Transferable III-V thin films, combined with light trapping structures, present several interests for photovoltaics: cost, material usage and weight reduction, flexible devices… To obtain such films, remote epitaxy consists in growing above a graphene covered III-V substrate, providing detachable mono-crystals. We report the fabrication of large-area graphene/GaAs substrates by a metal-assisted dry transfer with a high yield (<95%), reduced damage to the lattice, negligible doping, and stress relaxation. After the optimization of chemical etching steps, XPS reveals a residue-free surface with low oxidation levels compared to conventional transfers. Nucleation studies using MBE resulted in the formation of microcrystals, with partial alignment with the underlying GaAs(001).
In photovoltaic devices, thermalization of hot carriers generated by high energy photons is one of the major loss mechanisms, which limits the power conversion efficiency of solar cells. Hot carrier solar cells are proposed to increase the efficiency of this technology by suppressing phonon-mediated thermalization channels and extracting hot carriers isentropically. Therefore, designing hot carrier absorbers, which can inhibit electron-phonon interactions and provide conditions for the re-absorption of the energy of non-equilibrium phonons by (hot) carriers, is of significant importance in such devices. As a result, it is essential to understand hot carrier relaxation mechanisms via phonon-mediated pathways in the system. In this work, the properties of photo-generated hot carriers in an InGaAs multi-quantum well structure are studied via steady-state photoluminescence spectroscopy at various lattice temperatures and excitation powers. It is observed that by considering the contribution of thermalized power above the absorber band edge, it is possible to evaluate hot carrier thermalization mechanisms via determining the thermalization coefficient of the sample. It is seen that at lower lattice temperatures, the temperature difference between hot carriers and the lattice reduces, which is consistent with the increase of the quasi-Fermi level splitting for a given thermalized power at lower lattice temperatures. Finally, the spectral linewidth broadening of multiple optical transitions in the QW structure as a function of the thermalized power is investigated.
This paper describes a study of perimeter recombination on millimeter-scale multijunction photovoltaic cells using luminescence characterizations and a quasi-3D PV modeling. Device performance of controlled GaAs 1J cells shows an obvious perimeter dependency. By contrast, the performance of controlled InGaP 1J cells is less dependent on perimeters. Corresponding InGaP/GaAs 2J cells are assessed whose results exhibit combined contributions from both subcells. Luminescence method using current transport efficiency mapping and the PV modeling are able to reveal the influence of perimeter recombination and series resistance effect for individual subcells.
Photoluminescence spectroscopy is a powerful technique to investigate the properties of photo-generated hot carriers in materials in steady state conditions. Hot carrier temperature can be determined via fitting the emitted PL spectrum with the generalized Planck’s law. However, this analysis is not trivial, especially for nanostructured materials, such as quantum wells, with a modified density of states due to quantum confinement effects. Here, we present comprehensively different methods to determine carrier temperature via fitting the emitted PL spectrum with the generalized Planck’s law and discuss under what conditions it is possible to simplify the analysis.
This theoretical study sheds light on questions raised by inter-subband transition in quantum dot intermediate band solar cells. Based on a dedicated analytical model that correctly treats, from a quantum point-of-view, the trade-off between the absorption, the recombination and the electronic transport, we clearly show that it is essential to control the transit rate between the excited state of the quantum dot and the embedding semiconductor with a tunnel barrier. Such a barrier, matching the recombination and the tunnel rates, allows to strongly improve the current. On the other hand, by better controlling the retrapping, such a barrier can also improve the voltage. Finally this work, by giving a framework to design efficient inter-subband transitions, opens new opportunities for quantum dot intermediate-band solar cells.
Hot-carrier solar cells (HCSC) can potentially overcome the Shockley-Queisser limit, by having carriers at a higher temperature than the lattice. To this end, the carriers need to thermalize slower than power is generated by absorbing photons. In thin films, a hot-carrier distribution can only be achieved with very high incident power, by saturating the thermalization channels. Ultra-thin absorbers have a smaller thermalization rate, due to fewer channels. However, they typically absorb only a limited amount of light, which prevents them from reaching high efficiencies. Light trapping is an excellent way to increase significantly the amount of light absorbed in an ultra-thin material. Yet, studies on the coupling between light trapping and hot carriers are still lacking, due to the complexity of the whole system. We analyze numerically and experimentally how light trapping can enable high-efficiency HCSC. This manuscript presents the progress towards the experimental demonstration of the enhancement of the hot-carrier effect with light trapping. 280 nm-thick devices have successfully been reported on a gold mirror using epitaxial lift-off (ELO) and gold-gold bonding. These devices have been characterized by photoluminescence spectroscopy. Hot carriers with a temperature 37 K above lattice temperature were measured, in accordance with theoretical predictions. We are now working towards the ELO of absorbers 10 times thinner, on which we will implement light trapping to increase the carrier temperature.
