Browsing by Author "Fernandes, P. A."
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- Admittance spectroscopy of Cu2ZnSnS4 based thin film solar cellsPublication . Fernandes, P. A.; Sartori, A. F.; Salomé, P. M. P.; Malaquias, J.; Cunha, A. F. da; Graça, M. P. F.; González, J. C.In this report, we propose an AC response equivalent circuit model to describe the admittance measurements of Cu2ZnSnS4 thin film solar cell grown by sulphurization of stacked metallic precursors. This circuit describes the contact resistances, the back contact, and the heterojunction with two trap levels. The study of the back contact resistance allowed the estimation of a back contact barrier of 246 meV. The analysis of the trap series with varying temperature revealed defect activation energies of 45 meV and 113 meV. The solar cell’s electrical parameters were obtained from the J-V curve: conversion efficiency, 1.21%; fill factor, 50%; open circuit voltage, 360 mV; and short circuit current density, 6.8 mA/cm2.
- Annealing of RF-magnetron sputtered SnS2 precursors as a new route for single phase SnS thin filmsPublication . Sousa, M.G.; Cunha, A.F. da; Fernandes, P. A.Tin sulphide thin films have been grown on soda-lime glass substrates through the annealing of RF-magnetron sputtered SnS2 precursors. Three different approaches to the annealing were compared and the resulting films thoroughly studied. One series of precursors was annealed in a tubular furnace directly exposed to a flux of sulphur vapour plus forming gas, N2 + 5%H2, and at a constant pressure of 500 mbar. The other two series of identical precursors were annealed in the same furnace but inside a graphite box with and without elemental sulphur evaporation again in the presence of N2 + 5%H2 and at the same pressure as for the sulphur flux experiments. Different maximum annealing temperatures for each set of samples, in the range of 300–570 C, were tested to study their effects on the properties of the final films. The resulting phases were structurally investigated by X-Ray Diffraction (XRD) and Raman spectroscopy. Annealing of SnS2 precursors in sulphur flux produced films where SnS2 was dominant for temperatures up to 480 C. Increasing the temperature to 530 C and 570 C led to films where the dominant phase became Sn2S3. Annealing of SnS2 precursors in a graphite box with sulphur vapour at temperatures in the range between 300 C and 480 C the films are multi-phase, containing Sn2S3, SnS2 and SnS. For high annealing temperatures of 530 C and 570 C the films have SnS as the dominant phase. Annealing of SnS2 precursors in a graphite box without sulphur vapour at 300 C and 360 C the films are essentially amorphous, at 420 C SnS2 is the dominant phase. For temperatures of 480 C and 530 C SnS is the dominant phase but also same residual SnS2 and Sn2S3 phases are observed. For annealing at 570 C, according to the XRD results the films appear to be single phase SnS. The composition was studied using energy dispersive spectroscopy being then correlated with the annealing temperature. Scanning electron microscopy studies revealed that the SnS films exhibit small grain structure and the film surface is rough. Optical measurements were performed, from which the band gap energies were estimated. These studies show that the direct absorption transitions of SnS are at 1.68 eV and 1.41 eV for annealing in graphite box with and without elemental sulphur evaporation, respectively. For the indirect transition the values varied from 1.49 eV to 1.37 eV. The results of this work show that the third approach is better suited to produce single phase SnS films. However, a finer tunning of the duration of the high temperature plateau of the annealing profile is required in order to eliminate the b-Sn top layer.
