A big step in improving the efficiency of photovoltaic cells in on the horizon. A paper published over the weekend in Nature Physics describes the ability of a substance called Graphene to convert a high percentage of the energy from sunlight into electricity. Graphene uses more of each photon's energy, and a wider range of photons of different energy levels (using a broad spectrum of the Sun's energy), compared to existing solar cells. From the abstract of the paper:
The conversion of light into free electron–hole pairs constitutes the key process in the fields of photodetection and photovoltaics. The efficiency of this process depends on the competition of different relaxation pathways and can be greatly enhanced when photoexcited carriers do not lose energy as heat, but instead transfer their excess energy into the production of additional electron–hole pairs through carrier–carrier scattering processes. Here we use optical pump–terahertz probe measurements to probe different pathways contributing to the ultrafast energy relaxation of photoexcited carriers. Our results indicate that carrier–carrier scattering is highly efficient, prevailing over optical-phonon emission in a wide range of photon wavelengths and leading to the production of secondary hot electrons originating from the conduction band. As hot electrons in graphene can drive currents, multiple hot-carrier generation makes graphene a promising material for highly efficient broadband extraction of light energy into electronic degrees of freedom, enabling high-efficiency optoelectronic applications.
Peter Sinclair has summarized the info on Graphene and has links to various sources here.
K. J. Tielrooij,J. C. W. Song, S. A. Jensen, A. Centeno, A. Pesquera,A. Zurutuza Elorza, M. Bonn, L. S. Levitov & F. H. L. Koppens. Photoexcitation cascade and multiple hot-carrier generation in graphene. Nature Physics (2013) doi:10.1038/nphys2564. Source