Filter-Free Narrowband Photomultiplication-Type Planar Heterojunction Organic Photodetectors

Filter-free narrowband photomultiplication-type planar heterojunction (PHJ) organic photodetectors (PM-PHOPDs) are first realized by employing a thick front donor layer and an ultrathin PC₇₁BM layer. The thick front donor layer is employed as an optical field adjusting (OFA) layer. The sequentially coated PC₇₁BM will diffuse slightly into OFA layer, which works as interfacial electron traps to capture photogenerated electrons for assisting hole tunneling injection. The P3HT/PC₇₁BM-based PM-PHOPDs exhibit narrowband response with full-width of half-maximum of 32 nm and external quantum efficiency (EQE) of 1700% at 650 nm under −20 V bias. Due to the enhanced hole transport and reduced charge recombination in PHJ compared to those in bulk heterojunction (BHJ), the EQE of P3HT/PC₇₁BM-based narrowband PM-PHOPDs is twice as P3HT:PC₇₁BM BHJ-based narrowband PM-OPDs under the same bias. The response peak of PM-PHOPDs is adjusted from 650 to 695 or 745 nm by incorporating SMPV1 or DRCN5T in OFA layers due to the red-shifted absorption edge. The EQEs of 3600% at 695 nm and 870% at 745 nm are obtained for P3HT:SMPV1 and P3HT:DRCN5T-based PM-PHOPDs under −20 V bias, respectively. This work provides a smart strategy to achieve narrowband PM-OPDs by designing different OFA layers.     

Efficient red phosphorescent organic light emitting diodes based on solution processed all-inorganic cesium lead halide perovskite as hole transporting layer

An efficient red phosphorescent organic light emitting diode (PhOLED) has been realized by utilizing a composite hole transporting layer comprised of all-inorganic cesium lead halide perovskite CsPbBr₃ via spin-coating and 1,3-bis(9-carbazolyl) benzene (mCP) by vacuum depositing, in which CsPbBr₃ film is used as a hole transporting layer and mCP plays a dominant role in electron and exciton blocking. And this PhOLED shows a saturated red emission coordinated at CIE (0.65, 0.33) driven at 7.5 V, a maximum brightness of 20,750 cd/m², and a maximum current efficiency of 10.64 cd/A, which is as 1.87 times as that 5.68 cd/A of the reference PhOLEDs based on traditional small organic molecular hole transporting material N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzi (NPB). The electroluminescent (EL) spectra and the energy level alignment of different PhOLEDs are investigated. The enhanced EL performances are ascribed to improved hole injecting and transporting behaviors, and better electron and exciton confinements by introducing the composite hole transporting layer CsPbBr₃/mCP.     

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.     

H2O treatment-induced uniform NiOX interfacial layer boosting brightness and light-emitting efficiency of blue perovskite electroluminescence

The reported NiO_x interfacial layers in blue perovskite light-emitting diodes (PeLEDs) usually require high-temperature annealing and complex interface modification. Herein, we report a kind of uniform NiO_x anode interfacial layer induced by H₂O treatment, which effectively enhances the brightness and light-emitting efficiency of blue PeLEDs simultaneously. Compared to the as-prepared NiO_x anode interfacial layer, H₂O treatment renders uniform and pinhole-free NiO_x morphology. The solution-processed perovskite blue emissive layer prepared atop the H₂O-treated NiO_x interfacial layer demonstrates enhanced photoluminescent property and superior morphology with low trap density. The blue PeLEDs employing H₂O-treated NiO_x as anode interfacial layer show a maximum luminance of 9052 cd/m² and a maximum external quantum efficiency (EQE) of 1.80%, whereas the control device based on the as-prepared NiO_x anode interfacial layer merely exhibits a maximum luminance of 3850 cd/m² and an EQE of 0.88%, leading to about 135% and 104% increase in brightness and efficiency, respectively. The PeLEDs emit pure blue light with emission peak located at 482 nm and demonstrate superior spectral stability under different driving voltages and operating time.     

High performance top-emitting organic light-emitting diodes with copper iodide-doped hole injection layer

Efficient top-emitting organic light-emitting diodes were fabricated using copper iodide (CuI) doped 1,4-bis[N-(1-naphthyl)-N′-phenylamino]-4,4′-diamine (NPB) as a hole injection layer and Ir(ppy)₃ doped CBP as the emitting layer. CuI doped NPB layer functions as an efficient p-doped hole injection layer and significantly improves hole injection from a silver bottom electrode. The top-emitting device shows high current efficiency of 69 cd/A with Lambertian emission pattern. The enhanced hole injection is originated from the formation of the charge transfer complex between CuI and NPB.     

