Meso-scale model of transport in conjugated polymer materials:

Previous simulations have connected the microscopic properties of organic thin-films to its macroscopic electronic properties. However, because they treat each molecule explicitly, their computational cost limits them to small sample sizes. To reduce the computational cost, we develop three charge transport models, each relying on a different approximation to simplify the simulation. Under the correct conditions, each model accurately reproduces results from the fully detailed simulation. With these new models we plan to simulate experimentally-sized films, thus providing new insights into the factors that limit charge mobilities in organic electronic devices.

Three models which allow simulation of experimentally sized films

(a) The organic film is partitioned into cells. At any point in the simulation the only molecules considered are those within a cell that is occupied by a charge, thus allowing arbitrarily sized films to be simulated.
(b) Assuming that charge mobilities within grains are substantially higher than mobilities between grains, charge transport can be modelled as a Boltzmann weighted hop over grain boundaries, the rate of which is the probability of being at the grain boundary multiplied by the probability (per unit time) of hopping over it.
(c) A simplification of model (b), charges are assumed to follow the steepest energetic gradient, hopping over grain boundaries only when they have reached an energetic minimum within a grain.

Research performed by Joe Kwiatkowski
Collaboration with Alberto Salleo (Materials Science and Engineering, Stanford University)