Study of Triplet Exciton Dynamics in Small Organic Molecule Films Using Time Resolved Optical Spectroscopy

In recent years it has become clear that knowledge of triplet transport in single layer and multilayer films can be crucial in improving the efficiency of organic light emitting devices and solar cells. This thesis reports an investigation of triplet exciton dynamics in small organic molecule single and multilayer layer films using optical time resolved nanosecond spectroscopy. A diligent step by step approach, leading towards the investigation of complex donor/spacer/acceptor multilayer structures is used. First of all, single layer films to be a constituents of multilayer structures were studied by measuring fluorescence, delayed fluorescence and phosphorescence. 4,4’-N,N’-dicarbazolyl-1,1’-biphenyl (CBP) widely used in organic light emitting diodes is characterized. Evidence is provided that in some of these spectra emission comes from trap states rather than the CBP molecule itself. Also N,N’-diphenyl-N,N’-bis(1-naphthyl)-1,1’-biphenyl-4,4’’-diamine (NPB) has been investigated. Results indicate that bimolecular triplet recombination is dominant and that triplet transport has dispersive features even at room temperature in NPB films. Then films of heavy metal iridium complexes fac-tris(2-phenylpyridine) iridium (Ir(ppy)3) and iridium(III)tris(1-phenylisoquinoline) (Ir(piq)3) are put into the spotlight. New states previously not reported are identified and decay with the slope -1 characteristic of more than one iridium complex and previously not published in literature is observed. Triplet interface sites in bilayer Ir(piq)3/NPB films obstructing triplet migration are determined and triplet movement across interface is experimentally captured for the first time. The origin of these interface trap states is suggested. Then this system is upgraded into Ir(ppy)3/NPB/Ir(piq)3 and triplet transfer from Ir(ppy)3 to Ir(piq)3 via NPB is investigated. A model of triplet exciton dynamics in Ir(piq)3/NPB films using classical diffusion equations is presented with interface sites included. Computer simulations were performed and the results are in very good agreement with the experimental ones. Finally problems encountered are identified and main guidelines on how to do research in complicated multilayer structures are set

Study of triplet exciton dynamics in small organic molecule films using time resolved optical spectroscopy

In recent years it has become clear that knowledge of triplet transport in single layer and multilayer films can be crucial in improving the efficiency of organic light emitting devices and solar cells. This thesis reports an investigation of triplet exciton dynamics in small organic molecule single and multilayer layer films using optical time resolved nanosecond spectroscopy. A diligent step by step approach, leading towards the investigation of complex donor/spacer/acceptor multilayer structures is used. First of all, single layer films to be a constituents of multilayer structures were studied by measuring fluorescence, delayed fluorescence and phosphorescence. 4,4’-N,N’-dicarbazolyl-1,1’-biphenyl (CBP) widely used in organic light emitting diodes is characterized. Evidence is provided that in some of these spectra emission comes from trap states rather than the CBP molecule itself. Also N,N’-diphenyl-N,N’-bis(1-naphthyl)-1,1’-biphenyl-4,4’’-diamine (NPB) has been investigated. Results indicate that bimolecular triplet recombination is dominant and that triplet transport has dispersive features even at room temperature in NPB films. Then films of heavy metal iridium complexes fac-tris(2-phenylpyridine) iridium (Ir(ppy)3) and iridium(III)tris(1-phenylisoquinoline) (Ir(piq)3) are put into the spotlight. New states previously not reported are identified and decay with the slope -1 characteristic of more than one iridium complex and previously not published in literature is observed. Triplet interface sites in bilayer Ir(piq)3/NPB films obstructing triplet migration are determined and triplet movement across interface is experimentally captured for the first time. The origin of these interface trap states is suggested. Then this system is upgraded into Ir(ppy)3/NPB/Ir(piq)3 and triplet transfer from Ir(ppy)3 to Ir(piq)3 via NPB is investigated. A model of triplet exciton dynamics in Ir(piq)3/NPB films using classical diffusion equations is presented with interface sites included. Computer simulations were performed and the results are in very good agreement with the experimental ones. Finally problems encountered are identified and main guidelines on how to do research in complicated multilayer structures are set.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Deep Blue Exciplex Organic Light-Emitting Diodes with Enhanced Efficiency; P-type or E-type Triplet Conversion to Singlet Excitons?

Simple trilayer, deep blue, fluorescent exciplex organic light-emitting diodes (OLEDs) are reported. These OLEDs emit from an exciplex state formed between the highest occupied molecular orbital (HOMO) of N,N′-bis(1-naphthyl)N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) and lowest unoccupied molecular orbital (LUMO) of 1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (TPBi) and the NPB singlet manifold, yielding 2.7% external quantum efficiency at 450 nm. It is shown that the majority of the delayed emission in electroluminescence arises from P-type triplet fusion at NPB sites not E-type reverse intersystem crossing because of the presence of the NPB triplet state acting as a deep trap

Generating Light from Upper Excited Triplet States: A Contribution to the Indirect Singlet Yield of a Polymer OLED, Helping to Exceed the 25% Singlet Exciton Limit

The mechanisms by which light is generated in an organic light emitting diode have slowly been elucidated over the last ten years. The role of triplet annihilation has demonstrated how the “spin statistical limit” can be surpassed, but it cannot account for all light produced in the most efficient devices. Here, a further mechanism is demonstrated by which upper excited triplet states can also contribute to indirect singlet production and delayed fluorescence. Since in a device the population of these TN states is large, this indirect radiative decay channel can contribute a sizeable fraction of the total emission measured from a device. The role of intra- and interchain charge transfer states is critical in underpinning this mechanism

