Wagner Anacleto Pinheiro

Comparative study of CdTe sources used for deposition of CdTe thin films by close spaced sublimation technique

Unlike other thin film deposition techniques, close spaced sublimation (CSS) requires a short source-substrate distance. The kind of source used in this technique strongly affects the control of the deposition parameters, especially the deposition rate. When depositing CdTe thin films by CSS, the most common CdTe sources are: single-crystal or polycrystalline wafers, powders, pellets or pieces, a thick CdTe film deposited onto glass or molybdenum substrate (CdTe source-plate) and a sintered CdTe powder. In this work, CdTe thin films were deposited by CSS technique from different CdTe sources: particles, powder, compact powder, a paste made of CdTe and propylene glycol and source-plates (CdTe/Mo and CdTe/glass). The largest deposition rate was achieved when a paste made of CdTe and propylene glycol was used as the source. CdTe source-plates led to lower rates, probably due to the poor heat transmission, caused by the introduction of the plate substrate. The results also showed that compacting the powder the deposition rate increases due to the better thermal contact between powder particles.

Influence of deposition parameters on the properties of CdTe films deposited by close spaced sublimation

CdTe thin films are used as absorber layer in CdS/CdTe solar cells. The microstructure of this absorber layer plays a fundamental role in photovoltaic conversion and can be controlled by the deposition parameters used during the film growth. In this work, CdTe thin films were deposited by the CSS method onto glass substrates previously covered with In2O3:Sn. The effects of pressure, source temperature and substrate temperature on the microstructural properties of the films were studied. The properties were mainly influenced by the pressure, the presence of oxygen in the reaction chamber, and the substrate temperature. For films deposited under an argon atmosphere, an increase in grain size and a reduction of the texture were observed as the pressure and substrate temperature were increased. The introduction of oxygen in the atmosphere led to a decrease in the deposition rate and affected the microstructure and composition of the film. Films deposited under an argon-oxygen atmosphere have smaller grains than those deposited under argon and are richer in Te. The addition of oxygen to the atmosphere apparently did not result in the formation of oxides.

Fabrication of TCO/CdS/CdTe/Au solar cells using different CdCl 2 treatments

In this work CdS/CdTe solar cells were submitted to solution, sublimation and vapor CdCl2 treatments, followed by NP etch for several periods of time. The influence of these procedures on cell efficiency is discussed. The individual layers are analyzed and the results are compared with similar studies in the literature. The best NP etching time was 20 s for all CdCl2 treatments. The highest efficiency was achieved for the cell submitted to the solution treatment at 380 °C. Although the layers have shown good properties for a high efficiency cell, the maximum overall efficiency was only 7.8%, probably due to high series and low shunt resistances.

Construction of an in-line system to produce CdS/CdTe solar cells

This work involved the construction of an in-line system, where CdS and CdTe depositions, CdCl2 vapor treatment and ZnTe deposition were performed by close-spaced sublimation technique without the necessity of opening the system. The system allowed easy movement by rotating the substrate to the deposition regions. The time spent to produce solar cells in the new system (up to 120 min) was considerably shorter than those achieved in other works to perform similar steps with vacuum break. The new system allowed the substrate was positioned in the deposition region after the source and substrate graphite blocks were already heated, resulting in a high initial nucleation rate and the production of dense and continuous films, suitable for production of high efficiency cells. The first cells, with an area of 1 cm2, showed efficiencies up to 7%.

Thin film CdS/CdTe solar cells produced in a CSS in-line system

This work shows results for CdS/CdTe photovoltaic cells grown in a homemade in-line close-spaced sublimation equipment. In this system, most of the steps of cell fabrication were carried out continuously, using dry processing, without breaking vacuum. The efficiency values ranged from 3.5 to 7%. These cells were compared with unfinished devices prepared in the in-line system and finished (heat treatment and contact processing) at NREL. The results suggested that some steps in the cell fabrication should be optimized.

Alterações microestruturais e físicas em filmes de CdS submetidos a tratamento térmico em presença de CdCl2

O objetivo deste trabalho e descrever alteracoes microestruturais e fisicas em filmes finos de sulfeto de cadmio (CdS), decorrentes de um tratamento termico conduzido em presenca de cloreto de cadmio (CdCl2). Para isso, um filme de CdCl2 foi depositado sobre a superficie dos filmes de CdS antes do tratamento. A deposicao do filme de CdCl2 e o tratamento termico foram realizados em um sistema de sublimacao em espaco reduzido (CSS). Observou-se que o tratamento conduzido em temperaturas acima de 400 oC favorece a cristalizacao dos filmes de CdS. Mudancas na energia de banda proibida dos filmes tratados foram observadas, as quais foram atribuidas a modificacoes na estrutura cristalina das amostras.

INFLUÊNCIA DA TEMPERATURA DE REDUÇÃO EM FILMES DE ÓXIDO DE GRAFENO REDUZIDO PARA CONTATOS ELÉTRICOS EM CÉLULAS SOLARES

No presente artigo sao apresentados e discutidos resultados relativos a influencia da temperatura de reducao em filmes depositados por spray de oxido de grafeno (GO) e de oxido de grafeno reduzido quimicamente (rGO). O aumento da temperatura resultou em um maior grau de empolamento dos filmes de GO comparativamente aos de rGO, principalmente em temperaturas mais elevadas, devido a decomposicao de grupos funcionais. Contudo, esse mesmo aumento da temperatura tambem resultou em uma diminuicao da resistencia eletrica dos filmes. Assim, buscou-se determinar uma temperatura em que houvesse menor grau de empolamento e resistencia eletrica, a fim de produzir contatos eletricos traseiros adequados para celulas solares.

COMPÓSITOS COBRE-GRAFENO PRODUZIDOS POR METALURGIA DO PÓ COM COMPACTAÇÃO A FRIO

Resumo Foram fabricados compósitos de matriz de cobre, reforçados com grafeno a partir da mistura de uma dispersão de óxido de grafeno com pó de cobre eletrolítico. A dispersão foi produzida pelo método de Hummers modificado e caracterizada por espectroscopia Raman e difração de raios X. Os compósitos foram fabricados pelo processo de metalurgia do pó convencional. Os pós de compósitos foram caracterizados por microscopia eletrônica de varredura, os corpos de prova consolidados foram avaliados quanto a densificação a verde e do corpo sinterizado, e os corpos sinterizados foram submetidos a ensaio de compressão a fim de verificar a resistência dos compósitos. Palavras-chave: Óxido de grafeno; Compósitos de matriz metálica; Metalurgia do pó.

Microstructural and optical properties of CSS and CBD-CdS thin films for photovoltaic solar cells

This work compares the properties of CdS films deposited by CBD and CSS techniques. The influence of the deposition parameters on the properties of the films was investigated. The thickness of CBD films ranged from 80 to 100nm and was affected by the concentration of reactants in the solution. For CSS films, the thickness was controlled in the 100-300nm range, being strongly influenced by the source/substrate temperatures and pressure. Grain size in CSS films was larger than in CBD films. Transmittance of CBD and CSS films ranged from 80 to 90%, but the band gap in CSS films was narrower than in CBD films, which means that CSS films transmit less of the blue part of the solar spectrum than CBD films. However, the growth of CdS films by CSS technique may be advantageous because the CdS/CdTe device could be fully manufactured in the same system.