Solution-processed high-performance organic light-emitting diodes containing a green-emitting multiresonant thermally activated delayed fluorescent dendrimer

A multi-resonance thermally activated delayed fluorescence (MR-TADF) dendrimer emitter and a related reference MR-TADF compound were designed, synthesized, and characterized for use as narrowband emitters in solution-processed OLEDs. The 1 wt% doped films in PMMA film revealed that the compounds MR-D1 and MR-D2 showed narrowband green emission at λ_PL of 490 and 495 nm and with FWHM of 23 and 29 nm, respectively. The 50 wt% doped films in mCP still show narrowband green emission at λ_PL of 495 and 499 nm and with FWHM of 28 nm for MR-D1 and MR-D2 , respectively, while conserving the small ΔE_ST of 0.14 and 0.13 eV, respectively. OLEDs containing an emissive layer consisting of 50 wt% MR-D1 and MR-D2 in mCP showed high EQE_max of 27.7% and 21.0%, respectively, and low efficiency roll-off of 19% and 30% at a luminance of 2000 cd/m⁻².     

Highly efficient blue organic light‐emitting diode with high color purity using 4,4′‐N,N′‐dicarbazole‐biphyenyl (CBP) doped with 1,4‐bis[2‐(3‐N‐ethylcarbazoryl) vinyl]benzene (BCzVB)

Abstract— An efficient pure blue multilayer organic light‐emitting diode employing 1,4‐bis[2‐(3‐N‐ethylcarbazoryl)vinyl]benzene (BCzVB) doped into 4,4′‐N,N′‐dicarbazole‐biphyenyl (CBP) is reported. The device structure is ITO (indium tin oxide)/TPD (N,N′‐diphenyl‐N,N′‐bis (3‐methylphenyl)‐1,1′biphenyl‐4,4′diamine)/CBP:BCzVB/Alq₃ (tris‐(8‐hydroxy‐quinolinato) aluminum)/Liq (8‐hydroxy‐quinolinato lithium)/Al; here TPD was used as the hole‐transporting layer, CBP as the blue‐emitting host, BCzVB as the blue dopant, Alq₃ as the electron‐transporting layer, Liq as the electron‐injection layer, and Al as the cathode, respectively. A maximum luminance of 8500 cd/m² and a device efficiency of 3.5 cd/A were achieved. The CIE co‐ordinates were x = 0.15, y = 0.16. The electroluminescent spectra reveal a dominant peak at 448 nm and additional peaks at 476 nm with a full width at half maximum of 60 nm. The Föster energy transfer and, especially, carrier trapping models were considered to be the main mechanism for exciton formation on BCzVB molecules under electrical excitation.     

Electroluminescent properties of poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] doped with 1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene

The effect of 1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) doping on electroluminescent properties of poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (poly-TPD) was investigated. A series of organic light-emitting devices (OLEDs) integrated with (i) single-layer poly-TPD, (ii) blended single-layer poly-TPD:TPBi or (iii) bilayer poly-TPD/TPBi were fabricated and characterized. An excimer emission band at 500 nm was found in the poly-TPD film, poly-TPD:TPBi (1:1) blend film, and poly-TPD/TPBi bilayer film. It was observed that the planar geometry of poly-TPD was related to the formation of excimers. The electromer emission, which was absent in photoluminescence, was investigated by applying an external electrical field to devices with non-doped and TPBi-doped poly-TPD. Only the electromer emission was observed in the devices with TPBi-doped poly-TPD, due to the impeded intrinsic and excimer emissions. The planar geometry of poly-TPD molecules may be destroyed due to the longer inter-ion distance with the doping of TPBi.     

Low operating-voltage and high power-efficiency OLED employing MoO3-doped CuPc as hole injection layer

Effects of doping molybdenum oxide (MoO₃) in copper phthalocyanine (CuPc) as hole injection layer in OLEDs are studied. A green OLED with structure of ITO/MoO₃-doped CuPc/NPB/10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H, 11H-(1)-benzopyropyrano(6,7,8-i,j) quinolizin-11-one (C545T): tris(8-hydroxyquinoline) aluminum (Alq₃)/Alq₃/LiF/Al shows the driving voltage of 4.4 V, and power efficiency of 4.3 lm/W at luminance of 100 cd/m². The charge transfer complex between CuPc and MoO₃ plays a decisive role in improving the performance of OLEDs. The AFM characterization shows that the doped film owns a better smooth surface, which is also in good agreement with the electrical performance of OLEDs.     

