Organic - plasmonic hybrid systems for optoelectronic applications
ved Elzbieta Sobolewska

Over the last decade, the field of plasmonics has been gaining more and more attention in view of opportunities for controlling light-matter interaction on the nanoscale. The integration of active surface plasmon-based elements in nanophotonic circuits leads to new designs and applications of nanophotonic devicessince the resulting plasmonic hybrid systems are able to convert optical signals into plasmonic signals and vice versa.

Recent research demonstrated the existence of a loss channel for energy transfer between organic materials and plasmonic structures. This work is focused on investigating the optical energy transfer in plasmonic hybrid structures, by applying a crystalline organic material as an active surface plasmon polariton (SPP) component. The epitaxial grown organic material in the form of nanowires was roll-on transferred from the growth substrate and placed directly onto metal nano-aggregates, creating a system, which allows us to explore exciton-plasmon coupling. Exploiting the advantageous photoluminescence properties of organic materials, we investigate coupling principles by fluorescence-lifetime imaging microscopy (FLIM), supported by leakage radiation spectroscopy (LRS). We complement our experimental findings on the excitation of “hybrid modes” in plasmonic-metal systems theoretical considerations, carried out by the means of finite-difference-time-domain (FDTD) simulations. Our results lead to a better understanding and controlling of hybrid-mode systems, which are crucial elements in future designs of low-loss energy transfer devices.

Vejledere: Horst-Günter Rubahn, Jacek Fiutowski

Lifetime and stability of organic solar cells
ved Golnaz Sherafatipour

Organic photovoltaic (OPV) is an emerging economically competitive photovoltaic technology that has advantages over conventional inorganic PV technology including low fabrication cost, lightweight, semi-transparency and mechanical flexibility. Despite these advantages, OPVs have comparably low power conversion efficiencies and rather short lifetimes, which are the most critical factors hampering their application. In order to overcome these barriers and close the gap between laboratory achievements and industrial scale requirements, a detailed understanding of the device degradation mechanism is required. Charge transfer (CT) states, representing intermediate states between exciton dissociation and recombination at donor-acceptor interface, play hereby a crucial role.

The main purpose of this PhD project, as part of the FP7 ITN network THINFACE, is to study the degradation mechanism and charge transfer loss, taking place at the donor-acceptor heterointerface. We established some new methods for obtaining information about charge-transfer states in organic solar cells, which is a fundamental need for gaining a deeper insight into the physical working principle of the cells, here including also device efficiency and stability, which is strongly related to the aim of this PhD work. These investigations are done by means of sensitive external quantum efficiency and electroluminescence measurements, as well as surface-sensitive microscopy techniques.
Vejleder: Morten Madsen

Optical characterization of degradation mechanisms in organic solar cells
Ved Pawel Cielecki

This PhD project is focused on stability of organic solar cells (OSC), with respect to their degradation processes. The main idea is to utilize non-destructive optical characterization methods, to investigate the chemical and physical degradation of the devices and exploited materials. The project involves various optical techniques including: spatially resolved pump-probe and ultrafast fluorescence dynamics studies, photoluminescence-, photo reflectance-, and transmission spectroscopy as well as Coherent Anti Stokes Raman Scattering (CARS) microscopy. At the end of this project, it is expected to identify and characterize degradation processes involved in decreasing the lifetime of OSC. Additionally, the obtained data should exhibit possible ways to prevent or suppress degradation in organic photovoltaics.
 Vejledere: Morten Madsen, Jacek Fiutowski

mechanical stability of Organic Solar cells
Ved Michela Prete

Organic solar cells (OPVs) are considered, nowadays, a promising alternative to the inorganic PVs thanks to their low fabrication cost, the easy processability and tunability to different applications. In respect to the latest, organic materials allowed, in fact, the fabrication of flexible and stretchable devices opening the door to the possibility of mass production. These advantages are still limited by the ongoing improvement on the stability and efficiency of the organic devices. Although the efficiency has reached 12%, the final commercialization of the OPVs is still dependent on their stability. If different have been, in the past years, the photochemical stability studies, still few are the reported analysis on the mechanical properties of the organic devices. It is very important to understand the mechanical properties of the materials utilized, as much as understanding the results of the mechanical stress on flexible cells that is affecting the performances of the device over time.

The main goal of the research project is to gain insight into the mechanical properties of flexible organic solar cells and to explore the possibilities of increasing the mechanical durability and stability of the cells while maintaining scalability and high electrical device performance. The project therefore encompasses work and specific objectives on determining the influences from both photochemical and mechanical stressing of flexible organic solar cells, and on applying specific additives that stabilize the cells against these degradation factors, both individually and combined. The work builds upon initial work on photochemical stabilization of organic solar cells, from where the initially tested photochemically stabilizing additives are chosen.

Vejledere: Morten Madsen og Vida Engmann

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