Impact of optical properties of front glass substrates on Cu(In,Ga)Se2 solar cells using lift-off process

Transmittance of a front glass is one of the important factors in the development of high efficiency superstrate-type Cu(In,Ga)Se2 (CIGS) solar cells. In this study, we investigated the impact of optical properties of the front glass on the solar cell performance of the CIGS solar cells fabricated using the lift-off process. First, optical properties of quartz substrates and soda-lime glass (SLG) substrates with various thicknesses were investigated. Although optical properties of the SLG substrates depend on the thickness, those of the quartz substrates hardly depend on the thickness. Secondly, the superstrate-type CIGS solar cells were fabricated using 1-mm-thick SLG or 1-mm-thick quartz substrates. As a result, the short-circuit current density of the superstrate-type CIGS solar cell with 1-mm-thick quartz substrate was approximately 7% higher than that with 1-mm-thick SLG substrate, and its conversion efficiency was 7.1%. The external quantum efficiency of the solar cells was also improved using the quartz substrate as a front glass because transmittance and absorptance of the quartz substrate were superior to those of the SLG substrate. We therefore conclude that optical properties of the front glasses play an important role in the improvement of the superstrate-type solar cells.