Exciton and Polaron Quenching in Doping‐Free Phosphorescent Organic Light‐Emitting Diodes from a Pt(II)‐Based Fast Phosphor

By introducing a neat Pt(II)‐based phosphor with a remarkably short decay lifetime, a simplified doping‐free phosphorescent organic light‐emitting diode (OLED) with a forward viewing external quantum efficiency (EQE) and power efficiency of 20.3 ± 0.5% and 63.0 ± 0.4 lm W^?1, respectively, is demonstrated. A quantitative analysis of how triplet‐triplet annihilation (TTA) and triplet‐polaron annihilation (TPA) affect the device EQE roll‐off at high current densities is performed. The contributions from loss of charge balance associated with charge leakage and field‐induced exciton dissociation are found negligible. The rate constants k_TTA and k_TPA are determined by time‐resolved photoluminescence experiments of a thin film and an electrically‐driven unipolar device, respectively. Using the parameters extracted experimentally, the EQE is modeled versus electric current characteristics of the OLEDs by taking both TTA and TPA into account. Based on this model, the impacts of the emitter lifetime, quenching rate constants, and exciton formation zone upon device efficiency are analyzed. It is found that the short lifetime of the neat emitter is key for the reduction of triplet quenching.     

A Multifunctional Blue‐Emitting Material Designed via Tuning Distribution of Hybridized Excited‐State for High‐Performance Blue and Host‐Sensitized OLEDs

Actualizing full singlet exciton yield via a reverse intersystem crossing from the high‐lying triplet state to singlet state, namely, “hot exciton” mechanism, holds great potential for high‐performance fluorescent organic light‐emitting diodes (OLEDs). However, incorporating comprehensive insights into the mechanism and effective molecular design strategies still remains challenging. Herein, three blue emitters (CNNPI, 2TriPE‐CNNPI, and 2CzPh‐CNNPI) with a distinct local excited (LE) state and charge‐transfer (CT) state distributions in excited states are designed and synthesized. They show prominent hybridized local and charge‐transfer (HLCT) states and aggregation‐induced emission enhancement properties. The “hot exciton” mechanism based on these emitters reveals that a balanced LE/CT distribution can simultaneously boost photoluminescence efficiency and exciton utilization. In particular, a nearly 100% exciton utilization is achieved in the electroluminescence (EL) process of 2CzPh‐CNNPI. Moreover, employing 2CzPh‐CNNPI as the emitter, emissive dopant, and sensitizing host, respectively, the EL performances of the corresponding nondoped pure‐blue, doped deep‐blue, and HLCT‐sensitized fluorescent OLEDs are among the most efficient OLEDs with a “hot exciton” mechanism to date. These results could shed light on the design principles for “hot exciton” materials and inspire the development of next‐generation high‐performance OLEDs.     

Self‐Protective Room‐Temperature Phosphorescence of Fluorine and Nitrogen Codoped Carbon Dots

The unstable triplet excited state is a core problem when developing self‐protective room temperature phosphorescence (RTP) in carbon dots (CDs). Here, fluorine and nitrogen codoped carbon dots (FNCDs) with long‐lived triplet excited states, emitting pH‐stabilized blue fluorescence and pH‐responsive green self‐protective RTP, are reported for the first time. The self‐protective RTP of FNCDs arises from n–π * electron transitions for C? N/C?N bonds with a small energy gap between singlet and triplet states at room temperature. Moreover, the interdot/intradot hydrogen bonds and steric protection of C? F bonds reduce quenching of RTP by oxygen at room temperature. The RTP emission of FNCDs shows outstanding reversibility, while the blue fluorescence emission has good pH stability. Based on these FNCDs, a data encoding/reading strategy for advanced anticounterfeiting is proposed via time‐resolved luminescence imaging techniques, as well as steganography of complex patterns.     

Doping‐Free Organic Light‐Emitting Diodes with Very High Power Efficiency,Simple Device Structure,and Superior Spectral Performance

Today's state‐of‐the‐art phosphorescent organic light‐emitting diodes (PhOLEDs) must rely on the host‐guest doping technique to decrease triplet quenching and increase device efficiency. However, doping is a sophisticated device fabrication process. Here, a Pt(II)‐based complex with a near unity photoluminescence quantum yield and excellent electron transporting properties in the form of neat film is reported. Simplified doping‐free white PhOLED and yellow‐orange PhOLED based on this emitter achieve rather low operating voltages (2.2–2.4 V) and very high power efficiencies of approximately 80 lm W^?1 (yellow‐orange) and 50 lm W^?1 (white), respectively, without any light extraction enhancement. Furthermore, the efficient white device also exhibits high color stability. No color shift is observed during the entire operation of the device. Analysis of the device's operational mechanism has been postulated in terms of exciton and polaron formation and fate. It is found that using the efficient neat Pt(II)‐complex as a homogeneous emitting and electron transporting layer and an ambipolar blue emitter are determining factors for achieving such a high efficiency.     

