Spirobifluorene/sulfone hybrid: Highly efficient solution-processable material for UV–violet electrofluorescence,blue and green phosphorescent OLEDs

A high efficient UV–violet emission type material bis[4-(9,9′-spirobifluorene-2-yl)phenyl] sulfone (SF-DPSO) has been synthesized by incorporating electron deficient sulfone and morphologically stable spirobifluorene into one molecule. The steric and bulky compound SF-DPSO exhibits an excellent solid state photoluminescence quantum yield (Φ_PL = 92%), high glass transition temperature (T_g = 211 °C) and high triplet energy (E_T = 2.85 eV). In addition, the uniform amorphous thin film could be formed by spin-coating from its solution. These promising physical properties of the material made it suitable for using as UV–violet emitter in non-doped device and appropriate host in phosphorescent OLEDs. With SF-DPSO as an emitter, the non-doped solution processed device achieved an efficient UV–violet emission with the EL peak around 400 nm. By using SF-DPSO as a host, solution processed blue and green phosphorescent organic light emitting diodes showed a high luminous efficiency of 13.7 and 30.2 cd A⁻¹, respectively.     

Organic light-emitting diodes based on an ambipolar single crystal

Slice-like organic single crystals of 1,4-bis(2-cyano-2-phenylethenyl)benzene (BCPEB) are grown by the physical vapor transport (PVT) method, and exhibit a very high photoluminescence quantum efficiency (Φ_PL) of 75%. The ambipolar behavior of BCPEB single crystals are confirmed using the time of flight technique. The high efficiency and balanced (μ_h = 0.059 cm²/Vs and μ_e = 0.070 cm²/Vs) carriers’ mobility imply that the BCPEB single crystal is a promising light-emitting layer in the diodes structure. Intense green electroluminescence (EL) from a diode has been successfully demonstrated at an applied electric field of 2 × 10⁵ V/cm.     

Extremely low driving voltage electrophosphorescent green organic light-emitting diodes based on a host material with small singlet–triplet exchange energy without p- or n-doping layer

In order to achieve low driving voltage, electrophosphorescent green organic light-emitting diodes (OLEDs) based on a host material with small energy gap between the lowest excited singlet state and the lowest excited triplet state (ΔE_ST) have been fabricated. 2-biphenyl-4,6-bis(12-phenylindolo[2,3-a] carbazole-11-yl)- 1,3,5-triazine (PIC–TRZ) with ΔE_ST of only 0.11 eV has been found to be bipolar and used as the host for green OLEDs based on tris(2-phenylpyridinato) iridium(III) (Ir(ppy)₃). A very low onset voltage of 2.19 V is achieved in devices without p- or n-doping. Maximum current and power efficiencies are 68 cd/A and 60 lm/W, respectively, and no significant roll-off of current efficiency (58 cd/A at 1000 cd/m² and 62 cd/A at 10,000 cd/m²) have been observed. The small roll-off is due to the improved charge balance and the wide charge recombination zone in the emissive layer.     

Microwave dielectric properties of (1 − x)(Mg0.95Co0.05)TiO3– xCa0.6La0.8/3TiO3 ceramics with V2O5 addition