Nanodot‐Enhanced High‐Efficiency Pure‐White Organic Light‐Emitting Diodes with Mixed‐Host Structures

A relatively high‐efficiency, fluorescent pure‐white organic light‐emitting diode was fabricated using a polysilicic acid (PSA) nanodot‐embedded polymeric hole‐transporting layer (HTL). The diode employed a mixed host in the single emissive layer, which comprised 0.5 wt % yellow 5,6,11,12‐tetra‐phenylnaphthacene doped in the mixed host of 50 % 2‐(N,N‐diphenyl‐amino)‐6‐[4‐(N,N‐diphenylamino)styryl]naphthalene and 50 % N,N′‐bis‐(1‐naphthyl)‐N,N′‐diphenyl‐1,10‐biphenyl‐4‐4′‐diamine. By incorporating 7 wt % 3 nm PSA nanodot into the HTL of poly(3,4‐ethylene‐dioxythiophene)‐poly‐(styrenesulfonate), the efficiency at 100 cd m^–2 was increased from 13.5 lm W^–1 (14.7 cd A^–1; EQE: 7.2 %) to 17.1 lm W^–1 (17.6 cd A^–1; EQE: 8.3 %). The marked efficiency improvement may be attributed to the introduction of the PSA nanodot, leading to a better carrier‐injection‐balance.     

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.     

Thermal stability of devices with molybdenum oxide doped organic semiconductors

We demonstrate the thermal stability of transition-metal-oxide (molybdenum oxide; MoO₃)-doped organic semiconductors. Impedance spectroscopy analysis indicated that thermal deformation of the intrinsic 1,4-bis[N-(1-naphthyl)-N′-phenylamino]-4,4′-diamine (NPB) layer is facilitated when the MoO₃-doped NPB layer is deposited on the intrinsic NPB layer. The resistance of the intrinsic NPB layer is reduced from 300 kΩ to 3 kΩ after thermal annealing at 100 °C for 30 min. Temperature-dependent conductance/angular frequency–frequency (G/w-f-T) analysis revealed that the doping efficiency of MoO₃, which is represented by the activation energy (E_a), is reduced after the annealing process.     

Organic photovoltaic devices based on pentacene/N,N′-dioctyl-3,4,9,10-perylenedicarboximide heterojunctions

We have studied an organic photovoltaic cell based on an efficient donor/acceptor combination of pentacene/N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C₈) heterojunctions. Photocurrent spectra exhibited excellent light harvesting throughout the visible spectrum with maximum external quantum efficiency (EQE) of ～60%. PTCDI-C₈ layer provided significant contribution to the photocurrent due to its strong absorption properties and efficient exciton dissociation at pentacene/PTCDI-C₈ interface. Power conversion efficiency of about 1.2% has been achieved under AM 1.5 illumination. The device showed a low series resistance of 18 Ω cm² and a high shunt resistance of 2.5 kΩ cm², resulting in a high fill factor of 65%.     

Highly efficient and high transmittance semitransparent polymer solar cells with one-dimensional photonic crystals as distributed Bragg reflectors

We demonstrate that one-dimensional photonic crystals as distributed Bragg reflectors can effectively improve the performance of semitransparent polymer solar cells (PSCs) based on the blend of P3HT:ICBA. The one dimensional distributed Bragg reflectors (1D DBRs) are composed of N pairs of WO₃/LiF which are thermally evaporated on Ag anode. Due to its photonic bandgap, 1D DBRs can reflect the light totally back into the PSCs when the high reflectance range of 1D DBRs is well matched with absorption spectrum of the active layer. A maximum power conversion efficiency (PCE) of 4.12%, a highest transmittance of 80.4% at 660 nm and an average transmittance of 55.6% in the wavelength range of 600–800 nm are obtained in the case of N = 8, corresponding enhancement of 24.1% in PCE when compared with the device without the 1D DBRs.     

Solution-processed aqueous composite hole injection layer of PEDOT:PSS+MoOx for efficient ultraviolet organic light-emitting diode

Hole injection layer (HIL) plays a crucial role in governing external quantum efficiency (EQE) of ultraviolet organic light-emitting diodes (UV OLEDs). We develop a solution-processed aqueous composite HIL of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) incorporated MoO_x (PEDOT:PSS+MoO_x) and cast successful application to UV OLEDs. PEDOT:PSS+MoO_x is characterized in detail with scanning electron microscopy, atomic force microscopy, UV–visible absorption spectra, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and impedance spectroscopy measurements. The results show that PEDOT:PSS+MoO_x features superior film morphology and exceptional electronic properties such as enhanced surface work function and promoted hole injection capacity. With PEDOT:PSS+MoO_x as HIL, the UV OLED gives maximum EQE of 4.4% and radiance of 12.2 mW/cm² as well as improved durability. The electroluminescence peaks at 376 nm with full width at half maximum of 34 nm and stable voltage-dependent spectra. Our results pave a way for exploring efficient UV OLEDs with solution-processable techniques.     

