Hot Carriers in Quantum Wells for Photovoltaic Efficiency Enhancement

In a hot carrier solar cell, the steady-state carrier population is hot relative to the surrounding lattice. This requires an absorber material which restricts carrier-phonon interaction and, therefore, reduces entropic loss during thermalization. The limiting efficiency of these devices approaches 85%: the Carnot limit for a solar energy collector. A spectroscopic analysis of GaAsP/InGaAs quantum well structures shows that carrier cooling in single quantum well samples is dominated by the rate of radiative recombination, leading to unprecedented carrier cooling lifetime (tau = 5.8 +/- 0.1 ns). This exceptional lifetime arises due to state saturation, frustrating the carrier scattering processes. A steady-state carrier population temperature >100 K above the lattice temperature is measured under illumination equivalent to 10 000 Suns. We calculate the projected efficiency >40% for a device with these characteristics, amounting to a 3% efficiency enhancement over equivalent single-junction devices.