A blue organic emitting diode derived from new styrylamine type dopant materials

We have designed and synthesized new dopant materials based on the styrylamine moiety, 4-[(1,2-diphenyl)-4′-(N,N-diphenyl-4-vinylbenzenamine)]biphenyl (4) and 4-[(1,2-diphenyl)-4′-(N,N-diphenyl-4-vinylbenzenamine)]terphenyl (8). Blue OLEDs were obtained from new styrylamine dopant materials and compared with those of blue dopant bis[4-(di-p-N,N-diphenylamino)styryl]stilbene (DSA-Ph) and diphenyl[4-(2-terphenyl vinyl)phenyl]amine (R-BD). The ITO/DNTPD/NPB/MADN:dopant/Alq₃/Al-LiF device obtained from 4 shows blue EL spectrum at 469 nm and high efficiency 3.02 cd/A at 7 V. 8 also shows blue EL spectrum around λ_max = 468 nm, efficiency of 3.51 cd/A and a current density of 25.94 mA/cm² (855.7 cd/m²) at 7 V.     

Highly efficient red electroluminescence induced by efficient electron injection and exciton confinement

Highly efficient DCJTB-doped device was realized by enhanced electron injection and exciton confinement. A fluorine end-capped linear phenylene/oxadiazole oligomer 2,5-bis(4-fluorobiphenyl-4′-yl)-1,3,4-oxadiazole (1) and a trifluoromethyl end-capped oligomer 2,5-bis(4-trifluoromethylbiphenyl-4′-yl)-1,3,4-oxadiazole (2) were designed and incorporated as an electron transporting/hole blocking material in the device structure ITO/NPB (60 nm)/DCJTB:Alq₃ (0.5%, 10 nm)/1 or 2 (20 nm)/Alq₃ (30 nm)/LiF (1 nm)/Al (100 nm). The devices showed highly efficient red luminescence. In particular, the device based on 1 achieved pure red luminescence at 620 nm originating from DCJTB, with a narrow FWHI of 65 nm, maximal brightness of 13,300 cd/m² at voltage of 20.8 V and current density of ca. 355 mA/cm². High current and power efficiencies (>3.6 cd/A, 1.0 lm/W) were retained within a wide range of current densities. Our results show efficient and stable DCJTB-doped red electroluminescence could be anticipated for practical applications by taking advantage of the present approaches. The control experiments using BCP were also studied.     

Synthesis,photoluminescent and electroluminescent properties of a novel europium(III) complex involving both hole- and electron-transporting functional groups

A novel europium(III) complex involving a carbazole fragment as hole-transporting group and an oxadiazole fragment as electron-transporting group was synthesized and used as red light-emitting material in organic light-emitting diodes (OLEDs). The complex is amorphous, and exhibits high glass transition temperature (T_g = 157 °C) and high thermal stability with a 5% weight loss temperature of 367 °C. Two devices, device 1: ITO/NPB (40 nm)/Eu(III) complex (30 nm)/Alq₃ (30 nm)/LiF (1 nm)/Al (100 nm) and device 2: ITO/NPB (40 nm)/3% Eu(III) complex: CBP (30 nm)/BCP (10 nm)/Alq₃ (30 nm)/LiF (1 nm)/Al (100 nm), were fabricated, where NPB is N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine, Alq₃ is tris(8-hydroxyquinoline) Al(III), CBP is 4,4′-bis(carbazole-9-yl)-biphenyl, and BCP is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, respectively. In contrast with device 1, owing to less self-quenching and better charge confinement, device 2 shows improved performances: the maximum luminance of device 2 was dramatically increased from 199 to 1845 cd/m², the maximum current efficiency was increased from 0.69 to 2.62 cd/A, the turn-on voltage was decreased from 9.5 to 5.5 V, and higher color purity was attained.     

