BAND-BAND IMPACT IONIZATION AND SOLAR-CELL EFFICIENCY

The effect of impact ionization has been taken into account in the cal culation of the maximum solar cell efficiency in the thermodynamic lim it. A red shift of the optimum band gap is observed with respect to th e Shockley-Queisser result. A maximum solar cell efficiency of 60.3% a t E(g)=0.8 eV is predicted, which compares with 43.9% at E(g)=1.1 eV f or the usual case. In order to obtain a significant increase in eta, a n impact ionization probability of at least a few percent is required. A most pronounced effect is observed for bandgap energies below about 1 eV.