James Durrant, Tracey Clarke and colleagues from Imperial College London and Konarka Technologies Austria have used transient absorption spectroscopy and cyclic voltammetry to investigate the efficiencies of solar cells, which incorporate low bandgap polymers.

There is currently considerable need for the development of low bandgap polymers to improve performance of organic photovoltaic cells. The polymer attracting a lot of interest in the literature is dithiophene-benzothiadiazole (PCPDTBT) but the reasons for the high device efficiencies remain unclear.

Durrant employed transient absorption spectroscopy, correlated with cyclic voltammetry to address this issue. He found that PCPDTBT exhibits a charge generation yield, which indicates that the structure of the polymer (and most probably its charge transfer character) strongly favours charge photogeneration.

Durant explains that 'a new low bandgap PCPDTBT co-polymer, when blended with a soluble fullerene derivative, can achieve efficient charge separation with a much lower energetic cost than the previous state of the art polymer, P3HT. This observation thus provides a key insight into strategies to optimising the efficiency of polymer / fullerene solar cells. The overall motivation is to develop molecular design rules to enable the systematic optimisation of organic solar cell efficiency.'

The team are already working with synthetic chemistry colleagues, in academia and industry, to design, synthesise and test new polymers for organic photovoltaics. However, future challenges for this research area continue to be advancing the device efficiency and stability so that it enables commercial viability of organic photovoltaic cells for large scale power production.

Emma Shiells