High Efficiency Deep‐Blue Phosphorescent Organic Light‐Emitting Diodes with CIE x,y (≤ 0.15) and Low Efficiency Roll‐Off by Employing a High Triplet Energy Bipolar Host Material

Recently, bipolar host materials are the most promising candidates for achieving high performance phosphorescent organic light‐emitting diodes (PHOLEDs) in order to maximize recombination efficiency. However, the development of host material with high triplet energy (E _T) is still a great challenge to date to overcome the limitations associated with the present PHOLEDs. Herein, a highly efficient donor‐π‐acceptor (D‐π‐A) type bipolar host (4′‐(9H‐carbazol‐9‐yl)‐2,2′‐dimethyl‐[1,1′‐biphenyl]‐4‐yl)diphenylphosphine oxide (m‐CBPPO) comprising of carbazole, 2,2′‐dimethylbiphenyl and diphenylphosphoryl as D‐π‐A unit, respectively, is developed. Interestingly, a high E _T of 3.02 eV is observed for m‐CBPPO due to highly twisted conformation. Furthermore, the new host material is incorporated in PHOLEDs as emissive layer with a new carbene type Ir(cb)₃ material as a deep‐blue emitter. The optimized devices show an excellent external quantum efficiency (EQE) of 24.8% with a notable Commission internationale de l'éclairage (x, y) ≤ 0.15, (0.136, 0.138) and high electroluminescence performance with extremely low efficiency roll‐off. Overall, the above EQE is the highest reported for deep‐blue PHOLEDs with very low efficiency roll‐off and also indicate the importance of appropriate host for the development of high performance deep‐blue PHOLEDs.     

Mechanistic Study on High Efficiency Deep Blue AIE‐Based Organic Light‐Emitting Diodes by Magneto‐Electroluminescence

Aggregation‐induced emission (AIE) materials are highly attractive because of their excellent properties of high efficiency emission in nondoped organic light‐emitting diodes (OLEDs). Therefore, a deep understanding of the working mechanisms, further improving the electroluminescence (EL) efficiency of the resulting AIE‐based OLEDs, is necessary. Herein, the conversion process from higher energy triplet state (T₂) to the lowest singlet state (SS₁) is found in OLEDs based on a blue AIE material, 4′‐(4‐(diphenylamino)phenyl)‐5′‐phenyl‐[1,1′:2′,1′′‐terphenyl]‐4‐carbonitrile (TPB‐AC), obviously relating to the device efficiency, by magneto‐EL (MEL) measurements. A special line shape with rise at low field and reduction at high field is observed. The phenomenon is further clarified by theoretical calculations, temperature‐dependent MELs, and transient photoluminescence emission properties. On the basis of the T₂‐S₁ conversion process, the EL performances of the blue OLEDs based on TPB‐AC are further enhanced by introducing a phosphorescence doping emitter in the emitting layer, which effectively regulates the excitons on TPB‐AC molecules. The maximum external quantum efficiency (EQE) reaches 7.93% and the EQE keeps 7.57% at the luminance of 1000 cd m^?2. This work establishes a physical insight for designing high‐performance AIE materials and devices in the future.     

Enhancement of Phosphorescence by Surface‐Plasmon Resonances in Colloidal Metal Nanoparticles: The Role of Aggregates

The spectroscopic and near‐field scanning optical microscopy (NSOM) studies of phosphorescent films doped with colloidal gold nanoparticles (NPs) are presented. Films with a high concentration of 2,3,7,8,12,13,17,18‐octaethyl‐21H,23H‐porphine platinum(II ) dispersed in a neutral polymer poly[(methyl methacrylate)‐co‐(ethyl acrylate)] demonstrate a twofold increase of the phosphorescence quantum yield after the addition of aggregated NPs. In materials doped with unaggregated particles, a decrease of the emission yield is observed. Theoretical modeling of the phosphorescence transients suggests a minimization of the triplet–triplet quenching owing to the presence of fast processes that decrease the concentration of chromophores in the excited state and may be both of radiative and non‐radiative origin. NSOM examination of the films reveals increased light emission around large NP clusters. This observation demonstrates significant enhancement of the spontaneous emission rates by the large aggregates, although unaggregated NPs introduce mostly phosphorescence quenching sites.     

