Narrow-bandwidth solar upconversion: design principles, efficiency   limits, and case studies
  We employ a detailed balance approach to model a single-junction solar cell with a realistic narrow-band, non-unity-quantum-yield upconverter. As upconverter bandwidths are increased from 0 to 0.5 eV, maximum cell efficiencies increase from the Shockley-Queisser limit of 30.58% to over 43%. Such efficiency enhancements are calculated for upconverters with near-infrared spectral absorption bands, readily accessible with existing upconverters. While our model shows that current bimolecular and lanthanide-based upconverting materials will improve cell efficiencies by <1%, cell efficiencies can increase by several absolute percent with increased upconverter quantum yield - even without an increased absorption bandwidth. By examining the efficiency limits of a highly realistic solar cell-upconverter system, our model provides a platform for optimizing future solar upconverter designs. 
Justin A. Briggs, Ashwin C. Atre, Jennifer A. Dionne
physics.optics cond-mat.mtrl-sci