This work describes the relative contribution of intervalley scattering and phonon bottleneck effects in type-II InAs/AlAsSb quantum well solar cells. Moreover, recent predictions also suggest that altering the QW to barrier thickness ratio in these structures enables control of the phonon scattering rate, and therefore hot carrier relaxation may be inhibited by design. Experimental analysis of these predictions is presented in solar cell architectures, as well as, their effects upon both the optical and electrical performance of these devices.
Hot-carrier solar cells could overcome the Shockley-Queisser limit by having electrons and holes at a higher temperature than the lattice. To generate these hot carriers under concentrated sunlight, the thermalization rate should be as low as possible. Our objective in this presentation is to quantify the influence of different thermalization mechanisms. We determine the carrier temperature in ultrathin GaAs absorbers using continuous-wave photoluminescence and identify distinct surface and volume thermalization contributions. We explain the origin of these contributions using theoretical models involving non-equilibrium LO phonon populations and thermionic emission. We implement these mechanisms in detailed balance calculations for further understanding.
Short time carrier dynamics of transient photoluminescence decays contain valuable information on the optoelectronic properties of photovoltaic materials. We perform a theoretical analysis on short time dynamics to provide scaling laws for the time derivative of the transient photoluminescence signal as a function of both laser excitation power and wavelength . This innovative approach allowed us to extract in a simple and effective manner the external radiative recombination rate and was tested on different absorbers such as state-of-the-art triple cation mixed halide perovskite and III-V materials. Moreover, by coupling this analysis with the fitting of the whole PL decay, we have quantified different transport parameters and precisely estimated their uncertainties.
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.
A hot-carrier solar cell (HCSC) is a high-efficiency photovoltaic concept where electrons and holes are at a higher temperature than the lattice, allowing an additional thermoelectric energy conversion. There are two requirements for a HCSC: establishing a hot-carrier population and converting the temperature into extra voltage through energy-selective contacts. We focus on the generation of hot carriers, and the design of absorbers that can make this generation easier. Fundamentally, this requires to increase the power density absorbed per volume unit, so the photocarriers cannot fully thermalize (phonon bottleneck). Beyond simply increasing the light intensity, the main control knobs to favor hot carriers include reducing the thickness of the absorber, increasing its absorptivity, and reducing its bandgap. In this proceeding, we report the fabrication of structures that aim at measuring the influence of these different parameters. We justify our choices for sample structure and fabrication method from the need for high thermal conductivity, in order to prevent lattice heating. We characterize our structures in order to determine precisely the final thickness of all layers, and the absorptivity of the absorber layer. These samples are to be used for an analysis of the temperature with many variable parameters, in order to better understand the thermalization mechanisms and design better absorbers. Ultimately, our objective is to implement all solutions together in order to evidence a hot carrier population under concentrated sunlight illumination.
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.
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.
Double resonant tunneling barriers are considered for an application as energy selective contacts in hot carrier solar cells. Experimental symmetric and asymmetric double resonant tunneling barriers are realized by molecular beam epitaxy and characterized by temperature dependent current-voltage measurements. The negative differential resistance signal is enhanced for asymmetric heterostructures, and remains unchanged between low- and room-temperatures. Within Tsu-Esaki description of the tunnel current, this observation can be explained by the voltage dependence of the tunnel transmission amplitude, which presents a resonance under finite bias for asymmetric structures. This effect is notably discussed with respect to series resistance. Different parameters related to the electronic transmission of the structure and the influence of these parameters on the current voltage characteristic are investigated, bringing insights on critical processes to optimize in double resonant tunneling barriers applied to hot carrier solar cells.
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