Aprimoramento de fotoeletrodos de ZnS crescidos sobre ZnO, por meio do controle estrutural e eletrônico, para uso em células fotovoltaicas

Abstract

The search for alternative forms of energy as a substitute for fossil fuels is eminent. There are several forms of renewable energy capable of reducing the emission of pollutants in the atmosphere, including biomass, wind energy, photovoltaic energy, among others. In the case of photovoltaic energy, several types of photovoltaic cells and modules are being embraced and produced by companies consolidated in the market. However, it still presents a relationship between efficiency and operating cost that is not sufficiently attractive for the popularization of this means of electric energy generation. With this perspective, many researches around technologies that are not yet fully viable, technically and commercially, in this panorama stand out the dye-sensitized solar cells (DSSC’s), core-shells, multilayers and peroveskites. Many compounds have been studied for application in photoelectrodes of solar cells, among which stand out Zinc Oxide (ZnO) and Zinc Sulphide (ZnS). In this sense, the search for better answers and the segment of a work carried out by a student in our research group supported and motivated this work, aiming at the improvement of the ZnO/ZnS systems applied to photovoltaic cells, microwave-assisted solvothermal method (SAM). Two moments were established, one that converged on a theoretical-experimental work focused on the growth of ZnS crystals on ZnO, where the possibility of selective ZnS coatings was realized on the preferential plane (1011) of ZnO, when the ZnS stoichiometry was modified. Such factor influenced the surface energy of the planes in question and consequently presented different photovoltaic responses for each system, depending of the precursor used. Thus, the samples covered by ZnS in the plan (1011) meant an increase of 17% in the short-circuit current and 8% in the voltage open-circuit. The other moment was the realization of an experimental work on the growth of ZnS crystals on ZnO, modifying the sources of Zinc (Zn) and Sulfur (S), in the ZnO coating, using the SAM method. In this second moment, we include a second precursor for sulfur and a third precursor for Zinc, and we modified some steps of the synthesis process. As precursors of Zn, Zinc Acetate (ZA), Zinc Nitrate (ZN) and Zinc Chloride (ZC) were used, and as precursors of S thiourea (TIO) and thioacetamide (TAA). Six possibilities of ZnO/ZnS compounds plus pure ZnO were evaluated: (i) ZnO; (ii) ZnO/ZnS(ZA+TIO); (iii) ZnO/ZnS(ZA+TAA); (iv) ZnO/ZnS(ZN+TIO); (v) ZnO/ZnS(ZN+TAA); (vi) ZnO/ZnS(ZC+TIO)and (vii) ZnO/ZnS(ZC+TAA). In this set of samples, the cell that offered the better photovoltaic response was the one with the ZnO/ZnS(ZC+TAA) photoelectrode. The results obtained for ZnO/ZnS(ZC+TAA) indicated the absence of ZnO, which revealed a ZnO/ZnS(ZC+TAA) with different characteristics. Thus, such photovoltaic results were compared to that of a cell formed only by ZnS (ZC+TAA), however the results of photocurrent were 8 times higher (1.2 mA) to ZnO/ZnS(ZC+TAA), as well that of FF and VOC also were higher, with FF = 0:6594 and VOC = 0:63 V .