掺杂荧光染料的高亮度有机蓝光电致发光器件

利用蓝色发光材料DPVBi掺杂高荧光染料rubrene做发光层制备了蓝色发光器件。在掺杂浓度为1.5%(wt)左右的情况下,当改变掺杂层的总的厚度时,器件的亮度、效率和色坐标都有明显的改变。当掺杂层DPVBi和rubrene的厚度为40nm,电子传输层Alq3的厚度为20nm,器件所加的电压是13v时,其最大亮度为14000cd/m2,此时的色坐标为(0.24,0.24),为蓝光发射。这种掺杂明显的提高了蓝光器件的发光效率。使最大效率达到2.5cd/A。     

Efficiency enhancement of solution‐processed single‐layer blue‐phosphorescence organic light‐emitting devices having co‐host materials of polymer (PVK) and small‐molecule (SimCP2)

Abstract— A new type of single‐layer blue‐phosphorescence organic light‐emitting devices (OLEDs) containing poly(9‐vinylcarbazole) (PVK) and small‐molecule‐based amorphous ambipolar bis(3,5‐di(^9H‐carbazol‐9‐yl)phenyl) diphenylsilane (SimCP2) as the co‐host material have been demonstrated. All active materials [PVK, SimCP2, Flrpic (blue‐phosphorescence dopant), and OXD‐7 (electron transport)] were mixed in a single layer for solution processing in the fabrication of OLEDs. The SimCP2 small‐molecule host has adequate high electron and hole‐carrier mobiltieis of ～10^?4 cm²/V‐sec and a sufficiently large triplet state energy of ～2.70 eV in confining emission energy on FIrpic. Based on such an architecture for single‐layer devices, a maximum external quantum efficiency of 6.2%, luminous efficiency of 15.8 cd/A, luminous power efficiency of 11 lm/W, and Commision Internale de l'Eclairage (CIE^x,y) coordinates of (0.14,0.32) were achieved. Compared with those having PVK as the single‐host material, the improvement in the device performance is attributed to the balance of hole and electron mobilities of the co‐host material, efficient triplet‐state energy confinement on FIrpic, and the high homogeneity of the thin‐film active layer. Flexible blue‐phosphorescence OLEDs based on solution‐processed SimCP2 host material (withou PVK) have been demonstrated as well.     

Improved performance of organic light‐emitting devices with ultra‐thin hole‐blocking layers

Abstract— Tris‐(8‐hydroxyqunoline) aluminum (Alq₃)‐based organic light‐emitting devices (OLEDs) using different thickness of 2,9‐Dimethyl‐4,7‐diphenyl‐1,110‐phenanthorline (BCP) as a hole‐blocking layer inserted both in the electron‐ and hole‐transport layers have been fabricated. The devices have a configuration of indium tin oxide (ITO)/m‐MTDATA (80 nm)/BCP (X nm)/NPB (20 nm)/Alq₃ (40 nm)/BCP (X nm)/Alq₃ (60 nm)/Mg: Ag (200 nm), where m‐MTDATA is 4, 4′, 4″‐Tris(N‐3‐methylphenyl‐N‐phenyl‐amino) triphenylamine, which is used to improve hole injection and NPB is N,N′‐Di(naphth‐2‐yl)‐N,N′‐diphenyl‐benzidine. X varies between 0 and 2 nm. For a device with an optimal thickness of 1‐nm BCP, the current and power efficiencies were significantly improved by 47% and 43%, respectively, compared to that of a standard device without a BCP layer. The improved efficiencies are due to a good balance between the electron and hole injection, exciton formation, and confinement within the luminescent region. Based on the optimal device mentioned above, the NPB layer thickness influences the properties of the OLEDs.     

Role of triplet‐triplet annihilation in highly efficient fluorescent devices

Abstract— A study of delayed electroluminescence in model highly efficient OLEDs based on anthracene derivatives indicate that triplet‐triplet annihilation (TTA) contributes significantly to overall efficiency. Highly efficient devices (6–9% external quantum efficiencies) based on 9,10‐bis(2‐naphthyl)‐2‐phenylanthracene show that the TTA contribution depends primarily on operating current density, reaching as much as 20–30% of the overall emission intensity at moderate current densities (>5 mA/cm²). Revision of the classical estimates of maximum external quantum efficiency of fluorescent OLEDs to 8% and maximum internal quantum efficiency to 40% is recommended to account for TTA contribution (even further revision may be necessary to account for a better‐than‐20% optical outcoupling).     

