by Catriona Nguyen-Robertson MRSV
Science Engagement Officer

This article follows a presentation on 12 March, 2020 titled “Sustainable Solar Energy Through Exciton Control” by organic chemist Dr Wallace Wong, a Chief Investigator from the ARC Centre of Excellence in Exciton Science to the Royal Society of Victoria. The lecture was presented as a part of the Centre’s “Light Conversations” lecture series.

In Victoria, most of our electricity comes from the burning of brown coal mined in the Latrobe Valley. But the origins of that energy are from the Sun. Most of the energy on Earth originates from the Sun. Dr Wallace Wong harvests this energy directly to produce clean energy without reliance on fossil fuels.

Heat is generated at the 15-million-degree core of the Sun and it travels outwards to the surface, then radiates to our planet and throughout the Solar System. The energy we obtain from burning coal today comes from the energy that prehistoric vegetation absorbed from the Sun millions of years ago. But it instead only takes microseconds to convert sunlight directly into electricity.

Global electricity consumption continues to accelerate with economic growth and industrial demand. Around 23 trillion kilowatt hours of energy were consumed in the single year of 2018 – the equivalent power needed to turn on 1,800 billion LED bulbs for an hour. To provide our growing population with the level of energy the developed world is used to, we would need to generate 60 trillion kilowatts worldwide. Power plants as we know them cannot satisfy these demands; however, the sunlight energy striking the Earth’s surface in an area the size of Texas alone could provide up to 300 times the total power output of all the power plants in the world. “Solar energy has the greatest potential to fill this energy gap,” says Wong.

There are multiple ways to harvest solar energy: solar thermal technology, solar fuels, and photovoltaic cells. Solar thermal technology is efficient at heating water for use in pools and houses. Solar fuel will ideally become a new renewable source of fuel through the production of liquid hydrogen. Photovoltaic systems are the solar cells we are most familiar with that can convert photons from light energy into electricity.

Most commercial solar cells that are currently used for solar farms and residential rooftops are silicon-based. Silicon dioxide (sand) is purified into pure silicon and then the solar module is encased in glass. Silicon is a common element – there is no shortage of sand – however, the purification process is not cheap. With the current cost of electricity at around $1/Watt, it would take around 6 years to break even with the cost of solar panel installation by feeding electricity back into the grid. In addition, the amount of time it would take to harvest the same amount of solar energy as the energy cost of production would be a minimum of two years in a sunny location.

As an organic chemist, Wong is developing printed solar cells, composed of organic polymers – strings of repeating molecules, such as benzene. Benzene rings contain π bonds between carbon atoms, which allow electrons to flow freely through the molecule. By stringing benzene rings together in a large crystal, electrons can hop between individual molecules and this provides the material with semi-conductive properties. Rolling onto flexible, organic photovoltaic cells, the energy payback takes 1-3 years. Wong researches ways to improve the production and manufacturing of these printed solar cells, and with improved production, their energy payback can be as short as a single day.

To make solar cells even more efficient, we need to break the Shockley-Queisser (SQ) limit.

When solar energy is converted to electricity, some energy is lost as heat, and some light is reflected back or passes through the solar cell without being captured. The best modern production silicon cell efficiency commercially available is 24%. But we can do better.

The SQ limit calculates that a solar cell’s energy maximum conversion efficiency is 33%. The original calculation was 30% for a silicon solar cell, but current solar cell production efficiencies vary by the wavelengths of light the semiconductor material can absorb.

Sunlight provides electromagnetic radiation covering the entire light spectrum from infrared, to visible, to ultraviolet light. Solar cells are designed to absorb light at specific wavelengths and don’t capture the entire spectrum. 42% of solar energy is in the visible range (light that we can see), 5% in the UV range, and 52% is near-infrared. One way to make the most of all light photons is to “up-convert” the low-energy “waste photons” into high-energy photons. This is achieved by exciting the low-energy photon to an intermediate state for it to be excited again, thereby jumping up even further in energy. By combining techniques to up-convert photons and reduce thermal energy loss, Wong can boost the efficiency of silicon solar cells from 26.7% to 32.5%.

Wong is also developing luminescent solar concentrators (LSC), which capture and trap sunlight. Originally becoming a scientist because he liked “the beauty of art, objects and molecules”, this is the perfect opportunity for Wong to combine art and science. The concentrators contain a fluorescent dye dispersed in a plastic matrix – the dye absorbs light, which is trapped in the plastic. If connected to a solar cell, the emission trapped inside can be converted into a current. Wong and his colleagues are creating materials that could transform windows and walls into solar cells and are ideal for capturing light indoors. He has designed a coloured plastic wall feature for his own house to one day be connected to LSCs to power mobile phones, power banks, and more.

Ultimately, a combination of solar cell technologies and solar fuel will be crucial to creating our energy-efficient future. “We need to think about all renewable energies,” Wong says. He believes that we will no longer need to rely on fossil fuels. We will be living on sunshine.

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