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Interaction of UV Irradiation with Thin Films of Organic Molecules
Abstract
There is an ongoing interest in organic materials due to their application in various organic electronic devices. However stability of organic materials limits their potential use. They are prone to degradation both during the working life and storage. One of the main causes is extrinsic degradation, under the influence of oxygen and moisture. This problem can be solved by encapsulation of devices. However no encapsulation is perfect. This paper presents a study of interaction of thin films of well-known organic blue emitters, namely N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)benzidine (TPD) and 4,4′-bis(2,2-diphenylvinyl)-1,1′-biphenyl (DPVBi), with UV light in air. Films of both materials are stable in vacuum, but readily degrade in the presence of oxygen. Thus, the necessary condition for interaction (degradation) is the simultaneous presence of UV light and oxygen. Chemical analysis of irradiated films by mass and infrared spectroscopy revealed presence of oxidized species (impurities). These impurities are responsible for increased morphological stability of irradiated films and quenching of photoluminescence (PL). Only small amount of impurities, 0.4 % (0.2 %) for TPD (DPVBi), causes 50 % decrease of PL. This implies a non-trivial mechanism of quenching. For both molecules it was found that distance between impurities is smaller or equal to exciton diffusion length, which is the necessary condition for quenching. Following mechanism of quenching is proposed: exciton diffuses by hopping form one host molecule (DPVBi or TPD) to another through Förster resonant energy transfer in a random walk manner. If, during its lifetime, it comes to proximity of an impurity, a PL quenching process occurs. Findings of this study are important because they show that even a small amount of oxygen that penetrates a blue emitter layer would impair luminescence efficiency of a device. Moreover, the absorption of its own radiation would additionally contribute to the rate of degradation of a device. It is reasonable to expect that transport properties would also be affected when materials are used as a hole-transporting layer in OLEDs.
... The efficiency and durability of any multilayered organic device are influenced by the thermal stability, molecular structure, morphology, charge carriers' mobility of each layer [1,3]. Unfortunately, thin films of organic materials can be easily degrading under the effect of moisture, UV light exposure, and air exposure, consequently, most of the organic electronic devices are encapsulated [4]. Interestingly, the interaction between organic films and UV radiation may cause the formation of impurities which enhance the morphological and thermal stabilities as well as optical properties either absorption or photoluminescence [4]. ...
... Unfortunately, thin films of organic materials can be easily degrading under the effect of moisture, UV light exposure, and air exposure, consequently, most of the organic electronic devices are encapsulated [4]. Interestingly, the interaction between organic films and UV radiation may cause the formation of impurities which enhance the morphological and thermal stabilities as well as optical properties either absorption or photoluminescence [4]. ...
... Furthermore, an endothermic peak is observed in the DSC thermogram that is revealed in Fig. 2 (b) with the absence of glass transition. It is well known that TPD has a low glass transition temperature of about 60 • C which suppresses its stability due to the equality of non-covalent interaction energy and thermal fluctuations energy which induces structural and morphological changes by minimizing Gibbs free energy [4]. The absence of the glass transition in polycrystalline TPD powder was [33,36,37] *ν stretching, δ bending, ρ rocking and ω wagging, s symmetric, as asymmetric, ip in-plane, and op out of the plane. ...
Owing to the great importance of hole transport layer (HTL) in the performance of many optoelectronic and microelectronic applications, this report represents the investigations on the influence UV irradiation for 1, 3, and 5 min on some physical properties of a significant HTL known as N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) in the form of thin films. A stable amorphous structure with non-detectable IR variations in the molecular structure of the deposited films are observed in the XRD and FTIR results even under the irradiation for 5 min. But, the 1 min UV exposure caused a significant enhancement in the photoluminescence of TPD film with a conserved peak position at 437.5 nm. Furthermore, the surface morphology features are inspected by using the field emission electron microscopy technique (FESEM). The UV irradiation till 3 min induced stabilization of the film morphology and decreased the roughness of the films. The transmission, reflection, and absorption spectra of the pristine and irradiated TPD films are measured via spectrophotometric method from UV to NIR band. The spectral distribution of transmission is analyzed using the Swanepoel method to estimate the linear refractive index and film thickness. Moreover, the dispersion parameters of the films are estimated and interpreted in terms of the single oscillator model. The energy gap of the prepared films showed a little increase upon irradiation for 1 min (3.02 eV) and then decreased to 2.97 eV for 5 min irradiation. The values of molecular polarizability, plasmon frequency, Penn's energy, and Fermi energy are calculated for the pristine and irradiated films. Finally, the nonlinear refractive index and the third-order nonlinear optical susceptibility are estimated and the static value of both are found to be 9.667 × 10⁻¹³ esu, and 4.105 × 10⁻¹⁴ esu, respectively.
... Concerning the stability of the TPD films, even though most studies have been done with neat films, crystallization processes have been recognized as the main source of degradation [41,42]. In addition, there is evidence that photo-oxidation processes, which take place upon film irradiation with UV light in air, produce impurities that cause the PL quenching of TPD [43]. In fact, our results show that degradation is very fast under UV pulsed light. ...
The molecule N,N′-bis(3-methylphenyl)-N,N′-dyphenylbenzidine (TPD) has been widely used in optoelectronic applications, mainly for its hole-transporting properties, but also for its capability to emit blue light and amplified spontaneous emission, which is important for the development of organic lasers. Here, we report deep-blue-emitting distributed feedback (DFB) lasers based on TPD dispersed in polystyrene (PS), as active media, and dichromated gelatin layers with holographically engraved relief gratings, as laser resonators. The effect of the device architecture (with the resonator located below or on top of the active layer) is investigated with a dye (TPD) that can be doped into PS at higher rates (up to 60 wt%), than with previously used dyes (<5 wt%). This has enabled changing the index contrast between film and resonator, which has an important effect on the laser performance. With regards to thresholds, both architectures behave similarly for TPD concentrations above 20 wt%, while for lower concentrations, top-layer resonator devices show lower values (around half). Remarkably, the operational durability of top-layer resonator devices is larger (in a factor of around 2), independently of the TPD concentration. This is a consequence of the protection offered by the resonator against dye photo-oxidation when the device is illuminated with pulsed UV light.
