Low temperature pulsed direct current magnetron sputtering technique for single phase β-In₂S₃ buffer layers for solar cell applications

Highlights: • First report on the pulsed d.c magnetron sputtering (PdcMS) of β-In2S3 films. • Films deposited from powder target reduce the material wastage with race track effect. • Films found to be single phase with no additional heating. • Films showed a band gap of 2.77 eV, useful for solar cell applications. • PdcMS prevents temperature induced damages during cell integration process. - Abstract: This work explores the possibilities of using the pulsed direct current (dc) magnetron sputtering (PDCMS) process to deposit an alternative to the cadmium sulphide buffer layer in copper indium gallium diselenide – based solar cells. The main problems with the CdS layer are its toxic nature and its deposition using a chemical bath technique. These factors make it difficult to incorporate into in-line production and significant effort has been expended to find a suitable alternative buffer layer with in-line manufacturing capability. Towards this aim, the material properties of an In₂S₃ film, sputtered from a powder target, have been investigated. Films were deposited at different substrate temperatures ranging from "no additional substrate heating" to 250 °C. The deposition of a single phase β-In₂S₃ without substrate heating/annealing has not previously been reported. The films deposited by the ion-enhanced PdcMS technique without any additional heating were found to be single phase. The grain size increased with increase in substrate temperature. However, this led to a decrease in the sulphur content; as a result the band gap decreased. For solar cell applications, the CdS buffer layer (optical band gap ∼2.4 eV) needs to be replaced with a material which has a band gap wider than 2.4 eV for improved performance and reduction of absorption loss in the blue wavelength region. Ideally the band gap should be between 2.6 and 3.0 eV. Our PdcMS room temperature deposited In₂S₃ had a measured band gap of 2.77 eV.