A colourful (and illuminating) way of getting our work donePosted: Tuesday, 12th February, 2013
Our photovoltaics researchers at the Newcastle Energy Centre like to get right into the action when they’re in the lab.
Meet Kenrick Anderson, a photovoltaics experimental scientist. He gets to do fun science things – like monitoring how clean the lab is and filling out forms… no, I mean cool stuff like fabricating new solar cells and testing and comparing how they perform in the sunlight or indoors using a solar simulator.
Want to know more about ‘simulated sunlight’ and what we can do with it? Well, read on. Kenrick has given us his down-to-earth explanation of how one of our measurement tools – a monochromator – helps us understand how solar cells respond to sunlight.
Sunlight contains many different wavelengths of light – it’s a broad spectrum, polychromatic light source. Different types of solar cell respond to different parts of the solar spectrum. To compare these different cells we use monochromatic light – light of a single wavelength– as a means of seeing how a solar cell performs at a particular wavelength. For instance if we take just the light that we can see with our eyes, the wavelengths of visible light start at 400 nanometres and extend out to 720 nanometres.
Do you remember the spectrum by the following acronym?
ROYGBIV (Red Orange Yellow Green Blue Indigo Violet)
Actually, this is in reverse order as red light stops at 720 nanometres and violet starts at 400 nanometres. In nature we see white light being split into the spectrum. Have you noticed the reflection of light as it bounces off water droplets which produces rainbows, or in the interference patterns of an oil slick on water? To reproduce these effects in the laboratory we use a monochromator, like the one pictured below:
A monochromator works using a diffraction grating – a special surface with a series of very fine grooves (about 1000 parallel grooves every millimetre!). When light reflects off the surface the grooves cause the colours to separate out. If you turn a CD over you can see this effect for yourself: a rainbow-like spectrum of colours will be reflected off the disk – it’s a diffraction grating in real life using the even grooves of the CD. Similar surfaces are used within a monochromator to split the light. By changing the angle of the diffraction grating we can choose the wavelength coming from the monochromator. Fortunately, our system is computer controlled and all we need to do is type a number in and out comes the wavelength we are interested in. Job done!
Watch the short movie below showing the monochromator sweeping through the spectrum from 350 nm (in the UV part of the spectrum, just beyond violet) to 750 nm (in the infrared part of the spectrum, just beyond red).