Improving solar cell efficiencies by down-conversion of high-energy photons

TLDR

Thermalization of high‑energy photons limits solar‑cell efficiency, but generating multiple electron–hole pairs per photon can mitigate this loss. The authors propose a method to achieve multiple electron–hole pair generation per incident photon. High‑energy photons are absorbed by a luminescence converter that emits two or more lower‑energy photons, and the system’s theoretical efficiency limit is calculated via detailed balance for nonconcentrated sunlight. The approach can raise the maximum conversion efficiency to 39.63 % for a 6000 K blackbody spectrum—substantially above the 30.9 % limit of a conventional cell under the same assumptions.

Abstract

One of the major loss mechanisms leading to low energy conversion efficiencies of solar cells is the thermalization of charge carriers generated by the absorption of high-energy photons. These losses can largely be reduced in a solar cell if more than one electron–hole pair can be generated per incident photon. A method to realize multiple electron–hole pair generation per incident photon is proposed in this article. Incident photons with energies larger than twice the band gap of the solar cell are absorbed by a luminescence converter, which transforms them into two or more lower energy photons. The theoretical efficiency limit of this system for nonconcentrated sunlight is determined as a function of the solar cell’s band gap using detailed balance calculations. It is shown that a maximum conversion efficiency of 39.63% can be achieved for a 6000 K blackbody spectrum and for a luminescence converter with one intermediate level. This is a substantial improvement over the limiting efficiency of 30.9%, which a solar cell exposed directly to nonconcentrated radiation may have under the same assumption of radiative recombination only.