Identification of surface and volume hot-carrier thermalization mechanisms in ultrathin GaAs layers

Abstract

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.

Investigation of hot carrier thermalization mechanisms in quantum well structures

“…This effect indicates that by considering the amount of the thermalized power above the absorber band edge, it is possible to define a thermalization coefficient, which evaluates the performance of the sample in inhibiting the thermalization loss and is not dependent on the excitation wavelength. Similar behavior has been recently observed by Giteau et al 17 in thin GaAs absorbers and its origin is discussed in detail.…”

Theory of Non-equilibrium "Hot" Carriers in Direct Band-gap Semiconductors Under Continuous Illumination

“…We show this by extracting carrier temperatures using two generic experimental procedures frequently used, namely (i) extracting a temperature from the heat transfer between carriers and phonons (measured via, e.g., a floating thermal probe or a thermocouple 51,52 ), and (ii) by fitting the photoluminescence (PL) spectra. [53][54][55] These two temperatures are shown to be very different from one another.…”

“…We then extract the steady-state electron and hole temperatures, T e and T h , respectively, by adapting two generic experimental procedures frequently used, (i) from the exchange of power between the electron (hole) sub-systems and the phonons (see Supplementary Section S6), 51,52 and (ii) from the steady-state PL spectra (see Supplementary Section S7). [53][54][55]…”

Review of the mechanisms for the phonon bottleneck effect in III–V semiconductors and their application for efficient hot carrier solar cells

“…In 1996, Langot et al investigated the slowed hot electron thermalization due to the hot phonon effect in bulk GaAs by using femtosecond absorption saturation techniques at room temperature. 72 A quantitative analysis on the hot phonon effect was carried out by changing either the individual excess energy of the photoexcited carriers or the carrier density in the moderate range from 41 This work shows an affine dependence with the thickness of GaAs absorber, leading to the identification of distinct volume and surface contributions to thermalization. It is confirmed that volume thermalization is linked to LO phonon decay.…”