The microstructures and the microwave dielectric properties of the (1 − x)(Mg_0.95Co_0.05)TiO₃–xCa_0.6La_0.8/3TiO₃ ceramic system were investigated. In order to achieve a temperature-stable material, we studied a method of combining a positive temperature coefficient material with a negative one. Ca_0.6La_0.8/3TiO₃ has dielectric properties of dielectric constant ε_r ∼ 109, Q × f value ∼ 17,600 GHz and a large positive τ_f value ∼ 213 ppm/°C. (Mg_0.95Co_0.05)TiO₃ ceramics possesses high dielectric constant (ε_r ∼ 16.8), high quality factor (Q × f value ∼ 230,000 GHz), and negative τ_f value (−54 ppm/°C). As the x value varies from 0.1 to 0.8, (1 − x)(Mg_0.95Co_0.05)TiO₃–xCa_0.6La_0.8/3TiO₃ ceramic system has the dielectric properties as follows: 21.55 < ε_r < 75.44, 21,000 < Q × f < 90,000 and −10 < τ_f < 140. By appropriately adjusting the x value in the (1 − x)(Mg_0.95Co_0.05)TiO₃–xCa_0.6La_0.8/3TiO₃ ceramic system, zero τ_f value can be achieved. With x = 0.15, a dielectric constant ε_r ∼ 25.78, a Q × f value ∼ 84,000 GHz (at 9 GHz), and a τ_f value ∼ 2 ppm/°C were obtained for 0.85(Mg_0.95Co_0.05)TiO₃–0.15Ca_0.6La_0.8/3TiO₃ ceramics sintered at 1400 °C for 4 h. For practical application in communication systems, it is desirable to be able to sinter at lower temperatures. Therefore, V₂O₅ was as a sintering aid for lowering the sintering temperature of0.85(Mg_0.95Co_0.05)TiO₃–0.15Ca_0.6La_0.8/3TiO₃ ceramics. At the same time, the 0.85(Mg_0.95Co_0.05)TiO₃–0.15Ca_0.6La_0.8/3TiO₃ ceramic system with 0.5 wt% V₂O₅ can be obtained good properties at the microwave frequencies for 1200 °C.     

Post-annealing modification in structural properties of ZnO thin films on p-type Si substrate deposited by evaporation

Zinc oxide (ZnO) films of thickness ∼380 nm were deposited on p-type Si (1 1 1) substrate maintained at 300 °C under 3×10⁻⁶ Torr by a radio frequency (RF) heating source. Transmission Fourier transform infrared (FTIR) spectrum exhibited a clear Zn–O bond excitation frequency of ∼408 cm⁻¹. X-ray diffraction spectrum demonstrated four peaks (P₁–P₄) at 2θ (deg) ∼36±0.06, 40±0.09, 82±0.17 and 86±0.2, which originated from (1 0 0), (0 0 2), (2 0 1) and (0 0 4) hexagonal planes, respectively. P₂ being the highest intensity peak indicated that the growth of ZnO predominantly occurred along the c-axis i.e. (0 0 2) plane. Micrographs of the samples obtained from scanning electron microscopy (SEM) and atomic force microscopy (AFM) identically displayed scattered nanocrystallites, which grew bigger with the increase of sample annealing temperature (°C) in the range of 400–1000. AFM pictures, in particular, exposed the hexagonal structure of the deposited films along with voids. However, ZnO composition ∼6:1 (Zn:O) as calculated from the energy dispersive spectrum (EDS) revealed that the formation of ZnO was not stoichiometric, rather of Zincsuboxide structure ZnO_x (x<1). Arrhenius plot of the resistivity data yielded a donor level (zinc interstitial and/or Zn–on–O site) with ionization energy E_c–1.26 eV, thereby it supports our measured results, in general.     

Efficient red electrophosphorescence from a fluorene-based bipolar host material

We have prepared efficient red organic light-emitting diodes (OLEDs) incorporating 2,7-bis(diphenylphosphoryl)-9-[4-(N,N-diphenylamino)phenyl]-9-phenylfluorene (POAPF) as the host material doped with the osmium phosphor Os(fptz)₂(PPh₂Me)₂ (fptz = 3-trifluoromethyl-5-pyridyl-1,2,4-triazole). POAPF, which possesses bipolar functionalities, can facilitate both hole- and electron-injection from the charge transport layers to provide a balanced charge flux within the emission layer. The peak electroluminescence performance of the device reached as high as 19.9% and 34.5 lm/W – the highest values reported to date for a red phosphorescent OLED. In addition, we fabricated a POAPF-based white light OLED – containing red-[doped with Os(fptz)₂(PPh₂Me)₂] and blue-emitting {doped with iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C^2′] picolinate, FIrpic} layers – that also exhibited satisfactory efficiencies (18.4% and 43.9 lm/W).     

