As noted in Tech-On!, researchers at Kobe University (Kobe, Japan) have proposed a new solar-cell structure that uses two-photon upconversion to achieve a theoretical conversion efficiency higher than 50%.1 Experiments with a first prototype verified that the upconversion does indeed happen. The new structure enables absorption of sunlight's longer-wavelength spectral components, which have energies below the bandgap of the semiconductor and which normally pass through the photovoltaic cell and are lost.
The upper limit of the conversion efficiency of conventional single-junction solar cells is about 30%, meaning that most of the energy from incident sunlight is not absorbed by solar cells, either passing through the cells or turning into heat as surplus photon energy. The current world's highest solar cell efficiency is 46%, which was achieved with a quadruple-junction cell.
To realize the new solar-cell structure, the university used a hetero-interface consisting of two semiconductors, aluminum gallium arsenide (Al0.3Ga0.7As) and GaAs, having different bandgaps; the two-photon process happens at the boundary between the two semiconductors. The new structure generates a photoelectric current by converting two low-energy photons to one higher-energy photon that is absorbed by the cell. In theory, the conversion efficiency of the cell can reach up to 63%.
The new structure could become an ultrahigh-efficiency solar cell when improved by using optimal materials and refined structure. A conversion efficiency higher than 50% would lead to reductions in solar-cell installation areas of more than 50%, lowering both solar-cell and site construction costs.
Source: http://techon.nikkeibp.co.jp/atclen/news_en/15mk/041701281/
REFERENCE:
1. Shigeo Asahi et al., Nature Communications, 6 April 2017; doi:10.1038/ncomms14962