**8. Some thoughts on concentrated photovoltaics (CPV)**

Concentrated light on small solar cells can become of great advantage: a small size cell (~2 mm2 area) may be placed at the focal point of a Fresnel optical system. Concentrated light causes higher carrier absorption from the bulk of the device and therefore higher

High Efficiency Solar Cells via Tuned Superlattice Structures: Beyond 42.2% 339

highly efficient solar cells are the answer for our future energy needs. The 40% threshold has already been reached and current research shows that 50% photovoltaics will soon be a reality. It seems that such high conversion will be succeeded by means of small size highly efficient solar cells. By small size we mean from the mm level down to nano-sized PV particles mounted to Fresnel-type optical systems with high solar concentration. Global energy production based on high efficiency PV will solve the energy needs of all nations and will slow down planet pollution. No nuclear waste and zero chance for accidents will guide common sense in immediate future. The concept of tuned superlattices was outlined and its advantages have been presented. Well-understood and lattice-matched materials, such as GaAs/Alloy and Ge, along with improved growth techniques pave the way to high efficiency photovoltaic devices. Integrated circuit techniques are also available for cells of minute size (e.g. 5 mm2), which is a dramatic reduction of material and hence of cost. Reduced size photovoltaic cells, under high solar concentration (currently from 450 to 500 suns), have opened the avenue for a competitive PV industry in the near future. Concentrated Photovoltaics (CPV-farms) will eventually dominate the world energy production. PV system price range has been steadily reducing from \$0.40/KWh (mid-1990's) to mere \$0.20/KWh in 2008. Market penetration of the PV industry increases steadily (under 1GW in the US to 6GW by the year 2015). It is expected that the average KWh will be ~10 cents by or before 2015, with a steady GW plant installation. High efficiency solar cells (~50%) and parallel optical system advancement (total system at 30%), will lead to a very

Current modeling of the top structure has indicated top efficiency values in excess of 21% (power out vs. power in) while for the bottom cell preliminary calculations indicate collection efficiency in excess of 25%. The bottom cell is a GaAs-superlattice-Ge structure, where quantum size effects occur. Photo-excited carriers in the middle region are electrons trapped in quantum wells (thin germanium layers sandwiched by gallium arsenide layers). Thin Ge layers (20 nm) are tuned at 1eV. They act as quantum traps and confine electrons in a discrete set of energy levels (one or two at the most). From these traps photo-electrons escape to the conduction band (minus the lost ones). Some advantages of our design over other high-efficiency full-spectrum solar cells are: (a) No excess tunnel junctions are needed to connect the cells (b) The superlattice region includes germanium layers tuned to absorb photons near 1eV (or more, depending on the quantum well thickness) (c) High mobility of carriers in both cells (top, bottom); the latter is a direct advantage over existing III-N-V *high efficiency* competing (nitrogen based) solar cell structures (d) Perfect lattice matching among the layers (e) Parallel carrier transport via (i) tunneling (ii) hopping and (iii) thermionic carrier escape. In the case at hand, tunneling is not a part of the action; instead thermionic emission currents are of importance. Maximum efficiency over 40% is expected via the

[1] SM Sze, High-Speed Semiconductor Devices, John Wiley and Sons, 1990 [2] M Yamaguchi, Solar Energy Materials & Solar Cells 90 (2006) 3068–3077

[5] K Nishioka, Solar Energy Materials & Solar Cells 90 (2006) 1308–1321, RR King, Nature

strong PV industry, for the benefit of all.

synergistic action of the two cells.

[3] M Yamaguchi et al, Solar Energy 82, 173 (2008) [4] R. King, et al, 20th European PVSEC, 2005

Photonics, May 2008

**10. References** 

current generation. Currently III-V multijunction cells have shown to have the highest collection efficiency. Efficiency (%) increases logarithmically with solar power up to about 500 suns (one sun = 100mW/cm2). Currently, it seems that CPV cells show the highest efficiency (consistently above 38%) with latest record efficiencies at 41.1% (Fraunhofer Institute). As it can be seen from equation (11) and the efficiency expression: = (Voc Jsc FF)/Pin, the efficiency of a solar cell increases logarithmically with Jsc. Such a behavior has been observed, in fact, increases with increasing current generation (maximum value in the neighborhood of 550 suns).

Fig. 9. Current status of cell performance and improvement since the mid seventies. Note that MJ cells have taken the lead in the high efficiency race. Latest (2010) results: 41.1% collection efficiency (Fraunhofer Institute at 454 suns) [© 2009 Spectrolab, Inc. All rights reserved].

As seen from the figure above, multijunction cells, with more than one band gaps, take the lead in current and voltage production (recall that efficiency varies with open-circuit voltage and short-circuit current). CPV systems have given a boost of solar power production globally because they combine (a) highly efficient cells with small exposure area and (b) less costly optical system and components. As of 2009, CPV systems operate at 28 – 30% total efficiency (cell plus optics) and seem to be coming dynamically in the global PV market.