High electron mobility triazine for lower driving voltage and higher efficiency organic light emitting devices

The triazine compound 4,4′-bis-[2-(4,6-diphenyl-1,3,5-triazinyl)]-1,1′-biphenyl (BTB) was developed for use as an electron transport material in organic light emitting devices (OLEDs). The material demonstrates an electron mobility of ∼7.2 × 10⁻⁴ cm² V⁻¹ s⁻¹ at a field of 8.00 × 10⁵ V cm⁻¹, which is 10-fold greater than that of the widely used material tris(8-hydroxyquinoline) aluminum (AlQ₃). OLEDs with a BTB electron transport layer showed a ∼1.7–2.5 V lower driving voltage and a significantly increased efficiency, compared to those with AlQ₃. These results suggest that BTB has a strong potential for use as an OLED electron transport layer material.     

Functionalized terfluorene for solution-processed high efficiency blue fluorescence OLED and electrophosphorescent devices

A new multifunctional blue-emitting terfluorene derivative (TFDPA) featured with triphenylamine groups for hole-transportation and long alkyl chains for solution processability on the conjugation inert bridge centers was reported. TFDPA can give homogeneous thin film by solution process and exhibits high hole mobility (μ_h ≈ 10^?3 cm² V^?1 s^?1) and suitable HOMO for hole injection. Particularly, TFDPA performs efficient deep-blue emission with high quantum yield (～100% in solution, 43% in thin film) and suitable triplet energy (E_T = 2.28 eV), making solution-processed OLED devices of using TFDPA as blue emitter and as host for iridium-containing phosphorescent dopants feasible. The solution-processed nondoped blue OLED device gives saturated deep-blue electroluminescence [CIE = (0.17, 0.07)] with EQE of 2.7%. TFDPA-hosted electrophosphorescent devices performed with EQE of 6.5% for yellow [(Bt)₂Ir(acac)], 9.3% of orange [Ir(2–phq)₃], and 6.9% of red [(Mpq)₂Ir(acac)], respectively. In addition, with careful control on the doping concentration of [(Bt)₂Ir(acac)], a solution-processed fluorescence–phosphorescence hybrided two-color-based WOLED with EQE of 3.6% and CIE coordinate of (0.38, 0.33) was successfully achieved.     

Improving the Efficiency of Red Thermally Activated Delayed Fluorescence Organic Light‐Emitting Diode by Rational Isomer Engineering

The development of efficient red thermally activated delayed fluorescence (TADF) emitters with an emission wavelength beyond 600 nm remains a great challenge for organic light‐emitting diodes (OLEDs). Herein, two pairs of isomers are designed and synthesized by attaching electron‐donor 9,9‐diphenyl‐9,10‐dihydroacridine (DPAC) moiety to the different positions of two kinds of highly rigid planar acceptor cores (PDCN and PPDCN). Their TADF efficiencies and emission maxima (599–726 nm) are regulated by molecular isomer manipulation. Interestingly, the photoluminescence quantum yields (Φ_PLs) of trans‐isomers T‐DA‐1 and T‐DA‐2 (78% and 89%) are remarkably higher than those of their corresponding cis‐isomers C‐DA‐1 and C‐DA‐2 (12% and 14%). Significantly increased Φ_PL values can be explained by single crystal structures and theoretical simulation. As a result, a deep red TADF‐OLED based on T‐DA‐2 displays a maximum external quantum efficiency (EQE) of 26.26% at 640 nm. Notably, at a brightness of 100 cd m^?2, the EQE value of T‐DA‐2‐based device still remains at an extremely high level of 23.95%, representing the highest value for reported red TADF‐OLEDs at the same brightness. These results provide a reasonable pathway to optimize optoelectronic properties and thereby construct efficient red TADF emitters through rational isomer engineering.     

