The storage of charges and its optical application in organic light-emitting diodes measured by a transient electroluminescence method

Organic light-emitting diodes (OLEDs) device capable of stored charges with poly (methyl methacrylate) (PMMA) layer is studied by transient electroluminescence measurements. The mechanism and optical application of stored carriers are elucidated. A spike after a driven pulse is found in the device with PMMA layer, which is attributed to the drifting back of accumulated electrons and trapped ones in shallow states, and the detrapping of latter may result in a long decay tail. A reversed post-pulse is applied to release the electrons in deep traps as they are immobile unless under a strong reversed field. Since the stored charges can lead to a great loss of carriers and weaken the performance of device, we find a way to use them in the form of light emitting with an enhanced intensity more than 3 times as against steady-state. So we have a good reason to believe if in a proper way, we can make full use of the stored charges in optical application.     

Transient and stable electroluminescence properties of alternating-current biased organic light-emitting diodes

In this work,transient electroluminescence (EL) (brightness-voltage waveform curve) was utilized to investigate the working mechanism of alternating-current biased organic light-emitting diodes (AC-OLE...     

Analysis of alternating current driven electroluminescence in organic light emitting diodes: A comparative study

The alternating current (AC) responses of double-injection and double-insulated organic light-emitting diodes (OLEDs) were investigated and compared. To reveal the electroluminescent (EL) processes in these devices, the AC voltage and frequency dependence of the EL intensity and capacitive current were studied in the time domain with a focus on phase difference analysis. It was found that the voltage-dependent transit time and frequency-dependent carrier distribution were important for the AC-driven performance of the double-injection OLEDs. In contrast, although the double-insulated OLEDs shared some similarities with the double-injection OLEDs, they had some unique characteristics, which were the absence of resistive current and phase shift of EL profiles. It was revealed that the EL in the double-insulated OLEDs was driven by the displacement current generated by the ionization of the doped layers, which, however, formed space charge regions and undermined the EL emission. The space charge redistributed the electric field across the devices after the initiation of EL, making the EL maintain for a limited time interval. This effect was significant under low frequency and high AC voltage. Comparing the phase difference between both devices, it was indicated that the space charge effect was responsible for the observed EL phase shift and the asymmetric EL profiles at low frequency and high AC voltage in the double-insulated OLEDs. The proposed model was also of help to understand the EL saturation phenomena with AC frequency and voltage in those devices.     

Transient luminescence in organic light-emitting diodes explained by trap-assisted recombination of stored charges

We show that after voltage turn-off the recombination of the charges which are stored in the on-state may follow a trap-assisted mechanism instead of the Langevin formula in organic light-emitting diodes. A microscopic model based on this form of recombination is introduced, which not only fits the transient electroluminescence very well in the whole range of the off-state but also provides parameters quantitatively characterizing the stored charges in the on-state. In the last part, we briefly compare the work by Weichsel et al. [1] and our model, trying to constitute a comprehensive picture of the stored charges in OLEDs. As an OLED model system we have chosen a host/guest system co-doped with red and green phosphorescent emitters.     

Transient electroluminescence in heavy metal complex-based phosphorescent organic light-emitting diodes

In phosphorescent organic light-emitting diodes (PHOLEDs), both the rise time and decay time decrease with increasing amplitude of the applied voltage pulse. The rise time τ_r of the transient electroluminescence (TEL) increases linearly with increasing value of the ratio of voltage V to the current j, that is, with V/j. Using the equations for the dynamics of charge carriers an expression is derived for the rise time τ_r of the TEL in OLEDs. It is shown that τ_r should increase with increasing values of the ratio (V/j), dielectric constant ε, and area of cross-section of the emission layer, however, it should decrease with the thickness of emission layer. For higher values of the applied voltage nonlinearity occurs in the τ_r versus V/j plot because the increase in mobility of carriers at high electric field causes increase in the current flowing through the OLEDs. In fact, the rise time of TEL is related to the product of capacitance and effective resistance of the OLED. Considering the rate of generation and decay of radiative triplet excitons in the emission layer, an expression is derived for the decay time of TEL in PHOLEDs and it is shown that, for higher values of the time-constant of OLED, the decay time should be equal to the time-constant, however, for lower values of the time-constant, the decay time should be equal to the lifetime of radiative triplet excitons in the emission layer. A good agreement is found between the theoretical and experimental results.     

