Cambridge Photon Technology is a spin-off from the University of Cambridge, UK, and one of the final eight for The Spinoff Prize 2021.
With the world focusing more on renewable energy, the makers of solar cells are looking to squeeze every bit of electricity possible from their panels. Unfortunately, manufacturers face limits on how efficient they can make the devices. UK-based Cambridge Photon Technology thinks it has found a way to significantly boost how much electricity the photovoltaic material in solar cells can produce.
All solar cells work in essentially the same way: light strikes the device and energizes electrons in the cell, causing electric current to flow. The preferred photovoltaic (PV) material is silicon, which can absorb a large portion of incident sunlight and convert it into electricity. But silicon works best with photons in the red and near-infrared portion of the spectrum. Longer-wavelength, lower-energy photons — far infrared, microwaves and radio waves — don’t deliver enough energy to get the current flowing. Shorter-wavelength green and blue photons pack more energy than the silicon can handle, and the excess energy is wasted as heat.
Cambridge Photon Technology says it has found a way to stop this waste: converting higher-energy photons into lower-energy ones that the solar cell can use. “We’re trying to deal with this problem of how you improve solar PV performance and bring down costs significantly without throwing away the established silicon technology,” says David Wilson, who is head of business development at the company.
The maximum efficiency is determined by a phenomenon called the Shockley–Queisser limit. All PV materials have a property called a band gap that dictates how much energy can go into individual electrons; for silicon, it’s 1.1 electron-volts. That corresponds to photons in the near-infrared portion of the spectrum. Photons that have a higher energy than this band gap — the entire visible-light spectrum — can generate electrons, but any extra energy from the photon beyond the material’s band gap spills out as heat. Because of this limit, a conventional solar cell operating under ideal conditions can convert, at best, 29% of solar energy into electricity.
The new technique, which relies on a phenomenon called singlet exciton fission, was developed by physicist Akshay Rao and his team at the University of Cambridge. Rao is also the start-up’s chief scientific officer. When light strikes a PV material, it creates an exciton, in which a negatively charged electron and a positively charged electron vacancy are connected by an electrostatic charge. But if the material is an organic polymer semiconductor, the photon can create not just one, but two lower-energy excitons — both of which can be converted to electric current. “You’re preserving the total energy that comes in and out, but you’re making the silicon receive a higher photon flux in the portion of the spectrum that it’s good at converting into electricity,” Wilson says.
The idea of splitting photons is not unique. “People had for many years had an inkling that you could use this phenomenon of singlet exciton fission in organic semiconductors to get around that Shockley–Queisser limit,” Wilson says. But it wasn’t until 2014 that Rao and his colleagues, working in the lab of physicist Richard Friend at Cambridge, first worked out a practical way to do it1.
The plan from the outset was to look to commercialize this work, says Claudio Marinelli, an electrical engineer and entrepreneur who is the company’s chief executive. Rao spoke to a solar-panel manufacturer to understand what the industry needed and how his technology might help, and then approached people with business expertise, including Marinelli and Wilson, to help create a marketable product.
Rao developed a photon-multiplier film made up of a layer of an organic polymer called pentacene, studded with lead selenide quantum dots — small, light-emitting clumps of inorganic material. The polymer absorbs blue and green photons and converts them into pairs of excitons. These excitons flow to the quantum dots, which absorb them and emit lower-energy red or infrared photons. When the film is placed on top of a silicon solar cell, the light from the quantum dots shines onto the silicon (see ‘Colour shift’). Meanwhile, the red and infrared wavelengths directly from the Sun pass through the polymer film and hit the silicon as they normally would. The result is that more useable photons strike the silicon, increasing production of electrical current.
Rao calculates that this double-exciton technique could theoretically increase the potential conversion efficiency of solar cells to 35%2. The company hasn’t come anywhere near to that level yet, Wilson says, but, by the end of 2022, it is hoping to have created a prototype that converts about 31% of sunlight into electricity.
A simpler solution
Other approaches can also increase PV efficiency. Tandem solar cells, for example, use materials, such as a group of crystals known as perovskites, that can capture shorter-wavelength photons. The materials can be used to build solar cells, which can then be wired together with silicon cells, creating a hybrid device that produces more electricity. But the difficulty with such a set up, Wilson argues, is that making two devices work together while producing different currents could be complex. Building solar cells out of a different material also requires an extra manufacturing process and new equipment, which could drive costs up. “Our whole approach has been to avoid these problems and to make a simple, non-toxic material with no electrical connections that add very little complication to existing design,” Wilson says.
Cambridge Photon Technology’s idea seems feasible, says Christopher Bardeen, a chemist at the University of California, Riverside, who is not affiliated with the company. “It is a promising technology that provides a simple alternative to tandem cells,” he says.
The company’s photon-multiplier film could easily fit into existing manufacturing processes, Wilson says. A finished film could be sold to solar-panel manufacturers to place on their PV modules. A simpler approach might be to sell a precursor solution to the companies that make either the vinyl acetate layer that encapsulates the silicon or the glass panels that cover the solar cells. Panel manufacturers would then assemble the already-treated components into the final device. Whatever the approach, Wilson hopes a product will be ready for market within about three years.
Cambridge Photon Technology employs about a dozen people and has raised £1 million (US$1.4 million) in equity capital. It also has a number of research grants, and has access to researchers and facilities at the University of Cambridge to help develop the technology further. It has licensed four key patents from the university.
Although the company has made prototypes of the film and the quantum dots to show that they are efficient enough to work in a product, it has not assembled all the pieces into a working solar cell with improved efficiency. Once it proves its technology is viable, the potential pay-off could be great, Wilson says. “It’s really clear that there’s a fairly urgent need,” he says. “And this technology, if it works as promised, will go a long way to meeting that need.”
Tabachnyk, M. et al. Nature Mater. 13, 1033–1038 (2014).
Rao, A. & Friend, R. H. Nature Rev. Mater. 2, 17063 (2017).