Influence of plasma treatment of ITO surface on the growth and properties of hole transport layer and the device performance of OLEDs

Surface energy of indium tin oxide (ITO) surfaces treated by different plasmas, including argon (Ar–P), hydrogen (H₂–P), carbon tetrafluoride (CF₄–P), and oxygen (O₂–P), was measured and analyzed. The initial growth mode of hole transport layers (HTLs) was investigated by atomic force microscope observation of thermally deposited 2 nm thick N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) on the plasma treated ITO surfaces. The results show that different plasma treatments of ITO influence the growth of HTLs in significantly different ways through the modification of surface energy, especially the polar component. The O₂–P and CF₄–P were found to be most effective in enhancing surface polarity through decontamination and increased dipoles, leading to more uniform and denser nucleation of NPB on the treated ITO surfaces. It was further found that increased density of nucleation sites resulted in a decreased driving voltage of OLEDs. Under the same fabricating conditions, a lowest driving voltage of 4.1 V was measured at a luminance of 200 cd/m² for the samples treated in CF₄–P, followed by the samples treated in O₂–P (5.6 V), Ar–P (6.4 V), as-clean (7.0 V) and H₂–P (7.2 V) plasma, respectively. The mechanisms behind the improved performance were proposed and discussed.     

Transparent Al/WO3/Au film as anode for high efficiency organic light-emitting diodes

A transparent Al/WO₃/Au anode is introduced to fabricate high efficiency organic light-emitting devices (OLEDs). By optimizing the thicknesses of each layers of the Al/WO₃/Au structure, the transmittance of Al(7 nm)/WO₃(3 nm)/Au(13 nm) has reached over 55%. Concerning the performance of OLEDs using the optimized anode, the electroluminescence (EL) current efficiency and brightness are enhanced and the EL spectrum is greatly narrowed as compared to the OLEDs using indium-tin-oxide (ITO) as the anode. The results indicate that the metal/metal oxide/metal transparent electrode is a good structure for the anode of high performance OLEDs. In addition, Al/WO₃/Au can function as a composite transparent electrode for top-emitting OLEDs.     

Orange phosphorescent organic light-emitting diodes with high operational stability

In this article we report on the performances of phosphorescent orange organic light-emitting diodes (OLEDs) having a high operational stability. The fabricated devices all consist of a “hybrid” structure, where the hole-injection layer was processed from solution, while the rest of the organic materials were deposited by vacuum thermal evaporation. A device stack having an emissive layer comprising a carbazole-based host TCzMe doped with the orange phosphor tris(2-phenylquinoline)iridium(III) [Ir(2-phq)₃] shows improved efficiencies compared to a the same device with the standard N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB) as host material. External quantum efficiency (EQE) up to 7.4% and a power efficiency of 16 lm/W were demonstrated using TCzMe. Most importantly, the operational stability of the device was largely improved, resulting in extrapolated values reaching lifetimes well above 100,000 h at initial luminance of 1000 Cd/m².     

Influence of aluminium electrode preparation on PCE values of polymeric solar cells based on P3HT and PCBM

The main goal of the paper was investigation of influence of aluminum electrode preparation via thermal evaporation (TE) and the magnetron sputtering (MS) on power conversion efficiency (PCE) of polymeric solar cells. The photovoltaic properties of such three kinds devices based on poly(3-hexylthiophene-2,5-diyl) (P3HT) as ITO/P3HT/Al, ITO/P3HT:PCBM (1:1, w/w)/Al and ITO/PEDOT:PSS/P3HT:PCBM (1:1, w/w)/Al were investigated. For the constructed devices impedance spectroscopy were analyzed. For devices lack of PEDOT:PSS layer or lack of PCBM, photovoltaic parameters were very low and similar to the parameters obtained for device with Al electrode prepared by magnetron sputtering. The devices comprising PEDOT:PSS with P3HT:PCBM showed the best photovoltaic parameters such as a V_OC of 0.60 V, J_SC of 4.61 mA/cm², FF of 0.21, and PCE of 5.7 × 10^?1%.     