Transient electroluminescence determination of carrier mobility and charge trapping effects in heavily doped phosphorescent organic light-emitting diodes

Transient electroluminescence (EL) was used to measure the delay between the excitation pulse and onset of emission in OLEDs based on phosphorescent bis[3,5-bis(2-pyridyl)-1,2,4-triazolato] platinum(ΙΙ) doped into 4,4′-bis(carbazol-9-yl) triphenylamine (CBP), from which an electron mobility of 3.2 × 10⁻⁶ cm²/V s was approximated. Delayed recombination was observed after the drive pulse had been removed and based on its dependence on frequency and duty cycle, ascribed to trapping and de-trapping processes associated with disorder-induced carrier localization at the interface between the emissive layer and electron blocker. The data suggests that the exciton recombination zone is at, or close to the interface between the emissive layer and electron blocker. Despite the charge trapping effects, a peak power efficiency of 24 lm/W and peak external quantum efficiency of 10.64% were obtained. Mechanisms for the electroluminescence and delayed recombination are proposed.     

Plasmonic nanomeshes as large-area,low-resistive transparent electrodes and their application to ITO-free organic light-emitting diodes

Here we implement a systematic investigation of two-dimensional silver nanomeshes (AgNMs) for large-area, very low-resistive transparent electrodes and their successful application to ITO-free organic light-emitting diodes (OLEDs). Experimental results, as well as theoretical analyses with optical and electrical calculation, show that an optimized AgNM electrode fabricated via cost-effective colloidal lithography can have high transmittance and low sheet resistance simultaneously. By using an index-matching concept for further improvement of optical performance, highly transparent AgNM electrodes are achieved at very low sheet resistance (∼3.5 Ω/sq). Based on the study regarding transmittance variation of AgNM electrodes embedded in OLEDs, a pertinent index-matching layer is proposed for high performance AgNM-based OLEDs.     

Near infrared electroluminescence from Nd(TTA)3phen in solution-processed small molecule organic light-emitting diodes

We report on the near infrared electroluminescence properties of a Nd³⁺ complex with thenoyltrifluoroacetone and 1,10-phenantroline ligands in solution-processed organic light-emitting diodes. Spin-coated blends containing a 1,3-bis(9-carbazolyl)benzene host doped with the Nd³⁺ complex were found to exhibit a photoluminescence quantum yield of about 0.5%, regardless of the doping concentration level. Electroluminescent devices based on these small molecule blends showed the characteristic emission of Nd³⁺ at 890, 1060 and 1330 nm with an external quantum efficiency as high as 0.022%. These improved performances were mainly attributed to direct charge trapping and exciton formation on the near infrared emitter. Importantly, the efficiency roll-off at high current densities due to triplet-triplet exciton annihilation in the device containing 20 wt% of the complex was lower than what is typically observed in lanthanide complex-based electroluminescent devices. This is presumably due to the high triplet energy of the host material, which prevents guest-to-host energy-back transfer and thus host-guest triplet-triplet exciton annihilation.     

Magnetic field effects on electroluminescence in phosphorescence organic light emitting diodes

In this letter, we presented a method to study the MFEs on the triplets in phosphorescent OLEDs. The magnetic electroluminescence (MEL) was obtained by doping a red phosphorescent guest with low concentration into a fluorescent host, where the guest and host can simultaneously emit. Experimentally two different MEL shapes of Lorentz and linear were observed, depending on the used host materials. We presented two different mechanisms to explain their difference. The diffusion process of triplets from host to guest and prolonged lifetime of triplet by magnetic field were attributed to the formation of the Lorentz shape, and it is considered that the linear shape was caused by magnetic field increased Dexter energy transfer rate and determined by the triplet energy difference between guest and host. It can be seen that the competition of two processes lead to the formation of the two different MEL shapes in the phosphorescent OLEDs.     