Interfacial charges in organic hetero-layer light emitting diodes probed by capacitance–voltage measurements

The bias dependent capacitance of organic hetero-layer light emitting diodes (LEDs) based on N,N′-diphenyl-N,N′-bis(1-naphtyl)-1,1-biphenyl-4,4 diamine (NPB) and tris(8-hydroxyquinoline)aluminium (Alq₃) shows below the built-in voltage, a step-like change from a value corresponding to the total organic layer thickness to a higher value given by the Alq₃ layer thickness. The bias and frequency dependent behaviour of the capacitance can be explained by the presence of negative charges with a density of −6×10¹¹ e/cm² at the NPB–Alq₃ interface. This leads to an inhomogeneous potential distribution inside the device with a discontinuity of the electric field at the organic–organic interface.     

The use of cross-linkable interlayers to improve device performances in blue polymer light-emitting diodes

High efficiency blue PLEDs were fabricated by adding a thin interlayer between PEDT:PSS and emitting polymer layers. Two different cross-linkable alkoxysilane-based interlayer materials, X-NPB and X-PDA, were synthesized based on N,N′-bis(4-methylphenyl)-N,N′-diphenyl-1,4-phenylenediamine (PDA) and N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1-biphenyl-4,4-diamine (NPB) which are well-known OLED HTLs. The devices, with configuration of indium tin oxide (ITO)/PEDT:PSS (65 nm)/interlayer (10–20 nm)/emitting polymer layer (70 nm)/BaF₂ (2 nm)/Ca (50 nm)/Al (300 nm), were fabricated by spin coating and thermal evaporation. In this device structure, the cross-linked X-NPB or X-PDA interlayers are more adherent and mechanically robust as well as impervious to spin coating of next emitting polymer layer. In addition, the devices with these interlayers exhibit a higher luminescence and current efficiency than those without interlayers because interlayers have two functions which are blocking electrons and preventing from severe quenching by PEDT:PSS.     

Highly efficient organic light-emitting diodes using novel hole-transporting materials

A new class of tetraminobiphenyl derivatives, which contains a 3,3′,5,5′-tetraminobiphenyl core, has been synthesized and examined as a hole-transporting material for organic light-emitting diodes. We fabricated the organic light-emitting diode (OLED) cells with tetraminobiphenyl derivatives as hole-injecting layer, hole-transporting layer and hole-injecting and transporting layer for green device with tris(8-quinolinolato)aluminum (Alq₃) doped with 1% of Coumarin 545T (C545T) as green emitting layer. Tetraminobiphenyls were found to be useful as a novel hole-transporting material. The electroluminescent device with the 3,3′,5,5′-tetrakis(p-tolyldiamino)biphenyl (TTAB) as a hole-transporting layer was more efficient than that with the analogous triarylamine, N,N′-di(naphthalene-1-yl)-N,N′-diphenyl benzidine (NPB). The external luminous efficiency of the device IV having TTAB as one hole-injecting and transporting layer can reach 14.55 cd/A, which is higher than the standard device I (11.66 cd/A) using two layers, a hole-injecting layer and a hole-transporting layer.     

9,9-Bis{4-[di-(p-biphenyl)aminophenyl]}fluorene: a high Tg and efficient hole-transporting material for electroluminescent devices

A promising fluorene derivative 9,9-bis{4-[di-(p-biphenyl)aminophenyl]}fluorene (BPAPF) of high glass-transition temperature (T_g=167 °C) was synthesized and assessed as the hole-transporting material (HTM) in electroluminescent devices. Devices of various configurations, such as single-heterojunction, with or without dopant, and double-heterojunction devices were made. Superior performance was observed for devices based on BPAPF as the HTM, relative to that based on NPB (4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl). In particular, with a device structure of ITO/BPAPF/Alq:0.5% quinacridone/Alq/Mg:Ag, a maximum luminance of ∼140,000 cd/m² was obtained at 15 V and maximum luminance and external quantum efficiencies of 13.7 cd/A and 4.1%, respectively, were obtained at 5.5 V.     

A novel 1,5-naphthylenediamine derivative as potential organic blue light-emitting material

A novel 1,5-naphthylenediamine derivative, 1,5-bis[N-(1-naphthyl)-N-phenyl]naphthalene diamine (NND), was designed and synthesized. This amine exhibited high glass transition temperature (T_g=127 °C) and hole transporting ability. The device with a structure of ITO/N,N′-bis(naphthalene-1-yl)-N,N′-diphenyl-benzidine(NPB)/NND/2-(4-tert-butylphenyl)-5-(4-biphenyl)-1,3,4-oxadiazole (PBD)/Mg:Ag was fabricated and bright blue light emission was obtained with a peak wavelength of 432 nm, and the color coordinate in CIE chromaticity is (0.172, 0.126). The brightness of 250 cd/m² at 14 V was achieved.     