Evaluation of lateral barrier height of inhomogeneous photolithography-fabricated Au/n-GaAs Schottky barrier diodes from 80 K to 320 K

In order to evaluate current conduction mechanism in the Au/n-GaAs Schottky barrier diode (SBD) some electrical parameters such as the zero-bias barrier height (BH) Φ_bo(I–V) and ideality factor (n) were obtained from the forward bias current–voltage (I–V) characteristics in wide temperature range of 80–320 K by steps of 10 K. By using the thermionic emission (TE) theory, the Φ_bo(I–V) and n were found to depend strongly on temperature, and the n decreases with increasing temperature while the Φ_bo(I–V) increases. The values of Φ_bo and n ranged from 0.600 eV and 1.51(80 K) to 0.816 eV and 1.087 (320 K), respectively. Such behavior of Φ_bo and n is attributed to Schottky barrier inhomogeneities by assuming a Gaussian distribution (GD) of BHs at Au/n-GaAs interface. In the calculations, the electrical parameters of the experimental forward bias I–V characteristics of the Au/n-GaAs SBD with the homogeneity in the 80–320 K range have been explained by means of the TE, considering GD of BH with linear bias dependence.     

Computational studies on the radiative and nonradiative processes of luminescent N-heteroleptic platinum(II) complexes

Three N-heteroleptic Pt(II) complexes, [Pt(C^C)(O^O)] [O^O = acetylacetonate, C^C = 1-phenyl-1,2,4-triazol-5-ylidene (1), C^C = 4-phenyl-1,2,4-triazol-5-ylidene (2), C^C = 2-phenylpyrazine (3)] have been investigated with density functional theory (DFT) and time-dependent density functional theory (TDDFT). The radiative decay rate constants of complexes 1–3 have been discussed with the oscillator strength (f_n), the strength of spin–orbit coupling (SOC) interaction between the lowest energy triplet excited state (T₁) and singlet excited states (S_n), and the energy gaps between E(T₁) and E(S_n). To illustrate the nonradiative decay processes, the transition states between triplet metal-centered (³MC) and T₁ states have been optimized and were verified with the calculations of vibrational frequencies and intrinsic reaction coordinate (IRC). In addition, the minimum energy crossing points (MECPs) between ³MC and ground states (S₀) were optimized. At last, the potential energy curves relevant to the nonradiative decay pathways are simulated. The results show that complex 3 has the biggest photoluminescence quantum yield because the complex 3 has the biggest radiative decay rate constant and the smallest nonradiative decay rate constant in complexes 1–3.     

The temperature dependent analysis of Au/TiO2 (rutile)/n-Si (MIS) SBDs using current–voltage–temperature (I–V–T) characteristics

In this study, the main electrical parameters of Au/TiO₂(rutile)/n-Si Schottky barrier diodes (SBDs) were analyzed by using current–voltage–temperature (I–V–T) characteristics in the temperature range 200–380 K. Titanium dioxide (TiO₂) thin film was deposited on a polycrystalline n-type Silicon (Si) substrate using the DC magnetron sputtering system at 200 °C. In order to improve the crystal quality deposited film was annealed at 900 °C in air atmosphere for phase transition from amorphous to rutile phase. The barrier height (Φ_b) and ideality factor (n) were calculated from I–V characteristics. An increase in the value of Φ_b and a decrease in n with increasing temperature were observed. The values of Φ_b and n for Au/TiO₂(rutile)/n-Si SBDs ranged from 0.57 eV and 3.50 (at 200 K) to 0.82 eV and 1.90 (at 380 K), respectively. In addition, series resistance (R_s) and Φ_b values of MIS SBDs were determined by using Cheung's and Norde's functions. Cheung's plots are obtained from the donward concave curvature region in the forward bias semi-logarithmic I–V curves originated from series resistance. Norde's function is easily used to obtain series resistance as a function of temperature due to current counduction mechanism which is dominated by thermionic emission (TE). The obtained results have been compared with each other and experimental results show that R_s values exhibit an unusual behavior that it increases with increasing temperature.     

