一声叹息

Solar cells
1. Introduction and simple explanation:
A solar cell or photovoltaic cell is a device that converts light energy into electrical energy.
Fundamentally, the device needs to fulfill only two functions: photogeneration of charge carriers (electrons and holes) in a light-absorbing material, and separation of the charge carriers to a conductive contact that will transmit the electricity. This conversion is called the photovoltaic effect, and the field of research related to solar cells is known as photovoltaics.
When photons of sun light strike a PV cell, only the photons with a certain level of energy are able to free electrons from their atomic bonds to produce an electric current. This level of energy is known as the band-gap energy.
Materials with lower band-gap energies can exploit a broader range of the sun's spectrum of energies, creating greater numbers of charge carriers (greater current). We might conclude that material with the lowest band gap would thus make the best PV cell. But it isn't quite that simple.
The band-gap energy also influences the strength of the electric field, which determines the maximum voltage the cell can produce.
The power from an electrical device is equal to the product of the voltage (V) and the current (I). Low-band-gap cells have high current but low voltage; high-band-gap cells have high voltage and low current. A compromise is necessary in the design of PV cells. Cells made of materials with band gaps between 1 eV and 1.8 eV can be used efficiently in PV devices.
2. Three generations of development
2.1. First Generation
First generation refers to high quality and hence low defect single crystal photovoltaic devices. These have high efficiencies and are approaching the limiting efficiencies (Shockley and Queisser limit) for single band gap devices. However, the costs of such devices are not likely to get lower than US$1/W.
These cells are typically made using a silicon wafer. First generation photovoltaic cells  are the dominant technology in the commercial production of solar cells, accounting for more than 86% of the solar cell market.
2.2. Second Generation (Polycrystalline Thin Film)
The second generation of photovoltaic materials is based on the use of thin-film deposits of semiconductors. The advantage of using a thin-film of material was noted, reducing the mass of material required for cell design. This contributed to a prediction of greatly reduced costs for thin film solar cells. Such processes can bring costs down to a little under US$0.50.
Currently (2007) there are different technologies/semiconductor materials under investigation or in mass production, such as amorphous silicon, poly-crystalline silicon, micro-crystalline silicon, cadmium telluride, copper indium selenide/sulfide.
Typically, the efficiencies of thin-film solar cells are lower compared with bulk silicon solar cells, but manufacturing costs are also lower, so that a lower price in terms of $/watt of electrical output can be achieved.
However, decreasing the cell thickness implies a reduction in the absorption of light in the long wavelength region and a reduction in the minority carrier generation. To avoid this problem it is necessary to increase the optical path length in the cell sufficiently to preserve the photon absorption. This is made by different solution, mainly by process which yields films with large grain size or making use of schemes which trap light in the cell.
Another advantage of the reduced mass is that less support is needed when placing panels on rooftops and it allows fitting panels on light materials or flexible materials, even textiles.
2.3. Third Generation
Third generation photovoltaics are very different from the other two, broadly defined as semiconductor devices which do not rely on a traditional p-n junction to separate photogenerated charge carriers. These new devices include photoelectrochemical cells, Polymer solar cells, and nanocrystal solar cells.
Third generation solar cells able to exceed the Scockley - Queisser limit (efficiency > 31%):
The maximum efficiency one can reach with the conventional solar cell structure is given by the so-called single material or Shockley and Queisser (1961) limit (is the theoretical solar conversion efficiency limit for a single energy threshold material). Assuming that each photon above bandgap gives rise to just one electron – hole pair, while all photons with energy below the bandgap are lost, by using a detailed balance approach, one comes to a theoretical maximum energy conversion efficiency of about 30%.
The aim of the approaches of the third generation is to reduce the cost per Watt of “thin film” second generation technologies by increasing the efficiency of the photovoltaic devices with only a small increasing in area costs.
Means to overcome this limit have been variously quoted. They are divided into three generic categories, namely: multiple energy threshold devices; modification of the incident spectrum; and use of excess thermal generation to enhance voltages or carrier collection.
 
