Multijunction photovoltaic cell
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Multi-junction photovoltaic cells are a sub-class of solar cell or photovoltaic cell developed for higher efficiency. These multijunction cells consist of multiple thin films produced using molecular beam epitaxy and / or Metalorganic vapour phase epitaxy. Each type of semiconductor will have a characteristic band gap energy which, loosely speaking, causes it to absorb light most efficiently at a certain color, or more precisely, to absorb electromagnetic radiation 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.
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[edit] History
Multi junction solar cells first were developed and deployed for Satellite power applications where the high cost was offset by the weight savings offered by the higher efficiency.
Multi junction 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.
[edit] 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 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.
[edit] Device description
Multi junction photovoltaic cells use many layers of Epitaxy deposited films. By using differing alloys of III-V Semiconductors, 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 constant).
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 sufferers. Current match of each junction is a very important design consideration for multi junction cells.
[edit] Material Classification
Multi junction cells may be categorized by the substrate used for cell manufacture. Cells on Germanium and Gallium arsenide have been commercialized. Research into Indium Phosphide based cells for lower band gaps is ongoing.
[edit] Gallium arsenide substrate
Twin junction cells with Indium gallium phosphide 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.
[edit] Germanium substrate
Triple junction cells consisting of Indium gallium phosphide, Gallium arsenide or Indium gallium arsenide and [Germanium] 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 bang 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%.
[edit] 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 arsenide (In0.53Ga.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 filter.