Bistable electrical switching and nonvolatile memory effect in mixed composite of oxadiazole acceptor and carbazole donor

Bistable electrical switching and nonvolatile memory devices with the configuration of indium tin oxide (ITO)/active layer/aluminum (Al) are reported. The active layer were prepared from the mixed compositions of 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole, (PBD) and poly(N-vinylcarbazole) (PVK). The as-fabricated ITO/PBD:PVK/Al sandwiched devices exhibited rewriteable flash memory property. Due to the strong interaction between oxadiazole acceptor and carbazole donor, the devices demonstrate excellent performance. The memory devices can operate over a small voltage range, the absolute value of switching-on threshold voltage is less than 1 V and the switching-off threshold voltage is less than 3.5 V. The ON/OFF ratio of current switches in the range of 10⁴–10² during the variation of applied voltage and the two different resistance states can be maintained over 4 h.     

A novel dimesitylboron-substituted indolo[3,2-b]carbazole derivative: Synthesis,electrochemical, photoluminescent and electroluminescent properties

A novel indolo[3,2-b]carbazole derivative containing B(Mes)₂ groups, 5,11-dibutyl-2,8-bis(dimesitylboryl) indolo[3,2-b]carbazole (DBDMBICZ), was synthesized and structurally characterized by elemental analysis, NMR, MS. The thermal, electrochemical and photophysical properties of DBDMBICZ were characterized by thermogravimetric analysis, electrochemical methods, UV–vis absorption spectroscopy and fluorescence spectroscopy. DBDMBICZ exhibited high fluorescence quantum yields (Φ_max = 0.76) in solution and excellent thermal stability (T_d = 290 °C, T_g = 170 °C) and electrochemical stability. The multi-layered OLEDs devices with the configuration of ITO/NPB/CBP/light-emitting layer/Bphen/LiF/Al are fabricated by using DBDMBICZ as light-emitting layer. The devices show the same pure blue emissions at different voltages and relative good electroluminescent performances. The results indicate that DBDMBICZ has potential applications as an excellent optoelectronic material in optical field.     

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.     

Blue light-emitting OLED using new spiro[fluorene-7,9′-benzofluorene] host and dopant materials

A new spiro-type compound, 2-(10-biphenylanthracene)-spiro[fluorene-7,9′-benzofluorene] (BH-3B) containing anthracene moiety was prepared for the blue host material. Also new dopant materials, 2-[4′-(phenyl-4-vinylbenzeneamine)phenyl-spiro[fluorene-7,9′-benzofluorene] (BH-3BD) and 4-[2-naphthyl-4′(phenyl-4-vinylbenzeneamine)]phenyl (BD-1N) were successfully synthesized and a blue OLEDs were made from them. The structure of the device was as follows; ITO/DNTPD/α-NPD/Host:5% dopant/Alq₃/Al-LiF. Among all of the devices, the device obtained from BH-3B host doped with 5% BH-3BD showed the best electroluminescence characteristics. The emission peak of EL is at 456 nm and the CIE value is (0.15, 0.14). The brightness of the device is up to 5407 cd/m² at 10 V with the maximum EL efficiency of 3.4 cd/A.     

Deep blue/ultraviolet microcavity OLEDs based on solution-processed PVK:CBP blends

There is an increasing need to develop stable, high-intensity, efficient OLEDs in the deep blue and UV. Applications include blue pixels for displays and tunable narrow solid-state UV sources for sensing, diagnostics, and development of a wide band spectrometer-on-a-chip. With the aim of developing such OLEDs we demonstrate an array of deep blue to near UV tunable microcavity (μc) OLEDs (λ ∼373–469 nm) using, in a unique approach, a mixed emitting layer (EML) of poly(N-vinyl carbazole) (PVK) and 4,4′-bis(9-carbazolyl)-biphenyl (CBP), whose ITO-based devices show a broad electroluminescence (EL) in the wavelength range of interest. This 373–469 nm band expands the 493–640 nm range previously attained with μcOLEDs into the desired deep blue-to-near UV range. Moreover, the current work highlights interesting characteristics of the complexity of mixed EML emission in combinatorial 2-d μcOLED arrays of the structure 40 nm Ag/x  nm MoO_x/∼30 nm PVK:CBP (3:1 weight ratio)/y  nm 4,7-diphenyl-1,10-phenanthroline (BPhen)/1 nm LiF/100 nm Al, where x = 5, 10, 15, and 20 nm and y = 10, 15, 20, and 30 nm. In the short wavelength μc devices, only CBP emission was observed, while in the long wavelength μc devices the emission from both PVK and CBP was evident. To understand this behavior simulations based on the scattering matrix method, were performed. The source profile of the EML was extracted from the measured EL of ITO-based devices. The calculated μc spectra indeed indicated that in the thinner, short wavelength devices the emission is primarily from CBP; in the thicker devices both CBP and PVK contribute to the EL. This situation is due to the effect of the optical cavity length on the relative contributions of PVK and CBP EL through a change in the wavelength-dependent emission rate, which was not suggested previously. Structural analysis of the EML and the preceding MoO_x layer complemented the data analysis.     

