DOCUMENTATION
| ORGANIC ELECTRONICS IN TIBERCAD |
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In the last years, the field of Organic electronics has experienced a growing attention. Among the present and future applications are organic light emitting diodes (OLED), organic photovoltaics (OPV), organic field effect transistors (OFET). Research in this field is driven by the fact that organic devices are easier to fabricate and cheaper than traditional ones. Organic electronics is indeed an example of how chemistry enables innovations, by providing wide control on the tuning of material properties. It is nowadays evident that the correct modelling of these devices requires the full understanding of the energy structure of the organic material and of its charge transport mechanism. We have therefore included this detailed physical description in tiberCAD through an effective drift-diffusion model, which can be used to simulate many kinds of organic device structures, including features such as the effect of trap states and charge recombination. This comprehensive model can be indeed of great help for organic devices development and optimization. With TiberCAD it is in fact now possible to simulate multiple layer OLEDs by defining, for example, in each region of the device different distributions of electron and hole traps, setting their density and energy distribution (e.g. single-level, constant, exponential or gaussian). In addition, different models for recombination and generation, e.g. Shockley–Read–Hall (SRH), Auger and Langevin may be applied. This has been proven very important for a realistic simulation of the contact/semiconductor or semiconductor-A/semiconductor-B interfaces, where states inside the band gap are present which are crucial for the alignment of energy bands, that is for the correct determination of injection barriers. Two fundamental aspects of charge transport in organic semiconductors are in particular thoroughly take into account by TiberCAD models: Gaussian Density of States and Hopping mobility model. Gaussian Density of StatesWhile in inorganic semiconductors like Si or Ga the strong covalent bonds between atoms produce a band structure with electron delocalized in the whole solid, in organic materials molecules are held together by weak van der Waals forces. The visible consequence is that organic materials are soft and flexible. From the point of view of electronic structure, due to the weak bonds, electrons stay localized on their respective molecular sites and molecular disorder and random orientation of molecular electric dipoles determine energy levels, which are randomly distributed. Assuming there is no correlation between adjacent sites, the energy distribution that is the Density of States (DOS) has a gaussian profile. Thus, the conduction band is a gaussian centered on the LUMO level, while the valence band is centred on the HOMO. This model is commonly referred to as gaussian disorder model (GDM). Hopping mobility modelThe conduction in organic materials is due to phonon assisted hopping of charge carriers between localized states. The hopping rate can be calculated using the theoretical approach first proposed by Miller and Abrahams1. However, in order to simulate charge transport in organic semiconductors using an effective drift-diffusion model, we have implemented a model for an effective field dependent hopping mobility. This mobility model has been extended to contain also the dependence on the charge carrier density.
For further details see:
Charge trapping models of resistance switching in organic bistable devices with embedded nanoparticles
The relevance of correct injection model to simulate electrical properties of organic semiconductor
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