Kulwinder Kaur, Arivazhagan Valluvar Oli, Michele Melchiorre, Wolfram Hempel, Wolfram Witte, Jan Keller, Susanne Siebentritt
ABSTRACT Wide‐gap Cu(In,Ga)S 2 solar cells with In 2 O 3 :Sn (ITO) as transparent back contact are evaluated for the application as top cells in tandem devices. The effect of Na on the solar cell performance is investigated by supplying additional Na by NaF co‐evaporation or exclusively by Na diffusion from glass. An efficiency of 12.7% is achieved for a semitransparent solar cell with a band gap of 1.6 eV, with sufficient Na diffusion from glass only, allowed by a thin ITO layer. Absorber grown with additional NaF co‐evaporation during Cu(In,Ga)S 2 growth on thicker ITO show a comparable efficiency of 12%. High‐temperature growth at = 630°C enhances overall absorber quality and results in wide‐gap absorbers, with photoluminescence quantum yield improved to 1.5 × 10 −5 , two orders of magnitude higher than absorber grown at low temperature. NaF co‐evaporation is effective in suppressing deep defects, thereby reducing non‐radiative recombination and enhancing photoluminescence quantum yield further. A GaO x interfacial layer is formed at the rear contact, likely contributing to the passivation of the back contact. With the presence of thick GaO x layer, current blocking effects are visible in the current–voltage curves. On the contrary, a thinner ITO tends to result in thinner GaO x layer and no current blocking is observed.