Optical Responses of the Heterojunctions in Cu2ZnSn(S,Se)4 Solar Cells Studied by Electric Modulus Spectroscopy

The electrical properties of heterojunctions are essential to the charge transport and power conversion in Cu2ZnSn(S,Se)(4) (CZTSSe) thin-film solar cells. In the present study, the impedances of the primary CdS/CZTSSe p-n heterojunction and secondary ZnO/CdS isotype heterojunction in CZTSSe solar cells were characterized by electric modulus spectroscopy incorporated with the illumination of 650 and 473 nm lasers. At the CdS/CZTSSe junction, the decrease in junction resistance and increase in junction capacitance with illumination are attributed to the splitting of quasi-Fermi levels in the CZTSSe layer. The ZnO/CdS junction was the secondary junction verified by its optical responses to the 650 and 473 nm lasers. At the ZnO/CdS junction, the decreases in both resistance and capacitance with blue light are attributed to the photodoping phenomenon in the CdS layer. The direction of the ZnO/CdS junction is opposite to that of the CdS/CZTSSe junction, and the resistance of the ZnO/CdS junction is unfavorable to the collection of photogenerated electrons and power conversion of CZTSSe photovoltaics.