Calcogenetos fotovoltaicos: pontos quânticos, nanofios e filmes finos

• Ano de defesa: 2017
• Autor(a) principal: Henrique Limborço
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal de Minas Gerais; UFMG
• Palavras-chave em Português: Fisica; Calcogeneto; Nanofios; Pontos quânticos; Filmes finos; Celulas solares
• Link de acesso: http://hdl.handle.net/1843/BUOS-AUUL5T

The most efficient polycrystalline solar cells are based on the Cu(In,Ga)Se2 compound as a light absorption layer. In view of new nanostructured solar cell concepts, CuInSe2 0D and 1D nanostructures are of high interest. In this work, we report CuInSe2 nanowires (NWs) and nanodots (NDs) grown by a vacuum compatible growth process, coevaporation, on an amorphous surface. The presented growth process for the NWs results in a composite material consisting of CISe NWs on top of a polycrystalline CISe base layer.The nanostructures were extensively characterized by transmission electron microscopy, confirming their composition and atomic-scale crystal structure with a very low number of structural defects. From these analyses, we infer that the growth axis is along the [111] direction. The polycrystalline base layer has a tetragonal chalcopyrite structure and is optically active as confirmed by X-ray diffraction and photoluminescence (PL) analysis, respectively. Potential applications of this composite CISe NW/base-layer material for photovoltaic energy conversion are supported by the reduced reflectivity of the material and its strong PL intensity. The properties of CuInSe2 NDs samples grown by coevaporation, such as nanostructures areal density, mean size, and peak optical emission energy can be controlled by changing the growth temperature. Scanning transmission electron microscopy measurements confirmed the crystallinity of the NDs as well as chemical composition and structure compatible with tetragonal CuInSe2. Photoluminescence measurements of CdS-passivated NDs showed that the nanodots are optoelectronically active with a broad emission extending to energies above the CuInSe2 bulk band gap and in agreement with the distribution of nanostructures sizes. A blueshift of the luminescence is observed as the average size of the NDs get smaller, evidencing quantum confinement in all samples. By using simple quantum confinement calculations, we correlate the photoluminescence peak emission energy with the average size of theNDs. Cu2ZnSnSe4 (CZTS) solar cells technology, already in the first steps of implementation has shown a great potential for low cost solar energy harvesting in comparison with CIGS related compounds (due to In scarcity and high cost that limits its applicability). An important issue in the fabrication of CZTS solar cells is the crystalline phase and chemical composition control during deposition, since the formation of secondaryand ternary phases can be favored in out of stoichiometry growth conditions. Due to structural similarities, its a difficult task to detect the presence of unwanted crystalline phases, such as Cu2SnSe3 (CTSe) that has a low bandgap and can shunt the devices absorption layer. In this work we produced CTSe thin films by coevaporation and thesamples were analyzed by spectroscopic ellipsometry experiments. With the given dielectric function parametrization, the in situ identification of ternary compound formation in CZTS could be performed in a fast and nondestructive way. Furthermore, the presented growth method for CISe nanostructures and for CTSe thin films is based on elemental evaporation under vacuum conditions using Si wafers as substrates, which makes the process compatible with the fabrication of highefficiency photovoltaic devices and Si compatible, allowing for integration with the most of techniques used in semiconductor industry