Evidence of thermal transport anisotropy in stable glasses of vapour deposited organic molecules

Vapour-deposited organic glasses are currently in use in many optoelectronic devices. Their operation temperature is limited by the glass transition temperature of the organic layers and thermal management strategies become increasingly important to improve the lifetime of the device. Here we report the unusual finding that molecular orientation heavily influences heat flow propagation in glassy films of small molecule organic semiconductors. The thermal conductivity of vapour-deposited thin-film semiconductor glasses is anisotropic and controlled by the deposition temperature. We compare our data with extensive molecular dynamics simulations to disentangle the role of density and molecular orientation on heat propagation. Simulations do support the view that thermal transport along the backbone of the organic molecule is strongly preferred with respect to the perpendicular direction. This is due to the anisotropy of the molecular interaction strength that limit the transport of atomic vibrations. This approach could be used in future developments to implement small molecule glassy films in thermoelectric or other organic electronic devices.Comment: main manuscript: 17 pages and 7 figures; supplementary material: 6 pages and 7 figure

Introducing MR‐TADF emitters into Light‐Emitting Electrochemical Cells for narrowband and efficient emission

The Umeå University authors wish to acknowledge generous financial support from the Swedish Research Council, the Swedish Energy Agency, Bertil och Britt Svenssons stiftelse för belysningsteknik, Länsstyrelsen Västerbotten, Kempestiftelserna, Olle Engkvists Stiftelse, Wenner-Gren Foundations, and the Wallenberg Initiative Materials Science for Sustainability, WISE. The St Andrews authors thank the Engineering and Physical Sciences Research Council (EP/R035164/1).Organic semiconductors that emit by the process of multi‐resonance thermally activated delayed fluorescence (MR‐TADF) can deliver narrowband and efficient electroluminescence while being processable from solvents and metal‐free. This renders them attractive for use as the emitter in sustainable light‐emitting electrochemical cells (LECs), but so far reports of narrowband and efficient MR‐TADF emission from LEC devices are absent. Here, this issue is addressed through careful and systematic material selection and device development. Specifically, the authors show that the detrimental aggregation tendency of an archetypal rigid and planar carbazole‐based MR‐TADF emitter can be inhibited by its dispersion into a compatible carbazole‐based blend host and an ionic‐liquid electrolyte, and it is further demonstrated that the tuning of this active material results in a desired balanced p‐ and n‐type electrochemical doping, a high solid‐state photoluminescence quantum yield of 91%, and singlet and triplet trapping on the MR‐TADF guest emitter. The introduction of this designed metal‐free active MR‐TADF material into a LEC, employing air‐stabile electrodes, results in bright blue electroluminescence of 500 cd m−2, which is delivered at a high external quantum efficiency of 3.8% and shows a narrow emission profile with a full‐width‐at‐half‐maximum of 31 nm.Publisher PDFPeer reviewe

Relaxation dynamics of glasses along a wide stability and temperature range

While lots of measurements describe the relaxation dynamics of the liquid state, experimental data of the glass dynamics at high temperatures are much scarcer. We use ultrafast scanning calorimetry to expand the timescales of the glass to much shorter values than previously achieved. Our data show that the relaxation time of glasses follows a super-Arrhenius behaviour in the high-Temperature regime above the conventional devitrification temperature heating at 10 K/min. The liquid and glass states can be described by a common VFT-like expression that solely depends on temperature and limiting fictive temperature. We apply this common description to nearly-isotropic glasses of indomethacin, toluene and to recent data on metallic glasses. We also show that the dynamics of indomethacin glasses obey density scaling laws originally derived for the liquid. This work provides a strong connection between the dynamics of the equilibrium supercooled liquid and non-equilibrium glassy states

High-performance organic light-emitting diodes comprising ultrastable glass layers

