Hot carrier cooling in In_0.17Ga_0.83As/GaAs_0.80P_0.20 multiple quantum wells: the effect of barrier thickness

The hot carrier solar cell is an advanced concept photovoltaic device that is predicted to deliver efficiencies in excess of conventional single bandgap devices. The design requires the ability to concurrently have extended carrier thermalization times within an absorber material, giving a hot carrier population, and the ability to efficiently collect the hot carriers at an energy above the bandgap of the absorber material. In order to achieve this, we require an absorber material with a long-lived hot carrier population. We investigate the carrier thermalization rates of In _0.17Ga_0.83As/GaAs_0.80P_0.20 multiple quantum well samples with different barrier thicknesses. For a 40 quantum well strain-balanced structure, the cooling lifetime is found to be 1.23 ± 0.07 ns, but in samples which are not strain-balanced, defect-assisted carrier cooling increases the thermalization rate. Immediately following an ultrafast excitation, the initial carrier temperature is greater in samples with wider barriers. However, any gain in carrier temperature from utilizing wide barriers is negated by an increased thermalization rate as one deviates from strain-balanced conditions. We conclude that strain balancing is required for multiple quantum well hot carrier absorbers.