Properties of materials for organic light emitting diodes

Abstract

Organic light emitting diodes (OLED) promise new and exciting possibilities for display and lighting technologies. High performance phosphorescent materials can achieve 100% internal quantum efficiencies. This is highly attractive, but to achieve this phosphorescent complexes used in OLEDs must have a high photoluminescence quantum yield (PLQY), but also a suitable emission wavelength. Furthermore, the overall stability of the multilayer organic films that form the active component of OLEDs must also be highly durable against thermal degradation. In the first part of the thesis a study of the factors that affect the luminosity of phosphorescent iridium(III) complexes is reported. Blue phosphors, which typically have low performance or unsuitable colour characteristics, are particularly important for efficient OLEDs with a balanced emission spectrum. To learn more about the challenges associated with producing deep blue phosphors, a family of blue emitting iridium(III) complexes was investigated with magnetic circular dichroism spectroscopy and then described by relativistic quantum chemistry calculations. Heavy metals like iridium need relativistic corrections to properly describe their electronic properties. Density functional theory (DFT) calculations on the complex fac-tris(1-methyl-5-phenyl-3-n-propyl-[1,2,4]triazolyl)iridium(III) [Ir(ptz)3] showed that so-called scalar relativistic effects lead to an indirect destabilisation of the 5d orbitals. The core orbitals of the iridium are relativistically contracted, which electrostatically shields the nucleus and contributes a ~0.28 eV lowering of the energy of the frontier molecular orbitals. As a result the energies of the singlet and triplet excitations calculated by time-dependent density functional theory (TDDFT) are lowered by ~0.2 eV, compared to the non-relativistic calculations. Phosphorescence from heavy metal complexes is a direct result of spin-orbit coupling; spin-orbit coupling allows the formally forbidden singlet-triplet crossing to occur. In terms of emission efficiency, the radiative rate of emission should be maximised, while minimising non-radiative contributions. A perturbative TDDFT method was used to describe spin-orbit coupling in a family of fluorinated iridium(III) complexes based on the parent complex Ir(ptz)3. Fluorination drives the emission wavelength to deeper blue, but the PLQY decreases by an order of magnitude. The perturbative TDDFT treatment was found to give excellent agreement with more computationally expensive two-component fully relativistic methods. It was found that the radiative rate across the family of Ir(ptz)3 complexes is critically dependent on the S3(E)-T1(A) energy gap. These excitations arise predominantly from the highest occupied molecular orbital (HOMO), HOMO-1 and the lowest unoccupied molecular orbital (LUMO). Therefore the primary electronic feature for determining the radiative rate of emission is the energy of splitting between HOMO and HOMO-1. The second part of the thesis describes a study of film morphology where the films are comprised of small molecules typically found in phosphorescent OLEDs. For durable devices there is a need to better understand the morphology of small molecule based organic multilayer films that are used in phosphorescent OLEDs. In particular the active emissive layer often comprises phosphorescent molecules blended into a host material, which helps to reduce self-quenching and enhances charge transport. Using a combination of 6 wt% fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy)3] blended in 4,4′-bis(N-carbazolyl)biphenyl (CBP), it was found that lateral phase separation between the two components occurred after annealing at 80 °C. This phase separation was suppressed when the blend ratio between Ir(ppy)3 and CBP was increased to 12 wt%. Since low blend ratios tend to give higher device efficiency, there therefore exists a trade-off between device durability and performance. Interactions between layers in a multilayer film are difficult to investigate. Using neutron reflectometry (NR) a multilayer film of tris[4-(carbazoyl-9-yl)phenyl]amine (TCTA)/[Ir(ppy)3:CBP]/bathocuproine (BCP) was subjected to thermal annealing, and the photoluminescence of the film measured in situ. The film structure remained stable to 90 °C, but after heating to 100 °C the BCP and Ir(ppy)3:CBP layers rapidly interdiffused. This was accompanied by a significant decrease in the photoluminescence. Using time-dependent NR, the interdiffusion was characterised by a moving interfacial region which propagated according to the time dependence x ~ t^n with n = 0.34. This is indicative of anomalous Fickian interdiffusion. The electronic structure and film morphology of organic semiconductors has broad application in areas beyond OLEDs. Related fields such as dye sensitised solar cells; photocatalytic water splitting; polymer and small molecule photovoltaics; and organic field effect transistors all have similar challenges in understanding and optimising the fundamental electronic and morphological characteristics of their specific organic components.