Francisco Willian de Souza Lucas

Core Levels, Band Alignments, and Valence-Band States in CuSbS2 for Solar Cell Applications

The earth-abundant material CuSbS2 (CAS) has shown good optical properties as a photovoltaic solar absorber material, but has seen relatively poor solar cell performance. To investigate the reason for this anomaly, the core levels of the constituent elements, surface contaminants, ionization potential, and valence-band spectra are studied by X-ray photoemission spectroscopy. The ionization potential and electron affinity for this material (4.98 and 3.43 eV) are lower than those for other common absorbers, including CuInxGa(1-x)Se2 (CIGS). Experimentally corroborated density functional theory (DFT) calculations show that the valence band maximum is raised by the lone pair electrons from the antimony cations contributing additional states when compared with indium or gallium cations in CIGS. The resulting conduction band misalignment with CdS is a reason for the poor performance of cells incorporating a CAS/CdS heterojunction, supporting the idea that using a cell design analogous to CIGS is unhelpful. These findings underline the critical importance of considering the electronic structure when selecting cell architectures that optimize open-circuit voltages and cell efficiencies.

Diclofenac on Boron-Doped Diamond Electrode: From Electroanalytical Determination to Prediction of the Electrooxidation Mechanism with HPLC-ESI/HRMS and Computational Simulations

Using square-wave voltammetry coupled to the boron-doped diamond electrode (BDDE), it was possible to develop an analytical methodology for identification and quantification of diclofenac (DCL) in tablets and synthetic urine. The electroanalytical procedure was validated, with results being statistically equal to those obtained by chromatographic standard method, showing linear range of 4.94 × 10(-7) to 4.43 × 10(-6) mol L(-1), detection limit of 1.15 × 10(-7) mol L(-1), quantification limit of 3.85 × 10(-7) mol L(-1), repeatability of 3.05% (n = 10), and reproducibility of 1.27% (n = 5). The association of electrochemical techniques with UV-vis spectroscopy, computational simulations and HPLC-ESI/HRMS led us to conclude that the electrooxidation of DCL on the BDDE involved two electrons and two protons, where the products are colorful and easily hydrolyzable dimers. Density functional theory calculations allowed to evaluate the stability of dimers A, B, and C, suggesting dimer C was more stable than the other two proposed structures, ca. 4 kcal mol(-1). The comparison of the dimers stabilities with the stabilities of the molecular ions observed in the MS, the compounds that showed retention time (RT) of 15.53, 21.44, and 22.39 min were identified as the dimers B, C, and A, respectively. Corroborating the observed chromatographic profile, dimer B had a dipole moment almost twice higher than that of dimers A and C. As expected, dimer B has really shorter RT than dimers A and C. The majority dimer was the A (71%) and the C (19.8%) should be the minority dimer. However, the minority was the dimer B, which was formed in the proportion of 9.2%. This inversion between the formation proportion of dimer B and dimer C can be explained by preferential conformation of the intermediaries (cation-radicals) on the surface.

Combinatorial chemical bath deposition of CdS contacts for chalcogenide photovoltaics

Contact layers play an important role in thin film solar cells, but new material development and optimization of its thickness is usually a long and tedious process. A high-throughput experimental approach has been used to accelerate the rate of research in photovoltaic (PV) light absorbers and transparent conductive electrodes, however the combinatorial research on contact layers is less common. Here, we report on the chemical bath deposition (CBD) of CdS thin films by combinatorial dip coating technique and apply these contact layers to Cu(In,Ga)Se2 (CIGSe) and Cu2ZnSnSe4 (CZTSe) light absorbers in PV devices. Combinatorial thickness steps of CdS thin films were achieved by removal of the substrate from the chemical bath, at regular intervals of time, and in equal distance increments. The trends in the photoconversion efficiency and in the spectral response of the PV devices as a function of thickness of CdS contacts were explained with the help of optical and morphological characterization of the CdS thin films. The maximum PV efficiency achieved for the combinatorial dip-coating CBD was similar to that for the PV devices processed using conventional CBD. The results of this study lead to the conclusion that combinatorial dip-coating can be used to accelerate the optimization of PV device performance of CdS and other candidate contact layers for a wide range of emerging absorbers.

Characterization of defects in copper antimony disulfide

Copper antimony disulfide (CuSbS2) with the chalcostibite structure is a promising photovoltaic (PV) absorber material with several excellent measured optoelectronic properties, such as a solar matched band gap and tunable hole concentration. However, much less is known from an experimental perspective about defects in CuSbS2, even though the defects are critical for solar cell performance. Here, we explore the defect properties in CuSbS2 thin film materials and photovoltaic devices using photoluminescence and capacitance-based spectroscopies, as well as first principles theoretical calculations. We measured three electrically and optically active acceptor defects in CuSbS2, and assigned them to the copper vacancies, sulfur vacancies, and/or copper on antimony antisites by comparison with theoretical calculations. Their activation energies, concentrations, and capture cross sections have been determined and compared to other chalcogenide absorber materials. These fundamental parameters should enable more accurate simulations of CuSbS2 PV devices, paving the way for future improvements in CuSbS2 solar cell efficiencies.

