Nonequilibrium Hot-Carrier Transport in Type-II Multiple Quantum Wells for Solar-Cell Applications

Prototypes for hot-carrier solar cells based on type-II InAs/AlAs0.16Sb0.84 multiple quantum wells are examined with ac photoconductivity as a function of lattice temperature and photoexcitation energy to determine the photoexcited charge-carrier transport. These samples previously exhibited an excitation energy onset of a metastable regime in their short-time charge-carrier dynamics that potentially improves their applicability for hot-carrier photovoltaic cells. The transport results illustrate that the ac photoconductivity is larger in the regime corresponding to the metastability as a result of higher excitation photocarrier densities. In this excitation regime, the ac photoconductivity is accompanied by slightly lower carrier mobility, arising from the plasmalike nature of carriers scattered by Auger recombination. Outside of this regime, higher mobility is observed as a result of a lower excitation density that is more readily achievable by solar concentration. Additionally, at ambient temperatures, more scattering events are accompanied by slightly lower mobility, but the excitation dependence indicates that this is accompanied by an ambipolar diffusion length that is greater than half a micron. These transport properties are consistent with good quality in-organic elemental and III-V semiconductor solar cells and far exceed those of organic and perovskite materials. The transport results complement the dynamics observed in type-II InAs/AlAs0.16Sb0.84 and can guide the design of hot-carrier solar cells based on these and related materials.