Effect of ZnO Intermediate Layer Thickness on Performance of Cu2ZnSnS4 Solar Cells

Cu2ZnSnS4 (CZTS) thin films were sputtered on Mo/glass substrates using Cu, Sn and ZnS targets and sulfurized at 570°C. The effect of a ZnO intermediate layer between CZTS and Mo on the microstructure, morphology and photovoltaic properties was investigated. Three samples with different ZnO thickness, named as S1 (10 nm), S2 (50 nm) and S3 (211 nm), had the identical composition ratio of Cu/(Zn + Sn) = 0.67 and Zn/Sn = 1.05 for kesterite structure. No obvious secondary phase such as SnS, SnS2 and ZnS was found. XRD showed no ZnO peak which indicated that the ZnO layer was completely sulfurized and converted to ZnS and CZTS. The ZnO intermediate layer significantly inhibited the formation of MoS2 and the decomposition of CZTS. A porous interlayer with many voids appeared at CZTS/Mo in S1 and S2 which originated from two different formations of CZTS and resulted in the inferior properties. The efficiency of CZTS solar cells without ZnO was only 2.7%, obviously below that of the three samples with ZnO. The Voc and Jsc of S3 was 625 mV and 12.6 mA cm−2 respectively, which resulted in the highest efficiency of 4.16% in three samples. The results in S3 illustrate that the ZnS layer was substituted completely by ZnO which can effectively improve CZTS/Mo contact and the photovoltaic properties.