Grain size control of novel photoferroic absorber bournonite (CuPbSbS 3 ) Oliver Rigby, Prof. Marek Szablewski and Dr Budhika G. Mendis Durham University, UK Bournonite (CuPbSbS 3 ) was highlighted by Wallace et al . as a potential next-generation photoferroic absorber with optical properties suitable for PV applications [1]. Photoferroics are promising absorbers due to an additional electric field to aid charge separation which could reverse the trend of V OC deficits seen in most thin-film PV materials. Bournonite is of space group Pmn2 1 with an orthorhombic unit cell and a direct band gap of 1.3 eV. The best performing bournonite solar cell has achieved a champion power conversion efficiency (PCE) of 2.6% [2]. This is a promising start for a novel absorber but a key detriment to the PCE attained is the grain size which averaged 0.413±0.071μm. Smaller grain sizes have a higher density of grain boundaries which can act as recombination centres for photogenerated electron-hole pairs, reducing the overall PCE. We aim to improve the quality of bournonite absorber layers, specifically through studies on improving the grain size and morphology through variations in annealing temperature. We follow the methodology of Koskela et al . [3] and through optimised reaction conditions have produced phase pure bournonite films using spin-coating as a deposition technique. Bulk CuO, PbO and Sb 2 S 3 are dissolved with ethylenediamine, and adding ethanedithiol causes the corresponding sulfides (Cu 2 S, PbS and Sb 2 S 3 ) to form in a solution suitable for spin-coating. This is done at near ambient conditions (30°C and atmospheric pressure). After spin coating, the samples are placed in a tube furnace under N 2 flow and heating to ~450°C recovers phase pure bournonite. As bournonite is a quaternary, phase purity is particularly challenging and PbS impurities are commonly reported. The temperature of the tube furnace is key to controlling the phase purity of these films and their morphology. We have investigated the effects of temperature variation on annealed samples held at 1 hour over a temperature range from 400°C to 500°C and observed key changes to the structure, optical properties and surface morphology. Across the temperature range the majority phase is bournonite and at 400°C, 450°C and 500°C the films show high phase purity. At 425°C and 475°C degradation occurs and CuSbS 2 and PbS secondary phases are common (though still minority phases). At 450°C the bournonite grains show typical sizes of ~0.7μm. At 475°C and 500°C the grain sizes are significantly improved with typical sizes of ~3μm, though at the detriment of poor surface coverage. This is evidenced by backscattered imaging in scanning electron microscopy (SEM), energy- dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and UV-vis absorption spectroscopy. This work is important for establishing the optimum annealing temperature for bournonite absorber layers. References 1. S. Wallace., K. Svane., W., Huhn et al. , Sustainable Energy & Fuels, 1, 1339-1350, (2017). 2. M. Zhang., Y. Liu., B. Yang et al. , ACS Applied Materials & Interfaces, 13, 13273-13280, (2021). 3. K. Koskela, B. Melot, R. Brutchey, Journal of the American Chemical Society, 142, 6173-6179, (2020).
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