Fatores limitativos no Voc de células solares de CIGS: importância das flutuações de potencial

CIES2020 - XVII Congresso Ibérico e XIII Congresso Ibero-americano de Energia SolarRESUMO: Neste trabalho é apresentado um estudo experimental do papel dos defeitos na limitação do desempenho de células solares de filme fino baseadas em Cu(In,Ga)Se2 (CIGS). A existência de densidades elevadas de defeitos na rede do CIGS cria flutuações no potencial elétrico ao longo da rede, as quais se traduzem no aparecimento de caudas da densidade de estados no hiato. Diferentes modelos teóricos foram usados para descrever os vários tipos de caudas compatíveis com o CIGS. O estudo centrou-se em três células nas quais a fração de Cu foi intencionalmente variada. Os resultados experimentais, baseados na análise de propriedades óticas, mostram que o papel desempenhado pelas flutuações de potencial é determinante num dos parâmetros mais importantes para caracterizar o desempenho da célula solar, a tensão de circuito aberto (Voc). Por último, este trabalho mostra que as flutuações de potencial influenciam claramente o desempenho da célula solar à temperatura ambiente.ABSTRACT: This work presents a theoretical and experimental study of the role of defects in limiting the performance of thin-film solar cells based on Cu(In,Ga)Se2 (CIGS). The existence of high defect densities in the CIGS network creates fluctuations in the electrical potential along the network, which translate into the appearance of tails of the density of states in the bandgap. Different theoretical models were used to describe the various types of tails compatible with CIGS. The study focused on three solar cells in which the Cu fraction was intentionally varied. Experimental results based on the analysis of optical properties, showed the relevant role played by fluctuating potentials in one of the most important parameters that characterizes the performance of the solar cell, the open circuit voltage (Voc). Finally, this work shows that potential fluctuations clearly influence the performance of the solar cell at room temperature.info:eu-repo/semantics/publishedVersio

Effect of rapid thermal processing conditions on the properties of Cu2ZnSnS4 thin films and solar cell performance

In the present work, we have studied the effect of several sulphurization conditions on the properties of Cu2ZnSnS4 thin films obtained through rapid thermal processing (RTP) of multi-period precursors with 8 periods of Zn/SnS2/CuS. In this study we varied the heating rate, the maximum sulphurization temperature, the time at maximum temperature and the amount of evaporated sulphur. The samples were characterized through scanning electron microscopy, energy dispersive spectroscopy, Raman scattering spectroscopy, X-ray diffraction, photoluminescence and I–V measurements. We have observed that at heating rates above 0.5 1C/s the samples delaminated severely. As a result further tests were carried out at 0.2 1C/s heating rate. The morphological studies revealed that the samples sulphurized at higher temperatures, shorter times and higher amount of evaporated sulphur exhibited larger grain sizes. The structural analysis based on Raman scattering and XRD did not lead to a clear distinction between the samples. Photoluminescence spectroscopy studies showed an asymmetric broad band characteristic of CZTS, which occurs in the range of 1.0–1.4 eV and a second band, on the high energy side of the previous one, peaking at around 1.41 eV. The intensity of this latter band varies from sample to sample revealing substantial differences in their optical properties. This band appears to originate either from the surface of the absorber or from the CdS layer and has a clear correlation with cell efficiency. The higher the intensity of this band the lower the cell efficiency, presumably due to the increase in recombination resulting from CZTS surface decomposition and eventually from the CdS with modified optoelectronic properties. The cell results hint toward a detrimental effect of long sulphuriza-tion times and a positive effect of higher sulphur vapour pressure and higher sulphurization temperature. Solar cell efficiencies improved with increased grain size in the absorber layer. The highest cell efficiency obtained in this study was 3.1%.info:eu-repo/semantics/publishedVersio

Enhancement of up-conversion efficiency by combining rare earth-doped phosphors with PbS quantum dots

This paper aims to enhance the up-conversion phenomena observed in silicon solar cells by combining a rare earth-doped phosphor with PbS quantum dots. Two different ways of adhering the up-converter and the fluorescent material to a bifacial solar cell are implemented: dissolving the powder in a spin-on oxide and by dissolving it in a silicone gel. Characterization is carried out through photocurrent and photoluminescence measurements. The improvement in photocurrent detected by the combination of the up-converter and the PbS quantum dots is 60% better than without them, demonstrating that the absorption and emission characteristics of the quantum dots embedded both in the oxide or the silicone can be tuned to the desired spectral region

Anomalous persistent photoconductivity in Cu2ZnSnS4 thin films and solar cells

A persistent photoconductivity effect (PPC) has been investigated in Cu2ZnSnS4 thin films and solar cells as a function of temperature. An anomalous increase of the PPC decay time with temperature was observed in all samples. The PPC decay time activation energy was found to increase when temperature rises above a crossover value, and also to grow with the increase of the sulfurization temperature and pressure. Both the anomalous behavior of the PPC decay time and the existence of two different activation energies are explained in terms of local potential fluctuations in the band edges of CZTS

I Ciclo de Conferências : Conselho Técnico Científico : temas atuais em investigação