Herein, we represent the fabrication of self-biased visible-blind UV photodetector based on thermally deposited N,N′ -Bis(3-methylphenyl)-N,N′ -diphenylbenzidine (TPD) film over n-type silicon substrate giving rise to the structure Au/TPD/n-Si/Al heterojunction. The microelectronic parameters of the fabricated heterojunction Au/ TPD/n-Si/Al as well as the charge carriers’ transport mechanisms in forward and reverse bias conditions are analyzed and interpreted in detail under the dark conditions and the influence of temperature. The manufactured photodetector showed a significant response to UV illumination with high photocurrents and noticeable values of open-circuit voltage. The fabricated UV photodetector under self-biasing and illumination intensity (100 mW/
cm2) records values of responsivity, specific detectivity, linear dynamic range, signal to noise ratio, the response time, and ON/OFF ratio equals 147 mA/W, 6.4 × 1010 Jones, 58.69 dB, 945.5, 82/80 ms, and 861.98, respectively. This high, stable, and fast performance of the fabricated sensor suggest this architecture as an efficient integrated platform for self-biased UV photodetection.
The photodegradation behavior of four well-established iridium emitters was investigated. Irradiation of the samples in different solvents and under atmospheric as well as inert conditions helped to identify several pathways that can contribute to the deterioration of these compounds. Degradation via singlet oxygen or the excited states of the emitters as well as the detrimental influence of halogenated solvents are discussed for the different investigated iridium complexes. Some of the resulting degradation products could be identified by using LC-MS or other analytical techniques. The results show how even small structural changes can have a huge influence on rate and mechanism of the photodegradation. The observations from this study may help to better understand degradation processes occurring during the handling of the materials, but also during device processing and operation.
The relative quantum yields of the host and guest fluorescence in anthracene-tetracene mixed crystals have been measured for tetracene concentrations ct between 10-6 and 10-1 mol/mol and temperatures below and above the temperature region, where the energy transfer is partially quenched by shallow traps (35 °K). The concentration dependence of the quantum ratio is measured as QT/QA=k·CTP, where the exponent p is nearly 1 and k is measured as 6·104 and explained as the ratio τw/tH (τw is the decay time of the host fluorescence and th the hopping time). The fraction of transfer which is quenched at lowest temperatures is independent of the tetracene concentration. The results are compared with the exciton diffusion mechanism for energy transfer (hopping model).
Organic light-emitting diodes (OLEDs) based on small-molecule materials are currently developed for applications in flat panel displays and general lighting sources. However, a large number of efficient deep blue emitters still suffer from rather fast degradation and thus, requires further improvement. The aim of the present work is to gain a fundamental understanding of the intrinsic degradation processes causing the low stability of blue OLED emitters. For this purpose we study the photoluminescence (PL) degradation instead of the most often investigated electroluminescence (EL) degradation to separate electrically and optically generated effects. We show a newly developed PL lifetime measurement system which allows the study of degradation processes under the influence of either electron or hole currents. Using this set-up we demonstrate the very high PL stability of the highly efficient blue singlet emitter 2,2',7,7'-tetrakis(2,2-diphenylvinyl)spiro-9,9'-bifluorene (Spiro- DPVBi) under electron and hole currents and compare this to the lifetime of OLEDs using the same emitter material.
Tris (8-hydroxyquinoline) aluminum (Alq3) represents a material of significant interest for electron transport and/or light emitting layer applications in organic light emitting diodes (OLEDs). In spite of advances in Alq3 based devices, the knowledge and understanding of the optical properties of Alq3 and its chemical and environmental stability is still limited. With the reports of decreased turn-on voltage and increased efficiency of OLEDs, the issues of lifetime and stability of those devices are attracting increasing attention. The degradation of Alq3 based OLEDs and dark spots formation and growth have been intensively studied. The studies on degradation of optical properties of Alq3 itself remain scarce. We have investigated effects of atmosphere exposure to properties of tris (8-hydroxyquinoline) aluminum (Alq3) thin films by photoluminescence (PL) and absorption measurements. Alq3 films were evaportated on glass substrates at different temperatures. The influence of annealing to the environmental stability of the films has also been investigated. It has been found that deposition at higher substrate temperature and annealing of the samples deposited at room temperature yields improvement in environmental stability of the films, i.e. less decrease of the PL intensity over time with atmosphere exposure, as well as increased PL intensity. To investigate further effects of the air exposure, films deposited at room temprature were stored for four days in air, nitrogen, and oxygen. No decrease in PL intensity has been found for storage in nitrogen, while decrease for the film stored in oxygen was smaller than that for film stored in air, indicating that both humidity and oxygen play a role in PL intensity decrease in Alq3 thin films.
Efficient white light-emitting diodes (WOLEDs) were fabricated with a solution-processed single emission layer composed of a molecular and polymeric material mixed-host (MH). The main host used was a blue-emitting molecular material of 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) and the assisting host used was a hole-transport-type polymer of poly(9-vinylcarbazole) (PVK). By co-doping 4,4′-bis[2-(4-(N,N-diphenylamino)phenyl)vinyl]biphenyl and 5,6,11,12-tetraphenylnaphacene into the MH, the performances of the fabricated devices made with different mixing ratio of host materials were investigated, and were to depend on the mixing ratios. Under the optimal PVK:DPVBi ratio (3:7), we achieved a maximum luminance of 14 110cd/m2 and a maximum current efficiency of 9.5cd/A. These improvements were attributed to the MH structure, which effectively improved the thermal stability of spin-coated film and enhanced the hole-injection/transporting properties of WOLEDs.
We present a study of dark air-exposure degradation of organic solar cells based on photoactive blends of the conjugated polymer, poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) with [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Photovoltaic devices were fabricated on indium tin oxide (ITO) glass with or without a layer of poly (3,4-ethylenedioxythiophene):poly(4-styrene sulfonate) (PEDOT:PSS), and were studied without encapsulation. Photovoltaic performance characteristics were measured as a function of time for different ambient conditions (under white light irradiation and in the dark, and under air, dry oxygen and humid nitrogen atmospheres). It was found that a key cause of degradation under air exposure is light independent and results from water adsorption by the hygroscopic PEDOT:PSS layer. Measurements of the charge mobility and hole injection after air exposure showed that the degradation increases the resistance of the PEDOT:PSS/blend layer interface.
