Simulating exciton dynamics in organic semiconductors

Abstract

Organic semiconductors are carbon-based semiconductors with number of unique benefits over traditional semiconductors such as low production costs, versatile synthesis processes, and high portability. Unlike traditional crystalline semiconductors that exhibit high level of homogeneity, organic semiconductors are spatially and temporally heterogeneous due to the weak van der Waals intermolecular forces. In this thesis we utilize computational and theoretical methods to investigate how this heterogeneity affects the electronic properties in organic semiconductors. In particular, we focus on two microscopic processes fundamental to the performance of organic semiconductors: the transport of Frenkel exciton and dissociation of charge-transfer (CT) exciton. Frenkel excitons are tightly bound electron-hole pairs created upon photo-excitation of molecules and they carry the excess energy imparted by photons.

We employ theoretical approach that combines molecular dynamics and semi-empirical electronic structure calculations to reveal the effects of molecular disorder on Frenkel exciton transport in oligothiophene-based molecular semiconductors. Using this approach, we find that the magnitude and details of molecular disorder (i.e. spatial and temporal correlations) could have huge impact on exciton transport in this class of materials. CT excitons are electron-hole pairs partially separated across the donor-acceptor interface. To generate free charges, the oppositely charged electron and hole must overcome an electrostatic binding energy before they undergo ground state recombination. We explore the CT exciton dissociate mechanism and magnetic field effects through a model of quantum spin dynamics combined with a stochastic coarse-grained model of charge transport.

We demonstrate that simulations carried out on our model are capable of reproducing experimental results as well as generating theoretical predictions related to the efficiency of organic electronic materials. Next, we consider the effect of disorder in electronic energy levels on dissociation yield and demonstrate that it is maximized with a finite amount of disorder as a result of non-equilibrium effect.