Organic light-emitting diodes with ultrastable glass emission layers show increased efficiency and device stability. Organic light-emitting diodes (OLEDs) are one of the key solid-state light sources for various applications including small and large displays, automotive lighting, solid-state lighting, and signage. For any given commercial application, OLEDs need to perform at their best, which is judged by their device efficiency and operational stability. We present OLEDs that comprise functional layers fabricated as ultrastable glasses, which represent the thermodynamically most favorable and, thus, stable molecular conformation achievable nowadays in disordered solids. For both external quantum efficiencies and LT lifetimes, OLEDs with four different phosphorescent emitters show >15% enhancements over their respective reference devices. The only difference to the latter is the growth condition used for ultrastable glass layers that is optimal at about 85% of the materials' glass transition temperature. These improvements are achieved through neither material refinements nor device architecture optimization, suggesting a general applicability of this concept to maximize the OLED performance, no matter which specific materials are used

Organic vapour-deposited stable glasses: from fundamental thermal properties to high-performance organic light-emitting diodes

La deposició física de vapor ha sorgit recentment com una ruta alternativa per preparar vidres d’un ampli ventall d'estabilitats, juntament amb altres característiques. Concretament, ha fet possible l’obtenció de vidres amb propietats que superen les dels vidres convencionals i que, d’altra manera, requeririen temps de desenes a diversos milers d'anys de refredament lent o envelliment. És per aquesta raó que aquests vidres s’anomenen vidres de gran estabilitat o bé vidres ultraestables. S’ha demostrat com, per a moltes molècules orgàniques i bones formadores de vidre, la temperatura de dipòsit juga una paper fonamental a l’hora de determinar les propietats del vidre, com són l’estabilitat tèrmica, la densitat o l’orientació molecular entre altres, donant així la possibilitat d’incrementar l'inherent inestabilitat del vidres. Els vidres dipositats a partir de la fase vapor ofereixen tant noves perspectives als fenomen de transició vítria com també aplicacions potencials dins de diversos processos tecnològics, com és el cas de l’electrònica orgànica. Aquest treball té per objectiu aprofundir en el coneixement dels vidres dipositats utilitzant molècules orgàniques semiconductores. Per això, fem servir dues tècniques basades en membranes suspeses—la nanocalorimetria quasi-adiabàtica i ultra-ràpida de rastreig i el mètode 3ω-Völklein—per caracteritzar diversos aspectes d'aquests vidres. En primer lloc, mostrem que les capes amorfes més estables s’obtenen quan són evaporades sobre un substrat al 85 % de temperatura de transició vítria () del material en qüestió. Seguidament, mostrem com aquestes capes dipositades es transformen en vidre sota-refredat en forma d’un front de creixement que es propaga des de les regions altament mòbils (superfície i interfícies). Les característiques d'aquest mecanisme s’investiguen i es discuteixen respecte a les diferents propietats del vidre preparat. En tercer lloc, demostrem com aquesta transformació heterogènia es pot suprimir de manera eficaç quan la interfície amb la mobilitat més alta és bloquejada per una capa amb mobilitat més baixa, obtenint d’aquesta manera accés a la transformació homogènia en tot el volum. A més a més, veiem com l’estabilitat cinètica d’aquestes capes tapades millora quan utilitzem aquesta estratègia. Després de caracteritzar la transició vítria, també mesurem la conductivitat tèrmica d'aquestes capes. Observem com la conductivitat tèrmica en la direcció