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Triple-Junction Solar Cell hits 39.5% Efficiency

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In recent years, double and multiple junction solar cells have been breaking the 30% PCE mark and a new triple–junction solar cell developed at the U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) has been shown to have a record efficiency of 39.5%.

Solar cells have become the most broadly developed class of renewable energy technologies in the modern–day energy sector. While single junction solar cells have efficiency limitations around the 30% mark, developing solar cells with multiple junctions have helped push the boundaries of what is possible in terms of power conversion efficiency (PCE).

Adding Junctions to Solar Cells to Improve Efficiency

Modern–day solar cells come in many different forms, whether it be the materials used or the number of junctions used within the cell. Because the aim of commercial solar cells is to reduce the usage of fossil fuels, there is a desire to obtain the highest levels of efficiency as possible.

To do this, scientists and engineers have been creating solar cells with more than one junction, typically using III–V semiconductor materials such as gallium arsenide (GaAs). In the modern–day market, there are already several tandem (two junction) solar cells already used in the space, aerospace, and automotive sectors.

Graph of multijunction III-V cell technology
Multi–junction III–V cells (Source: U.S. Office of
Energy Efficiency & Renewable Energy) (Click image to enlarge)

To improve efficiencies further, and to get the most out of solar cells, there is a push to create solar cells with multiple junctions. However, this brings in a lot of challenges because the more junctions added to a solar cell, the harder it becomes to fabricate the cell and make it commercially feasible. Multi–junction solar cells require materials that have specific electronic bandgaps, but they also require excellent carrier collection and low non–radiative recombination properties.

The efficiency of multi–junction solar cells has been steadily increasing over the years as scientists add more and more junctions and trial different materials, all while trying to obtain the ideal bandgap combination. This has led to the highest efficiencies being obtained for a one–sun (i.e., standard non–enhanced illumination) of 39.2%.

To achieve this high efficiency, up to six junctions were used. So, while the efficiencies were impressive, the sheer number of junctions required to obtain this efficiency brought its own set of problems that would present barriers to commercialization and real–world use. These challenges include mismatched alloys, the need to use challenging and high bandgap materials such as aluminum gallium indium phosphide, and transparent tunnel junctions that require anti–reflection coatings to work effectively.

A New Triple–Junction Cell that Outperforms Multi–Junction Cells

While it’s possible to utilize many junctions to improve the efficiency of a solar cell, the induced challenges often offset the good efficiencies of the device. This is especially true for devices which have a lot of junctions, because the efficiency gained by adding extra junctions after the third junction gets progressively smaller due to material challenges.

So, white tandem solar cells are used in a majority of cases, triple–junction solar cells could well be the sweet spot and trade–off between obtaining a high efficiency and ensuring that the device is not overly complex that it becomes commercially unfeasible.

This is the approach that the team at the U.S. DOE NREL took, and they have now managed to create a triple–junction solar cell that has an efficiency of 39.5% ― outperforming the six–junction solar cell previously created with a lot less challenges.

Instead of adopting the approach of adding junctions to improve the efficiency of the solar cell, the researchers opted to improve the bandgap recombination properties of triple–junction solar cells.

The improvements were based on using an existing triple–junction solar cell with an inverted metamorphic multi–junction architecture that was invented at NREL. The original triple–junction cell is composed of a gallium indium phosphide top layer, a GaAs middle layer and a gallium indium arsenide (GaInAs) bottom layer.

These devices had already shown high efficiency levels of 37.9%, but GaAs was found to not have the ideal bandgap for this triple–junction design ― meaning that the balance of the photocurrents between the three cells was not optimal. It was thought that a lower band gap material would perform better here, so the team set out to optimize the bandgap of the central material and they used quantum wells to aid them in this pursuit.

Quantum wells are nanometer–thin layers that can trap electrons and confine them in one dimension (in the dimension perpendicular to the layer surface), allowing the electrons to move in two dimensions. For these solar cells, the quantum wells were sandwiched between a lower bandgap quantum well layer with two higher bandgap layers acting as barrier layers.

In place of a pure GaAs central cell material, the researchers introduced optically thick GaInAs/GaAsP strain–balanced quantum well superlattices to modify the bandgap of the depleted region of the cell, with minimal loss, high voltage, and a high level of light absorption.

This allowed the triple–junction to have an optimized bandgap across all the junctions ― with a slightly reduced bandgap in the middle layer over similar devices of its kind. By optimizing the bandgap, it allowed more solar energy to be harvested by the device, leading to a higher efficiency.

Potential Applications

Even though the NREL’s research is on the fundamental, more academic side of developments, when the research comes out of a government research facility, the commercialization potential can often carry more weight than pure academic research.

The researchers have a lot of experience in creating advanced solar cell architectures and have stated that the solar cells will be the most useful for area–constrained terrestrial applications and low–radiation space missions.

The solar cells were tested to see how efficient they might be in space applications, where the conditions are vastly different than here on Earth. While the efficiency is likely to be lower in space, it’s thought that the efficiencies could still be as high as 34.2%. Regardless of applications, these cells have now set the standard for one–sun efficiency across all solar cell devices, and the race will be on to see who can push these triple–junction solar cells even further.

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