Pseudo-halide Incorporated Defect Passivation for Stable Perovskite Solar Cells

Abstract

Metal halide perovskites are promising materials in photovoltaic technology with their fascinating advantages such as bandgap tunability, high absorption coefficient, high defect tolerance, low exciton binding energy and ambipolar charge transporting ability. Over 20 years of effort have raised the power conversion efficiency of perovskite solar cells (PeSCs) to 27%, which is comparable to that of crystalline silicon solar cells. However, PeSCs are prone to undergo degradation due to the instability of perovskite materials. Intrinsic point defects—particularly halide vacancies—play a major role, together with extrinsic triggers such as moisture and heat. Therefore, suppression of halide vacancies is one of the most efficient and facile way to improve the efficiency and stability of PeSCs. Pseudo-halides are regarded as powerful passivation agents because of their Lewis basicity towards undercoordinated Pb²⁺. In my thesis, pseudo-halides are utilized to suppress defects and thereby address the instability of PeSCs. First, the pseudo-halide anion, formate, is introduced into PeSCs as an anion engineering additive. With the highest binding affinity to iodide vacancies calculated through simulation, formate anions suppress vacancy defects that are present at grain boundaries and at the surface of the perovskite films thereby augmenting the crystallinity of the films. The resulting PeSCs have long-term operational stability and show intense electroluminescence with external quantum efficiencies of more than 10 %. These findings provide a direct route to eliminate the most abundant and deleterious lattice defects present in metal halide perovskites, providing facile access to solution-processable films. Secondly, lead formate (PbFo₂) is used to introduce the formate anion into the buried interface between the electron transport layer and the perovskite layer. The buried interface induces adverse lattice strain, leading to poor crystallinity. Particularly, residual lead iodide (PbI₂) at the buried interface impedes electron transport and induces severe photodecomposition of perovskite during operation. The carboxylate functionality of formate ions exerts multifaceted effects, influencing not only the electrical properties of the electron transport layer, but also the crystallization mechanism of the overlying perovskite film by stabilizing intermediate phases. Finally, pseudo-halides are applied as a post-treatment on the surface of the perovskite films. To avoid halide migration caused by common passivating agents such as octylammonium iodide, we extended the passivation concept beyond halide chemistry and synthesized three non-halide sulfonate compounds are synthesized. The new materials with high binding energy include octylammonium functionalized with sulfanilate (OAS), p-toluenesulfonate (OAT), and camphorsulfonate (OAC). By passivating the surface with the non-halide passivators, halide migration is effectively prevented, and the passivated perovskite films show high stability under humid or illumination conditions. Among them, OAC is demonstrated to be the most efficient passivator, giving reduced hysteresis. Through these studies, pseudo-halides are applied at every critical region—bottom interface, grain bulk, and top interface. Owing to their strong Lewis basicity, they coordinate strongly to Pb²⁺, controlling the crystallization process of perovskite to suppress the creation of crystallographic defects and further eliminate the existing defects efficiently. The defect elimination considerably stabilizes the PeSCs under operating conditions. These pseudo-halide-based approaches could provide a breakthrough pathway toward highly stable PeSCs.