Correlation Between Triplet–Triplet Annihilation and Electroluminescence Efficiency in Doped Fluorescent Organic Light‐Emitting Devices

Triplet–triplet annihilation (TTA) is studied in a wide range of fluorescent host:guest emitter systems used in organic light‐emitting devices (OLEDs). Strong TTA is observed in host:guest systems in which the dopant has a limited charge‐trapping capability. On the other hand, systems in which the dopant can efficiently trap charges show insignificant TTA, an effect that is due, in part, to the efficient quenching of triplet excitons by the trapped charges. Fluorescent host:guest systems with the strongest TTA are found to give the highest OLED electroluminescence efficiency, a phenomenon attributed to the role of TTA in converting triplet excitons into additional singlet excitons, thus appreciably contributing to the light output of OLEDs. The results shed light on and give direct evidence for the phenomena behind the recently reported very high efficiencies attainable in fluorescent host:guest OLEDs with quantum efficiencies exceeding the classical 25% theoretical limit.     

Is Poly(vinylcarbazole) a Good Host for Blue Phosphorescent Dopants in PLEDs? Dimer Formation and Their Effects on the Triplet Energy Level of Poly(N‐vinylcarbazole) and Poly(N‐Ethyl‐2‐Vinylcarbazole)

The detailed measurement and analysis of the delayed emission from poly(vinylcarbazole) (PVK) and poly(N‐ethyl‐2‐vinyl‐carbazole) (P2VK) thin films is described. PVK has rapidly become a “polymer of choice” for hosting phosphorescent dopants in PLEDs, especially blue emitters. In this respect it is important to have a full understanding of the triplet properties of this host. It is concluded that in films, the electronic 0–0 peak energy of PVK phosphorescence is found at 2.88 eV (14 K). With an increase of temperature, >44 K, increasing emission from new long lived, lower energy species, previously ascribed to “trap states” in the literature, is observed. Increasing temperature enables thermally assisted triplet exciton hopping to these trap states. Critically it is shown that some of these triplet trap species are ground state triplet dimers in origin for both PVK (2.46 eV) and P2VK (2.1 eV), and not all of them are of excimer nature as previously thought. These species can quench the emission of blue heavy metal complexes doped in PVK and drastically effect performance over lifetime if the dimer formation increases over time and at elevated operating temperature. It is therefore concluded that PVK might not be such an ideal host material for blue phosphorescent emitters.     

Efficient Persistent Room Temperature Phosphorescence in Organic Amorphous Materials under Ambient Conditions

Persistent emission with a long lifetime (>1 s) from organic materials can only be observed at a low temperature, because of the significant nonradiative deactivation pathway that occurs at room‐temperature (RT). If organic materials with persistent RT emission in air could be developed, they could potentially be utilized for a variety of applications. Here, organic host‐guest materials with efficient persistent RT phosphorescence (RTP) are developed by minimizing the nonradiative deactivation pathway of triplet excitons. The nonradiative deactivation pathway is dependent on both nonradiative deactivation of the guest and quenching by diffusional motion of the host. The rigidity and oxygen barrier properties of the steroidal compound used as the host suppressed the quenching, and the aromatic hydrocarbon used as the guest is highly deuterated to minimize nonradiative deactivation of the guest. Red‐green‐blue persistent RTP with a lifetime >1 s and a quantum yield >10% in air is realized for a pure organic material.     

Confinement of Long‐Lived Triplet Excitons in Organic Semiconducting Host–Guest Systems

Long‐lived triplet excitons on organic molecules easily deactivate at room temperature because of the presence of thermally activated nonradiative pathways. This study demonstrates long‐lived phosphorescence at room temperature resulting from suppression of the nonradiative deactivation of triplet excitons in conventional organic semiconducting host–guest systems. The nonradiative deactivation pathway strongly depends on the triplet energy gap between the guest emitting molecules and the host matrices. The triplet energy gap required to confine the long‐lived triplet excitons (≈0.5 eV) is much larger than that of conventional host–guest systems for phosphorescent emitters. By effectively confining the triplet excitons, this study demonstrates long‐lived room‐temperature phosphorescence under optical and electrical excitation.     

