The sunlight that powers solar panels also damages them. ‘Gallium doping’ is providing a solution


Matthew Wright, UNSW; Brett Hallam, UNSW, and Bruno Vicari Stefani, UNSWSolar power is already the cheapest form of electricity generation, and its cost will continue to fall as more improvements emerge in the technology and its global production. Now, new research is exploring what could be another major turning point in solar cell manufacturing.

In Australia, more than two million rooftops have solar panels (the most per capita in the world). The main material used in panels is silicon. Silicon makes up most of an individual solar cell’s components required to convert sunlight into power. But some other elements are also required.

Research from our group at the University of New South Wales’s School of Photovoltaics and Renewable Energy Engineering shows that adding gallium to the cell’s silicon can lead to very stable solar panels which are much less susceptible to degrading over their lifetime.

This is the long-term goal for the next generation of solar panels: for them to produce more power over their lifespan, which means the electricity produced by the system will be cheaper in the long run.

As gallium is used more and more to achieve this, our findings provide robust data that could allow manufacturers to make decisions that will ultimately have a global impact.

The process of ‘doping’ solar cells

A solar cell converts sunlight into electricity by using the energy from sunlight to “break away” negative charges, or electrons, in the silicon. The electrons are then collected as electricity.

However, shining light on a plain piece of silicon doesn’t generate electricity, as the electrons that are released from the light do not all flow in the same direction. To make the electricity flow in one direction, we need to create an electric field.

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In silicon solar cells — the kind currently producing power for millions of Australian homes — this is done by adding different impurity atoms to the silicon, to create a region that has more negative charges than normal silicon (n-type silicon) and a region that has fewer negative charges (p-type silicon).

When we put the two parts of silicon together, we form what is called a “p-n junction”. This allows the solar cell to operate. And the adding of impurity atoms into silicon is called “doping”.

An unfortunate side effect of sunlight

The most commonly used atom to form the p-type part of the silicon, with less negative charge than plain silicon, is boron.

Boron is a great atom to use as it has the exact number of electrons needed for the task. It can also be distributed very uniformly through the silicon during the production of the high-purity crystals required for solar cells.

But in a cruel twist, shining light on boron-filled silicon can make the quality of the silicon degrade. This is often referred to as “light-induced degradation” and has been a hot topic in solar research over the past decade.

The reason for this degradation is relatively well understood: when we make the pure silicon material, we have to purposefully add some impurities such as boron to generate the electric field that drives the electricity. However, other unwanted atoms are also incorporated into the silicon as a result.

One of these atoms is oxygen, which is incorporated into the silicon from the crucible — the big hot pot in which the silicon is refined.

When light shines on silicon that contains both boron and oxygen, they bond together, causing a defect that can trap electricity and reduce the amount of power generated by the solar panel.

Unfortunately, this means the sunlight that powers solar panels also damages them over their lifetime. An element called gallium looks like it could be the solution to this problem.

A smarter approach

Boron isn’t the only element we can use to make p-type silicon. A quick perusal of the periodic table shows a whole column of elements that have one less negative charge than silicon.

Adding one of these atoms to silicon upsets the balance between the negative and positive charge, which is needed to make our electric field. Of these atoms, the most suitable is gallium.

Gallium is a very suitable element to make p-type silicon. In fact, multiple studies have shown it doesn’t bond together with oxygen to cause degradation. So, you may be wondering, why we haven’t been using gallium all along?

Well, the reason we have been stuck using boron instead of gallium over the past 20 years is that the process of doping silicon with gallium was locked under a patent. This prevented manufacturers using this approach.

Gallium-doped silicon heterojunction solar cell.
Robert Underwood/UNSW

But these patents finally expired in May 2020. Since then, the industry has rapidly shifted from boron to gallium to make p-type silicon.

In fact, at the start of 2021, leading photovoltaic manufacturer Hanwha Q Cells estimated about 80% of all solar panels manufactured in 2021 used gallium doping rather than boron — a massive transition in such a short time!

Does gallium really boost solar panel stability?

We investigated whether solar cells made with gallium-doped silicon really are more stable than solar cells made with boron-doped silicon.

To find out, we made solar cells using a “silicon heterojunction” design, which is the approach that has led to the highest efficiency silicon solar cells to date. This work was done in collaboration with Hevel Solar in Russia.

We measured the voltage of both boron-doped and gallium-doped solar cells during a light-soaking test for 300,000 seconds. The boron-doped solar cell underwent significant degradation due to the boron bonding with oxygen.

Meanwhile, the gallium-doped solar cell had a much higher voltage. Our result also demonstrated that p-type silicon made using gallium is very stable and could help unlock savings for this type of solar cell.

To think it might be possible for manufacturers to work at scale with gallium, producing solar cells that are both more stable and potentially cheaper, is a hugely exciting prospect.

The best part is our findings could have a direct impact on industry. And cheaper solar electricity for our homes means a brighter future for our planet, too.

