Study of photophysical processes in organic light-emitting diodes based on light-emission profile reconstruction

Abstract

Organic light-emitting diodes (OLEDs) are emerging as a promising option for energy-efficient, flexible light sources. A key factor that needs to be measured and controlled is the shape of the emission profile, i.e. the spatial distribution of the emitting excitons across the active layer thickness. Being able to accurately measure the emission profile makes it possible to understand the fundamental (photo)physical processes involved in the device operation, providing a basis for further improving the efficiency. In order to investigate state-of-the-art devices, containing 10-20 nm thick emitting layers, emission profile measurements should provide nanometer-scale resolution. In this thesis, a method is presented and applied to reconstruct the light-emission profile, with nanometer resolution, from the measurement of wave-length, angle and polarization-dependent electroluminescence spectra. The method is introduced in chapter 2 and it is used to investigate the photophysics of OLEDs. It uses a fit-profile approach within which the shape of the profile is constrained by making use of our understanding of the recombination process, while still allowing more freedom than in previous studies. The method is first applied to blue-emitting and orange/red-emitting single-layer polymer-based devices. We show that a 5 nm shift of the emission profile within the emissive layer from the cathode-side to the anode-side by increasing the applied voltage can be resolved, and provide a formalism within which the resolution limits can be analyzed. Subsequently, in chapter 3 the resolution is compared to that of a more standard inverse-problem solving approach and analyzed for single-layer, double-layer and multilayer OLEDs. In all cases, the resolution is found to be in the range 1-10 nm. As a next step, in chapter 4 this method is used to determine the singlet exciton fraction in OLEDs. From standard statistical physics considerations a value of 25% is expected. Since in fluorescent materials only singlet excitons can decay radiatively, this fraction limits the maximum achievable efficiency of fluorescent OLEDs. In recent years several studies have indicated that deviations from this value may occur, in particular for polymers. The development of an accurate method for determining the singlet exciton fraction has thus become a topic of intensive discussion and great interest in the literature. We have extended a method presented by Segal et al. (2003) by exploiting the possibility to reconstruct the light-emission profile in OLEDs, and show that for the specific case of intensively studied polyfluorene-based copolymers and for a polyphenylene-vinylene-based polymer the singlet fraction is only 8-25%. The light-emission profiles obtained for the polyfluorene-based copolymers were further used to investigate the validity of charge transport and recombination models. This investigation indicates that the mobility is strongly anisotropic. A study of the emission profile in more complex double-layer small-molecule-based fluorescent OLEDs (chapter 5) is shown to provide novel insights in the photophysical processes near organic-organic interfaces. Increasing the thickness of one of the two layers is found to give rise to an emission profile shift from one side of the interface to the other, and to the occurrence of charge-transfer exciton emission. A delicate balance is shown to govern the exciton emission at both sides of the interface and the charge-transfer exciton emission from the interface itself. Since the corresponding three emission spectra are different, controlling the light-emission profile by varying one of the layer thicknesses results in the possibility to tune the emission color. As a final step, in chapter 6 an analysis is presented of the emission profile in a multilayer white-emitting OLED, investigated within the framework of the European project AEVIOM. The light-emission profile obtained provides a measure of the balance between the generation of excitons in the three emissive layers (red, green and blue). It furthermore enables making an accurate assessment of the validity of the charge transport and recombination models developed in the project, and it is shown to provide deeper insight in the role of exciton transfer processes within and between the emitting layers. The method presented in this thesis is expected to be a fundamental tool for the experimental validation of device models and for designing OLEDs with increased efficiency. The possibility to apply the method as a tool to validate state-of-the-art charge-transport and recombination models (both one-dimensional drift-diffusion models and three-dimensional Monte Carlo models) is discussed in chapter 4 and in the outlook of the thesis (chapter 7). The results presented are not only important for OLEDs, but may also be applied to other organic optoelectronic devices containing disordered organic semiconductors, such as light-emitting electrochemical cells and light-emitting field-effect transistors, as discussed in chapter 7.