Six-Junction III–V Solar Cell Breaks Record for Efficiency

Article By : Maurizio Di Paolo Emilio

The solar cell, designed at NREL, gets close to a 50% conversion rate, by far surpassing silicon-based solar cells...

A US research group has developed a new solar cell based on six photosensitive active layers able to capture sunlight. These new cells could potentially reach a light conversion efficiency of 50%.

The conversion efficiencies of traditional flat-plate and single-junction solar cells max out at about 30%. Experiments have shown that increasing the number of junctions and techniques to concentrate light improve the results. The new six-junction solar cell, developed by NREL (National Renewable Energy Laboratory), converts 47.1% of incident light into electricity when combined with optical concentration. Indications thus far suggest solar cells of this type can reach an efficiency rate of 50%.

Multi-junction photovoltaics were developed with the aim of overcoming the efficiency limits of traditional solar cells. The new NREL device  device contains a total of about 140 layers of various III-V alloys. That includes six different photoactive layers. Each photoactive layer captures light from a specific part of the solar spectrum.

In the not-too-distant future the cell could be used in photovoltaic applications.

It is a device that “really shows the extraordinary potential of multi-junction solar cells,” as reported by John Geisz, one of the scientists involved in the project and the main author of the study that describes the device itself and that appeared inNature Energy.

Each of the cell’s six junctions (the photoactive layers) captures light from a specific part of the solar spectrum [Source Nature]

It should be noted that the record was obtained by using special lenses capable of focusing sunlight by increasing the intensity of 143 times, but even without using this technique, the conversion rate is still very high.

The tests with natural light carried out by the research team showed an efficiency of 39.2%, which is also a very high value. According to the researchers, it is also possible to equip solar cells with mirrors to focus sunlight on the surface, leading to the higher efficiency rates.

Internal resistance in series still too high

At the moment, there are a number of obstacles to commercialization, though NREL believes that will probably be overcome in the near future. One of these is the presence of a resistive barrier inside the cell, which prevents the flow of a high percentage of current. This problem does not allow to achieve a 50% efficiency. Another obstacle to consider is the high cost for the production of the materials needed for the devices.

One way to lower costs is to reduce the active lighting area involved. You could, for example, use a mirror or concentrator to capture light and concentrate it on a specific point. This problem exists on Earth, while on other planets, perhaps the ones closer to the sun, the luminous flux is much more powerful.

This solution could reduce the amount of photosensitive material needed by as much as a factor of a hundred or even a thousand. Furthermore, it is well known that efficiency increases when light is concentrated. Prior to this, four-junction solar cells have demonstrated the highest solar conversion efficiency levels, but now, with the adoption of six junctions, the results have greatly improved. A further reduction in series resistance within this structure could realistically allow efficiency levels in excess of 50%. The use and exploitation of light are extremely important and should take place at the highest level and under the best conditions.

NREL scientists John Geisz (left) and Ryan France

The production of solar energy systems is highly dependent on cloud coverage. Weather forecasts can be useful to predict the amount of sunlight that will reach terrestrial solar collectors. This will increase the number of geostationary satellites aimed at accurate and precise forecasting of cloud phenomena. These estimates will affect the amount of sunlight reaching the Earth’s surface by determining the height of the cloud, its thickness, and optical depth. Water, in fact, takes on multiple liquid forms or ice crystals of various sizes. This modifies its absorption by influencing the optical depth of a cloud.

The tests were carried out by manufacturing, characterizing, and analyzing a solar cell with an IMM 6J concentrator with an efficiency of 35.8%. As said before, the limitations concerned the high internal resistive barrier, caused by the diffusion of Zn. The operating temperature also had a major influence on the high resistance of the barrier. The devices were made with electrolytic gold plated contacts combined with a low viscosity epoxy resin silicone handle.

multi-junction solar cells
multi-junction solar cells [Source: Nature]

Solar Energy

The production of electricity from the sun is constantly increasing and is expected to be the main source of electricity supply in the coming decades.

Any type of equipment would work, even better, with more efficient solar cells that, with the same space, would guarantee more power or, vice versa, with the same power, would be smaller. In any case, the new solar cells are able to extract more energy from the Sun.

Lately, in many parts of the world, pollution-free skies have contributed to the increased productivity of photovoltaic power plants. In Britain, on 20th April, solar generation peaked at 9.7 gigawatts. This represents almost 30% of the country’s electricity supply. In Germany, the share of solar energy reached 23% for a whole week in April, compared to an average of around 8% in 2019. Although temporary, these figures are impressive. Solar energy is now ready for new challenges. Further research is needed and technology will certainly make its contribution to ensuring that the world is powered by cleaner electricity.

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