When the first commercial solar photovoltaic panel was unveiled in 1954, it was only capable of converting into electricity six per cent of the Sun’s light.
Efficiency has increased since then, making solar a viable option for homes and power plants alike. The gains started plateauing in the 1990s and today’s panels, which convert about a quarter of the sunlight they capture into electricity, are only slightly more productive than the ones made in 1995.
This is, at least, the case for panels made out of crystalline silicon, says Prof Ammar Nayfeh, associate professor of electrical engineering and computer science at the Masdar Institute for Science and Technology.
The search for alternatives has yielded results, with new types of efficient cells and new uses for solar energy.
One approach has been to layer materials on top so as to better harvest lightwaves of different frequencies. These are called multi-junction cells and, together with colleagues from the Massachusetts Institute of Technology (MIT), Prof Nayfeh employs this approach.
“Multi-junction cells have high efficiencies – lab measurements point to 35 per cent and higher – but are too costly for mainstream applications,” he says, explaining that these panels are used in the aerospace industry, where cost is less of an issue.
Prof Nayfeh and his colleagues are hoping to one day change this. The team, which includes Dr Sabina Abdul Hadi, a post-doctoral fellow at the institute, has developed a highly-efficient cell that can also be manufactured cheaply. The design uses silicon as a base, making it more affordable compared to the alternatives. It also has a top layer of gallium arsenide phosphide, which absorbs photons in the blue, green and yellow spectrum of light, while the bottom silicon layer absorbs the photons from red light. In addition, part of the bottom silicon cell is exposed so it absorbs the entire visible light spectrum.
A laboratory-scale device, one by one centimetre, has confirmed the concept and work is now under way to demonstrate it on a larger scale.
The approach aims to increase efficiency by improving panels’ ability to absorb light at different wavelengths. An alternative approach employed in thermophotovoltaic systems is to collect the solar energy as heat, which is then transformed into light waves that can be captured by a photovoltaic cell more effectively.
“There are many wavelengths that are emitted from the sun and some of them are below what we call the band gap of the solar cell, that means those photons that are below the band gap don’t have enough energy to generate electricity,” says Evelyn Wang, associate professor at MIT.
In a photovoltaic system only the wavelengths that are above the band gap are converted to electricity. In a thermophotovoltaic system all the sunlight converts to heat and is then targeted at a particular wavelength that gives the best kind of performance for the photovoltaic cell.
“While the converter is still a solar cell, essentially we’ve added new components to the front side of the solar cell that allow us to achieve more efficient conversion,” Prof Wang says. “Theoretically, you can achieve very high efficiencies in the order of almost 90 per cent.”
Such systems consist of a selective absorber emitter and photovoltaic cells. Because the thermal conversion happens at temperatures of about 1,000°C, the system is contained in a vacuum tube, minimising heat losses.
Prof Wang and a team of colleagues have already achieved efficiencies of six per cent in a laboratory-scale device and are now scaling up the project. One day, such systems can power residential buildings and provide electricity in remote areas, she says. However, in the meantime more work awaits.
“There is still a lot of research and development that needs to be undertaken before we can really move forward and really utilise this technology,” said Prof Wang.
Another new area is organic photovoltaics, which are composed of organic semiconductors – a special class of carbon-based molecules. They are cheap to produce, flexible and may in the future lend themselves to fabrication via 3D printing technology, says Paul Berger, professor at Ohio State University, in the US.
Organic photovoltaics are among Prof Berger’s research interests and, despite some challenges – such as low efficiency and the need for hermetic sealing to prevent them from reacting with water and oxygen – these devices can be a game-changer in terms of enabling new uses for solar power.
Rather than deploying power through utility transmission lines, it is better to attach them to everyday objects so they scavenge small amounts of energy, providing point-of-use power without the cords, says Prof Berger.
“I do not see the organic [cells] competing against coal, I do not see organic being out in the Arizona desert for 30 years producing energy,” he says.
Instead, they can be attached to backpacks or the exterior of cars, providing point-of-use electricity. Together with colleagues at Tampere University of Technology in Finland, Prof Berger is working on a project to incorporate organic solar cells into simple electronic sensor devices that can be attached to sensor grids and everyday objects, like grocery products, helping with stocktaking and also giving customers information about the products.
“It can tell you whether the milk got too hot and maybe it has gotten damaged, it can tell you expiration dates, maybe it should warn you that you need to buy some more milk,” he says.
Work is under way to integrate the different parts into one single unit and, in the future, it could be printed in sheets and cut to size, just like a Sunday paper, says Prof Berger.
