Open Access

Stefano Sem

• The need of high quality and efficient displays is continuously increasing. The organic light-emitting diode (OLED) technology is certainly one of the most important in this sense, thanks to their high contrast, excellent color purity and wide viewing angle. Despite being already widely used in commercial products, scientific research on OLED materials is still ongoing to improve their efficiency and durability. A new technology which might replace currently used emissive materials in OLED is the so called thermally-activated delayed fluorescence (TADF). With these emitters, the display efficiency can be improved without the need of expensive and pollutant heavy metal atoms. This PhD project is focused on these materials and their use in OLEDs. The study of TADF OLEDs presented in this thesis has been structured in three main parts. The first study allows to clarify a frequent misconception about these emitters: the portion of excitons leading to an emissive event, usuallyThe need of high quality and efficient displays is continuously increasing. The organic light-emitting diode (OLED) technology is certainly one of the most important in this sense, thanks to their high contrast, excellent color purity and wide viewing angle. Despite being already widely used in commercial products, scientific research on OLED materials is still ongoing to improve their efficiency and durability. A new technology which might replace currently used emissive materials in OLED is the so called thermally-activated delayed fluorescence (TADF). With these emitters, the display efficiency can be improved without the need of expensive and pollutant heavy metal atoms. This PhD project is focused on these materials and their use in OLEDs. The study of TADF OLEDs presented in this thesis has been structured in three main parts. The first study allows to clarify a frequent misconception about these emitters: the portion of excitons leading to an emissive event, usually approximated to 100%, can actually be much lower when electrically excited. A method to estimate this value is provided from the analysis of transient and steady state optical measurements. In a state-of-the-art OLED, the emission layer (EML) consists of two or more components. The adjustment of each material component and the optimization of the concentration largely impact the OLED performance. In the second study, OLEDs containing different concentrations of the TADF molecule in the emissive layer are investigated. Several experimental techniques are used and, with the use of software simulations, the effect of emission layer composition on the charge and excitonic processes is analysed. The key aspect which must be improved in order to make TADF a suitable technology in commercial products, is the lifetime. Two studies about this topic have been included in this thesis. To effectively measure the lifetime of emissive devices, one would need to operate them for several thousands of hours. This approach is definitely not applicable on a large scale, when a multitude of different devices need to be tested, since it requires a lot of time and resources. Such characterization is therefore typically done under accelerated stressing conditions, with high currents and/or temperatures. The use of appropriate scaling laws allows to estimate the durability of the device in standard operating conditions from the accelerated ones. In the first study described in this work, several identical TADF OLEDs have been stressed with different current at different temperature, and the complete set of luminance decay is fed into a global fitting algorithm. With this approach the expected lifetime can be estimated in a shorter amount of time, yet with a high accuracy. To improve the device lifetime, a detailed understanding of the processes causing it is necessary. The second study on device lifetime goes more into detail of the degradation processes occurring in a specific TADF OLED stack. The devices are stressed with constant current, and during stressing interruption a series of experimental techniques are used. Electrical device simulations are used to model these OLEDs and qualitatively identify the degradation causes. Specifically, it is found that the generation of trap states causes a variation of the charge injection and accumulation inside the device.…