Bifacial Wide-Gap (Ag,Cu)(In,Ga)Se2 Solar Cell with 13.6% Efficiency Using In2O3:W as a Back Contact Material

This study evaluates In2O3:W as a transparent back contact material in wide-gap (bandgap range = 1.44-1.52 eV) (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells for potential application as a top cell in a tandem device. High silver concentrations and close-stoichiometric absorber compositions result in a complete depletion of free charge carriers, allowing for decent electron collection, despite the low diffusion length. Remarkable efficiencies of 13.6% and 7.5% are reached using 1 mu m- and 400 nm-thick absorbers, respectively. At rear illumination (i.e., superstrate backwall), the best cell shows an efficiency of 8.7%. For each of the four analyzed samples, the short-circuit current at rear illumination reaches at least 60% of the value at front illumination. Losses arise from recombination at the back contact and a too low drift/diffusion length. The parasitic absorption by the transparent electrodes for photon energies close to the bandgap of a potential Si bottom cell (1.1 eV) is close to 15%. Strategies to reduce this value and to further increase the efficiency are discussed.