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

High Resolution Image Download MS PowerPoint Slide 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 (FAPbI 3 and FAPbBr 3 ) 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. FAPbI 3 and FAPbBr 3 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 FAPbI 3 and FAPbBr 3, where these states are linked to the decomposition of FA + . The FAPbI 3 /ALD SnO x interface also showed the presence of lead iodide (PbI 2 ) through additional lead states other than that from FAPbI 3 by using SOXPES measurements. Concerning the FAPbBr 3 /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, FAPbI 3 and FAPbBr 3 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 PbI 2 at the FAPbI 3 /ALD SnO x interface and the incorporation of Br in SnO x at the FAPbBr 3 /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.