Effect of Joule heating on transient current and electroluminescence in p-i-n organic light-emitting diodes under pulsed voltage operation

The transient current and electroluminescent characteristics of p-i-n organic light-emitting diodes driven using short voltage pulses with various amplitudes and pulse widths were investigated to understand high current behavior (>10 A/cm²). Even under short voltage pulse operation, Joule heating was found to strongly affect the transient characteristics and lead to temperature rises estimated to be over 100 K in the high current density region (>400 A/cm²). This results in a large increase in both current density and EL intensity within the pulse width. In addition, the Joule heating was found to have an effect on the external quantum efficiency. However its contribution was found to be limited compared with the other quenching mechanism, singlet-polaron quenching.     

Degradation pattern of contact resistance and characteristic trap energy in blue organic light-emitting diodes

Thin and lightweight organic light-emitting diodes (OLEDs) are promising candidates for next-generation rollable displays; they offer numerous advantages, such as scalable manufacturing, high color contrast ratio, flexibility, and wide viewing angle. Despite the numerous merits of OLEDs, the insufficient lifetime and stability of blue OLEDs remain unresolved, thereby necessitating a feedback strategy for lifetime extension. Herein, we propose a simple yet effective methodology to determine the contact resistance (R_CT) and characteristic trap energy (E_T) of OLEDs simultaneously in the trapped-charge-limited-conduction regime, where electroluminescence occurs primarily. To validate our approach, the extracted R_CT and E_T values are directly compared with each other by connecting a commercial resistor (R_C) to a blue OLED in series. The percent errors discovered in R_C and E_T are less than 7% and 4%, demonstrating the high feasibility and accuracy of our approach. We further employ this method to study the degradation mechanism of a blue OLED by presenting the electrical stress time- and cycle-dependent R_CT, E_T, ideality factor, and turn-on voltage, revealing different degradation patterns of the metal-to-transport layer interface and emission layer, respectively. Our results provide better insights into the electrical parameter extraction method and electrical current degradation mechanism in blue OLEDs.     

Electron injection and interfacial trap passivation in solution-processed organic light-emitting diodes using a polymer zwitterion interlayer

Here we describe the use of a polymer zwitterion as a solution-processable material that serves as the key component of the electron injection layer (EIL) in solution processed organic light-emitting diodes (OLEDs). Poly(sulfobetaine methacrylate) (PSBMA) was employed in both regular and inverted device configurations as a work-function modifier for Al and ZnO cathodes, respectively. For both architectures, PSBMA significantly improved the OLED performance when compared to reference devices without EIL in terms of turn-on voltage and luminance. In inverted devices, PSBMA showed a passivation effect on ZnO surface trap states, producing better performing and more stable devices.     

Fully solution-processed and multilayer blue organic light-emitting diodes based on efficient small molecule emissive layer and intergrated interlayer optimization

Efficient and fully solution-processed blue organic light-emitting diodes (OLEDs) based on fluorescent small-molecule and methanol/water soluble conjugated polymer as electron-injection material are reported. The emitting layer is 3,6-bis(9,9,9′,9′-tetrakis (6-(9H-carbazol-9-yl)hexyl)-9H,9′H-[2,2′-bifluoren]-7-yl)dib-nzo[b, d]thiophene 5, 5-dioxide (OCSoC) with a blue-fluorescent small-molecule, and a methanol/water soluble polymer poly[(9,9-bis(30-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl-fluorene)] (PFN) acted as electron-injection layer (EIL). All the organic layers are spin-coated from solution. The multilayer device structure with emitting layer/electron-injection layer is achieved by solution-processed method without the dissolution problem between layers. The performances of the devices show that the maximum luminous efficiency of the multilayer device is increased about 43%, compared to the single-layer device. PFN acting as the EIL material plays a key role in the improvement of the device performance when used in solution-processed small-molecule OLEDs.     

Molecular design of large-bandgap host materials and their application to blue phosphorescent organic light-emitting diodes

