Multijunction photovoltaic cells are a sub-class of
solar cellA solar cell is a device that converts the energy of sunlight directly into electricity by the photovoltaic effect. Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the light source is...
or photovoltaic cell developed for higher efficiency. These multijunction cells consist of multiple
thin filmThin films are thin material layers ranging from fractions of a nanometre to several micrometres in thickness. Electronic semiconductor devices and optical coatings are the main applications benefiting from thin film construction....
s produced using
molecular beam epitaxyMolecular beam epitaxy , is one of several methods of depositing single crystals. It was invented in the late 1960s at Bell Telephone Laboratories by J. R. Arthur and Alfred Y. Cho.-Method:...
and / or
metalorganic vapour phase epitaxyMetalorganic vapour phase epitaxy is a chemical vapour deposition method of epitaxial growth of materials, especially compound semiconductors from the surface reaction of organic compounds or metalorganics and metal hydrides containing the required chemical elements...
. Each type of semiconductor will have a characteristic
band gapIn solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states exist...
energy which, loosely speaking, causes it to absorb light most efficiently at a certain color, or more precisely, to absorb
electromagnetic radiationElectromagnetic radiation is a ubiquitous phenomenon that takes the form of self-propagating waves in a vacuum or in matter. It consists of electric and magnetic field components which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation...
over a portion of the spectrum. The semiconductors are carefully chosen to absorb nearly all of the solar spectrum, thus generating electricity from as much of the solar energy as possible.
In short, in the multijunction structure, several layers each capture part of the sunlight passing through the cell. These layers allow the cell to capture more of the solar spectrum and convert it into electricity .
History
Multijunction solar cells first were developed and deployed for
SatelliteIn the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavor. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
power applications where the high cost was offset by the weight savings offered by the higher efficiency.
Multijunction cells have recently seen application in terrestrial applications in Concentrated photovoltaics. The combination of the higher efficiency and concentration has resulted in a price competitive with silicon flat panel arrays.
This technology is currently being utilized in the Mars rover missions.
Tandem solar cells based on monolithic, series connected, gallium indium phosphide (GaInP), gallium arsenide GaAs, and germanium Ge pn junctions, are seeing demand rapidly rise. In just the past 12 months (12/2006 - 12/2007), the cost of 4N gallium metal has risen from about $350 per kg to $680 per kg. Additionally, germanium metal prices have risen substantially to $1,000-$1,200 per kg this year. Those materials include gallium (4N, 6N and 7N Ga), arsenic (4N, 6N and 7N) and germanium, pyrolitic boron nitride (pBN) crucibles for growing crystals, and boron oxide, these products are critical to the entire substrate manufacturing industry.
Triple-junction GaAs solar cells were also being used as the power source of the Dutch four-time
World Solar ChallengeThe World Solar Challenge is a solar-powered car race which covers 3021 km through the Australian Outback, from Darwin to Adelaide....
winners
NunaThe Nuna4 is a solar car developed by the Delft University of Technology in 2006-2007 for the 2007 World Solar Challenge.It succeeded the Nuna3, the solar car that scored a hat trick by winning the World Solar Challenge for the third time in a row...
in 2005 and 2007.
Scientists at the U.S. Department of Energy's
National Renewable Energy LaboratoryThe National Renewable Energy Laboratory , located in Golden, Colorado, as part of the U.S. Department of Energy, is the United States' primary laboratory for renewable energy and energy efficiency research and development.-History:...
(NREL) have set a world record in solar cell efficiency with a photovoltaic device that converts 40.8 percent of the light that hits it into electricity. This is the highest confirmed efficiency of any photovoltaic device to date. The inverted metamorphic triple-junction solar cell was designed, fabricated and independently measured at NREL.
Theory of operation
In a single band gap solar cell, efficiency is limited due to the inability to efficiently convert the broad range of energy that photons possess in the solar spectrum. Photons below the band gap of the cell material are lost; they either pass through the cell or are converted to only heat within the material. Energy in the photons above the band gap energy is also lost, since only the energy necessary to generate the hole-electron pair is utilized, and the remaining energy is converted into heat.
By utilizing multiple junctions with several band gaps, different portions of the solar spectrum may be converted by each junction at a greater efficiency.
Device description
Multijunction photovoltaic cells use many layers of
epitaxiallyEpitaxy refers to the method of depositing a monocrystalline film on a monocrystalline substrate. The deposited film is denoted as epitaxial film or epitaxial layer. The term epitaxy comes from the Greek roots epi, meaning "above", and taxis, meaning "in ordered manner". It can be translated "to...
deposited films. By using differing alloys of III-V
SemiconductorA semiconductor is a material that has an electrical resistivity between that of a conductor and an insulator, that is, generally in the range 10
3 Siemens/cm to 10
−8 S/cm. Devices made from semiconductor materials are the foundation of modern electronics, including radio,...
s, the band-gap of each layer may be tuned to absorb a specific band of the solar electromagnetic radiation. The ability to optimize the respective band gaps of the various junctions is hampered by the requirement that each layer must be lattice matched to all other layers. (See
Lattice constantThe Lattice Constant [or lattice parameter] refers to the constant distance between unit cells in a crystal lattice. Lattices in three dimensions generally have three lattice constants, referred to as a, b, and c. However, in the special case of cubic crystal structures, all of the constants are...
).
Each layer is optically in series, with the highest band gap material at the top. The first junction receives all of the spectrum. Photons above the band gap of the first junction are absorbed in the first layer. Photons below the band gap of the first layer pass through to the lower layers to be absorbed there.
