Fluorene-based cathode interlayer polymers for high performance solution processed organic optoelectronic devices

A hydrophilic polyfluorene-based conjugated polyelectrolyte (CPE) Poly[9,9-bis(4′-(6″-(diethanolamino)hexyloxy) phenyl)fluorene], PPFN-OH (Scheme 1) has been synthesized and utilized as cathode interlayer for both polymer light emitting diodes (PLEDs) and solar cells (PSCs). For comparison, another CPE namely Poly[9,9-bis(6′-(diethanolamino)hexyl)fluorene] (PFN-OH) has also been investigated. They comprise the same polyfluorene backbone structures with, respectively, diethanolaminohexyl (PFN-OH) and diethanolaminohexoxyphenyl (PPFN-OH) substituents attached to the C9 carbon of the fluorene repeat unit. In comparison to reference devices with more reactive Ca/Al cathodes, utilizing these CPEs as interlayers allowed an Al cathode to be used for blue light emission PLEDs, yielding 51% and 92% enhancement of maximum luminous efficiency (LE) for PFN-OH and PPFN-OH, respectively. The PLEDs with PPFN-OH showed both higher maximum LE and maximum luminance (L) (LE = 2.53 cd/A at 6.2 V, L = 9917 cd/m² at 8.3 V) than devices with PFN-OH (2.00 cd/A at 4.1 V, 3237 cd/m² at 7.2 V). The PPFN-OH PLEDs also showed no significant roll-off in efficiency with increasing current density up to 400 mA/cm², indicating excellent electron injection ability and stability for this interlayer. The insertion of alkoxy-phenyl groups at the C9-position in PPFN-OH is clearly advantageous. This simple modification significantly improves the CPE cathode interlayer performance. Parallel investigations of the electron extraction properties of PPFN-OH in inverted architecture PSCs with PCDTBT:PC₇₀BM bulk heterojunction active layers demonstrated a power conversion efficiency enhancement of ∼19% (from 4.99% to 5.95%) for indium tin oxide cathode devices compared with reference devices using Ca/Al cathodes. These results confirm PPFN-OH to be a promising interlayer material for high performance solution processed organic optoelectronic devices.     

Alcohol-soluble polyfluorenes containing dibenzothiophene-S,S-dioxide segments for cathode interfacial layer in PLEDs and PSCs

A series of alcohol-soluble amino-functionalized polyfluorene derivatives (PF-N-S, PF-N-SC8 and PF-N-SOC8) comprising various ratios of dibenzothiophene-S,S-dioxide segments (S/SC8/SOC8) in the main chains, respectively, were synthesized and utilized as cathode interfacial layer (CIL) in polymer light-emitting diodes (PLEDs) and polymer solar cells (PSCs) with high-work-function Al (or Au) electrode. The polymers possess LUMO/HOMO levels at −2.78 to −3.53 eV/−5.69 to −6.32 eV. Multilayer PLEDs and PSCs with device configurations of ITO/PEDOT:PSS (40 nm)/P-PPV or PFO-DBT35:PCBM = 1:2 (80 nm)/CIL (3–15 nm)/Al (or Au) (100 nm) were fabricated. The PF-N-S-10/Al (or Au) cathode PLEDs displayed maximum luminous efficiency of 24.4 cd A⁻¹ (or 11.9 cd A⁻¹), significantly higher than bare Al (or Au) cathode device, exceeding well-known Ba/Al and poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN)/Al (or PFN/Au) cathode devices. The enhanced open-circuit voltages (V_ocs), electron reflux and reduced work functions clarify that the electron injection barrier from the Al (or Au) electrode can be lowered by inserting the polymers as CIL. The resulted PSCs also show device performances exceeding Al and PFN/Al cathode devices. The results indicate that PF-N-S, PF-N-SC8 and PF-N-SOC8 are excellent CIL materials for PLEDs and PSCs with high-work-function Al or Au electrode.     

