Benchmarking photoactive thin-film materials using a laser-induced steady-state photocarrier grating

In this study, we assess the charge carrier diffusive transport quality of traditional and emerging thin-film photoactive absorber materials used for photovoltaic applications. We use a steady-state photocarrier grating technique, which has so far been predominantly used for amorphous silicon-based materials, to obtain ambipolar diffusion lengths as well as minority and majority carrier mobility-lifetime products. The measurements are performed at volume-averaged generation rates of G = 1020–1021 cm⁻³ s⁻¹ and low electric field strengths of E = 20–200 V cm⁻¹. The absorbing capability of the materials is analysed by calculating an effective optical absorption depth, and we compare its value with the obtained electronic ambipolar diffusion length. The effective absorption depths are independent of the band-gap values so that our assessment is also relevant for multijunction solar cells. We observe that for silicon-based thin-film materials, the ambipolar diffusion length (with a value lower than 150 nm) is more than twice as short as their effective absorption depth, while for copper indium gallium selenide chalcopyrite and halide perovskite materials, the diffusion length (with a value up to 367 nm) is similar or larger than the effective absorption depth. The presented method can be used as a rapid assessment of the optoelectronic quality of photoactive thin-film materials.