QUANTUM EFFICIENCIES EXCEEDING UNITY IN SILICON LEADING TO NOVEL SELECTION PRINCIPLES FOR SOLAR-CELL MATERIALS

Solar cells utilize carriers which are generated by absorption of photons with an energy above the fundamental band gap of the semiconductor. A maximum number of one electron/hole pair per photon has thus far been taken for granted under the tacit assumption that the amount of the photon energy that exceeds the band gap is wasted into lattice vibrations, i.e., phonons. We present experimental evidence that this excessive photon energy can be utilized for the generation of a second electron/hole pair via an additional Auger-type generation. The hot carriers of the primary electron/hole pair generate secondary pairs and thus lead to values of the internal quantum efficiency exceeding unity; we find experimental values of up to 1.3 for our silicon solar cells. This Auger generation has to be included into theories of the maximum efficiency of solar cells. For example, the so-called ultimate solar cell efficiency for the AM0 and the AM1.5G spectrum is increased to maximum values of 75% and 79%, and the optimum band gap shifts to values below 1 eV if we allow for quantum efficiencies of two instead of one. The simple optimum band gap criterion has to be complemented by more detailed demands upon the band structure of the semiconductor if one wants to make effective use of Auger generation.