Since the second generation of solar panels were designed seven years ago, the energy efficiency has risen from 16.7% to just 22.1%. Further efficiencies have been thwarted by a lack of understanding on why the cadmium and tellurium cells showed improved efficiencies when combined with selenium.
A break-through in understanding these efficiencies was recently made by PhD student Tom Fiducia at Loughborough University. The paper, published in Nature Energy, will help drive more advances in solar technology and lower costs for green energy.
Fiduccia starts by providing some much-needed history, explaining the successive generations of solar panel prototypes. First on the scene were silicon cells, which have better efficiencies – up to 24.2% – but are expensive to produce as they rely on the time-consuming and tricky formation of a single sheet of silicon crystals. Second generation ‘Cad’ cells, Fiduccia explains, are thin-film technology, and are much quicker and easier to mass produce, keeping costs low. There is also a new kid on the block, third generation perovskites, which have very exciting efficiency potentials but are not on the market yet as researchers are battling with their instability under light. “Obviously a bit of a problem with solar panels,” he adds. “You’ve got people making their cells and running downstairs to the lab to get them tested before they degrade.”
So Cad cells look like the frontrunner – but they have some catching up to do in terms of efficiency. In a large part, this is because the fast manufacturing means the thin films are produced with more defects – which that can effectively swallow the energy they generate. Defects are structural problems at the chemical level; a hole that traps electrons excited by the sun’s energy. “Think of these holes like bubbles in water. You never have a perfect crystal, so there can be extra or missing atoms in the lattice work. This means that electrons can be lost as they move through them.” The aim of second gen solar panel research is to minimise these defects. Adding selenium to the cadmium and tellurium panels has increased the efficiency, but no one has looked at why.
And that’s where Fiduccia comes in. As the focus of his PhD project, he went into it drawn by the variety of physics involved. “It starts from the sun, it’s got a lots of electrophysiological mathematics going through the atmosphere, […] the optics to actually get there, and then you’ve got your sold-state physics at the cell level.” He describes himself as very lucky; the project had strong connections with solar panel manufacturer company First Solar, and Colorado State University, who both lead the field on Cad cell research, and he held a good level of autonomy over his work. It was from direct consequence of bringing his own ideas to the table that he had his break-through. “I was doing literature reading, and some guys in the US at the NREL, the National Renewable Energy Laboratory, had […] done some great work with this technique, cathodoluminescence. But they hadn’t related it back, hadn’t done any compositional mapping, and they only looked at Cad cells without the selenium. So I saw an opportunity there.”
Cathodoluminescence fires a beam of electrons at the cells, exciting the cell’s electrons and making them fluoresce. Areas of the cell with defects instead absorb the electrons, and don’t fluoresce – making it a perfect way to measure the efficiency of a solar panel. By aligning the fluorescence pattern against the layout of selenium in the panels, the mysterious link between selenium and efficiency began to come clear.
It didn’t disappoint. When Fiduccia saw the results, he had the first inkling he was onto something. “The selenium layer just lit up. I knew then. My heart started racing, I thought, okay, there may be something in this.” Still, it was whirlwind of several ups and downs before he knew for certain. “There was about five or six moments of ‘oh, I’m onto something’, then, ‘no, it’s not a thing’.”
He reached out to experts at Durham, Warwick, and Oxford University to help corroborate the results. This took the data from one measurement on one cell, to more measurements on different machines. As Tom puts it, it “gave us a bit more ammunition on this hypothesis.” Their collaborative results found that selenium reduces the defects in Cad cells, accounting for the increased efficiency.
The team hope that this understanding could help push efficiency even higher; perhaps by increasing the amount of selenium in the cells, or by altering the distribution. With the record at 22.1% efficiency, up from 16.7%, these increases in efficiency might sound tiny. To help put this into perspective, Tom explains that a 1% increase in efficiency of the cell in lab testing equates to around a 5% increase in the power output. A 5% power increase means that much less panels needed for the same amount of energy, and so costs can come down. At the end, it really is worth chasing every last percent – or penny – for cleaner energy!
On his experience coming to the end of his PhD, Tom reflects that while he enjoyed the in-depth research opportunity it gave him, he couldn’t have gotten through the ups and downs without that starting interest. “You need an organic interest. You’re only really going to be able to sustain it if you’re actually interested in it.” His advice to future PhD students hinges around building a career on that enjoyment. “If you’re going to do something, it should be useful or it should be fun, or some combination of the two. If it’s not, what’s the point?”
Paper on the issue: https://www.nature.com/articles/s41560-019-0389-
Christine Parry is currently studying for a MSc in Science Communication at Imperial College London.
Banner image: Andreas Gücklhorn, Unsplash