Fine tuning hole injection for high-performance polyfluorene-based blue emitting device by adjusting work function of anode via deposition of CFx layer with proper ionization potential on indium tin oxide

By introducing CF_x thin film as hole injection layer on top of indium tin oxide (ITO) anode via plasma polymerization of CHF₃, the device with poly(9,9-dioctylfluorene) (PFO) as emitting layer, ITO/CF_x(35 W)/PFO/CsF/Ca/Al, is prepared. At the optimal C/F atom ratio using the radio frequency power 35 W, the device performance is optimal having the maximum current efficiency 3.1 cd/A and maximum brightness 8400 cd/m². This is attributed to a better balance between hole and electron fluxes, resulting from a decrease in hole injection barrier as manifested by ultraviolet photoelectron spectroscopy and scanning surface potential microscopy.     

Single layer deep blue polymer light emitting diodes with chlorinated Indium Tin Oxide after surface modification for high performance

Chlorinated-Indium Tin Oxide (Cl-ITO) has been found to have higher work function than the pristine ITO. When used as anode for polymer light-emitting diodes (PLEDs) with deep blue emitting β-phase poly(9,9-di-n-octylfluorene) (β-PFO) as the emitting layer, it allows hole injection without barrier. However, the presence of chlorine free radical on the surface of Cl-ITO leads to an exciton quenching effect. The surface modification on anode by dipping into 1% ammonium (NH₄OH) aqueous solution to remove free chlorine radicals on surface can enhance device performance significantly. The single layer device Cl-ITO (treated)/β-PFO/CsF/Al gives the maximum brightness 16773 cd/m² and maximum luminance efficiency 2.40 cd/A, which are higher than those with untreated Cl-ITO, (2519; 0.27) and CFx treated ITO, (7800; 1.8) and PEDOT:PSS coated ITO, (12884; 1.43). The ease of the present anode treatment allows the one-layer-only device to be a promising candidate for practical application.     

Role of LiF in polymer light-emitting diodes with LiF-modified cathodes

We report on high-efficiency polymer light-emitting diodes (PLEDs) based on poly [2-methoxy-5-(3′,7′-dimethyloctyloxyl)]-1,4-phenylene vinylene (OC1C10) with LiF-modified cathodes. Devices with different cathodes are made and characterized by the electroabsorption technique to measure their built-in voltage. Devices with a LiF/Al bilayer cathode or a LiF:Al composite cathode, all show significantly improved performance as compared to those with bare Al cathodes. The improvement is correlated with enhanced electron injection due to a decrease of the electron injection barrier, which is also indicated by the electroabsorption measurements. The same effect is also observed with LiF(0.6 nm)/Mg cathodes. However, inserting the same LiF thin film between Ag and OC1C10 does not improve the device performance. Cathodes composed of ultra-thin films of LiF(0.6 nm)/Al(1 nm) or LiF:Al(2 nm) covered by Ag (100 nm) show the same performance as LiF(0.6 nm)/Al bilayer cathode or a LiF:Al composite cathode, indicating that the enhancement is specific to LiF and Al. Our experiments can be explained by assuming that Li-ions can dissociate from LiF and diffuse into the OC1C10 layer, leading to an n-type zone close to the polymer/cathode interface. This n-doped layer at the interface facilitates electron injection at the cathode/polymer interface and eventually leads to the formation of an Ohmic contact.     

Improved electron injection from silver electrode for all solution-processed polymer light-emitting diodes with Cs2CO3:conjugated polyelectrolyte blended interfacial layer

This study demonstrates the incorporation of a Cs₂CO₃:conjugated polyelectrolyte blended interfacial layer between the emissive layer and a silver (Ag) cathode, for realizing all-solution processed polymer light-emitting diodes. For a device with poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) as the emissive layer, this approach improves the maximum luminance of approximately 80,000 cd/m² and maximum current efficiency of 10.6 cd/A. It is clarified that the interfacial layer prevents Ag nanoparticles from penetrating into the emissive layer, resulting in yellow–green emission from F8BT. We also demonstrate the possibility of all-solution processed polymer light-emitting diodes utilizing solution-processed Cs₂CO₃:conjugated polyelectrolyte interfacial layer and Ag nano-ink.     