del pla canvia en funció de la temperatura de dipòsit, un comportament que atribuïm a variacions en l’orientació molecular. Finalment, presentem un senzill díode orgànic d’emissió de llum (OLED) fosforescent consistent tan sols de dues capes orgàniques, per comprovar la influència que la temperatura de dipòsit té en el rendiment del dispositiu. Demostrem com l’eficiència i temps de vida útil del dispositiu milloren quan les seves capes funcionals s’evaporen a . Aquests resultats s’aconsegueixen considerant només la temperatura de transició vítria i, per tant, en principi es poden generalitzar a qualsevol dispositiu OLED. Aquest treball contribueix al coneixement actual dels vidres dipositats a partir de la fase vapor aportant tant noves perspectives sobre les seves propietats tèrmiques i mecanismes de devitrificació com un exemple exitós sobre l’aplicació en els dispositius d'OLED moderns.Physical vapour deposition has recently emerged as an alternative route to prepare glasses that span a broad range of stabilities, together with other features. Particularly, it is possible to achieve glasses with properties that outperform conventional glasses, and that would otherwise require times from tenths to several thousands of years of slowly-cooling or ageing. For this reason, these glasses are referred as highly stable glasses or ultrastable glasses. In particular, it has been shown that for many molecular organic glass-formers, the deposition temperature plays a crucial role in determining glass properties, such as thermal stability, density or molecular orientation among others, giving the possibility to enhance the inherent instability of glasses. Vapour-deposited glasses offer new insights into the glass transition phenomenon but also potential applications in many technological processes such as in organic electronics. This work is committed to further deepen the knowledge on vapour-deposited glasses using organic semiconductor materials. We use two silicon nitride membrane-based techniques---fast-scanning quasi-adiabatic nanocalorimetry and the 3ω-Völklein method---to characterise several facets of these glasses. Firstly, we show that the most stable amorphous films are obtained when evaporated at 85 % of its corresponding glass transition temperature (). Secondly, we show how vapour-deposited films transform into the supercooled liquid via a propagating growth front that starts at the highly-mobile regions (surface and interfaces). The characteristics of this mechanism are examined and rationalised regarding the different glass properties. Thirdly, we demonstrate how this heterogeneous transformation can be effectively suppressed when the high-mobility interface is capped with a lower mobility layer, gaining access to the bulk transformation. We see how the kinetic stability of the capped layers is improved using this strategy. After characterising the glass transition, we look at the thermal conductivity of these glasses. We observe how the in-plane thermal conductivity changes with the deposition temperature and we attribute this behaviour to variations in the molecular alignment. Finally, we present a simple phosphorescent organic light-emitting diode device (OLED), consisting only of two organic layers, to check the influence of the deposition temperature on the device performance. We demonstrate how its efficiency and lifetime are enhanced when its functional layers are evaporated at 0.85. These results are achieved considering only the glass transition temperature and, therefore, they could be generalised to any OLED device. This work contributes to the existing knowledge of vapour-deposited glasses by providing new insights into their thermal properties and devitrification mechanisms and by exploring their potential application in the state-of-the-art OLED devices

