Realistic upconverter-enhanced solar cells with non-ideal absorption and recombination efficiencies

Upconverting materials can be used to increase the energy conversion efficiency of a solar cell. Such materials convert low-energy transmitted photons to higher-energy photons that can be absorbed by the cell, substantially reducing the spectral mismatch between the cell and the solar spectrum. Previously, the performance enhancements achievable with an ideal upconverter-solar cell system were theoretically investigated. Here, we perform a comprehensive analysis to determine the effects of non-ideal cell and upconverter systems, accounting for non-ideal absorption and radiative recombination. We also allow for realistic nonradiative relaxation within the upconverter. The system is modeled using a detailed balance approach, with the upconverter treated as a series connection of two small-bandgap solar cells and a large-bandgap light emitting diode. We demonstrate that significant improvements in efficiency are possible even for nonconcentrated light, as long as the upconverter includes a small nonradiative relaxation pathway. Furthermore, we show that the existence of a nonradiative relaxation event in the upconverter is necessary for improved power conversion when cell absorption efficiency is low. Our results indicate that the efficiencies of both conventional-Si and thin film photovoltaic cells can be substantially improved with upconverting materials, even including non-idealities. (C) 2011 American Institute of Physics. [doi:10.1063/1.3610522]