- Annealing of RF-magnetron sputtered SnS2 precursors as a new route for single phase SnS thin filmsPublication . Sousa, M. G.; Cunha, A. F.; Fernandes, P. A.Tin sulphide thin films have been grown on soda-lime glass substrates through the anneali ng of RF-mag netron sputtered SnS2 precursors . Three different approaches to the annealing were compared and the resulti ng films thoroughly studied. One series of precursors was annealed in a tubular furnace directly exposed to a flux of sulphur vapour plus formi ng gas, N 2 + 5%H2, and at a constant pressure of 500 mbar. The other two series of identical precursors were annealed in the same furnace but inside a graphite box with and without elemental sulphur evaporation again i n the presence of N 2 + 5%H2 and at the same pres sure as for the sulphur flux experiments. Different maximum annealing temperatures for each set of sam ples , in the range of 300-570 °c,were tested to study their effects on the properties of the final films.The resulti ng phases were structurally investigated by X-Ray Diffraction (XRD) and Raman spectroscopy. Annealing of SnS2 precursors in sulphur flux produced films where SnS2 was dominant for temperatures up to 480 °C. Increasing the temperature to 530 °c and 570 °c led to films where the dominant phase became Sn2S3• Annealing of SnS2 precursors in a graphite box with sulphur vapour at temperatures in the range between 300 °c and 480 °c the films are multi-phase , containing Sn2 S3, SnS2 and SnS. For high annealing temperatures of 530 °c and 570 °c the films have SnS as the dominant phase.Annealing of SnS2 precursors in a graphite box without sulphur vapour at 300 °c and 360 °c the films are essentially amor phous, at 420 °c SnS2 is the dominant phase. For temperatures of 480 °c and 530 °c SnS is the dominant phase but also same residual SnS2 and Sn2S3 phases are observed. For annealing at 570 °c, according to the XRD results the films appear to be single phase SnS. The composition was studied usi ng energy dis persive spectroscopy being then correlated with the annealing temperature. Scanning electron micros copy studies revealed that the SnS films exhibit small grain structure and the film surface is rough. Optical measurements were performed, from which the band gap energies were estimated .These studies show that the direct absorption transitions of SnS are at 1.68 eV and 1.41 eV for annealing in graphite box with and without elemental sulphur evaporation, respectively . For the indirect transition the val ues var ied from 1.49 eV to 1.37 eV.The results of this work show that the third approach is better suited to pro duce single phase SnS films. However, a finer tunning of the duration of the high temperature plateau of the anneali ng profile is required in order to eliminate the -Sn top layer.
- Assessment of the potential of tin sulphide thin films prepared by sulphurization of metallic precursors as cell absorbersPublication . Malaquias, J.; Fernandes, P. A.; Salomé, P. M. P.; Cunha, A. F. daIn this work, SnxSy thin films have been grown on soda-lime glass substrates by sulphurization of metallic precursors in a nitrogen plus sulphur vapour atmosphere. Different sulphurization temperatures were tested, ranging from 300 °C to 520 °C. The resulting phases were structurally investigated by X-Ray Diffraction and Raman spectroscopy. Composition was studied using Energy Dispersive Spectroscopy being then correlated with the sulphurization temperature. Optical measurements were performed to obtain transmittance and reflectance spectra, from which the energy band gaps, were estimated. The values obtained were 1.17 eV for the indirect transition and for the direct transition the values varied from 1.26 eV to 1.57 eV. Electrical characterization using Hot Point Probe showed that all samples were p-type semiconductors. Solar cells were built using the structure: SLG/Mo/SnxSy/CdS/ZnO:Ga and the best result for solar cell efficiency was 0.17%.
- Comparison of fluctuating potentials and donor-acceptor pair transitions in a Cu-poor Cu2ZnSnS4 based solar cellPublication . Teixeira, J. P.; Sousa, R. A.; Sousa, M. G.; Cunha, A. F. da; Fernandes, P. A.; Salomé, P. M. P.; González, J. C.; Leitão, J. P.The structure of the electronic energy levels of a single phase Cu2ZnSnS4 film, as confirmed by Raman Scattering and x-ray diffraction, is investigated through a dependence on the excitation power of the photoluminescence (PL). The behavior of the observed asymmetric band, with a peak energy at ∼1.22 eV, is compared with two theoretical models: (i) fluctuating potentials and (ii) donor-acceptor pair transitions. It is shown that the radiative recombination channels in the Cu-poor film are strongly influenced by tail states in the bandgap as a consequence of a heavy doping and compensation levels. The contribution of the PL for the evaluation of secondary phases is also highlighted.
