SOLAR-CELL EFFICIENCY AND CARRIER MULTIPLICATION IN SI1-XGEX ALLOYS

Crystalline Si1-xGex compounds offer the possibility for tuning the el ectronic energy band structure with the chemical composition of the al loy in order to adapt the material for devices utilizing the energy of solar photons at an optimum. We concentrate on the efficiency enhance ment due to carrier multiplication by impact ionization. We calculate the internal quantum efficiency and the possible solar cell efficiency for this material system. The number of impact-generated charge carri ers is obtained by a simulation of the competing carrier-carrier and c arrier-photon scattering processes. These calculations show that the w ave vector dependence of the scattering processes is unimportant for g ood agreement between theoretical and experimental quantum efficiencie s in Si and Ge. Finally, we calculate solar cell efficiencies under th e ideal assumption of unity collection efficiency and radiative recomb ination only. Impact ionization enhances the theoretical conversion ef ficiency by 0.5 percentage point; this improvement is curtailed by the strong phonon emission probability of hot carriers. (C) 1998 American Institute of Physics.