The Sensitivity of Hopping Transport in Dipolar Glasses and Molecularly Doped Polymers to Localised Dipole Ordering

David M Goldie

doi:10.5923/j.materials.20120203.10

Abstract

The influence of ordered dipole regions upon hopping transport through dipolar glasses and molecularly doped polymers has been evaluated via Monte Carlo simulation. Ordered dipole regions were introduced as a set of randomly dis- tributed cubes throughout disordered dipolar lattices where the number and size of the cubes could be independently varied. Within each cube neighbouring dipoles were anti-aligned to minimise the local energetic disorder but the specific dipole orientation for each inserted cube was randomly selected. Whilst the underlying density of states appears to become ener- getically narrower as the overall proportion of disordered dipole sites is diluted by inserted ordered cubes this is not generally reflected in the associated transport properties. It is demonstrated that the lattice potential that is associated with background non-aligned dipoles in dipolar glasses have a controlling influence upon the observed macroscopic mobility. Confirmation of the importance of such non-aligned dipoles is provided by complimentary simulations using molecularly doped polymers where the dopant dipoles are agglomerated and ordered into cubic regions. The implication of the simulation results for the interpretation of experimental transport data in dipolar glasses and molecularly doped polymers is evaluated.

Highlights

The use of organic materials as the active elements in a diverse range of plastic electronic products has become commercially attractive due to the possibility of synthetically optimising the constituent materials to the target application
An interesting subset of this data is focussed upon organic materials whose transport occurs via hopping processes which are controlled by energetic disorder due to background dipole electric fields[2,3,4]
The stability of the TPD mobility to crystallisation would indicate that a significant fraction of dipoles sites remain unordered according to the present Monte Carlo characteristics and that μ0 ~ 10-3 cm2V-1s-1 is determined by the underlying σDOS(NOC) magnitude of these disordered TPD dipoles

Summary

Introduction

The use of organic materials as the active elements in a diverse range of plastic electronic products has become commercially attractive due to the possibility of synthetically optimising the constituent materials to the target application. By contrast the generation of energy landscapes in the present study has been achieved using a fundamental electrostatic calculation of the dipole potential, where local variations of the potential have been introduced using dipoles that are locally ordered within cubic regions that have a specified volume. Using this approach, the sensitivity of hopping transport to the total concentration of ordered dipoles, and the associated ordered volume size that is employed, has been independently evaluated. Due to possible overlap of the OC site locations due to the random seeding procedure it was generally found that C ≤ m(n/N) 3 with equality only holding for dilute OC

Simulation of Dipolar Glasses

Experimental Dipolar Glass Data

Simulation of Molecularly Doped Polymers

Experimental Molecularly Doped Polymer Data

Conclusions