The photophysics of singlet, triplet, and degradation trap states in 4,4-N,N-'-dicarbazolyl-1,1(')-biphenyl

In this paper we report the results of optical characterization of 4,4-N,N-'-dicarbazolyl-1,1(')-biphenyl (CBP), known as a host material for phosphorescent light emitting devices. Using absorption, steady state, and time-resolved spectroscopy, we explore the singlet and triplet states in solid and solution samples of CBP. In solutions we observe two distinct short-lived states with well-resolved emission originating from individual molecule singlet states (at 365 and 380 nm) and "quenching" low energy (LE) states (at 404 and 424 nm). The latter are seen only in saturated solutions and solid samples. Both of those species have different lifetimes. After UV exposure of very concentrated degassed solution the intensities of the LE bands starts to decrease. The longer the solution is exposed to UV, the less emission is seen at 404 and 424 nm, until it is totally gone. The spectrum of the highly concentrated solution is then the same as the spectrum of dilute solution, i.e., only emission at 365 and 380 nm is present. An increase in intensities of the singlet emission peaks correlates with an increase in UV exposure time. Similar behavior is observed in evaporated CBP film. We propose that this behavior is due to chemical instability of the weak N-C bonding of carbazolyl moiety-this creates new degradational species over time which dissociate after exposure to UV. We believe this to be the reason for variation in CBP fluorescence and delayed fluorescence spectra recorded by various research groups. Further, we detected two types of very long-lived states. One of these states (higher energy) is ascribed to molecular phosphorescence emission, the other to emission from low energy triplet trap states which we relate to degradational species. We propose that triplets are more easily caught by these latter sites when their hopping rate increases, and they emit inefficiently from these lower energy sites

The role of exciplex states in phosphorescent OLEDs with poly(vinylcarbazole) (PVK) host

Polymer light emitting diodes (PLEDs) may revolutionize lighting and display industries. PLEDs would enable printing of display or lighting panels on large area substrates that could substantially reduce fabrication costs by avoiding expensive vacuum processes presently used in OLED technologies. PVK is one of the most popular hosts for blue PLEDs. However, PVK has very poor electron transport properties and oxadiazole based electron dopants, e.g. PBD or OXD-7, are used to improve charge transport. This is generally ascribed to capture and transport of electrons on the PBD or OXD-7. Here we show that this is not necessarily the only reason for improved efficiency upon PVK doping. We demonstrate that devices with PVK doped with PBD or OXD-7 have emission lasting up to 1 ms which in some cases may be greater than prompt emission from excitons formed initially on the dopant. This long-lived emission is arising mainly due to formation of an exciplex between the PVK and PBD/OXD-7. This exciplex state then repopulates dopant iridium complexes over a long period of time giving very long-lived emission. We also note that this exciplex-fed long-lived emission from heavy metal complexes is observed in several PLEDs with PBD and PVK (and also OXD-7) doped with blue or green iridium phosphors indicating this to be a general phenomenon

Ultrahigh Efficiency Fluorescent Single and Bi-Layer Organic Light Emitting Diodes: The Key Role of Triplet Fusion

A new family of anthracene core, highly fluorescent emitters is synthesized which include diphenylamine hole transport end groups. Using a very simple one or two layer organic light emitting diode (OLED) structure, devices without outcoupling achieve an external quantum efficiency of 6% and photonic efficiencies of 20 cd/A. The theoretical maximum efficiency of such devices should not exceed 3.55%. Detailed photophysical characterization shows that for these anthracene based emitters 2T1≤Tn and so in this special case, triplet fusion can achieve a singlet production yield of 0.5. Indeed, delayed electroluminescence measurements show that triplet fusion contributes 59% of all singlets produced in these devices. This demonstrates that when triplet fusion becomes very efficient, fluorescent OLEDs even with very simple structures can approach an internal singlet production yield close to the theoretical absolute maximum of 62.5% and rival phosphorescent‐based OLEDs with the added advantage of much improved stability

Triplet–Triplet Annihilation in 9,10-Diphenylanthracene Derivatives: The Role of Intersystem Crossing and Exciton Diffusion

Triplet–triplet annihilation (TTA) is an attractive way to boost the efficiency of conventional fluorescent organic light-emitting diodes (OLEDs). TTA-active anthracene derivatives are often considered as state-of-the-art emitters due to the proper energy level alignment. In this work, TTA properties of a series of highly fluorescent nonsymmetrical anthracene compounds bearing 9-(4-arylphenyl) moiety and 10-(4-hexylphenyl) fragments were assessed. Two different methods to enhance the TTA efficiency are demonstrated. First, the intensity of TTA-based delayed fluorescence directly depended on the intersystem crossing (ISC) rate. This ISC rate can be significantly enhanced in more conjugated compounds due to the resonant alignment of S₁ and T₂ energy levels. While enhanced ISC rate slightly quenches the intensity of prompt fluorescence, the rise of the triplet population boosts the intensity of resultant delayed fluorescence. Second, the triplet annihilation rate can be significantly enhanced by optimization of triplet exciton diffusion regime in the films of anthracene derivatives. We show that the proper layer preparation technology has a crucial influence on uniformity and energetic disorder of the film. This enhances the nondispersive triplet diffusion and increases the resulting delayed fluorescence intensity