Two ideal compositions for kesterite-based solar cell devices

We report a compositional paradigm in kesterite compounds, where V_OC and J_SC must be optimized independently from point of view of composition. Device-grade Cu₂ZnSnSe₄ thin films in solar cells are characterized by non-stoichiometric compositions. The reason for such a broad range is not entirely understood, and a precise “ideal” composition is not known. In this work compositionally graded Cu₂ZnSnSe₄ films are prepared and composition is correlated to Raman spectral features and optoelectronic properties of a device based on the film. In the very Cu-poor and Zn-rich range a V_OC maximum is obtained, while in the less Cu-poor and Zn-rich range a J_SC maximum is obtained. In the ideal compositional range most cells are above 5% efficiency, with a maximum of 6.9% at Cu/(Zn+Sn) = 0.76 and Zn/Sn = 1.34, between the two points with maxima in V_OC and J_SC.