Carrier Recombination Zone Research Articles

Correction: Corrigendum: Promising operational stability of high-efficiency organic light-emitting diodes based on thermally activated delayed fluorescence

Organic light-emitting diodes (OLEDs) are attractive for next-generation displays and lighting applications because of their potential for high electroluminescence (EL) efficiency, flexibility and low-cost manufacture. Although phosphorescent emitters containing rare metals such as iridium or platinum produce devices with high EL efficiency, these metals are expensive and their blue emission remains unreliable for practical applications. Recently, a new route to high EL efficiency using materials that emit through thermally activated delayed fluorescence (TADF) was demonstrated. However, it is unclear whether devices that emit through TADF, which originates from the contributions of triplet excitons, are reliable. Here we demonstrate highly efficient, stable OLEDs that emit via TADF by controlling the position of the carrier recombination zone, resulting in projected lifetimes comparable to those of tris(2-phenylpyridinato)iridium(III)-based reference OLEDs. Our results indicate that TADF is intrinsically stable under electrical excitation and optimization of the surrounding materials will enhance device reliability.

Promising operational stability of high-efficiency organic light-emitting diodes based on thermally activated delayed fluorescence

Organic light-emitting diodes (OLEDs) are attractive for next-generation displays and lighting applications because of their potential for high electroluminescence (EL) efficiency, flexibility and low-cost manufacture. Although phosphorescent emitters containing rare metals such as iridium or platinum produce devices with high EL efficiency, these metals are expensive and their blue emission remains unreliable for practical applications. Recently, a new route to high EL efficiency using materials that emit through thermally activated delayed fluorescence (TADF) was demonstrated. However, it is unclear whether devices that emit through TADF, which originates from the contributions of triplet excitons, are reliable. Here we demonstrate highly efficient, stable OLEDs that emit via TADF by controlling the position of the carrier recombination zone, resulting in projected lifetimes comparable to those of tris(2-phenylpyridinato)iridium(III)-based reference OLEDs. Our results indicate that TADF is intrinsically stable under electrical excitation and optimization of the surrounding materials will enhance device reliability.

UV light-emitting electrochemical cells based on an ionic 2,2′-bifluorene derivative

UV light-emitting electrochemical cells (LECs) were, for the first time, achieved by the ionic 2,2′-bifluorene derivative, 1, which was synthesized through covalent tethering of methylimidazolium moieties as pendent groups. LEC devices incorporating ionic bifluorene 1 without (Device I) and with (Device III) the presence of poly(methyl methacrylate) (PMMA) exhibited UV EL emissions centered at 388 and 386nm with maximum external quantum efficiencies and power efficiencies of 1.06% and 7.44mWW−1 for Device III and 0.15% and 1.06mWW−1 for Device I, respectively. Transmission electron microscopy (TEM) images showed that 1 tends to form nanospheres due to amphiphilic nature. The presence of PMMA unified the size of nanospheres which greatly reduced the void area in films, suppressing the current leakage and enhancing the device efficiency. Furthermore, thicker thickness of the emissive layer of LECs increases the distance between carrier recombination zone and electrodes to avoid exciton quenching. Thus, a sevenfold increase in device efficiency was obtained in thicker UV LECs containing PMMA (Device III) as compared to thinner UV LECs based on neat films of 1 (Device I). The EL emissions in the UV region are successfully achieved by LECs based on 1, which are so far the shortest emission wavelength achieved in LECs.

Tailoring carrier injection efficiency to improve the carrier balance of solid-state light-emitting electrochemical cells

