Effect of Solar Concentration on the Thermodynamic Power Conversion Efficiency of Quantum-Dot Solar Cells Exhibiting Multiple Exciton Generation

Thermodynamic calculations show that all solar cells can convert solar photons into electricity or fuel with higher theoretical power conversion efficiencies under concentrated sunlight. For conventional (viz, present day) single-junction solar cells that produce at most one electron–hole pair per absorbed photon, the theoretical increase in efficiency is relatively small (absolute values of 38% at 500× vs 33% at 1×). However, when solar concentration is combined with multiple exciton generation (MEG) in semiconductor quantum dots, the increase in theoretical power conversion efficiency is greatly enhanced. For the ideal MEG case, where the threshold for exciton multiplication is twice the bandgap, E g, the maximum thermodynamic efficiency increases to 75% at 500×, but the optimum E g shifts to smaller values. If E g is fixed at the 1-sun optimal level, then the maximum theoretical efficiency still increases markedly, becoming 62% at 500× for the staircase MEG characteristic (defined as producing N electron–hole pairs when the photon energy is N × E g) and 47% for a linear MEG characteristic that has a threshold photon energy of 2E g. The bandgaps in these two cases are 0.70 and 0.93 eV, respectively.