Efficiency boost of bifacial Cu(In,Ga)Se2 thin-film solar cells for flexible and tandem applications with silver-assisted low-temperature process

Bifacial Cu(In,Ga)Se2 (CIGS) solar cells are attractive for a range of applications, but their low power conversion efficiency is a limitation. To improve their efficiency, the formation of GaOx at the CIGS/transparent-conducting-oxide interface and charge recombination near this interface under rear illumination need to be suppressed. In the study reported here we prevented the formation of GaOx by silver-promoted low-temperature growth of the CIGS layer. This process also led to an improvement in the absorber quality, a steep Ga gradient near the back interface and reduced the absorption of the transparent conducting oxide. We also report here a certified bifacial solar cell on a glass substrate with efficiencies of 19.77% and 10.89% under front and rear illumination, respectively. We also fabricated bifacial solar cells directly on flexible substrates. Finally, we prepared bifacial perovskite/CIGS tandem solar cells in a four-terminal configuration, achieving a power generation density of 28.0 mW cm-2 under 1 Sun and 30% albedo illumination.