Observation and implications of the Franz-Keldysh effect in ultrathin GaAs solar cells

Abstract

Voltage-dependencies were observed in the external quantum efficiency (EQE) spectra of ultrathin GaAs solar cells. The subbandgap tail was shown to increase going from forward to reverse bias, while at energies above the bandgap, voltage-dependent oscillations in the EQE were measured. Using optical simulations, it is irrefutably shown that the voltage-dependencies are caused by the Franz-Keldysh effect, that is, an electric field-dependent absorption coefficient near the bandgap. The dependency on voltage of the subbandgap tail is demonstrated to be strongest in thin-film cells with a textured rear mirror, since the absorptivity below the bandgap is enhanced by light trapping. The voltage-dependent subbandgap tail has important implications for the use of the reciprocity relation between photovoltaic quantum efficiency and electroluminescence. It is shown that the radiative limit for the open-circuit voltage of thin-film cells integrated with light management schemes can be underestimated by more than 25 mV. Consequently, these cells may be assumed to be closer to the radiative limit than they really are.