The organic light emitting devices (OLEDs) material tris-(8-hydroxyquinoline) aluminum (Alq3) is synthesized and deposited in thin film form by thermal evaporation on silicon and glass substrates. Thin films are studied for environmental related degradation. The light induced effects on Alq3 films exposed to white light in air and vacuum are characterized by atomic force microscopy (AFM) for surface morphology and phases present. The photoluminescence (PL) and spectroscopic ellipsometry (SE) techniques have been used to study optical properties. The AFM images, both topography and phase contrast, indicate a second phase with enhanced crystallinity for the vacuum exposed samples. This observation is supported by PL experiments. The phase with increased lifetime and hence enhanced crystallinity has also been found for vacuum exposed films by time correlated single photon counting (TCSPC) measurements.
Molecules with delocalized p-electrons possess attractive electroluminescent properties, and have been recently incorporated as the active emissive layers in organic light-emitting devices. In particular, electroluminescent molecular composites using a highly fluorescent guest, with/without carrier trapping properties, and a host with dual luminescent and carrier transport properties are the focus of this study. Devices based on these luminescent functional p-electron molecular systems exhibited high electroluminescence quantum efficiency, and good thermal and temporal stability, with ease of color tunability. The paper discusses the mechanisms of light-emission that lead to superior performance in these molecular devices. Efficient Forster energy transfer from the host to the guest molecules, and direct carrier recombination at the highly fluorescent guest molecules are proposed as the two mechanisms responsible for high efficiency.
First published as an Advance Article on the web 6th August 2003 Theoretical optimization of triphenylamine geometry, carried out at DFT(B3LYP) level using 6-31G** and aug-cc-pVDZ basis sets, predicted a propeller-like structure of the compound with D 3 overall symmetry. In this structure, the central NCCC atoms are coplanar and the phenyl rings are symmetrically twisted from this plane by 41.5 (6-31G**) or 41.6 (aug-cc-pVDZ). The experimental FTIR spectrum of triphenylamine monomers isolated in an argon matrix was measured and interpreted by comparison with theoretical spectra calculated at the DFT(B3LYP) level with 6-31G** or aug-cc-pVDZ basis sets. The good agreement between the experimental and theoretical spectra allowed a positive assignment of the observed infrared absorption bands. Conformational flexibility of triphenylamine was investigated by carrying out a series of theoretical scans of the potential energy hypersurface of the system. Special attention was granted to the minimal energy pathway between the left-hand rotating and right-hand rotating symmetry identical structures of the compound. A route conserving a C 2 symmetry axis was identified as implying an energy barrier of 20 kJ mol À1 only, whereas the calculated barrier for the concerted twist of all the phenyl rings (the route with conservation of the C 3 symmetry axis) was as high as 54 kJ mol À1 .
A saturated-red emitting system suitable for solution-processable LEDs (light-emitting diodes) is reported. In their approach the authors dope the blue-emitting conjugated polymer poly(9,9-dioctylfluorene) (see Figure) with the red dye tetraphenylporphyrin, which leads to an efficient single-step Förster transfer from blue to red. The fabrication and efficiency of LEDs based on this system are also described.
Exciton generation, migration, and dissociation are key processes that play a central role in the design and operation of many organic optoelectronic devices. In organic photovoltaic cells, charge generation often occurs only at an interface, forcing the exciton to migrate from the point of photogeneration in order to be dissociated into its constituent charge carriers. Consequently, the design and performance of these devices is strongly impacted by the typically short distance over which excitons are able to move. The ability to engineer materials or device architectures with favorable exciton transport depends strongly on improving our understanding of the governing energy transfer mechanisms and rates. To this end, this review highlights recent efforts to better characterize, understand and ultimately engineer exciton transport.
Oxygen causes reversible and irreversible detrimental effects to the performance of organic (opto-)electronic devices. In order to get some insight into the mechanisms of these effects, we investigated the kinetics of fluorescence quenching (FQ) in thin films (d≈100 nm–12 μm) of regioregular polyalkylthiophenes upon exposure to oxygen.The kinetics of FQ consists of a fast component, which is fully reversible, as well as a slow component, which is partially irreversible. The fast reversible component leads to a loss of fluorescence intensity of approximately 2% at an oxygen partial pressure of 1 bar within milliseconds. It is independent of the intensity of the exciting light and is ascribed to collisional quenching of excited singlet states after oxygen diffusion into the bulk of the film. The diffusion coefficients of oxygen and singlet excitons were determined as D(O2)=1.5(3)×10−7 cm2 s−1 and D(S1)=5×10−4 cm2 s−1, respectively. The slow reversible component, whose amplitude depends on light intensity, occurs on a time-scale of minutes. It is assigned to the formation of charge-transfer complexes between excited singlet states of polythiophene and oxygen. Femtosecond pump-probe experiments show that effective quenching of excited singlet states by oxygen takes place on a picosecond time-scale, leading to the enhanced formation of charged states.
From time-resolved luminescence measurements it is demonstrated that the background concentration of excitonquenching defects in a range of organic semiconductors is the same as their electron-trap concentration. This observation suggests that the exciton-quenching defects and the electron traps share the same origin. The typical exciton diffusion length of 5-8 nm for organic semiconductors is therefore governed by the distance between the universal electron traps.
Films of regioregular poly(3-hexylthiophene) [P3HT] were monitored for degradation by means of in situ fluorescence quenching measurements. The films were found to be stable under UV or VIS energy illumination in vacuo and stable to the air when kept in the dark. Under illumination in air however, degradation products were formed immediately as seen by the fluorescence quenching. The quantum yield for defect product formation was calculated to be around 3 × 10−6 per incident photon of above band-gap energy, a relatively rapid reaction that could create over 1016 defects per cm3 in optically thin films of P3HT in less than a second under sunlight, or in only a matter of minutes under room lighting. The degradation pathway is thought to involve attack of ground state molecular oxygen on the P3HT excited state. The reaction itself is self-inhibiting, as the P3HT excited state is thought to be effectively quenched by the increasing formation of defects, leading to slower and slower defect formation until the film enters a ‘quasi-stable’ state where no further degradation by this mechanism will occur. The upper limit to the number of defects to be found in films of P3HT formed by this mechanism is estimated to be around 2 × 1017 cm−3.