New large-bandgap host materials with carbazole and carboline moieties were designed and synthesized for high-performance blue phosphorescent organic light-emitting diodes (PhOLEDs). The two kinds of host materials, 9-(4-(9H-carbazol-9-yl)phenyl)-6-(9H-carbazol-9-yl)-9H-pyrido[2,3-b]indole (pP2CZCB) and 9-(3-(9H-carbazol-9-yl)phenyl)-6-(9H-carbazol-9-yl)-9H-pyrido[2,3-b]indole (mP2CZCB), displayed promisingly high triplet energies of ∼2.92–2.93 eV for enhancing the exothermic energy transfer to bis[2-(4,6-difluorophenyl)pyridinato-C²,N](picolinato)iridium(III) (FIrpic) in PhOLED devices. It was found that the blue PhOLEDs bearing the new host materials and the FIrpic dopant exhibited markedly higher external quantum efficiencies (EQEs) than a device made with 1,3-bis(N-carbazolyl)benzene (mCP) as the host. In particular, the PhOLED device made with 3 wt% FIrpic as the dopant and mP2CZCB as the host material displayed a low driving voltage of 4.13 V and the high EQE of 25.3% at 1000 cd m⁻².     

Straight-forward control of the degree of micro-cavity effects in organic light-emitting diodes based on a thin striped metal layer

We investigated the control of micro-cavity (MC) effects in organic light-emitting diodes (OLEDs) with the introduction of a striped thin metal layer between the indium tin oxide (ITO) layer and the hole transporting layer (HTL). With an enhanced MC effect obtained through the inserted metal layer, the forward emission of the OLED became stronger and the angular distribution became more forward-directed, leading to a current efficiency (CE) that was nearly 1.45 times higher than that of the reference device without the inserted metal layer. The net CE of the OLEDs with a striped metal layer was found to be determined by the area-weighted average of the CE’s of full-cavity-enhanced OLEDs and non-cavity OLEDs. It was also observed that the trade-off between resonance enhancement in efficiency and angle-dependent color stability, often found problematic in MC-based OLEDs, could be mitigated in a straight-forward manner by changing the relative portion of the metal-covered area.     

Solution processed single-emissive-layer white organic light-emitting diodes based on fluorene host: Balanced consideration for color quality and electroluminescent efficiency

Cost-effective fabrication of white organic light-emitting diodes (WOLED) is meaningful toward commercial application of environment-friendly solid-state lighting sources. Electroluminescent efficiency and color quality are two opposite performance characteristics facing solution processed WOLEDs requiring balanced consideration. Herein, a recently synthesized molecule of 4,4’-(9,9’-(1,3-phenylene)bis(9H-fluorene-9,9-diyl))bis(N,N-diphenylaniline) (DTPAFB) is introduced as a host material for solution processed all-phosphor WOLEDs, embracing four well-known molecules which are blue iridium (III) bis(2-(4,6-difluorophenyl)pyridinato-N,C²)(picolinate) (FIrpic), green iridium (III) bis[2-(2-pyridinyl-N)phenyl-C](2,4-pentanedionato-O²,O⁴) [Ir(ppy)₂(acac)], and orange iridium (III) bis(2-phenyl-benzothiazole-C²,N)(acetylacetonate) [Ir(bt)₂(acac)] plus a home-made red phosphor of iridium (III) tris(1-(2,6-dimethylphenoxy)-4-(4-chlorophenyl)phthalazine) [Ir(MPCPPZ)₃]. Illumination quality white light with high brightness, high efficiency, suitable correlated color temperature (CCT), high color-rendering index (CRI), and stable electroluminescent (EL) emission is obtained. A stable white emission with a CRI over 70, Commission Internationale de L'Eclairage (CIE) of (0.37, 0.42), and high EL efficiency of 19.6 lm W⁻¹ at high luminance of 2000 cd m⁻² for blue/orange complementary color WOLEDs is demonstrated. The optimized red/green/blue three primary color WOLEDs show improved CRI up to 81, moderate high efficiency of 25.8 cd A⁻¹, 14.4 lm W⁻¹, and EQE of 13.9%. Furthermore, the red/green/blue/orange four primary color WOLEDs show the optional balance between color quality and EL efficiency with high CRI of around 81–83 and medium CCT of 3755–3929 K which is warm and soft to human eyes. At an illumination relevant luminance of 1000 cd m⁻², the total power efficiency reaches 33.6 lm W⁻¹, and still remains 30.2 lm W⁻¹ at 3000 cd m⁻², approaching the efficiency of state-of-the-art fluorescent-tube (40–70 lm W⁻¹), potentially suitable as an environment-friendly solid-state lighting source. This work indicates that developing high performance host materials and highly efficient phosphors and carefully combining them with common phosphors is an effective way toward high performance WOLEDs.     