We study the influence of the carrier injection efficiency on the performance of light-emitting electrochemical cells (LECs) based on a hole-preferred transporting cationic transition metal complex (CTMC) [Ir(dfppz)(2)(dtb-bpy)](+)(PF(6)(-)) (complex 1) and an electron-preferred transporting CTMC [Ir(ppy)(2)(dasb)](+)(PF(6)(-)) (complex 2) (where dfppz is 1-(2,4-difluorophenyl) pyrazole, dtb-bpy is 4,4'-di(tert-butyl)-2,2'-bipyridine, ppy is 2-phenylpyridine and dasb is 4,5-diaza-9,9'-spirobifluorene). Experimental results show that even with electrochemically doped layers, the ohmic contacts for carrier injection could be formed only when the carrier injection barriers were relatively low. Thus, adding carrier injection layers in LECs with relatively high carrier injection barriers would affect carrier balance and thus would result in altered device efficiency. Comparison of the device characteristics of LECs based on complex 1 and 2 in various device structures suggests that the carrier injection efficiency of CTMC-based LECs should be modified according to the carrier transporting characteristics of CTMCs to optimize device efficiency. Hole-preferred transporting CTMCs should be combined with an LEC structure with a relatively high electron injection efficiency, while a relatively high hole injection efficiency would be required for LECs based on electron-preferred transporting CTMCs. Since the tailored carrier injection efficiency compensates for the unbalanced carrier transporting properties of the emissive layer, the carrier recombination zone would be located near the center of the emissive layer and exciton quenching near the electrodes would be significantly mitigated, rendering an improved device efficiency approaching the upper limit expected from the photoluminescence quantum yield of the emissive layer and the optical outcoupling efficiency from a typical layered light-emitting device structure.

间隔层对双发光层白色有机电致发光器件性能的影响

White organic light-emitting devices(WOLEDs) with double light-emitting layers(EMLs) were fabricated,which were based on phosphorescent blue bis[3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl) iridium Ⅲ](FIrpic) and yellow bis[2-(4-tertbutylphenyl)benzothiazolato-N,C2′] iridium(acetylacetonate) [(t-bt)2Ir(acac)].Device structure was given as:ITO/TAPC(30 nm)/host:(t-bt)2Ir(acac)[(10-x) nm,4%]/spacer(x nm)/host:FIrpic(15 nm,8%)/Bphen(40 nm)/Mg∶Ag(200 nm),while p-type 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane(TAPC) and n-type tris[3-(3-pyridyl)-mesityl]borane(3TPYMB) was separately employed as the host,and device optimization was made by introducing a spacer between the two EMLs.The results showed that,compared with that of spacer-without device,the devices employed a spacer yielded higher device performance as well as stable white emission.For TAPC as the host,a device with spacer obtained maximum luminance of 19 550 cd/m2 and power efficiency of 8.3 cd/A,while 3TPYMB as the host,a device with spacer achieved maximum luminance of 1 950 cd/m2 and power efficiency of 30.7 cd/A.It was suggested that by incorporating a spacer in the WOLEDs,charge carrier recombination zone was broadened,and electron-hole balance was improved in the EMLs.

Enhanced efficiency in mixed host red electrophosphorescence devices

Enhanced efficiency of red phosphorescent organic light-emitting devices is observed by using a bis[2-(2′-benzothienyl)pyridinato-N,C3′] iridium(acetylacetonato) doped 4,4′-N,N′-dicarbazole-biphenyl (CBP) and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI) mixed host emitting layer. The CBP:TPBI mixed host device shows a maximum external quantum efficiency of 9.1%, which is dramatically improved compared to that of the CBP (6.6%) and TPBI (5.4%) single host devices. Such a mixed host strategy can also be exploited in red phosphor dibenzo[f,h]quinoxaline iridium (acetylacetonate) doped devices. Investigations reveal that the position of charge carrier recombination zone of the mixed host devices predominantly locates in the electron blocking layer/emitting layer interface. The efficiency enhancement is attributed to the optimized hole and electron injection balance and hence increased charge carrier recombination rate in the emitting layer.

Dual-functional conjugated polymers based on trifluoren-2-yl-amine for RGB organic light-emitting diodes

A series of blue-light-emitting conjugated polymers that possess high-lying HOMO energy levels were synthesized by introducing trifluoren-2-yl-amine (TFA) as a building block. The emission color could be effectively tuned in the region of deep-blue and light-blue by introducing various substituents onto the TFA as the pendants. Deep-blue light emission was achieved for the device based on P4FNCz as an emitting layer without using any hole-transport layer (HTL), giving a maximum current efficiency (CE) of 2.44 cd A−1, corresponding to an external quantum efficiency of 3.00%, with Commission Internationale de L'Eclairage (CIE) coordinates of (0.16, 0.12). Thanks to the improved hole injection and transport ability, red- and green-light-emitting devices based on P4FNCz as an HTL were also successfully fabricated, giving a maximum CE of 7 and 25 cd A−1 with CIE coordinates of (0.64, 0.34) and (0.29, 0.64), respectively, which are comparable to those of the devices based on a general HTL of 4,4′-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (NPB). The CIE coordinates of the RGB devices are very close to those of the National Television System Committee (NTSC) and High Definition Television (HDTV) standard colors. The RGB devices achieved by tuning the carrier recombination zone with different electron-transport layers will facilitate the development of the polymer-based full-color flat-panel displays in an alternative process.

