Unveiling the Structure and Role of Point and Planar Defects in Metal Halide Perovskite Solar Cells Using Low-dose Transmission Electron Microscopy

Metal halide perovskite semiconductors are at the forefront of solar cell research, known for their high power conversion efficiencies, long carrier lifetimes, reduced recombination rates, and cost-effective production. However, practical applications are still constrained by issues such as structural instability, ion migration, hysteresis, and structural defects. Addressing these challenges requires an atomic-scale understanding of relationships between structure and performance. However, due to the extreme electron beam sensitivity of perovskites, atomic-scale studies via electron microscopy have been limited [ 1]. Here, we develop and apply low-dose electron microscopy techniques to uncover critical insights into structural defects that govern their unique properties. We present a sequence of atomic-level structural changes that precede degradation in Cs 1-xFA xPbI 3 quantum dots, a technologically important composition [ 2]. Initially, random formation of cation and anion vacancy pairs triggers a migration of remaining ions, unit cell by unit cell, into an ordered, stable intermediate superstructure ( Fig. 1). Further vacancy creation induces phase transitions via octahedral tilting, which is influenced by cation composition [ 3]. These insights reveal the underlying instability mechanisms in hybrid perovskites and point toward strategies to enhance device lifespan. Additionally, we explore the role of Ruddlesden-Popper (RP) planar defects in the formation and photophysical properties of CsPbI 3 perovskite films. Achieving phase-stable CsPbI 3 solar cells with high efficiency remains a major challenge. By varying the composition in CsPbI 3 prepared by thermal vapour deposition [ 4], we manipulate RP defect prevalence to access their impact on corresponding phase stability and optoelectronic performance [ 5]. Our findings indicate that RP defects effectively stabilise the perovskite phase by acting as internal free surfaces, although antisite defects at RP turning points may negatively impact electronic properties ( Fig. 2). Fig. 1 Low dose STEM-ADF images show A-site and I- vacancy ordering in ( A, B) FAPbI 3 and ( C, D) Cs 0.5FA 0.5PbI 3 [ 3]. Fig. 2. Ruddlesden-Popper planar defects in CsPbI 3 perovskite films [ 5].