Can Gallium Arsenide Batteries, Which Are Expensive and Powerful, Become Cheaper in the Future?

The 12th China Electronic Information Expo was held in Shenzhen from April 9 to April 11. We all know that semiconductor materials play a vital role in emerging electronic information industries such as 5G, artificial intelligence, and the Internet of Things. Among them, there is a semiconductor material called gallium arsenide, and batteries made of it have more powerful and broad applications.

The most important indicator of solar cells is the photoelectric conversion efficiency. The solar cells laid in the Gobi Desert, which are mainly made of silicon materials, usually have a photoelectric conversion efficiency of about 20%. However, the solar cells used on the Chinese space station have a conversion efficiency of up to 32%. They are gallium arsenide solar cells.

While bringing amazing conversion efficiency, it also brings amazing prices. So, can gallium arsenide batteries become cheaper in the future? Before answering this question, let's first understand the core advantages of this battery.

Can be made very thin

To absorb 95% of sunlight, silicon needs to be more than 150 microns thick, while gallium arsenide only needs 5 to 10 microns.

The benefit is that solar cells made of gallium arsenide can be greatly reduced in weight, which is exactly what is favored by space launches.

Even the lowest-cost, reusable SpaceX Falcon 9 rocket has a launch cost of around 20,000 yuan per kilogram of payload, while other non-reusable rockets have even higher costs for launching each kilogram of payload into space.

In addition to helping reduce the cost of space launches, another advantage of gallium arsenide that is also beneficial to the space environment is its super strong radiation resistance.

All kinds of radiation in outer space are very strong, and in terms of radiation resistance, gallium arsenide is much stronger than silicon solar cells.

High theoretical conversion efficiency

According to a public document from the "China Academy of Engineering Consulting Research Project", the theoretical efficiency of single-junction and multi-junction gallium arsenide solar cells is 27% and 50% respectively, which far exceeds that of silicon material cells.

Screenshot from the China Photovoltaic Industry Development Roadmap (2022-2023) released by the China Photovoltaic Industry Association

The screenshot above shows that the current average conversion efficiency of the three-junction GaAs small cell is about 37%, and it is expected to increase the conversion rate to more than 40% by 2030.

The main benefit of high photoelectric conversion rate for ground applications is that more electricity can be generated in the same area. However, for aerospace applications, there are too many additional benefits.

Gallium arsenide triple-junction batteries are laid on the two wings of the "Tianhe Core Module" of the Chinese space station. Image from Wikipedia, author: Shujianyang.

To generate the same amount of electricity, if silicon batteries are used, the area of ​​the two wings of the Tianhe core module will increase significantly. The problem is that the launch cost will increase. At the same time, the huge wings will also bring a series of difficulties in design and maintenance.

From the high end to the low end

The huge advantages and high price of GaAs batteries mean that their applications can only extend from high-end to low-end. For example:

On November 16, 1965, the Soviet Union launched the Venera 3 to explore the surface of Venus. It used two 2 square meters of solar cells, which was the first use of gallium arsenide solar cells in space. On March 1, 1966, Venera 3 crashed on Venus, becoming the first man-made object to hit the surface of another planet.

Opportunity Mars Rover, image from Wikipedia.

Spirit Mars Rover. Image courtesy of NASA.

A photo of China's Zhurong Mars rover and lander on the surface of Mars, taken by a detachable camera released by the rover. Image from Wikipedia.

During the Mars exploration, the United States' Opportunity and Spirit, as well as China's Zhurong Mars rover, also used gallium arsenide batteries.

In addition to Venus and Mars exploration, lunar probes, such as China's Yutu lunar rover, also use gallium arsenide solar cells.

In addition, the Hubble Space Telescope, the Chinese Space Station, and many satellites also use gallium arsenide solar cells as the main means of obtaining electricity in space.

China's Yutu lunar rover on the lunar surface. Image from Wikipedia.

Hubble Space Telescope, image via NASA.

Chinese space station, image from Wikipedia

After talking about space applications, let’s take a look at the use of near-space.

