Band-band impact ionization and solar cell efficiency

The effect of impact ionization has been taken into account in the calculation of the maximum solar cell efficiency in the thermodynamic limit. A red shift of the optimum band gap is observed with respect to the Shockley–Queisser result. A maximum solar cell efficiency of 60.3% at Eg=0.8 eV is predicted, which compares with 43.9% at Eg=1.1 eV for the usual case. In order to obtain a significant increase in η, an impact ionization probability of at least a few percent is required. A most pronounced effect is observed for bandgap energies below about 1 eV.