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Up-and Down-Conversion,and Multi-Exciton Generation for Improving Solar Cells:A Reality Check

  • Hagay Shpaisman (a1), Olivia Niitsoo (a2), Igor Lubomirsky (a3) and David Cahen (a4)

Abstract

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.

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References

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[1] Shockley, W. and Queisser, H. J.Detailed Balance Limit of Efficiency of p-n Junction Solar Cells,” Journal of Applied Physics, vol. 32, pp. 510519, 1961.
[2] Green, A. M. Third Generation Photovoltaics: Ultra-High Efficiency at Low Cost. Berlin: Springer-Verlag, 2003.
[3] Jenny, N. The Physics of Solar Cells. London: Imperial College Press, 2003.
[4] Conibeer, G.Third-generation photovoltaics,” Materials Today, vol. 10, pp. 42, 2007.
[5] Jackson, E. D. “Areas for improvement of the semiconductor solar energy converter,” presented at Trans. Conf. on the use of Solar Energy, Tucson, Arizona, 1955.

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