Efficient bluish-green phosphorescent iridium complex for both solution-processed and vacuum-deposited organic light-emitting diodes

Iridium(III)bis(4,6-(difluorophenyl)pyridinato-N,C^2′)picolinate (Firpic) is one typical bluish-green phosphor widely used in phosphorescent organic light-emitting diodes (PhOLEDs). In order to optimize its electroluminescent performance, 3,6-(di-tert-butyl)carbazolyl was introduced into the pyridine ring of the 2,4-difluorophenyl-pyridine ligand via a non-conjugated CH₂ linkage. The generated 3,6-di-tert-butyl-9-((6-(2,4-difluorophenyl)pridine-3-yl)methyl)-9H-carbazole (Cz-CH₂-dfppy) was used as cyclometalating ligand to prepare iridium complex 1, (Cz-CH₂-dfppy)₂Ir(pic). In comparison with the case to attach carbazole directly on pyridine, this non-conjugated CH₂ linking strategy avoids the unwanted bathochromic shift of the phosphorescence and improves the solubility of the iridium complex. (Cz-CH₂-dfppy)₂Ir(pic) (1) was used as doped emitter to fabricate OLEDs by both spin-coating and vacuum evaporation methods. Efficient bluish-green electrophosphorescence was obtained with maximum luminance efficiency of 22 cd/A (14 lm/W, 8.7%) and 26 cd/A (12 lm/W, 9.5%) for the solution-processed and vacuum-deposited devices, respectively, which far exceed those of the parent Firpic based device. The improved performance for (Cz-CH₂-dfppy)₂Ir(pic) was interpreted in terms of improved charge balance brought by the presence of the carbazole groups in the ligands.     

Highly efficient styrylamine-doped blue and white organic electroluminescent devices

We have developed highly efficient blue and white organic electroluminescent devices based on a blue fluorescent styrylamine dopant EBDP. The blue and white organic light emitting diodes (OLEDs) with the structures: Indium–tin oxide (ITO)/copper phthalocyanine(CuPc)/N,N′-bis-(1-naphenyl)-N,N′-biphenyl-1,1′-bipheny1-4-4′-diamine (NPB)/2-t-butyl-9,10-di-(2-naphthyl)anthracene (TBADN):EBDP/tris(8-hydroxyquinoline)aluminum(Alq₃)/LiF/Al and ITO/CuPc/NPB/TBADN:EBDP: 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB)/Alq₃/LiF/Al were studied by using EBDP as blue dopant. For the blue device, the maximum luminance and maximum efficiency were 26961 cd/m² and 8.29 cd/A, respectively, the luminance at a current density 20 mA/cm² was 1597 cd/m². For the white device, the maximum luminance of 32,291 cd/m², maximum efficiency 8.31 cd/A and the luminance of 1413 cd/m² at a current density 20 mA/cm² were obtained. The slow decrease of efficiency with the increase of current density indicates weak exciton–exciton annihilation, which is resulted from the large steric hindrance due to the non-planar structure of the fluorescence dye EBDP.     

Synthesis of Bi and Gd doped ZnB2O4 for evaluation as potential materials in luminescent display applications

A series of Bi³⁺ and Gd³⁺ doped ZnB₂O₄ phosphors were synthesized with solid state reaction technique. X-ray diffraction technique was employed to study the structure of prepared samples. Excitation and emission spectra were recorded to investigate the luminescence properties of phosphors. The doping of Bi³⁺ or Gd³⁺ with a small amount (no more than 3 mol%) does not change the structure of prepared samples remarkably. Bi³⁺ in ZnB₂O₄ can emit intense broad-band purplish blue light peaking at 428 nm under the excitation of a broad-band peaking at 329 nm. The optimal doping concentration of Bi³⁺ is experimentally ascertained to be 0.5 mol%. The decay time of Bi³⁺ in ZnB₂O₄ changes from 0.88 to 1.69 ms. Gd³⁺ in ZnB₂O₄ can be excited with 254 nm ultraviolet light and yield intense 312 nm emission. The optimal doping concentration of Gd³⁺ is experimentally ascertained to be 5 mol%. The decay time of Gd³⁺ in ZnB₂O₄ changes from 0.42 to 1.36 ms.     

