Insights from Transient Absorption Spectroscopy into Electron Dynamics Along the Ga?Gradient in Cu(In,Ga)Se ₂ Solar Cells

Cu(In,Ga)Se2 solar cells have markedly increased their efficiency over the last decades currently reaching a record power conversion of 23.3%. Key aspects to this progress are engineered bandgap gradient profile across absorber depth, along with controlled incorporation alkali atoms via post-deposition treatments. Whereas impact these treatments on carrier lifetime has been extensively studied in ungraded films, role Ga-gradient mobility less explored. Here, transient absorption spectroscopy (TAS) is utilized investigate charge dynamics. Minority carriers excited large grains [Ga]/([Ga]+[In]) ratio between 0.2–0.5 found drift-diffuse ?1/3 layer minimum within 2 ns, which corresponds range 8.7–58.9 cm2 V?1 s?1. In addition, recombination times strongly depend Ga-content, ranging from 19.1 ns energy 85 ps high Ga-content region near Mo-back contact. An analytical model, as well drift-diffusion numerical simulations, fully decouple transport and behaviour complex composition-graded structure, demonstrating potential TAS.