Alta Solar Cells Up Efficiency by Slimming Down
July 11th, 2011 by 
Alta Devices, a Santa Clara, California solar PV company, recently announced that its gallium arsenide solar cells have set a new efficiency record of 28.2%.
The company’s press release quotes Alta co-founder Eli Yablonovitch, who says the trick to maximizing voltage is to generate more photons inside the solar cell.
According to Cleantech PatentEdge™, a couple of Alta’s patent families relate to a PV cell structure that increases voltage using a thin absorber layer to trap light.
U.S. Patent Application Publication Nos. 2010/0132780, entitled “Photovoltaice device” (’780 Application) and No. 2010/0126570, entitled “Thin absorber layer of a photovoltaic device” (’570 Application) are representative of these patent families.
Both applications are directed to a PV unit (100) having several epitaxial layers including a buffer layer (102), a release layer (104), a window layer (106), a base layer (108), and an emitter layer (110). The base layer (108) may comprise n-doped gallium arsenide semiconductor material, and the emitter layer (110) may comprise a p-doped aluminum gallium arsenide semiconductor material.
The combination of the base layer (108) and the emitter layer (110) may form an absorber layer for absorbing photons.
According to the ’780 and ’570 Applications, this structure provides an absorber layer that is much thinner than those found in conventional solar cells (<500 nm versus up to several micrometers).
This is important because the thickness of the absorber layer is proportional to “dark current” levels in the cell, so a thinner absorber layer reduces dark current levels. Dark current is the small electric current that flows through a PV cell even when no photons are entering the cell.
According to the ’780 and ’570 Applications, a lower dark current level means higher voltage and greater efficiency:
Because the open circuit voltage (Voc) increases as the dark current is decreased in a photosensitive semiconductor device, a thinner absorber layer may most likely lead to a greater (Voc) for a given light intensity and, thus, increased efficiency. As long as the absorber layer is able to trap light, the efficiency increases as the thickness of the absorber layer is decreased.
Alta’s web site says it is a development stage company. The company certainly excels at development.
Eric Lane is a patent attorney at Luce, Forward, Hamilton & Scripps in San Diego and the author of Green Patent Blog. Mr. Lane can be reached at elane@luce.com.
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