- Coupling of plasmonic nanoparticles on a semiconductor substrate via a modified discrete dipole approximation methodPublication . Carvalho, Diogo F.; Martins, Manuel A.; Fernandes, P. A.; Correia, M. Rosário P.Understanding the plasmonic coupling between a set of metallic nanoparticles (NPs) in a 2D array, and how a substrate affects such coupling, is fundamental for the development of optimized optoelectronic structures. Here, a simple semi-analytical procedure based on discrete dipole approximation (DDA) is reported to simulate the far-field and near-field properties of arrays of NPs, considering the coupling between particles, and the effect of the presence of a semiconductor substrate based on the image dipole approach. The method is validated for Ag NP dimers and single Ag NPs on a gallium nitride (GaN) substrate, a semiconductor widely used in optical devices, by comparison with the results obtained by the finite element method (FEM), indicating a good agreement in the weak coupling regime. Next, the method is applied to square and random arrays of Ag NPs on a GaN substrate. The increase in the surface density of NPs on a GaN substrate mainly results in a redshift of the dipolar resonance frequency and an increase in the near-field enhancement. This model, based on a single dipole approach, grants very low computational times, representing an advantage to predict the optical properties of large NP arrays on a semiconductor substrate for different applications.
- Cu(In,Ga)Se2 based ultrathin solar cells the pathway from lab rigid to large scale flexible technologyPublication . Lopes, T. S.; Teixeira, J. P.; Curado, M. A.; Ferreira, B. R.; Oliveira, A. J. N.; Cunha, J. M. V.; Monteiro, M.; Violas, A.; Barbosa, J. R. S.; Sousa, P. C.; Çaha, I.; Borme, J.; Oliveira, K.; Ring, J.; Chen, W. C.; Zhou, Y.; Takei, K.; Niemi, E.; Deepak, F. L.; Edoff, M.; Brammertz, G.; Fernandes, P. A.; Vermang, B.; Salomé, P. M. P.The incorporation of interface passivation structures in ultrathin Cu(In,Ga)Se2 based solar cells is shown. The fabrication used an industry scalable lithography technique—nanoimprint lithography (NIL)—for a 15 × 15 cm2 dielectric layer patterning. Devices with a NIL nanopatterned dielectric layer are benchmarked against electron-beam lithography (EBL) patterning, using rigid substrates. The NIL patterned device shows similar performance to the EBL patterned device.The impact of the lithographic processes in the rigid solar cells’ performance were evaluated via X-ray Photoelectron Spectroscopy and through a Solar Cell Capacitance Simulator. The device on stainless-steel showed a slightly lower performance than the rigid approach, due to additional challenges of processing steel substrates, even though scanning transmission electron microscopy did not show clear evidence of impurity diffusion. Notwithstanding, time-resolved photoluminescence results strongly suggested elemental diffusion from the flexible substrate. Nevertheless, bending tests on the stainless-steel device demonstrated the mechanical stability of the CIGS-based device.
