Third generation photovoltaic cell
Third generation photovoltaic cells are solar cells that are potentially able to overcome the Shockley–Queisser limit of 31-41% power efficiency for single bandgap solar cells. This includes a range of alternatives to the so-called "first generation solar cells" (which are solar cells made of semiconducting p-n junctions) and "second generation solar cells" (based on reducing the cost of first generation cells by employing thin film technologies). Common third-generation systems include multi-layer ("tandem") cells made of amorphous silicon or gallium arsenide, while more theoretical developments include frequency conversion, hot-carrier effects and other multiple-carrier ejection.
Solar cells can be thought of as visible light counterparts to radio receivers. A receiver consists of three basic parts; an antenna that converts the radio waves (light) into wave-like motions of electrons in the antenna material, an electronic valve that traps the electrons as they pop off the end of the antenna, and a tuner that amplifies electrons of a selected frequency. It is possible to build a solar cell identical to a radio, a system known as an optical rectenna, but to date these have not been practical.
Instead, the vast majority of the solar electric market is made up of silicon-based devices. In silicon cells, the silicon acts as both the antenna (or electron donor, technically) as well as the electronic valve. Silicon is almost ideal as a solar cell material; it is widely available, relatively inexpensive, and has a bandgap that is ideal for solar collection. On the downside it is energetically expensive to produce silicon in bulk, and great efforts have been made to reduce or eliminate the silicon in a cell. Moreover it is mechanically fragile, which typically requires a sheet of strong glass to be used as mechanical support and protection from the elements. The glass alone is a significant portion of the cost of a typical solar module.
According to the Shockley–Queisser limit, the majority of a cell's theoretical efficiency is due to the difference in energy between the bandgap and solar photon. Any photon with more energy than the bandgap can cause photoexcitation, but in this case any energy above and beyond the bandgap energy is lost. Consider the solar spectrum; only a small portion of the light reaching the ground is blue, but those photons have three times the energy of red light. Silicon's bandgap is 1.1 eV, about that of red light, so in this case the extra energy contained in blue light is lost in a silicon cell. If the bandgap is tuned higher, say to blue, that energy is now captured, but only at the cost of rejecting all the lower energy photons.
It is possible to greatly improve on a single-junction cell by stacking extremely thin cells with different bandgaps on top of each other - the "tandem cell" or "multi-junction" approach. Traditional silicon preparation methods do not lend themselves to this approach. There has been some progress using thin-films of amorphous silicon, notably Uni-Solar's products, but other issues have prevented these from matching the performance of traditional cells. Most tandem-cell structures are based on higher performance semiconductors, notably gallium arsenide (GaAs). Three-layer GaAs cells hold the production record of 41.6% for experimental examples.
Numerical analysis shows that the "perfect" single-layer solar cell should have a bandgap of 1.13 eV, almost exactly that of silicon. Such a cell can have a maximum theoretical power conversion efficiency of 33.7% - the solar power below red (in the infrared) is lost, and the extra energy of the higher colors is also lost. For a two layer cell, one layer should be tuned to 1.64 eV and the other at 0.94 eV, with a theoretical performance of 44%. A three-layer cell should be tuned to 1.83, 1.16 and 0.71 eV, with an efficiency of 48%. A theoretical "infinity-layer" cell would have a theoretical efficiency of 64%.
The third generation is somewhat ambiguous in the technologies that it encompasses, though generally it tends to include, among others, non-semiconductor technologies (including polymer cells and biomimetics), quantum dot, tandem/multi-junction cells, intermediate band solar cell, hot-carrier cells, photon upconversion and downconversion technologies, and solar thermal technologies, such as thermophotonics, which is one technology identified by Green as being third generation.
It also includes:
- Silicon nanostructures
- Modifying incident spectrum (concentration), to reach 300-500 suns and efficiencies of 32% (already attained in Sol3g cells) to +50%.
- Use of excess thermal generation (caused by UV light) to enhance voltages or carrier collection.
- Use of infrared spectrum to produce electricity at night.
Expected market shift
There has been a lot of hype circling around the possibilities of advanced solar technology in recent years. Major companies and investors such as Google, have invested hundreds of millions of dollars towards this new generation of solar power. They are counting on the likely possibility that the new technologies could compete with not only traditional solar cells, but more importantly with fossil fuels and nuclear energy (to reach and surpass grid parity). This would revolutionize our energy market; as said, in order for this to happen, third generation solar cells will need to be more efficient and cheaper. There are also further issues with durability (for solar cells to be able to reach their full lifetime in adverse outdoor conditions), safety (some solar manufacturing techniques involve dangerous chemicals), and energy storage (solar energy is only available at limited times, and almost unavailable all winter in non-tropical climates).
The new materials that solar energy can be harnessed with is one of the most exciting elements of the new technology. The flexible and lightweight physical characteristics of the different types of third generation solar cells makes many new applications possible.
There is the possibility that solar cells could be integrated into clothing which would allow us to have personal wireless power without batteries.
Another plausible application could be a type of automobile paint that is blended with polymer solar cells. This could help maintain the lightweight form of a solar car while still providing ample energy to power it.
Types of third generation solar cells
While the new solar technologies that have been discovered center around nanotechnology, there are several different material methods currently used.
CdTe (second generation)
CIGS (copper indium gallium selenide) (second generation)
- Band gap
- Grid parity
- Low-cost solar cell
- Organic electronics
- Printed electronics
- Solar Cell
- Silicon vs. CIGS: With solar energy, the issue is material
- Start-up targets thin-film silicon solar cells
- Spray-On Solar-Power Cells Are True Breakthrough
- Solar Cells: The New Light Fantastic
- Honda to Mass Produce Next-Generation Thin Film Solar Cell
- Shockley W. and Queisser H.J., J. Appl. Phys. 32 p. 510 (1961)
- Green, M.A., Third generation photovoltaics: Ultra-high conversion efficiency at low cost, Progress in Photovoltaics: Research and Applications 9(2), 123 (2001).
- A. Martí and A. Luque (Eds.), Next generation photovoltaics: high efficiency through full spectrum utilization, Series in Optics and Optoelectronics, Institute of Physics Publishing, Bristol (2003).
- G. Conibeer, Third generation photovoltaics, Materials today 10(11), 42 (2007)
- David Biello, "New solar-cell efficiency record set", Scientific American, 27 August 2009
- Weiming Wang, Albert S. Lin, Jamie D. Phillips (2009). "Intermediate band photovoltaic solar cell based on ZnTe:O". Appl. Phys. Lett. 95: 011103.
- Green, Martin (2003). Third Generation Photovoltaics: Advanced Solar Energy Conversion. Springer Science+Business Media. ISBN 3-540-40137-7.
- UNSW School for Photovoltaic Engineering. "Third Generation Photovoltaics". Retrieved 2008-06-20.
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