InGaP top cell often limits the efficiency of multi-junction solar cells. Its efficiency is expected to increase by introducing strain-balanced In1-xGaxP/In1-yGayP multiple-quantum-wells since the concentration of photoexcited carriers into the wells enhances radiative recombination. To keep carrier collection efficiency to be similar to the case of a bulk InGaP cell, effective mobilities of carriers through the cascaded quantum wells are evaluated. The critical mobilities for maintaining good carrier transport as a photovoltaic device are discussed, which provides a general design principle for the multiplequantum- well architecture.
The Intermediate Band Solar Cell is an advanced concept, which has been predicted to overcome the Shockley-Queisser limit, despite efficiencies remaining below the best single junctions so far. Practical realizations with nanostructures suffer from two intrinsic deficiencies: narrow absorption widths and low radiative efficiencies. We evaluate in this paper the theoretical efficiency expectations with respect to those two properties, and consider in addition the possibility of including an electronic ratchet. We observe that an intermediate band solar cell using a ratchet becomes highly tolerant to non-ideal nanostructures, so that any combination of low absorption and low radiative efficiency becomes compatible with optimized performances above the Shockley-Queisser limit. We conclude that future practical realization may take advantage of quantum wells, which have been less considered so far than quantum dots, due to relatively higher nonradiative recombination rates. Such realizations would take advantage of the higher absorption properties of quantum wells.
Multijunction solar cells are currently the devices offering the largest conversion efficiencies of the solar radiation, which could be further increased by limiting their series resistances. A clear evaluation of the impact of those resistances is therefore required, and provided in this paper by introducing a mapping method of the current transport efficiency from luminescence images. This method brings finer information on the cell than electroluminescence methods, widely used so far for multi-junction cells, and offers much faster acquisition time than what could be obtained with a light beam induced current setup. While it has been theoretically and experimentally developed for single junction solar cells, its application to multijunction cells remains to be demonstrated. The purpose of this communication is to assess its validity and to explain some results that can be counterintuitive at a first sight. Two different triple-junction architectures are investigated and successfully compared with electrical measurements and calculations.
Insertion of InGaAs/GaAsP strain-balanced multiple quantum wells (MQWs) into i-regions of GaAs p-i-n solar cells show several advantages against GaAs bulk p-i-n solar cells. Particularly under high-concentration sunlight condition, enhancement of the open-circuit voltage with increasing concentration ratio in thin-barrier MQW cells has been reported to be more apparent than that in GaAs bulk cells. However, investigation of the MQW cell mechanisms in terms of I-V characteristics under high-concentration sunlight suffers from the increase in cell temperature and series resistance. In order to investigate the mechanism of the steep enhancement of open-circuit voltage in MQW cells under high-concentration sunlight without affected by temperature, the quasi-Fermi level splitting was evaluated by analyzing electroluminescence (EL) from a cell. Since a cell under current injection with a density Jinjhas similar excess carrier density to a cell under concentrated sunlight with an equivalent short-circuit current Jsc = Jinj, EL measurement with varied Jinj can approximately evaluate a cell performance under a variety of concentration ratio. In addition to the evaluation of quasi-Fermi level splitting, the external luminescence efficiency was also investigated with the EL measurement. The MQW cells showed higher external luminescence efficiency than the GaAs reference cells especially under high-concentration condition. The results suggest that since the MQW region can trap and confine carriers, the localized excess carriers inside the cells make radiative recombination more dominant.
Having shown some demonstration of two photon absorption and multiband emission processes in quantum dots (QD), multiquantum wells (MQW), and highly mismatched alloys, intermediate band solar cells are currently the subject of numerous studies. To better understand the underlying mechanisms, our objective is to experimentally probe the multiband operation of this device. We used photoluminescence recorded with a calibrated hyperspectral imager which provides spectrally resolved images with a spatial resolution of 2 μm and spectral resolution of 2 nm on proof of concept QD and MQW solar cells samples. Device emission can be described with the generalized Planck’s law from which the quasi-Fermi level splitting of the three bands can be determined. The advantage of the technique is that it can be used to investigate the intermediate band material without the need to make contacts or a full device structure. We also discuss the usefulness of a dual-beam method.