A obra é constituída pelos resumos das comunicações apresentadas pelos docentes da Escola Superior Agrária do Instituto Politécnico de Castelo Branco e são referentes aos projetos de investigação nos quais estão envolvidos.O Conselho Técnico-Científico (CTC) da Escola Superior Agrária tomou posse no dia 9 de Fevereiro de 2010. As suas primeiras acções orientaram-se no sentido de dar cumprimento ao estabelecido nos Estatutos da Escola Superior Agrária do Instituto Politécnico de Castelo Branco, que entraram em vigor a partir de 14 de Janeiro de 2010. Estas acções centraram-se na re-organização interna da estrutura técnica e científica, na definição das estratégias de formação e de investigação, desenvolvimento e inovação (ID&I) da Escola Superior Agrária. Enquadrada nesta estratégia, o CTC considerou oportuno promover um ciclo de conferências, com o principal objectivo de criar um meio de apresentação e debate público do trabalho de ID&I desenvolvido por docentes e técnicos que possibilitasse, nas áreas de intervenção da ESA, o desenvolvimento de novas ideias e a criação de sinergias para estabelecimento de trabalhos pluridisciplinares e parcerias, não só a nível interno como também com stakeholders. A publicação das conferências realizadas neste primeiro mandato (2010-2012), disponibiliza a informação para um público mais alargado, visando contribuir de uma forma concreta para o desenvolvimento da região, aspecto de interesse prioritário na missão do IPCB/ESA

Structural and optical properties of Er implanted AlN thin films: green and infrared photoluminescence at room temperature

In this work erbium ions were implanted into AlN films grown on sapphire with fluence range: (0.5-2) × 1015 at/cm-2, ion energy range: 150-350 keV and tilt angle: 0°, 10°, 20°, 30°. The optical and structural properties of the films are studied by means of photoluminescence and Raman spectroscopy in combination with Rutherford backscattering/channeling (RBS/C) measurements. The photoluminescence spectra of the Er3+ were recorded in the visible and infrared region between 9 and 300 K after thermal annealing treatments of the samples. The emission spectrum of the AlN:Er films consists of two series of green lines centered at 538 and 558 nm with typical Er3+ emission in the infrared at 1.54 μm. The green lines have been identified as Er3+ transitions from the 2H11/2 and 4S3/2 levels to the 4I15/2 ground state. Different erbium centers in the matrix are suggested by the change of infrared photoluminescence relative intensity of some of the emission lines when different excitation wavelengths are used. The relative abundances of these centers can be varied by using different implantation parameters. The Raman and RBS/C measurements show good crystalline quality for all the studied films.PTDC/CTM/100756/2008SFRH/BD/45774/2008Portuguese Agency GRICESBrazilian Agency CAPES the Grant 172/0

Células solares ultrafinas de Cu (In,Ga)Se2 : passivação de interfaces

CIES2020 - XVII Congresso Ibérico e XIII Congresso Ibero-americano de Energia SolarRESUMO: A comunidade de Cu(In,Ga)Se2 (CIGS) tem focado grande parte da sua investigação no estudo e melhoramento das propriedades cristalinas do CIGS.A última estratégia utilizada, que tem permitido aumentar o valor de eficiência das células solares, passa pela implementação de elementos alcalinos através de tratamentos pós-deposição (PDT). Para se atingir valores de conversão de eficiência competitivos é necessário melhorar as interfaces do CIGS. Neste estudo, focamo-nos no estudo das propriedades morfológicas, estruturais e optoelectrónicas entre o CIGS e a alumina (Al2O3), que tem o potencial de ser usada como camada passivadora frontal. Pode-se concluir que as propriedades morfológicas e estruturais não são alteradas devido à deposição do Al2O3. O Al2O3 não resiste ao banho químico usado para a deposição do CdS. O Al2O3 apresenta um valor de densidade de defeitos baixos, uma propriedade desejada destas camadas. Este estudo demonstra a potencialidade de se utilizar a Al2O3, para camadas buffer alternativas, que não usem processos químicos durante a sua deposição.ABSTRACT: Cu(In,Ga)Se2 (CIGS) community have been focusing the research line in the study and improvement of the crystalline properties of CIGS. The last trend, to increase the light to power conversion efficiency values, is the use of fluoride-alkaline post-deposition treatments. (PDT). To reach competitive efficiency values, it is necessary to focus on the improvement of CIGS interface. In this work, we focus on the study of the structural, morphological and optoelectronic properties in the interface of CIGS and alumina (Al2O3) which has the potential to be used as front passivation layer. We can conclude that the structural and morphological properties of CIGS remain the same with the deposition of Al2O3. When it was deposited the CdS, on Al2O3, the Al2O3 layer does not resist to the CdS chemical bath deposition. The interface Al2O3/CIGS has a low density of defects value, which is one of the desired properties of a passivation layer. This study demonstrates the potential of using Al2O3 as a front passivation layer with alternative buffer layers to CdS that do not use chemical processes during the deposition.info:eu-repo/semantics/publishedVersio