Tailoring Donor-Acceptor Strengths to Modulate Excited State Dynamics in TADF Emitters: A Pathway to High-Efficiency SP-OLEDs

The advancement of organic light-emitting diodes (OLEDs) for future display technologies holds immense industrial importance and necessitates collaborative, interdisciplinary efforts to achieve successful outcomes. Thermally activated delayed fluorescence (TADF) has emerged as one of the most promising and efficient approaches for converting non-emissive triplet excitons into emissive singlet states. These cost-effective and environmentally sustainable TADF-based OLEDs, with internal quantum efficiencies (IQEs) approaching 100%, have been extensively studied and are widely employed in commercial applications. However, many aspects of the underlying mechanisms remain unclear and require further investigation to facilitate the design of novel and more efficient TADF molecules capable of emitting across various regions of the spectrum. This PhD project focuses on the physics and engineering of these innovative materials and their application in state-of-the-art blue and red OLEDs. Throughout the PhD research, the photophysical and chemical properties of the TADF mechanism were explored in various organic molecules, including novel donor-acceptor (D-A), double donor-acceptor (D2-A), and multi-resonance TADF (MR-TADF) systems. The core of the research presented in this thesis examines different TADF emitters in terms of their ability to achieve significant lightmatter interaction. First, the structure-property relationships of TADF emitters were investigated by modifying the donor and acceptor functionalities using both theoretical and experimental approaches. The aim was to gain deeper insights into the design of efficient red and near-infrared (NIR) TADF emitters. This was accomplished through comprehensive theoretical, photophysical, and electrochemical studies. Solution- processed (SP) OLEDs were then fabricated and optimized using these novel TADF materials. First, SP-OLEDs incorporating different transport layers in multiplayer device stack were evaluated, and their influence on overall device efficiency was assessed. The issue of conductivity within the device stack, which can lead to inefficient charge balance in multilayered structures, was also investigated and optimized. Using the novel TADF emitter CF3BP2PXZ, SP-OLEDs achieved a maximum external quantum efficiency (EQEmax) of 9.0%, demonstrating remarkable stability by maintaining an EQE of 8.4% at a high brightness of 100 cdm-². In OLEDs, in addition to the multilayer arrangement of organic molecules, fine-tuning the material composition within each layer is crucial for enhancing performance. In our final study, SP-OLEDs were engineered at varying concentrations of TADF molecules in the emissive layer (EML). These cutting-edge devices, incorporating a novel TADF material (DMACDOB) as a host matrix, exhibited narrow blue emission with an emission wavelength (λEL) of 471 nm and a full width at half maximum (FWHM) of 30 nm at an optimal dopant P age | iv concentration of 2 wt%. These devices also demonstrated efficient performance with an EQEmax of 13.6%. Since blue TADF emitters face significant challenges in current display technology, including low efficiency, limited stability, and poor color purity, this advancement represents a pivotal development for future blue-OLEDs