ABSTRACT Thin-film solar cells with a Cu-based chalcopyrite absorber achieve high conversion effi... more ABSTRACT Thin-film solar cells with a Cu-based chalcopyrite absorber achieve high conversion efficiencies (up to 20%). Their technology being more cost effective than the crystalline silicon technologies, they are expected to replace Si-based solar cells. But a best cost-performance ratio requires first a knowledge of the parameters which ascertain the electrical quality of the solar cell. The first of them is the minority carrier diffusion length in the absorber and the second one is the collection efficiency of the p–n junction space charge region (SCR) located within the absorber. A low value of at least one of them drastically reduces the efficiency of the cell. In this paper we present an electron-beam-induced-current (EBIC) determination of these two parameters in CIS solar cells.
In this work, single layers of TCO thin-film antireflection coatings over GaSb substrates have be... more In this work, single layers of TCO thin-film antireflection coatings over GaSb substrates have been computer-designed for their use with thermophotovoltaic cells. The optimal parameters of the thin film TCO layers over GaSb TPV cell structures, i.e., optimal thickness, refractive index and also the reflectivity of the whole structure as a function of the wavelength have been first theoretically determined
ABSTRACT One of the limitations of online quality control processes is the inability of detecting... more ABSTRACT One of the limitations of online quality control processes is the inability of detecting electronic defects in semiconductor materials. This is particularly important in low cost processes such as electrodeposited Cu(In,Ga)(S,Se)2 solar cells, due to the large number of inhomogeneities that can be expected. Standard techniques for characterizing electronic defects are not suitable for online control. However, optical techniques such as Raman scattering can be specially suited for online process monitoring applications. In this paper we analyze the correlation between the Raman spectral features and the presence of electronic defects. Our results confirm the potential of Raman spectroscopy for the indirect detection of point defects which are important for device performance.
ABSTRACT Cu (In1-x,Ga-x) S-2 was studied using photoreflectance spectroscopy. In this study, effo... more ABSTRACT Cu (In1-x,Ga-x) S-2 was studied using photoreflectance spectroscopy. In this study, efforts are devoted to optimizing PR set-up for measuring CIGS grown by electrodeposition: issues such as photoluminescence perturbation, high roughness and scattering are addressed. Dual frequency photoreflectance, where both probe and pump beams are modulated, is proposed here to over come the poor signal to noise ratio. Considering the low electric field regime, material parameters are extracted by employing the third derivative functional form of dielectric functions to fit data.The reliability of the technique is finally tested by measuring PR spectra on a specific 15 x 15cm(2) wafer and explanations of PR line-shape evolution on this wafer are discusse.
ABSTRACT Using structural analyses means of ex-situ Raman spectroscopy and X-ray diffraction comb... more ABSTRACT Using structural analyses means of ex-situ Raman spectroscopy and X-ray diffraction combined with electrical measurements, we study the phase evolution in the growth by electrodeposition technique of CuInSe2 on polycrystalline Mo. For this purpose the growth was stopped at different stages, and then the different layers were analysed. First growth steps seem to be controlled by the deposition of secondary phases, like elemental Se and Cu2Se binary. After the deposition of approximately 300 nm of material, CuInSe2 ternary and ordered vacancy compounds start to adequately form. At a thickness close to 2000 nm, the formation of binary CuxSe is observed, remaining up to the final growth process (4350 nm). All these results are compared with the kinetic model of the system under the consideration of the experimental composition evolution.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2004
ABSTRACT Thermophotovoltaic (TPV) cells convert the energy from non-solar thermal sources into el... more ABSTRACT Thermophotovoltaic (TPV) cells convert the energy from non-solar thermal sources into electricity. Therefore, low bandgap semiconductor materials are necessary for TPV cells. Materials based on GaSb cells and on ternary and quaternary antimonide compounds are being investigated as candidates for use in the next generation of TPV systems. We grew GaSb/AlxGa1−xSb thin films on single crystalline GaSb substrates using Metal-Organic Vapour Phase Epitaxy performed in an industrial size reactor at 848 K, with a V/III ratio of 1–2. Some of the films include several layers, each with a different Al content. RBS of the films was performed at different angles of incidence, from normal to grazing incidence. The stoichiometry and thickness of the deposited layers was determined, and the dependence of Al incorporation on the orientation of the GaSb crystal was investigated. The density of the layers was determined.
CdTe films were deposited by closed space sublimation from different CdTe:Bi targets (from non-do... more CdTe films were deposited by closed space sublimation from different CdTe:Bi targets (from non-doped up to 0.16 at.%) previously sintered by the Bridgman method. X-ray diffraction measurements demonstrate that CdTe films are formed and Bi is incorporated. Electrical and optical characterizations show that thin films reproduce the bulk material behaviour with a decrease in resistivity and an increase in photoconductivity.
