Identification number: lzp-2022/1-0331
Type: Latvian Council of Science
Project duration: 2023 - 2025
Project manager: Dr.phys. Jeļena Butikova, Institute of Solid State Physics, University of Latvia
Total funding: 300 000 EUR
Project aim: The main goal of this fundamental research project is to investigate physicochemical properties of the interfaces, and their dependence on the deposition conditions and their role in the performance and degradation kinetics of organic electronic devices. Interfaces between thin films of different organic materials and multilayer structures will be investigated to understand the interface contribution to interfacial transport of electrons and the degradation of OLEDs, and to explore the possibilities of interfacial tuning with the aim to improve the OLEDs performance.
Project summary:
Organic materials are used as promising active layer in advanced electronic, optoelectronic, and energy harvesting devices, e.g. organic light-emitting diodes (OLEDs), polymer-based photovoltaic devices. They contain multilayer structures that incorporate electrode materials and polymer layers. The operation of these devices is strongly influenced by interfaces, optical gradient, and anisotropy of the conjugated polymer films. Interfaces play crucial role in the optical performance of photonic devices opening the way to new multiphase polymer nanomaterials. Accurate knowledge of optical constants of these materials is important for interface engineering and modelling of light emitted and absorbed by OLEDs and photodiodes. The main goal of the project is to investigate the physicochemical properties of interfaces and their dependence on deposition conditions and role in the functioning of organic electronic devices. With the development of these devices, the complexity of polymer multistacks increases, demanding novel and more sensitive characterisation techniques. We apply advanced UV–VIS–NIR spectroscopic ellipsometry capable to measure all Müller matrix elements as an alternative spectroelectrochemical method to determine the electron orbital structure, interface mixture, roughness, anisotropy, intra- and interchain order of polymers. The characterisation of layered systems will be tailored to kinetics during post‐deposition thermal and solvent vapour treatment of the film.
Expected scientific results
During the implementation of the project, the folllwing scientific results are expected to obtain:
- Knowledge on the interface mechanisms and degradation kinetics for organic multilayer devices;
- Manufacturing OLEDs with enhanced characteristics (in terms of efficiency and durability);
- Developing stable deposition process to control interfaces between the layers of an organic material;
- Providing a detailed report on influence of the deposition and post-deposition treatments on organic thin films optical properties and interfaces;
- Providing a detailed report on degradation kinetics of organic compounds and interfaces.
Project progress
31.12.2023.
1. Regular meetings of the project team are held in Zoom every other week.
2. We have started to deposit two layers of low-molecular-weight compounds onto the glass substrates. During the sample preparation, it has been found that the layer of the transparent electrode ITO has not been etched from the glass substrate completely, which makes the modelling of the ellipsometry data more difficult. It has been decided to modify the deposition procedure in order to increase the modelling accuracy.
3. We continue spectroscopic ellipsometry measurements and data modelling. To facilitate the modelling process, additional measurements are being made: the thickness of the low-molecular-weight compounds is measured with a Dektak profilometer, and their absorbance with a Cary spectrophotometer.
4. The literature on OLED spectroscopic ellipsometry is reviewed in order to develop a paper concept with a sufficient novelty element.
30.09.2023.
1. Regular meetings of the project team are held in zoom every other week.
2. The modelling of spectroscopic ellipsometry data for single layers has been completed. A database of material optical constants (n, k) was compiled for further measurements of the samples with multiple layers deposited.
3. When modelling samples on glass, special attention should be paid to the roughness parameter.
30.06.2023.
1. Regular meetings of the project team are held in zoom every other week. Project participant Dr. phys. Laima Trinklere has been trained to use the spectroscopic ellipsometer J.A. Woollam RC2.
2. We continue depositing single organic layers on the sample substrate. Applying polymer thin films with another spin-coater has significantly improved the quality of the samples. The thickness variations have significantly decreased.
3. Spectroscopic ellipsometry measurements and data modelling are ongoing. When modelling, it is important to avoid the correlation of thickness with optical constants (n, k). Attention should be paid to the thickness non-uniformity parameter in the case of depolarisation.
Organic films for the working OLED device will be measured in the inverted configuration during its operation. This would be equivalent to flipping the single-layer sample upside down, i.e. the thin film would be at the bottom, and the substrate at the top; therefore, the light reflects from the substrate first. Therefore, the reverse data modelling has been performed.
31.03.2023.
1. The project implementation is started. The project team has meet to discuss and clarify the tasks of each project participant. A regular meeting time for the project team is set. We will meet every other week in the zoom environment. Out-of-the-schedule discussions will take place in person when required.
2. We have started to synthesise single-layer samples. Polymer thin films (polyTPD, PVK) are being deposited using spin-coating method, while low-molecular-weight organic compounds (IR(ppy), TPBi, CBP) are being deposited using thermal evaporation method. The typical thicknesses of the organic thin films are around 55-60 nm.
Low-molecular-weight organic compounds have a tendency to crystallize, which could negatively affect the quality of future OLED samples and the accuracy of the results of spectroscopic ellipsometry measurements. Severe degradation and exfoliation of the CBP sample has been observed.
At this stage, we have decided to focus on polymer samples. When measuring the thickness of the layers with a profilometer, a significant variation of thickness has been observed. In some areas the samples, it reaches up to 100%. It has been decided to deposit polymer layers using another spin-coater.
3. We have started to perform spectroscopic ellipsometry measurements and data modelling. The reflection and transmission spectra of the samples are recorded. The samples are mostly modelled as a transparent B-spline with an absorbing region, which is then converted to a model containing multiple Gaussian oscillators.