Non-equilibrium hot-carrier transport in type-II multiple-quantum wells for solar-cell applications
Prototypes for hot-carrier solar cells based on type-II InAs/AlAsSb multiple quantum wells are examined for AC photoconductivity as a function of lattice temperature and photoexcitation energy to determine the photoexcited charge carrier transport. These samples previously exhibit an excitation energy onset of a metastable regime in their short time charge carrier dynamics that potentially improves their applicability for hot-carrier photovoltaic applications. The transport results illustrate that the AC photoconductivity is larger in the dynamic 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 plasma-like 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 inorganic elemental and III-V semiconductor solar cells and far exceed those of novel materials. The transport results complement the dynamics observed in type-II InAs/AlAsSb and can guide the design of hot-carrier solar cells based on these and related materials.