technology. ‘Our sprayed nanocrystal devices last for several years and show little signs of degradation because they are inherently stable,’ Dr Townsend said. ‘While there are still lingering stability issues with this technology, many companies are emerging right now to take this to market because of the efficiency and the solution processing capability. Research is currently focused on improving the air stability of the layers and the device encapsulation to prevent water exposure.’ The research has also recently

Spray-on inorganic nanocrystal test device

as opposed to organic materials usually found in spray and other liquid-based applications. Inorganic materials are typically more stable and have more attractive electro-optical properties than their organic counterparts, hence their widespread adoption in the traditional solar panel sector. There are issues associated with using

inorganic materials in a spray form, as Dr Townsend explained: ‘The major challenge for inorganics is that they – unlike organics – are not soluble in liquid solvents. In order to spray or print inorganic materials, they must either be synthesised as nanocrystal inks or formulated as a combustible salt solution. Both of these end up with an intermediate nanocrystalline film.’ A better device is also usually formed

from large crystal grains, as Dr Townsend added: ‘The next challenge will be to find a way to convert these nanometre-sized crystals into micrometre-size grains – which is a 1,000-fold size increase – with enhanced film qualities.’ Another challenge is that the efficiency

is limited to between 6 and 12 per cent because of the need for crystal size growth. However, chemists and engineers have recently made great strides with a new type of material known as perovskites. These are metal-organic hybrids that can be solution-processed with minimal heating to produce devices with more than 20 per cent efficiency. Despite these challenges, solar sprays

are a real contender in the alternative solar space thanks to the stability of the

transitioned to nanodevices, and this poses new challenges and opportunities for photonics companies, as Dr Townsend explained: ‘Our devices, for example, are less than one micrometre thick, which is 50 times thinner than the human hair. This leads to ultra-thin and lightweight devices that can be deposited cheaply and quickly. Not only would this save money on materials used, but would also open new applications to start thinking about where we could deposit these devices and how many we could pack into a smaller area. This includes LEDs and transistors, which are also thin film layered electronics.’

Solar foliage Taking inspiration from nature, the Noël research group at the Eindhoven University of Technology has developed a ‘solar leaf’ technology to harvest the sun’s light. Timothy Noël, assistant professor at Eindhoven University of Technology, explained: ‘We looked at nature’s biomachinery – leaves, bacteria, algae. These organisms harvest light very efficiently by capturing the light with so- called antenna molecules. Next, the light is brought to the reaction centre where the actual photosynthesis process occurs.’ g

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