Two-stage processed Cu4SnS4 thin films for photovoltaics - Effect of (N2 + S2) pressure during annealing

Abstract

In this paper, we report the fabrication of Cu4SnS4 thin films by annealing chemically deposited SnS–CuS precursors at 823 K for 90 min, and we studied the effect of the (N2 + S2) pressure during annealing on the growth and properties of the Cu4SnS4 films. Films prepared at a (N2 + S2) pressure of 1.3 kPa exhibited an orthorhombic crystal structure with lattice parameters of a = 1.371 nm, b = 0.766 nm, and c = 0.643 nm, a grain size of 3–6 μm, a direct optical band gap of 1.0 eV, p-type electrical conductivity, and a hole mobility of 69.5 cm2 V−1 s−1. Increasing the (N2 + S2) pressure from 1.3 kPa to 66.7 kPa increased the grain size to more than 6 μm and the hole mobility to 150 cm2 V−1 s−1 at 26.7 kPa and then decreased to 86 cm2 V−1 s−1 at 66.7 kPa. Further increasing the (N2 + S2) pressure to 101.3 kPa resulted in the formation of a monoclinic Cu2SnS3 secondary phase. This study reveals that an annealing temperature of 823 K and a (N2 + S2) pressure of 1.3–66.7 kPa are the optimized conditions to obtain large-grained Cu4SnS4 films free of secondary phases with good optical and electrical properties.