A SURPRISING DISCOVERY is set to accelerate the global search for better solar materials. Total darkness. A large black table sprouts mirrors like some kind of extra-terrestrial mushroom farm. Orange laser light bounces from one mirror to the next, hits a small square of plastic and disappears into a black box. The laser has just taken a ‘photograph’ of the mechanics of light being converted to electricity in the plastic target—a solar cell. The exposure time was mind-bendingly short. One day, this new plastic may replace the standard material that’s busy generating electricity in the solar panels directly above Dr Justin Hodgkiss’ Ultrafast Laser lab, on the roof of the Alan MacDiarmid Building at Victoria University. The field of solar energy is bristling with researchers looking for cheaper, more efficient and more functional materials. Until now, a fundamental question about the mechanics of the process has remained elusive and slowed down progress in the industry: what actually happens when an electron is flicked out of its molecular home by a photon of light and becomes part of an electrical current generated by the material? Getting to the answer required Hodgkiss’ designer laser kit and a novel approach. “It all happens very fast. In the first 100 femtoseconds (less than a trillionth of a second) after irradiation, the electron is in a very delocalised state – it’s everywhere at once. We were able to measure the volume of this delocalised area, something that’s never been done before,” he says. It turns out that the volume (around 20 nanometres3) is much bigger than first thought. But it’s precisely by occupying this large volume that the electron is able to rapidly move away from its home to enable a current to flow. “In the last year or two, people have realised that the electron moves away very quickly, but we’ve provided a mechanism to show how that happens.” Presenting this discovery at recent conferences has caused quite a stir. “People are very surprised by it, and tend to sit up and pay attention. It’s definitely got some lively discussion going.” Their work has been written up and appeared this month in the prestigious Journal of the American Chemical Society, JACS. Hodgkiss explains that a chemistry journal was chosen (when the fundamental nature of the discovery might make it more suited to a physics journal) because the new information has the potential to transform the way chemists design and make new solar materials. It’s directly relevant to everyone working in the field. Now that we know what matters on a molecular scale, we can give chemists very specific instructions about how to make the best solar cells.” Fortuitously, the result also answers questions posed by Hodgkiss in both his Marsden Fast Start and Rutherford research grants. “Laser spectroscopy has become an indispensable tool for plastic solar cell development in recent years, but routine laser experiments had still failed to answer key questions. Answering the really important questions required us to go back and develop new tools and devise new experiments.”
Kai Chen was already experienced in ultrafast spectroscopy before he came to New Zealand from Taiwan for his PhD in physics. That knowledge enabled him to design, model and build an experiment to explore how light is absorbed in a solar cell. “Initially I thought it wasn’t going to be a difficult experiment because of the equipment we had. But after I built the system I couldn’t find a signal—there was nothing! So we had to start again. Fortunately Justin and I had a breakthrough that enabled us to understand the whole process properly. Then we got the results and repeated the experiment over and over to make sure they were right.” Chen is the lead author of the publication. His data answered the light absorption question and at the same time revealed the extent of electron delocalisation in the cell. Before then, it wasn’t known that the processes were synchronised. “We built this unique system, and while we were studying an ordinary material, we found something totally new.
Two, six, 10, 14—Joe Gallaher counts in fours. The first year of his chemistry PhD has been spent in the lab, building a library of cut-down versions of standard polymeric solar cell material. Starting with two units, he clips on units of four either side, and builds it up to 14. The colour of the molecule deepens as the size of the chain grows—two units are pale yellow, six are orange, while 10 and above are deep red. Gallaher is looking forward to seeing how his molecules perform under the laser. “Being able to take something I’ve made, analyse it and see how it could relate to a new material in the real world is very exciting.” His results will enable the group to get a detailed understanding of how the length of a molecule affects its electronic delocalisation properties—results that will also be verified in theoretical calculations.
Principal Investigator Keith Gordon , a collaborator from the University of Otago freely admits that working with solar cells is a challenge. “The materials are interesting, but their behaviour is quite complicated. You can make very small changes to the structure of the material that have quite big effects on the performance of the solar cell, so it’s important to understand the quantum niceties of what’s going on.”
As well as contributing to the recent work, Gordon will analyse the small molecules made by Gallaher, using his expertise in theoretical modelling and resonance Raman spectroscopy. “As the strings of molecules get longer, they tend to want to curl up a little. We can see this in our models. A curving structure is not as efficient as a linear one, and it’s a problem in commercial solar plastics. As we search for better materials, checking their performance using theoretical experiments first will make the process much more efficient.”