Highly efficient blue electrophosphorescent devices with a novel host material

Highly efficient blue electrophosphorscent light emitting diodes with a new host material N,N′-dicarbazolyl-1,4-dimethene-benzene (DCB) were demonstrated. The energy transfer mechanism of the host–guest material system consisting of DCB and bis[(4,6-difluorophenyl)-pyridinato-N,C²′] (picolinato) Ir(III) (FIrpic) is an exothermic process. The device with a configuration of indium tin oxide/ N,N′-diphenyl-N,N′-bis(1,1′-biphenyl)-4,4′-diamine (NPB)/DCB:FIrpic/4,7-diphenyl-1,10-phenanthroline(BPhen)/Mg:Ag was optimized by adjusting the thickness of emitting layer and the dopant concentration. The device with the 8% (weight ratio) FIrpic and 30 nm emitting layer exhibits the maximum external quantum efficiency and current efficiency of 5.8% and 9.8 cd/A, respectively, at the luminance of 22 cd/m² driven at the voltage of 6.0 V.     

Efficient blue-to-violet organic light-emitting diodes

Organic light-emitting diodes emitting in the range of 400 nm (violet) to 460 nm (blue) are reported. The basic device structure consists of indium–tin oxide/N,N′-diphenyl-N,N′-bis-(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/lithium fluoride (LiF)/aluminum. Offset of the energy levels at the TPD/BCP interface favors blocking of holes on the TPD side of the interface. Voltage-induced color change is observed and explained in terms of a switching from emission dominated by interfacial exciplex-induced recombination at low applied bias to one dominated by bulk exciton-induced recombination at high applied bias. With the addition of copper(II) phthalocyanine (CuPc) as an anode buffer layer and tris-8-(hydroxyquinoline) aluminum (Alq₃) as a cathode buffer layer, external quantum efficiencies as high as 0.5% at blue emission and 0.4% at violet emission have been obtained.     

Role of mixed hole transport layer with exciton blocking properties in phosphorescent organic light-emitting diodes

Improved efficiency in green phosphorescent organic light-emitting diodes using a composite hole transport layer (HTL) with hole transport and exciton blocking function was investigated. Mixed layer of (4,4′-N,N′-dicarbazole)biphenyl (CBP) and N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine (NPB) was used as a HTL and quantum efficiency could be enhanced from 4.3% to 8.1% at 100 cd/m². Best performances could be obtained in the device with 50% CBP and 50% NPB in the HTL.     

Polypyridyl Ru(II)-sensitizers with extended π-system enhances the performance of dye sensitized solar cells

New polypyridyl ruthenium(II) complexes “cis-Ru(4,4′-dimesityl-2,2′-bipyridine) (Ln) (NCS)₂ H102” and “cis-Ru(4,4′-bis(2,3,6-tri-isopropylphenyl)-2,2′-bipyridine) (Ln) (NCS)₂ H105”, where Ln = 4,4′-dicarboxylic acid-2,2′-bipyridine; were synthesized and successfully applied to sensitization of nano-crystalline TiO₂ based solar cells (DSSCs). The DSSCs of H102 and H105 fabricated from 0.16 cm² TiO₂ electrodes exhibited broader comparable photocurrent action spectra with almost identical solar-to-electrical energy conversion efficiency (η) as compared to N719 sensitizer. The incident photon-to-current conversion efficiency (IPCE) values of 98% and 95% were obtained for H102 and H105 sensitizers respectively. Under 1 sun condition, η-values of 8.39% (short-circuit photocurrent (J_SC) = 16.4 mA/cm², open-circuit photo voltage (V_OC) = 692 mV, fill factor = 0.734), 8.76% (J_SC = 16.3 mA/cm², V_OC = 735 mV, fill factor = 0.734) and 9.12% (J_SC = 16.1 mA/cm², V_OC = 745 mV, fill factor = 0.753) were obtained for H102, H105 and N719 sensitizers respectively.     

