Conjugated polymers have found a wide range of applications as charge and energy transporting hosts in organic devices such as light emitting diodes (LEDs) for displays and lighting, solar cells, field effect transistors and chemical sensors. Understanding how morphology and chemical structure synergetically influence charge and exciton transport is essential in order to fully exploit the potential applications of these devices. In addition to that, fundamental interest arises from the difficulty in predicting transport properties of disordered molecular solids from the chemical structure. We present a hierarchical approach for charge and exciton transport modelling in conjugated polymers that relates the chemical structure to the physical morphology via quantum chemical calculations of charge and exciton hopping rates on an predetermined rod-like film morphology. Charge and exciton transfer rates, calculated using small polaron non adiabatic Marcus-Hush theory and an improved Forster model respectively, are fed in a kinetic Monte Carlo scheme that enables us to follow the trajectories of the individual quasiparticles. Important quantities such as the charge carrier mobility and exciton diffusion length are calculated. Our charge transport calculations focusing on poly(9,9-dioctylfluorene) show that the points of closest electronic contact between chains rate-limit charge transport, with the result that positional disorder in polymer chains can even increase mobility [1]. We find a good agreement with experimental time of flight mobility data in alligned PFO films [2]. On the exciton migration studies in polyindenofluorene our predictions for the energy and time evolution of the 0-1 photoluminescence peak agree well with time-resolved measurements [3]. Moreover we demonstrate that traps are the main limitation to exciton diffusion. Depending on the device application this sensitivity is problematic in some situations, such as solar cells but it acts as an advantage in others, such as sensors.
[1] S. Athanasopoulos, J. Kirkpatrick, D. Martinez, J. M. Frost, C. M. Foden, A. B. Walker, J. Nelson, Nano Lett., 7 1785 (2007)
[2] M. Redecker, D. D. C. Bradley, M. Inbasekaran, E. P. Woo, Appl. Phys. Lett., 74 1400 (1999)
[3] L. M. Herz, C. Silva, A. C. Grimsdale, K. Mullen, R. T. Phillips, Phys. Rev. B, 70 165207 (2004)
Conjugated polymers have found a wide range of applications as charge and energy transporting hosts in organic devices such as light emitting diodes (LEDs) for displays and lighting, solar cells, field effect transistors and chemical sensors. Understanding how morphology and chemical structure synergetically influence charge and exciton transport is essential in order to fully exploit the potential applications of these devices. In addition to that, fundamental interest arises from the difficulty in predicting transport properties of disordered molecular solids from the chemical structure. We present a hierarchical approach for charge and exciton transport modelling in conjugated polymers that relates the chemical structure to the physical morphology via quantum chemical calculations of charge and exciton hopping rates on an predetermined rod-like film morphology. Charge and exciton transfer rates, calculated using small polaron non adiabatic Marcus-Hush theory and an improved Forster model respectively, are fed in a kinetic Monte Carlo scheme that enables us to follow the trajectories of the individual quasiparticles. Important quantities such as the charge carrier mobility and exciton diffusion length are calculated. Our charge transport calculations focusing on poly(9,9-dioctylfluorene) show that the points of closest electronic contact between chains rate-limit charge transport, with the result that positional disorder in polymer chains can even increase mobility [1]. We find a good agreement with experimental time of flight mobility data in alligned PFO films [2]. On the exciton migration studies in polyindenofluorene our predictions for the energy and time evolution of the 0-1 photoluminescence peak agree well with time-resolved measurements [3]. Moreover we demonstrate that traps are the main limitation to exciton diffusion. Depending on the device application this sensitivity is problematic in some situations, such as solar cells but it acts as an advantage in others, such as sensors.
[1] S. Athanasopoulos, J. Kirkpatrick, D. Martinez, J. M. Frost, C. M. Foden, A. B. Walker, J. Nelson, Nano Lett., 7 1785 (2007)
[2] M. Redecker, D. D. C. Bradley, M. Inbasekaran, E. P. Woo, Appl. Phys. Lett., 74 1400 (1999)
[3] L. M. Herz, C. Silva, A. C. Grimsdale, K. Mullen, R. T. Phillips, Phys. Rev. B, 70 165207 (2004)
