Understanding the degradation mechanisms of single layer OLEDs to establish structure-stability relationships of blue TADF emitters

• Applicant: Rishabh Saxena, Ph.D.
• Subject Area: Experimental Condensed Matter Physics ; Experimental and Theoretical Physics of Polymers
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 547125865

This project is devised from a simple industrial necessity, high efficiency blue organic light emitting diodes (OLEDs) with long operational device lifetime, as realization of such a device will revolutionize the display industry. This is not an easy problem to solve, as although OLEDs have been developed into a multi-billion market dominating pixelated displays in smartphones and televisions, blue pixels still operate far below the performance and efficiency of red and green pixels to achieve an acceptable lifetime. The project addresses this enormous challenge using the latest generation of OLED materials, namely the thermally activated delayed fluorescence (TADF) emitters. TADF emitters allow for an internal quantum efficiency of up to 100%, owing to the small energy gap between the first excited singlet and triplet states which can facilitate a thermally activated transition from non-emissive triplet to emissive singlet state. However, TADF OLEDs still suffer greatly from stability issues. The investigations will be performed on single layer OLEDs which can compete in efficiency with OLEDs consisting of very complex multilayer structures. The reduced complexity of single layer OLEDs as compared to the generally adopted multilayer OLED structure is advantageous in terms of performing a quantitative analysis of degradation mechanisms. Within the course of this project, firstly the role of various mechanisms to the overall degradation of single layer TADF OLEDs, via the generation of luminance quenchers, will be clarified. Subsequently this information will be utilized to identify the structural factors which govern the degradation of molecules to establish the structure-stability relationships in this class of purely organic OLED emitters. On the one hand, OLEDs will be fabricated, characterized, and modelled to elucidate the quencher generation mechanisms. In parallel, the molecular fragmentation products (i.e., the degraded chemical species) generated during the electrical operation of OLEDs will be analyzed to comprehend the molecular degradation pathways. Furthermore, quantum chemical calculations will be performed to characterize the molecular fragmentation process from a thermodynamic point of view. Overall, the findings of this project will be utilized to establish clear correlations between molecular structure of TADF emitters, quencher generation mechanisms and device lifetime of single layer blue TADF OLEDs, consequently leading to the deduction of chemical design rules for the engineering of improved TADF emitters with better stability and longer device lifetime.

• DFG Programme: Research Grants