Estudo das propriedades estruturais e eletrônicas das interfaces ZnO/ZnX (X=S, Se, Te e F2) via Teoria do Funcional da Densidade

Abstract

The design of materials suitable for application in electronic devices such as electrodes in solar cells of the Gr¨atzel with or without dye requires a detailed understanding of their structural and electronic properties. The vast majority of works were based on experimental approaches, but ab initio calculations have emerged as a powerful approach that complements the experiment and serves as a predictive tool for the identification and characterization of solar cells materials, allowing access and understanding of phenomena of electronic origin. In this thesis, the Density Functional Theory (DFT) was used to study the structural, electronic properties of the ZnO/ZnX compounds (X = S, Se, Te and F2) initially in the bulk configuration and finally as interfaces along of the [10-10] ZnO direction. For this theoretical study, as stated, we used the DFT, with the exchange and correlation functional PBE, since our studied compounds have complex properties (they are strongly correlated materials), which means that the properties described by conventional DFT (with the functional PBE) are inaccurate. The strong location of electrons in the d orbitals of ZnX compounds complicates the description by methods that use pseudopotentials, so that they are particularly well described with all electron schemes. In this scenario, we have two alternative methodologies, the first is to use hybrid functional (HSE), which has a high computational cost, otherwise there is the DFT+U (Hubbard approximation) method with the advantage of having considerably lower computational cost. Therefore, we used both non-local approaches to better discuss the properties of our compounds. Both methodologies have been shown to be viable for ZnX materials, but not generally for all ZnX compounds. Using hybrid functionals, we analyzed the structural and electronic properties of ZnX compounds and at the same time compared our results to DFT+U calculations. Our study of first principles calculations reveals that due to the understanding of the charge transfer processes at the ZnO/ZnX interface, we are able to propose which is the best interface for application in solar cells without dyes, where ZnX aims to replace it. For the interfaces, we have used the same number of covering layers after convergence tests. In the end, the ZnO/ZnSe system proved to be the best candidate for application, respecting the conditions presented in this thesis.