W.G.J.H.M. van Sark

Luminescent Solar Concentrators - A review of recent results

Luminescent solar concentrators (LSCs) generally consist of transparent polymer sheets doped with luminescent species. Incident sunlight is absorbed by the luminescent species and emitted with high quantum efficiency, such that emitted light is trapped in the sheet and travels to the edges where it can be collected by solar cells. LSCs offer potentially lower cost per Wp. This paper reviews results mainly obtained within the framework of the Full-spectrum project. Two modeling approaches are presented, i.e., a thermodynamic and a ray-trace one, as well as experimental results, with a focus on LSC stability.

Upconverter solar cells: materials and applications

Spectral conversion of sunlight is a promising route to reduce spectral mismatch losses that are responsible for the major part of the efficiency losses in solar cells. Both upconversion and downconversion materials are presently explored. In an upconversion process, photons with an energy lower than the band gap of the solar cell are converted to higher energy photons. These higher photons are directed back to the solar cell and absorbed, thus increasing the efficiency. Different types of upconverter materials are investigated, based on luminescent ions or organic molecules. Proof of principle experiments with lanthanide ion based upconverters have indicated that the benefit of an upconversion layer is limited by the high light intensities needed to reach high upconversion quantum efficiencies. To address this limitation, upconverter materials may be combined with quantum dots or plasmonic particles to enhance the upconversion efficiency and improve the feasibility of applying upconverters in commercial solar cells.

Fluorescence lifetime imaging in scanning microscopes: acquisition speed, photon economy and lifetime resolution

In this paper a detailed discussion is presented of the factors that affect the fluorescence lifetime imaging performance of a scanning microscope equipped with a single photon counting based, two‐ to eight‐channel, time‐gated detection system. In particular we discuss the sensitivity, lifetime resolution, acquisition speed, and the shortest lifetimes that can be measured. Detection systems equipped with four to eight time‐gates are significantly more sensitive than the two time‐gate system. Only minor sensitivity differences were found between systems with four or more time‐gates. Experiments confirm that the lifetime resolution is dominated by photon statistics. The time response of the detector determines the shortest lifetimes that can be resolved; about 25 ps for fast MCP‐PMTs and 300–400 ps for other detectors. The maximum count rate of fast MCP‐PMTs, however, is 10–100 times lower than that of fast PMTs. Therefore, the acquisition speed with MCP‐PMT based systems is limited. With a fast PMT operated close to its maximum count rate we were able to record a fluorescence lifetime image of a beating myocyte in less than one second.

A self-consistent fluid model for radio-frequency discharges in SiH4–H2 compared to experiments

A one-dimensional fluid model for radio-frequency glow discharges is presented which describes silane/hydrogen discharges that are used for the deposition of amorphous silicon (a-Si:H). The model is used to investigate the relation between the external settings (such as pressure, gas inlet, applied power, and frequency) and the resulting composition of the gas and the deposition rate. In the model, discharge quantities such as the electric field, densities, and fluxes of the particles are calculated self-consistently. Look-up tables of the rates of the electron impact collisions as a function of the average electron energy are obtained by solving the Boltzmann equation in a two term approximation for a sequence of values of the reduced electric field. These tables are updated as the composition of the background neutral gas evolves under the influence of chemical reactions and pumping. Pumping configuration and gas inlet are taken into account by adding source terms in the density balance equations. The e...

Modeling improvement of spectral response of solar cells by deployment of spectral converters containing semiconductor nanocrystals

A planar converter containing quantum dots as wavelength-shifting moieties on top of a solar cell was studied. The highly efficient quantum dots are to shift the wavelengths where the spectral response of the solar cell is low to wavelengths where the spectral response is high in order to improve the conversion efficiency of the solar cell. It was calculated that quantum dots with an emission at 603 nm increase the multicrystalline solar cell short-circuit current by nearly 10%. Simulation results for planar converters on hydrogenated amorphous silicon solar cells show no beneficial effects, due to the high spectral response at low wavelength.