High-Efficiency Nondoped Blue Organic Light-Emitting Devices with Reduced Efficiency Roll-Off

Nondoped blue organic light-emitting devices with a structure of ITO/hole transporting layer (HTL)/2-diphenylamino-7-(2,2′′-diphenylvinyl)-9,9′- spirobifluorene/electronic transporting layer (ETL)/LiF/Al are fabricated. The performances of the devices are dependent on the charge mobility, charge injection, and energy level characteristics of HTL and ETL. The device with 4,4′,4′′-tris(3-methylphenylphenylamino)triphenylamine and 4,7-diphenyl-1,10- phenanthroline as HTL and ETL shows high efficiency and brightness at low voltage (5.8 cd/A and 1000 cd/m2 at 4.6 V). Furthermore, the device shows a slight efficiency roll-off of 16% from the brightness at maximum current efficiency to 10 000 cd/m2. We attributed this to the formation of a broader carrier recombination zone and relative charge-balancing in the device.

Tuning of threshold voltage by interfacial carrier doping in organic single crystal ambipolar light-emitting transistors and their bright electroluminescence

Organic light-emitting field-effect transistors, based on a p-bis[(p-styryl)styryl] benzene (P5V4) single crystal, that possess high mobilities of over 0.1 cm2/V s for both electrons and holes were fabricated. For a small charge injection barrier and successive formation of high exciton density in the carrier recombination zone, the hole accumulation threshold voltage was significantly reduced by interfacial hole doping based on electron transfer from P5V4 molecules to a molybdenum oxide layer. The threshold voltage for hole accumulation was drastically decreased from −80±3 to 2±3 V, leading to dual charge injection and accumulation of a very high current density of J>100 A cm−2 with intense edge electroluminescence.

P‐151: Low‐Voltage, High‐Efficiency Blue Phosphorescent OLED Using Double Emission Layers

AbstractEmploying double emission layers (DELs) into blue phosphorescent organic light‐emitting devices (PHOLED), the device shows improved current efficiency by a factor of 1.8 compared to that of device using single emission layer (SEL). Furthermore, by doping additional carrier‐transporting material into DELs, the device shows a slight efficiency roll‐off of 24% from low (1 mA/cm2 and 400 cd/m2) to high current density (40 mA/cm2 and 10000 cd/m2). The dramatic improvement in device performances can be attributed to the formation of a broader carrier recombination zone and the elimination of carrier accumulation at interface. A blue PHOLED with a current efficiency of 29.5 cd/A, a power efficiency of 21 lm/W and a low driving voltage of 4.4 V with a CIEx,y of (0.16, 0.35) at a practical brightness of 1000 cd/m2 can be achieved.

Controlling the carrier recombination zone for improved color stability in a two-dopant fluorophore/phosphor white organic light-emitting diode

A white organic light-emitting diode (WOLED) is produced upon systematic introduction of deep-blue fluorescence from 4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-biphenyl to broad-band yellow phosphorescence from bis[3,5-bis(2-pyridyl)-1,2,4-triazolato]platinum(II) in the common host 4,4′-bis(carbazol-9-yl)biphenyl. The carrier recombination zone is first identified upon investigating alternative device structures, then optimized by varying the thickness of the electron-transport layer. The WOLED exhibits striking stability of color and efficiency, as manifest by parameters at high brightness of 1000 cd/m2 sustaining 94%–122% their values at 50 cd/m2.