Multi-Quantum well solar cells (MQWSC) have been shown to present several advantages, among which are low dark currents and tunable bandgaps. They are especially suited for implementation in multi-junction cells, and are highly promising for absorbers in Hot Carrier Solar Cells (HCSC). Such applications require high concentration ratio, which arises the issue of collection efficiency. Whereas it is usually considered that collection in MQW is very close to unity at one sun, it has been shown to not be the case under high concentration at the maximum power point. We propose in this work to take advantage of the luminescence spectral variation to investigate the depth collection efficiency. In order to validate the model, a series of strain compensated InGaAs/GaAsP MQW solar cells with intentional variation of the MQW doping concentration are grown. This has the effect of switching the space charge region position and width as well as the electric field intensity. Recording the luminescence spectra at various illumination intensities and applied voltages, we show that the in-depth quasi-Fermi level splitting and thus collection properties can be probed. Other measurements (EQE, luminescence intensity variation) are shown to be consistent with these results. Regarding their use as HCSC, the luminescence of MQW solar cells has been mainly used so far for investigating the quasi-Fermi level splitting and the temperature. Our results improve our understanding by adding information on carrier transport.
Light trapping enhancement is a major research field in photovoltaics. Scarce and expensive resources for
semiconductor material drive the research on light management in thin absorber layer. This paper reviews some of the
known techniques, from back reflector to nanophotonic technologies such as nanowires or plasmonic-enhanced
photovoltaic devices. Light trapping enhancement can reach ~100 fold and experimental demonstrations of device
exceeding the ray optics limits have been reported.
CIGS is a material showing high efficiencies in photovoltaic devices although numerous questions remain about its physical mechanisms. Among them is the influence of the polycrystalline nature on global efficiencies. In order to study the spatial fluctuations of the optoelectronic parameters, two original setups are developed. Firstly a Hyperspectral Imager is used to investigate spectrally resolved luminescence images. It is also possible to calibrate it in absolute values, which means that the signal is measured in number of photons. From photoluminescence measurement we deduce maps of the quasi-Fermi level splitting with variation of 30 meV. These results are compared with a more common confocal microscope, which shows that the carrier transport has to be taken into account for the interpretation of these experiments. Using electroluminescence and reciprocity relations, we calculate map of the External Quantum Efficiency with the Hyperspectral Imager. For this investigation a second setup is developed, using Light Beam Induced Current with different excitation wavelengths. Therefore mapping of the diffusion lengths is possible, exhibiting a distribution around 1.09 μm with standard deviation of 0.10 μm.
We analyze photoluminescence (PL) and electroluminescence (EL) of a GaAs solar cell using a hyperspectral imager that records spectrally resolved images. Thanks to the absolute calibration of the setup, we first investigate the reciprocity relations between solar cells and light-emitting diode and determine the external quantum efficiency from EL images. Spatial variations are observed due to series resistance effect that we can evaluate. Second, the PL experiment allows us to plot the recombination current at a given spatial location versus the quasi-Fermi level splitting at the same location. Under reasonable assumptions, this can be linked to the classical measurement of the short circuit current versus the open circuit voltage. Therefore we perform a contactless mapping of optoelectronic properties such as the saturation currents. The assumptions made in these experiments are discussed in order to correctly investigate polycrystalline solar cells in the future where strong lateral variations exist.
We analyze photoluminescence (PL) and electroluminescence (EL) using a hyperspectral imager that records spectrally
resolved luminescence images of a GaAs solar cell. Thanks to the absolute calibration, we first investigate the reciprocity
relations between Solar Cell and LED and determine the External Quantum Efficiency (EQE) from EL images for a
specific range of voltage. Spatial variations are observed due to series resistance effect that we can evaluate. Second, the
PL experiment allows us to plot the recombination current at a given spatial location versus the quasi Fermi level
splitting at the same location. Indeed, under reasonable assumptions a link can be done with the classical plot of the short
circuit current versus the open circuit voltage. We therefore can optically determine optoelectronic properties such as the
saturation currents. The assumptions made in this experiment are discussed in order to correctly investigate
polycrystalline solar cells in the future where strong lateral variations exist.
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