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.     

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.     

White OLED based on a temperature sensitive Eu3+/Tb3+ β-diketonate complex

A mixed lanthanide β-diketonate complex of molecular formula [Eu_0.45Tb_0.55(btfa)₃(4,4′-bpy)(EtOH)] (btfa^– = 4,4,4–trifluoro–1–phenyl–1,3–butanedionate; 4,4′-bpy = 4,4′-dipyridyl; EtOH = ethanol) was synthesized and its structure was elucidated by single crystal X-ray diffraction. The temperature dependence of the complex emission intensity between 11 and 298 K is illustrated by the Commission Internacionale l’Éclairage (CIE) (x, y) color coordinates change within the orange-red region, from (0.521, 0.443) to (0.658, 0.335). The existence of Tb³⁺-to-Eu³⁺ energy transfer was observed at room temperature and as the complex presents a relatively high emission quantum yield (0.34 ± 0.03) it was doped in a 4,4′-bis(carbazol-9-yl)biphenyl (CBP) organic matrix to be used as emitting layer to fabricate a white organic light-emitting diode (WOLED). Continuous electroluminescence emission was obtained varying the applied bias voltage showing a wide emission band from 400 to 700 nm. The white emission results from a combined action between the Eu³⁺ and Tb³⁺ peaks from the mixed Eu³⁺/Tb³⁺ complex and the other organic layers forming the device. The intensity ratio of the peaks is determined by the layer thickness and by the bias voltage applied to the OLED, allowing us to obtain a color tunable light source.     

Sequentially doped blue electrofluorescent organic light-emitting diodes

We report on a blue fluorescent [4,4′-bis(9-ethyl-3-carbazovinylene)-1-1′-bisphenyl] organic light-emitting diode with a sequentially doped device architecture introduced recently. The emission layer consists of a few repeating cells, similar to a multiple quantum well structure, which are made of sequentially evaporated host and guest layers. An external quantum efficiency as high as 2.8% photons/electron was obtained, comparable with that fabricated using the conventional doping method. Without degrading the efficiency, devices with varied emission spectra (peak wavelength shifted from 472 to 488 nm) were produced by simply varying the guest layer thickness. The spectrum shift is due to changes in local order or aggregate state, or both.     

Active layer thickness effect on the recombination process of PCDTBT:PC71BM organic solar cells

We investigated the effect of active layer thickness on recombination kinetics of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C71-butyric acid methyl ester (PC₇₁BM) based solar cells. Analysis of the fitted Lambert W-function of illuminated current density–voltage (J–V) characteristics revealed increased recombination processes with increased active layer thicknesses. The ideality factor extracted from PCDTBT:PCBM solar cells continuously increased from 1.89 to 3.88 when photoactive layer thickness was increased from 70 to 150 nm. We found that such increase in ideality factor is closely related to the defect density which is increased with increased photoactive layer thickness beyond 110 nm. Therefore, the different density of defect states in PCDTBT:PCBM solar cells causes the different recombination paths where solar cells with a thicker active layer (?110 nm) are considered to undergo coupled trap-assisted recombination processes while single-defect trap-assisted recombination is dominant for thinner (70–90 nm) PCDTBT:PCBM solar cells. As a result, we found that the optimal efficiencies of PCDTBT:PC₇₁BM solar cells were limited to the active layers between 70 and 90 nm. Particularly, when PCDTBT:PC₇₁BM solar cells were optimized with an active layer thickness of 70 nm, energy conversion efficiency reached 6.5% while an increase in thickness led to the reduction of efficiency to 4.7% at 133 nm but then an increase to 5.02% at 150 nm.     

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.     

