Solar cells: essential for a brighter future

You wouldn’t recognise the precursor to the modern solar panel if you saw it, and who knows what they’ll look like in the future?

The precursor to the first solar panel wasn’t really a panel, and it didn’t even use the sun’s light. But the physical processes first observed by French scientist Antoine César Becquerel, in his laboratory in 1839 and then in bars of selenium by Willoughby Smith when checking telegraph cables to be submerged under the Atlantic Ocean, are essentially the same as what happens in solar cells everywhere today.

In a nutshell: light shines onto a semiconductor material, which then produces an electric current – no moving parts, no steam, no turbines.

The first true solar cell prototype was created in 1883 by Charles Fritts, a New York inventor – who, incidentally, also tinkered with watch springs, curtain fixtures and train car couplings (oh to be a 19^th century inventor!) – and consisted of a thin layer of selenium spread onto a metal plate, coated with a layer of gold-leaf film, so thin it was nearly transparent. Less than 1% of the light energy absorbed by this first solar cell was converted into electrical energy – a fact that did not escape the attention of many of the engineers, physicists and entrepreneurs of the day newly obsessed with the promise of coal-fired turbines.

Fast forward through the years…

We flick through 1905, past the iconic Albert Einstein publishing the physics behind the photoelectric effect (it’s all in the name, folks), through to the introduction of the copper oxide solar cell in 1927 and finally to 1941 when silicone brightened the future of solar cells considerably. Over the next decade or so – helped considerably by the advent of transistors – the 1% energy conversion impasse was finally overcome. Energy conversion rates inched up to 6% and then, in the late 1980s, to around 20% – pretty much what you can expect from the average rooftop panel (an array of joint solar cells) today.

Well, the ones we put on roofs, anyway.

No doubt you’ve quite a few of them around you, adorning houses, supermarkets and solar farms.

They were installed on the White House roof in 1979 by then President Jimmy Carter before Ronald Reagan came along and turned the light off on renewables for the next few decades.

These rooftop-type panels consist of silicon based cells, explains Dr. Cameron Shearer, a research fellow in chemistry at the University of Adelaide. “They consist of two types of silicon. One is very good at transporting electrons, while the other is very good at transporting holes” (or absences of electrons). When light is absorbed, the electrons started to move, creating a current.

A second kind of solar cell is typically found in space applications, where weight and efficiency considerations reign supreme – in spacecraft, satellites and space observatories. These have a series of layers which absorb many different energies of light – not just visible light – from the Sun, making them far more efficient (with energy conversions clawing towards 48%), but the materials used are far more expensive.

Our energy needs

In 2021, a total of 1,032.5 TWh (terawatt-hours) of solar energy was generated globally. Of that, Australia, with its plentiful sunlight and open space generated 31.19 TWh. By comparing this to the total global energy generation from all energy sources of 28214 TWh in 2021 and taking into account the average amount of sunlight reaching Central Australia, the area of solar panels needed to generate the electricity used in 2021 for the entire globe would be a touch over 55,000 km², calculates Shearer. This is about 80% of the state of Tasmania, or slightly less than the area of Croatia.

To produce the 13340 TWh of energy consumed globally in 2021 (which includes non-electrical forms of energy, such as diesel, petrol and other fuels, for instance), would require an area of around 340,000 km². It sounds like a lot, but the area covered by deserts on the Australian content alone is around 2.7 million km², and this says nothing of the area covered by suitable roofs.

But, according to the International Roadmap for Pholtovoltaic, the world is going to need to generate more than 60 TWh via solar energy to reach net zero emissions.

There are a few bottlenecks for solar at the moment.

Firstly, it is that same problem from 150 years ago: efficiency. Current rooftop solar cells are limited by the material from which they are made, silicon. The energy conversion efficiency of a given solar cell is limited by what’s called the Shockley Quiesser limit, which takes into account how well the material absorbs energy and how much energy is lost through the whole process. For silicon, at absolute zero, the energy conversion efficiency is 33%, but in real-world conditions, it tops out at around 29%.

