‘Light-trapping’ technique could enable thinner solar cells

Norwegian scientists claim they could make solar cells thinner using a layer of plastic microbeads to trap light inside the cells.

Traditional silicon solar cells must be at least as thick as the wavelength of light they are trying to capture in order to maximise the amount of electricity they produce.

But a team from Oslo University has found that attaching microbeads (uniform polymer beads less than 500 micrometres in diameter) to the surface of cells increases their efficiency, meaning they can be made thinner while maintaining their electrical output.

‘We use the microbeads to build perfectly regular and periodic structures (one-layered crystals),’ lead researcher Erik Marstein told The Engineer via email.

‘Such structures, when designed properly, have the ability to both reflect light extremely efficiently, but also to allow for a controlled scattering of the light.

‘In practice, this means that we, by the proper design of the microbead-based structure, can force light that hits a solar cell perpendicularly to continue its travel within the solar cell in a lateral direction.’

This ‘light-trapping’ technique can enable the use of solar cells so thin that they are almost transparent, he added.

The researchers are experimenting with finding and manufacturing the best structure to trap the light, but have calculated they could make solar cells with electrical output equivalent to cells that are 25 times thicker.

They are also examining ways of increasing the amount of light the cells trap by making asymmetrical micro indentations on the back of the silicon.

‘Cylinders, cones and hemispheres are symmetrical shapes,’ said Marstein. ‘We have proposed a number of structures that break the symmetry. Our calculations show that asymmetrical micro indentations can trap even more of the sunlight.

This technique alone could reduce the amount of silicon needed by another 20 per cent. For example, 20-micrometre solar cells with symmetrical micro indentations are as effective as 16 micrometre plates with asymmetrical indentations.

The next stage of research will be to develop ways of mass-producing the structures without their current high costs.

‘On paper, our structures represent an almost ideal solution,’ said Marstein. ‘However, solar cell manufacture is an extremely challenging environment where large-area processing and high throughput must be combined with very low costs.’