A highly efficient OLED based on terbium complexes

A neutral ligand 9-(4-tert-butylphenyl)-3,6-bis(diphenylphosphineoxide)-carbazole (DPPOC) and its complex Tb(PMIP)₃DPPOC (A, where PMIP stands for 1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone) were synthesized. DPPOC has a suitable lowest triplet energy level (24,691 cm^?1) for the sensitization of Tb(III) (⁵D₄: 20,400 cm^?1) and a significantly higher thermal stability (glass transition temperature 137 °C) compared with the familiar ligand triphenylphosphine oxide (TPPO). Experiments revealed that the emission layer of the Tb(PMIP)₃DPPOC film could be prepared by vacuum co-deposition of the complex Tb(PMIP)₃(H₂O)₂ (B) and DPPOC (molar ratio = 1:1). The electroluminescent (EL) device ITO/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-diphenyl-4,4′-diamine (NPB; 10 nm)/Tb(PMIP)₃ (20 nm)/co-deposited Tb(PMIP)₃DPPOC (30 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP; 10 nm)/tris(8-hydroxyquinoline) (AlQ; 20 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm) exhibited pure emission from terbium ions, even at the highest current density. The highest efficiency obtained was 16.1 lm W^?1, 36.0 cd A^?1 at 6 V. At a practical brightness of 119 cd m^?2 (11 V) the efficiency remained above 4.5 lm W^?1, 15.7 cd A^?1. These values are a significant improvement over the previously reported Tb(PMIP)₃(TPPO)₂ (C).     

Electrical and interfacial characteristics of nanolaminate (Al2O3/ZrO2/Al2O3) gate stack on fully depleted SiGe-on-insulator

The structural and electrical characteristics of a novel nanolaminate Al₂O₃/ZrO₂/Al₂O₃ high-k gate stack together with the interfacial layer (IL) formed on SiGe-on-insulator (SGOI) substrate have been investigated. A clear layered Al₂O₃ (2.5 nm)/ZrO₂ (4.5 nm)/Al₂O₃ (2.5 nm) structure and an IL (2.5 nm) are observed by high-resolution transmission electron microscopy. X-ray photoelectron spectroscopy measurements indicate that the IL contains Al-silicate without Ge atom incorporation. A well-behaved C–V behavior with no hysteresis shows the absence of Ge pileup or Ge segregation at the gate stack/SiGe interface.     

Improved efficiency and stability in polymer solar cells using a Tetra-nuclear Zinc(II) complex interfacial layer

Tetra-nuclear Zinc(II) complex Zn₄O(AID)₆ [AID = 7-azaindolate] is a wide band gap luminescent material that exhibits efficient emission matching the absorption spectra of organic donor materials such as polythiophene (P3HT). This work demonstrates polymer solar cells (PSC) based on P3HT:PCBM (phenyl-C₆₁-butyric acid methyl ester) blend active layer with a Zn₄O(AID)₆ cathode interfacial layer achieving a power conversion efficiency (PCE) significantly higher than that of the reference devices. The energy level and impedance spectroscopy analysis show that the Zn₄O(AID)₆ cathode interfacial layer acts as an efficient exciton/hole blocking layer, and reduces charge recombination rate with more efficient electron extraction. The Zn₄O(AID)₆ interfacial layer also helps achieve longer lifetime of PSC devices. The improved efficiency and stability combined with low cost and nontoxicity of Zn₄O(AID)₆ make it a promising cathode interfacial material for high-performance and stable PSC devices.     

To observe bidirectional negative differential resistance at room temperature by narrowing transport channels for charge carriers in vertical organic light-emitting transistor

Bidirectional negative differential resistance (NDR) at room temperature with high peak-to-valley current ratio (PVCR) of ～10 are observed from vertical organic light-emitting transistor indium-tin oxide (ITO)/N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine) (α-NPD)(60 nm)/Al(30 nm)/α-NPD(60 nm)/tris-(8-hydroxyquinoline) aluminium (Alq₃)(50 nm)/Al by narrowing the transport channels for charge carriers with a thick-enough middle Al gate electrode layer to block charge carriers transporting from source electrode to drain electrode. When the transport channel for charge carriers gets large enough, the controllability of gate bias on the drain–source current gets weaker and the device almost works as an organic light-emitting diode only. Therefore, it provides a very simple way to produce NDR device with dominant bidirectional NDR and high PVCR (～10) at room temperature by narrowing transport channels for charge carriers in optoelectronics.     

