Excited state processes in molecular materials of OLEDs

Organic Light Emitting Diodes (OLEDs) devices utilize organic molecules to generate light. The emissive layer, which is at the heart of OLEDs, directly converts electrical energy into light. This thesis focuses on investigating components of this layer.

The emissive layer comprises two types of molecules: dopant and host molecules. These molecules possess captivating photophysical properties, yet their properties to meet the specific requirements for commercial OLEDs devices need to be finely tuned. Specifically, in the framework of second-generation OLEDs devices, where transition metal complexes are employed as dopants, controlling their excited state properties remains challenging in nature. Moreover, operational stability issues arise from the inherent instability of host molecules during device operation.

To address these intricacies, computational chemistry emerges as a saviour, offering a cost-effective means of working with such systems by modelling them rather than relying solely on experimental tools. However, not all computational methods can adequately elucidate the properties of such complex systems, particularly their excited state characteristics. Therefore, it is imperative to select a computational protocol with utmost care in order to accurately describe these systems.

In this thesis, we have developed accurate computational protocol for computing the photophysical properties of Pt(II) and Ir(III) complexes, which serve as dopant materials in OLEDs devices. We employ local coupled-cluster methods to calculate phosphorescence energies and further validate our results by comparing them with different variants of density functional theory (DFT) functionals. Additionally, utilizing our computational approach, including state-of-the-art vibronic calculations and excited state characterization tools we have unambiguously elucidated the excited states responsible for their photoluminescence properties. Regarding organic host molecules, we have conducted degradation studies using high-level ab initio methods, non-adiabatic dynamics, and machine learning techniques. Our degradation study encompasse two aspects: static and dynamic analyses, providing valuable insights for the design of host molecules with enhanced operational stability for OLEDs devices. Therefore, in general, this thesis impacts not only the computational photochemistry community by providing new protocols and good practices, but also the experimental communities with interests in photoluminescence materials and OLEDs.