A Multifunctional Iridium‐Carbazolyl Orange Phosphor for High‐Performance Two‐Element WOLED Exploiting Exciton‐Managed Fluorescence/Phosphorescence

By attaching a bulky, inductively electron‐withdrawing trifluoromethyl (CF₃) group on the pyridyl ring of the rigid 2‐[3‐ (N‐phenylcarbazolyl)]pyridine cyclometalated ligand, we successfully synthesized a new heteroleptic orange‐emitting phosphorescent iridium(III) complex [Ir( L ₁ )₂(acac)] 1 ( HL ₁ = 5‐trifluoromethyl‐2‐[3‐(N‐phenylcarbazolyl)]pyridine, Hacac = acetylacetone) in good yield. The structural and electronic properties of 1 were examined by X‐ray crystallography and time‐dependent DFT calculations. The influence of CF₃ substituents on the optical, electrochemical and electroluminescence (EL) properties of 1 were studied. We note that incorporation of the carbazolyl unit facilitates the hole‐transporting ability of the complex, and more importantly, attachment of CF₃ group provides an access to a highly efficient electrophosphor for the fabrication of orange phosphorescent organic light‐emitting diodes (OLEDs) with outstanding device performance. These orange OLEDs can produce a maximum current efficiency of ～40 cd A^?1, corresponding to an external quantum efficiency of ～12% ph/el (photons per electron) and a power efficiency of ～24 lm W^?1. Remarkably, high‐performance simple two‐element white OLEDs (WOLEDs) with excellent color stability can be fabricated using an orange triplet‐harvesting emitter 1 in conjunction with a blue singlet‐harvesting emitter. By using such a new system where the host singlet is resonant with the blue fluorophore singlet state and the host triplet is resonant with the orange phosphor triplet level, this white light‐emitting structure can achieve peak EL efficiencies of 26.6 cd A^?1 and 13.5 lm W^?1 that are generally superior to other two‐element all‐fluorophore or all‐phosphor OLED counterparts in terms of both color stability and emission efficiency.     

Tuning the Emitting Color of Organic Light‐Emitting Diodes Through Photochemically Induced Transformations: Towards Single‐Layer,Patterned, Full‐Color Displays and White‐Lighting Applications

Photochemically induced emission tuning for the definition of pixels emitting the three primary colors, red, green, blue (RGB), in a single conducting polymeric layer is investigated. The approach proposed is based on an acid‐induced emission shift of the (1‐[4‐(dimethylamino)phenyl]‐6‐phenylhexatriene) (DMA‐DPH) green emitter and acid‐induced quenching of the red fluorescent emitter (4‐dimethylamino‐4′‐nitrostilbene) (DANS). The two emitters are dispersed in the wide bandgap conducting polymer poly(9‐vinylcarbazole) (PVK), along with a photoacid generator (PAG). In the unexposed film areas, red emission is observed because of efficient energy transfer from PVK and DMA‐DPH to DANS. Exposure of selected areas of the film at different doses results in quenching of the red emitter's fluorescence and the formation of green, blue, or even other color‐emitting pixels, depending on the exposure dose and the relative concentrations of the different compounds in the film. Organic light‐emitting diodes having the PVK polymer containing the appropriate amounts of DMA‐DPH, DANS, and PAG as the emitting layer are fabricated and electroluminescence spectra are recorded. The time stability of induced emission spectrum changes and the color stability during device operation are also examined, and the first encouraging results are obtained.     

White Organic Light‐Emitting Diodes with Evenly Separated Red,Green, and Blue Colors for Efficiency/Color‐Rendition Trade‐Off Optimization

A novel yellowish‐green triplet emitter, bis(5‐(trifluoromethyl)‐2‐p‐tolylpyridine) (acetylacetonate)iridium(III) (1), was conveniently synthesized and used in the fabrication of both monochromatic and white organic light‐emitting diodes (WOLEDs). At the optimal doping concentration, monochromatic devices based on 1 exhibit a high efficiency of 63 cd A^?1 (16.3% and 36.6 lm W^?1) at a luminance of 100 cd m^?2. By combining 1 with a phosphorescent sky‐blue emitter, bis(3,5‐difluoro‐2‐(2‐pyridyl)phenyl)‐(2‐carboxypyridyl)iridium(III) (FIrPic), and a red emitter, bis(2‐benzo[b]thiophen‐2‐yl‐pyridine)(acetylacetonate)iridium(III) (Ir(btp)₂(acac)), the resulting electrophosphorescent WOLEDs show three evenly separated main peaks and give a high efficiency of 34.2 cd A^?1 (13.2% and 18.5 lm W^?1) at a luminance of 100 cd m^?2. When 1 is mixed with a deep‐blue fluorescent emitter, 4,4′‐bis(9‐ethyl‐3‐carbazovinylene)‐1,1′‐biphenyl (BCzVBi), and Ir(btp)₂(acac), the resulting hybrid WOLEDs demonstrate a high color‐rendering index of 91.2 and CIE coordinates of (0.32, 0.34). The efficient and highly color‐pure WOLEDs based on 1 with evenly separated red, green, blue peaks and a high color‐rendering index outperform those of the state‐of‐the‐art emitter, fac‐tris(2‐phenylpyridine)iridium(III) (Ir(ppy)₃), and are ideal candidates for display and lighting applications.     

