Increasing energy demands and concerns about global warming mean that the sun is an important inexhaustible and clean energy source. Dye sensitised solar cells (DSSCs) have proved to be a highly efficient and low-cost option for conversion of sunlight to energy. 

Ruthenium dye photosensitisers have given the highest power conversion efficiency in the past but they are not very environmentally friendly and supply is limited, explains Eric Diau at the National Chiao Tung University, Hsinchu, Taiwan. Diau and his team took inspiration from nature by developing a dye based on porphyrin - a component of chlorophyll, which gives plants their green colour and allows them to convert sunlight into energy. They also replaced ruthenium with zinc, which is more abundant and costs less.  

An efficient artificial porphyrin dye is used to harvest solar energy in an organic photovoltaic device

A light scattering layer is often added to a DSSC to improve the efficiency of the device. But Diau's group found that their porphyrin dye performed better than a ruthenium dye in devices without adding a scattering layer. This also made the device see-through meaning it could be integrated into windows in the buildings of the future, explains Diau.

Emilio Palomares from the Institute of Chemical Research of Catalonia, Tarragona, Spain, says this represents 'a

step further on the long-standing research into bio-inspired artificial leaves for photovoltaic applications'. He adds that 'the high efficiency achieved using fully transparent devices makes the dye suitable for applications in smart windows where people can see through, while the window acts as an active light-to-energy conversion system.' 

Diau says that this type of light-weight, see-through and colourful DSSC could be very useful in the building industry. However, he sees some challenges ahead before it is commercialised: solar power conversion efficiency and the stability of the devices need to be improved, he says. 

Fay Nolan-Neylan

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