Abstract:In this study, detailed temperature dependent simulations for absorption and photogenerated recombination of hot electrons are compared with experimental data for an InAs/AlAsSb multi-quantum well. The simulations describe the actual photoluminescence (PL) observations accurately; in particular, the room temperature e1-hh1 simulated transition energy of 805 meV closely matches the 798 meV transition energy of the experimental PL spectra, a difference of only 7 meV. Likewise, the expected energy separations bet… Show more
“…If electrons and holes are at different temperatures, Eq. (2) is still valid, with related to the temperature of the carrier populations 30,31 . We will consider here, as is usually assumed, that the carrier temperatures are the same.…”
Hot-carrier solar cells offer the opportunity to harvest more energy than the limit set by the Shockley-Queisser model by reducing the losses due to the thermalization of photo-generated carriers. Previous reports have shown lower thermalization rates in thinner absorbers, but the origin of this phenomenon is not precisely understood. In this work, we investigate a series of ultrathin GaAs absorber layers sandwiched between AlGaAs barriers and transferred on host substrates with a gold back mirror. We perform power-dependent photoluminescence characterizations at different laser wavelengths from which we determine the carrier temperature in four absorber thicknesses between 20 and 200 nm. We observe a linear relationship between the absorbed power and the carrier temperature increase. By relating the absorbed and thermalized power, we extract a thermalization coefficient for all samples. It shows an affine dependence with the thickness, leading to the identification of distinct volume and surface contributions to thermalization. We confirm that volume thermalization is linked to LO phonon decay. We discuss the origin of the interface-related thermalization, showing that the effect of LO phonon transport is negligible. Overall, this work sheds new light on thermalization processes in ultrathin semiconductor layers and introduces a method to compare the performance of hot-carrier absorbers.
“…If electrons and holes are at different temperatures, Eq. (2) is still valid, with related to the temperature of the carrier populations 30,31 . We will consider here, as is usually assumed, that the carrier temperatures are the same.…”
Hot-carrier solar cells offer the opportunity to harvest more energy than the limit set by the Shockley-Queisser model by reducing the losses due to the thermalization of photo-generated carriers. Previous reports have shown lower thermalization rates in thinner absorbers, but the origin of this phenomenon is not precisely understood. In this work, we investigate a series of ultrathin GaAs absorber layers sandwiched between AlGaAs barriers and transferred on host substrates with a gold back mirror. We perform power-dependent photoluminescence characterizations at different laser wavelengths from which we determine the carrier temperature in four absorber thicknesses between 20 and 200 nm. We observe a linear relationship between the absorbed power and the carrier temperature increase. By relating the absorbed and thermalized power, we extract a thermalization coefficient for all samples. It shows an affine dependence with the thickness, leading to the identification of distinct volume and surface contributions to thermalization. We confirm that volume thermalization is linked to LO phonon decay. We discuss the origin of the interface-related thermalization, showing that the effect of LO phonon transport is negligible. Overall, this work sheds new light on thermalization processes in ultrathin semiconductor layers and introduces a method to compare the performance of hot-carrier absorbers.
“…The same calculation could be performed with different hole and electron temperatures and/or effective masses [10]. Also, it has been experimentally confirmed that in polar semiconductors, electrons (with a much lower effective mass) heat much more than holes [11]. However, considering different electron and hole behaviors makes the interpretation of the results much more difficult, while it does not bring any further insight into this study.…”
Section: Absorptivity: Band Filling and Light Path Enhancementmentioning
Hot-carrier solar cells could enable an efficiency gain compared to conventional cells, provided that a high current can be achieved, together with a hot-carrier population. Because the thermalization rate is proportional to the volume of the absorber, a fundamental requirement is to maximize the density of carriers generated per volume unit. In this work, we focus on the crucial role of light trapping to meet this objective. Using a detailed balance model taking into account losses through a thermalization factor, we obtained parameters of the hot-carrier population generated under continuous illumination. Different absorptions corresponding to different light path enhancements were compared. Results are presented for open-circuit voltage, at maximum power point and as a function of the applied voltage. The relation between the parameters of the cell (thermalization rate and absorptivity) and its characteristics (temperature, chemical potential, and efficiency) is explained. In particular, we clarify the link between absorbed light intensity and chemical potential. Overall, the results give quantitative values for the thermalization coefficient to be achieved and show that in the hot-carrier regime, absorptivity enhancement leads to an important increase in the carrier temperature and efficiency.
“…These properties all occur in InAs/AlAs Sb multiple-quantum wells (MQWs), where a hot-carrier distribution is shown along with extended carrier lifetimes as a result of inhibited phonon–phonon interactions 8 . In steady-state photoluminescence (PL), these MQWs have shown evidence of hot carriers 14 , non-monotonic emission energy as a function of lattice temperature due to the complicated valence band structure 15 and even intervalley scattering of electrons to the long-lived L-valley states 16 . Transient absorption directly shows long-lived carriers complementing the evidence of hot carriers and the non-monotonic temperature dependence 8 .…”
Section: Introductionmentioning
confidence: 99%
“… Figure 1 Schematic of the optical pump and THz probe experimental geometry ( EOS = electro-optic sampling). ( b ) Band structure 15 of the InAs MQW superlattice. The inset shows the band minima and maxima of a single QW with confined energy levels at cm .…”
A type-II InAs/AlAs$$_{0.16}$$
0.16
Sb$$_{0.84}$$
0.84
multiple-quantum well sample is investigated for the photoexcited carrier dynamics as a function of excitation photon energy and lattice temperature. Time-resolved measurements are performed using a near-infrared pump pulse, with photon energies near to and above the band gap, probed with a terahertz probe pulse. The transient terahertz absorption is characterized by a multi-rise, multi-decay function that captures long-lived decay times and a metastable state for an excess-photon energy of $$>100$$
>
100
meV. For sufficient excess-photon energy, excitation of the metastable state is followed by a transition to the long-lived states. Excitation dependence of the long-lived states map onto a nearly-direct band gap ($$E{_g}$$
E
g
) density of states with an Urbach tail below $$E{_g}$$
E
g
. As temperature increases, the long-lived decay times increase $$<E{_g}$$
<
E
g
, due to the increased phonon interaction of the unintentional defect states, and by phonon stabilization of the hot carriers $$>E{_g}$$
>
E
g
. Additionally, Auger (and/or trap-assisted Auger) scattering above the onset of the plateau may also contribute to longer hot-carrier lifetimes. Meanwhile, the initial decay component shows strong dependence on excitation energy and temperature, reflecting the complicated initial transfer of energy between valence-band and defect states, indicating methods to further prolong hot carriers for technological applications.
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