Optical Properties and Surface morphology of ZnTe thin films prepared by multiple potential steps

Nesse trabalho foram obtidos eletrodepositos de filmes finos de ZnTe sobre substrato de Pt, usando pulsos potenciotasticos. A influencia do numero de pulsos, tempo de deposicao para cada elemento (Zn ou Te) e a ordem de deposicao das camadas (Zn/Te ou Te/Zn) na morfologia, propriedades opticas e na foto-corrente foi avaliada. Os dados de microanalise mostraram que a razao Zn/Te variou entre 0,12 e 0,30 sendo que o filme nao era estequiometrico. Entretanto, o valor de energia da banda proibida obtido, em todas as condicoes experimentais utilizadas neste trabalho, foi de 2,28 eV, indicando a formacao do filme de ZnTe. As amsotras com maior porcentagem de Zn apresentaram maior fotocorrente, a qual foi da ordem de 2,64 µA cm-2 e uma morfologia do tipo dendritica.

Research Update: Emerging chalcostibite absorbers for thin-film solar cells

Copper antimony chalcogenides CuSbCh2 (Ch=S, Se) are an emerging family of absorbers studied for thin-film solar cells. These non-toxic and Earth-abundant materials show a layered low-dimensional chalcostibite crystal structure, leading to interesting optoelectronic properties for applications in photovoltaic (PV) devices. This research update describes the CuSbCh2 crystallographic structures, synthesis methods, competing phases, band structures, optoelectronic properties, point defects, carrier dynamics, and interface band offsets, based on experimental and theoretical data. Correlations between these absorber properties and PV device performance are discussed, and opportunities for further increase in the efficiency of the chalcostibite PV devices are highlighted.Copper antimony chalcogenides CuSbCh2 (Ch=S, Se) are an emerging family of absorbers studied for thin-film solar cells. These non-toxic and Earth-abundant materials show a layered low-dimensional chalcostibite crystal structure, leading to interesting optoelectronic properties for applications in photovoltaic (PV) devices. This research update describes the CuSbCh2 crystallographic structures, synthesis methods, competing phases, band structures, optoelectronic properties, point defects, carrier dynamics, and interface band offsets, based on experimental and theoretical data. Correlations between these absorber properties and PV device performance are discussed, and opportunities for further increase in the efficiency of the chalcostibite PV devices are highlighted.

Thermal treatment improvement of CuSbS2 absorbers

Thermal treatment in Sb₂S₃ vapor was used to improve the quality of CuSbS₂ thin films, a promising non-toxic and earth-abundant absorber. A change in the CuSbS₂ crystallographic texture and a decrease in the lattice stress were observed, as well as increases in the grain size, photoluminescence intensity and photoconductivity. To eliminate the influence of the possible Sb₂S₃ rich surface layer on photovoltaic performance, a selective chemical etching with KOH was developed.

Electrochemical Deposition of the Single Phase TlxCu3 – xSe2 Thin Films

In spite of selenide semiconductors having a wide application area, TlCu2Se2 (TCSe) is poorly studied and its synthesis by non-vacuum or thermic methodology in film form is non-existent. Generally, the main problems of applying these TCSe compounds are related to complicated single phase growth, and the great time consuming and non-scalable fabrication method. In this work, the facile, fast, scalable and new electrochemical deposition of this material was proposed as a workaround for these main problems and obtaining single phase thin film. Thus, the electrodeposited TlxCu3 – xSe2 films were characterized by scanning electron microscopy, X-ray diffraction (with Rietveld refinement) and energy dispersive X-ray spectroscopy. They showed interesting variation of composition (x =1.1, 1.2, and 1.25) and morphology as function of the electrodeposition potential, electrolytic bath temperature and thermal treatment.

Copper antimony chalcogenide thin film PV device development

Emerging ternary chalcogenide thin film solar cell technologies, such as CuSbS₂ and CuSbSe₂, have recently attracted attention as simpler alternatives to quaternary Cu₂ZnSnS₄ (CZTS). Despite suitable photovoltaic properties, the initial energy conversion efficiency of CuSbS₂ is rather low (0.3%). Here, we report on our progress towards improving the efficiency of CuSbS₂ solar cells using a high throughput approach. The combinatorial methodology quickly results in baseline solar cell prototypes with 0.6% efficiency, and then modification of the back contact architecture leads to 1% PV devices. We then translate the optimal CuSbS₂ synthesis parameters to CuSbSe₂ devices, which show 3% efficiencies.