Laser-induced transient grating (LITG) spectroscopy has been used to measure the carrier transport properties of perovskite materials and devices, including the diffusion coefficient and carrier lifetime [1]. These measurements are important for understanding the electronic properties of materials and how they can be optimized for different applications. LITG has the advantage of being a non-destructive method, which can be important for protecting the integrity of the material during measurement.

LITG method itself was realized several decades ago [2, 3]. The principle scheme of a LITG measurement is depicted in Fig.1. A pair of ultrashort pulses are overlapped both spatially and temporally to create an interference pattern on the sample – a transient grating with a spatial period Λ that depends on the pump wavelength and the beam intersection angle. Excitation by periodic pattern excites a spatially-modulated carrier distribution and, effectively, a periodic modulation of the refractive index, thus, diffracting the delayed probe beam.

Over time, the laser-induced grating decays due to carrier recombination (electronic decay with the rate of τ_R) and carrier diffusion (spatial decay with the rate of τ_D). The diffusion term depends on the transient grating period: fine gratings diffuse faster than coarser ones. Accordingly, if we measure the temporal behavior of the diffracted signal over a series of different periods Λ, we can determine the carrier diffusion coefficient D [2].

The device presented in this paper allows automated continuous tuning of the excitation grating period Λ and selection of excitation wavelength from 340 to 560 nm, which allows measuring carrier diffusion in the range from 0.1 to 50 cm²/s and carrier lifetime in the range from 1 ps to ca. 80 ns.

Exemplary carrier diffusion and lifetime measurements were performed with a thin film of metal-halide perovskite Cs_0.05(FA_0.85MA_0.15)Pb(I_0.85Br_0.15)₃ with Et2NH additive in a wide range of excitation densities. The results showed that the charge carriers of the material are in a trap-filling and de-trapping diffusion regime, from which carriers easily escape with increasing excitation density.

The extensive experience in optomechanical engineering and high-level automation enabled the all-optical measurement for carrier diffusion and carrier lifetime. It was extensively tested with different samples, including but not limited to the aforementioned perovskites. The results will be presented in detail during the conference.