Efficiency and cost projections for first-, second- and third-generation photovoltaic technology
(wafers, thin-films, and advanced thin-films, respectively).
3. Implementation of the solutions
3.1. Second Generation
3.1.1. Thin film silicon solar cells
3.1.2. Thin film CdTe solar cells
3.1.3. Spherical solar cells
3.1.4. ORGANIC SOLAR CELLS
3.1.5. DYE SENSITIZED SOLAR CELLS (DSSCs)
3.1.6. Holographic Solar Cells
3.2. Third Generation
3.2.1. Nanostructure solar cells [13]
3.2.2. Quantum dots structure and formation [18][13]
3.2.3. Tandem solar cell [13][16]
3.2.4. Multiple Exciton Generation in Quantum Dot Solar Cells [17]
3.2.5. Quantum dot intermediate band solar cell (QD-IBSC) [14]
3.2.6. Hot Carrier Solar Cells [13][15]
3.2.7. Surface Plasmons [17]
3.2.8. Up/Down Converters [13][17]
3.2.9. Thermophotonics Solar Cells [17]

References
[1] Martin A. Green, Center of Excellence for Advanced Silicon Photovoltaics and Photonics , University of New South Wales, Sydney, Australia. Consolidation of Thin-film Photovoltaic Technology : The coming Decade of Opportunity. Prog. Phothovolt: Res. Appl. 2006; 14: 383-392
[2] J.Poortmans and V.Arkhipov. Epitaxial Thin Film Crystalline Silicon Solar Cells on low Cost Silicon Carriers. Thin Film Solar Cells November 16, 2006
[3] Wenjing Wang, Ying Xu and Hui Shen. Polycrystalline Silicon Thin-Film Solar Sells on various Substrates. Phys. Stat. Sol. (a) 203, No 4, 721-731 (2006)
[4] M. Fonrodona, J. Escarré, F. Villar, D. Soler, J. M. Asensi, J. Bertomeu, J. Andreu. PEN as substrate for new solar cell technologies. Solar Energy Materials & Solar Cells 89 (2005) 37-34
[5] IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 53, NO. 6, JUNE 2006, title: Three-Dimensional Modeling and Simulation of p-n Junction Spherical Silicon Solar Cells. Pages 1355-1363
[6]???
[7] Sophie E. Gledhill, Brian Scott, and Brian A. Gregg: Organic and nano-structured composite photovoltaics: An overview, J. Mater. Res., 20, 3167 (Dec 2005)
[8] Travis L. Benanti & D. Venkataraman, Organic solar cells: An overview focusing on active layer morphology, Photosynthesis research, 87, 73 (2006)
[9] Prism Solar Technologies, Inc. web site. http://www.prismsolar.com/index.html
[10] National Renewable Energy Laboratory (NREL) web site. http://www.nrel.gov/
[11] United States Patent # 5,877,874. Device for concentrating optical radiation. Http://patft1.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=5877874.PN.&OS=PN/5877874&RS=PN/5877874
[12] United States Patent # 6,274,860. Device for concentrating optical radiation. http://patft1.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=6274860.PN.&OS=PN/6274860&RS=PN/6274860
[13] Thin Solid Films, Volumes 511-512, 26 July 2006, Pages 654-662
Gavin Conibeer, Martin Green, Richard Corkish, Young Cho, Eun-Chel Cho, Chu-Wei Jiang, Thipwan Fangsuwannarak, Edwin Pink, Yidan Huang, Tom Puzzer, et al.
Title: Silicon nanostructures for third generation photovoltaic solar cells
[14] A. Martí, N. López, E. Antolín, E. Cánovas, C. Stanley, C. Farmer, L. Cuadra and A. Luque: Thin Solid Films, Volumes 511-512, 26 July 2006, Pages 638-644, Title: Novel semiconductor solar cell structures: The quantum dot intermediate band solar cell
[15] Ryne P. Raffaelle, Stephanie L. Castro, Aloysius F. Hepp, Sheila G. Bailey: Progress in Photovoltaics: Research and Applications Volume 10, Issue 6, Date: September 2002, Pages: 433-439, Title: Quantum dot solar cells
[16] Physica E: Low-dimensional Systems and Nanostructures, Volume 14, Issues 1-2, April 2002, Pages 115-120 Title: Quantum dot solar cells
[17] Evident Technologies  http://www.evidenttech.com/
[18] Center For Quantum Devices  http://cqd.ece.northwestern.edu/
[19] National renewable Energy laboratory Golden, Co, and University of Colorado, Boulder, Title : Third generation solar photon conversion