Introduction

My research is based around modeling charge and energy transport in organic devices such as organic light emitting diodes (OLEDS) , organic photovoltaics (OPVS) and organic field effect transistors (OFETS).

In terms of basic operation, organic devices differ from their inorganic counter parts by using organic semiconductors instead of inorganic semiconductors. This essentially means that the main component of organic devices is some from of plastic.

More specifically I have been looking at how adding certain conductive organic layers to the structure of OLEDs affects the current and charge recombination zones within the device.

Motivation

Organic devices are attractive to researchers as they hold the potential to completely revolutionize modern gadgets and offer rich commercial integration prospects that will be worth investment.

In general, devices made from organic materials can be flexible due to their plastic mature, easy to produce with production methods such as ink jet printing and cheap due to the low temperature processing.

These assets along with many others specific to the type of device make organic based electronics very desirable for the future.

However organic devices suffer from two main flaws. The first being that plastic doesn't conduct electricity very well, meaning organic semiconductors typically have low mobilities compared to inorganic semiconductors.

The second problem is the lifetime of organic devices. Organic devices suffer from internal degradation simply due to the operation of the device.

Organic devices are entering a market which is already dominated by similar inorganic devices that currently outperform their competitors. Huge industry exists around silicon electronics and for organic devices to penetrate this market huge research efforts are required to improve the efficiency and lifetime so that these devices can compete alongside their inorganic counterparts.

Theory

There are books (see resources section) that describe the theory much more elegantly then I can, but I will supply a very brief outline as to how charge and energy is transported through organic semiconductors.

The conjugated bonding between the carbon atoms means that three sp2 hybridized orbitals are formed creating a coplanar structure where two of the bonds link carbon atoms together and the third attaches hydrogen or some other R group to the carbon atoms.

This sp2 hybridized bonding leaves spare p_z orbitals per carbon atom normal to the coplanar structure. Overlapping of the p_z orbitals occurs over extended conjugated bonding, creating a delocalized sea of electrons above and below the coplanar structure known as a pi band.

These electrons can be viewed as being in the highest occupied molecular orbital, and are therefor easily excitable across a band gap into a pi* band, where the excitation is referred to as a polaron. This is analogous to electrons in an inorganic semiconductor being excited from the valence band to the conduction band, leaving behind a hole.

However, due to typically low dielectric constants in organic semiconductors, these excitations formed are still coulombically bound to the respective 'holes' left behind, creating a stable neutral state known as an exciton.

It is through the movement, generation and recombination of these excitons that most of the optical and electrical characteristics of organic semiconductors are generated

It is also believed that through the generation, recombination and movement of excitons, physical degradation of the polymers that carry these excitations can occur.

KMC Code

The Kinetic Monte Carlo (KMC) code is a method of stochastically choosing events to occur based on how quickly that event can take place. The type of events that occur in our organic devices involve charge and exciton movement, charge injection (from electrodes) and exciton formation and recombination.

These simulations utilize a full 3D user created morphology, and makes the model very versitile at exploring the effect different device structures and geometries have on device performance. This is coupled with the ability to view and track all particle species within the device through time.

This method has been used to successfully investigate the efficiency of gyroid shaped polymer structures in OPVs by Robin Kimber, and is currently my primary tool in investigating the effect of interlayers in OLEDs

Drift Diffusion Model

A drift diffusion model has been created to solve the transport equations in 1D for charge and energy movement within organic devices. The model is a quick way of testing new ideas, and an excellent tool to run alongside the KMC model.

Optical Model

An optical model is used to analyze the output power from a layered OLED. The model works by assuming light is generated somewhere in the organic semiconductor by an oscillating dipole representing an exction. Coupled with the exciton profiles generated with either the KMC or drift diffusion codes, it is possible to work out the efficiency and light intensity emitted from OLEDs.

The primary aim of my research is to use the models to help shed light onto the mechanisms behind the degradation of OLEDs. Using the KMC and Drift Diffusion models it is possible to investigate how different structures effect the positions of exciton generation and recombination zones.

Using this, it is then possible to probe different degradation mechanisms, and, using the optical model, compare the change in light intensity due to degradation with experimental data.