A research team at the University of Manchester in the UK is investigating improving the properties of solar cells by creating microscopic structures on photovoltaic thin films using a laser.
Dr Olivier Allegre at the University of Manchester's School of Mechanical, Aerospace and Civil Engineering discussed the topic of laser induced periodic surface structures (LIPSS) and how these can alter material surface properties at the Industrial Laser Applications Symposium, held in Grantham, UK from 22 to 23 March.
LIPSS are microscopic ripple structures that are produced in materials when ultrashort laser beams interact and scatter across their surface. If produced uniformly, LIPSS can be used to alter properties such as electrical charge transfer, friction and light diffraction. Allegre outlined how this change in properties could be applied to certian industrial applications.
LIPSS alter the diffractive properties of materials, enabling multicoloured images to be produced in various metals. (Credit: O Allegre et al.)
‘There are many examples where LIPSS are beneficial. However, it can be quite difficult to control the formation of LIPSS when processing with ultrashort lasers,’ Allegre said. ‘For industrial processes, high uniformity, reliability and speed are all needed.’
LIPSS can be produced in materials such as steel, copper, fused silica and thin films such as titanium and zinc oxide. In each case, according to Allegre, the direction of LIPSS is determined by the direction of the incident radiation’s polarisation vector and the shape of its wave front.
Armed with this knowledge, multiple research groups, including Allegre’s, are now manipulating LIPSS production and exploring how they can be used in a number of industrial applications.
‘We wanted to see if we could use LIPSS to improve the properties of photovoltaic cells,’ said Allegre. ‘We textured an aluminium-doped zinc oxide film which was used to build a photovoltaic cell and showed that it was possible to achieve a good uniformity of LIPSS on the film, and that this changed the electrical properties of the photovoltaic cell. We saw a significant difference between the untextured surface and the textured surface modified with LIPSS.’
According to Allegre, this change in electrical property was only achievable using very high-uniformity LIPSS. The formation therefore had to be done very slowly – the Manchester team required four hours to texture a single cell.
A separate research group has successfully textured the surface of a bio-incubator with LIPSS and proved that their presence promoted cell growth, compared to regions that were not textured with LIPSS. Allegre also hinted that LIPSS could be used to improve the mechanical lubrication of certain materials.
LIPSS have additionally been proven to alter the diffractive properties of materials, according to Allegre. White light can be used to illuminate the textured surfaces, and the colour of the reflected light varies depending on the direction of the LIPSS. ‘By inducing LIPSS with different directions into a material, a variety of images could be imprinted that produce a range of different colours,’ Allegre explained. LIPSS can also be formed as different sized rings, and when these are applied to a metal plate, applications such as encoding serial numbers into metal equipment can be envisioned for anti-counterfeit purposes.
In the production of LIPSS, according to Allegre, each material has a pulse threshold value that must first be identified before texturing can begin. If the pulse energy or duration of the ultrafast laser being used is too high then the material itself may be ablated rather than any LIPSS being formed. ‘For processing materials such as steel, the processing window tends to be wider, and we found that with thin films the processing window becomes much narrower, so that’s where you have to be really careful with the pulse duration.’
Achieving both speed and uniformity over larger surface structures still remains a challenge in LIPSS production, according to Allegre. With the introduction of higher power spatial light modulators in the future, however, Allegre believes that LIPSS production methods will be scalable to higher speeds.