at the FAPbBr3/ALD SnO x interface prove detrimental toward device performance. Therefore, careful interfacial engineering that can mitigate the formation of these products should be utilized to enhance the performance of perovskite solar cells.