Investigation of the charge transport through disordered organic molecular heterojunctions

Abstract

We develop a new three-dimensional multiparticle Monte Carlo (3DmpMC) approach in order to study the hopping charge transport in disordered organic molecular media. The approach is applied here to study the charge transport across an energetically disordered organic molecular heterojunction, known to strongly influence the characteristics of the multilayer devices based on thin organic films. The role of energetic disorder and its spatial correlations, known to govern the transport in the bulk, are examined here for the bilayer homopolar system where the heterojunction represents the bottleneck for the transport. We study the effects of disorder on both sides of the heterojunction, the effects of the spatial correlation within each material and among the layers. Most importantly, the 3DmpMC approach permits us to treat correctly the effects of the Coulomb interaction among carriers in the region where the charge accumulation in the device is particularly important and the Coulomb interaction most pronounced. The Coulomb interaction enhances the current by increasing the electric field at the heterojunction as well as by affecting the thermalization of the carriers in front of the barrier. Our MC simulations are supplemented by the master equation (ME) calculations in order to build a rather comprehensive picture of the hopping transport over the homopolar heterojunction.

Monte Carlo study of charge transport in organic sandwich-type single-carrier devices: Effects of Coulomb interactions

“…Therefore, it is important to study the effects of Coulomb interactions in devices where high carrier densities can be reached at the electrodes in the case of low injection barriers and where the carrier density can vary over several orders of magnitude within a distance of only a few nanometers from the electrodes. The effect of Coulomb interactions on transport across organic heterojunctions has been studied using MC simulations by Houili et al 23 At these heterojunctions the carrier concentration can also become very large, leading to important effects of Coulomb interactions. Multiparticle MC simulation methods have also been used to study diffusion-limited recombination in polymer-fullerene blends.…”

Simulating charge transport in organic semiconductors and devices: a review

“…Although one has to be careful to ensure that charges are not double-counted [129], this approach allows one to trade-off speed and the accuracy of long-range Coulomb interactions through choice of rc. An advantage of this approach is that it is possible to maintain the full accuracy of short range Coulomb interactions, which have been shown to assist detrapping of charge [129] and hence have a significant effect on predicted current [121]. Still further gains in efficiency are possible by borrowing ideas from interacting particle simulations.…”

“…Arkhipov et al [120] showed using a KMC model that one cannot simply model the process as thermionic emission over a potential barrier due to a 'backflow' of charges which depends on electric field and potential barrier. Houlli et al [121] demonstrated that that a 1D continuum approach significantly overestimates the effect of Coulomb interactions on current passing a heterojunction due to the effect of energetic disorder. This is inconvenient from a modelling perspective since DD models are computationally efficient, particularly at the large charge densities found in OLEDs.…”

“…This is important when considering high charge density (i.e. approaching 1 charge per 100 sites) [128] and charge transport across a heterojunction [121] where short-range Coulomb interactions [129] have been shown to be important. Casalegno et al [127] describe a method which allows partial consideration of Coulomb interactions in ME, although this poses additional numerical challenges to implement self-consistently.…”

Photocurrent Limiting Mechanisms in Organic Bulk Heterojunction Solar Cells

“…The Marcus model [40,41] and the Miller-Abrahams [42] model provide insight into how to calculate the hopping rates and macroscopic properties of charge transport in such disordered systems. The theoretical value of charge mobility can be derived by Monte Carlo simulations [43] or hopping master equations [44]. Charge mobilities can also be determined by various experimental techniques, such as time of flight (TOF) [41][42][43], space charge limited current (SCLC) [45][46][47], and source-drain current versus gate voltage in field effect transistors (FETs) [48,49].…”