Barium concentration controlled phase evolution in molecular solution processing of Cu2BaSnS4 thin films for solar cells with improved optical and electrical properties

Recently, Cu ₂BaSnS ₄ (CBTS) has emerged as a leading candidate to improve cationic ordering, which limits performance of solar cells, in Cu ₂ZnSnS ₄ (CZTS) absorber layers. While non-toxic solution based approaches appear very attractive for the deposition of the CBTS layers, the formation of secondary phases stemming from poor control of composition must be suppressed. While preparing CBTS thin films from 2-methoxy ethanol based molecular solutions, we find critical dependence of phase evolution on the Ba concentration in the solution. From the systematic variation in molar concentration of cations and sulfurization parameters we have established the favourable reaction pathway leading to the formation of the single phase CBTS via intermediate solid-state binary and tertiary compounds. The resultant films exhibited significantly reduced effects of cation antisite disorder compared to CZTS as reflected from a symmetric room temperature photoluminescence peak. The films had a band gap of ∼2.0 eV and showed considerable white light sensitivity. Detailed electrical and electro-impedance measurements were carried out that revealed p-type conductivity with a carrier concentration of 1.7×10 ¹⁴ cm ⁻³.