We compare the degradation of organic light emitting diodes (OLEDs) by UV light and by electrical driving. We prove that the exponential dependence of the half-lifetime on the current density known from electrical aging is also valid for UV-degradation. The influence of excitons on the degradation of OLEDs is determined and we experimentally distinguish between the influence of singlet and triplet excitons. We conclude that singlet excitons are the main cause of degradation for Spiro-DPVBi(2,2',7,7'-tetrakis(2,2-diphenylvinyl)spiro-9,9'-bifluorene)-based OLEDs by a comparison of the degradation of electrically driven and UV-excited OLEDs.
In this study, blue organic light-emitting diodes (OLEDs) using polymer-dopant systems as a light-emitting layer were fabricated. The light-emitting layer was produced via a spin-coating process. The materials used in the light-emitting layer included: poly(9-vinylcarbazole) (PVK) as a matrix material, and blue-emission 4,4'-bis(2,2-diphenylvinyl)biphenyl (DPVBi) as a light-emitting material. The blue OLED was fabricated by blending PVK and DPVBi according to the proportion of mass as specified for a light-emitting layer. Process relevant photophysical mechanisms and energy transfer phenomena were studied using absorption, photoluminescence (PL), photoluminescent excitation, and electroluminescence (EL) spectra. We have learned that there was energy transfer phenomenon between PVK and DPVBi. More specifically, the EL spectrum of PVK:DPVBi was significantly different from the PL spectra of PVK, DPVBi, and PVK:DPVBi, shifting to a longer wavelength and showing multiple boarding emission peaks (maximum emission at 465 and 486 nm). The electro-optical characteristics of the blue OLED and the possible origin of the difference between the EL and PL spectra were discussed.
Photoluminescence spectra of N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)-benzidine (TPD) are studied in a temperature range from 10K to room temperature using a neat TPD film, and 5wt.% TPD doped films with polystyrene (PS), 4,4′-N,N′-dicarbazole-biphenyl (CBP), and polycarbonate (PC) as hosts. The photo-excitation occurred in the singlet absorption region of TPD. The blue fluorescence and green phosphorescence quantum yields, ϕF and ϕP, of TPD are determined from their quantum distributions, EF(λ) and EP(λ). The yields are found to be ϕF=0.39 and ϕP=0.012 for neat films at room temperature, while ϕF=0.78 and ϕP=0.026 are measured for a TPD doped PS film. The lower luminescence quantum yield in the neat film is caused by self-quenching.
N,N′-biphenyl-N,N′-bis-(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine (TPD) and N,N′-biphenyl-N,N′-bis-(1-naphenyl)-[1,1′-biphenyl]-4,4′-diamine (NPB) are typical hole transporting materials, which are widely used in current organic eletroluminescence (EL) research. NPB is regarded as a better hole transporting material than TPD, since it has a higher glass transition temperature (Tg). In this paper, the photostability and morphological stability of TPD and NPB have been studied in film states. It is concluded that NPB thin film is a better hole transporting material for organic EL than TPD, not only because it has a higher Tg, but also because it is more photostable and has a better morphological stability in thin film state.
Numerical simulations of exciton migration along a one-dimensional lattice via Forster energy transfer are reported. The roles of static Gaussian energetic disorder and transition dipole orientational disorder are investigated. Both disorder mechanisms result in subdiffusive behavior during the initial phase of energy migration, tending asymptotically to normal diffusion. The dependence of the subdiffusion parameters on the inhomogeneous linewidth and intersite spacing is examined. Depending on the exciton lifetime, subdiffusive motion may dominate the exciton displacement behavior, and it becomes problematic to use normal diffusion theory to make even qualitative predictions about the exciton motion.
Enhanced stability against heat and moisture in organic vapor-deposited TPD thin film having hole transport property has been achieved by UV irradiation. The crystallization of TPD film without any treatment was started within a week even at RT and accelerated with increasing temperature under the air. By photoirradiation using UV light, however, the TPD thin film was found to be difficult to form crystallized state and showed good contact with substrate even at 60 °C and in water for more than one month. Furthermore, the molecular interdiffusion by heat between organic layers in Alq3/TPD bilayer was remarkably suppressed by the UV irradiation due to the enhanced stability of the TPD film.
The fluorescence of solid solutions of aromatic hydrocarbons has been investigated with the object of studying the transfer of excitation energy from solvent to impurity. Anthracene and pyrene were used as solvents, and the solutes comprised nine larger condensed hydrocarbons. The concentration ranged from 10-5 to 10-1 M. Fluorescence spectra were observed by photographic spectrophotometry, and the degree of quenching and quantum efficiency of the transfer process were obtained by comparison with the pure solvent. The dependence of quenching on concentration of impurity may be deduced from a simple theory of excition migration. The experimental results are in good agreement with this model. The efficiency of a particular impurity species in capturing the excitation cannot be predicted on any simple basis, but there is some evidence that it depends on the symmetry properties of the excited states of solvent and impurity. It is not directly determined by the overlap of solvent emission and impurity absorption, as is the case with sensitized fluorescence.
Described is the application of a combinatorial physical vapor deposition (CPVD) method for studying the growth dynamics of epitaxial films. The CPVD method takes advantage of the angle-dependent evaporation rate from a point source to produce thin film libraries whose deposition rate changes continuously for a factor of 50 across a 70-mm long-substrate. The link between the deposition rate and the resulting thin film morphology was made by spatially correlated absorption and atomic force microscopy measurements. It is shown that the growth of tryphenyldiamine derivate on a silica surface proceeds by three-dimensional growth of isolated islands which, at some critical coverage, coalesce to form uniform amorphous film. While the critical coverage of such films depends on the deposition rate in the 0.015–0.4 nm/s region, the particle size distribution function does not.
