Exploring the Electrodeposition, the Termal Treatment, and the Photoelectrochemical Application of Non-Doped and Co-Doped Sb2Se3 Thin Films

Abstract

Among different kinds of materials used for photovoltaic (PV) applications, emerging thin film-based light absorbers with more common and less toxic elements are of increasing interest. In this scenario, Cu2ZnSn(Se,S)4 is one of the most promising in terms of PV efficiency (certified 12.6%),1,2 but the further improvements may be hindered by its chemical complexity. These considerations have led to a resurgent interest in earth-abundant less-complex non-toxic absorbers, such as Cu3P(S,Se)4,3 CuSbS2,5 and Sb2Se3.6–8 Recently, several groups6–8 have reported studies on Sb2Se3 as an alternative material for PV device, highlighting its excellent properties; such as, its direct band gap in a range from 1.0 to 1.2 eV and high absorption coefficient >105 cm-1 at 1.2 eV. However, there are only few works of photoelectrochemical application, and about the effects of the thermal treatment and of the elemental doping on the properties of this semiconductor. In addition, in the most part of the papers, this material has been obtained by vacuum-based processes, which are often not desirable for the production of a low cost technology. Based on aforementioned perspectives, this work proposes the obtaining of Sb2Se3 thin films by electrodeposition, a non-vacuum method, and shows an exploring study of the effects of the thermal treatment and of the addition of dopants. The main goal is to improve the optoelectronic quality and photoactivity toward hydrogen gas production of this material. Firstly, several Sb2Se3 films were electrodeposited from SeO2 and K(SbO)C4H4O6 solutions at different concentrations, varying the supporting electrolytes, bath temperature, substrate, deposition potential and the total charge. Also, they were submitted to thermal treatment under Se atmosphere, at different temperatures and times. To evaluate the effects of each parameter on the morphological, structural and optoelectronic properties of the films, they were characterized by SEM, EDX, UV-vis spectroscopy, XRD and by photoelectrochemical measurements toward hydrogen gas evolution reaction, under simulated sun light (100 mW cm-2). The optimized condition of electro-obtaining of this material was achieved using the intensity of photocurrent of the films as guide. The optimized Sb2Se3 film was obtained from a bath composed of 2.0 mM Se-source and 2.5 mM Sb-source in 0.5 M Na2SO4/H2SO4 – pH 2, at 25 ºC and using a total charge of 600 mC cm-2 (deposition at -0.6 V vs. Ag/AgCl(Sat. KCl)). The films thermal threated under Se atmosphere at 300 ºC for 3h, as well as, those deposited on FTO substrate were more homogenous and stoichiometric, showing the highest photocurrent among the other tested conditions. The band gap of the films did not significantly changed at different deposition and treatment conditions, being this of 1.1 eV. Some variations in morphology were observed as function as the supporting electrolyte, the bath temperature and the deposition potential, but the most part of the films showed globular morphology. All films were phase pure of Sb2Se3, independent of the small variation in atomic composition. The higher photocurrent for H2 evolution (at the standard H+/H2 potential) achieved at the optimized condition was of 168.15 µA cm-2, value three times higher than the previously observed for this material.9 At last, based on the optimized conditions of electrodeposition and thermal treatment, the effect of the addition of Co2+-salt source as an dopant in the electrolytic bath was studied, where different concentrations of dopant were evaluated and the optimal condition was achieved at 5% atomic. At this dopant level, the photocurrent of the film doubled in relation to the undoped film; however, its band gap, morphology and crystalline structure did not change. In conclusion, undoped and Co-doped Sb2Se3 thin films can be obtained by electrodeposition: a non-vacuum, fast, easy and cheap method. These films showed good optoelectronic properties and photoactivity toward hydrogen gas production, and can be promising earth-abundant less-complex non-toxic absorbers for photoelectrochemical applications. Acknowledgment: Grant #2016/10513-3 and grant #2013/07296-2, São Paulo Research Foundation (FAPESP).