Improved performance of electroplated CZTS thin-film solar cells with bifacial configuration

Annealing in S vapor greatly improves the performance of electroplated Cu₂ZnSnS₄ (CZTS) solar cells based on the bifacial configuration of Al-doped ZnO (AZO, front contact)/ZnO/CdS/ CZTS/indium tin oxide (ITO, back contact), as compared to H₂S annealing in our previous works. S-vapor annealing does not cause severe damage to the conductivity of the ITO back contact. The highest device efficiency of 5.8% was reached under 1 sun illumination from the AZO side. The well-preformed devices based on the ITO back contact demonstrate smaller series resistances and better fill factors, as compared to our sub-strate-type devices using Mo back contacts. An interfacial reaction at the ITO back contact has been revealed in experiments, which contributes to the formation of SnO₂-enriched interfacial layer and diffusion of In from ITO into CZTS through the Sn sites. Incorporation of In does not significantly change the optical and structural properties or the grain size of CZTS absorbers.