Structural and interfacial properties of high-k HfOxNy gate dielectric films

The thermal stability and interfacial characteristics for hafnium oxynitride (HfO_xN_y) gate dielectrics formed on Si (1 0 0) by plasma oxidation of sputtered HfN films have been investigated. X-ray diffraction results show that the crystallization temperature of nitrogen-incorporated HfO₂ films increases compared to HfO₂ films. Analyses by X-ray photoelectron spectroscopy confirm the nitrogen incorporation in the as-deposited sample and nitrogen substitution by oxygen in the annealed species. Results of FTIR characterization indicate that the growth of the interfacial SiO₂ layer is suppressed in HfO_xN_y films compared to HfO₂ films annealed in N₂ ambient. The growth mechanism of the interfacial layer is discussed in detail.     

Four-wavelength white organic light-emitting diodes using 4,4′-bis-[carbazoyl-(9)]-stilbene as a deep blue emissive layer

White organic light-emitting diodes (WOLEDs) with four wavelengths were fabricated by using three doped layers, which were obtained by separating recombination zones into three emitter layers. Among these emitters, blue emissions with two wavelengths (456 and 487 nm) were occurred in the 4,4′-bis(carbazoyl-(9))-stilbene (BCS) host doped with a perylene dye. Also, a green emission was originated from the tris(8-quinolinolato)aluminum (III) (Alq₃) host doped with a green fluorescent of 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-[1]benzopyrano [6,7,8-ij]-quinolizin-11-one (C545T) dye. Finally, an orange emission was obtained from the N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) host doped with a 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) dye. The white light could be emitted by simultaneously controlling the emitter thickness and concentration of fluorescent dyes in each emissive layer, resulting in partial excitations among those three emitter layers. Electroluminescent spectra of the device obtained in this study were not sensitive to driving voltage of the device. Also, the maximum luminance for the white OLED with the CIE coordinate of (0.34, 0.34) was 56,300 cd/m² at the applied bias voltage of 11.6 V. Also, its external quantum and the power efficiency at about 100 cd/m² were 1.68% and 2.41 lm/W, respectively.     

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.     

Bulk and interface properties of molybdenum trioxide-doped hole transporting layer in organic light-emitting diodes

Effects of doping molybdenum trioxide (MoO₃) in N,N′-diphenyl-N,N′-bis(1,1′-biphenyl)-4,4′-diamine (NPB) are studied at various thicknesses of doped layer (25–500 Å) by measuring the current–voltage characteristics, the capacitance–voltage characteristics and the operating lifetime. We formed charge transfer complex of NPB and MoO₃ by co-evaporation of both materials to achieve higher charge density, lower operating voltage, and better reliability of devices. These improved performances may be attributed to both bulk and interface properties of the doped layer. The authors demonstrated that the interface effects play more important role in lowering the operating voltage and increasing the lifetime.     

Water‐Soluble Polyfluorenes as an Interfacial Layer Leading to Cathode‐Independent High Performance of Organic Solar Cells

Novel poly[(9,9‐bis((6′‐(N,N,N‐trimethylammonium)hexyl)‐2,7‐fluorene)‐alt‐(9,9‐bis(2‐(2‐(2‐methoxyethoxy)ethoxy)ethyl)‐9‐fluorene)) dibromide (WPF‐6‐oxy‐F) and poly[(9,9‐bis((6′‐(N,N,N‐trimethylammonium)hexyl)‐2,7‐fluorene)‐alt‐(9,9‐bis(2‐(2‐methoxyethoxy)ethyl)‐fluorene)] dibromide (WPF‐oxy‐F) compounds are developed and the use of these water‐soluble polymers as an interfacial layer for low‐cost poly(3‐hexylthiophene):phenyl‐C₆₁ butyric acid methyl ester (P3HT:PCBM) organic solar cells (OSCs) is investigated. When WPF‐oxy‐F or WPF‐6‐oxy‐F is simply inserted between the active layer and the cathode as an interfacial dipole layer by spin‐coating water‐soluble polyfluorenes, the open‐circuit voltage (V_oc), fill factor (FF), and power‐conversion efficiency (PCE) of photovoltaic cells with high work‐function metal cathodes, such as Al, Ag, Au, and Cu, dramatically increases. For example, when WPF‐6‐oxy‐F is used with Al, Ag, Au, or Cu, regardless of the work‐function of the metal cathode, the V_oc is 0.64, 0.64, 0.58, and 0.63 V, respectively, approaching the original value of the P3HT:PCBM system because of the formation of large interfacial dipoles through a reduction of the metal work‐function. In particular, introducing WPF‐6‐oxy‐F into a low‐cost Cu cathode dramatically enhanced the device efficiency from 0.8% to 3.36%.