Efficiency Roll‐Off in Blue Emitting Phosphorescent Organic Light Emitting Diodes with Carbazole Host Materials

The efficiency roll‐off in blue phosphorescent organic light emitting diodes (OLEDs) using different carbazole compounds as the host is systematically studied. While there is no significant difference in device efficiency, OLEDs using ter‐carbazole as the host show a reduction in efficiency roll‐off at high luminance. Data from transient photoluminescence and electroluminescence measurements show that the lower triplet–triplet annihilation (TTA) and triplet–polaron quenching (TPQ) rates in devices with the ter‐carbazole host compared with other carbazole hosts are the reasons for this reduced efficiency roll‐off. It is also found that the host materials with low glass transition temperatures are more susceptible to the efficiency roll‐off problem.     

Highly Efficient Simple‐Structure Blue and All‐Phosphor Warm‐White Phosphorescent Organic Light‐Emitting Diodes Enabled by Wide‐Bandgap Tetraarylsilane‐Based Functional Materials

Two host materials of {4‐[diphenyl(4‐pyridin‐3‐ylphenyl)silyl]phenyl}diphenylamine (p‐PySiTPA) and {4‐[[4‐(diphenylphosphoryl)phenyl](diphenyl)silyl]phenyl}diphenylamine (p‐POSiTPA), and an electron‐transporting material of [(diphenylsilanediyl)bis(4,1‐phenylene)]bis(diphenylphosphine) dioxide (SiDPO) are developed by incorporating appropriate charge transporting units into the tetraarylsilane skeleton. The host materials feature both high triplet energies (ca. 2.93 eV) and ambipolar charge transporting nature; the electron‐transporting material comprising diphenylphosphine oxide units and tetraphenylsilane skeleton exhibits a high triplet energy (3.21 eV) and a deep highest occupied molecular orbital (HOMO) level (‐6.47 eV). Using these tetraarylsilane‐based functional materials results in a high‐efficiency blue phosphorescent device with a three‐organic‐layer structure of 1,1‐bis[4‐[N,N‐di(p‐tolyl)‐amino]phenyl]cyclohexane (TAPC)/p‐POSiTPA: iridium(III) bis(4′,6′‐difluorophenylpyridinato)tetrakis(1‐pyrazolyl)borate (FIr6)/SiDPO that exhibits a forward‐viewing maximum external quantum efficiency (EQE) up to 22.2%. This is the first report of three‐organic‐layer FIr6‐based blue PhOLEDs with the forward‐viewing EQE over 20%, and the device performance is among the highest for FIr6‐based blue PhOLEDs even compared with the four or more than four organic‐layer devices. Furthermore, with the introduction of bis(2‐(9,9‐diethyl‐9H‐fluoren‐2‐yl)‐1‐phenyl‐1H‐benzoimidazol‐N,C³)iridium acetylacetonate [(fbi)₂Ir(acac)] as an orange emitter, an all‐phosphor warm‐white PhOLED achieves a peak power efficiency of 47.2 lm W^?1, which is close to the highest values ever reported for two‐color white PhOLEDs.     

A Novel Fluorene‐Triphenylamine Hybrid That is a Highly Efficient Host Material for Blue‐, Green‐, and Red‐Light‐Emitting Electrophosphorescent Devices