There is hope for increasing the efficiency of rooftop solar cells though, both in the panel design and the solar cell itself.

Solar panels consist of many solar cells linked together, which leads to dead spots of space and energy losses in the connections, so no matter how careful the design, panels will inevitably introduce some amount of inefficiency.

New solar cell technology

One of the most promising materials researchers have been working with are perovskites – a class of mineral structures, explains Shearer. The one showing the most promise is a lead halide, which when combined with silicon in tandem solar cells, has been shown in laboratories to be capable of a whopping 31.25% energy conversion.

Before you say, “that’s not that much”, the average energy efficiency of a coal power plant in the US is about 33%.

Perovskite in layers with silicone maybe the way of the future for solar cells. Perovskite tends to work better with high-energy visible light (those wavelengths that appear bluer to us), whereas silicon tends to work better in the lower energy, or redder, wavelengths. Perovskite can also be tuned to absorb specific colours (that is, red, green and blue) by varying its chemistry, meaning that if sensitively aligned, stacks of tuned perovskite and silicone might even push the whole stacked cell to efficiencies of over 50%.

There are other stackable materials being developed, which you can read more about here.

Interviewed in the Sydney Alumni Magazine, Professor Anita Ho-Baillie has been working with perovskites and is thrilled with the progress. “It took people 40 years to double the efficiency of silicon,” she says. “Perovskite caught up with silicon in just 10 years.”

What’s more, perovskite can be printed, making the manufacturing process much quicker. trying to make them more stable to degradation and appropriate for use commercially. “It’s just easier to handle than silicon,” says Ho-Baillie. “It used to take me four weeks to make a silicon cell in the lab. With perovskite, it takes only two days.”

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Read More: Another promising material in solar cell development are conductive organic polymers

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Solar panels must also collect as much light as possible. Usually that means tracking the Sun and moving the panels via machinery. Recently, a group from Stanford University in the US has produced a device that collects and concentrates the Sun’s light removing the necessity to constantly move the panels. Fewer moving parts increases the longevity of such systems.

The issue of waste and resource management is a serious concern. “Although the raw materials needed – silicon, silver, glass and plastic – are not rare, it’s still easier to produce en mass solar cells from sand than it is to produce them from old solar cells”, says Shearer. But, there are already companies in Australia jumping at the commercial potential in recycling solar cells and even the aluminium framing is part of the conversation.

Storing power from solar cells

Finally, one of the biggest advances we can expect to see in this space over the next decade or so is less to do with solar cells, specifically, and more to do with how we store and use power from them and other renewables, ensuring that demands can be met out of production hours.

Large lithium ion batteries like the 100 megawatt Tesla battery connected to the Hornsdale Wind Farm in the mid north of South Australia, are one development from the past decade that has cropped up to support renewable energy.

Many residents of Adelaide, including Shearer, are also connected to the ‘virtual power plant’.

This means solar panels on his roof occasionally deliver power to the grid. But, as Shearer explains, “battery storage, as a whole is very heavy, and we need electricity to power things other than just our home or our electricity grid, such as long-haul transport or heavy industry”.

Shearer’s own research focusses on storing energy another way – by using the energy to split apart water into its constituents: hydrogen and oxygen. “When they recombine, the create water and release energy”, he says, noting that the hydrogen can also then be used as a fuel. “It’s a completely renewable process, with no carbon emission at any stage.” To Shearer, the future of renewable energy and solar cells in particular, is bright. As we head towards a carbon-neutral future, “we are going to see the benefits of having our own solar panels, having our own batteries’, he says. With advances into solar cells and other renewables, plus technology such as batteries and other methods of chemical energy storage arising to support them, the next decade or two may look very different from the last few – and certainly different from 150 years ago.

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