Abstract:
Nanostructural control via processing kinetics in plastic solar cells Rajeev Dattani1, 2, Carlos G. Lopez2, James H. Bannock1, 3, Siva H. Krishnadasan1, 3, John C. de Mello1, 3, João T. Cabral1, 2, Alisyn J. Nedoma1, 2, 4 1Centre for Plastic Electronics, Imperial College London, London, UK 2Department of Chemical Engineering, Imperial College London, London, UK 3Department of Chemistry, Imperial College London, London, UK 4Department of Chemical and Materials Engineering, University of Auckland, Auckland, NZ Plastic solar cells promise inexpensive, flexible energy generation; however, pioneering attempts to scale-up production have generated solar cells with power conversion efficiencies that decay rapidly over short device lifetimes. The growth of micron-sized fullerene crystals is largely responsible for the loss of performance during the operation of plastic solar cells. Generally, plastic solar cells comprise a two-phase mixture with a polymeric electron donor and a fullerene-based electron acceptor. The best performing solar cells exhibit nanoscale dispersions of these two phases, which maximises the surface area available for charge production. The glass transition temperature of the polymer, commonly poly-3- hexylthiophene, is ~40 °C, well below the expected operation temperature of solar cells, 80 °C. During operation, kinetically trapped fullerene crystals continue to grow within the rubbery polymer matrix, reducing the amount of interfacial area available for charge separation and decreasing the cell efficiency. Two general strategies are proposed for inhibiting fullerene crystal growth: 1) nucleating a large population of nanoscale fullerene crystals, which prevents any individual crystal from growing to the micron scale and 2) inducing a higher degree of crystallinity in the polymer matrix, which mechanically prevents the growth of fullerene crystals. Both these strategies depend on the processing conditions of the polymer/fullerene composite. The two most common processing techniques, spincoating and printing, were translated from thin films to bulk samples to minimise the effects of surface forces; rapid quenching into a non- solvent and drop-casting, respectively, were chosen as the analogous bulk techniques. Small- angle neutron scattering was used to examine the structures that resulted in polymer/fullerene blends. Slowly dried samples exhibited a small population of polymer crystals and a homogenously mixed polymer/fullerene phase. Upon annealing, these samples evolved micron-sized fullerene crystals. Rapidly quenched samples exhibited a large population of both polymer and fullerene crystals; upon annealing, no structural changes were observed. These results suggest that rapid phase transitions can be harnessed to build a population of fullerene nuclei that controls further growth. Current work seeks to translate these fast-quenches into a scalable deposition technique for thin films.