Optoelectronic characteristics of MEH-PPV polymer LEDs with thin,doped composition-graded a-SiC:H carrier injection layers

The optoelectronic characteristics of poly(2-methoxy-5-(2′ethyl-hexoxy)-1,4-phenylene-vinylene) (MEH-PPV) polymer LEDs (PLEDs) have been improved by employing thin doped composition-graded (CG) hydrogenated amorphous silicon–carbide (a-SiC:H) films as carrier injection layers and O₂-plasma treatment on indium–tin-oxide (ITO) transparent electrode, as compared with previously reported ones having doped constant-optical-gap a-SiC:H carrier injection layers. For PLEDs with an n-type a-SiC:H electron injection layer (EIL) only, the electroluminescence (EL) threshold voltage and brightness were improved from 7.3 V, 3162 cd/m² to 6.3 V, 5829 cd/m² (at a current density J = 0.6 A/cm²), respectively, by using the CG technique. The enhancement of EL performance of the CG technique was due to the increased electron injection efficiency resulting from a smoother barrier and reduced recombination of charge carriers at the EIL and MEH-PPV interface. Also, surface modification of the ITO transparent electrode by O₂-plasma treatment was used to further improve the EL threshold voltage and brightness of this PLED to 5.1 V, 6250 cd/m² (at J = 0.6 A/cm²). Furthermore, by employing the CG n[p]-a-SiC:H film as EIL [hole injection layer (HIL)] and O₂-plasma treatment on the ITO electrode, the brightness of PLEDs could be enhanced to 9350 cd/m² (at a J = 0.3 A/cm²), as compared with the 6450 cd/m² obtained from a previously reported PLED with a constant-optical-gap n-a-SiCGe:H EIL and p-a-Si:H HIL.     

Hydroxyethyl cellulose filled with M2+ chelate complexes with ethylenediaminetetraacetic acid (EDTA) as an effective electron-injection layer for polymer light-emitting diodes

An effective electron-injection layer (EIL) is crucial to the development of highly efficient polymer light-emitting diodes (PLEDs) using stable, high work-function aluminium as the cathode. This work presents the first investigation using hydroxyethyl cellulose (HEC), filled with chelate complexes [(CH₃COO)₂-M, EDTA-M; M: Ca²⁺, Mg²⁺], as an electron-injection layer (EIL) to fabricate multilayer polymer light-emitting diodes (ITO/PEDO:PSS/HY-PPV/EIL/Al) by spin-coating processes. Devices based on HEC doped with EDTA-M provided the best performance. The maximum luminance and maximum current efficiency of polymer light-emitting diodes with EDTA-Ca in an HEC layer were 7502 cd/m² and 2.85 cd/A, respectively, whereas those with EDTA-Mg were 8443 cd/m² and 3.12 cd/A, which was approximately seven- to eight-fold of that without EIL. This performance enhancement was attributed to electron donation from the chelator that reduces metal cations to a “pseudo-metallic state”, enabling it to act as an intermediate step to facilitate electron injection. The results demonstrate that chelates of bivalent cations with EDTA can potentially serve as electron-injection materials for optoelectronic applications.     

High performance top-emitting and transparent white organic light-emitting diodes based on Al/Cu/TcTa transparent electrodes for active matrix displays and lighting applications

It is challenging to obtain broadband emission covering as much of the visible light spectrum as possible in top-emitting white organic light-emitting diodes (TEWOLEDs) due to the well known microcavity effects. In this work, we achieved TEWOLED with three separate peak and negligible angular dependence by employing a high transmittance stack cathode Al (2 nm)/Cu (18)/TcTa (60 nm). The TEWOLED shows an efficiency of 25.6 cd/A, 20.1 Lm/W at 1000 cd/m², and low voltage of 4.2 V for 1222 cd/m². Synchronously, we achieved transparent white organic light-emitting diode (TWOLED) using this high transmittance stack cathode, the TWOLED exhibits similar spectrum and comparable luminance from both sides, and the maximum total efficiencies of the TWOLED are 28.6 cd/A, 24.9 Lm/W.     