Efficient Deep-Blue Organic Light-Emitting Diodes Based on 9,9-Bis(4-biphenylyl)fluorene Derivatives

We fabricated highly efficient deep-blue organic light-emitting diodes (OLEDs) composed of 2,7-bis(4′-diphenylamino-1,1′-biphenyl-4-yl)-9,9-bis(1,1′-biphenyl-4-yl)-9H-fluorene (FLBD) as an emitter doped into a wider energy gap host of 2,7-bis(1,1′-biphenyl-4-yl)-9,9-bis(1,1′-biphenyl-4-yl)-9H-fluorene (FLBH). We investigated the dominant factors for external electroluminescence quantum efficiency (ηext) in the deep-blue OLEDs by focusing on the carrier balance in the emitting layer (EML), the location and width of the carrier recombination zone, and carrier-induced exciton quenching. The ηext value of the OLEDs strongly depended on the hole injection efficiency from a hole transport layer (HTL) to the EML. By controlling the hole injection barrier into the EML and balancing the number of electrons and holes in the EML using HTLs having various highest occupied molecular orbital levels, we obtained the highest ηext of 5.4% with deep-blue Commission International De L’Eclairage coordinates of x = 0.15 and y = 0.09. Additionally, we found that the width of the carrier recombination zone tightly correlates to the hole injection efficiency into the EML.

Suppression of efficiency roll off in blue phosphorescent organic light-emitting devices using double emission layers with additional carrier-transporting material

By employing double emission layers (DELs) into blue phosphorescent organic light-emitting device (PHOLED), the device shows improved current efficiency by a factor of 1.8 as compared with that of device using single emission layer. Furthermore, by doping additional carrier-transporting material into DELs, the device shows only a slight efficiency roll off of 24% from low (1 mA/cm2 and 400 cd/m2) to high current density (40 mA/cm2 and 10 000 cd/m2). The dramatic improvement in device performances can be attributed to the formation of a broader carrier recombination zone and the elimination of carrier accumulation at the interface. A blue PHOLED with a current efficiency of 29.5 cd/A, a power efficiency of 21 lm/W, and a low driving voltage of 4.4 V with a Commission Internationale deL’Eclairage (CIEx,y) of (0.16, 0.35) at a practical brightness of 1000 cd/m2 can be achieved.

Organic Light Emitting Diodes Using Doped Alq3 as the Hole-transport Layer

Effects of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) doping on the hole conductivity of Alq3layer are measured. In the hole-only device of Alq3, the current densities increase in 1–3 orders of magnitude upon doping with F4TCNQ, suggesting that the doping can effectively enhance the hole-injection and hole-transport ability of Alq3. An organic light-emitting device using an F4TCNQ doped Alq3 layer as the hole-injection and hole-transport layer, and pristine Alq3 as the electron-transport and emitting layer is fabricated and characterized. Bright emission is achieved in the simple OLED with p-doped Alq3 as the hole-transport layer and the intrinsic Alq3 as the electron-transport and emitting layer. The emitting efficiency and brightness of the device are further improved by inserting a thin electron block layer to confine the carrier recombination zone in the middle of the organic layers.

Blue and green organic light-emitting devices with various film thicknesses for color tuning

Blue and green organic light-emitting devices with a structure of indium tin oxide (ITO) /N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB)/aluminum(III) bis(2-methyl-8-quinolinato)4-phenylphenolato (BAlq)/tris(8-hydroxyquinolate)-aluminum (Alq3)/Mg:Ag have been fabricated. Blue to green light emission has been achieved with the change of organic film thickness. Based on energy band diagram and charge carrier tunneling theory, it is concluded that the films of different thicknesses play a role as a color-tuning layer and the color-variable electroluminescence (EL) is ascribed to the modulation function within the charge carrier recombination zone. In the case of heterostructure devices with high performance, the observed EL spectra varies significantly with the thickness of organic films, which is resulted from the shift of recombination region site. It has not been hitherto indicated that the devices compose of identical components could be implemented to realize different color emission by changing the film thickness of functional layers.

Light Extraction From Solution-Based Processable Electrophosphorescent Organic Light-Emitting Diodes

Molecular dye dispersed solution processable blue emitting organic light-emitting devices have been fabricated and the resulting devices exhibit efficiency as high as 25 cd/A. With down-conversion phosphors, white emitting devices have been demonstrated with peak efficiency of 38 cd/A and luminous efficiency of 25 lm/W. The high efficiencies have been a product of proper tuning of carrier transport, optimization of the location of the carrier recombination zone and, hence, microcavity effect, efficient down-conversion from blue to white light, and scattering/isotropic remission due to phosphor particles. An optical model has been developed to investigate all these effects. In contrast to the common misunderstanding that light out-coupling efficiency is about 22% and independent of device architecture, our device data and optical modeling results clearly demonstrated that the light out-coupling efficiency is strongly dependent on the exact location of the recombination zone. Estimating the device internal quantum efficiencies based on external quantum efficiencies without considering the device architecture could lead to erroneous conclusions

