Unraveling the varied nature and roles of defects in hybrid halide perovskites with time-resolved photoemission electron microscopy

Abstract

Hybrid halide perovskites are found to contain multiple types of nanoscale defects that play varied roles in charge trapping – from highly detrimental to relatively benign.

Stabilized tilted-octahedra halide perovskites inhibit local formation of performance-limiting phases

“…Nanoscale domains of hexagonal-phase impurities can persist even in high-performing films that appear otherwise cubic in macroscopic measurements (15). These trace hexagonal domains induce clusters of deep trap states that are detrimental to performance (16,17) and induce photodegradation under operational conditions (15). Eliminating these hexagonal-phase impurities will be essential for commercial viability of these cells, but doing so requires a fundamental atomic-level understanding of why and how they form.…”

“…This octahedral tilt-stabilized (ots) phase is induced through the alloying of cations and acts as an inherent photoactive material stabilizer by frustrating the transformation from the photoactive noncubic phase to hexagonal widebandgap phases. Although this alloying approach provides apparent phase purity if viewed macroscopically, spatial heterogeneity in cation distribution in the film is associated with local nanoscopic regions that are not tilted and thus form residual hexagonalphase impurities (15,17).…”

“…Even if these phases appear stabilized when probed macroscopically through small additions of other cations (2,32), typically used filmprocessing approaches for devices can leave phase impurities that persist on the nanoscale (in general unobservable in macroscopic techniques) (16,17) and induce degradation under operation (15). The pathway to highly stable and efficient FA-rich perovskite devices is through the slight distortion of octahedra across the sample to a degree that frustrates the transition from corner-sharing to facesharing structures but does not compromise optoelectronic properties for example by widening the bandgap or reducing carrier lifetimes (33).…”

FAPbI3 Perovskite Solar Cells: From Film Morphology Regulation to Device Optimization

“…Furthermore, systems containing MA have the risk of long-term thermal instability, Br/I mixture will undergo severe ion separation under long-term illumination, and FACs-based perovskite is difficult to achieve high efficiency but also prone to phase transition. [9,10] Therefore, developing pure FAPbI 3 has become the important direction to achieve high-efficiency and stable PSCs. In the past years, pure FAPbI 3 -based PSC has achieved important progress in high efficiency due to the film quality improvement and device optimization.…”

Understanding the Impacts of Grain Size Variation, Distribution, and Recombination Losses in Halide Perovskites: A Generalized Semi‐Analytical Model from Thin‐Film to Photovoltaics

“…[ 31,32 ] There could be a complex interplay among the defect network formed by the grain boundaries causing such discrepancy. [ 33 ] For instance, how the defect network impacts the thin‐film properties that are influenced by the nature of the defect phase (3D, 2D, 1D, or 0D perovskite phases), [ 34 ] degree of crystallinity owned by the defects (polymorph, or crystalline), [ 35 ] type of atomic defects (vacancy, interstitial), [ 36 ] scale of defect clusters (nano‐ to micro‐scale), [ 37 ] distribution of defect clusters, [ 34 ] local misorientation of grains (highly faceted, twinning, or mono‐crystalline), [ 38,39 ] and impurity that affects the choice of passivation strategies (dangling bond or field‐effect passivation). [ 40,41 ] Such an area of research is recently becoming active, but many questions are still unanswered.…”