We report the photodegradation of poly(p‐phenylenevinylene) polymer when the film was irradiated with laser light at a wavelength corresponding to the peak wavelength of electroluminescence. Degradation in photoluminescent properties was significant in an air environment but not under vacuum. This indicates that the oxygen in air aids photodegradation and this hypothesis was confirmed by optical spectroscopy. This degradation may be associated with long‐term stability of the electroluminescence device. © 1995 American Institute of Physics.
The term ``sensitized luminescence'' in crystalline phosphors refers to the phenomenon whereby an impurity (activator, or emitter) is enabled to luminesce upon the absorption of light in a different type of center (sensitizer, or absorber) and upon the subsequent radiationless transfer of energy from the sensitizer to the activator. The resonance theory of Förster, which involves only allowed transitions, is extended to include transfer by means of forbidden transitions which, it is concluded, are responsible for the transfer in all inorganic systems yet investigated. The transfer mechanisms of importance are, in order of decreasing strength, the overlapping of the electric dipole fields of the sensitizer and the activator, the overlapping of the dipole field of the sensitizer with the quadrupole field of the activator, and exchange effects. These mechanisms will give rise to ``sensitization'' of about 103−104, 102, and 30 lattice sites surrounding each sensitizer in typical systems. The dependence of transfer efficiency upon sensitizer and activator concentrations and on temperature are discussed. Application is made of the theory to experimental results on inorganic phosphors, and further experiments are suggested.
Energy transfer in highly-efficient doped organic light-emitting devices (OLEDs) is described and discussed. The OLEDs include a hole transport layer (HTL) and an electron transport layer composed of an efficient blue emitter. A region of the HTL adjacent to the host blue-emitting layer was doped with an efficient guest red dye. The blue emitter-to-red dye energy transfer probability PHGη was determined by comparing the emission from the two fluorophores and its dependence on the applied field. PHGη decreases with increasing field, probably due to an increasing fraction of dye molecules which are positively charged, i.e., which trap a hole. It is also estimated that at fields as low as 0.4 MV/cm, ∼50% of the dye emission is due to trap emission rather than Förster energy transfer. The analysis yields a Förster energy transfer radius R0 = 33.5±3.5 Å. © 2004 American Institute of Physics.
We present measurements of the absolute photoluminescence (PL) quantum yield, ϕPL, for a wide variety of organic compounds in solid films, pure and molecularly doped with strongly fluorescent materials. The procedure, which uses an integrating sphere, does not entail comparison to other standards, and provides accurate measure of the photoluminescence efficiency for submicron thick films, prepared by high vacuum vapor deposition. Host materials include N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4-4′-diamine (TPD), a common hole transport material for light emitting diodes, tris (8-hydroxyquinolinolato) aluminum (III) (Alq3) and its methyl derivative, Almq3, two aluminum chelates used as electron transport and/or green emitting materials. Dopants include tetraphenylnapthacene (rubrene) and N,N′-diethyl quinacridone (DEQ). Doping results in a substantial increase (∼a factor 2–4) of ϕPL in comparison with that of the pure host. For instance, measured ϕPL increases from 0.25 and 0.42 for pure Alq3 and Almq3, respectively, to near unity upon doping with rubrene at a concentration of ∼1 mol %. The above data are discussed within the framework of Förster energy transfer from host to guest. © 1999 American Institute of Physics.
The energy transfer in amorphous polyvinyl carbazole (PVCA) has been studied by fluorescence quenching experiments using perylene, trinitrofluorenone, and hexachloro‐p‐xylene as guest molecules. The PVCA shows excimer fluorescence. The results are discussed in terms of exciton diffusion and competition between excimer‐forming sites and guest molecules for the trapping of excitons. Single‐step transfer from the excimer state by dipole resonance is considered as an alternative mechanism of energy transfer; in all three cases, however, exciton migration is the more probable energy‐transfer mechanism. The concentration of excimer‐forming sites is estimated to be 10−3 mole/mole basic unit of the polymer. In the pure polymer the exciton covers about a thousand basic units during its lifetime.
We report UV-light-induced photodegradation of N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1-'biphenyl-4,4�'
diamine (TPD) dye. When a halomethane solution of TPD is exposed to UV light it first oxidizes TPD and subsequently with further exposure leads to irreversible transformation of TPD to an acridine derivative as photoproduct. Exclusive use of halomethane solvents like chloroform and dichloromethane suggests solvent radical initiated photochemical transformation of TPD to a stable acridine-based photoproduct. The resultant
photoproduct of TPD showed broad-band dual (both blue and green) emissions in various solvents and also from the sublimed thin film. Based on solvent- and concentration-dependent photoluminescence (PL) of the photoproduct, PL excitation and lifetimes of blue and green bands, the origin of emission in the green region is due to an intermolecular excimer / aggregate of acridine-based photoproduct. A film of acridine-based photo-
product dispersed in polystyrene also showed substantial improvement in electrical conductivity compared to the parent compound TPD. The technique of UV-light-induced molecule structure transformation and material processing under specific environmental conditions may be considered as an efficient means of obtaining materials with enhanced optoelectronic properties.
Changes in current–voltage characteristics of N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD) dispersed in polymer films are studied when solutions of TPD
are exposed to UV light. Higher current density is observed in films cast with solutions exposed
under UV for an optimum time. Observations are rationalized on the basis of the photoinduced
oxidation of TPD when chlorinated solvents are used. Photoinduced generation of the TPD
cation (TPD+) is confirmed using linear absorption spectroscopy when compared with salt-based generation (using CuCl 2 ) of TPD+. Results suggest another possible route for generating TPD+ and the possibility of unintentional oxidation of TPD when chlorinated solvents like chloroform or dichloromethane are used for making devices.