Green and blue-green phosphorescent heteroleptic iridium complexes containing carbazole-functionalized β-diketonate for non-doped organic light-emitting diodes

Two novel iridium complexes both containing carbazole-functionalized β-diketonate, Ir(ppy)₂(CBDK) [bis(2-phenylpyridinato-N,C²)iridium(1-(carbazol-9-yl)-5,5-dimethylhexane-2,4-diketonate)], Ir(dfppy)₂(CBDK) [bis(2-(2,4-difluorophenyl)pyridinato-N,C²)iridium(1-(carbazol-9-yl)-5,5-dimethylhexane-2,4-diketonate)] and two reported complexes, Ir(ppy)₂(acac) (acac = acetylacetonate), Ir(dfppy)₂(acac) were synthesized and characterized. The electrophosphorescent properties of non-doped device using the four complexes as emitter, respectively, with a configuration of ITO/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-diphenyl-4,4′-diamine (NPB) (20 nm)/iridium complex (20 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (5 nm)/tris(8-hydroxyquinoline)aluminum (AlQ) (45 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm) were examined. In addition, a most simplest device, ITO/Ir(ppy)₂(CBDK) (80 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm), and two double-layer devices with configurations of ITO/NPB (30 nm)/Ir(ppy)₂(CBDK) (30 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm) and ITO/Ir(ppy)₂(CBDK) (30 nm)/AlQ (30 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm) were also fabricated and examined. The results show that the non-doped four-layer device for Ir(ppy)₂(CBDK) achieves maximum lumen efficiency of 4.54 lm/W and which is far higher than that of Ir(ppy)₂(acac), 0.53 lm/W, the device for Ir(dfppy)₂(CBDK) achieves maximum lumen efficiency of 0.51 lm/W and which is also far higher than that of Ir(dfppy)₂(acac), 0.06 lm/W. The results of simple devices involved Ir(ppy)₂(CBDK) show that the designed complex not only has a good hole transporting ability, but also has a good electron transporting ability. The improved performance of Ir(ppy)₂(CBDK) and Ir(dfppy)₂(CBDK) can be attributed to that the bulky carbazole-functionalized β-diketonate was introduced, therefore the carrier transporting property was improved and the triplet–triplet annihilation was reduced.     

Improving the performance of organic thin-film transistor with a doped interlayer

Top contact organic thin-film transistors (TC OTFTs) based on pentacene are fabricated. For improving the contact characteristics between the organic semiconductor thin-film and gold electrodes, we doped the starburst molecular 4,4′,4″-tris{N,(3-methylpheny)-N-phenylamino}-triphenylamine) (m-MTDATA), which is an excellent hole injection material for the organic light-emitting devices (OLEDs), into the interlayer contact with the electrodes. Compared with conventional TC OTFT, the performances of the organic transistor with the doped interlayer are improved. The field-effect mobility increases from 0.16 to 0.51 cm²/V s, and threshold voltage downshifts from –11 to –2.8 V for the linear region. The on/off current ratio is more than 10⁴ when the gate voltage varies from 0 to –20 V. We ascribe the improvements to the doped interlayer for which the contact resistance is reduced and the hole injection is enhanced.     

