Exciton dynamics in organic light emitting diodes

Abstract

Organic light emitting diode (OLED) technology has gained significant attention over the past few decades due to its self-emissive displays, high contrast ratios, and energy efficiency compared to traditional lighting sources. However, substantial challenges remain in achieving OLED emitters with long lifetimes and high efficiency while also having narrowband emissions. Current high-performance thermally activated delayed fluorescence (TADF) emitters, which can achieve 100% internal quantum efficiency (IQE), predominantly utilise charge-transfer (CT) excited states that exhibit inherently broad emissions, typically with a full-width half maximum (FWHM)of70-120 nm. This broad emission necessitates the use of filters or optical microcavities to enhance colour purity, which often introduces trade offs in efficiency and lifespan. To address this, multi-resonant (MR) emitters have been developed, offering narrow emissions but large singlet-triplet gaps, resulting in less TADF. Additionally, advancements in computational methods are needed to study these molecules effectively. This thesis is structured into three key parts that aim to tackle these challenges by investigating the optimisation of compounds used in OLED devices, focusing on the interplay between structural and electronic properties, thoroughly understanding the mechanisms involved, benchmarking computational methods, and designing novel organic molecules for OLEDs, thereby contributing to the broader goal of making OLED technology a superior alternative to current display technologies. The first part of the thesis explores the conformational control of TADF emitters using non-covalent interactions. TADF emitters typically require a specific Donor-Acceptor (D-A) framework, where the highest and lowest energy states have minimal spatial overlap, achieved by maintaining a near 90-degree angle between the D and A units. However, overly rigid D-A bonds can hinder TADF efficiency by limiting molecular movement necessary for vibrational coupling, while excessive flexibility can lead to dispersed TADF rates, broader emissions, and increased non-radiative decay rates. Introducing explicit chemical bonds to increase rigidity often excessively alters the electronic structure of these emitters, hindering their ability to exhibit TADF. One strategy is introducing steric hindrance between D-A groups, such as methylation; however, this can lead to enforcing orthogonal dispositions between D-A groups. We’ve demonstrated by examining a series of D-A molecules with a B-N bond that the introduction of non-covalent interactions via oxygen and sulphur atoms significantly stabilises the twisted conformer necessary for efficient TADF. This stabilisation enhances spin-orbit coupling, thus improving intersystem crossing (ISC) and reverse intersystem crossing (rISC) rates. The studyhighlighted theimportanceofmethoxygroups at donor in enhancing conformational control, leading to more stable and efficient TADF properties, particularly in solid-state applications. The second part of thesis addresses the challenge of achieving narrowband emission in luminescent materials for high-resolution and energy-efficient OLED displays. Calculations of emission spectra typically require ground and excited state geometries and Hessians, making them challenging for large molecule due to the high computational costs involved. The DHO model was employed in this study to predict the emission FWHM for various organic molecules including π-π∗, charge transfer (CT), and multiple-resonance (MR) without extensive excited state optimisations, making it a valuable tool for high-throughput screening. In addition, it can also be extended to include off-diagonal coupling between excited states, accounting for non-Condon and non-Born-Oppenheimer effects, which are crucial for functional organic molecules, especially those exhibiting TADF. Furthermore, by combining quantum chemistry at both TDDFT and CC2 levels of theory with rate calculations within the semi-classical Marcus formalism, we demonstrate that incorporating heteroatoms like oxygen and sulphur into the B-N framework slightly increases emission FWHM but significantly enhances the ISC pathway compared to the spin-vibronic mechanism, simplifying triplet harvesting mechanisms. This, along with the DHO model, offers new avenues for designing high-efficiency MR-TADF emitters and improving high-throughput screening procedures. The third part of thesis focuses on multi-resonance TADF (MR-TADF) materials, which offer high colour purity and photoluminescence quantum yield but suffer from slow rISC rates, resulting in long-delayed fluorescence lifetimes. Previous studies have highlighted the importance of double excitations, not accounted for within the framework of Linear Response Time Dependent Density Functional Theory (LR-TDDFT). This study employs Mixed-Reference Spin-Flip Time-Dependent Density Functional Theory (MRSF-TDDFT) to overcome these limitations, providing accurate excited state energetics, including the crucial ∆EST, and outperforming traditional LR-TDDFT methods. By increasing the density of states to enhance coupling between singlet and triplet states, this method was used to explore the excited state properties of these materials. The results in this work set the foundation for computationally efficient in silico development of high-performing MR-TADF materials within the framework of MRSF TDDFT. Despite progress in developing MR-TADF, narrow blue MR-TADF emitters face significant challenges due to the high energy required for blue emission, leading to energy losses and efficiency roll-off. Inverse design and high-throughput computational screening offer powerful, resource-efficient approaches to discover new deep blue MR-TADF emitters compared to traditional methodologies. However, the scarcity of documented MR-TADF emitters complicates this process. To address this, we have used the stoned algorithm with SELFIES molecular representation to generate a diverse candidate dataset, followed by high-throughput computational screening. This multi-stage funnel approach systematically narrowed the candidate pool, but the structures at the end showed minimal variations from the initial hypothesis. Nevertheless, the SF-TDDFT and MRSF-TDDFT approach proves promising for studying the excited state properties of MR-TADF emitters. In summary, this thesis presents significant advancements in the optimisation of compounds for OLED devices, enhancing the understanding of structural and electronic properties, and developing computational methods for designing novel organic molecules.