Pre-annealing of metal stack precursors and its beneficial effect on kesterite absorber properties and device performance

Abstract A variety of approaches is being used to fabricate kesterite absorbers, such as sputtering, co-evaporation and solution based techniques. Annealing at high temperatures is a common processing step to stimulate elemental mixing and grain growth. This study investigates the effect of pre-annealing of metal stack precursors at 150 °C, 200 °C, 300 °C and 450 °C on kesterite absorber and solar cell properties. Structural, morphological and vibrational properties of the thin films were investigated. It was found that pre-annealing at 450 °C exhibited a structural mixture distinct from precursors annealed at lower temperatures. The absorber showed improved thickness uniformity, compositional surface homogeneity and absence of Sn-S phases. Lower temperatures resulted in Sn-S compounds in the absorber. In situ XRD is used to monitor phase transitions during pre-annealing. It is shown that implementing a pre-annealing step can improve the absorber properties and the device efficiency. The best performing solar cell had a 4.02% efficiency and was achieved by pre-annealing the precursor layer at 450 °C.