Monolithically integrated CIGS submodules fabricated on flexible substrates

Monolithically integrated Cu(In,Ga)Se₂ (CIGS) submodules with energy conversion efficiencies over 15% have been demonstrated on two kinds of substrates. One of them is flexible zirconia-based ceramic sheet, which contains no alkali elements and needs an extrinsic Na-doping control to obtain high solar cell efficiencies. We have demonstrated a technique to allow fine control of the Na doping level in CIGS absorber layers grown on alkali-free substrates using alkali-silicate glass thin layers of various thicknesses deposited on substrates prior to the sputtering of the Mo back contact layer. Using this technique, a submodule efficiency of 15.9% (17 cells, aperture area 75.7 cm²) has been demonstrated on flexible ceramic substrates. Another substrate used in this study was 250-μm thick soda-lime glass (SLG). Thin glass sheets are also potential flexible and lightweight substrates. Such very thin SLG substrates, however, often distorted due to the high temperatures used for CIGS growth and therefore lower growth temperatures or thinner Mo back contact layers are necessary to reduce the thermal expansion induced stress from the Mo back contact layers. Substrate distortion can lead to difficulties in the subsequent mechanical scribing processes for cell integration. In this study, CIGS absorber layers were grown at the relatively low growth temperature of 500°C on thin SLG substrates using a rf-plasma cracked Se radical beam (R-Se) source and 15%-submodule efficiencies have been demonstrated.