Fluorescence, quantum dots – future solar cells

By Lynley Hargreaves 16/07/2014

justin-hodgkissPlastic solar cells – which may start appearing on hardware store shelves in the next decade – do things a little differently from the silicon-based photovoltaic variety. They’re bendy, less efficient but getting steadily more so, and they also emit tiny bursts of fluorescent light. Dr Justin Hodgkiss, Senior Lecturer at Victoria University and Principal Investigator at the MacDiarmid Institute, explains why fluorescing is not what you want electrons to do and how his team can measure this ultra-fast behaviour.

What does your laboratory look like?

Black curtains shut out all of the sunlight from my solar cell lab, and instead we use femtosecond laser pulses (one femtosecond is a millionth of a billionth of a second) to understand how the solar cells generate electricity from light. A jungle of hundreds of mirrors and lenses directs different coloured laser pulses onto the sample at precise times. The experiment is analogous to strobe photography, only with much shorter flashes. By measuring how long it takes for certain spectral features (colours) to emerge or diminish, we learn how different solar cell materials convert light to electricity, ultimately aiming to design more efficient new materials.

Which are the best materials you’ve worked with?

We have focused mainly on conducting polymer (plastic) materials because plastic solar cells could be made very cheaply in the future. We’ve looked at commercially available polymers, as well as custom polymers that collaborators send us from around the world. The efficiency of polymer solar cells has doubled over the past few years, with new polymers becoming available all the time. Our collaborators in Korea have recently broken the efficiency record for polymer solar cells at nearly 10%. That may not sound a lot, but the thermodynamic limit is only 30%, and the standard silicon solar cells found on rooftops today typically have 12-15% efficiency.

Doesn’t plastic have trouble conducting electricity?

That is the main challenge; although conducting polymers are excellent at absorbing light, there is a strong tendency for electrons to get stuck, which limits the electricity generated. We have found the first step of current generation is the trickiest – ensuring that negatively charged electrons are separated from the positive charges so that they don’t just stay together. We’re trying to understand how this step relates to the material structure so that we can design better materials.

We’re also starting to look at other promising new solar cell materials. A solar cell material called perovskites was discovered only two years ago and already has efficiencies of over 15%. Quantum dots are another exciting direction that our expert collaborators here at Victoria University are leading.

A quantum dot?

A quantum dot is a nanoparticle of material (in our case inorganic semiconductors) that is so small that their electrons are squeezed, giving them entirely new properties. For example, light absorption and electrical conduction can be tuned by their size, making them very promising for solar cells, especially if you imagine printing large scale solar cells from quantum dot inks. Of course, there are different drawbacks with quantum dot solar cells that we are trying to resolve. For example, since they have high surface areas, electrons have a propensity to get stuck on particle surfaces rather than jumping between particles.

Do stuck electrons emit fluorescence?

Solar cells made from polymers, perovskites and quantum dots will actually act as an LED if you drive a current through them in reverse. While fluorescence is an unwanted side reaction in solar cells, we can usually still see a tiny ultrafast burst; not enough to be detrimental to the efficiency, but a fleeting signature for us to understand what the electrons are doing (how quickly they are moving, how ‘hot’ they are). We have developed an optical shutter to measure how fluorescence develops on femtosecond timescales. This very powerful and unique new tool has led to a lot of collaboration interest from labs around the world.

These interviews showcase researchers supported by the Marsden Fund which, since 1994, has been supporting fundamental, investigator-led research in New Zealand.