Impurity photovoltaic effect with defect relaxation: Implications for low band gap semiconductors such as silicon

The impurity photovoltaic solar cell can, in principle, increase the sunlight to electricity conversion efficiency of a conventional single junction solar cell by the introduction of optically active impurities or defects into the device. These defects ideally allow electrons to be excited from the valence band to the conduction band via the mid-gap defect level through the absorption of previously wasted sub-band-gap photons. In this work the maximum efficiency limits for such a device are calculated for the special case where the energy of the partly excited electron relaxes to a lower energy partly through the two-stage excitation process. This relaxation in energy by the electron when occupying the defect state is shown to give an efficiency improvement over the case where no defect relaxation occurs. In the case of silicon, an efficiency limit of 39.7% under the airmass 1.5 G solar spectrum is obtained, compared to 33.0% when no defects are present and 30.5% when a defect is present but no relaxation is allowed. (C) 2004 American Institute of Physics.