Synthesis of simple, low cost and benign sol–gel Cu2InxZn1−xSnS4 alloy thin films: influence of different rapid thermal annealing conditions and their photovoltaic solar cells

Abstract

Cu₂In_xZn_1−xSnS₄ (x = 0.4) alloy thin films were synthesized on soda lime glass (SLG) substrate by a simple low-cost sol–gel method followed by a rapid annealing technique. The influence of sulfurization temperature and sulfurization time on the structure, morphology, optical and electrical properties of Cu₂In_xZn_1−xSnS₄ thin films was investigated in detail. The XRD and Raman results indicated that the crystalline quality of the Cu₂In_xZn_1−xSnS₄ alloy thin films was improved, accompanied by metal deficiency, particularly tin loss with increasing the sulfurization temperature and sulfurization time. From absorption spectra it is found that the band gaps of all Cu₂In_xZn_1−xSnS₄ films are smaller than that (1.5 eV) of the pure CZTS film due to In doping, and the band gap of the Cu₂In_xZn_1−xSnS₄ films can be tuned in the range of 1.38 to 1.19 eV by adjusting the sulfurization temperature and sulfurization time. Hall measurement results showed that all Cu₂In_xZn_1−xSnS₄ alloy thin films showed p-type conductivity characteristics, the hole concentration decreased and the mobility increased with the increase of sulfurization temperature and sulfurization time, which is attributed to the improvement of the crystalline quality and the reduction of grain boundaries. Finally, the Cu₂In_xZn_1−xSnS₄ film possessing the best p-type conductivity with a hole concentration of 9.06 × 10¹⁶ cm⁻³ and a mobility of 3.35 cm² V⁻¹ s⁻¹ was obtained at optimized sulfurization condition of 580 °C for 60 min. The solar cell using Cu₂In_xZn_1−xSnS₄ as the absorber obtained at the optimized sulfurization conditions of 580 °C for 60 min demonstrates a power conversion efficiency of 2.89%. We observed an increment in open circuit voltage by 90 mV. This work shows the promising role of In in overcoming the low V_oc issue in Cu-kesterite thin film solar cells.