Ferroelectrics
Apart from being physically rich systems, ferroelectric materials are relevant for a wide variety of applications like data storage (e.g. FeRAM), actuation, transduction and actuation, and energy harvesting. The unique feature of organic ferroelectrics is the possibility to change, improve and add functionality through modification of the molecular structure. As an example, we were the first to demonstrate that ferroelectric polarization can couple to the bulk charge carrier mobility, giving rise to a fundamentally new resistive switching effect.
Switching behavior of dip-coated organic ferroelectrics
Organic ferroelectrics, such as BTAs, are of great interest due to their switching properties. However, the strong electric fields that are often required to achieve saturation polarization limit their possible applications. A possible solution is to pre-align the molecules, i.e. via dip-coating. The already present results suggest that dip-coating in general enhances the ferroelectric switching of pristine films. We expect the results to be transferable to other small molecular ferroelectrics.
Contact: Andrey Butkevich
Investigation of organic molecules on ferroelectricity
In cooperation with the Kivala group, dielectric measurements are applied to their synthesized Tritridecanamide molecules in order to investigate possible ferroelectric behavior. This is done via searching for indices of phase transitions in the measured capacitance spectrum. The measurements are done for different frequencies and temperatures. When a potential phase transition is discovered, it is further investigated if the material has a Curie-Weiss behavior which is a prerequisite for ferroelectricity. With this method, it can be determined whether or not a material is a ferroelectric.
Contact: Andrey Butkevich
Bulk photovoltaic effect and photostriction in organic ferroelectrics
The anomalous photovoltaic effect arises as a specific case from the conventional photovoltaic effect. Bulk materials can have an inherent non-centrosymmetry leading to a shift current and thus an unusually high photovoltage. This is known as the bulk photovoltaic effect. Due to their asymmetry, all ferroelectrics may show this anomaly. Until now, mainly inorganic materials were studied in this field. We are looking forward to investigate the bulk photovoltaic effect of organic ferroelectrics, such as BTTTA-DA which showed research potential as above band-gap photovoltages were documented.
All ferroelectrics show piezoelectricity, a coupling between the charge and strain in the material. The converse piezoelectric effect means that an electric field in the material leads to deformation. The combination of the bulk photovoltaic effect and converse piezoelectricity can thus lead to light induced movement, also called photostriction.
Contact: Andrey Butkevich and Maximilian Litterst
Bulk Photovoltaic and Photostriction of Organic-Inorganic Ferroelectrics
Bulk photovoltaic and photostriction of organic-inorganic ferroelectrics.: The anomalous photovoltaic effect arises as a specific case from the conventional photovoltaic effect. Bulk materials can have an inherent non-centrosymmetry leading to a shift current and thus an unusually high photovoltage. This is known as the bulk photovoltaic effect. Due to their asymmetry, all ferroelectrics may show this anomaly. Until now, mainly inorganic materials were studied in this field. We are looking forward to investigate the bulk photovoltaic effect of organic-inorganic hybrid ferroelectrics which showed research potential as above band-gap photovoltages were documented.
All ferroelectrics show piezoelectricity, a coupling between the charge and strain in the material. The converse piezoelectric effect means that an electric field in the material leads to deformation. The combination of the bulk photovoltaic effect and converse piezoelectricity can thus lead to light induced movement, also called photostriction.
Contact: Yuzhong Hu
Investigation of novel organic ferroelectrics
Ferroelectric materials exhibit a ferro- to paraelectric phase transition at a material specific temperature, called the Curie temperature. This transition is accompanied by a divergence of the dielectric permittivity, which can be obtained by placing the material in a capacitive device and measuring the capacitance over a temperature and frequency range. When such a transition is found, the material is further investigated regarding its ferroelectric properties. Electrical measurements like polarisation hysteresis loops and capacitance-voltage measurements give information about characteristic ferroelectric parameters and switching kinetics, while structural characterisation via atomic force microscopy and x-ray diffraction can allow for insights in morphology changes related to ferroelectric behaviour.
Contact: Heiko Mager
Barkhausen Noise in Organic Ferroelectrics
The switching of the polarization in ferroelectric materials is often described using a hysteresis loop which look smooth suggesting that the polarization domains switch one by one. Taking a closer look at the Hysteresis it can be seen that the switching happens in domain clusters of varying sizes. This effect is known as the Barkhausen noise and can be seen in bistable materials such as ferromagnets or ferroelectrics. In this project the Barkhausen noise in organic feroelectrics is investigated experimental in thin film PVDF-TrFE capacitor devices and theoretical by simulating the switching process in the molecule BTA.
Contact: Marcel Hecker