Medidas de tempo de vôo, em cinco diferentes temperaturas, foram realizadas em enxofre ortorrômbico para vários campos elétricos, tanto positivos como negativos. O trânsito dos portadores negativos foi mais extensivamente analisado e para baixas intensidades de luz, isto é, o caso do sinal fraco, os seguintes fatos foram verificados; a) não foi detectada dependência da mobilidade com o campo elétrico; b) a razão da corrente no tempo t para a corrente inicial mostrou não ser uma função universal do tempo, (isto é, independente do campo aplicado). Transporte dispersivo, discernível através da dependência da mobilidade com o campo elétrico, foi detectado pelo fato a. Contudo, para o transporte não dispersivo nós devemos esperar que a corrente normalizada leve a uma função universal do tempo, somente dependente do tempo de captura e, eventualmente, do tempo de soltura dos portadores, contrário ao que foi encontrado em b. A parte principal desta tese foi dedicada a testar nossos resultados com modelos existentes e o efeito esperado da falha das suposições nos resultados. Nós fomos então levados a um novo modelo, o qual explicou razoavelmente os resultados; a luz joga portadores, não só no canal normal de condução, mas também em um nível extra, confinado à região superficial, caracterizado por uma mobilidade dependente da profundidade. Soluções numéricas foram obtidas para confirmar a validade das soluções analíticas aproximadas obtidas. Um modelo aproximado foi desenvolvido para explicar o efeito do campo elétrico na extração de portadores da zona iluminada / Time off light measurements, in five different temperatures, were carried out in orthorhombic sulphur for various electric fields, both positive and negative. The transit of negative carriers were more thoroughly analyzed and at low light intensities, that is, the small signal case, the following facts were well established; a) no electric field dependence of the mobility was detected, b) the ratio of the current at a time t to the initial current was found not to be a universal function of the time (that is, independent of the applied field). Dispersive transport, discernible through its field dependent electron mobility, is ruled out by a. However, for non dispersive transport, we should expect the normalized current to lead to a universal function of time, only dependent of the trapping and, eventually, de-trapping times of the carriers, contrary to what was found in b. The main part of this thesis was devoted to test our results with the existing models and the expected effect of the breakdown of the simplified assumptions on the results. We were thus lead to a new model, which reasonable explained the results: light put carriers, not only in the normal conduction channel, but also in an extra level, confined to the surface region, characterized by a depth dependent mobility. Computer solution were carried out to confirm the validity of approximate analytic solutions thus obtained. An approximate model was also developed to explain the effect of the electronic field on the extraction of carriers from the illuminated zone

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This thesis contains primarily a study of CdTe/CdS heterojunction solar cells^ chiefly using photoluminescence spectroscopy. These solar cells show a good potential for commercial power generation in the near Aiture and are of interest to several major companies. A vital but little understood step in the manufacturing process is: annealing the cells in the presence of chlorine prior to back contact application. Studies are performed on a selection of thin film CdTe/CdS cells subjected to CdCl(_2) anneals of different duration. A chemical bevel etch was used to study the spectra at different depths into the sample and laser intensity arid sarhple temperature variations to identify the mechanisms behind the observed photoluminescence peaks. Evidence was found for the CdCl(_2) anneal promoting sulphur diffusion and subsequent grain boundary passivation in the CdTe, leading to increased minority carrier lifetimes and device efficiencies Attempts to obtain electroluminescence from the CdTe/CdS solar cells were madei By using current pulses electroluminescence was obtained in the 780-850nm range with discernible spectral features. Photoluminescence (PL) studies were also performed on a single crystal of CdTe grown to an unprecedented size of approx. 5 cm diameter at Durham university by a multi-tube seeded vapour method of crystal growth. Much higher resolution spectra were: obtained for this than for the solar cells. Several peaks were identified arid the mechanisms responsible were theorised. By taking slices of the crystal boule the PL spectra at different points-throughout the bulk of the crystal were determined. A series of high quality ion-implanted CdTe crystals were also studied! by intensity and temperature dependent PL in order to obtain a better understanding of the effects of known concentrations of known impurities on the PL spectra of CdTe. Specific PL features associated with certain dopants were observed.

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Made available in DSpace on 2014-10-09T12:50:58Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:01:48Z (GMT). No. of bitstreams: 0 / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP / FAPESP:01/07337-3

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