Titanatos de zinco e cobalto para células fotovoltaicas

Abstract

Zinc titanates are materials that have properties that make it possible to apply them as semiconductors. Reactions via solid state, polymeric precursor method and hydrothermal method are some of the synthesis methods used to obtain these materials, as reported in the literature. In the present work, the microwave-assisted hydrothermal method (HAM) is used to synthesize high purity zinc titanate and with the addition of cobalt (20, 40 and 60%). The physical properties were evaluated using structural and optical characterization techniques. Each of the samples obtained were applied to dye sensitized solar cells (DSSC). After synthesis and subsequent heat treatment, in samples with and without cobalt, X-ray diffractograms demonstrated that the crystallization of the material begins at 500°C. For the samples without cobalt addition, the formation of a cubic phase of ZnTiO3, of the spinel type was detected, considered metastable by the literature but which remained stable at all the heat treatment temperatures tested. The formation of the Zn2TiO4 phase was detected using Raman spectroscopy. For samples containing cobalt, X-ray diffractograms did not detect changes in crystallinity after heat treatment at 500°C. The predominance of rhombohedral ZnTiO3 was detected after heat treatment with temperatures above 500°C. Samples with 20% cobalt were treated at higher temperatures with a predominance of rhombohedral ZnTiO3. The formation of phases containing cobalt was only possible to detect by means of Raman spectroscopy. The most important results are related to optical characterization, based on the techniques of photoluminescence spectroscopy and UV-visible spectroscopy, demonstrating that the photoluminescent activity and the band gap energy had the lowest value for the sample with 60% cobalt, heat treated at 500°C. The value obtained, of 2.56eV, is lower than many semiconductors already consolidated, such as TiO2 and ZnO, and that promising semiconductors such as Nb2O5 and BaTiO3. Another important point refers to the results obtained with the characterization of DSSC’s, where it was concluded that the addition of cobalt was able to sensitize zinc titanate, since only samples with the addition of cobalt were able to produce photovoltaic responses. The results of the photovoltaic parameters obtained, although not very expressive, show a new path for the sensitization of DSSC’s, through a simple and efficient synthesis method, and with the advantages of low cost and low degradation, main problems of the dyes used in CSSC’s, especially those based on ruthenium.