All-small-molecule efficient white organic light-emitting diodes by multi-layer blade coating

Blue and white small-molecule organic light-emitting diodes are fabricated by multi-layer blade coating on hot plate at 80 °C with hot wind. Uniform multi-layer structures are made without dissolution due to rapid drying. Only small molecules originally developed for vacuum deposition are used. For hole transport layer of, 4′,4″-tris(carbazol-9-yl)triphenylamine (TCTA), electron transport layer of 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TBPI), emissive layer host of, 6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy), triplet emitters of bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(III) (FIrpic), and cathode of LiF/Al, the peak current efficiency for blue emission is 25.1 cd/A (10.8% and 9.3 lm/W). Orange emitter iridium(III)bis (4-(4-t-butylphenyl) thieno[3,2-c]pyridinato-N,C2′)acetylacetonate (PO-01-TB) is added to obtain white emission with CIE coordinate of (0.39, 0.46) [1]. The current efficiency is 34.2 cd/A (11.6% and 12 lm/W) at maximum, 32.4 cd/A at 1000 cd/m², and 31 cd/A at 10,000 cd/m².     

Carrier transport of inverted quantum dot LED with PEIE polymer

An inverted-type quantum-dot light-emitting-diode (QD LED), employing low-work function organic material polyethylenimine ethoxylated (PEIE) as electron injection layer, was fabricated by all solution processing method, excluding anode electrode. From transmission electron microscopy (TEM) and scanning electron microscopy (SEM) studies, it was confirmed that CdSe@ZnS QDs with 7 nm size were uniformly distributed as a monolayer on PEIE layer. In this inverted QD LED, two kinds of hybrid organic materials, [poly (9,9-di-n-octyl-fluorene-alt-benzothiadiazolo)(F8BT) + poly(N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine (poly-TPD)] and [4,4′-N,N′-dicarbazole-biphenyl (CBP) + poly-TPD], were adopted as hole transport layer having high highest occupied molecular orbital (HOMO) level for improving hole transport ability. At a low-operating voltage of 8 V, the device emits orange and red spectral radiation with high brightness up to 2450 and 1420 cd/m², and luminance efficacy of 1.4 cd/A and 0.89 cd/A, respectively, at 7 V applied bias. Also, the carrier transport mechanisms for the QD LEDs are described by using several models to fit the experimental I–V data.     

Enhanced pure red electroluminescence intensity of Eu(o-BBA)3(phen) by red dye co-doping and inorganic semiconductor as charge carrier function layer

There is an emission peak at 494 nm in the electroluminescence (EL) of PVK [poly(n-vinylcarbazole)]: Eu(o-BBA)₃(phen) besides PVK exciton emission and Eu³⁺ characteristic emissions. Both the peaking at 494 nm emission and PVK emission influenced the color purity of red emission from Eu(o-BBA)₃(phen). In order to restrain these emissions and obtain high intensity red emission, 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7,-tetramethyljulolidy-9-enyl)-4Hpyran (DCJTB) and Eu(o-BBA)₃(phen) were co-doped in PVK solution and used as the active emission layer. The EL intensity of co-doped devices reached to 420 cd/m² at 20 V driving voltage. The chromaticity coordinates of EL was invariable (x = 0.55, y = 0.36) with the increase of driving voltage. For further improvement of EL intensity, organic–inorganic hybrid devices (ITO/active emission layer/ZnS/Al) were fabricated. The EL intensity was increased by a factor of 2.5 [(420 cd/m²)/(168 cd/m²)] when the Eu complex was doped with an efficient dye DCJTB, and by a factor of ≈4 [(650 cd/m²)/(168 cd/m²)] when in addition ZnS layer was deposited on such an emitting layer prior to evaporation of the Al cathode.     

