Materials Processing Routes to Trap-Free Halide Perovskites

TLDR

Lead‑iodide perovskite photovoltaics have rapidly advanced, reaching 17.9 % certified efficiency, yet the influence of growth conditions and precursors on device performance remains poorly understood. The authors fabricated films under iodine‑poor conditions using a lead acetate precursor, demonstrating a diffusion length of 600 ± 40 nm. Iodine‑rich growth produces deep electronic traps, whereas chloride precursors eliminate Pb‑by‑I defects and the crystal’s lowest‑energy surfaces are trap‑free, leading to long carrier diffusion lengths and superior photovoltaic performance.

Abstract

Photovoltaic devices based on lead iodide perovskite films have seen rapid advancements, recently achieving an impressive 17.9% certified solar power conversion efficiency. Reports have consistently emphasized that the specific choice of growth conditions and chemical precursors is central to achieving superior performance from these materials; yet the roles and mechanisms underlying the selection of materials processing route is poorly understood. Here we show that films grown under iodine-rich conditions are prone to a high density of deep electronic traps (recombination centers), while the use of a chloride precursor avoids the formation of key defects (Pb atom substituted by I) responsible for short diffusion lengths and poor photovoltaic performance. Furthermore, the lowest-energy surfaces of perovskite crystals are found to be entirely trap-free, preserving both electron and hole delocalization to a remarkable degree, helping to account for explaining the success of polycrystalline perovskite films. We construct perovskite films from I-poor conditions using a lead acetate precursor, and our measurement of a long (600 ± 40 nm) diffusion length confirms this new picture of the importance of growth conditions.