MATERIALS | PHOTOVOLTAICS
Right: Ellen Moons of Karlstad University leads a project to raise the efficiency of organic solar cells
polymer surface, and letting it dry. “To improve efficiency, we need to better understand how these new molecules distribute in the layer – and how both donor and acceptor contribute to the absorption of the sunlight and the generation of electric current,” she said. The research team includes a range of disciplines. Some will create computer models of the molecules, and simulation tools to optimise structures and properties. Others will optimise the molecules, to build specific structures into each layer. The research is supported by a grant from the Swedish Research Council.
Quantum boost Also in Sweden, Karl Börjesson of the University of Gothenburg is applying quantum mechanics to organic solar cells – in an attempt to make them more cost-effective. “There are excellent opportunities to use quantum efficiencies to change different chemical and material characteristics,” he said. In the study, the researchers present a way to
increase energy diffusion in organic materials. “This allows us to create organic solar cells with simpler structure,” he said.
Quantum physics sounds a long way from plastic
formulation, but the project could help to simplify the manufacturing process. Börjesson says that transferring energy within the cell is always in danger of being lost as heat. To overcome this, formulators typically blend two key materials together – but this makes cells less durable. By ‘coupling’ materials at the quantum level,
there is no need to physically blend them together, he says. According to Börjesson, the discovery could allow cells to be made with a simple layered structure – increasing their durability. He also noted that the research is an extension
of a concept already found in nature. “Nature uses strong coupling between mol- ecules to effectively transfer solar energy in photosynthesis,” he said. “In principle, we have shown that the same basic concept can be applied to organic solar cells.” The research was published in Nature Communications.
Better solvents Meanwhile, researchers at Linkoping University – also in Sweden – have managed to manufacture long-lasting organic solar cells using ‘green’ solvents. According to Feng Gao, professor in the depart-
ment of physics, chemistry and biology at the university, cells are typically made using “toxic solvents, with a relatively low boiling point”. The low
26 FILM & SHEET EXTRUSION | April 2022
boiling point can cause problems during manufac- ture, as the solution evaporates too rapidly. In a paper published in Nature, Gao and
colleagues describe the manufacture a solar cell – using a solution with a high boiling point and without toxic ingredients – that has an energy efficiency above 17%. “This is a major step towards large-scale industrial manufacture of efficient and stable organic solar cells,” said Gao. In addition, the research identified a ‘guest’ molecule that boosts efficiency. It builds on work from a few years ago, when Chinese researchers developed a new acceptor material – called Y6 – which can raise cell efficiency. Now, Gao’s team – with co-researchers from Soochow University in China – have identified a ‘guest molecule’ called BTO, which keeps Y6 molecules packed closely together, allowing a photocurrent to be generated efficiently. Adding BTO also enables larger areas of the solar cells to be manufactured with high efficiency, said the researchers.
Raising efficiency Researchers from nine research institutions have identified a key mechanism that reduces efficien- cies in organic solar cells – and suggested a possible solution. The teams – from institutions including Cambridge University in the UK and UC Santa Barbara in the USA – published their findings in a recent issue of Nature. The researchers identified a pathway in organic solar cells where current is lost – making them less efficient than silicon-based cells at converting sunlight into electricity. They discovered a way to supress this – by manipulating molecules inside the solar cell.
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IMAGE: KARLSTAD UNIVERSITY
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