032-033_EETE-VF

EETE JULAUG 2013

Fig. 1: A typical modeling study includes three major steps: looking at the molecular material level, experimenting with device fabrication and assessing device operation. with different doping concentrations were constructed by introducing F4TCNQ molecules in the vacancies of the host crystal. To make this structure physically realistic, molecular mechanics was used to find the most likely orientation of doping molecules in the host crystal. Energetics for the doping-host charge donation and transport of these donated charges were modeled using micro-electrostatics and charge transport simulations. The ability to monitor these doping-associated electronic processes at the molecular scale provided several new insights into the efficiency of the doping mechanism and its sub-events. It allowed, for example, to complement the existing vision of charge donation with information about how the chemical and structural properties of doping and host molecules impact the charge donation efficiency. Even more importantly, the previously obscure process of free-charge generation has been clarified microscopically as a function of the doping concentration. These findings were compared to the experimental measurements showing excellent qualitative agreement. The overall mechanism of molecular doping in organic semiconductors has been found to differ strongly from the conventional wisdom of inorganic materials. In organics, a strong correlation between the dopant efficiency and its concentration is demonstrated along with the threshold concentration of the doping activation. Strategies towards the design of more efficient doping involve a proper tuning of the host-dopant geometric and electronic interfaces. Outlook to further applications To give an outlook to further applications of the multi-scale methodology, it is important to notice that similar models can be used to investigate the influence of impurities or effects of environmental degradation of organic materials, for example due to oxygen or water penetration into organic films. Generally speaking, one can envisage many more applications of our methodology for in silico optimization of hetero-interfaces in organic optoelectronic devices or, alternatively, for de novo molecular design of interfaces with tailored properties. For example, we are currently studying how charge generation in organic solar cells depends on the material selection and nano-architecture of their donor-acceptor interfaces; we perform similar modeling for dielectric-semiconductor interfaces in organic thin-film transistors. We also foresee multiple avenues for the further development of the multi-scale toolbox of methods. A natural leap in development will be done towards easier integration of new materials. Comparing different materials is the primary practical duty of the developed methodology. An easy, modeling-friendly procedure is therefore required to automate the integration of new molecules into simulations. Given the skyrocketing progress in the chemical engineering of new organic molecules, rapid material succession is likely to be the case, and will probably even accelerate in the future. Therefore, modeling tools must be capable of quick and reliable integration and testing of new (maybe even not-yet-existing) materials. We anticipate that this new approach to modeling will assist the progress of experimental and theoretical work on the aforementioned problems, and many more of those we have not (yet) conceived of. We established evidence that this methodology provides a computationally efficient framework to link the molecular properties of the constituent material to the resultant electronic device performance. This should fuel further studies of electronic processes in the heart of OTFT, OPV and OLED device operation, with the long-term goal of becoming an instrument to facilitate the development of materials and devices. Multi-scale modeling can inform the rational design of optimized molecular materials for organic electronics, eventually advancing better performance and more durable materials. Application in device building may also lead to improved electrical performance and perhaps even to novel device concepts and architectures. It will ultimately facilitate the experimental validation cycles that are needed to improve on existing organic electronic materials and devices. www.electronics-eetimes.com Electronic Engineering Times Europe July/August 2013 31


EETE JULAUG 2013
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