A novel topological strategy is described for designing amorphous molecular solids suitable for optoelectronic applications. In this approach, chromophores are attached to a tetrahdral point of convergence. Stilbenoid units were covalently linked to tetraphenylmethane, tetraphenyladamantane, or tetraphenylsilane cores using palladium catalyzed coupling methodology. Thus, reaction of E(C6H5X)4 (E = C and adamantane, X = I; E = Si, X = Br) with styrene or 4,4‘-tert-butylvinylstilbene under Heck coupling conditions yields the corresponding tetrakis(stilbenyl) (E(STB)4) and tetrakis(4-tert-butylstyrylstilbenyl) (E(tBuSSB)4) compounds. Similarly, reaction of 1,1-diphenyl-2-(4-dihydroxyboronphenyl)ethene or 2-(4-pinacolatoboronphenyl)-3,3-diphenylacrylonitrile with tetrakis(4-bromophenyl)methane using Suzuki coupling methodology gives tetrakis(4,4‘-(2,2-diphenyl-vinyl)-1,1‘-biphenyl)methane (C(DPVBi)4) or tetrakis(4,4‘-(3,3-diphenylacrylonitrile)-1,1‘-biphenyl)methane (C(DPAB)4), respectively, in good yields. Compounds with more extended conjugation can also be prepared. Thus, reaction of excess 1-(4‘-tert-butylstyryl)-4-(4‘-vinylstyryl)benzene with C(C6H4I)4 provides tetrakis(4-(4‘-(4‘ ‘-tert-butylstyryl)styryl)stilbenyl)methane (C(4R-tBu)4) in low yield (20%). The more soluble analogue, tetrakis(4-(4‘-(3‘ ‘,5‘ ‘-di-tert-butylstyryl)styryl)stilbenyl)methane (C(4R-2tBu)4) is prepared similarly using 1-(3‘,5‘-di-tert-butylstyryl)-4-(4‘-vinylstyryl)benzene and in better yield (80%). Alkoxy substituents can also be used to increase solubility. Tetrakis((4-(2‘,5‘-dioctyloxy-4‘-styryl)styryl)stilbenyl)methane, C(4R-(OC8H17)2)4, was prepared by treatment of C(C6H4I)4 with excess 2,5-dioctyloxy-1-styryl-4-(4‘-vinylstyryl)benzene (yield 73%). The simple stilbenyl-derivatives were found by DSC measurements and powder diffraction experiments to be crystalline compounds. Comparison of single-crystal X-ray diffraction data shows that C(STB)4 and Si(STB)4 form isomorphous crystals. The larger E(tBuSSB)4, C(DPVBi)4, and C(DPAB)4 compounds readily form amorphous glasses with elevated glass transition temperatures (Tg = 142−190 °C) in the absence of solvent. Extending the conjugation length of the arm leads to more stable glasses. For example, the glass transition temperature of C(4R-tBu)4 was measured at 230 °C. Solution phase optical spectroscopic data of E(tBuSSB)4 (E = C, adamantane, and Si) are characteristic of the parent distyrylbenzene chromophore. Films, however, show broad and significantly red-shifted emission spectra. In contrast, C(DPVBi)4 gives absorption and emission spectra which are nearly identical between dilute solution phase samples and neat solid films. The emission of C(DPAB)4 is broad and structureless, reminiscent of exciplex or excimer emission. Films of the tetramers with longer arms (C(4R-tBu)4, C(4R-2tBu)4, and C(4R-(OC8H17)2)4) show emission properties which are dependent on sample history. Annealing the sample at elevated temperature leads to red-shifted emission as a result of better interdigitation between the optically active fragments.
The molecular structures of low-molecular-weight organic compounds and their amorphous properties have been investigated to obtain a design rule for uniform amorphous films with high thermal stability. The glass transition temperature (T(g)/K), maximum crystal-growth velocity (MCV/m s-1), and maximum crystal-growth temperature (T(c,max)/K) are key parameters for characterizing the amorphous properties of organic materials. Some quantitative relations between these parameters and thermodynamic parameters were examined from both theoretical and experimental viewpoints. The equation for T(g) of various aromatic compounds expressed as T(g) = a - bSIGMADELTAS(tr,m)/N has been found to be nearly established, where SIGMADELTAS(tr,m) is the sum of the entropies of fusion and of phase transitions between T(g) and the melting point (T(m)/K), N is the number of heavy atoms per molecule except hydrogen atoms, and a and b are constants. The relation can be successfully explained by using the Adam-Gibss theory on the viscosity of supercooled liquids. It has also been found that MCV for aromatic compounds nearly followed the equation log (MCV) = c - dN/(T(m)SIGMADELTAH(tr,m)), where c and d are constants and SIGMADELTAH(tr,m) is the sum of the enthalpies of fusion and of phase transitions between T(c,max) and T(m). This can be explained by a potential barrier model for molecular diffusion both at a crystal/supercooled liquid interface and in a bulk supercooled liquid. Consequently, molecules preferably used for amorphous films should have a symmetric globular structure with a large molecular weight and small intermolecular cohesion. According to these findings, it has been revealed that high T(g) and T(c,max) and low MCV yield stable organic glasses with high thermal stability.
Hole mobilities have been measured in a series of donor compounds with different dipole moments. The results have been described by the disorder formalism, due to Bassler and co-workers. The formalism is based on the assumption that charge transport occurs by hopping through a Gaussian manifold of states with superimposed positional disorder. Key parameters of the model are the width of the density-of-hopping states and the degree of positional disorder. The width of the hopping site manifold increases with increasing dipole moment. This results in a sharp decrease in mobility with increasing dipole moment. The origin of this effect has been attributed to random internal fields associated with the dipole moments. The positional disorder has been attributed to packing constraints.
The organic semiconductor pentacene (1) has shown the highest field effect mobilities in thin films of any organic semiconductor, yet suffers from instability toward oxidation. 6,13-Bis(triisopropylsilylethynyl)pentacene (2) has been reported as an interesting functionalized pentacene which is soluble in common organic solvents and exhibits high carrier mobility (>0.1 cm2/Vs) in thin film transistor devices. In our investigations of 2, we were surprised by its remarkable stability in solution. Using UV−vis spectroscopy we observe that under ambient light conditions, 2 is approximately 50× more stable toward degradation in air-saturated tetrahydrofuran solution as compared to unsubstituted pentacene. Previous investigators have implicated oxygen in the mechanism of photodegradation of pentacene. In this study, quantum chemical calculations have been performed which demonstrate that alkynyl functionalization at the 6 and 13 positions reduces the rate of photooxidation in two ways. First, alkynyl substitution reduces the triplet energy of 2 considerably, thereby preventing singlet oxygen sensitization. Second, alkynyl substitution lowers the LUMO energy for 2 as compared to that of pentacene. We propose that the lower LUMO energy hinders photooxidation by reducing the rate of electron transfer from photoexcited 2 to oxygen. In thin films, pentacene is more stable to photooxidation than 2 when exposed to UV irradiation. The stabilization of pentacene in the solid state is discussed in the context of solid-state interactions.