Interface electronic structures of organic light-emitting diodes with WO3 interlayer: A study by photoelectron spectroscopy

The energy level alignment and chemical reaction at the interface between the hole injection and transport layers in an organic light-emitting diode (OLED) structure has been studied using in-situ X-ray and ultraviolet photoelectron spectroscopy. The hole injection barrier measured by the positions of the highest occupied molecular orbital (HOMO) for N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB)/indium tin oxide (ITO) was estimated 1.32 eV, while that with a thin WO₃ layer inserted between the NPB and ITO was significantly lowered to 0.46 eV. This barrier height reduction is followed by a large work function change which is likely due to the formation of new interface dipole. Upon annealing the WO₃ interlayer at 350 °C, the reduction of hole injection barrier height largely disappears. This is attributed to a chemical modification occurring in the WO₃ such as oxygen vacancy formation.     

Synthesis and electroluminescent properties of highly efficient anthracene derivatives with bulky side groups

Three new asymmetric light emitting organic compounds were synthesized with diphenylamine or triphenylamine side groups; 10-(3,5-diphenylphenyl)-N,N-diphenylanthracen-9-amine (MADa), 4-(10-(3,5-diphenylphenyl)anthracen-9-yl)-N,N-diphenylaniline (MATa), and 4-(10-(3′,5′-diphenylbiphenyl-4-yl)anthracen-9-yl)-N,N-diphenylaniline (TATa). MATa and TATa had a PL_max at 463 nm in the blue region, and MADa had a PL_max at 498 nm. MADa and MATa had T_g values greater than 120 °C, and TATa had a T_g of 139 °C. EL devices containing the synthesized compounds were fabricated in the configuration: ITO/4,4′,4′′-tris(N-(2-naphthyl)-N-phenyl-amino)-triphenylamine (2-TNATA) (60 nm)/N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) (15 nm)/MADa or MATa or TATa or 9,10-di(2′-naphthyl)anthracene (MADN) (30 nm)/8-hydroxyquinoline aluminum (Alq₃) (30 nm)/LiF (1 nm)/Al (200 nm). The efficiency and color coordinate values (respectively) were 10.3 cd/A and (0.199, 0.152; bluish-green) for the MADa device, 4.67 cd/A and (0.151, 0.177) for the MATa device, and 6.07 cd/A and (0.149, 0.177) for the TATa device. The TATa device had a high external quantum efficiency (EQE) of 6.19%, and its luminance and power efficiencies and life-time were more than twice those of the MADN device.     

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.     

Solution-processable and thermal-stable triphenylamine-based dendrimers with truxene cores as hole-transporting materials for organic light-emitting devices

Two solution processable π-conjugated triphenylamine-based dendrimers, Tr-TPA3 and Tr-TPA9 were served as hole-transporting materials (HTMs) for organic light-emitting devices (OLEDs). The two dendrimers exhibit similar absorption and emission behaviors in solutions and thin films, which demonstrate that these dendrimers can form amorphous states in their films. The dendrimers showed excellent solubility, which are soluble in common organic solvents such as chloroform, tetrahydrofuran, and 1,1,2,2-tetrachloroethane, high thermal stability with high glass-transition temperature (T_g) of 115 °C for Tr-TPA3 and 140 °C for Tr-TPA9, high the highest unoccupied molecular orbital (HOMO) energy level (?5.12 eV for Tr-TPA3 and ?4.95 eV for Tr-TPA9, respectively) and good film forming property. When we employed these dendrimers as hole transport layer (HTL) in tris-(8-hydroxyquinoline) aluminum (Alq₃)-emitting electroluminescence (EL) devices, the Tr-TPA9-based double-layer device exhibited the turn-on voltage of 2.5 V, the maximum luminance of about 11,058 cd m^?2 and the maximum current efficiency of 4.01 cd A^?1. The comparison of the properties between the EL devices with dendrimers as HTL and the EL device with 1,4-bis(1-naphthylphenylamino)biphenyl (NPB) as HTL indicated that this series of dendrimers can be good candidates for HTM in OLEDs.     

