Leila Rosa de Oliveira Cruz

The influence of post-deposition treatment on the physical properties of CdTe films deposited by stacked elemental layer processing

Cadmium telluride thin films have been deposited on glass and glass/SnO2/CdS substrates using the stacked elemental layer (SEL) technique. After the films were deposited, they were subjected to the conventional heat treatment in the presence of a CdCl2/methanol solution. The samples were analyzed, before and after treatment, by X-ray diffraction, atomic force microscopy, cathodoluminescence and energy dispersive X-ray spectrometry. The results suggest that the treatment promotes grain growth, decreases defect concentration, and gives rise to the formation of a CdTe1−xSx solid solution, which replaces the CdTe original layer. The existence of this solid solution was evidenced by a reduction in the lattice parameter relative to untreated samples and confirmed by EDS analysis of treated and untreated glass/SnO2/CdS/CdTe structures.

Fundamental Studies of Adhesion of Dust to PV Module Surfaces: Chemical and Physical Relationships at the Microscale

Photovoltaic (PV) module soiling is a growing area of concern for performance and reliability. This paper provides evaluations of the fundamental interactions of dust/soiling particles with several PV module surfaces. The purpose is to investigate the basic mechanisms involving the chemistry, morphology, and resulting particle adhesion to the first photon-incident surface. The evaluation and mapping of the chemistry and composition of single dust particles collected from operating PV module surfaces are presented. The first correlated direct measurements of the adhesive force of individual grains from field-operating collectors on identical PV module glass are reported, including correlations with specific compositions. Special microscale atomic force microscopy techniques are adapted to determine the force between the particle and the module glass surface. Results are presented for samples under dry and moisture-exposed conditions, confirming the effects of cementation for surfaces having soluble mineral and/or organic concentrations. Additionally, the effects of hydrocarbon fuels on the enhanced bonding of soiling particles to surfaces are determined for samples from urban and highly trafficked regions. Comparisons between glass and dust-mitigating superhydrophobic and superhydrophilic coatings are presented. Potential limitations of this proximal probe technique are discussed in terms of results and initial proof-of-concept experiments.

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.

The formation of CdTe thin films by the stacked elemental layer method

Cadmium telluride thin films have been produced using the Stacked Elemental Layer technique. The films were characterized using X-ray diffraction, optical transmittance and reflectance, and atomic force microscopy. The evolution of the thin film reaction and compound formation were studied using X-ray data. The results show that the growth is diffusion controlled and the activation energy is 81.8 kJ/mol. In addition, some physical properties of the films produced are reported.

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.

Fundamental studies of the adhesion of dust to PV module chemical and physical relationships at the microscale

PV module soiling is a growing area of concern for performance and reliability. This paper provides evaluations of the fundamental interactions of dust/soiling particles with a several PV module surfaces. The purpose is to investigate the basic mechanisms involving the chemistry, morphology and resulting particle adhesion to that first photon-incident surface The first-time evaluation and mapping of the chemistry of single dust particles from operating PV module surfaces is presented. The first direct measurements of the adhesive force of individual grains are reported, including correlations to the specific surface chemistry. Special nanoscale techniques using atomic force microscopy (AFM) are adapted to determine the force between the particle and the surface. Results are presented for samples under dry and moisture-exposed conditions confirming the effects of cementation for surfaces having organic/soluble mineral concentrations. Additionally, the effects of hydrocarbon fuels on the enhanced bonding and adhesive force of soiling particles to surfaces are determined for samples from urban and highly-trafficked regions. Comparisons between glass and superhydrophobic and superhydrophilic coatings are presented, showing the effectiveness of the lower-surface energy conditions on the particle adhesion. The potential, limitations of this novel proximal probe technique are discussed in terms of the results and initial, proof-of-concept experiments.

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.

Soiling particle interactions on PV modules: Surface and inter-particle adhesion and chemistry effects

The understanding of the fundamental physics and chemistry of dust and the interaction of these soiling fragments with the PV module surface and each other is potentially important to developing viable mitigation approaches. This paper builds on our previous reports and observations investigating individual soiling particle adhesion on PV module glass using microscale proximal probe techniques. Specifically, in this presentation we report on the refinement of those adhesive force measurements by gaining and including information on the contact area of those particles with the surface, the specific chemistry of interactive surfaces, and the quantification of the force values using materials standard. We also investigate the adhesive forces holding the soiling particles together and the effects of the critical parameters of surface compositional properties, moisture (humidity), and hydrocarbons. This allows for the comparisons of the inter-particle adhesion to the adhesive force holding the particle to the glass module surface. These evaluations are performed on soiling particles collected from operating modules in differing climate zones in Brazil and Middle-East Gulf regions.

Propriedades óticas e microestruturais de filmes finos eletrodepositados de CdTe

CdTe thin films were obtained by cathodic electrodeposition from a solution of CdSO4, H2SO4, and Te. The films were deposited at room temperature onto glass substrates covered with SnO2 or In2O3:Sn. The dependence of the microstructural and optical properties of the films on deposition potential was investigated. The bandgap values ranged from 1.4 to 1.5 eV, as in bulk CdTe. An optimum value for the deposition potential wasdetermined for each substrate. Heating the electrolyte to 50oC did not change the film crystallinity, although it decreased the defect concentration. This suggests that a post-deposition heat treatment may lead to a better performance of the films in photovoltaic devices.

Processing of CdTe thin films by the stacked elemental layer method: compound formation and physical properties

Abstract Cadmium telluride (CdTe) thin films have been deposited using the stacked elemental layer (SEL) technique. This process consists of sequentially depositing tellurium and cadmium layers and then annealing the stacks in order to synthesize the compound. The films were characterized using X-ray diffraction (XRD), optical transmittance and reflectance, scanning electron microscopy (SEM) and atomic force microscopy (AFM). The evolution of the thin film reaction and compound formation were studied using X-ray data. The results show that the growth is diffusion-controlled and the activation energy is (82±2) kJ/mol. The effect of the conventional post-synthesis CdCl 2 heat treatment on the physical properties of the films produced is also reported.