TFTPA (tris[4‐(9‐phenylfluoren‐9‐yl)phenyl]amine), a novel host material that contains a triphenylamine core and three 9‐phenyl‐9‐fluorenyl peripheries, was effectively synthesized through a Friedel‐Crafts‐type substitution reaction. Owing to the presence of its sterically bulky 9‐phenyl‐9‐fluorenyl groups, TFTPA exhibits a high glass transition temperature (186 °C) and is morphologically and electrochemically stable. In addition, as demonstrated from atomic force microscopy measurements, the aggregation of the triplet iridium dopant is significantly diminished in the TFTPA host, resulting in a highly efficient full‐color phosphorescence. The performance of TFTPA ‐based devices is far superior to those of the corresponding mCP‐ or CBP‐based devices, particularly in blue‐ and red‐emitting electrophosphorescent device systems. The efficiency of the FIrpic‐based blue‐emitting device reached 12 % (26 cd A^–1) and 18 lm W^–1 at a practical brightness of 100 cd m^–2; the Ir(piq)₂acac‐based red‐emitting device exhibited an extremely low turn‐on voltage (2.6 V) and a threefold enhancement in device efficiency (9.0 lm W^–1) relative to those of reference devices based on the CBP host material.     

Bright and Efficient,Non‐Doped,Phosphorescent Organic Red‐Light‐Emitting Diodes

Ir^(III) metal complexes with formula [(nazo)₂Ir(Fppz)] ( 1 ), [(nazo)₂Ir(Bppz)] ( 2 ), and [(nazo)₂Ir(Fptz)] ( 3 ) [(nazo)H = 4‐phenyl quinazoline, (Fppz)H = 3‐trifluoromethyl‐5‐(2‐pyridyl) pyrazole, (Bppz)H = 3‐t‐butyl‐5‐(2‐pyridyl) pyrazole, and (Fptz)H = 3‐trifluoromethyl‐5‐(2‐pyridyl) triazole] were synthesized, among which the exact configuration of 1 was confirmed using single‐crystal X‐ray diffraction analysis. These complexes exhibited bright red phosphorescence with relatively short lifetimes of 0.4–1.05 μs in both solution and the solid‐state at room temperature. Non‐doped organic light‐emitting diodes (OLEDs) were fabricated using complexes 1 and 2 in the absence of a host matrix. Saturated red electroluminescence was observed at λ_max = 626 nm (host‐emitter complex 1 ) and 652 nm (host‐emitter complex 2 ), which corresponds to coordinates (0.66,0.34) and (0.69,0.31), respectively, on the 1931 Commission Internationale de l'Eclairage (CIE) chromaticity diagram. The non‐doped devices employing complex 1 showed electroluminance as high as 5780 cd m^–2, an external quantum efficiency of 5.5 % at 8 V, and a current density of 20 mA cm^–2. The short phosphorescence lifetime of 1 in the solid state, coupled with its modest π–π stacking interactions, appear to be the determining factors for its unusual success as a non‐doped host‐emitter.     

Triplet Harvesting in Hybrid White Organic Light‐Emitting Diodes

White organic light‐emitting diodes (OLEDs) are highly efficient large‐area light sources that may play an important role in solving the global energy crisis, while also opening novel design possibilities in general lighting applications. Usually, highly efficient white OLEDs are designed by combining three phosphorescent emitters for the colors blue, green, and red. However, this procedure is not ideal as it is difficult to find sufficiently stable blue phosphorescent emitters. Here, a novel approach to meet the demanding power efficiency and device stability requirements is discussed: a triplet harvesting concept for hybrid white OLED, which combines a blue fluorophor with red and green phosphors and is capable of reaching an internal quantum efficiency of 100% if a suitable blue emitter with high‐lying triplet transition is used is introduced. Additionally, this concept paves the way towards an extremely simple white OLED design, using only a single emitter layer.     

The Use of Substituted Iridium Complexes in Doped Polymer Electrophosphorescent Devices: The Influence of Triplet Transfer and Other Factors on Enhancing Device Performance

The problem of phosphorescence quenching by the host polymer of a dopant in a polyfluorene‐based electrophosphorescent device has been extensively studied. This paper concentrates on reduction of the rate of triplet‐energy transfer from the dopant to the host by making inert t‐butyl substitutions to the ligands of the well‐understood fac‐trisphenylpyridine iridium phosphorescent dopant. These substitutions introduce steric bulk to the dopant that approximately halves the rate of energy transfer compared to the unsubstituted dopant, and a concomitant increase in device performance is observed. This is attributed to the strong distance dependence of the Dexter‐type energy transfer involved, the steric bulk of the t‐butyl groups effectively preventing the energy transfer from emissive dopant to the host. In addition, through the use of specific substitutions on either the pyridyl or phenyl ring, the pathway of the energy transfer has been identified as being through the pyridyl ring of the ligand. Employing this technique of steric prevention of the triplet‐energy transfer to the host reduces the need for development of hosts with a high triplet level for electrophosphorescent devices.