Losses due to Quantum Size Effects in Thermionic Currents in III-V MQW Solar Cells

Abstract:- Quantum wells in solar cells are important probes for the study of free carrier behavior. Photo-generated carriers in quantum wells contribute to transport and hence current only via thermionic escape, and once they are in the conduction band continuum, they can be treated as propagated plane waves subject to constraints of the crystal lattice. It is pointed out that neighboring quantum wells act as scattering centers causing back-scattering and trapping (standing waves). On the other hand, thermionically escaping electrons comprise traveling quantum mechanical waves liable to trapping and backscattering due to nearest neighbor quantum wells in a multi-layer photovoltaic device. Such losses due to scattering and trapping are taken into consideration in this communication, where transmitted waves are calculated after scattering and trapping take place. In such lossy multiquantum well “lines”, computations show that only 16 to 25 % of thermionically escaping carriers get through, and the rest remain trapped in quantum wells and/or reflect back to neighboring unit cells. For 6nm-width (GaAs-AlGaAs) un-doped quantum wells, illuminated at room temperature, current densities are expected to drop from 0.4 mA/cm2/qu. well (with recombination losses but with no scattering) down to a range between 0.064 to 0.100 mA/cm2/qu. well (with both recombination losses and overall scattering included in this study)