Monte Carlo Simulation of Carrier Diffusion in Organic Thin Films with Morphological Inhomogeneity

Abstract

Monte Carlo simulation was carried out to understand the influence of morphological inhomogeneity on carrier diffusion in organic thin films. The morphological inhomogeneity was considered in the simulation by incorporating the regions of low energetic disorder in a host lattice of high energetic disorder which decreases the overall energetic disorder of the system.For the homogeneous films, the carrier diffusion was found to decrease upon decreasing the energetic disorder. In contrast to this, in the case of inhomogeneous films the carrier diffusion enhanced upon decreasing the overall energetic disorder, up to an optimum value and beyond which the carrier diffusion decreased. Through our simulation, we observed that the behavior of carrier diffusion in the inhomogeneous case is due to the morphology dependent carrier spreading, which acts in addition to the thermal and non-thermal field assisted diffusion mechanisms. This morphological dependence of carrier spreading arises due to the generation of packets of carriers with different jump rates, which is after effect of slow relaxation of the carriers generated in the less disordered regions of inhomogeneous system. Our simulation of morphology dependent carrier spreading and its influence on the basic diffusion process provide deeper insight into the charge transport mechanisms in organic thin films.

Influence of Film Morphology on Transient Photocurrent Pulse Shape in Organic Thin Films: A Monte Carlo Study

“…The Monte Carlo simulation code, developed in house using FORTRAN, is based on the commonly used algorithm reported earlier [4,[25][26][27]. A 3D array with size 70 × 70 × 10000 along x, y and z directions is considered as the lattice.…”

“…Thus simulation is performed for a sample thickness of 6 μm along the applied field direction. For homogeneous system (HS), the site energies in the lattice are assumed to be correlated and follow single Gaussian distribution [27]. The width of the Gaussian distribution is the measure of energetic disorder (σ).…”

“…In simulation this can be implemented by considering the site specific contribution as Γ ij = Γ i + Γ j = 2γa, where, Γ i and Γ j are the site specific contribution each taken randomly from Gaussian distribution with variance σ Γ = Σ 2 4 and Σ is the variance of Γ ij [4]. Throughout the simulation the positional disorder is neglected (Σ = 0) and the value of overlap parameter taken to be 2γa = 10, which is a realistic approximation for organic solids with weak intermolecular interaction [4,[25][26][27][28]. Using MA equation hopping probability (Pij) of carrier from the present site i to any site j around within a cube of size 7 × 7 × 7 (343 sites) is first calculated.…”

“…Contradictory to this expectation, we find that the value of W initially increases and reaches a maximum value with increase of concentration of ordered regions beyond which it starts decreasing with further increase in the concentration of the ordered regions. A plausible explanation for the intriguing observation is provided on the basis of morphology dependent carrier diffusion mechanism [27].…”

Organic light-emitting diodes: multiscale charge transport simulation and fabrication of new thermally activated delayed fluorescence (TADF) materials

“…This is one of the most successful studies for charge transports. However, this approach, including related ones [10][11][12][13][14][15][16][17], does not consider the organic molecule explicitly.…”