Efficient solution-processed electrophosphorescent devices using ionic iridium complexes as the dopants

Efficient solution-processed electrophosphorescent devices using two blue-emitting ionic iridium complexes (complex 1 and complex 2) were fabricated, with poly(N-vinylcarbazole) (PVK):1,3-bis(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)benzene (OXD-7) as the host and Cs₂CO₃/Al as the cathode. Using complex 1 as the dopant, we obtained efficient blue-green electrophosphorescence from single-layer devices with a maximum efficiency of 12.2 cd A^?1, a maximum brightness of 12,600 cd m^?2 and CIE (Commission Internationale de l’Éclairage) coordinates of (0.19, 0.45). And the maximum efficiency of the device based on complex 1 can be further improved to 20.2 cd A^?1, when a thin 1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzene (TPBI) layer was inserted between the light-emitting layer and the cathode. Using complex 2 as the dopant, we obtained deep-blue electrophosphorescence with the emission peak at 458 nm and CIE coordinates of (0.16, 0.22). Our work suggests that ionic iridium complexes are promising phosphors for obtaining efficient electrophosphorescence in the blue region.     

White emission polymer light-emitting devices with efficient electron injection from alcohol/water-soluble polymer/Al bilayer cathode

We demonstrate highly efficient white emission polymer light-emitting diodes (WPLEDs) from multilayer structure formed by solution processed technique, in which alcohol/water-soluble polymer, poly [(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) was incorporated as electron-injection layer and Al as cathode. It was found that the device performance was very sensitive to the solvents from solution of which the PFN electron-injection layer was cast. Devices with electron-injection layer cast from methanol solution show degraded performance while the best device performance was obtained when mixed solvent of water and methanol with ratio of 1:3 was used. We attribute the variation in device performance to washing out the electron transport material in the emissive layer due to rinse effect. As a result of alleviative loss of electron transport material in the emissive layer, the optimized device with a peak luminous efficiency of 18.5 cd A^?1 for forward-viewing was achieved, which is comparable to that of the device with same emissive layer but with low work-function metal Ba cathode (16.6 cd A^?1). White emission color with Commission International de I’Eclairage coordinates of (0.321, 0.345) at current 10 mA cm^?2 was observed.     

Deep blue light-emitting diode based on high molecular weight poly(9,9-dioctylfluorene) with high efficiency and color stability

A highly efficient deep blue polymer light-emitting diode based on poly(9,9-dioctylfluorene) is demonstrated. The performance is found to increase significantly with the molecular weight. Two different molecular weights are compared, one is 71,000 and the other is 365,000. The electroluminescent efficiency and color stability are improved by slightly doping hole traps into the emission layer and bilayer structure. The maximum efficiency is 3.8 cd/A with the corresponding external quantum efficiency of 3.7% at deep blue with Commission Internationale de L’Eclairage (CIE) coordinate at (0.15, 0.09). Stable blue emission is maintained up to 6600 cd/m² without growth of green shoulder in emission spectrum.     

Color stable phosphorescent white organic light-emitting diodes with double emissive layer structure

In this paper, we report color stable phosphorescent white organic light-emitting diodes (OLEDs) based on a double emissive layer (EML) structure composed of blue and red/green phosphorescent units. Deep hole trapping situation of red and green dopants at the red/green EML could induce less voltage dependent white spectral characteristics by restricting the change of exciton generation zone. A wide band-gap host material, 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy), was used for achieving such deep-trap generation. Fabricated phosphorescent white OLED shows a slight color coordinate change of (?0.002, +0.002) from 1000 cd/m² to 5000 cd/m² with power efficiency of 38.7 lm/W and current efficiency of 46.4 cd/A at 1000 cd/m². In addition, negligible color changes were observed by delaying red dopant saturation time using optimum red dopant concentration.     

