Efficiency enhancement of flexible CIGS solar cells using alkali-silicate glass thin layers as an alkali source material

We have demonstrated a technique to allow fine control of the alkali doping levels in CIGS absorber layers using alkali-silicate glass thin films (ASTL) of various thicknesses deposited on substrates prior to the sputtering of the Mo back contact layer. This technique can be applied for a wide variety of substrates. Using this technique, energy conversion efficiencies of 17.4% and 17.7% have been demonstrated using a Ti-foil substrate and a flexible zirconia sheet substrate, respectively (the CIGS absorber layers were grown at 550°C). In addition, a 14.7%-efficiency has been demonstrated using a polyimide substrate with the use of a low temperature (400°C)-grown CIGS layer. Variations in the structural, optical, and electrical properties of CIGS films grown on ASTL and the Na-effect observed from the CIGS films are discussed.