Color tunable organic light-emitting diodes by using europium organometallic complex

Using the rare-earth special feature of a sharp emission spectrum, voltage-controlled continuous color tuning of organic light-emitting diodes is achieved. Europium(dibenzoylmethanato)3(bathophenanthroline) is used as the strategic starting point close to the red corner of the Commission International de I’Eclairage chromaticity diagram for a wide color tuning. The end point and path of the color tuning can be engineered by doping the hole-transport emitting layer with dyes. The mechanisms of color tuning have been investigated and explained by the efficiency reduction of the europium complex and the extension of carrier recombination zone with driving voltage. The effect of exciplex on the color tuning is also studied.

Investigation of the recombination zone in the structure of red organic electroluminescent devices

A hole-blocking layer (HBL) is used to obtain red organic electroluminescence (OEL) from 4-dicyanomethylene-2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-8-yl) vinyl]-4H-pyran (DCM2) fluorescent dye. Indium tin oxide (ITO) was used as a glass substrate and N, N′-bis-(1-naphthyl)- N, N′-diphenyl-1,1-biphenyl-4-4′-diamine (NPB) as a hole transport layer and an emissive layer. DCM2, as a red fluorescent dye, was doped into the NPB layer. Bathocuproine (BCP) was used as a hole-blocking layer (HBL); tris-(8-hydroxy-quinoline) aluminum (Alq 3) was used in the electron transport layer, and aluminum (Al) was used as a cathode in red organic electroluminescent (OEL) devices with the configuration ITO/NPB (30 − x nm)/NPB:DCM2 (1.5 nm)/NPB ( x nm)/BCP (30 nm)/Alq 3 (40 nm)/Al. BCP acted as HBL, sp the electrons and holes recombine in the NPB:DCM2 layer. Accordingly, the OEL device emits red light by energy transfer from NPB to DCM2. The recombination zone of electrons and holes was investigated by varying the NPB:DCM2 location in the OEL device. This investigation indicates that the optimum thickness of the carrier recombination zone was x = 0∼3 nm, as measured from the interface of the BCP layer.

Role of diffusion on SCLC transport in double injection devices

A theoretical study of SCLC transport in double injection insulators is presented. It will be demonstrated, that the inclusion of charge carrier diffusion, neglected in many previous studies of transport in organic light emitting diodes (OLEDs), is essential to obtain physical meaningful spatial charge carrier densities and field distributions. Only the knowledge of such correct spatial distributions enables one to compute the correct position of the charge carrier recombination zone. In previous calculations without diffusion the recombination process often takes place in the vicinity of both electrodes, even for equal mobilities of holes and electrons. In the present calculation including diffusion it is demonstrated that only one recombination zone exists. For equal mobilities of electrons and holes the recombination zone is found as expected in the centre of the device whereas for different mobility values it may be strongly shifted to one of the electrodes. The resulting I– V characteristics indicate that, in double injection devices, the well-known Mott–Gurney law holds only at sufficiently high voltages and only if recombination is taken into account. For small voltages, an ohmic-like behavior is observed in any case, however, if no recombination is assumed a transition to an I ∼ V 3 law is obtained for higher voltages. Due to the inclusion of diffusion, all I– V characteristics exhibit temperature dependence.

Green to white to blue OLEDs by using PBD as a chromaticity-tuning layer

Blue to white to green light-emitting diodes with structure ITO/PVK:DPPhP(5%)/PBD( x nm)/Alq 3(5 nm)/LiF(0.3 nm)/Al have been obtained. DPPhP (1,3-diphenyl-5-(9-phenanthryl)-4,5-dihydro-1-H-pyrazole), which performs good blue-emitting and hole-transporting properties, is synthesized and used as dopant. When the thickness of PBD layer varies from 0 to 5 to 10 nm, green to white to blue emission is achieved. Based on the energy band theory and carries tunneling theory, we consider that PBD layer plays a chromaticity-tuning role and the color-variable EL is ascribe to its modulation function on carrier recombination zone.