Organic vapour-deposited stable glasses : from fundamental thermal properties to high-performance organic light-emitting diodes /

Bibliografia.La deposició física de vapor ha sorgit recentment com una ruta alternativa per preparar vidres d'un ampli ventall d'estabilitats, juntament amb altres característiques. Concretament, ha fet possible l'obtenció de vidres amb propietats que superen les dels vidres convencionals i que, d'altra manera, requeririen temps de desenes a diversos milers d'anys de refredament lent o envelliment. És per aquesta raó que aquests vidres s'anomenen vidres de gran estabilitat o bé vidres ultraestables. S'ha demostrat com, per a moltes molècules orgàniques i bones formadores de vidre, la temperatura de dipòsit juga una paper fonamental a l'hora de determinar les propietats del vidre, com són l'estabilitat tèrmica, la densitat o l'orientació molecular entre altres, donant així la possibilitat d'incrementar l'inherent inestabilitat del vidres. Els vidres dipositats a partir de la fase vapor ofereixen tant noves perspectives als fenomen de transició vítria com també aplicacions potencials dins de diversos processos tecnològics, com és el cas de l'electrònica orgànica. Aquest treball té per objectiu aprofundir en el coneixement dels vidres dipositats utilitzant molècules orgàniques semiconductores. Per això, fem servir dues tècniques basades en membranes suspeses-la nanocalorimetria quasi-adiabàtica i ultra-ràpida de rastreig i el mètode 3ω-Völklein-per caracteritzar diversos aspectes d'aquests vidres. En primer lloc, mostrem que les capes amorfes més estables s'obtenen quan són evaporades sobre un substrat al 85 % de temperatura de transició vítria () del material en qüestió. Seguidament, mostrem com aquestes capes dipositades es transformen en vidre sota-refredat en forma d'un front de creixement que es propaga des de les regions altament mòbils (superfície i interfícies). Les característiques d'aquest mecanisme s'investiguen i es discuteixen respecte a les diferents propietats del vidre preparat. En tercer lloc, demostrem com aquesta transformació heterogènia es pot suprimir de manera eficaç quan la interfície amb la mobilitat més alta és bloquejada per una capa amb mobilitat més baixa, obtenint d'aquesta manera accés a la transformació homogènia en tot el volum. A més a més, veiem com l'estabilitat cinètica d'aquestes capes tapades millora quan utilitzem aquesta estratègia. Després de caracteritzar la transició vítria, també mesurem la conductivitat tèrmica d'aquestes capes. Observem com la conductivitat tèrmica en la direcció del pla canvia en funció de la temperatura de dipòsit, un comportament que atribuïm a variacions en l'orientació molecular. Finalment, presentem un senzill díode orgànic d'emissió de llum (OLED) fosforescent consistent tan sols de dues capes orgàniques, per comprovar la influència que la temperatura de dipòsit té en el rendiment del dispositiu. Demostrem com l'eficiència i temps de vida útil del dispositiu milloren quan les seves capes funcionals s'evaporen a . Aquests resultats s'aconsegueixen considerant només la temperatura de transició vítria i, per tant, en principi es poden generalitzar a qualsevol dispositiu OLED. Aquest treball contribueix al coneixement actual dels vidres dipositats a partir de la fase vapor aportant tant noves perspectives sobre les seves propietats tèrmiques i mecanismes de devitrificació com un exemple exitós sobre l'aplicació en els dispositius d'OLED moderns.Physical vapour deposition has recently emerged as an alternative route to prepare glasses that span a broad range of stabilities, together with other features. Particularly, it is possible to achieve glasses with properties that outperform conventional glasses, and that would otherwise require times from tenths to several thousands of years of slowly-cooling or ageing. For this reason, these glasses are referred as highly stable glasses or ultrastable glasses. In particular, it has been shown that for many molecular organic glass-formers, the deposition temperature plays a crucial role in determining glass properties, such as thermal stability, density or molecular orientation among others, giving the possibility to enhance the inherent instability of glasses. Vapour-deposited glasses offer new insights into the glass transition phenomenon but also potential applications in many technological processes such as in organic electronics. This work is committed to further deepen the knowledge on vapour-deposited glasses using organic semiconductor materials. We use two silicon nitride membrane-based techniques---fast-scanning quasi-adiabatic nanocalorimetry and the 3ω-Völklein method---to characterise several facets of these glasses. Firstly, we show that the most stable amorphous films are obtained when evaporated at 85 % of its corresponding glass transition temperature (). Secondly, we show how vapour-deposited films transform into the supercooled liquid via a propagating growth front that starts at the highly-mobile regions (surface and interfaces). The characteristics of this mechanism are examined and rationalised regarding the different glass properties. Thirdly, we demonstrate how this heterogeneous transformation can be effectively suppressed when the high-mobility interface is capped with a lower mobility layer, gaining access to the bulk transformation. We see how the kinetic stability of the capped layers is improved using this strategy. After characterising the glass transition, we look at the thermal conductivity of these glasses. We observe how the in-plane thermal conductivity changes with the deposition temperature and we attribute this behaviour to variations in the molecular alignment. Finally, we present a simple phosphorescent organic light-emitting diode device (OLED), consisting only of two organic layers, to check the influence of the deposition temperature on the device performance. We demonstrate how its efficiency and lifetime are enhanced when its functional layers are evaporated at 0.85. These results are achieved considering only the glass transition temperature and, therefore, they could be generalised to any OLED device. This work contributes to the existing knowledge of vapour-deposited glasses by providing new insights into their thermal properties and devitrification mechanisms and by exploring their potential application in the state-of-the-art OLED devices

An Amorphous Spirobifluorene-Phosphine-Oxide Compound as the Balanced n-Type Host in Bright and Efficient Light-Emitting Electrochemical Cells with Improved Stability

