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 (VOC) 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 2CuCd + SnCd 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 III and III 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.