Engineering the Band Offsets at the Back Contact Interface for Efficient Kesterite CZTSSe Solar Cells

Interfacial properties play a significant role in the photovoltaic performance of kesterite solar cells. Different from its predecessor of Cu(In,Ga)S(e)(2), the interface between Cu2ZnSnS(e)(4) (CZTSSe) and the back contact electrode of Mo is chemically unstable during selenization of the absorbing layer at high temperature. Raman spectra reveal that the MoS2 interfacial layer is easily formed because of more negative change of free energy. However, in reality, the band offset between CZTSSe and MoS2 is unfavorable for hole transfer. By selenizing the Mo electrode, the as-prepared MoSe2 interfacial layer can suppress the diffusion of S and improve the band structure, which is beneficial for charge carrier separation and transfer. Therefore, the conversion efficiency of CZTSSe solar cells is increased from 10.28 to 11.46%.