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

Autoría

Descargar referencia