Parasitic photon process versus productive photon process: a theoretical study of free-carrier absorption in conventional and hot-carrier solar cells

Abstract The effects of free-carrier absorption on conventional and hot-carriers solar cells are theoretically investigated in this work. The common view that free-carrier absorption in solar cells is ‘parasitic’ is re-examined, with the assistance of a theoretical framework and formulation developed and verified for calculating free-carrier absorption coefficients. In the case of spatial partitioning with photon absorption selectivity (e.g. solar cells with embedded quantum structures), free-carrier-absorption can facilitate and enhance carrier escape processes and increases photocurrents, especially in deep potential wells. Carrier heating resulting from free-carrier absorption is shown to be extremely beneficial to hot-carrier solar cells, especially for heavily-doped wide-band-gap optical absorbers. The energy conversion processes from carrier heating of free-carrier absorption could potentially make ideal hot-carrier solar cells function like solar thermal converters. As a result, their energy conversion efficiency is closer to the thermodynamic limit, regardless of optical absorbers’ band gap energy. It is illustrated that, as an optical process which is not limited by band gap energy, free-carrier absorption could benefit possible materials of hot-carrier solar cells regardless of their band gap energy. From this perspective, free-carrier absorption is far from a ‘parasitic’ process. Its usefulness depends on how we turn it into productive work.