We describe encapsulated passive matrix, video rate organic light-emitting diode (OLED) displays on flexible plastic substrates using a multilayer barrier encapsulation technology. The flexible OLED (FOLED(TM)) displays are based on highly efficient electrophosphorescent OLED (PHOLED(TM)) technology deposited on barrier coated plastic (Flexible Glass(TM) substrate) and are hermetically sealed with an optically transmissive multilayer barrier coating (Barix(TM) encapsulation). Preliminary lifetime to half initial luminance (L(0)similar to100 cd/m(2)) of order 200 h is achieved on the passive matrix driven encapsulated 80 dpi displays; 2500 h lifetime is achieved on a dc tested encapsulated 5 mm(2) FOLED test pixel. The encapsulated displays are flexed 1000 times around a 1 in. diameter cylinder and show minimal damage. (C) 2003 American Institute of Physics.
The first edition of Pope and Swenberg’s Electronic Processes of Organic Crystals, published in 1982, became the classic reference in the field. It provides a tutorial on the experimental and related theoretical properties of aromatic hydrocarbon crystals and includes emerging work on polymers and superconductivity. This new edition has been expanded to cover the major theoretical and experimental advances over the last fifteen years. It contains a unified description of what is known in almost every aspect of the field. The basic phenomena covered in the first edition included fluorescence, exciton and charge carrier generation, transport, recombination, and photoemission; the new edition adds solitons, polarons, bipolarons, spin waves, and charge density waves. It provides in-depth coverage of such model polymers such as polyacetylene, polydiacetylene, poly (phenylene-vinylene), polyanilines, polysilanes, and fullerenes. It also provides detailed treatments of the expanding areas of electroluminescence, non-linear optics, organic magnets, organic superconductors, and Langmuir-Blodgett films. In addition, it contains a chapter on major applications, including LED’s, photocopiers, photoconductors, batteries, transistors, liquid crystals, photorefractive devices, and sensors. As in the first volume, the authors take informed positions in controversial areas. This book will be an essential reference for organic material scientists, whether they are experienced researchers or just entering the field. It will also be a reliable guide to anyone interested in this rapidly growing field
Flexible organic light emitting diode (OLED) will be the ultimate display technology to customers and industries in the near future but the challenges are still being unveiled one by one. Thin-film encapsulation (TFE) technology is the most demanding requirement to prevent water and oxygen permeation into flexible OLED devices. As a polymer substrate does not offer the same barrier performance as glass, the TFE should be developed on both the bottom and top side of the device layers for sufficient lifetimes. This work provides a review of promising thin-film barrier technologies as well as the basic gas diffusion background. Topics include the significance of the device structure, permeation rate measurement, proposed permeation mechanism, and thin-film deposition technologies (Vitex system and atomic layer deposition (ALD)/molecular layer deposition (MLD)) for effective barrier films.
A system of a blue light-emitting host polymer, poly(9,9-dioctylfluorene) (PFO) and a red light-emitting molecular guest molecule, tetraphenylporphyrin (TPP), suitable for use in solution-processable light emitting diodes (LEDs) was developed. The large overlap between the emission from the PFO and the Soret band of the TPP resulted in very efficient Forster transfer. A large Forster transfer radius R0 = 5.4 nm was deduced. Between a TPP weight concentration of 1 and 10%, 95% of the photogenerated excitons were transferred from the PFO to TPP. Finally, in LED devices, external electroluminescence quantum efficiencies of 0.9% at a luminance of 90 cd m-2.
A study of the photo-oxidation of films of poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) blended with [6,6]-phenyl C-61-butyric acid methyl ester (PCBM), and solar cells based thereon, is presented. Solar-cell performance is degraded primarily through loss in short-circuit current density, J(SC). The effect of the same photodegradation treatment on the optical-absorption, charge-recombination, and charge-transport properties of the active layer is studied. It is concluded that the loss in J(SC) is primarily due to a reduction in charge-carrier mobility, owing to the creation of more deep traps in the polymer during photo-oxidation. Recombination is slowed down by the degradation and cannot therefore explain the loss in photocurrent. Optical absorption is reduced by photo-bleaching, but the size of this effect alone is insufficient to explain the loss in device photocurrent.
We have investigated the combination effect of host-guest materials in an emitting layer on the transient property of an organic light-emitting diode (OLED). We found that an efficient energy transfer owing to the large overlap between the photoluminescence spectrum of host material and the absorp-tion spectrum of guest material was important factor to improve the response speed of the OLED. As a result, the rise time of optical response was mainly affected by the combination of host-guest materials, and it increased using the optimal guest material, 1,4-bis[2-[4-N,N-di(p-tolyl)amino]phenyl]vinyl]benzene (DSB). A maximum -3 dB cutoff frequency of 15.8 MHz was achieved for an OLED with DSB as a guest material.