Ultra-thin and graded sliver electrodes for use in transparent pentacene field-effect transistors

The authors report the fabrication of efficient and transparent pentacene field-effect transistors (FETs) using a graded structure of ultra-thin silver (Ag) source and drain (S–D) electrodes. The S–D electrodes were prepared by thermal evaporation with a controlled deposition rate to form Ag layer with a graded structure, leading to a reduced injection barrier and smoothing the contact surface between the electrode and the pentacene channel. The sheet resistance of such Ag electrode was found to be as low as 9 Ω/sq. In addition, a hole-only behavior of device with Ag electrode characterized by current–voltage measurement and conductive atomic-force microscopy shows the injection property of high current flowing as compared with device using Au electrode, resulting in an efficient injection condition existing at the interface of the graded Ag/pentacene. Device characterization indicates the transparent pentacene FET with a graded ultra-thin Ag electrode and organic capping layer of N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine exhibits a high transmission rate of ∼75% in the range of visible light from 400 to 550 nm, a threshold voltage of −6.0 V, an on–off drain current ratio of 8.4 × 10⁵, and a field-effect mobility of 1.71 cm²/V s, thus significantly outperforming pentacene FETs with multilayer oxide electrodes or other transparent thin metal layers.     

High-performance inverted top-emitting green electrophosphorescent organic light-emitting diodes with a modified top Ag anode

Green electrophosphorescent inverted top-emitting organic light-emitting diodes with a Ag/1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) anode are demonstrated. A high current efficacy of 124.7 cd/A is achieved at a luminance of 100 cd/m² when an optical outcoupling layer of N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl-4,4′-diamine (α-NPD) is deposited on the anode. The devices have a low turn-on voltage of 3.0 V and exhibit low current efficacy roll-off through luminance values up to 10,000 cd/m². The angle dependent spectra show deviation from Lambertian emission and color change with viewing angle. Hole-dominated devices with Ag/HAT-CN electrodes show current densities up to three orders of magnitude higher than devices without HAT-CN.     

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.     

Low and small resistance hole-injection barrier for NPB realized by wide-gap p-type degenerate semiconductor,LaCuOSe:Mg

LaCuOSe:Mg is a wide-gap p-type semiconductor with a high conductivity and a large work function. Potential of LaCuOSe:Mg as a transparent hole-injection electrode of organic light-emitting diodes (OLEDs) was examined by employing N,N′-diphenyl-N,N′-bis (1,1′-biphenyl)-4,4′-diamine (NPB) for a hole transport layer. Photoemission spectroscopy revealed that an oxygen plasma treated surface of LaCuOSe:Mg formed a hole-injection barrier as low as 0.3 eV, which is approximately a half of a conventional ITO/NPB interface. Hole-only devices composed of a LaCuOSe:Mg/NPB/Al structure showed a low threshold voltage ∼0.2 V and high-density current drivability of 250 mA cm⁻² at 2 V, which is larger by two orders of magnitude than that of ITO/NPB/Al devices. These results demonstrate that LaCuOSe:Mg has great potential as an efficient transparent anode for OLEDs and other organic electronic devices.     

The effect of oxide layer thickness on the quantification of 1.5 MeV γ–radiation induced interface traps in the Ag/SiO2/Si MOS devices

This work presents the effect of varied thickness of oxide layer and radiation dose on electrical characteristics of Ag/SiO₂/Si MOS devices irradiated by 1.5 MeV γ–radiations of varied doses. SiO₂ layers of 50, 100, 150 and 200 nm thickness were grown on Si substrates using dry oxidation and exposed to radiation doses of 1, 10 and 100 kGy. The exposure to radiation resulted in generation of fixed charge centers and interface traps in the SiO₂ and at the Si/SiO₂ interface. Capacitance-conductance-voltage (C-G-V) and capacitance-conductance-frequency (C-G-f) measurements were performed at room temperature for all MOS devices to quantify the active traps and their lifetimes. It is shown that accumulation and minimum capacitances decreased as the thickness of SiO₂ layer increased. For the unexposed MOS devices, the flat band voltage V_FB decreased at a rate of −0.12 V/nm, density of active traps increased by 4.5 times and depletion capacitance C_DP, increased by 2.5 times with the increase of oxide layer thickness from 50 to 200 nm. The density of active traps showed strong dependence on the frequency of the applied signal and the thickness of the oxide layer. The MOS device with 200 nm thick oxide layer irradiated with 100 kGy showed density of active interface traps was high at 50 kHz and was 3.6×10¹⁰ eV⁻¹ cm⁻². The relaxation time of the interface traps also increased with the exposure of γ–radiation and reached to 9.8 µs at 32 kHz in 200 nm thick oxide MOS device exposed with a dose of 100 kGy. It was inferred that this was due to formation of continuum energy states within the band gap and activation of these defects depended on the thickness of oxide layer, applied reverse bias and the working frequency. The present study highlighted the role of thickness of oxide layer in radiation hard environments and that only at high frequency, radiation induced traps remain passivated due to long relaxation times.     

