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).
We present a study on the role and optimization of diffraction gratings used as back reflector/scatterer in multiresonant GaAs ultrathin solar cells. We show the influence of parameters variation for the grating and for the pattern on the diffraction efficiencies. With an optimized square pattern, we show a record-high absorption of 92.5% in a 100 nm-thick GaAs absorber. Accounting for parasitic absorption, the estimated short-circuit current is 26.4 mA/cm2. We also discuss routes towards even higher currents by breaking the degeneracy of the modes with non-symmetric structures.
We demonstrated recently a record 19.9%-efficient GaAs solar cell with an absorber thickness of only 200 nm. Our next step is to optimize the device to reach a 25% efficiency. In this contribution we will present our latest simulation and experimental results based on an extensive analysis of the optical and electrical losses. The benefits brought by the contacts optimization and the improvement of the nanostructured design at the rear side of the solar cell will be emphasized.
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.
KEYWORDS: Solar cells, Nanowires, Silicon, Doping, Photovoltaics, Multijunction solar cells, Group III-V semiconductors, Beryllium, Electron beams, Microscopy
Today, the record in photovoltaic (PV) conversion efficiency is detained by multi-junction solar cells based on III-V semiconductors. However, the wide adoption of these devices is hindered by their high production cost, especially the expensive III-V substrates. As an alternative, a hybrid solar cell was proposed by LaPierre et al.1 The cell geometry, which combines a 2D Si bottom-cell with a nanowire (NW) top-cell in a tandem device, presents a theoretical efficiency record of 34% when the top-cell band gap lies around 1.7 eV[1],[2].
In this work, we report the elaboration, nanoscale characterization and device fabrication of solar cells based on axial junction GaAsP NWs. Organized GaAsP NWs were grown on patterned SiO2/Si(111) substrates by MBE. Junction was axially created during the growth by incorporating different doping impurities (Be for p- and Si for n-doping). In-situ surface passivation using a radial GaP shell was applied to reduce non-radiative recombinations on surface states[3]. Local I-V characteristics and electron beam induced current (EBIC) microscopy under different biases were used to probe the electrical properties and the generation patterns of individual NWs. The doping concentrations and the minority carrier diffusion lengths were extracted from the EBIC generation profiles. Macroscopic devices based on NW arrays were fabricated by dielectric encapsulation and ITO contacting. Top view EBIC analyses were applied to probe the device homogeneity.
References
[1] R.R. LaPierre et al., J. Appl. Phys. 110 (2011), 014310.
[2] S. Bu et al., Appl. Phys. Lett. 102 (2013), 031106.
[3] C. Himwas et al., Nanotechnology. 28 (2017), 495707.
KEYWORDS: Solar cells, Photovoltaics, Solar energy, Quantum wells, Energy efficiency, Semiconductors, Heterojunctions, Thermodynamics, Electrons, Luminescence
In single junction solar cells a large part of the incident energy ends up as heat which limits their maximum achievable efficiency. Thus the achievement of maximum power conversion efficiencies relies on complex multijunction devices. Here we show the possibility to harvest the available solar energy using hot carrier devices and evidence a positive contribution of the hot carrier effect on photovoltaic performances. We investigated a semiconductor heterostructure based on a single InGaAsP quantum well using quantitative optoelectrical characterization, especially luminance measurements. The quantitative thermodynamic study of the hot carrier population allows us to discuss the hot carrier contribution to the solar cell performance. We demonstrate that voltage and current are enhanced due to the presence of the hot carrier population in the quantum well. These experimental results substantiate the potential of increasing photovoltaic performances in the hot carrier regime.
Moreover, by developing a suitable analytic theoretical framework, we show how to obtain separate (hot) temperatures of electrons and holes from photoluminescence spectra analysis. The individual thermalization coefficients of each carrier type are also discussed. The method developed in this article paves the way towards the design of new energy harvesting devices and to the development of advanced characterization tools.
Finaly, to increase the PV performance enhancement and reduce the concentration factor, an optimize design is investigated.
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.
Efficiency of state-of-the-art single-junction solar cells is approaching the Shockley-Queisser limit (c-Si, GaAs). In contrast, the thickness of state-of-the-art solar cells is far from its theoretical limits and could be reduced by more than one order of magnitude with efficient light-trapping.
In this talk, I first present a benchmark of recent advances of ultra-thin solar cells (c-Si, CIGS, GaAs), using short-circuit current as a function of absorber thickness. I show that current state-of-the-art solar cells operate close to single-pass absorption and I highlight different light-trapping strategies proposed in the literature to approach the Lambertian limit. I then introduce our strategy for efficient light-trapping based on multi-resonant absorption. This approach overcomes the 4n2 limit, making use of coherent scattering of a discrete number of resonant modes.
In the second part, I apply these concepts to CIGS and GaAs solar cells. The goal is to reduce the thickness of the semiconductor absorber by one order of magnitude while preserving the short-circuit current. This study is not only pertinent from an academic point of view, but is of practical relevance to CIGS manufacturers for reducing material consumption and time deposition and for space power applications, where ultra-thin solar cells based on III-V semiconductors outperform long-term efficiency and power production of thicker cells due to their intrinsically higher radiation tolerance.
We propose a simple and scalable light-trapping architecture based on nanostructured TiO2/silver back-mirror fabricated by direct nanoimprint of sol-gel derived films over large surface areas. Electromagnetic simulations predict a short-circuit current of 36.3 mA/cm2 for a 150 nm thick CIGS solar cell, and I present our roadmap for implementing these concepts in an industrial CIGS solar cell fabrication process. Finally, I detail the design and fabrication of a 205 nm thick GaAs solar cell featuring certified efficiency of 19.9%, and I discuss possible improvements to achieve 25% efficiency.
Conventional light trapping techniques are inefficient at the sub-wavelength scale. This is the main limitation for the thickness reduction of thin-film solar cells below 500nm. We propose a novel architecture for broadband light absorption in ultra-thin active layers based on plasmonic nano-cavities and multi-resonant mechanism. Strong light enhancement will be shown numerically for photovoltaic materials such as CIGSe and GaAs. First experiments on ultrathin nano-patterned CIGSe solar cells will be presented.
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