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  • Insights from Transient Abs...
    Chang, Yu‐Han; Carron, Romain; Ochoa, Mario; Bozal‐Ginesta, Carlota; Tiwari, Ayodhya N.; Durrant, James R.; Steier, Ludmilla

    Advanced energy materials, 02/2021, Letnik: 11, Številka: 8
    Journal Article

    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 ≈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. In Cu(In,Ga)Se2 solar cells, a Ga‐concentration dependent bandgap gradient has long been implemented as a strategy to achieve high device efficiencies. Here, employing transient absorption spectroscopy, minority carrier dynamics are tracked along the gradient allowing the development of a rigorous physical model that decouples charge carrier recombination and electron transport throughout the absorber layer.