Introduction: The development of intermediate band solar cells is associated with materials with an intermediate band between the valence and conduction bands which can be used the stepping stone for generating electron-hole pair. These materials are generally produced artificially, which generates practical challenges on the synthesis side.

Methods and Results: In this theoretical study, we investigate a natural intermediate band material, Cu2CdGeS4, which has the intermediate band in the main band gap. Cu2CdGeS4 is I2-II-IV-VI4 group quaternary Cu-based chalcogenide semiconductor, which has been proposed as promising high efficiency light-absorber layer. For the first time, we found the isolated conduction band of Cu2CdGeS4 in the zincblende or wurtzite structure, which can be regarded as a natural intermediate band by employing a hybrid functional model. We have demonstrated the absorption coefficient of both valance band to intermediate band and intermediate band to conduction band transition using Bethe-Salpeter equation (BSE) method. These steps of optical transitions are corresponding to electronic transitions.

Conclusions: Hence, the results imply that Cu2CdGeS4 being two-photon absorption in the visible light range. Additionally, the hole and electron effective masses of Cu2CdGeS4 are systematically studied using first-principle calculations. We show that Cu2CdGeS4 could make possible intermediate band solar cells with improved efficiency.