ABSTRACT Transparent conducting oxides (TCOs) are evaluated as selective filters in thermophotovo... more ABSTRACT Transparent conducting oxides (TCOs) are evaluated as selective filters in thermophotovoltaic (TPV) devices using theoretical considerations. The TCOs plasma frequency and the optical properties in the near infrared are calculated using the Drude theory for free carriers. To improve the radiation transmission for energies higher than the band gap of the cells and the reflection back to the emitter for sub-band gap radiation the TCO's optical properties are studied as a function of electron concentration. Taking into account the photocell and emitter spectral response radiation, the optimal free carrier concentration and the thus thickness of the TCO to be used as selective filter are calculated.
We show that the Photo-Acoustic Spectroscopy (PAS) is an useful alternative method for the determ... more We show that the Photo-Acoustic Spectroscopy (PAS) is an useful alternative method for the determination of the optical-absorption coefficient of CdTe thin films, in the spectral region near to the fundamental absorption edge, ranging from 1.0 eV to 2.4 eV, using an open cell in the transmission configuration. We applied this method to the optical characterization of CdTe layers for several values
CdTe:Bi crystals were grown by the vertical Bridgman method, varying the nominal Bi-dopant concen... more CdTe:Bi crystals were grown by the vertical Bridgman method, varying the nominal Bi-dopant concentration in the range of 1×1017–1×1019at/cm3. Bi and Bi1.8−2.3Te precipitates are the most characteristic structural defects in these crystals. Raman spectroscopy studies have shown that Bi traps Te from the CdTe host lattice. Low-temperature photoluminescence in the 1.2–1.6eV energy region is presented. Several new emissions related to
CdTe pure and Bi-doped crystals were grown by the Markov method using an insulating element betwe... more CdTe pure and Bi-doped crystals were grown by the Markov method using an insulating element between the ampoule and the furnace. The numerical simulation performed shows that the presence of a graphite element causes a diminution of the axial temperature gradient, reducing the natural convection inside the growth chamber. This leads to a more stable growth process, improving the crystal
Defect levels in CdTe doped with Bi are studied by low temperature photoluminescence, photoinduce... more Defect levels in CdTe doped with Bi are studied by low temperature photoluminescence, photoinduced current transient spectroscopy, photoconductivity measurements, and optical absorption. Two centers associated with the doping with Bi are reported. The first one, a deep level ...
ABSTRACT Thin-film solar cells with a Cu-based chalcopyrite absorber achieve high conversion effi... more ABSTRACT Thin-film solar cells with a Cu-based chalcopyrite absorber achieve high conversion efficiencies (up to 20%). Their technology being more cost effective than the crystalline silicon technologies, they are expected to replace Si-based solar cells. But a best cost-performance ratio requires first a knowledge of the parameters which ascertain the electrical quality of the solar cell. The first of them is the minority carrier diffusion length in the absorber and the second one is the collection efficiency of the p–n junction space charge region (SCR) located within the absorber. A low value of at least one of them drastically reduces the efficiency of the cell. In this paper we present an electron-beam-induced-current (EBIC) determination of these two parameters in CIS solar cells.
In this work, single layers of TCO thin-film antireflection coatings over GaSb substrates have be... more In this work, single layers of TCO thin-film antireflection coatings over GaSb substrates have been computer-designed for their use with thermophotovoltaic cells. The optimal parameters of the thin film TCO layers over GaSb TPV cell structures, i.e., optimal thickness, refractive index and also the reflectivity of the whole structure as a function of the wavelength have been first theoretically determined
ABSTRACT One of the limitations of online quality control processes is the inability of detecting... more ABSTRACT One of the limitations of online quality control processes is the inability of detecting electronic defects in semiconductor materials. This is particularly important in low cost processes such as electrodeposited Cu(In,Ga)(S,Se)2 solar cells, due to the large number of inhomogeneities that can be expected. Standard techniques for characterizing electronic defects are not suitable for online control. However, optical techniques such as Raman scattering can be specially suited for online process monitoring applications. In this paper we analyze the correlation between the Raman spectral features and the presence of electronic defects. Our results confirm the potential of Raman spectroscopy for the indirect detection of point defects which are important for device performance.
ABSTRACT Cu (In1-x,Ga-x) S-2 was studied using photoreflectance spectroscopy. In this study, effo... more ABSTRACT Cu (In1-x,Ga-x) S-2 was studied using photoreflectance spectroscopy. In this study, efforts are devoted to optimizing PR set-up for measuring CIGS grown by electrodeposition: issues such as photoluminescence perturbation, high roughness and scattering are addressed. Dual frequency photoreflectance, where both probe and pump beams are modulated, is proposed here to over come the poor signal to noise ratio. Considering the low electric field regime, material parameters are extracted by employing the third derivative functional form of dielectric functions to fit data.The reliability of the technique is finally tested by measuring PR spectra on a specific 15 x 15cm(2) wafer and explanations of PR line-shape evolution on this wafer are discusse.
