The 40 edition of the Efficiency Tables (Green et al., 2012) announces a new record of power conversion efficiency of any solar cell made with a single semiconductor absorber. An efficiency of 28.8% has been measured at the National Renewable Energy Laboratory (NREL) for a 1-cm2 thin film GaAs (gap 1.43 eV) device, with photovoltage 1.122 V, photocurrent 29.68 mA cm-2 and FF 86.5. The device was fabricated by Alta Devices, a Santa Clara-based ‘start-up’ seeking to develop low cost, 30% efficient solar modules. Alta Devices was founded by renowned solar cell scientists Harry Atwater of Cal Tech and Eli Yablonovitch of UCB. Yablonovitch, author of fundamental contributions such as the maximum increase of absorption 2 n^2 by randomization of light reflection (Yablonovitch, 1982), recently issued a preprint (Miller et al., 2011) in which the connection between photovoltaic performance and light emitting properties of a solar cell device was emphasized. The idea refers to develop solar cells that can actually approach the fundamental limits set by (Shockley and Queisser, 1961) based on the balance between light absorption and radiative emission in the solar cell. The connection between radiative properties, for the solar cell working as a a LED, and photovoltaic performance, has become a popular idea (Green et al., 2003,Kirchartz and Rau, 2008) and seems to give important payoffs, as shown by the Alta device.
In the figure I show how the best silicon solar cell and 28.8% record cell approach the maximal theoretical photocurrent and the maximal efficiency under 1 sun illumination. For a general discussion of the realization of efficiencies see (Nayak et al., 2011)
Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. “Solar cell efficiency tables (version 40)”. Progress in Photovoltaics: Research and Applications 2012, 20, 606–614.
Green, M. A.; Zhao, J.; Wang, A.; Trupke, T. “High-efficiency silicon light emitting diodes”. Physica E: Low-dimensional Systems and Nanostructures 2003, 16, 351-358.
Kirchartz, T.; Rau, U. “Detailed balance and reciprocity in solar cells”. physica status solidi (a) 2008, 205, 2737-2751.
Miller, O. D.; Yablonovitch, E.; Kurtz, S. R. “Intense Internal and External Fluorescence as Solar Cells Approach the Shockley-Queisser Efficiency Limit”. arXiv:1106.1603v3 2011.
Nayak, P. K.; Bisquert, J.; Cahen, D. “Assessing Possibilities and Limits for Solar Cells”. Advanced Materials 2011, 23, 2870-2876.
Shockley, W.; Queisser, H. J. “Detailed balance limit of efficiency of p-n junction solar cells”. Journal of Applied Physics 1961, 32, 510.
Yablonovitch, E. “Statistical ray optics”. J. Opt. Soc. Am. 1982, 72, 899-907.