| AVS 54th International Symposium | |
| The Industrial Physics Forum 2007: The Energy Challenge | Monday Sessions |
| Session IPF-MoM |
| Session: | Energy Efficiency |
| Presenter: | R.R. King, Spectrolab, Inc. |
| Authors: | R.R. King, Spectrolab, Inc. D.C. Law, Spectrolab, Inc. K.M. Edmondson, Spectrolab, Inc. C.M. Fetzer, Spectrolab, Inc. G.S. Kinsey, Spectrolab, Inc. H. Yoon, Spectrolab, Inc. D.D. Krut, Spectrolab, Inc. J.H. Ermer, Spectrolab, Inc. R.A. Sherif, Spectrolab, Inc. N.H. Karam, Spectrolab, Inc. |
| Correspondent: | Click to Email |
Solar cell efficiency is one of the most enabling device parameters for widespread implementation of solar electricity generation on Earth, since high efficiency dramatically reduces not only the cell area needed to generate a given power, but also the cost of all area-related components in a photovoltaic system. The efficiency of a solar cell with a single energy band gap Eg in unconcentrated sunlight is quite limited by fundamental considerations, such as thermalization of photogenerated electrons and holes, non-absorption of low energy photons, and the limited quasi-Fermi level splitting at one sun. Multijunction concentrator cells are able to overcome these fundamental efficiency limits, and as a result have attracted much attention recently for cost-effective terrestrial photovoltaics. If the subcell bandgaps for the multijunction solar cell are chosen from metamorphic semiconductors that are lattice-mismatched to the growth substrate, theoretical efficiencies can be raised even higher than for lattice-matched designs. Advances in the design of metamorphic subcells to reduce carrier recombination and increase voltage, wide-bandgap tunnel junctions, metamorphic buffers to transition to the lattice constant of the active subcells, concentrator cell anti-reflection coating and grid design, and integration into current-matched 3-junction cells have resulted in new heights in solar cell performance. A metamorphic Ga0.44In0.56P/ Ga0.92In0.08As/ Ge 3-junction solar cell has reached a record 40.7% efficiency at 240 suns, under the standard reporting spectrum for terrestrial concentrator cells (AM1.5 direct, low-AOD, 24.0 W/cm2, 25°C). This metamorphic 3-junction device is the first solar cell to reach over 40% in efficiency, and has the highest solar conversion efficiency for any type of photovoltaic cell to date. Experimental lattice-matched 3-junction cells have now also achieved over 40% efficiency, with 40.1% measured at 135 suns. The multijunction structure of these cells and their operation at concentration allow efficiencies substantially above the Shockley-Queisser limit1 of 30% for a single-band-gap device at one sun, and above the theoretical limit of 37% for single-band-gap cells at 1000 suns,2 to now be achieved in practice.
1W. Shockley and H. J. Queisser, J. Appl. Phys., 32, 510 (1961).
2C. H. Henry, J. Appl. Phys., 51, 4494 (1980).