Bifunctional organic materials for OLEDs based on Tröger’s base: Subtle structural changes and significant differences in electroluminescence

Three bifunctional organic materials based on Tröger’s base (TB) scaffold have been designed, synthesized and their functional utility in OLEDs demonstrated through fabrication of devices. The diarylamino-functionalized TBs, i.e., A-TB, PA-TB and P-TB, are all amorphous with considerably high T_gs and permit modulation of electroluminescence with variation of one of the N-aryl rings, e.g., anthryl to 10-phenylanthryl to pyrenyl. Subtle structural modifications are found to manifest in rare pure yellow EL emission from PA-TB.     

Novel spiro-based host materials for application in blue and white phosphorescent organic light-emitting diodes

Two novel spiro-based host materials, namely 3-(9,9′-spirobi[fluoren]-6-yl)-9-phenyl-9H-carbazole (SF3Cz1) and 9-(3-(9,9′-spirobi[fluoren]-6-yl)phenyl)-9H-carbazole (SF3Cz2) were designed and synthesized. Due to the meta-linkage of spirobifluorene backbone, both SF3Cz1 and SF3Cz2 possess triplet energies over 2.70 eV, indicating they could serve as suitable hosts for blue and even white phosphorescent organic light-emitting diodes (PHOLEDs). The fabricated bis(4,6-(difluorophenyl)-pyridinato -N,C′)picolinate (FIrpic) based PHOLEDs hosted by SF3Cz1 and SF3Cz2 exhibited excellent performance with maximum external quantum efficiencies (EQEs) of 18.1% and 19.7%, respectively. Two-color warm white PHOLEDs fabricated by utilizing SF3Cz1 and SF3Cz2 as hosts also achieved high EQEs and low efficiency roll-offs. The results demonstrate that SF3Cz1 and SF3Cz2 are promising hosts for blue and white PHOLEDs.     

Hole mobilities of thermally polymerized triaryldiamine derivatives and their application as hole-transport materials in organic light-emitting diodes (OLEDs)

This paper describes the synthesis of three triaryldiamine derivatives presenting two thermally polymerizable trifluorovinyl ether groups that can be polymerized through thermal curing to form perfluorocyclobutyl (PFCB) polymers. These PFCB polymers, studied using time-of-flight techniques for the first time, exhibited remarkable non-dispersive hole-transport properties, with values of μ_h of ca. 10^?4 cm² V^?1 s^?1. When we employed these thermally polymerized polymers as hole-transport layers (HTLs) in electroluminescence devices containing tris(8-hydroxyquinolate) aluminum (Alq₃) as the emission layer, we obtained high current densities (ca. 3400 mA cm^?2), impressive brightnesses (5 × 10⁴ cd m^?2), and high external quantum efficiencies (EQEs = 1.43%). These devices exhibited the same turn-on voltage, but higher EQEs, relative to those incorporating the vacuum-processed model compound N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine (α-NPD) (EQE = 1.37%) as the HTL under the same device structure.     

Heat evolution and dissipation in organic light-emitting diodes on flexible polymer substrates

We investigated the effect of the heat generated in organic light-emitting diodes (OLEDs) on their performance with a focus on the thermo-physical properties of the substrates. OLEDs were fabricated on polyethylene terephthalate (PET), polyimide (PI), and glass substrates, and the instantaneous and time-resolved performances of the OLEDs were compared. The device operation temperature (T) was predicted via heat transfer analysis using the finite element method (FEM). The value of T during operation was experimentally measured using an infrared (IR) camera, and the results were compared with the numerically calculated values. The effects of T on the time-resolved electroluminescence (EL) spectra of the OLEDs on the different substrates were also investigated. The experimental results of this study agreed well with the numerical predictions that the T of the OLEDs on the polymers are higher than the T of the OLEDs on glass during operation due to the relatively low thermal diffusivity (α) of the polymer substrates used in this study. The results also showed that the performance and reliability of the flexible OLEDs (f-OLED) are highly dependent on the heat extraction capabilities of the substrates; thus, the current density (J), luminescence (L), voltage (V) characteristics, and efficiencies (η) over time of the OLEDs on PET and PI are inferior to the OLEDs on the glass substrate.