Because conventional photovoltaic (PV) cells are threshold systems in terms of optical absorption, “photon management“is an obvious way to improve their performance.
Calculations to optimize photon utilization in a single-junction PV cell show ˜1.4 eV to be the optimal bandgap for terrestrial solar to electrical power conversion. For Si, with a slightly sub-optimal gap, continuous efforts have yielded single-junction laboratory cells, quite close to the theoretical limit.
One of the repeatedly proposed directions to improve photon management is that of up- and down-conversion of photon energy. In up-conversion two photons with energy hv < EG (the band gap) create one photon with hv > EG, while in down-conversion one photon with energy hv > 2EG, yields two photons with energy hv > EG.
Multi-exciton generation (MEG), although not a "photon management" process, can achieve effects like down-conversion, which, though, is more limited than MEG. In MEG one photon with energy hv > nEG yields n electron-hole pairs with energy EG. Because MEG has clear advantages over down-conversion, in the following we will, instead of considering both, consider MEG.
We find that a straightforward analysis of this approach to “photon management” for a single junction cell under the detailed balance limit shows clearly that, even if we assume (highly unrealistic) 100% efficient up-conversion and MEG, a new theoretical PV conversion limit of 49 %, instead of 31% is arrived at, a maximum possible gain of ≈60%. The main attractive feature of the combination of up-conversion and MEG is a significant broadening of the optimal band-gap range. Rough estimates for the very highest possibly feasible efficiencies for up-conversion and MEG (25% and 70% respectively), yield at most slightly less than 40% PV conversion efficiency, i.e., only a ˜25% gain over conventional single band gap semiconductor.
These results show that up-conversion or MEG are fascinating scientific areas of research, whose implementation can indeed improve PV cell performance. However, truly formidable challenges need to be met to have UC + MEG lead to the type of radical decrease in the (cost)/ (efficiency × lifetime × yield) ratio that we need to allow large-scale economic introduction of PV cells. Parallel pursuit of alternative approaches to improved photon management, such as, for example, lowering the costs of arrangements with multiple solar absorbers and/or multi-junction systems, appears, therefore, critical for the future of PV.