Enhancement of solar cell performance by employing planar spectral converters

The effect on solar cell performance of planar converters containing quantum dots (QDs) as wavelength-shifting entities on top of multicrystalline silicon cells was investigated by means of model studies with varying incident spectra. These included global, direct, and diffuse spectra with Air Mass (AM) values ranging from 1 to 10. In case of AM1.5, a planar converter with QDs emitting at 603nm yields a short-circuit current increase of 6.3%, 9.6%, and 28.6% for direct, global, and diffuse irradiation, respectively, as a result of the larger blue/green content of diffuse spectra with respect to direct and global ones. For other AM values, similar results are calculated, with a lower increase toward high AM values.

Local structure and bonding states in a‐Si1−xCx:H

Infrared spectra of a‐Si1−xCx:H deposited in a glow discharge of a silane/methane mixture have been measured. Comparison with elastic recoil detection and Rutherford backscattering spectrometry shows that the mean number of hydrogen atoms attached to silicon per silicon atom ([HSi]/[Si]) increases with higher carbon content and that more Si—H2 bonding configurations are formed. Hydrogen is preferentially bonded in (Si—H2)n clusters, which partly explains the observed apparent shift of the Si—H stretching mode to higher energy. The remaining contribution to this shift is believed to result from Si—H on surfaces of voids instead of an inductive effect. From composition measurements we observe that for each carbon atom, three hydrogen atoms are incorporated in the material, suggesting that during deposition carbon is initially incorporated in CH3 groups. However, the mean number of C—H bonds per carbon atom decreases from about 2.2±0.4 to 1.4±0.3 with increasing carbon content, indicating that the majority o...

A new method for the evaluation of solar cell parameters

Abstract A new method is presented that is capable of resolving the parameters in a double-exponential model with which the electrical characteristics of a crystalline-silicon solar cell are analysed. This method gives not only open-circuit voltage, short-circuit current, fill factor and efficiency, but also diode saturation currents, light-generated current, series resistance and shunt resistance, all from one measurement under AM 1 illimination. The experimental set-up used for I-V measurement and automated data handling is described. A fast computer fit procedure is introduced which resolves all parameters from one measurement. The errors in the parameter values obtained are studied. A comparison of these values for a number of I-V measurements of solar cells with different internal physical properties is given, in order to illustrate the utility of the method for unravelling various electrical processes in a solar cell.

Structural properties of a-Si:H related to ion energy distributions in VHF silane deposition plasmas

Abstract We present measurements on typical silane-hydrogen RF/VHF deposition plasmas and the corresponding a-Si:H films deposited from these plasmas. A range of process settings was used, covering both the α and the γ′ regime of the discharge. Mass resolved ion energy distributions were measured at the grounded electrode to determine the ion flux at the growing surface. Although the main precursors are radicals, in the lower pressure α regime the flux of ions towards the surface can account for at least 10% of the observed growth rate. In the γ′ regime this contribution to the growth of the film is less. We measured internal stress, hydrogen concentration, hydrogen bonding configuration, and refractive index to determine the effects of the ion bombardment on the structure of the deposited a-Si:H films. Good structural properties, i.e. a refractive index of about 4.25 at 600 nm and a minimum number of SiH2 bonds, are found above a threshold energy of 5 eV per deposited atom. This observation is explained in terms of knock-on processes of the deposited atoms by ions and an increased mobility of the growth precursors at the surface. Both these processes promote the formation of a dense film.

Luminescence quenching in erbium‐doped hydrogenated amorphous silicon

Hydrogenated amorphous silicon thin films are doped with erbium by ion implantation. Room‐temperature photoluminescence at 1.54 μm, due to an intra‐4f transition in Er4+, is observed after thermal annealing at 300–400 °C. Excitation of Er3+ is shown to be mediated by photocarriers. The Er3+ luminescence intensity is quenched by a factor of 15 as the temperature is raised from 10 K to room temperature. Codoping with oxygen (1 at. %) reduces the luminescence quenching to a factor of 7. The quenching is well correlated with a decrease in luminescence lifetime, indicating that nonradiative decay of excited Er3+ is the dominant quenching mechanism as the temperature is increased.