MATERIALS | PHOTOVOLTAICS


light to electricity. Although the power they produce is low, it could be enough to supply the many ‘internet of things’ devices – such as multiple sensors – that are likely to be used in future. The researchers have developed a new combi-


nation of donor and acceptor materials, with a carefully determined composition, that can be used as the active layer in an OPV device. The combina- tion absorbs exactly the exact wavelengths of light that are found in indoor lighting. The researchers describe two variants of an


organic solar cell in an article in Nature Energy, where one has an area of 1cm2


Above: Researchers from Riken in Japan have used a


post-annealing process to create an ultrathin organic solar cell that is efficient and durable


eters,” said the researchers. This effect, which is strong at ambient tempera-


tures, can significantly reduce the efficiency of the OPV. With this information, the authors say they are now working on ways to reduce these vibration- induced voltage losses.


Quantum effect At Linköping University in Sweden, researchers have identified a quantum phenomenon that influences the formation of free charges in OPVs. “If we can properly understand what’s going on, we can increase the efficiency,” said Olle Inganäs, professor of biomolecular and organic electronics at the university. The research, published in Nature Communica-


tions, came about after doctoral student Qingzhen Bian obtained unexpected results in an experiment to optimise a solar cell material. In several repeated experiments, a brief periodic waveform appeared as a photocurrent flowed in the solar cell material – which seemed to undermine existing understanding. Inganäs theorised that ‘coherence’ – a link between two different parts of the system – was taking place. “This helps to create the charges that [generate] the photocurrent at room temperature, but we don’t yet know why or how,” he said. He said that understanding these fundamental details could have practical significance. “If we understood better how the charge carriers


are formed – and how the process is controlled – we should be able to increase the efficiency of organic solar cells,” he said. “If we can design mole- cules that help increase the photocurrent, we can use the phenomenon to our advantage.”


Staying indoors Separate to this, Linköping researchers have teamed up with scientists from China to develop OPVs that are optimised to convert ambient indoor


34 FILM & SHEET EXTRUSION | May 2020 and the other 4cm2 .


The smaller cell was exposed to ambient light at an intensity of 1000 lux, and up to 26% of the light energy was converted to electricity. The larger cell maintained an energy efficiency of 23%. “This shows great promise for organic solar cells to be widely used in our daily life for powering the internet of things,” said Feng Gao, senior lecturer in the division of biomolecular and organic electron- ics at Linköping University.


Thin and durable Physical structure is critical for organic solar cells. Now, scientists from Riken in Japan have created an ultrathin organic solar cell that is both efficient and durable. Ultrathin flexible solar cells could find use in a


variety of applications – such as wearable electron- ics – but tend to be relatively inefficient, with energy conversion ratios of 10-12%. They also tend to degrade rapidly under the influence of sunlight, heat, and oxygen.


Using a simple post-annealing process, the


researchers have created a flexible OPV that degrades by less than 5% over 3,000 hours in atmospheric conditions and has an energy conver- sion ratio of 13%. The group began with a semiconductor polymer for the donor layer developed by Toray Industries and experimented with the idea of using a non- fullerene acceptor – which increased the thermal stability. In addition, they added a simple post- annealing process, where the material was heated to 150°C after an initial annealing at 90°C. This created a stable interface between the layers. “Our research shows that ultra-thin organic solar cells supply high power in a stable way over long periods of time, and even used under severe conditions such as high temperature and humidity,” said Kenjiro Fukuda, one of the authors of the study. The research was published in Proceedings of


the National Academy of Sciences of the United States of America.


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