- Cu(In,Ga)Se2 based ultrathin solar cells the pathway from lab rigid to large scale flexible technologyPublication . Lopes, T.S.; Teixeira, J. P.; Curado, M. A.; Ferreira, B. R.; Oliveira, A. J. N.; Cunha, J. M. V.; Monteiro, M.; Violas, A.; Barbosa, J. R. S.; Sousa, P. C.; Çaha, I.; Borme, J.; Oliveira, K.; Ring, J.; Chen, W. C.; Zhou, Y.; Takei, K.; Niemi, E.; Deepak, F. L.; Edoff, M.; Brammertz, G.; Fernandes, P. A.; Vermang, B.; Salomé, P. M. P.The incorporation of interface passivation structures in ultrathin Cu(In,Ga)Se2 based solar cells is shown. The fabrication used an industry scalable lithography technique—nanoimprint lithography (NIL)—for a 15 × 15 cm2 dielectric layer patterning. Devices with a NIL nanopatterned dielectric layer are benchmarked against electron-beam lithography (EBL) patterning, using rigid substrates. The NIL patterned device shows similar performance to the EBL patterned device.The impact of the lithographic processes in the rigid solar cells’ performance were evaluated via X-ray Photoelectron Spectroscopy and through a Solar Cell Capacitance Simulator. The device on stainless-steel showed a slightly lower performance than the rigid approach, due to additional challenges of processing steel substrates, even though scanning transmission electron microscopy did not show clear evidence of impurity diffusion. Notwithstanding, time-resolved photoluminescence results strongly suggested elemental diffusion from the flexible substrate. Nevertheless, bending tests on the stainless-steel device demonstrated the mechanical stability of the CIGS-based device.
- Cu2ZnSnS4 absorber layers obtained through sulphurization of metallic precursors: Graphite box versus sulphur fluxPublication . Sousa, M. G.; Cunha, A. F. da; Fernandes, P. A.; Teixeira, J. P.; Leitão, J. P.; Salomé, P. M. P.In this work we employed a hybrid method, combining RF-magnetron sputtering with evaporation, for the deposition of tailor made metallic precursors, with varying number of Zn/Sn/Cu (ZTC) periods and compared two approaches to sulphurization. Two series of samples with 1×, 2× and 4× ZTC periods have been prepared. One series of precursors was sulphurized in a tubular furnace directly exposed to a sulphur vapour and N2+5% H2 flux at a pressure of 5.0×10+4 Pa. A second series of identical precursors was sulphurized in the same furnace but inside a graphite box where sulphur pellets have been evaporated again in the presence of N2+5% H2 and at the same pressure as for the sulphur flux experiments. The morphological and chemical analyses revealed a small grain structure but good average composition for all three films sulphurized in the graphite box. As for the three films sulphurized in sulphur flux grain growth was seen with the increase of the number of ZTC periods whilst, in terms of composition, they were slightly Zn poor. The films' crystal structure showed that Cu2ZnSnS4 is the dominant phase. However, in the case of the sulphur flux films SnS2 was also detected. Photoluminescence spectroscopy studies showed an asymmetric broad band emission whichoccurs in the range of 1–1.5 eV. Clearly the radiative recombination efficiency is higher in the series of samples sulphurized in sulphur flux. We have found that sulphurization in sulphur flux leads to better film morphology than when the process is carried out in a graphite box in similar thermodynamic conditions. Solar cells have been prepared and characterized showing a correlation between improved film morphology and cell performance. The best cells achieved an efficiency of 2.4%.
- Cu2ZnSnS4 solar cells prepared with sulphurized dc-sputtered stacked metallic precursorsPublication . Fernandes, P. A.; Salomé, P. M. P.; Cunha, A. F. da; Schubert, Björn-ArvidIn the present work we report the details of the preparation and characterization results of Cu2ZnSnS4 (CZTS) based solar cells. The CZTS absorber was obtained by sulphurization of dc magnetron sputtered Zn/Sn/Cu precursor layers. The morphology, composition and structure of the absorber layer were studied by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and Raman scattering. The majority carrier type was identified via a hot point probe analysis. The hole density, space charge region width and band gap energy were estimated from the external quantum efficiency measurements. A MoS2 layer that formed during the sulphurization process was also identified and analyzed in this work. The solar cells had the following structure: soda lime glass/Mo/CZTS/CdS/i-ZnO/ZnO:Al/Al grid. The best solar cell showed an opencircuit voltage of 345 mV, a short-circuit current density of 4.42 mA/cm2, a fill factor of 44.29% and an efficiency of 0.68% under illumination in simulated standard test conditions: AM 1.5 and 100 mW/cm2.