The EXTMOS model looks at the mesoscale (1-100 nm), capturing the physics of organic semiconductors at different levels of resolution and complexity. Rather than relying on empirical parameters fitted to experiment, parameters generated by quantum chemistry are used. This development allows application to a wide range of novel materials and structures, reducing development costs. The code generates outputs for continuum models which operate at length scales exceeding 1 micron. A commercial Technology Computer-Aided Design, TCAD, package was used to investigate multilayer OLEDs and OTFTs. Thus the project spans atomistic to device length scales.
To enable the translation of these methods to industry, the command-line based software modules were incorporated in the workflow platform SimStack. This integration provides an easy-to-use graphical user interface, GUI, for each module. The GUI hides the complexity connected to high-performance computing, including job-submission and monitoring, from the end-user, and enables the drag-and-drop combination of various modules into seamless workflows, connecting molecular chemistry to circuit design.
Validation and a proof of the concepts underpinning the code came from tests in an experimental feedback loop. Devices were fabricated and characterised to test model predictions. Measurements included scanning electron and conductive atomic force microscopy, optical measurements of emission zone profiles, Helium pycnometry for film densities and ultraviolet, inverse photoemission and optical spectroscopy.
Key insights were obtained from the project. Dopant guidelines were suggested from a series of virtual experiments using the EXTMOS model that predict the electron affinity of p-type dopants in different hosts using structures measured by X Ray diffraction on dopants provided by the consortium. EXTMOS partners demonstrated that the electron affinity of a dopant strongly depends on the molecular host as a result of intermolecular electrostatic interactions. Morphologies, i.e. molecular arrangements, were predicted with a coarse-grained Molecular Dynamics model using interactions obtained from electronic structure calculations. Charge transport studies were made on these morphologies with a Kinetic Monte Carlo, KMC, code on a system of 0.5 million molecules. These studies show that increasing the size of host and dopant molecules improves transport properties. Simulations with a lattice KMC model show that due to disorder compensation, the conductivity of doped organic materials is insensitive to the degree of intrinsic disorder of the host material.
Impurities and charge traps are present in most organic semiconductors and are introduced when doping the material. These defects limit the charge transport and thus the efficiency of the devices. The EXTMOS model was used to characterize the influence of impurities on the density of states and charge transport in small-molecule amorphous organic semiconductors and applied to study the influence of water molecules and water-oxygen complexes.
OLEDs based on thermally activated delayed fluorescence, TADF, were studied with the EXTMOS model that clarified how TADF molecules can achieve high OLED efficiencies, and suggest new TADF emitters. Experimental studies showed that it is possible to have Ohmic injection into ohmic semiconductors allowing highly efficient single layer devices using TADF.
EXTMOS demonstrated that the molecular weight of polymers or equivalently the average length of the polymer backbone chains can impact the doping efficiency. Electronic characteristics therefore depend on the host polymer structure and a deeper understanding of how polymer chains interact with dopants can help with engineering of the semiconductor polymer films and their processing for optimizing the performance of future organic semiconductor devices. A model of charge transport in polymer films was developed assuming fast intrachain transport with interchain hops at the close contact points.
