Impact of Zn Alloying to CsZnxPb1–xI3 Charge Carrier Diffusion Coefficient, Diffusion Length, and Recombination Parameters

Abstract

The generation and transport parameters of charge carriers are critical for the evaluation of optoelectronic devices. In this study, we alloyed CsPbI₃ perovskite by partially substituting Pb²⁺ with Zn²⁺ in a simple solution process technique. The spin-coated thin films were investigated to analyze the effect of Zn²⁺ substitution on the charge carrier lifetimes and diffusion coefficients. Samples with moderate levels of Zn exhibited improved morphological characteristics and a higher absorption coefficient. No significant band gap shift was observed, even in highly Zn²⁺-alloyed thin films. A photoluminescence (PL) lifetime of up to 710 ns was recorded in samples with 40% Zn alloying, suggesting enhanced morphology and fewer trap states. The diffusion coefficient, diffusion length, and charge carrier lifetime were measured across a wide range of excitation densities using both fast and slow light-induced transient grating (LITG) methods. At higher carrier densities, an increase in both the carrier recombination rate and diffusivity was observed. Bimolecular and Auger recombination processes were characterized by coefficients B₀ = 5–8.5 × 10^–10 cm³s^–1 and C = 4–8 × 10^–29 cm⁶s^–1, respectively, in 30–40% Zn-alloyed CsPbI₃ samples. The LITG results indicated a direct correlation between higher carrier densities, increased diffusion coefficients (∼20 cm²/s), and a reduction in charge carrier lifetime, which can be attributed to carrier degeneracy and localized state saturation. The highest diffusion lengths, related to carrier density, were found in samples with ∼40% Zn alloying. Additionally, the slow LITG method revealed a 3 orders of magnitude smaller localized exciton diffusion coefficient, which initially increased and further decreased across varying Zn concentrations. Slow LITG and PL decays showed similar lifetime values related to localized excitons, providing high PL efficiency.