Chips, cell phones, and other electronic devices are traditionally based on silicon, easily the most recognized, commercially advantageous, and abundant class of semiconductors. The majority of solar technology is still built on silicon, and that’s likely to remain the case for the foreseeable future given how cheap it is. But in order to pave the way for a solar energy revolution that is adopted worldwide, researchers are going to face several barriers scaling up existing silicon-based solar technology. Developing very high-efficiency solar cells — the kind that can be used to power electric vehicles, drones, and cell phones — will require a different class of semiconductor material that can more efficiently accept and transfer sunlight into usable electrical energy.
Gallium arsenide solar cells can harness more of the sun’s energy than silicon
One such material being used in solar cell production that can accomplish this feat is gallium arsenide (GaAs). Gallium is a soft, silvery metal used primarily in electronic circuits, semiconductors, and light-emitting diodes. It is also useful in high-temperature thermometers, barometers, pharmaceuticals and nuclear medicine tests. A gallium arsenide wafer can better absorb and convert higher-energy photons, making it optimal for solar energy conversion.
Some electronics and EV’s, for instance, rely on gallium arsenide chips to process the high-frequency radio signals. This is because thin-film, single-junction GaAs solar cells have a record efficiency of 28.8%. This high efficiency is one of the primary reasons why it’s the material of choice for the Mars Exploration Rover and other NASA applications.
There are 5 distinct ways GaAs is a marked improvement over silicon in solar cell production:
1. Electron Mobility
Electron mobility refers to how quickly an electron can move through a metal or semiconductor. Electrons race through a GaAs crystalline structure much faster than they can through silicon, allowing for much greater solar energy conversion. Semiconductor mobility depends on a number of factors that include the impurity concentrations (including donor and acceptor concentrations), defect concentration, temperature, and electron and hole concentrations.
2. Single Junction Bandgap
In semiconductors and insulators, electrons are confined to a number of bands of energy and forbidden from other regions. The term “band gap” refers to the energy difference between the top of the valence band and the bottom of the conduction band. The GaAs bandgap lies at the energy for the theoretical maximum efficiency of single junction (SJ) solar cells. Furthermore, a high-quality GaAs crystal can be mass produced in a similar method as a silicon crystal, meaning it’s the ideal material for high-efficiency solar cells.
3. High Efficiency
According to the Department of Energy, the maximum theoretical efficiency that a single-bandgap solar cell can achieve with non-concentrated sunlight is about 33.5%, primarily because of the broad distribution of solar emitted photons. This means GaAs produces more power in a given surface area than any other existing solar cell (record efficiency rating for silicon is 26.3%). You can access more semiconductor efficiency levels via the National Renewable Energy Lab (NREL) Solar Efficiency Chart.
4. Heat and Moisture Resistance
The thermal stability of a semiconductor is imperative as it has a direct effect on the performance of a solar cell. Understanding the environmental factors affecting the degradation and eventual failure of PV modules is critical to our understanding of solar cell manufacturing.
Gallium arsenide holds many advantages over silicon in this regard — it is naturally resistant to damage from moisture, radiation and ultraviolet light, making it a better choice for solar energy applications where it must withstand harsh conditions.
5. Superior Flexibility and Size
Because of the higher efficiency level, a GaAs wafer can absorb more sunlight than its silicon counterpart, and do so in a much smaller solar cell. Just a sliver of GaAs placed on a thin flexible substrate can maintain the lightweight and flexible properties and still maintain the efficiency levels needed to charge cell phones, headphones, drones, and many more electronic devices — without the assistance of grid electricity.
What do affordable GaAs solar cells mean for a sustainable future?
High-efficiency solar cells made from gallium arsenide would allow electronics to gain power from ambient light, including indoor lighting, and potentially take EVs off of the grid. EVs have limited range due to battery capacity, but by embedding high-efficiency, lightweight GaAs solar cells into EVs, a single day’s charge can extend their range by 35 miles. Given that the average US driver’s daily commute is 30 miles, this will allow EVs to run on pure clean energy in most cases.
But in order for this to happen, we must address the commercial disadvantage of using GaAs, one of the primary reasons it’s used in a handful of niche applications that can justify the cost.
How Rayton Solar is making gallium arsenide solar cells affordable
Now there is a solar cell manufacturing process that could dramatically reduce the cost of making gallium arsenide solar devices. Rayton Solar is developing high-efficiency solar cells that are cost-effective by using ion implantation technology. This technology reduces the amount of raw gallium arsenide material that is currently wasted (kerf waste) using the prevailing cutting technique and reduces required raw materials by 95%+, thereby significantly reducing costs. Rayton Solar is attempting to develop solar cells consisting of a thin two-micron layer of gallium arsenide film exfoliated from an ingot using an ion-cutting technique.
Thanks to its direct bandgap property, GaAs only needs minimal material to absorb light. The new approach only uses two-micron thick GaAs, which is only roughly 1/300th of a regular wafer (which is usually 600 microns thick).
Below are the four major steps of thin GaAs solar cell fabrication in the Rayton Solar process:
- Proton implantation: a beam of protons is accelerated and directed at the GaAs ingot. The protons penetrate to a depth of two microns below the surface.
- Bonding: the implanted ingot is bonded to a less expensive carrier such as glass using wafer bonding technology such as anodic bonding.
- Exfoliation: the bonded material is annealed in a furnace or on a hot plate; during the annealing, the implanted protons recombine into hydrogen platelets and cause stress inside the crystal. This stress creates micro-cracks below the surface which propagates and spreads along a plane parallel to the surface and then separates the top thin layer from the ingot. The remaining GaAs ingot is put back to the first step until all the material is completely exfoliated.
- Device fabrication: the exfoliated thin GaAs film on glass is fabricated to create solar cells using standard semiconductor processing including metallization, doping, and anti-reflective coating.
Learn more about how this exciting Rayton solar cell manufacturing process aims to make high efficiency solar panel production more affordable.
Originally published at raytonsolar.com on April 27, 2018.