ABSTRACT Using structural analyses means of ex-situ Raman spectroscopy and X-ray diffraction comb... more ABSTRACT Using structural analyses means of ex-situ Raman spectroscopy and X-ray diffraction combined with electrical measurements, we study the phase evolution in the growth by electrodeposition technique of CuInSe2 on polycrystalline Mo. For this purpose the growth was stopped at different stages, and then the different layers were analysed. First growth steps seem to be controlled by the deposition of secondary phases, like elemental Se and Cu2Se binary. After the deposition of approximately 300 nm of material, CuInSe2 ternary and ordered vacancy compounds start to adequately form. At a thickness close to 2000 nm, the formation of binary CuxSe is observed, remaining up to the final growth process (4350 nm). All these results are compared with the kinetic model of the system under the consideration of the experimental composition evolution.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2004
ABSTRACT Thermophotovoltaic (TPV) cells convert the energy from non-solar thermal sources into el... more ABSTRACT Thermophotovoltaic (TPV) cells convert the energy from non-solar thermal sources into electricity. Therefore, low bandgap semiconductor materials are necessary for TPV cells. Materials based on GaSb cells and on ternary and quaternary antimonide compounds are being investigated as candidates for use in the next generation of TPV systems. We grew GaSb/AlxGa1−xSb thin films on single crystalline GaSb substrates using Metal-Organic Vapour Phase Epitaxy performed in an industrial size reactor at 848 K, with a V/III ratio of 1–2. Some of the films include several layers, each with a different Al content. RBS of the films was performed at different angles of incidence, from normal to grazing incidence. The stoichiometry and thickness of the deposited layers was determined, and the dependence of Al incorporation on the orientation of the GaSb crystal was investigated. The density of the layers was determined.
CdTe films were deposited by closed space sublimation from different CdTe:Bi targets (from non-do... more CdTe films were deposited by closed space sublimation from different CdTe:Bi targets (from non-doped up to 0.16 at.%) previously sintered by the Bridgman method. X-ray diffraction measurements demonstrate that CdTe films are formed and Bi is incorporated. Electrical and optical characterizations show that thin films reproduce the bulk material behaviour with a decrease in resistivity and an increase in photoconductivity.
ABSTRACT Transparent conducting oxides (TCOs) are evaluated as selective filters in thermophotovo... more ABSTRACT Transparent conducting oxides (TCOs) are evaluated as selective filters in thermophotovoltaic (TPV) devices using theoretical considerations. The TCOs plasma frequency and the optical properties in the near infrared are calculated using the Drude theory for free carriers. To improve the radiation transmission for energies higher than the band gap of the cells and the reflection back to the emitter for sub-band gap radiation the TCO's optical properties are studied as a function of electron concentration. Taking into account the photocell and emitter spectral response radiation, the optimal free carrier concentration and the thus thickness of the TCO to be used as selective filter are calculated.
We show that the Photo-Acoustic Spectroscopy (PAS) is an useful alternative method for the determ... more We show that the Photo-Acoustic Spectroscopy (PAS) is an useful alternative method for the determination of the optical-absorption coefficient of CdTe thin films, in the spectral region near to the fundamental absorption edge, ranging from 1.0 eV to 2.4 eV, using an open cell in the transmission configuration. We applied this method to the optical characterization of CdTe layers for several values
CdTe:Bi crystals were grown by the vertical Bridgman method, varying the nominal Bi-dopant concen... more CdTe:Bi crystals were grown by the vertical Bridgman method, varying the nominal Bi-dopant concentration in the range of 1×1017–1×1019at/cm3. Bi and Bi1.8−2.3Te precipitates are the most characteristic structural defects in these crystals. Raman spectroscopy studies have shown that Bi traps Te from the CdTe host lattice. Low-temperature photoluminescence in the 1.2–1.6eV energy region is presented. Several new emissions related to
CdTe pure and Bi-doped crystals were grown by the Markov method using an insulating element betwe... more CdTe pure and Bi-doped crystals were grown by the Markov method using an insulating element between the ampoule and the furnace. The numerical simulation performed shows that the presence of a graphite element causes a diminution of the axial temperature gradient, reducing the natural convection inside the growth chamber. This leads to a more stable growth process, improving the crystal
Defect levels in CdTe doped with Bi are studied by low temperature photoluminescence, photoinduce... more Defect levels in CdTe doped with Bi are studied by low temperature photoluminescence, photoinduced current transient spectroscopy, photoconductivity measurements, and optical absorption. Two centers associated with the doping with Bi are reported. The first one, a deep level ...
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Papers by Carmen Ruiz