Development of high-efficiency flexible Cu(In,Ga)Se₂ solar cells: A study of alkali doping effects on CIS, CIGS, and CGS using alkali-silicate glass thin layers

CuInSe₂ (CIS), Cu(In,Ga)Se₂ (CIGS), and CuGaSe₂ (CGS) solar cells were fabricated on flexible 50-μm thick zirconia sheet substrates. Alkali doping into CIS, CIGS, and CGS absorber layers was demonstrated using alkali-silicate glass thin layers (ASTL) deposited on substrates prior to the sputtering of the Mo back contact layer. Enhanced cell efficiencies with the use of ASTL were demonstrated regardless of the In/Ga composition ratio in CIGS. The external quantum efficiency (EQE) curves of CIGS solar cells fabricated with ASTL showed an enhanced absorption in the long wavelength region, whereas the EQE curves of CIS and CGS cells showed no such variation. This result implies that the presence of alkali elements in CIGS lead to a reduction in elemental inter-diffusion of In and Ga during growth, resulting in a decrease of the nominal band-gap energy of the CIGS layer due to the steep Ga composition gradient present in the CIGS layer.