High-performance diketopyrrolopyrrole-based organic field-effect transistors for flexible gas sensors

We demonstrate high-performance flexible polymer OFETs with P-29-DPP-SVS in various geometries. The mobilities of TG/BC OFETs are approximately 3.48 ± 0.93 cm²/V s on a glass substrate and 2.98 ± 0.19 cm²/V s on a PEN substrate. The flexible P-29-DPP-SVS OFETs exhibit excellent ambient and mechanical stabilities under a continuous bending stress of 1200 times at an R = 8.3 mm. In particular, the variation of μ_FET, V_Th and leakage current was very negligible (below 10%) after continuous bending stress. The BG/TC P-29-DPP-SVS OFETs on a PEN substrate applies to flexible NH₃ gas sensors. As the concentration of NH₃ increased, the channel resistance of P-29-DPP-SVS OFETs increased approximately 100 times from ∼10⁷ to ∼10⁹ Ω at V_SD = −5 V and V_GS = −5 V.     

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.     

Highly efficient bluish green phosphorescent organic light-emitting diodes based on heteroleptic iridium(III) complexes with phenylpyridine main skeleton

A new series of heteroleptic iridium(III) complexes, bis(2-phenylpyridinato-N,C2′)iridium (2-(2′,4′-difluorophenyl)-4-methylpyridine), (ppy)₂Ir(dfpmpy) and bis(2-(2′,4′-difluorophenyl)-4-methylpyridinato-N,C2′)iridium (2-phenylpyridine) (dfpmpy)₂Ir(ppy), have been synthesized by using phenylpyridine as a main skeleton for bluish green phosphorescent organic light-emitting diodes (PhOLEDs). The Ir(III) complexes showed high thermal stability and high photoluminescent (PL) quantum yields of 95% ± 4% simultaneously. As a result, the PhOLEDs with the heteroleptic Ir(III) complexes showed excellent performances approaching 100% internal quantum efficiency with a very high external quantum efficiency (EQE) of ∼27%, a low turn-on voltage of 2.4 V, high power efficiency of ∼85 lm/W, and very low efficiency roll-off up to 20,000 cd/m².     

Properties of fluorine-doped tin oxide films prepared by an improved sol-gel process

Fluorine-doped tin oxide (FTO) films were prepared by an improved sol-gel process, in which FTO films were deposited on glass substrates using evaporation method, with the precursors prepared by the conventional sol-gel method. The coating and sintering processes were combined in the evaporation method, with the advantage of reduced probability of films cracking and simplified preparation process. The effects of F-doping contents and structure of films on properties of films were analyzed. The results showed the performance index (Φ_TC=3.535×10⁻³ Ω⁻¹ cm) of the film was maximum with surface resistance (R_sh) of 14.7 Ω cm⁻¹, average transmittance (T) of 74.4% when F/Sn=14 mol%, the reaction temperature of the sol was 50 °C, and the evaporation temperature was 600 °C in muffle furnace, and the film has densification pyramid morphology and SnO_2−xF_x polycrystalline structure with tetragonal rutile phase. Compared with the commercial FTO films (Φ_TC=3.9×10⁻³ Ω⁻¹ cm, R_sh=27.4 Ω cm⁻¹, T=80%) produced by chemical vapor deposition (CVD) method, the Φ_TC value of FTO films prepared by an improved sol-gel process is close to them, the electrical properties are higher, and the optical properties are lower.     

Structural characterization of Zn1−xCdxO (0≤x≤0.20) microrods grown by spray pyrolysis

Zn_1−xCd_xO (x= 0.00, 0.05, 0.10, 0.15 and 0.20) thin films were obtained by spray pyrolysis and characterized by XRD, SEM, EDAX and optical measurements. The Zn_1−xCd_xO microrods are in the wurtzite crystallographic phase with (0 0 2) preferred orientation. A narrowing of the fundamental band gap from 3.30 to 3.10 eV was observed with the increasing nominal Cd content up to 20 at% due to the direct modulation of the band gap caused by Cd substitution. The undoped ZnO film showed two emission bands in the spectra: one sharp UV luminescence at ∼382 nm and one broad visible emission ranging from 430 to 600 nm. The sharp peak at ∼382 nm is split into two at 376 and 400 nm upon Cd doping at levels of 5 and 10 at%. However this splitting is not observed in the doped ZnO samples containing 15 at% Cd and more. It should also be mentioned that the broad peak at the range of 430–600 nm has almost disappeared in the films containing 5, 10 and 15 at% Cd.     