Charge injection in metal/organic/metal structures with ZnO:Al/organic interface modified by Zn1−xMgxO:Al layer

Aluminum-doped zinc oxide (ZnO:Al, AZO) electrodes were covered with very thin (∼6 nm) Zn_1−xMg_xO:Al (AMZO) layers grown by atomic layer deposition. They were tested as hole blocking/electron injecting contacts to organic semiconductors. Depending on the ALD growth conditions, the magnesium content at the film surface varied from x = 0 to x = 0.6. Magnesium was present only at the ZnO:Al surface and subsurface regions and did not diffuse into deeper parts of the layer. The work function of the AZO/AMZO (x = 0.3) film was 3.4 eV (based on the ultraviolet photoelectron spectroscopy). To investigate carrier injection properties of such contacts, single layer organic structures with either pentacene or 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine layers were prepared. Deposition of the AMZO layers with x = 0.3 resulted in a decrease of the reverse currents by 1–2 orders of magnitude and an improvement of the diode rectification. The AMZO layer improved hole blocking/electron injecting properties of the AZO electrodes. The analysis of the current-voltage characteristics by a differential approach revealed a richer injection and recombination mechanisms in the structures containing the additional AMZO layer. Among those mechanisms, monomolecular, bimolecular and superhigh injection were identified.     

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.     

Fabrication of inverted pyramid structure by Cesium Chloride self-assembly lithography for silicon solar cell

Inverted pyramids were fabricated through a method combining cesium chloride (CsCl) self-assembly technology and anisotropy corrosion of silicon solar cells. Ti film with nanoporous masks was formed by lift-off the CsCl nanoislands for the inverted pyramids. The pyramids were then formed by anisotropy corrosion of alkaline solution. The average diameter and morphology of the pyramids were controlled by varying the average diameter of CsCl nanoislands from 400 nm to 1.5 µm and by varying the etching time of alkaline solution from 2 to 8 min. The inverted-pyramid texture could suppress reflection to below 10% at wavelengths from 400 to 1000 nm, which was much lower than that of planar wafer. A solar cell fabricated from the pyramids had higher short-circuit current density (J_sc) and photovoltaic conversion efficiency (PCE) compared with those of planar solar cells for the good antireflection property. The solar cell showed a PCE of 15.25%, a J_sc of 38.35 mA/cm², and an open-circuit voltage of 555.7 mV.     

1 Volt organic transistors with mixed self-assembled monolayer/Al2O3 gate dielectrics

Control of the threshold voltage and the subthreshold swing is critical for low voltage transistor operation. In this contribution, organic field-effect transistors (OFETs) operating at 1 V using ultra-thin (∼4 nm), self-assembled monolayer (SAM) modified aluminium oxide layers as the gate dielectric are demonstrated. A solution-processed donor–acceptor semiconducting polymer poly(3,6-di(2-thien-5-yl)-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione)thieno[3,2-b]thiophene) (PDPP2TTT) is used as the active layer. It is shown that the threshold voltage of the fabricated transistors can be simply tuned by carefully controlling the composition of the applied SAM. The optimised OFETs display threshold voltages around 0 V, low subthreshold slopes (150 ± 5 mV/dec), operate with negligible hysteresis and show average saturated field-effect mobilities in excess of 0.1 cm²/V s at 1 V.     

