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This work reports the study of semiconductor nanocrystals, also known as quantum dots, focusing specifically on zinc sulfide (ZnS). Two different capping agents were used (glutathione and N-acetyl-L-cysteine) for the preparation of ZnS nanocrystals via aqueous route. The study aimed specifically at evaluating the efficacy of the capping agents in the stabilization of semiconductor nanocrystal suspensions towards coalescence as well as in controlling nanocrystal size and optical properties. In addition the effect of doping the ZnS nanocrystals with transition metal ions (Cu2+ and Co2+) on the photoluminescence properties has also been studied. Finally the possibility of energy transfer between the semiconductor nanocrystals and the safranine dye was also evaluated. Spherical-shaped glutathione and N-acetyl-L-cysteine-capped ZnS nanocrystal were obtained with diameters below 5 nm free from coalescence, showing that both iv capping agents were efficient as stabilizers. Both capping agents lead to the formation of ZnS nancrystals with blue fluorescence, typical of the involvement of surface defect states of ZnS. However, samples prepared with glutathione exhibited higher fluorescence intensities than those obtained with N-acetyl-L-cysteine. Upon doping glutathione-capped ZnS nanocrystals with both copper and cobalt the fluorescence intensities decreased gradually following the increase in nominal concentration of dopants, suggesting that cobalt ions played a similar role as copper. Considering both the effect on the intensities and the absence of d-d metal transitions this study suggests that doping reduced the concentration of cation vacancies as well as the involvement of at least one cobalt state in the transition processes. Changes in emission wavelength with different dopant concentrations were not observed probably owing to lack of influence on the nanocrystal size. Finally the preliminary study of fluorescence quenching of semiconductor nanocrystals by safranine dye indicated that significantly low concentrations were able to quench the emissions. Different components of the emission band were distinctly affected. Data analysis by Stern-Volmer plots suggested the occurrence of more than one transfer processes (energy and/or electron transfer). This study will be refined in future works. / No presente trabalho foram estudados nanocristais semicondutores, tambem conhecidos como pontos quanticos ou quantum dots, selecionando-se especificamente o sulfeto de zinco (ZnS). Foram utilizados ois diferentes agentes estabilizantes (glutationa e N-acetil-L-cisteina) na obtencao de nanocristais de ZnS por via aquosa. Buscou-se avaliar, especificamente, a eficiencia dos agentes tiois na estabilizacao das suspensoes de nanocristais frente a agregacao, no controle e distribuicao de tamanhos das particulas, bem como nas propriedades opticas. Estudou-se, alem disto, o efeito da dopagem com ions de metais de transicao (Cu2+ e Co2+) nas propriedades de fluorescencia. Por fim, foi avaliada a possibilidade de transferencia de energia entre os nanocristais semicondutores dopados e o corante safranina. Os nanocristais semicondutores de ZnS estabilizados por glutationa e por N-acetil-L-cisteina foram obtidos com tamanhos abaixo de 5 nm, formas aproximadamente esfericas e livres de agregacao, evidenciando que ambos agentes ii estabilizantes foram eficientes. Ambos agentes estabilizantes levaram a formacao de nanocristais com emissoes na regiao do azul, caracteristicas do envolvimento de estados de defeito de superficie do ZnS. No entanto, as amostras preparadas com glutationa apresentaram maiores intensidades de fluorescencia, quando comparadas com aquelas preparadas com N-acetil-L-cisteina. A dopagem dos nanocristais semicondutores ZnS/Glu com ions cobre e cobalto teve um efeito de diminuir as intensidades de fluorescencia dependente da concentracao nominal dos dopantes em ambos os casos, sugerindo que o cobalto atua de modo analogo ao cobre. Considerando-se tanto o efeito sobre as intensidades de emissao do ZnS quanto a ausencia de transicoes d-d do metal, o estudo sugeriu que a dopagem reduz a concentracao de vacancias de cations, bem como o envolvimento de pelo menos um dos estados eletronicos do cobalto nos processos de transicao. Nao se observou variacoes nos comprimentos de onda para diferentes concentracoes dos dopantes, provavelmente pela ausencia de interferencia no tamanho dos nanocristais semicondutores formados. Por fim, o estudo preliminar da supressao de fluorescencia dos nanocristais semicondutores pelo efeito de diferentes concentracoes do corante safranina mostrou que concentracoes significativamente baixas do corante foram suficientes para diminuir a intensidade de fluorescencia. Diferentes componentes das bandas de emissao dos nanocristais semicondutores foram influenciados de modo distinto. A analise dos dados pelos graficos de Stern-Volmer sugeriu a ocorrencia de mais de um processo de transferencia (energia e/ou eletrons). Este estudo sera aprofundado nos trabalhos futuros.

Semiconductors such as CuInGaSe2 and CdTe have been investigated as absorber layer materials for thin film solar cells since their band gap matches with the solar spectrum. Films as thin as 2m are sufficient for the absorption of the visible part of solar radiation, because they are characterized by a high absorption coefficient. However, the scarcity and high costs of Indium, Gallium and Tellurium have led to concerns on the sustainability of these technologies. The semiconductor Cu2ZnSnS4 (Copper Zinc Tin Sulphide) consisting of abundantly available elements promises to be an excellent photovoltaic absorber material. The present study is focused on the growth and characterization of CZTS/ZnS thin film heterostructure suitable for PV applications. Ultrasonic Spray Pyrolysis (USP), a variation of Spray Pyrolysis is a thin film deposition technique where the solution to be sprayed is atomized by ultrasonic frequencies. The details of the USP experimental set up and the deposition principle are presented in the thesis. The active layers of the solar cell, viz. the CZTS absorber layer and ZnS emitter layer were grown by this technique. The metal top contact was deposited using e-beam evaporation. The effects of copper concentration and sodium diffusion on the Cu2ZnSnS4 film properties were investigated. The films have shown preferred orientation along (112) direction confirming kesterite structure. The optical studies revealed that a reduction of copper in the films will bring the band gap energy to 1.5eV, which will match with the solar spectrum. Sodium diffusion in the CZTS films is found to passivate the grain boundaries and enhance the electrical conductivity. These properties render CZTS films as good photovoltaic absorber layers. ZnS has a high band gap and is non toxic unlike CdS. The influences of variation in substrate temperature and spray duration on the ZnS film properties were examined. The optical studies conducted on ZnS films revealed that they are highly transparent in the visible region of the solar spectrum. The films were found to possess a band gap of 3.5 eV. These properties make them potential candidates as solar cell emitter layers. The CZTS/ZnS heterojunction solar cell was fabricated and subjected to electrical characterization in dark and illuminated conditions. A conversion efficiency of 1.16% was achieved for the device.