Scientists from Japan and India have created a dye-sensitised solar cell (DSSC) with the highest recorded efficiency of 11.4%, breaking the record set five years ago.

In the search for alternative energy sources to silicon-based photovoltaic cells, DSSCs have been heavily researched. They are a promising option, as they have a low manufacturing cost and the potential for high efficiency. Unfortunately, their conversion efficiency is currently still below that of silicon cells and the research to improve them is extensive.

A DSSC usually comprises a dye-sensitised nanocrystalline TiO₂ film deposited on a transparent conducting oxide glass, a platinum counter electrode and an electrolyte solution with a dissolved I^-/I₃^- ion redox couple. Liyuan Han at the National Institute for Materials Science, Tsukuba, and Malapaka Chandrasekharam from CSIR-Indian Institute of Chemical Technology, Hyderabad,  and colleagues have created a novel donor-acceptor type co-adsorbent that increases the efficiency of the photovoltaic cell to 11.4%.

'We have successfully developed a new small co-adsorbent to solve the problems that induce loss of light harvesting and charge recombination,' says Han. 'We aim to increase the efficiency of DSSCs to at least be as high as that of silicon-based solar cells, so that DSSCs may become more competitive.'

The team designed two simple donor-acceptor type co-adsorbents that had intense absorption maxima. They also improved the incident photon-to-electron conversion efficiency (IPCE) by offsetting the competing visible light absorption. By introducing butyloxyl chains into one of the molecules, they were able to adjust the distance between dyes and study the cell performance.   

'This work has shown an attractive new route to the development of an efficient type of co-adsorbent for a very well known sensitiser for DSSC, namely the "black dye",' comments Tomas Torres from the Autonoma University of Madrid, Spain, who specialises in molecular organic materials. 'Once more, pure organic small molecules are paving the way to future dye-sensitised solar cells.'

Han and his colleagues intend to continue improving their design. 'The efficiency will be further improved by developing new co-adsorbents with additional functions so that the IPCE in the IR wavelength region is enhanced.' 

Rebecca Brodie

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