Organic photovoltaics are a promising alternative to traditional silicon-based solar cells. As well as being relatively low cost to produce, they are more environmentally friendly. However, they generally can’t match the efficiency of silicon cells, which can have energy conversion ratios as high as 25 per cent. Rigid organic PVs can reach around 17 per cent, while ultrathin organic cells are restricted to about 10-12 per cent. Ultrathin films also tend to degrade rapidly under the influence of sunlight, heat, and oxygen, and it was these problems that the Riken team set out to address.
Described in the Proceedings of the National Academy of Sciences, the organic solar cell started life with a semiconductor polymer for the donor layer, with the team then using a non-fullerene acceptor to increase thermal stability. The researchers then experimented with a simple post-annealing process, where the material was heated to 150 degrees Celsius after an initial annealing at 90 degrees. This step proved to be critical in increasing the durability of the cells by creating a stable interface between the layers. The final product demonstrated an energy conversion ratio of 13 per cent and degraded by less than five per cent over 3,000 hours in atmospheric conditions
"By combining a new power generation layer with a simple post-annealing treatment, we have achieved both high energy conversion efficiency and long-term storage stability in ultra-thin organic solar cells,” said Kenjiro Fukuda, one of the authors of the study.
“Our research shows that ultra-thin organic solar cells can be used to supply high power in a stable way over long periods of time, and can be used even under severe conditions such as high temperature and humidity. I very much hope that this research will contribute to the development of long-term stable power supply devices that can be used in wearable electronics such as sensors attached to clothes."