Theoretical Study for Drastic Improvement of Solar Cell Efficiency

Abstract

A new technique to improve the energy conversion efficiency of thin-film solar cells such as amorphous silicon cells is theoretically studied. In the technique, thin trap layers are introduced in a depletion region of p-n or p-i-n structure. The traps act as stepping stones in two-step excitation of electrons from valence to conduction bands, and thus, sunlight with photon energy less than the energy of the band gap can be utilized to improve cell efficiency. It is found that if the energy level of traps is located far from the center of the band gap, the probability of two-step excitation through traps becomes dominant over that of carrier recombination. This new technique is effective to improve solar cell efficiency drastically, for instance, the efficiency of amorphous silicon solar cells is expected to be about twice by this technique in some special cases.