Efficiency enhancement and doping type inversion in Cu₂CdSnS₄ solar cells by Ag substitution

The main limiting factor of kesterite-related solar cells is the low open-circuit voltage (V_OC) relative to their bandgap. This drawback has been correlated with the easily formed anti-site defects caused by similar ionic radii and/or chemical valence. Recent success in suppressing Sn-related defects by using Cd to replace Zn in Cu₂CdSnS₄ was attributed to the higher formation energy of the 2Cu_Cd + Sn_Cd defect complex compared to its counterpart in Cu₂ZnSnS₄. This has motivated the use of bigger ions to replace Cu in Cu₂CdSnS₄ to reduce the possibilities of I_II and II_I defect formation. In this work, we substitute Cu in Cu₂CdSnS₄ with larger Ag at various concentrations and investigate the structural, optoelectronic, and photovoltaic properties of (Cu,Ag)₂CdSnS₄. Higher concentrations of Ag lead to peak splitting in XRD spectra, which is attributed to mixed phases and marks the transition towards Ag₂CdSnS₄. This is also complemented by Raman scattering analysis, the first time the Raman spectrum of Ag₂CdSnS₄ is reported. Doping type inversion was observed for pure n-type Ag₂CdSnS₄ instead of the p-type of Cu₂CdSnS₄, accompanied by high carrier mobility and sharp absorption onset. Further optoelectronic and photovoltaic characterization reveals that adding 5% Ag concentration improves Cu₂CdSnS₄ device performance to 7.72%, mainly due to superior film quality and improved interface properties. As a result, better carrier collection contributes to the short-circuit current improvement of the champion device.