Novel carbazole/anthracene hybrids for efficient blue organic light-emitting diodes

Two novel carbazole/anthracene hybrided molecules, namely 2-(anthracen-9-yl)-9-ethyl-9H-carbazole (AnCz) and 2,7-di(anthracen-9-yl)-9-ethyl-9H-carbazole (2AnCz), were designed and synthesized via palladium catalyzed coupling reaction. The anthracene was attached either at the 2-site (AnCz) or at both 2,7-sites (2AnCz) of the central carbazole core to tune the conjugation state and the optoelectronic properties of the resultant molecules. Both of them show good solubility in common organic solvents. They also possess relatively high HOMO levels (−5.39 eV, −5.40 eV) that would facilitate efficient hole injection and be favorable for high power efficiencies when used in organic light-emitting devices (OLEDs). AnCz and 2AnCz were used as non-doped emitter to fabricate OLEDs by vacuum evaporation. Good performance was achieved with maximum luminance efficiency of 2.61 cd A⁻¹ and CIE coordinates of (0.15, 0.12) for AnCz, and 9.52 cd A⁻¹ and (0.22, 0.37) for 2AnCz.     

Highly efficient and stable white organic light emitting diode with triply doped structure

A triply doped white organic light emitting diode with red and blue dyes in the light emitting layer and a green dye in another layer is proposed. The device structure was CuPc(12 nm)/NPB(40 nm)/ADN:DCJTB(0.2%):TBPe(1%)(50 nm)/Alq:C545(0.5%)(12 nm)/LiF(4 nm)/Al. Here copper phthalocyanine (CuPc) is a buffer layer, N,N′-di(naphthalene-1-y1)-N,N′-dipheyl-benzidine (NPB) is a hole transporting layer, 9,10-di-(2-naphthyl) anthracene (ADN) is blue emitting layer, tris (8-quinolinolato)aluminium complex (Alq) is an electron transporting layer, 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidy1-9-enyl)-4H- pyran (DCJTB), 2,5,8,11-tetra-butylperylene (TBPe), Coumarin6 and deveriative (C545) are red, blue and green dyes, respectively. This device shows a luminance of 21200 cd/m² at driving current of 400 mA/cm² and 1026 cd/m² at 20 mA/cm². Its efficiency is 6 cd/A and 3.11 Lm/W. It also shows a higher operating stability: the half lifetime is 22,245 h at an initial luminance of 100 cd/m², while the driving voltage increased only 0.3 V.     

Printable phosphorescent organic light‐emitting devices

Abstract— A new approach to full‐color printable phosphorescent organic light‐emitting devices (P²OLEDs) is reported. Unlike conventional solution‐processed OLEDs that contain conjugated polymers in the emissive layer, the P²OLED's emissive layer consists of small‐molecule materials. A red P²OLED that exhibits a luminous efficiency of 11.6 cd/A and a projected lifetime of 100,000 hours from an initial luminance of 500 cd/m², a green P²OLED with a luminous efficiency of 34 cd/A and a projected lifetime of 63,000 hours from an initial luminance of 1000 cd/m², a light‐blue P²OLED with a luminous efficiency of 19 cd/A and a projected lifetime 6000 hours from an initial luminance of 500 cd/m², and a blue P²OLED with a luminous efficiency of 6.2 cd/A and a projected lifetime of 1000 hours from an initial luminance of 500 cd/m² is presented.     

Solution‐processable double‐layered ionic p‐i‐n organic light‐emitting diodes

Abstract— Solution‐processed double‐layered ionic p‐i‐n organic light‐emitting diodes (OLEDs), comprised of an emitting material layer doped with an organometallic green phosphor and a photo‐cross‐linked hole‐transporting layer doped with photo‐initiator is reported. The fabricated OLEDs were annealed using simultaneous thermal and electrical treatments to form a double‐layered ionic p‐i‐n structure. As a result, an annealed double‐layered OLED with a peak brightness over 20,000 cd/m² (20 V, 390 mA/cm²) and a peak efficiency of 15 cd/A (6 V, 210 cd/m²) was achieved.     

Spectral studies of white organic light-emitting devices based on multi-emitting layers

White organic light emitting devices (WOLEDs) with an RBG stacked multilayer structure were demonstrated. In RGB stacked OLEDs, blue emitting, 2-t-butyl-9,10-di-(2-naphthyl)anthracene (TBADN) doped with p-bis(p-N,N-diphenyl-amono-styryl)benzene (DSA-Ph), green emitting, tris-(8-hydroxyquinoline)aluminum (Alq) doped with C545, and red emitting, tris-[8-hydroxyquinoline]aluminum (Alq) doped with 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)- 4H-pyran (DCJTB), were used. By adjusting the order and thickness of emitting layer in RBG structure, we got a white OLED with current efficiency of 5.60 cd/A and Commission Internationale De L’Eclairage (CIE) coordinates of (0.34, 0.34) at 200 mA/cm². Its maximum luminance was 20,700 cd/m² at current density of 400 mA/cm². The results have been explained on the basis of the theory of excitons generation and diffusion. According to the theory of excitons generation and diffusion, an equation has been set up which relates EL spectra to the thickness of every layer and to the exciton diffusion length.     

