Effect of conducting polymer molecular weight on nanocrystal growth size for photovoltaic applications

Organic photovoltaics promise a number of key advantages over conventional silicon, namely: ease of processing, low cost, physical flexibility and large area coverage. However, the solar power conversion efficiencies of pure polymer devices are poor. When electron acceptor nanocrystals are blended with a donor conducting polymer to create a bulk heterojunction structure, the optical and electronic properties of both materials combine synergistically to enhance overall performance. We use a novel single pot process to fabricate the nanocomposite photovoltaic material, where PbS nanocrystals are grown directly in a solution of the conducting polymer MEH-PPV. This study investigates the dependence of nanocrystal growth size and subsequent power conversion efficiency as a function of polymer molecular weight. It was found that a higher molecular weight polymer resulted in the formation of a broken percolation of smaller nanocrystals that act to enhance the charge separation of excitons generated at the low energy band edge of MEH-PPV.