Theoretical studies on electronic and electron blocking properties of iridium complexes with phenylpyrazolato ligands

The tris(1-phenylpyrazolato,N,C2′)iridium(III) Ir(ppz)₃, (fac-Ir(ppz)₃, 1; mer-Ir(ppz)₃, 2) and iridium(III)bis(1-phenylpyrazolato,N,C2′) (2,2,6,6-tetramethyl-3,5-heptane-dionato-O,O) ppz₂Ir(dpm) (C-cis,N-trans-ppz₂Ir(dpm), 3; C-cis,N-cis-ppz₂Ir(dpm), 4) have been investigated theoretically to explore their electronic structures, spectroscopic and electron blocking properties. A detailed comparison of the electronic structure characteristics of the two isomers has been addressed for pointing out differences in absorption and emission properties. The geometries and electronic structures are investigated at B3LYP and CIS levels for ground and excited states, respectively. At the TD-DFT and PCM levels, 1–4 give rise to absorptions at 329, 346, 355 and 347 nm, respectively, and phosphorescent emissions at 377, 461 and 405 nm for 1–3, respectively. The transitions of 1–2 are attributed to [d(Ir) + π(phenyl)] → [π*(pyrazolyl)] charge transition, whereas 3–4 are related to [d(Ir) + π(phenyl)] → [π*(pyrazolyl) + π*(dpm)]. The reorganization energies computed for hole (λ_hole) except 2 are smaller than that of N,N′-diphenyl-N,N′-bis(1,1′-biphenyl)-4,4′-diamine which is a typical hole transport material. Fac-Ir(ppz)₃ is the most efficient electron blocking material among the four complexes.     

Phenothiazine conjugated bipyridine as ancillary ligand in Ru(II)-complexes for application in dye sensitized solar cell

A new high molar extinction coefficient ruthenium(II)-bipyridine complex “cis-Ru(4,4′-bis((E)-2-(10-decyl-10H-phenothiazin-3-yl)vinyl)-2,2′-bipyridine)(4,4′-dicarboxylic acid-2,2′-bipyridine)(NCS)₂ PTZ1″ was synthesized through conjugation of phenothiazine unit with bipyridine and characterized by FT-IR, ¹H-NMR and ESI-MASS spectroscopes. Absorption measurements and time dependent-density functional theory (TD-DFT) calculations show increased spectral response for the ancillary ligand and the corresponding complex. The dye upon anchoring onto mesoporous nanocrystalline TiO₂ solar cells exhibited solar-to-electric energy conversion efficiency (η) of 3.77% short-circuit photocurrent density (J_SC) = 7.79 mA/cm², open-circuit voltage (V_OC) = 640 mV, fill factor = 0.750) under air mass 1.5 sunlight, the reference Z907 and HRS1sensitized solar cells, fabricated and evaluated under identical conditions exhibited η-value of 7.02% (J_SC = 15.25 mA/cm², V_OC = 650 mV, fill factor = 0.705) and 3.05% (J_SC = 8.20 mA/cm², V_OC = 610 mV, fill factor = 0.620) respectively. The lower film absorption of PTZ1on TiO₂ surface could be probably due to larger molecular diameter and planarity of phenothiazine prone to aggregate in solution as well as on TiO₂ surface. The DFT calculations show that the first three HOMOs of PTZ1 have t2g character as observed in case of Z907, while HOMO-4 and HOMO-5 have π-orbitals with major component on phenothiazine moieties of L1.     