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The Conversation

Matthew Wright, Postdoctoral Researcher in Photovoltaic Engineering, UNSW; Brett Hallam, Scientia and DECRA Fellow, UNSW, and Bruno Vicari Stefani, PhD Candidate, UNSW

This article is republished from The Conversation under a Creative Commons license. Read the original article.

When tree planting actually damages ecosystems

Giraffes prefer the open space and scattered trees of the African savanna.
Volodymyr Burdiak/Shutterstock

Kate Parr, University of Liverpool and Caroline Lehmann, University of Edinburgh

Tree planting has been widely promoted as a solution to climate change, because plants absorb the climate-warming gases from Earth’s atmosphere as they grow. World leaders have already committed to restoring 350m hectares of forest by 2030 and a recent report suggested that reforesting a billion hectares of land could store a massive 205 gigatonnes of carbon – two thirds of all the carbon released into the atmosphere since the Industrial Revolution.

Many of those trees could be planted in tropical grassy biomes according to the report. These are the savannas and grasslands that cover large swathes of the globe and have a grassy ground layer and variable tree cover. Like forests, these ecosystems play a major role in the global carbon balance. Studies have estimated that grasslands store up to 30% of the world’s carbon that’s tied up in soil. Covering 20% of Earth’s land surface, they contain huge reserves of biodiversity, comparable in areas to tropical forest. These are the landscapes with lions, elephants and vast herds of wildebeest.

Gorongosa, Mozambique. The habitat here is open, well-lit and with few trees.
Caroline Lehmann, Author provided

Savannas and grasslands are home to nearly one billion people, many of whom raise livestock and grow crops. Tropical grassy biomes were the cradle of humankind – where modern humans first evolved – and they are where important food crops such as millet and sorghum originated, which millions eat today. And, yet among the usual threats of climate change and wildlife habitat loss, these ecosystems face a new threat – tree planting.

It might sound like a good idea, but planting trees here would be damaging. Unlike forests, ecosystems in the tropics that are dominated by grass can be degraded not only by losing trees, but by gaining them too.

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Where more trees isn’t the answer

Increasing the tree cover in savanna and grassland can mean plant and animal species which prefer open, well-lit environments are pushed out. Studies from South Africa, Australia and Brazil indicate that unique biodiversity is lost as tree cover increases.

This is because adding trees can alter how these grassy ecosystems function. More trees means fires are less likely, but regular fire removes vegetation that shades ground layer plants. Not only do herbivores like zebra and antelope that feed on grass have less to eat, but more trees may also increase their risk of being eaten as predators have more cover.

A mosaic of grassland and forest in Gabon.
Kate Parr, Author provided

More trees can also reduce the amount of water in streams and rivers. As a result of humans suppressing wildfires in the Brazilian savannas, tree cover increased and the amount of rain reaching the ground shrank. One study found that in grasslands, shrublands and cropland worldwide where forests were created, streams shrank by 52% and 13% of all streams dried up completely for at least a year.

Grassy ecosystems in the tropics provide surface water for people to drink and grazing land for their livestock, not to mention fuel, food, building materials and medicinal plants. Tree planting here could harm the livelihoods of millions.

Losing ancient grassy ecosystems to forests won’t necessarily be a net benefit to the climate either. Landscapes covered by forest tend to be darker in colour than savanna and grassland, which might mean they also absorb more heat. As drought and wildfires become more frequent, grasslands may be a more reliable carbon sink than forests.

Read more:
Exaggerating how much CO₂ can be absorbed by tree planting risks deterring crucial climate action

Redefine forests

How have we reached the point where the unique tropical savannas and grasslands of the world are viewed as suitable for wholesale “restoration” as forests?

At the root of the problem is that these grassy ecosystems are fundamentally misunderstood. The Food and Agricultural Organisation of the UN defines any area that’s half a hectare in size with more than 10% tree cover as forest. This assumes that landscapes like an African savanna are degraded because they have fewer trees and so need to be reforested. The grassy ground layer houses a unique range of species, but the assumption that forests are more important threatens grassy ecosystems across the tropics and beyond, including in Madagascar, India and Brazil.

A flowering aloe in Madagascan grassland.
Caroline Lehmann, Author provided

“Forest” should be redefined to ensure savannas and grasslands are recognised as important systems in their own right, with their own irreplaceable benefits to people and other species. It’s essential people know what degradation looks like in open, sunlit ecosystems with fewer trees, so as to restore ecosystems that are actually degraded with more sensitivity.

Calls for global tree planting programmes to cool the climate need to think carefully about the real implications for all of Earth’s ecosystems. The right trees need to be planted in the right places. Otherwise, we risk a situation where we miss the savanna for the trees, and these ancient grassy ecosystems are lost forever.The Conversation

Kate Parr, Professor of Tropical Ecology, University of Liverpool and Caroline Lehmann, Senior Lecturer in Biogeography, University of Edinburgh

This article is republished from The Conversation under a Creative Commons license. Read the original article.