Simplified phosphorescent organic light-emitting diodes with a WO3-doped wide bandgap organic charge transport layer

The effects of p-type doping of wide bandgap ambipolar 4,4′-N,N′-dicarbazolebiphenyl (CBP) with WO₃ were investigated through detailed electrical device characterization. It was found that, to achieve effective doping for improved hole injection and transport, the doping level should be greater than 20 mol% and the doped layer should be at least 10 nm thick. A large downward shift of the Fermi level in WO₃-doped CBP causes band bending and depletion at the doped/undoped CBP interface, resulting in an additional energy barrier which hampers hole transport. Simplified green phosphorescent organic light-emitting diodes (PhOLEDs) with CBP as the hole transport and host material were fabricated. With a WO₃-doped hole transport layer, the PhOLEDs attained brightness of 11,163 cd/m² at 20 mA/cm², and exhibited an improved reliability under constant-current stressing as compared to undoped PhOLEDs.     

High performance near infrared photosensitive organic field-effect transistors realized by an organic hybrid planar-bulk heterojunction

Compared with organic photodiodes, photoresponsive organic field-effect transistors (photOFETs) exhibit higher sensitivity and lower noise. The performance of photOFETs based on conventional single layer structure operating in the near infrared (NIR) is generally poor due to the low carrier mobility of the active channel materials. We demonstrate a high performance photOFETs operating in NIR region with a structure of hybrid planar-bulk heterojunction (HPBHJ). PhotOFETs with the structures of single layer [lead phthalocyanine (PbPc) or copper phthalocyanine (CuPc)], single planar heterojunction (PHJ) of CuPc/PbPc, double PHJ of CuPc/PbPc/3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and HPBHJ of CuPc/PbPc:PTCDA were fabricated and characterized. It is concluded that the photOFET with HPBHJ structure showed superior performance compared to that with other structures, and for NIR light of wavelength 808 nm, the photOFET with HPBHJ structure exhibited a large photoresponsivity of 322 mA/W, a high external quantum efficiency of around 50%, and a maximal photosensitivity of 9.4 × 10². The high performance of HPBHJ photOFET is attributed to its high exciton dissociation efficiency and excellent hole transport ability. For 50-nm thick CuPc layer, the optimal thickness of the PbPc:PTCDA layer is found to be around 30 nm.     

Gravure printed organic rectifying diodes operating at high frequencies

Organic rectifier diodes operating at 10 MHz made using roll-to-roll compatible mass printing processes to define patterns and deposit inks are reported. The diodes consist of a layer of poly(triarylamine) sandwiched between layers of silver and copper. No high resolution prepatterning of any surfaces was performed, thus the entire process could be carried out on large-scale roll-to-roll production lines. The organic diode based rectifier circuit generates a DC output voltage of approximately 2.7 V at 10 MHz, using an input signal with zero-to-peak voltage amplitude of 10 V. The result demonstrates the possibility of printed organic diodes for RFID applications.     

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.     

Electrical and optical characteristics of phosphorescent organic light-emitting device with thin-codoped layer insertion

In this paper, we demonstrated the changes of electrical and optical characteristics of a phosphorescent organic light-emitting device (OLED) with tris(phenylpyridine)iridium Ir(ppy)₃ thin layer (4 nm) slightly codoped (1%) inside the emitting layer (EML) close to the cathode side. Such a thin layer helped for electron injection and transport from the electron transporting layer into the EML, which reduced the driving voltage (0.40 V at 100 mA/cm²). Electroluminescence (EL) spectral shift at different driving voltage was observed in our blue OLED with [(4,6-di-fluoropheny)-pyridinato-N,C^2′]picolinate (FIrpic) emitter, which came from the recombination zone shift. With the incorporation of thin-codoped Ir(ppy)₃, such EL spectral shift was almost undetectable (color coordinate shift (0.000, 0.001) from 100 to 10,000 cd/m²), due to the compensation of Ir(ppy)₃ emission at low driving voltage. Such a methodology can be applied to a white OLED which stabilized the EL spectrum and the color coordinates ((0.012, 0.002) from 100 to 10,000 cd/m²).