When a high electric field is applied to a disordered semiconductor the electrons are theorized to behave as if they had a higher than lattice effective temperature. We hope to prove this effect by showing that it can produce a Seebeck voltage. The main difficulty lies in the high necessary field strengths which require very small mesurement structures. If successful this might be useful for high frequency rectifiers.
Project of: Anton Kompatscher
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.
Project of: Andrey Butkevich
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.
Project of: Andrey Butkevich
Organic solar cells are photovoltaic generators made predominantly from carbon-based materials, such as plastic substrates, conductive and semiconducting molecules or polymers. While organic solar cells can be described in some ways similar to their inorganic -- commonly Silicon-based -- relatives, they work on fundamentally different principles. Those include non-bandlike transport, excitonic absorption properties, or charge separation at the phase interfaces of donor:acceptor semiconductor blends, that are yet to be fully understood. We are utilizing a highly sensitive, electro-optical absorption spectroscopy method to directly map the density of states in the blended phases and at their interface in the steady state to study how energetic disorder, vibrational transitions, and the effective temperature of
generated charge carriers influence the solar cell performance and efficiency.
Project of: Alexander Flamm, Dr. Clemens Göhler
Image Source: Göhler & Deibel, ACS Energy Lett. 2022, 7, 6, 2156–2164, 2022. https://doi.org/10.1021/acsenergylett.2c00303
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.
Project of: Andrey Butkevich and Maximilian Litterst
Noise, although often perceived as an unwanted artefact in a measurement, contains essential information about the charge transport in materials. Noise spectroscopy has been used as a non-destructive tool to study about dynamics of charge carriers, defects and correlations in the system. This project aims to set up sensitive noise spectroscopy and, hence, measure theoretical thermal (Johnson-Nyquist) noise and shot noise in organic semiconductors. Often in literature, the shot noise deviates from the theoretical value. This deviation, quantified as the Fano factor, is an exciting quantity that can be used to understand charge transport.
Project of: Sebastian Klein and Priya Viji
The question whether charge transport in operational organic photovoltaic (OPV) occurs far-from-equilibrium or not is of significant practical and fundamental importance. The charge transport in OPV modelled using kinetic Monte Carlo has shown that charge carriers undergo slow thermalisation and, hence, are ‘hotter’ than their inorganic counterparts. Probing the effective temperature of photo-created charge carriers in OPV using noise spectroscopy is expected to offer a unique way forward to measure the ‘hotness’ experimentally.
Project of: Priya Viji
Most state-of-the-art organic solar cells are based on the simple bulk heterojunction morphology, for which donor and acceptor materials are dissolved and processed in a common solvent. While this approach has enabled power conversion efficiencies above 18%, the open circuit voltage VOC of these organic photovoltaics still falls considerably below the theoretical maximum. A prominent yet incompletely understood loss channel decreasing VOC is the thermalization of photogenerated charge carriers in the density of states, that is broadened by energetic disorder. Symmetric morphologies like classic bulk heterojunctions do not provide a preferential direction to the thermalizing photogenerated charge carriers and the excess energy of the photogenerated charge carriers is lost in an undirected, diffusive motion. Carefully designed morphologies like funnels or compositional gradients in the donor:acceptor ratio on the other hand can mitigate this loss channel by rectifying the diffusive motion. Understanding the relationship between morphology and solar cell performance is a key step towards future organic photovoltaics with power conversion efficiencies beyond 20%.
Project of: Constantin Tormann
Image Source: Upreti, Tormann & Kemerink
J. Phys. Chem. Lett. 2022, 13, 28, 6514–6519. https://doi.org/10.1021/acs.jpclett.2c01565
Organic Solar Cells provide an interesting and low-cost alternative to conventional inorganic photovoltaics. Upon illumination, excitons are created that separate into free electrons and holes at the donor-acceptor-interface and can be extracted to deliver electricity. The opposing loss mechanism is the recombination of charge carriers, which happens in the bulk both prior to and after charge separation. To understand these processes, the recombination order is crucially important, which we determine via kinetic Monte Carlo Simulations under variation of different parameters such as morphology, contacts and delocalisation. Simulation results are compared to experimental data obtained via the Steady State Bias Assisted Charge Extraction (BACE) method on P3HT:PCBM and PM6:Y6 solar cells using different light sources.
Jana Seiler and Kathrin Brocker
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.
Project of: Heiko Mager
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.
Project of: Toni Seiler
The low temperature behavior of organic solar cells yields important information about the underlying processes like charge transfer and exciton formation. The open circuit voltage is an experimentally well accessible proxy for these mechanisms, as it is deeply linked to the energy level of the bound charge transfer states which decay to the free charge carriers constituting the device current. Measuring the open circuit voltage at varying temperatures allows us to extrapolate it down to 0K, where it should coincide with the charge transfer state energy as all losses due to thermalization are suppressed. Varying the light intensity as well allows us to further challenge our theoretical understanding of these devices.
Project of: Tobias Krebs
Networks of semiconducting SWCNTs are interesting thermoelectric materials due to the interplay between CNT and network properties. As a one-dimensional (1D) material with a tunable density of states, SWCNT offers stability, flexibility, and solution processability along with a unique combination of properties, such as high carrier mobilities and high Seebeck coefficients, which makes it attractive for thermoelectric applications.Understanding the underlying physics of charge and energy transport in networks of nanotubes as a function of morphology and doping is necessary to improve the performance of such systems at the lowest possible cost and to establish design rules for experimentalists.
Project of: Aditya Dash
Image Source: Dash, Scheunemann, &Kemerink (2022).
Physical Review Applied,2022, 18(6), 064022.
https://doi.org/10.1103/PhysRevApplied.18.064022
The field of thermoelectric energy conversion has held great promise to convert heat to electrical power, using a technology that is free of moving parts and thereby silent and extremely durable. The objectives are (i) to describe electronic charge and heat transport in disordered, partially ordered and hybrid systems using kinetic Monte-Carlo calculations of hopping transport, and (ii) to provide recommendations for material selection and device design for especiallypolycrystalline and hybrid systems. Hybrid and organic materials provide an excellent alternative to inorganic counterparts with rare and/or toxic elements which are brittle and require energy-intensive fabrication processes.
Project of: Aditya Dash
Organic Semiconductors show a relatively pronounced change in resistance when exposed to magnetic fields. Different theories have been proposed to explain this effect. One explanation relies on the Pauli exclusion principle preventing two electrons with the same spin from entering a site effectively hindering conduction. This effect should become stronger the more dimensionality is limited towards a single path trough the device. We attempt to create such a geometry in order to verify the theoretical predictions. If successful this might open up applications for various electronic switching circuits.
Project of: Anton Kompatscher
Along with thermal and electrical conductivity, the Seebeck coefficient is a parameter that indicates the efficiency of a thermoelectric device. It describes how much voltage builds up in the material due to a temperature gradient. So far, mostly DC measurement methods are used to determine the Seebeck coefficient of a material, but the AC method promises to be a faster and more efficient method. An oscillating voltage is used to heat an electrode on a silicon sample. The heat wave is attenuated within the sample and will set up a temperature gradient between two electrodes, where the active material is located. This results in a thermal voltage, which can be measured and, knowing the temperature of both electrodes, the Seebeck coefficient can be calculated.
Project of: Ejona Syla
