Interfacial engineering to improve Cu2ZnSnX4 (X = S, Se) solar cell efficiency

Interfacial properties between metal back contacts and solar cell absorbers play important roles in determining efficiencies, but studies of such properties in CZTS and CZTSe based solar cells are quite lacking from theoretical aspects. To fill such a blank and explore insights for improving energy conversion efficiencies, we have studied interfacial properties in CZTS and CZTSe solar cells. The natural band offsets between CZTX and the spontaneously formed MoX2 layer (X = S, Se) are obtained using our recently developed intermediate-phase method. We find that the band alignment between CZTS and MoS2 is actually harmful while the band alignment between CZTSe and MoSe2 is beneficial for carrier extractions. We further propose to engineer the back contact interface to improve CZTS solar cell efficiency by depositing a thin layer of Se on the Mo back contact prior to the CZTS deposition, thus avoiding the formation of the undesirable MoS2 layer.