Stabilizing α-FAPbI₃ at FAPbI₃/SnO_2-X Interface By Reducing Oxygen Vacancies in SnO_2-X

CH 3 NH 3 PbI 3 (CH 3 NH 3 + is denoted MA hereinafter) was selected as the photoactive material in the early stage of perovskite solar cell (PSC) research. However, MAPbI 3 shows poor thermal stability due to high volatility of the MA organic cation. Replacing the MA organic cation with a CH(NH 2 ) 2 + (CH(NH 2 ) 2 + is denoted FA hereinafter) organic cation was suggested to resolve the thermal stability issue. Additionally, FAPbI 3 holds higher potential for high-efficiency solar cells with a broader photoresponse range in the solar spectrum than MAPbI 3 . However, fabricating FAPbI 3 thin films is more challenging than MAPbI 3 . FA exhibits a larger ionic size (2.56 Å) than MA (2.17 Å), making incorporation of FA into PbI 6 octahedra difficult. Incorporating FA induces tilting of PbI 6 octahedra, which causes reversible deformation of perovskite-phase FAPbI 3 (α-phase) into unfavorable phases (δ-phase and PbI 2 ) at room temperature. Several reports on stabilizing the α-phase of FAPbI 3 , such as through heterogeneous cation or anion mixing, have been presented. For example, mixing smaller ions such as MA, Cs, and Br reduces the unit cell volume of FAPbI 3 , enhancing hydrogen bonding between FA and the PbI 6 octahedron. This hydrogen bonding prevents reversible deformation of and stabilizes the α-phase of FAPbI 3 in ambient atmosphere. In conventional PSCs, the perovskite layer usually forms an interface with an electron or hole transport layer (ETL or HTL). The compatibility of materials forming the interface is significant for separating charges from the perovskite with minimal loss. Nevertheless, although stabilizing FAPbI 3 in a bulk manner is widely studied, stabilizing FAPbI 3 at the interface is relatively overlooked. Stabilizing FAPbI 3 at the interface is more important since the interface contains a high portion of structural defects compared to the bulk, almost 100-fold. Unfavorable phases, such as δ-phase FAPbI 3 and PbI 2 , formed at the interface will hinder the charge extraction process from FAPbI 3 to the transport layer, similar to defects. Especially, in the n-i-p structure, the perovskite is epitaxially deposited over the ETL, and tuning the interface after deposition is nearly impossible, which stresses the importance of the surface structure of the ETL. In the case of FAPbI 3 /SnO 2-x , Sn and O are the key atoms for stabilizing the PbI 6 octahedron and organic cation, respectively, which affects the growth of the perovskite at the interface. Oxygen vacancies in the surface of SnO 2-x can cause distortion of the perovskite structure at the interface, inducing unfavorable phases, such as the δ-phase, PbI 2 , or other unknown phases. Furthermore, oxygen vacancies are more prevalent issue in SnO 2-x than TiO 2-x due to the stronger multivalency of Sn, which further emphasizes control of oxygen vacancies in SnO 2-x . Since the open-circuit voltage (V oc ) and fill factor (FF) show greater potential for improvement than the short-circuit current density (J sc ), stabilizing α-FAPbI 3 at the interface may be a necessary step for researchers to further increase the power conversion efficiency (PCE) and stability of PSCs. In this work, we systematically studied the formation of δ-phase FAPbI 3 and PbI 2 induced by oxygen vacancies of the SnO 2-x surface at the FAPbI 3 /SnO 2-x interface. Using X-ray diffraction (XRD) and transmission electron microscopy (TEM), we observed PbI 2 segregation and δ-phase FAPbI 3 formation adjacent to SnO 2-x . The generation of iodine interstitials at the interface was revealed as the driving force of the unfavorable phase transition. The absence of oxygen atoms interacting with tin atoms lowered the energy barrier for iodine interstitial generation almost by half, and the iodine interstitials accelerated the unfavorable phase transition of FAPbI 3 . For the first time, organic cation loss at the FAPbI 3 /SnO 2-x interface was also observed as a severe drawback to forming an ideal heterojunction. Oxygen vacancies in the SnO 2-x surface, which act as hydrogen bonding sites for FA cations, induced withdrawal of FA cations, which was revealed by X-ray photoelectron spectroscopy (XPS) measurements. Inspired by this phenomenon, we fabricated a SnO 2-x layer with reduced oxygen vacancies by using oxidized black phosphorus quantum dots (O-BPs). The multiple P=O bonds of O-BPs effectively assisted the formation of a SnO 2-x layer with minimal oxygen vacancies. After P=O passivation, most of the detrimental effects were mitigated, showing highly stable α-FAPbI 3 crystal structures. Consequently, we achieved a maximum PCE of 23.43% with enhanced thermal and operational stability.