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.