High performance red and green phosphorescent emitters suitable for the BT.2020 color gamut

In this work, we investigated the potential for phosphorescent emitters to achieve the BT.2020 color standard in displays, where the CIE coordinates for red and green are (0.709, 0.292) and (0.170, 0.797), respectively. Optical simulations were performed for both green and red top emission organic light emitting devices (OLEDs). For the green emitter, it is possible to reach (0.170, 0.785) using a spectrum with a peak wavelength (λ_max) at 526 nm and a full width at half maximum (FWHM) less than 30 nm. For the red emitter, in order to achieve (0.708, 0.292) while maintaining a high current efficiency (CE), it is important to decrease the FWHM instead of red-shifting the spectrum. Following the guidance of these simulation results, we designed and synthesized novel deep green (DGD) and deep red phosphorescent (DRD-II) emitters. The photoluminescent (PL) spectrum of DGD shows an FWHM of 30 nm and a λ_max of 523 nm. A top-emission green OLED built using DGD reached a CE of 171 cd/A at an operating voltage of 3.3 V and a lifetime of 95% of initial brightness (LT₉₅) > 1300 h at 10 mA/cm² with a CIE (x, y) = (0.170, 0.777). This is, to our knowledge, the best device performance ever reported for a green phosphorescent OLED at this CIE y. The PL spectrum of DRD-II has a λ_max of 630 nm with an FWHM of 30 nm. A top-emission red OLED built with DRD-II achieved a CE of 59 cd/A, an operating voltage of 3.2 V and an LT₉₅ over 20,000 h at a drive current of 10 mA/cm² with a CIE (x, y) = (0.708, 0.292). We also studied the angular dependence of the above devices and found they were comparable to devices with commercial emitters for the Digital Cinema Initiative P3 (DCI-P3) standard that had a wider FWHM. Combining these green and red emitters with a commercial blue OLED at (0.131, 0.046), we are able to cover 97% of the BT.2020 color gamut. The results using DGD and DRD-II suggest that they have great potential to satisfy BT.2020 in an organic phosphorescent system.     

New blue phosphorescent iridium complexes for OLEDs

Abstract— A new class of ligands for complexation with Ir(III) has been developed. Tris‐homoleptic complexes derived from these ligands have been found to exhibit highly efficient blue phosphorescence with photoluminescent quantum yields in solution at room temperature of >0.9. These complexes have been applied as the emissive materials in OLEDs to give devices with efficiencies of up to 26 cd/A and an E.Q.E. of 1 7.4% at 1 mA/cm².     

Efficient organic light-emitting diodes with C60 buffer layer

Organic light-emitting diodes (OLEDs) with C₆₀ buffer layer were fabricated. The effect of C₆₀ buffer layer on the performance of the devices was investigated by inserting C₆₀ buffer layer at the interface between the electrode and organic layers. The device structures were (1) ITO/C₆₀ (0.0, 0.4, 0.7 and 1.0 nm)/NPB/Alq₃/LiF/Al and (2) ITO/NPB/Alq₃/C₆₀ (0.0, 0.4, 0.7 and 1.0 nm)/LiF/Al. The highest brightness and efficiency of the device (1) with 0.7 nm-thick C₆₀ layer reached 6439 cd/m² at 16 V and 1.80 cd/A at 6.4 V, respectively. The enhancements in brightness and efficiency are attributed to an improved balance of hole and electron injections due to C₆₀ layer blocking parts of the injected holes. On the contrary, the brightness and efficiency of the devices with the structure (2) had been hardly enhanced.     

Effect of thickness of platinum catalyst clusters on response of SnO2 thin film sensor for LPG

This paper reports the sensing response characteristics of rf-sputtered SnO₂ thin films (90 nm thick) loaded with platinum catalyst cluster of varying thickness (2-20 nm) for LPG detection. The enhanced response (5 × 10³) was obtained for 200 ppm LPG with the presence of 10 nm thin and uniformly distributed Pt catalyst clusters on the surface of SnO₂ thin film at a relatively low operating temperature (220 °C). The high response for LPG is shown to be primarily due to the enhanced catalytic activity for adsorbed oxygen on the surface of SnO₂ thin film besides the spill over mechanism at elevated temperature.