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One of the main loss mechanisms in photovoltaic solar cells is the thermalization of photogenerated hot carriers via phonon-mediated relaxation. By inhibiting these relaxation mechanisms and reducing thermalization losses, it may be possible to improve the power conversion efficiency of solar cells beyond the single gap limit. Here, type-II InAs/AlAsSb multi-quantum well (MQW) structures are investigated to study the impact of the phononic properties of the AlAsSb barrier material in hot carrier thermalization. Experimental and theoretical results show that by increasing the barrier thickness (increasing the relative contribution of AlAsSb content in the superlattices), the relaxation of hot carriers is reduced as observed in power-dependent photoluminescence and thermalization analysis. This is attributed to an increase in the phononic bandgap of the MQW with increasing AlAsSb composition reducing the efficiency of the dominant Klemens mechanism as the phononic properties shift toward a more AlSb-like behavior.

Simultaneous continuous wave (CW) photoluminescence and monochromatic current densityvoltage (J-V) measurements of InAs/AlAsSb QW p-i-n diodes reveal stable hot carriers observed at relatively low power (nearly independent of power). This behavior is attributed to preferential scattering of high energy carriers to the upper satellite L-and X-valleys, which inhibits carrier thermalization via LO phonon emission. Although both high electric field and optical excitation are shown to enable stable hot carrier generation in the quantum wells (QWs), the extraction of these carriers is inhibited by the mismatch in valley degeneracy (L to Γ) across the InAs QW/n-AlInAs layers resulting in carrier localization in the QWs in the operating regime of the solar cell.

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