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May 1, 2016

Nano power: a new boost for solar?

Researchers from Rice University in the US have found a more effective way of harvesting sunlight for energy through the use of tiny light-activated gold nanoparticles. New technologies such as this are contributing to the falling costs of solar power implementation, which could solidify its place in future energy production.

By Lindsay Dodgson

Electron carrier

A discovery from Rice University in Texas could boost the efficiency of solar cells and reduce their costs. Researchers have developed a new way of capturing sunlight and converting it into energy by splitting water molecules into oxygen and hydrogen. This technique makes clean, renewable power and is made possible by light-activated gold nanoparticles.

Modern solar panels have an efficiency of around 15%, which is the percentage of sunlight they are capable of turning into usable electricity when it hits the panel. The higher a panel’s efficiency, the less surface area it needs to absorb the same amount of light, and thus, potentially, the less expensive it will be. So, this new method developed by the Rice team could help reduce the cost of solar electricity generation worldwide.

Harnessing sunlight

Rice researchers used nanoparticles to capture sunlight and create electron carriers and electron holes, which were then used to split water molecules in to oxygen and hydrogen. The electron carriers are described as ‘hot’ because they are usually arranged in the same way around the nucleus, but with a higher temperature.

An electron leaves a gap behind called an ‘electron hole’ when it changes to a higher energy state and becomes what’s known as a ‘hot electron’. These are then absorbed by water molecules, causing them to separate.

However, hot electrons disappear very rapidly, in a few trillionths of a second, so they can be difficult to use for processes such as water-splitting. They do this because holes within atoms don’t like to be empty. So, hot electrons have a tendency to quickly recombine with the holes, immediately releasing their energy, and returning to their low-energy state.

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In current solar panels, most of the energy lost is through hot electrons cooling and releasing wasted heat. To address this, lead researcher Dr Isabell Thomann and her team wanted to develop a means of capturing the high-energy electrons before they had the chance to revert. The only way to avoid recombination is to devise a system whereby hot electrons and electron holes are immediately separated.

In previous methods, hot electrons and holes were divided by the Schottky Junction-based carrier extraction process, which is a common and trusted practice used by engineers. In this process, an electric field is used to separate and extract hot electrons and apply them to target molecules, such as in water, for light to be absorbed.

This can be an inefficient technique, says a member of Thomann’s team Shah Mohammad Bahauddin, and they decided to try something new. Instead of using a magnetic field, the Rice team based their structure on extremely small gold nanoparticle discs. When sunlight hits the discs, the electron gas density within them produces hot electrons and holes.

"We took an unconventional approach," Thomann said in a statement. "Rather than driving off the hot electrons, we designed a system to carry away the electron holes."

"Rather than driving off hot electrons, we designed a system to carry away the electron holes."

The system is composed of layers. It has a thin sheet of shiny aluminium which is coated with ‘hole scavenging’ nickel oxide, with gold nanoparticle discs scattered on the top. The device is then covered with water so that the nanoparticles can act as catalysts for subsequent water molecule splitting.

The nickel oxide removes all of the electron holes from the nanoparticles and leaves the hot electrons, acting much like a sieve. Hot electrons are blocked and left available on the surface of the gold discs.

Once the hot electrons and holes are separated they cannot recombine, and the hot electrons move to the only place available; to bind with water molecules.

Being unable to recombine reduces energy loss from hot electrons. This means that solar panels using this technology could lose less heat and therefore be more efficient and effective.

Clean and cost-effective

Building structures inside solar panels could be more cost-effective long-term through Rice’s technology. The efficiencies of their experiment were equal to those of other structures with more expensive components, meaning their method could be preferable when mass producing solar panels.

"This work can provide the pivot for the design and fabrication of simple and cost-effective nanostructures," says Bahauddin, referring to technologies in solar panels that rely on water-splitting. "Since the carriers in this case are hot electrons, they can produce higher voltage in the solar cells than the conventional architectures."

The technique could be used to incorporate light-capturing nanomaterials in future designs of solar panels, which could result in designs that generate higher levels of electricity from the same amount of sunlight.

"Since we have designed our structure to increase the efficiency of carrier extraction," says Bahauddin, "It can be used in solar cells which work in [a] similar mechanism."

The team is optimistic that this success is only the start, as Thomann says: "We are confident that we can optimise our system to significantly improve upon the results we have already seen."

Solar over time: efficiency and costs

The record for solar cell efficiency stands at about 40%, which was recorded in a laboratory. It was made using cells composed of multiple layers of silicon, each tuned to trap different frequencies, or colours, of light. Cells like these would be hugely expensive to mass produce, and have so far mainly been used in space where efficiency outweighs financial costs.

The price of solar panels has fallen considerably since the 1970s. The cost has decreased from about $76.67 per watt to $0.613 per watt, which is more than 100 times lower than in 1977. In addition, technological advances such as the research at Rice University are making renewable options more efficient so the price of generating clean energy is steadily dropping.

It is being recognised that solar power and other renewable technologies are no longer more expensive than fossil fuels, in many countries. Sustainable energy plays a big part in countries aiming to reduce their carbon footprint, so present realistic ways to make the technology cheaper and better at harnessing energy from sunlight, and so more mainstream.

"The plummeting price of renewables is creating a historic opportunity to build a clean, sustainable energy system."

At its annual conference in Abu Dhabi this year, The International Renewable Energy Agency, and other organisations, predicted that solar costs are going to fall considerably over the next few years.

"The game has changed," says director-general of the International Renewable Energy Agency (IRENA) Adnan Z Amin. "The plummeting price of renewables is creating a historic opportunity to build a clean, sustainable energy system and avert catastrophic climate change in an affordable way."

Research is contributing to renewables more affordable and accessible. By enabling solar energy providers to significantly increase their solar-to-electric power conversion efficiency, the team at Rice believes its method will aid nations to meet their goals of achieving low-cost solar electricity.

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