retired scientist, former Head of Dept. Optical materials, Academy of Sciences of the Czech Republic, Prague. Present: consulting - optics, photovoltaics, materials research
ABSTRACT We evaluate the potential of three-dimensional (3D) thin-film silicon solar cells in the... more ABSTRACT We evaluate the potential of three-dimensional (3D) thin-film silicon solar cells in the superstrate configuration deposited on hexagonally ordered arrays of ZnO nanocolumns (NCs). These nanostructures are prepared by hydrothermal growth, which is an effective and versatile method to obtain ZnO NCs of high optical and electrical quality at low temperature. For the periods P investigated, varied between 0.9 and 1.4 μm, 3D solar cells based on hydrogenated amorphous silicon (a-Si:H) exhibit a photocurrent (JSC) boost in the red wavelength range as compared to flat cells; this JSC gain (by more than 1.5 mA cm−2 for P = 0.9 μm) is explained mostly by the increased effective optical thickness of the absorber layer grown on the vertical walls of the NCs. Combining this 3D concept with randomly textured interfaces, rigorous 3D optical simulations based on the finite element method predict that photocurrents significantly higher than those obtained with state-of-the-art substrates (up to 20 mA cm−2) are within reach, if the experimental obstacles specific for 3D design are overcome.
Characteristic features in photocurrent (PC) and electron paramagnetic resonance (EPR) spectra ar... more Characteristic features in photocurrent (PC) and electron paramagnetic resonance (EPR) spectra are discussed and attributed to main defects in the gap of optical-quality chemical vapor deposited diamond. A shoulder in the PC spectra with an onset at about 2.2 eV is attributed to the single-substitutional nitrogen defect (EPR P1 resonance at g=2.0024). A second feature in the PC spectra with an onset of about 1.3 eV is observed on ``as-grown'' samples with a hydrogen terminated surface. The defect level associated with this feature is hydrogen related, and this defect disappears after oxidation of the diamond sample surface. The EPR g=2.0028, which was also suggested to be H-related, is discussed.
Recent Constant Photocurrent Method (CPM) results and a new model of a-Si:H density of states are... more Recent Constant Photocurrent Method (CPM) results and a new model of a-Si:H density of states are briefly reviewed. The small but systematic changes of subgap absorption (α) with the applied external stress have been found. When light-soaking is done under the applied stress the strong increase of α is observed. The μτ (mobility-lifetime) versus α duality is demonstrated. The increase of the dangling bond density, combined with the change of its charge state, is used for the explanation of duality. The light induced creation of dangling bond-impurity intimate pairs is speculated to be the driving force for the Fermi level shift.
We measure subgap absorption on n-type amorphous silicon using the “absolute” constant photocurre... more We measure subgap absorption on n-type amorphous silicon using the “absolute” constant photocurrent method. We find that for typical monochromator probe beam intensities the measurement is not significantly influenced by lifetime changes. When the measurement is performed under light bias, an apparent increase in the defect absorption coefficient is observed, but no change in the photoexcitation threshold or spectral shape of the absorption band is seen. We show that this increase is likely due to a bias-light amplification of spectrally dependent lifetime changes. Our measurements suggest a larger electron capture cross section of positive valence band tail states compared to neutral dangling bonds.
ABSTRACT We evaluate the potential of three-dimensional (3D) thin-film silicon solar cells in the... more ABSTRACT We evaluate the potential of three-dimensional (3D) thin-film silicon solar cells in the superstrate configuration deposited on hexagonally ordered arrays of ZnO nanocolumns (NCs). These nanostructures are prepared by hydrothermal growth, which is an effective and versatile method to obtain ZnO NCs of high optical and electrical quality at low temperature. For the periods P investigated, varied between 0.9 and 1.4 μm, 3D solar cells based on hydrogenated amorphous silicon (a-Si:H) exhibit a photocurrent (JSC) boost in the red wavelength range as compared to flat cells; this JSC gain (by more than 1.5 mA cm−2 for P = 0.9 μm) is explained mostly by the increased effective optical thickness of the absorber layer grown on the vertical walls of the NCs. Combining this 3D concept with randomly textured interfaces, rigorous 3D optical simulations based on the finite element method predict that photocurrents significantly higher than those obtained with state-of-the-art substrates (up to 20 mA cm−2) are within reach, if the experimental obstacles specific for 3D design are overcome.
Characteristic features in photocurrent (PC) and electron paramagnetic resonance (EPR) spectra ar... more Characteristic features in photocurrent (PC) and electron paramagnetic resonance (EPR) spectra are discussed and attributed to main defects in the gap of optical-quality chemical vapor deposited diamond. A shoulder in the PC spectra with an onset at about 2.2 eV is attributed to the single-substitutional nitrogen defect (EPR P1 resonance at g=2.0024). A second feature in the PC spectra with an onset of about 1.3 eV is observed on ``as-grown'' samples with a hydrogen terminated surface. The defect level associated with this feature is hydrogen related, and this defect disappears after oxidation of the diamond sample surface. The EPR g=2.0028, which was also suggested to be H-related, is discussed.
Recent Constant Photocurrent Method (CPM) results and a new model of a-Si:H density of states are... more Recent Constant Photocurrent Method (CPM) results and a new model of a-Si:H density of states are briefly reviewed. The small but systematic changes of subgap absorption (α) with the applied external stress have been found. When light-soaking is done under the applied stress the strong increase of α is observed. The μτ (mobility-lifetime) versus α duality is demonstrated. The increase of the dangling bond density, combined with the change of its charge state, is used for the explanation of duality. The light induced creation of dangling bond-impurity intimate pairs is speculated to be the driving force for the Fermi level shift.
We measure subgap absorption on n-type amorphous silicon using the “absolute” constant photocurre... more We measure subgap absorption on n-type amorphous silicon using the “absolute” constant photocurrent method. We find that for typical monochromator probe beam intensities the measurement is not significantly influenced by lifetime changes. When the measurement is performed under light bias, an apparent increase in the defect absorption coefficient is observed, but no change in the photoexcitation threshold or spectral shape of the absorption band is seen. We show that this increase is likely due to a bias-light amplification of spectrally dependent lifetime changes. Our measurements suggest a larger electron capture cross section of positive valence band tail states compared to neutral dangling bonds.
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Papers by Milan Vanecek