The basic principle on which solar cells work is Photovoltaic ie. they convert the photons from the sunlight into usable electricity. However, in this process neither is all the light absorbed nor does all the photons get converted into electricity. High energy blue light photons do not have all of their energy converted into electricity, some is wasted as heat & low energy red light photons are not absorbed at all. Sunlight is made up of a broad spectrum and depending upon these various wavelengths, photons of light also have different energy levels.Hence, to minimize this wastage & to enhance the efficiency of solar cells, scientists came up with the concept of multijunction solar cells.
Efficiency and Spectrum
Theoretically, efficiencies of over 70% could be reached by using hundreds of different layers, each one tuned to a small range of light wavelengths varying from ultraviolet to infrared. However due to difficulties in manufacturing such complicated crystals, the idea could not be put into practice. The theoretical efficiency of multi-junction solar cells is about 86.8% for an infinite number of junctions, implying that more junctions increase efficiency.Efficiency of solar cells made from silicon normally varies from 15-24%
Multijunction Cell Composition
Practically, a multi-junction solar cell is made up of a two or more layers of semi-conductor material – for example, one layer that can absorb blue light, and a second layer that can absorb red light. Hence, the overall efficiency is better than a single material solar cell. Multi-junction cells were conceived to get the maximum output of energy by converting the maximum number of photons from sunlight. To achieve this purpose these should be designed in a way to absorb the ample amount of the light energy. Hence, the sequencing of the layers of a multi-junction solar cells is very important. It is decided by their individual capacity to absorb the different wavelengths of light. The topmost layer, which is the closest to the sun is the layer with the largest band gap. Subsequent layers are then positioned in descending order of their band gaps. The highest energy photons like blue light are captured by the top layer, and the bottom layer captures the lower energy photons like the red to infra red which passes through the other layers.
Layers and Materials used in Multijunction Solar Cells
The majority of multi-junction cells that have been produced to date use three layers, tuned to blue (on top), yellow and red (on the bottom). Most of them are made of gallium arsenide (GaAs) compounds, which can be tuned to specific frequencies. Light concentrators increase efficiencies and reduce the cost. The two types of light concentrators are refractive lenses like Fresnel lenses and reflective dishes, because of these devices, light reaching the large surface can be concentrated on a smaller cell.
Challenges in Producing Multijunction Cells
It is a difficult task to produce a Multi-junction cell, because of the thinness of the materials and the difficulties in extracting the current between the layers. The technique used in amorphous silicon solar cells, is simpler where two mechanically separate thin film solar cells are used and are connected together outside the cell. Another technique is where the cell consists of a number of layers that are mechanically and electrically connected. This is known as monolithically integrated cell. However, this a more complex set-up because the photocurrent generated in each layer has to be matched, otherwise electrons will be absorbed between layers.
List of Multijunction Solar Cell Manufacturers
- Spire Semiconductor which is a small solar equipment company based in the USA has been working with US National Renewable Energy Laboratory (NREL) under an 18 month incubator project.The company has managed to produce a triple-junction cell with 42.3 per cent conversion efficiency.
- Emcore Corporation (NASDAQ: EMKR) is a commercial provider of Solar Cells for Spacecraft.With a sunlight-to-electricity conversion efficiency of 30%, the ZTJ solar cell is the highest performance space qualified multi-junction solar cell. Production of the multi-junctionsolar panels will take place at EMCORE’s state-of-the-art manufacturing facilities located in Albuquerque, New Mexico.
- Amonix is the leading designer and manufacturer of concentrated photovoltaic (CPV)commercial solar power systems.. Originally developed for space-based applications,Amonix III-V multijunction cells use three different photovoltaic materials in a single cell, they extract more energy from the range of wavelengths in sunlight.. Under concentration, this can exceed 39% in volume production (over 41% in world-record demonstrations).
- Cyrium Technologies Inc., a designer and producer of high efficiency photovoltaic solar cells, The company is privately-held and received its first venture capital funding in 2004. Cyrium’s first generation solar cells offer efficiencies of 40 per cent or higher together with a nearly constant conversion efficiency for solar concentrations from 200 to greater than 1000 suns.
- Spectrolab, headquartered in California is a subsidiary of The Boeing Company that manufacturers space solar cells and panels. . The company claims its “NeXt Triple Junction” high efficiency solar cellshave a minimum average efficiency of 29.9%. In 2006 testing at the National Renewable Energy Laboratory demonstrated an efficiency of 40.7% using triple-junction solar cells developed by Spectrolab.
Cost and Price of Multi-junction Solar Cells
Multijunction Solar Cells cost much more than the mainstream silicon solar cells .The reason is that most of the production is done in labs instead of the massive factories which means that their is no scale advantages.They also use materials which are not as cheap as silicon in making these cells.The process and equipment required for making the multi-junction solar cells is also much higher.This means that some of the multi-junction solar cells can cost many multiples of the PV solar Cells available for sale.These days the cheapest Solar Cells can be bought at 75c/watt which is almost 50% lower than the price available a year ago.The price of Multijunction Solar Cells depends on the materials for example some companies using reflectors and cheap materials say that they can produce high efficiency Multijunction Solar Cells for as low as $3/watt in the future.