Vertical structure permeable-base hybrid transistors based on multilayered metal base for stable electrical characteristics

In this work we present a permeable-base transistor consisting of a 60 nm thick N,N′-diphenyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine layer or a 40 nm thick 2,6-diphenyl-indenofluorene layer as the emitter, a Ca/Al/Ca multilayer as the metal base, and p-Si as collector. In the base, the Ca layers are 5 nm thick and the Al layer was varied between 10 and 40 nm, the best results obtained with a 20 nm thick layer. The devices present common-base current gain with both organic layer and silicon acting as emitter, but there is only observable common-emitter current gain when the organic semiconductor acts as emitter. The obtained common-emitter current gain, ～2, is independent on collector-emitter voltage, base current and organic emitter in a reasonable wide interval. Air exposure or annealing of the base is necessary to achieve these characteristics, indicating that an oxide layer is beneficial to proper device operation.     

Fluorene-based Tröger’s base analogues: Potential electroluminescent materials

Two new Λ-shaped fluorene-based Tröger’s base (TB) analogues with aryl substitutions are successfully synthesized and their photophysical and electroluminescent properties are examined in detail. Both compounds exhibit strong fluorescence emission in dilute solutions and aggregated states. Some abnormal photophysical behaviors have been observed; that is, the amorphous films of the two TB analogues show multiple blue–green emissions similar to the emissions of some polyfluorenes and oligofluorenes, while both the dilute solutions and the polycrystalline powders of two compounds show single blue–violet emission. Furthermore, the emissions of the amorphous film are obviously red-shifted in comparison with the polycrystalline powders. Organic light emitting diodes (OLEDs) using the two compounds as non-doped emitters with device structure of ITO/NPB (30 nm)/TBFB-BP or TBFB-FB (40 nm)/TPBI (40 nm)/LiF (1 nm)/Al (80 nm) were fabricated and high brightness (22047 cd/m² for TBFB-BP and 13434 cd/m² for TBFB-FB), high efficiency (2.78 cd/A, 1.82 lm/W for TBFB-BP and 2.76 cd/A, 1.93 lm/W for TBFB-FB) and low turn-on voltage (4.6 V for TBFB-BP and 4.5 V for TBFB-FB) were obtained. Our studies suggest that TB analogues could be excellent light emitting materials for OLED applications.     

Efficient hole-transporter for phosphorescent organic light emitting diodes with a simple molecular structure

N,N-diphenyl-4-(quinolin-8-yl)aniline (SQTPA), which composes a triphenylamine group and a quinoline group, has been synthesized and employed as a hole-transporter in phosphorescent OLEDs. It has been proved that SQTPA has efficient hole-transport property with a hole-mobility of 3.60 × 10⁻⁵ cm²/V s at the electric field of 800 (V/cm)^1/2, which is higher than that of NPB (1.93 × 10⁻⁵ cm²/V s). Blue, orange and green phosphorescent OLEDs have been fabricated based on FIrpic, Ir(2-phq)₃, Ir(ppy)₃ with typical structures by using SQTPA as the hole-transporter. The SQTPA-based devices show maximum external quantum efficiencies and power efficiencies of 17.5%, 32.5 lm/W for blue, 12.3%, 20.5 lm/W for orange and 20.3%, 64.5 lm/W for green. The performances of SQTPA-based devices are much better than that of NPB-based phosphorescent OLEDs with similar structures. Thought of its very simple molecular structure and easy synthetic route, SQTPA should be an efficient hole-transporter for phosphorescent OLEDs.     

Color stable white organic light-emitting diode based on a novel triazine derivative

A novel red–orange emitting material with a branched molecular structure, 2,4,6-tris[2-(N-ethyl-3-carbazole)carboxethenyl]-1,3,5-s-triazine (TC3), has been synthesized and characterized using UV–visible, photoluminescence (PL) and electroluminescence (EL) spectroscopy. White EL devices were fabricated using TC3 as a red–orange emitter and 8-hydroxyquinolinolato lithium (Liq) as a blue–green emitter. N,N-bis(3-methylphenyl)-N,N-diphenylbenzidine (TPD) as the adjustor for charge carrier mobility was introduced between the two emitting layers to improve the stability of the white emission color on bias voltage. The EL devices of ITO/poly(N-vinylcarbazole) (PVK):TC3 (56 nm)/TPD (5 nm)/Liq (30 nm)/Mg:Ag exhibited good quality white emission. The Commission Internationale De L’Eclairage chromaticity coordinates are (0.34, 0.39) and are stable on the bias voltage.     

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.