Impurity photovoltaic effect in GaAs solar cell with two deep impurity levels

The impurity photovoltaic (IPV) effect has been extensively investigated for silicon solar cells with indium impurities. A small positive effect on the efficiency was theoretically predicted, provided that efficient light trapping schemes are applied. Since IPV effects are predicted to be more pronounced in wider band gap materials and possibly also by introducing more than one impurity level, we carried out a numerical study on GaAs solar cells with copper impurities. Indeed, copper introduces multiple acceptor type mid-gap levels in GaAs, two of which are investigated here: one at 0.14 eV and another at 0.40 eV above the valence band edge. We used a solar cell device simulator (SCAPS) specially adapted to include the IPV effect. We varied the impurity concentration and the light trapping parameters of the device, and calculated the occupation probability of the impurity levels, the generation and recombination through these levels (thermal and optical) and the solar cell characteristics: quantum efficiency, QE(λ), short-circuit current Jsc, open-circuit voltage Voc and efficiency η.A significant IPV effect with extended response in the infrared is calculated when efficient light trapping is present. When an internal reflection coefficient R=0.99 can be reached at both the front and the back surface of the solar cell, an increase of the short-circuit current densities 2 mA/cm2 can be obtained. The consequences of two levels instead of one are calculated and discussed, leading to a trade-off between Jsc improvement and a Voc decrease.