A rational host–guest concept design for the attainment of high efficiency at strong luminance from light‐emitting electrochemical cells (LECs) by suppression of exciton‐polaron quenching [Tang et al., Nature Communications 2017, 8, 1190] has been reported. However, a practical drawback with the presented host–guest LEC devices was that the operational stability is insufficient for many applications. Here, a systematic study is performed, revealing that a major culprit for the limited operational stability is that the employed n‐type host, 1,3‐bis[2‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazo‐5‐yl]benzene (OXD‐7), has a strong propensity for crystallization and that this crystallization results in a detrimental phase separation of the constituents in the active material during device operation. The authors, therefore, identify an alternative class of concept‐functional n‐type hosts in the form of spirobifluorene‐phosphine‐oxide compounds, and report that the replacement of OXD‐7 with amorphous 2,7‐bis(diphenylphosphoryl)‐9,9′‐spirobifluorene results in a much improved operational lifetime of 700 h at >100 cd m−2 during constant‐bias driving at an essentially retained high current efficacy of 37.9 cd A−1 and a strong luminance of 2940 cd m−2

In situ determination of the orientation of the emissive dipoles in light-emitting electrochemical cells

The orientation of the emissive dipoles in thin-film devices is important since it strongly affects the light outcoupling and thereby the device emission efficiency. The light-emitting electrochemical cell (LEC) is particularly interesting in this context because its emissive dipoles are located in a high electric-field p-n junction, which is formed in situ by redistribution of bulky ions. This implies that the dipole orientation could be distinctly different in the driven LEC compared to the pristine device. This study develops the destructive-interference microcavity method for the accurate in situ determination of the orientation of the emissive dipoles during LEC operation and apply it on a common LEC device comprising an amorphous conjugated polymer termed Super Yellow as the emitter. It is found that ≈95% of the emissive dipoles are oriented in the horizontal direction with respect to the thin-film plane in both the pristine LEC and during steady-state light emission. This finding is attractive since it enables for efficient outcoupling of the generated photons, and interesting because it shows that a horizontal orientation of the emissive dipoles can remain despite the existence of a strong perpendicular electric field and the nearby motion of bulky ions during LEC operation

Relaxation dynamics of glasses along a wide stability and temperature range

While lots of measurements describe the relaxation dynamics of the liquid state, experimental data of the glass dynamics at high temperatures are much scarcer. We use ultrafast scanning calorimetry to expand the timescales of the glass to much shorter values than previously achieved. Our data show that the relaxation time of glasses follows a super-Arrhenius behaviour in the high-Temperature regime above the conventional devitrification temperature heating at 10 K/min. The liquid and glass states can be described by a common VFT-like expression that solely depends on temperature and limiting fictive temperature. We apply this common description to nearly-isotropic glasses of indomethacin, toluene and to recent data on metallic glasses. We also show that the dynamics of indomethacin glasses obey density scaling laws originally derived for the liquid. This work provides a strong connection between the dynamics of the equilibrium supercooled liquid and non-equilibrium glassy states

A metal-free and transparent light-emitting device by sequential spray-coating fabrication of all layers including PEDOT:PSS for both electrodes

The concept of a metal-free and all-organic electroluminescent device is appealing from both sustainability and cost perspectives. Herein, we report the design and fabrication of such a light-emitting electrochemical cell (LEC), comprising a blend of an emissive semiconducting polymer and an ionic liquid as the active material sandwiched between two poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) conducting-polymer electrodes. In the off-state, this all-organic LEC is highly transparent, and in the on-state, it delivers uniform and fast to turn-on bright surface emission. It is notable that all three device layers were fabricated by material- and cost-efficient spray-coating under ambient air. For the electrodes, we systematically investigated and developed a large number of PEDOT:PSS formulations. We call particular attention to one such p-type doped PEDOT:PSS formulation that was demonstrated to function as the negative cathode, as well as future attempts towards all-organic LECs to carefully consider the effects of electrochemical doping of the electrode in order to achieve optimum device performance