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Investigating charge transport in organic semiconductors with electrochemical methods and modelling

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Description

Organic semiconductors are promising candidates in many different applications such as solar cells, transistors, sensors, memory elements and, most importantly, in organic light emitting diodes (OLEDs). OLEDs outperform many classic LCD based displays, and find entrance into lighting applications where large area OLEDs have demonstrated excellent efficiencies and lifetimes [1].

To further improve the commercial success of organic semiconductors, a better understanding and better models of charge transport in those semiconductors are required. In the past few years, electrochemical methods have been shown to yield valuable insight into charge transport in organic semiconductors by providing a method to measure the density-of-states (DOS) distribution and related charge transport parameters. Those methods are not widely adopted in the OLED community yet, and also the numerical models typically used to optimize OLEDs and other devices are less sophisticated and use simpler models to describe charge transport in organic semiconductors.

In this project, we like to use electrochemical methods to improve numerical models of charge transport in organic semiconductors. In particular, we like to adapt our numerical drift diffusion models that include ion transport to describe the electrochemical measurements such as cyclic voltammetry and electrochemical impedance spectroscopy. By performing dedicated experiments, we like to elucidate diffusion constants of ions from the electrolyte inside the semiconductor film and quantify the charge transport across the semiconductor/electrolyte interface. These measurements determine which processes need to be considered in the numerical models and provide the basis for a quantitative model.

The numerical model will then be used to extract the DOS from electrochemical measurements to arrive at a better understanding and description of charge transport in the chosen material systems. By using a detailed numerical model that accounts for drift-diffusion, ion movement, charge transport between semiconductor and electrolyte and possibly some additional mechanism, we expect more accurate results than obtained with simple equivalent circuit models used to date. Additionally, we foresee a chance to elucidate trapping and de-trapping rates from the difference between the first and the second cyclovoltammogram, and with the help of the numerical model, it should be possible to determine those rates or discard the idea.

With the improved description of charge transport inside the semiconductor, charge carrier injection from the metal electrode to the semiconductor will be investigated closer using dedicated electrochemical measurements. Together with parameters describing charge transport, the parameters describing charge injection are most relevant for device simulations. Devising a viable way to reliably determine injection properties in real devices with a simple technique would benefit the entire organic electronics community. The material systems we like to investigate throughout the project are OLED materials as they are of greatest near-term commercial interest. The materials will be selected according to the experimental requirements and range from well-studied materials (e.g.  NPB, CBP:Ir(ppy)2(acac), super yellow) to state-of-the-art exciplex forming host materials (e.g. TCTA:B3PYMPM).

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