0.0001 cm ~ Device thickness

1. - Differences in electron affinity generate a diode: current can only flow in one direction + Simulations of disorder in packing of poly-pyrrole and resulting disorder in the transfer integrals [4] Multi-scale models of charge transport for organic photovoltaics James Kirkpatrick, Department of Physics, Clarendon Laboratory, University of Oxford Solar energy: a planet wide opportunity • Global warming: UK committed to 80% cuts by 2050 • Energy security: solar energy wide spread • Solar energy largest energy source on earth Renewable energy sources compared to consumption [1] Organic photovoltaics: why plastic is fantastic • Traditional photovoltaics expensive • Plastic electronics: from Nobel prizes (Heeger, Shirakawa, McDiarmid 2000) to commercial reality (Sony XEL-1 OLED TV, 2008) • Offers low manufacturing costs, using cheap and abundant raw materials • Efficiencies are still low because of poor charge transport and of lack of fundamental understanding 120 g of active plastic material are sufficient to pave an Olympic swimming pool in organic solar cells = Abstract Organic electronics allows to easily create new materials by chemical synthesis Vast numbers of new chemicals could be made, they cannot all be tested by trial and error Models need to describe both the molecular length-scale and the device scales 0.0001 cm ~ Device thickness • On a large scale charges density is continuous • Jumps in electron affinity drive charge generation • Developed a mathematical model of the junction between donor and acceptor materials [2] • Identified two regimes: • Small current: junction acts as a diode • Large current: bulk acs as a resistor 0.0000001 cm ~ Molecule size • On a quantum mechanical scale, charges are discrete • Motion of charges occurs in “hops” • Hopping rates can be computed [3] • Transfer integrals set the overall rate for charge transfer Knowing the chemical structure of a material allows us to compute the wave-function of the electrons in it 0.0000001 cm - 0.00001 cm ~ Scale of disorder • Molecular orientation is not perfect • Disorder in charge transfer rates exist • Simulating packing allows to compute several important distributions: • In transfer integrals (off-diagonal disorder) • In site energies (diagonal disorder – density of states) • Such models have been applied to many systems [4] Acknowledgements My research at Oxford is supported by the James Martin School for the 21st Century. I would like to acknowledge my colleagues at the university of Southampton (Giles Richardson and Colin Please); at Imperial College (Jenny Nelson and Jarvist Frost) and at the Max Planck Institute for Polymer Research (Denis Andrienko, Valentina Marcon and Victor Ruehle) References: [1] Global Exergy resource chart, gcep.stanford.edu [2] G. Richardson, C.R. Please, J. Kirkpatrick in press [3] J. Kirkpatrick, Int J Quant Chem 108, 51 (2008) [4] V. Ruehle, J. Kirkpatrick, D. Andrienko, J Chem Phys 132, 134103 (2010)