Thin film encapsulation for organic light emitting diodes using a multi-barrier composed of MgO prepared by atomic layer deposition and hybrid materials

We demonstrated an organic/inorganic multi-barrier and encapsulation for flexible OLED devices. The multi-barrier consisted of a silica nanoparticle-embedded hybrid nanocomposite, in short, S-H nanocomposite, and MgO, which were used as organic and inorganic materials, respectively. The S-H nanocomposite was spin-coated followed by UV curing. The thickness of the S-H nanocomposite was 200 nm, and 40 nm of MgO was deposited by atomic layer deposition (ALD) using Mg(CpEt)₂ and H₂O at 70 °C. The results of a Ca test showed that the 4.5 dyads of the MgO/S-H nanocomposite had a low water vapor transmission rate (WVTR) of 4.33 × 10^?6 g/m²/day and an optical transmittance of 84%. The normalized luminance degradation of the thin film encapsulated OLED was also identical to that of glass-lid encapsulation after 1000 h of the real operation time. We proposed low temperature ALD as a deposition method to create relatively thin film for OLED passivation without degradation, such as creation of dark spots. The results confirmed that it may be feasible for our multi-barrier to passivate flexible OLEDs devices.     

Efficient and stable red organic light emitting devices from a tetradentate cyclometalated platinum complex

An efficient and stable red phosphorescent organic light emitting diode was developed using a tetradentate cyclometalated platinum complex. Devices employing the phosphorescent molecule, platinum(II)-9-(4-methylpyridin-2-yl)-2-(3-(quinolin-2-yl)phenoxy)-9H-carbazole (PtON11Me), yielded high external quantum efficiencies and high operational lifetimes. A maximum EQE of 12.5% and color coordinates CIE (x = 0.61, y = 0.36) was achieved in devices employing efficient hole blocking and transporting materials and a high operational lifetime of T_0.97 ∼ 3200 h was achieved in devices utilizing electrochemically stable hole blocking and transporting materials.     

A high-yield vacuum-evaporation-based R2R-compatible fabrication route for organic electronic circuits

Advances are described in a vacuum-evaporation-based approach for the roll-to-roll (R2R) production of organic thin film transistors (TFTs) and circuits. Results from 90-transistor arrays formed directly onto a plasma-polymerised diacrylate gate dielectric are compared with those formed on polystyrene-buffered diacrylate. The latter approach resulted in stable, reproducible transistors with yields in excess of 90%. The resulting TFTs had low turn-on voltage, on–off ratios ∼10⁶ and mobility ∼1 cm²/V s in the linear regime, as expected for dinaphtho[2,3-b:2′,3′-f] thieno[3,2-b]thiophene the air stable small molecule used as the active semiconductor. We show that when device design is constrained by the generally poor registration ability of R2R processes, parasitic source–drain currents can lead to a >50% increase in the mobility extracted from the resulting TFTs, the increases being especially marked in low channel width devices. Batches of 27 saturated-load inverters were fabricated with 100% yield and their behaviour successfully reproduced using TFT parameters extracted with Silvaco’s UOTFT Model. 5- and 7-stage ring oscillator (RO) outputs ranged from ∼120 Hz to >2 kHz with rail voltages, V_DD, increasing from −15 V to −90 V. From simulations an order of magnitude increase in frequency could be expected by reducing parasitic gate capacitances. During 8 h of continuous operation at V_DD = −60 V, the frequency of a 7-stage RO remained almost constant at ∼1.4 kHz albeit that the output signal amplitude decreased from ∼22 V to ∼10 V. Over the next 30 days of intermittent operation further degradation in performance occurred although an unused RO showed no deterioration over the same period.