Abstract
AbstractVoltage‐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.
Highlights
In 1958, Franz and Keldysh described in two separate publications the effect of a uniform electric field on the absorbing properties of semiconductors and dielectrics.[1,2] The electric field distorts the band structure of the material, thereby changing the joint density of states in the valence and conduction band
Between photovoltaic quantum efficiency measured at short circuit and electroluminescence
Since the Franz-Keldysh effect only affects the absorption of photons in the depletion region, we expect it to be more significant in an ultrathin cell structure
Summary
In 1958, Franz and Keldysh described in two separate publications the effect of a uniform electric field on the absorbing properties of semiconductors and dielectrics.[1,2] The electric field distorts the band structure of the material, thereby changing the joint density of states in the valence and conduction band. This leads to oscillations in the absorption coefficient at energies above the bandgap while below the bandgap an exponentially decaying tail is introduced as a result of photon-induced tunneling. We discuss the implications of this observation for the use of the reciprocity relation
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