Near-space, also known as near-space, refers to the area between the flight space of ordinary aircraft and the orbital space of spacecraft. It is generally defined as the space 20 kilometers to 100 kilometers from the ground.

Today, large solar-powered drones can fly for long periods of time in near-space, with the longest continuous flight time being up to two months.

Zephyr drone, image from Wikipedia.

The Zephyr, a large solar-powered drone from abroad, can reach a maximum altitude of 23.2 kilometers, and is already in near-space. In 2022, the Zephyr drone achieved a record of more than two months of continuous flight for the first time.

In terrestrial applications, GaAs solar cells are mainly used in some newly developed high-end solar cars. Some data also show that radar stations and microwave communication stations in remote mountainous areas also partially use this battery to provide power.

What are single knots and multiple knots?

As mentioned earlier, gallium arsenide cells are divided into single junction and multi-junction. So, what exactly do single junction and multi-junction mean?

A prism disperses white light into various colors of light. Image from Wikipedia.

As we all know, Newton divided the spectrum into 7 colors, namely red, orange, yellow, green, blue, indigo and violet. After recalling these, the single junction and multi-junction in solar cells are easy to understand.

That is to say, any solar cell with one junction cannot absorb all colors of light, it can only focus on one color, either absorbing red and orange light the best, or absorbing yellow and green light the best, or absorbing blue and indigo light the best.

However, if the solar cell is made into multiple junctions, such as triple junctions, since each junction has a different absorption focus, the triple junction solar cell can maximize the absorption of the various wavelengths of light in the sunlight.

From one junction to two junctions, and then to three junctions, this is the core means to continuously improve the photoelectric conversion efficiency of solar cells.

Will it be cheaper in the future?

Gallium arsenide has a far-leading photoelectric conversion efficiency and is currently mainly used in high-end scenarios. So, can it be cheaper in the future?

To answer this question, we first need to understand why gallium arsenide is expensive.

If the reason for the high price is the extreme scarcity of materials, such as the global production of only 1,000 tons of gallium, then it is basically impossible to make it. This 1,000 tons of gallium can only be used in a small number of high-end applications.

But that’s not the case.

The United States Geological Survey estimates that known bauxite and zinc ore reserves contain more than 1 million tons of gallium.

Gallium accumulates in sodium hydroxide solutions during the processing of bauxite into alumina, which can be extracted by a variety of methods. This is the main source of gallium metal.

Whether or not humans need gallium, they certainly need aluminum, and huge amounts of aluminum are produced every year. In the process of obtaining aluminum, gallium is a byproduct.

In other words, although gallium production is low, making the metal more expensive, it is not extremely scarce.

Otherwise, we will not see gallium arsenide in some low-end products.

When light shines on gallium arsenide, it can generate electric current. Conversely, when the electric current acts on gallium arsenide in reverse, gallium arsenide will emit light, which is a light-emitting diode.

The LED bulbs used in homes are a type of light-emitting diode, most of which use gallium arsenide.

Commonly used LED lights on home ceilings, image from Wikipedia.

The current reality is that more than 80% of GaAs is mainly used in chips, LED lighting, and LED displays, while less than 2% is used to make solar cells.

Therefore, the high cost of GaAs solar cells is only partly due to the gallium material, while the larger part is due to the preparation process.

Gallium arsenide solar cell manufacturing technology, especially triple-junction gallium arsenide cells, is a comprehensive technology that integrates physical design, process implementation, equipment, and testing.

Since the high cost is mainly reflected in the preparation, we have the confidence to answer the questions raised in the previous article:

The price of gallium arsenide solar cells will inevitably become cheaper in the future. It is difficult to predict how much cheaper it will be, but it will definitely be cheaper than it is now.

Because various manufacturing technologies are constantly improving.

Author: Hanmu Diaomeng, a popular science writer and winner of the "National Excellent Popular Science Work Award" from the Ministry of Science and Technology

Reviewer : Chen Changshui, Researcher, School of Information and Optoelectronics, South China Normal University

Produced by: Science Popularization China

Producer: China Science and Technology Publishing Co., Ltd., China Science and Technology Publishing (Beijing) Digital Media Co., Ltd.