Dinâmica de femtossegundos em pontos quânticos de CdTe

Orientador: Carlos Henrique de Brito CruzTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb WataghinResumo: Estudamos os fenômenos ultrarápidos em Pontos Quânticos (PQs) de CdTe em Vidros Borosilicatos usando técnicas laser de pulsos curtos (femtossegundos). Em medidas de Espectroscopia Diferencial de Transmissão Resolvida no Tempo observamos deslocamentos para o infravermelho dos espectros. Para a interpretação dos dados estudamos a Estrutura Eletrônica do CdTe com Confinamento Quântico zero dimensional (0-D). Estabelecemos que o processo responsável pelo fenômeno é a relaxação intrabanda entre os estados discretos do material. Estudamos e modelamos este e outros processos para uma discussão exaustiva. Medimos também diretamente os tempos ultracurtos de relaxação da absorção para diferentes tipos de amostras. As amostras que apresentam bandas de luminescência no infravermelho têm resposta mais rápida que as que não a apresentam. Os processos ultrarápidos das que têm essas bandas são devidos à captura dos portadores por estados de armadilhas profundas. As outras apresentam bandas de absorções induzias, diferentes dos "bleachings" das primeiras. A diferença é devida ao efeito de fotoenegrecimento, que muda permanentemente a dinâmica dos PQs por exposição prolongada dos mesmos ao feixe óptico de excitação. Estudamos a passagem das amostras "frescas" para as que se estabilizam, e não mudam mais suas propriedades. Estudando a dependência com potência reconhecemos dois processos como responsáveis por estes sinais: Recombinação "Auger", quando se tem mais que um par elétron-buraco excitado por ponto quântico; e outro processo mais lento independente da excitação óptica, provavelmente captura por estados "rasos"Abstract: We study the ultrafast phenomena in CdTc Quantum Dots in Borosilicate Glasses using short laser pulses (femtoseconds) methods. Measuring the Time Resolved Differential Transmission Spectroscopy we find infrared shifts of the bleached spectra. For the interpretation of these data we study the Electronic Structure of CdTe with 0-D Quantum Confinement. We determine that the process responsible for this phenomenum is intraband relaxation between the discrete states of the material. We study and modelate this and other processes for a detailed discussion. We also measure, in a direct way, the ultrashort relaxation times in the absorption for different samples. The samples showing luminescence bands in the infrared have faster responses than those which do not show them. The ultrafast processes of those that have these bands are due to carrier capture by deep trap states. The others show induced absorption bands, different from the bleaching of the previous ones. This difference is caused by the photodarkening effect: it changes permanently the QDs dynamics by long time exposure of them to the optical excitation beam. We study the passage from "fresh" samples to stabilized ones thit do not change their properties anymore. Studying the dependence with the optical power, we can recognize two processes causing these signals: Auger Recombination, when we have more than one electron-hole pair excited per QD; and another slower process, independent of optical excitation, probably caused by shallow state trappingDoutoradoFísicaDoutor em Ciência

Carrier recombination and transport dynamics in superstrate solar cells analyzed by modeling the intensity modulated photoresponses

The dynamics of carrier recombination and transport of two CuInS2 superstrate solar cells was studied by intensity modulated photovoltage and photocurrent spectroscopy (IMVS and IMPS respectively). For the analysis of the resulting data two different approaches were implemented. In the first approach, the typically used analysis in Dye Sensitized Solar Cells (DSSC) was adapted to obtain the characteristic times of the processes involved. The second approach was based on the fittings of both the IMVS and IMPS data to the solution of the continuity equation. These fittings allow the calculation of different dynamic parameters of the cells. Moreover, consistency between the obtained parameters was observed, in good agreement with the typical analysis for DSSC. The resulting dynamics was associated with the presence and distribution of defect states among the samples. Moreover, from the performed analysis, a relation between the results and the post-treatment applied to the solar cells could be established. The difference in the dynamics of the cells is mainly observed in the difference between the electron lifetimes of both solar cells.Fil: Pereyra, Carlos J.. Universidad de la Republica. Facultad de Ingeniería. Departamento de Física; UruguayFil: Di Iorio, Yesica Dolores. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Berruet, Mariana. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Vazquez, Marcela Vivian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Marotti Priero, Ricardo Enrique. Universidad de la Republica. Facultad de Ingeniería. Departamento de Física; Urugua

Influence of a nanostructured ZnO layer on the carrier recombination and dynamics in chalcopyrite solar cells

The influence of the incorporation of a nanostructured ZnO layer on the carrier recombination and dynamics of chalcopyrite solar cells was studied. Intensity-modulated photocurrent and photovoltage spectroscopy (IMPS and IMVS, respectively) were used for the charge carrier dynamics characterization of two ZnO/In2S3/CuInS2-based solar cells. The charge carrier dynamics on a cell with a ZnO nanorod (ZnO-NR) layer was compared with a similar sample without the nanostructured layer. The IMPS and IMVS responses were measured at different continuous light intensities, and a multitrapping behavior was observed. Higher recombination and transport times were obtained for the cell including the NR layer. Moreover, an enhancement in the charge carrier collection efficiency was observed for the cell with the NR additional layer. The increased surface-to-volume ratio of the NR layer and the passivation of defects in the ZnO/In2S3 interface could be associated with the observed charge dynamics enhancement.Fil: Pereyra, C. Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; Argentina. Universidad de la República; UruguayFil: Di Iorio, Yesica Dolores. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Berruet, Mariana. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Vazquez, Marcela Vivian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Marotti Priero, Ricardo Enrique. Universidad de la Republica; Urugua

CVD Growth of Hematite Thin Films for Photoelectrochemical Water Splitting: Effect of Precursor-Substrate Distance on Their Final Properties

The development of photoelectrode materials for efficient water splitting using solar energy is a crucial research topic for green hydrogen production. These materials need to be abundant, fabricated on a large scale, and at low cost. In this context, hematite is a promising material that has been widely studied. However, it is a huge challenge to achieve high-efficiency performance as a photoelectrode in water splitting. This paper reports a study of chemical vapor deposition (CVD) growth of hematite nanocrystalline thin films on fluorine-doped tin oxide as a photoanode for photoelectrochemical water splitting, with a particular focus on the effect of the precursor–substrate distance in the CVD system. A full morphological, structural, and optical characterization of hematite nanocrystalline thin films was performed, revealing that no change occurred in the structure of the films as a function of the previously mentioned distance. However, it was found that the thickness of the hematite film, which is a critical parameter in the photoelectrochemical performance, linearly depends on the precursor–substrate distance; however, the electrochemical response exhibits a nonmonotonic behavior. A maximum photocurrent value close to 2.5 mA/cm2 was obtained for a film with a thickness of around 220 nm under solar irradiation