Highly efficient energy transfer to a novel organic dye in OLED devices

The neutral 4,4-difluoro-8-(2,2′:6′,2″-terpyridine-4′-yl)-1,3,5,7-tetramethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene (Boditerpy) molecule was synthesized and incorporated as dopant (<1%) in double-layer organic light emitting diodes (OLEDs) with the configuration ITO/α-NPD(60 nm)/Alq₃(60 nm):Boditerpy (0.4 nm)/LiF(0.02 nm)/Al(80 nm). This device exhibits green emission with a brightness of 545 cd/m² at 8 V and a maximum power efficiency of 0.9 lm/W. A full quantitative energy transfer process is indicated by a complete quenching of light emission from Alq₃ in photoluminescence. However, I–V characteristics indicate some losses during the charge transfer processes in OLED configuration     

New host materials with high triplet energy level for blue-emitting electrophosphorescent device

A series of new 9-phenylcarbazole (Cz-Ph)-based host materials with 1,2,4-trizole (TAZ) were synthesized for blue-emitting electrophosphorescent device. The substitution position of Cz-Ph on TAZ ring did not influence photoluminescence maximum (band gap) but the triplet energy level (T₁). The electron mobility of 3,5-bis-(3-(9-carbazoyl)-phenyl)-4-(4-butyl-phenyl)-4H-[1,2,4]triazole (6) is 10 times higher than the hole mobility. This is due to the electron transporting/hole blocking characteristics of TAZ moiety. The triplet energy level of the new host materials ranges from 2.8 to 3.0 eV which are suitable for blue-emitting electrophosphorescent devices. The time-resolved photoluminescence decay curve of 4% of FIrpic (iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate) doped in the film of compound 6 showed a single exponential decay curve with a lifetime of 1.2 μs. The absolute PL quantum efficiency (η_PL) of 6 doped with 4% of FIrpic was (82 ± 2%), which is significantly higher than the case of commonly used CBP (4,4′-bis-(9-carbazoyl)-biphenyl) (44 ± 2%). These results also strongly support that triplet excitons formed in FIrpic was not transferred to 6. For a device based on 6 (ITO/PEDOT:PSS (40 nm)/NPB (15 nm)/6:6% FIrpic (30 nm)/BAlq (35 nm)/LiF (1 nm)/Al (100 nm), the maximum photometric efficiency was 14.2 cd/A at a current density of 1.1 mA/cm², which is higher than that observed with a device based on CBP (ITO/PEDOT:PSS (40 nm)/NPB (30 nm)/CBP:6% FIrpic (40 nm)/BAlq (30 nm)/LiF (1 nm)/Al (100 nm)).     

Doped and non-doped organic light-emitting diodes based on a yellow carbazole emitter into a blue-emitting matrix

A new carbazole derivative with a 3,3′-bicarbazyl core 6,6′-substituted by dicyanovinylene groups (6,6′-bis(1-(2,2′-dicyano)vinyl)-N,N′-dioctyl-3,3′-bicarbazyl; named (OcCz2CN)₂, was synthesized by carbonyl-methylene Knovenagel condensation, characterized and used as a component of multilayer organic light-emitting diodes (OLEDs). Due to its π-donor–acceptor type structure, (OcCz2CN)₂ was found to emit a yellow light at λ_max = 590 nm (with the CIE coordinates x = 0.51; y = 0.47) and was used either as a dopant or as an ultrathin layer in a blue-emitting matrix of 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi). DPVBi (OcCz2CN)₂-doped structure exhibited, at doping ratio of 1.4 weight %, a yellowish–green light with the CIE coordinates (x = 0.31; y = 0.51), an electroluminescence efficiency η_EL = 1.3 cd/A, an external quantum efficiency η_ext = 0.4 % and a luminance L = 127 cd/m² (at 10 mA/cm²) whereas for non-doped devices utilizing the carbazolic fluorophore as a thin neat layer, a warm white with CIE coordinates (x = 0.40; y = 0.43), η_EL = 2.0 cd/A, η_ext = 0.7%, L = 197 cd/m² (at 10 mA/cm²) and a color rendering index (CRI) of 74, were obtained. Electroluminescence performances of both the doped and non-doped devices were compared with those obtained with 5,6,11,12-tetraphenylnaphtacene (rubrene) taken as a reference of highly efficient yellow emitter.     