are known to be unstable under ambient conditions. The criticisms of the tech-nology focused mainly on the fact that OLEDs require low-work-function elec-trodes as the cathode. Many early skeptics did not believe that the OLED fabrication process could be manufacturable, because the entire process requires stringent envi-ronmental control to avoid exposure of the cathode to oxygen and moisture. Exposure of devices to ambient conditions would lead to cathode degradation and genera-tion of non-emissive areas—commonly known as dark spots in the OLED commu-nity today. Throughout the 1990s, numerous pub-lications [ 7 , 15–18 ] were devoted to the study of dark-spot growth and its growth mecha-nism. In general, dark spots occur if the devices are not suffi ciently protected from the environment, and their growth would accelerate when the devices are under electrical stress. Dark sports in general originate from areas with pre-existing particle defects on the substrate. Subsequent deposition of the cathode layer leads to pin-hole formation in the metal thin fi lm at the defect sites and results in diffusion of moisture through the pin holes, leading to localized delamination of the cathode and for-mation of non-emissive areas. These non-emissive areas often lead to the formation of bubbles or domelike [ 19 ] structures at the cathode interface. It has been reported that gas evolved because of the electrochemical processes at the cathode/organic inter-face causes the formation of these bubbles. Under continuous operation the delamination of the cathode at the defect sites would continue to grow and results in nonuniform emission from the devices. It should be noted that the dark-spot growth is mainly related to the degradation of the cathode, not the under-lying organic layers. In fact, one can peel off the cathode layer from a degraded device and redeposit a fresh cathode layer on the same organic surface, and the resulting device would still function. Since the late 1990s, many researchers learned how to encapsulate the devices; growth of dark spots can be easily con-trolled if the devices are properly protected. It is interesting that initially many were skeptical about OLED technology because of the reactive cathodes used. Now, encapsulation has become a common practice in the fi eld of OLEDs and dark spots no longer appear to be an issue. With improved encapsulations, corrosion of reactive metals no longer affects the lifetime of OLEDs, which is contrary to the initial concern. During the early development, dark-spot growth was an obvious problem, and much attention was paid to the Degradation in organic light-emitting diodes (OLEDs) is a complex problem. Depending upon the materials and the device architectures used, the degra-dation mechanism can be very different. In this Progress Report, using exam-ples in both small molecule and polymer OLEDs, the different degradation mechanisms in two types of devices are examined. Some of the extrinsic and intrinsic degradation mechanisms in OLEDs are reviewed, and recent work on degradation studies of both small-molecule and polymer OLEDs is pre-sented. For small-molecule OLEDs, the operational degradation of exemplary fl uorescent devices is dominated by chemical transformations in the vicinity of the recombination zone. The accumulation of degradation products results in coupled phenomena of luminance-effi ciency loss and operating-voltage rise. For polymer OLEDs, it is shown how the charge-transport and injection properties affect the device lifetime. Further, it is shown how the charge bal-ance is controlled by interlayers at the anode contact, and their effects on the device lifetime are discussed.
The influence of environmental factors on the degradation process of P3HT film has been investigated quantitatively. The decay kinetics of the polymer absorption during variation of intensity and spectral distribution of the incident light, oxygen concentration, humidity level as well as temperature are monitored using infrared and UV/vis absorption spectroscopy. Additionally, the oxygen diffusion into the polymer film has been investigated using fluorescence spectroscopy under the same experimental conditions. The degradation process is light initiated with a strong increase of the effectiveness toward the ultraviolet region of the spectrum. The observed photo oxidation is not oxygen diffusion limited although an activation energy of 26 kJmol(-1) was observed for both degradation and oxygen diffusion. The observed kinetics, especially its dependence on wavelength of the incident light, point to a radical-based degradation process in the solid state rather than a singlet oxygen-based mechanism as it is observed in the liquid phase. Furthermore the presence of humidity strongly affects the degradation process although water itself does not decompose the polymer. Changing the structure of the polymer from regioregular to regiorandom significantly accelerates the degradation, probably due to the higher triplet yield of the regiorandom polymer.
We report UV-light-induced photodegradation of N,N’-diphenyl-N,N’-bis(3-methylphenyl)-1,1’-biphenyl-4,4’ diamine (TPD) dye.
When a halomethane solution of TPD is exposed to UV light it first oxidizes TPD and subsequently with further exposure leads
to irreversible transformation of TPD to an acridine derivative as photoproduct. Exclusive use of halomethane solvents like
chloroform and dichloromethane suggests solvent radical initiated photochemical transformation of TPD to a stable acridine-based
photoproduct. The resultant photoproduct of TPD showed broad-band dual (both blue and green) emissions in various solvents
and also from the sublimed thin film. Based on solvent- and concentration-dependent photoluminescence (PL) of the photoproduct,
PL excitation and lifetimes of blue and green bands, the origin of emission in the green region is due to an intermolecular
excimer/aggregate of acridine-based photoproduct. A film of acridine-based photoproduct dispersed in polystyrene also showed
substantial improvement in electrical conductivity compared to the parent compound TPD. The technique of UV-light-induced
molecule structure transformation and material processing under specific environmental conditions may be considered as an
efficient means of obtaining materials with enhanced optoelectronic properties.
Highly sensitive alternate current (ac) impedance measurements with variable temperature have been performed to investigate the optical and electrical failure mechanisms during the glass transition phenomena in the archetypal ITO / TPD / Alq 3/ Al organic light emitting diode (OLED) structure. Since the device degradation is mainly related to the lower glass transition temperature (Tg) of the N,N′ -Bis(3-methylphenyl)- N,N′ -diphenylbenzidine (TPD), this study is focused on the frequency response of thin TPD films approaching the glassy region. The related experimental data are discussed in the framework of the universal dielectric response model. By ac measurements, TPD glass transition temperature is located and temperature regions with different OLED behaviors are evidenced. The relation between the behaviors of TPD frequency response and of the OLED electro-optical response, while the temperature approaches the glass transition region, is discussed.
We measure the current–voltage and electroluminescence characteristics of single‐heterojunction, vacuum‐deposited organic light‐emitting devices (OLEDs) over a wide range of materials, temperatures, and currents. We find that the current is limited by a large density of traps with an exponential energy distribution below the lowest unoccupied molecular orbital. The characteristic trap depth is 0.15 eV. Furthermore, in metal–quinolate‐based devices, electroluminescence originates from recombination of Frenkel excitons, and its temperature dependence is consistent with the excitons being formed by Coulombic relaxation of the trapped electrons with holes injected from the counter electrode. By semiempirical molecular orbital modeling, we find that the trap distribution obtained from the current–voltage characteristics is consistent with a distribution in the metal–quinolate molecular conformations which result in a continuous, exponential distribution of allowed states below the lowest unoccupied molecular orbital. We discuss the implications of the intrinsic relationship between electroluminescence and current transport in OLEDs for the optimization of efficiency and operating voltage in these devices. © 1996 American Institute of Physics.