Stabilizing α-FAPbI3 at FAPbI3/SnO2-X Interface By Reducing Oxygen Vacancies in SnO2-X

Abstract

CH3NH3PbI3 (CH3NH3 + is denoted MA hereinafter) was selected as the photoactive material in the early stage of perovskite solar cell (PSC) research. However, MAPbI3 shows poor thermal stability due to high volatility of the MA organic cation. Replacing the MA organic cation with a CH(NH2)2 + (CH(NH2)2 + is denoted FA hereinafter) organic cation was suggested to resolve the thermal stability issue. Additionally, FAPbI3 holds higher potential for high-efficiency solar cells with a broader photoresponse range in the solar spectrum than MAPbI3. However, fabricating FAPbI3 thin films is more challenging than MAPbI3. FA exhibits a larger ionic size (2.56 Å) than MA (2.17 Å), making incorporation of FA into PbI6 octahedra difficult. Incorporating FA induces tilting of PbI6 octahedra, which causes reversible deformation of perovskite-phase FAPbI3 (α-phase) into unfavorable phases (δ-phase and PbI2) at room temperature. Several reports on stabilizing the α-phase of FAPbI3, 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 FAPbI3, enhancing hydrogen bonding between FA and the PbI6 octahedron. This hydrogen bonding prevents reversible deformation of and stabilizes the α-phase of FAPbI3 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 FAPbI3 in a bulk manner is widely studied, stabilizing FAPbI3 at the interface is relatively overlooked. Stabilizing FAPbI3 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 FAPbI3 and PbI2, formed at the interface will hinder the charge extraction process from FAPbI3 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 FAPbI3/SnO2-x, Sn and O are the key atoms for stabilizing the PbI6 octahedron and organic cation, respectively, which affects the growth of the perovskite at the interface. Oxygen vacancies in the surface of SnO2-x can cause distortion of the perovskite structure at the interface, inducing unfavorable phases, such as the δ-phase, PbI2, or other unknown phases. Furthermore, oxygen vacancies are more prevalent issue in SnO2-x than TiO2-x due to the stronger multivalency of Sn, which further emphasizes control of oxygen vacancies in SnO2-x. Since the open-circuit voltage (Voc) and fill factor (FF) show greater potential for improvement than the short-circuit current density (Jsc), stabilizing α-FAPbI3 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 FAPbI3 and PbI2 induced by oxygen vacancies of the SnO2-x surface at the FAPbI3/SnO2-x interface. Using X-ray diffraction (XRD) and transmission electron microscopy (TEM), we observed PbI2 segregation and δ-phase FAPbI3 formation adjacent to SnO2-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 FAPbI3. For the first time, organic cation loss at the FAPbI3/SnO2-x interface was also observed as a severe drawback to forming an ideal heterojunction. Oxygen vacancies in the SnO2-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 SnO2-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 SnO2-x layer with minimal oxygen vacancies. After P=O passivation, most of the detrimental effects were mitigated, showing highly stable α-FAPbI3 crystal structures. Consequently, we achieved a maximum PCE of 23.43% with enhanced thermal and operational stability.