Efficient simplified orange and white phosphorescent organic light-emitting devices with reduced efficiency roll-off

Efficient orange phosphorescent organic light-emitting devices based on simplified structure with maximum efficiencies of 46.5 lm/W and 51.5 cd/A were reported. One device had extremely low efficiency roll-off with efficiencies of 50.6 cd/A, 45.0 cd/A and 39.2 cd/A at 1000 cd/m², 5000 cd/m² and 10,000 cd/m² respectively. The reduced efficiency roll-off was attributed to more balanced carrier injection and broader recombination zone. The designed simplified white device showed much lower efficiency roll-off than the control one based on multiple emitting layers. The efficiency of simplified white device was 40.8 cd/A at 1000 cd/m² with Commission Internationale de I’Eclairage coordinates of (0.39, 0.46).     

New hole blocking material for green-emitting phosphorescent organic electroluminescent devices

The new amorphous molecular material, 2,5-bis(4-triphenylsilanyl-phenyl)-[1,3,4]oxadiazole, that functions as good hole blocker as well as electron transporting layer in the phosphorescent devices. The obtained material forms homogeneous and stable amorphous film. The new synthesized showed the reversible cathodic reduction for hole blocking material and the low reduction potential for electron transporting material in organic electroluminescent (EL) devices. The fabricated devices exhibited high performance with high current efficiency and power efficiency of 45 cd/A and 17.7 lm/W in 10 mA/cm², which is superior to the result of the device using BAlq (current efficiency: 31.5 cd/A and power efficiency: 13.5 lm/W in 10 mA/cm²) as well-known hole blocker. The ITO/DNTPD/α-NPD/6% Ir(ppy)₃ doped CBP/2,5-bis(4-triphenylsilanyl-phenyl)-[1,3,4]oxadiazole as both hole blocking and electron transporting layer/Al device showed efficiency of 45 cd/A and maximum brightness of 3000 cd/m² in 10 mA/cm².     

Work function modification of solution-processed tungsten oxide for a hole-injection layer of polymer light-emitting diodes

We report a solution-processed tungsten oxide hole injection layer for polymer light-emitting diodes (PLEDs). Unlike vacuum evaporated tungsten oxide, solution-processed tungsten oxide has a reduced work function due to contamination of the ambient atmosphere. To increase the work function of tungsten oxide layer, an organic ionic solution containing poly(ethylene oxide) and tetra-n-methylammonium tetrafluoroborate was coated on the tungsten oxide layer. Organic ions infiltrated into the tungsten oxide layer and formed an interfacial dipole, increasing the layer’s work function by 0.4 eV. As a result, a PLED, which had a hole-injection layer of the tungsten oxide and organic ionic solution, showed a performance equivalent to that of the PEDOT:PSS-based device with even higher maximum luminance of 27,560 cd/m² at 10 V.     

Bright and efficient white stacked organic light-emitting diodes

High brightness and efficient white stacked organic light-emitting diodes have been fabricated by connecting individual blue and red emissive units with the anode–cathode layer (ACL) consisting of LiF (1 nm)/Ca (25 nm)/Ag (15 nm). Use 1,3-bis(carbazol-9-yl)benzene (mCP):bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl) iridium III (FirPic) as the blue emitter and tris(8-hydroxy-quinolinato)aluminium (Alq₃):4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran (DCJTB) as the red emitter, white light emission with CIE coordinates of (0.32, 0.38) was obtained at a driving voltage of 26 V with a luminance of 40,000 cd/m². By replacing the red fluorescent emitter with a phosphorescent one, the color coordinates were improved to (0.33, 0.31). The peak external quantum efficiency was enhanced from 5.3% (at 28.2 mA/cm²) to 10.5% (at 1.4 mA/cm²) as well.