Highly efficient blue electroluminescence based on a new anthracene derivative

A novel blue-light-emitting material, 2,3,6,7-tetramethyl-9,10-dinaphthyl-anthracene (TMADN), was synthesized and characterized. Organic light-emitting diode (OLED), which has a double-layer structure, has been fabricated. In this OLED, the homemade TMADN was used as the light-emitting material and 4,7-diphenyl-1,10-phenanthroline (DPA) was used as the hole blocking/electron transporting material, N,N′-biphenyl-N,N′-bis-(1-naphenyl)-[1,1′-biphenyl]-4,4′-diamine (NPB) was used as the hole transporting material. The peak emission of electroluminescence (EL) is at about 456 nm and the CIE coordinates are (0.171, 0.228). The brightness of the device is up to 5600 cd/m² at 17 V with the maximum EL efficiency of 2.2 cd/A.     

Syntheses and characterization of new low-band gap polymers containing 4H-cyclopenta[def]phenanthrene unit and 4,7-di(thien-2-yl)-2H-benzimidazole-2-spirocyclohexane for photovoltaic device

Two new low-band gap polymers, poly[(2,6-(4,4-bis(2′-ethylhexyl)-4H-cyclopenta[def]phenanthrene))-alt-(5,5-(4′,7′-di(thien-2-yl)-2H-benzimidazole-2′-spirocyclohexane))] (PCPP-DTCHBI) and poly[(2,6-(4,4-bis(4-((2-ethylhexyl)oxy)phenyl)-4H-cyclopenta[def]phenanthrene))-alt-(5,5-(4′,7′-di(thien-2-yl)-2H-benzimidazole-2′-spirocyclohexane))] (PBEHPCPP-DTCHBI), were synthesized and characterized for the photovoltaics. These polymers showed typical characteristics of low-band gap polymers through the internal charge transfer (ICT) between 4H-cyclopenta[def]phenanthrene as the electron-rich unit and di(thien-2-yl)-2H-benzimidazole-2′-spirocyclohexane as the electron-deficient unit. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are ?5.52 and ?3.82 eV for PCPP-DTCHBI, and ?5.36 and ?3.76 eV for PBEHPCPP-DTCHBI, respectively. Optical band gaps of PCPP-DTCHBI and PBEHPCPP-DTCHBI are 1.70 and 1.60 eV, respectively. As compared to the case of poly[(2,6-(4,4-bis(2′-ethylhexyl)-4H-cyclopenta[def]phenanthrene))-alt-(5,5-(4′,7′-di(thien-2-yl)-2,1,3-benzothiadiazole))] (PCPP-DTBT), PCPP-DTCHBI shows deeper HOMO energy levels by 0.12 eV and lower band gap by 0.3 eV. The FET mobilities of PCPP-DTCHBI and PBEHPCPP-DTCHBI are 1.19 × 10^?4 and 5.11 × 10^?5 cm²/V s, respectively, and the power conversion efficiencies of the solar cell devices of the PCPP-DTCHBI and PBEHPCPP-DTCHBI blended with [6,6]phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) are 1.01% and 0.53%, respectively. The newly designed DTCHBI unit can be used as the electron-deficient moiety inducing efficient ICT for low band gap generation while keeping deep HOMO energy level of the polymer.     

Polymer light-emitting diodes with a phenyleneethynylene derivative as a novel hole blocking layer for efficiency enhancements

This paper reports on the use of an electron transport layer (ETL) in polymer light-emitting diodes based on poly(2,5-bis(3′,7′-dimethyl-octyloxy)1,4-phenylene-vinylene) (BDMO-PPV). This ETL is inserted between BDMO-PPV and a calcium cathode as a hole blocking layer (HBL). A novel phenyleneethynylene derivative (ImPE) is proposed and compared to well-known materials such as tris(8-hydroxyquinoline) aluminum (Alq₃) and bathocuproïne (BCP). Efficient hole blocking is achieved leading to yield improvements at low luminances. With a 8 nm-thick ImPE layer, at 1 cd/m², the power efficiency reaches 1.2 lm/W whereas a BDMO-PPV-only PLED exhibits a 0.13 lm/W power efficiency. ImPE enables to reach higher performances than Alq₃ for low luminances (<20 cd/m²). However, for luminances higher than 350 cd/m², it is demonstrated that the hole blocking in no more efficient because of a too strong electric field.