All currently commercialized cells utilize tandem electrical connection. This means that they are electrically connected in series and the composite cell has two terminals. A major constraint placed upon tandem cells is that because of the series connection, the current through each junction will be the same. If the maximum power point current of each junction is not the same, then efficiency suffers. Current match of each junction is a very important design consideration for multijunction cells.
Material classification
Multijunction cells may be categorized by the substrate used for cell manufacture. Cells on
GermaniumGermanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. Germanium has five naturally occurring isotopes ranging in atomic mass number from 70 to 76...
and Gallium arsenide have been commercialized. Research into Indium Phosphide based cells for lower band gaps is ongoing.
Gallium arsenide substrate
Twin junction cells with
Indium gallium phosphideIndium gallium phosphide , also called gallium indium phosphide , is a semiconductor composed of indium, gallium and phosphorus...
and gallium arsenide can be made on gallium arsenide wafers. Alloys of In
.5Ga
.5P through In
.53Ga
.47P may be used as the high band gap alloy. This alloy range provides for the ability to have band gaps in the range of 1.92eV to 1.87eV. The lower GaAs junction has a band gap of 1.42eV.
The considerable quantity of photons in the solar spectrum with energies below the band gap of GaAs results in a considerable limitation on the achievable efficiency of GaAs substrate cells.
In spacecraft applications, the cells have a poor current match due to a greater photon flux of photons above 1.87eV vs. those between 1.87eV and 1.42eV. This results in too little current in the GaAs junction, and hampers the overall efficiency since the InGaP junction operates below MPP current and the GaAs junction operates above MPP current. To improve current match, the InGaP layer is intentionally thinned to allow additional photons to penetrate to the lower GaAs layer.
In terrestrial concentrating applications, the scatter of blue light by the atmosphere reduces the photon flux above 1.87eV, better balancing the junction currents.
Germanium substrate
Triple junction cells consisting of
Indium gallium phosphideIndium gallium phosphide , also called gallium indium phosphide , is a semiconductor composed of indium, gallium and phosphorus...
, Gallium arsenide or
Indium gallium arsenideIndium gallium arsenide is a semiconductor composed of indium, gallium and arsenic. It is used in high-power and high-frequency electronics because of its superior electron velocity with respect to the more common semiconductors silicon and gallium arsenide. InGaAs bandgap also makes it the...
and
GermaniumGermanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. Germanium has five naturally occurring isotopes ranging in atomic mass number from 70 to 76...
can be fabricated on germanium wafers. Early cells used straight gallium arsenide in the middle junction. Later cells have utilized In
.015Ga
.985As, due to the better lattice match to Ge, resulting in a lower defect density.
Due to the huge band gap difference between GaAs (1.42eV), and Ge (.66eV), the current match is very poor, with the Ge junction operated significantly current limited.
Current efficiencies for InGaP/GaAs/Ge cells are in the mid 30% range.
Research into methods to produce band gaps in the range between the Ge and GaAs is ongoing. Lab cells using additional junctions between the GaAs and Ge junction have demonstrated efficiencies above 40%.
Indium phosphide substrate
Indium Phosphide may be used as a substrate to fabricate cells with band gaps between 1.35eV and 0.74eV. Indium Phosphide has a band gap of 1.35eV.
Indium gallium arsenideIndium gallium arsenide is a semiconductor composed of indium, gallium and arsenic. It is used in high-power and high-frequency electronics because of its superior electron velocity with respect to the more common semiconductors silicon and gallium arsenide. InGaAs bandgap also makes it the...
(In
0.53Ga
.47As) is lattice matched to Indium Phosphide with a band gap of 0.74eV. A quaternary alloy of Indium gallium arsenide phosphide can be lattice matched for any band gap in between the two.
Indium Phosphide based cells are being researched as a possible companion to gallium arsenide cells. The two differing cells may be either optically connected in series (with the InP cell below the GaAs cell), or through the use of spectra splitting using a
Dichroic filterA dichroic filter or thin-film filter is a very accurate color filter used to selectively pass light of a small range of colors while reflecting other colors. By comparison, dichroic mirrors and dichroic reflectors tend to be characterized by the color of light that they reflect, rather than the...
.
Efficiency
SpectrolabSpectrolab, headquartered in Sylmar, California, is a subsidiary of The Boeing Company that manufacturers space solar cells and panels. It is also a subsidiary of the Boeing Satellite Development Center, which is a unit of Boeing Integrated Defense Systems. It was founded in 1956 by Alfred E. Mann,...
has developed a new multijunction concentrator solar cell with a sunlight-to-electricity conversion efficiency of 40.7 percent, a new world record in solar cell efficiency, costing as little as $3 per watt to install and producing electricity at a cost of 8 to 10 cents per kilowatt-hour. In the 1980s, multijunction solar cells achieved about 16 percent efficiency, and USDOE's
National Renewable Energy LaboratoryThe National Renewable Energy Laboratory , located in Golden, Colorado, as part of the U.S. Department of Energy, is the United States' primary laboratory for renewable energy and energy efficiency research and development.-History:...
broke the 30 percent barrier in 1994..
See also
- High efficiency solar cells
- List of semiconductor materials
- Organic photovoltaic cell
- p-i-n and n-i-p
Typically, amorphous silicon thin-film cells use a p-i-n structure, whereas CdTe cells use an n-i-p structure.The basic scenario is as follows: A three-layer sandwich is created, with a middle intrinsic layer between an n-type layer and a p-type layer...
- Tandem cell
Tandem cell is a photovoltaic cell that works in a fashion similar to those for single-junction cells, with the caveat that some of the light will be converted to other frequencies and re-emitted within the structure....
External links