ESR studies of ambipolar charge carriers in metal–insulator–semiconductor diodes of regioregular poly(3-hexylthiophene)/PCBM composites

We have applied field-induced electron spin resonance (FI-ESR) method to organic metal–insulator–semiconductor (MIS) diodes using regioregular poly(3-hexylthiophene) (RR-P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) composites as the active layer, Al₂O₃ as the gate insulator and gold (Au) or aluminum (Al) as the top electrode. In the case of Al electrode, clear FI-ESR signals are observed with g ～ 2.002 and 1.999 for negative and positive gate voltage (V_G), respectively, demonstrating that these signals with former and the latter g-values correspond to field-induced positive polarons of RR-P3HT and PCBM radical anions, respectively. Both RR-P3HT and PCBM signals monotonically increase as |V_G| increases. The threshold voltages (V_th) for field-induced ambipolar charge carriers coincide well with those of capacitance measurements with hysteresis behaviors. On the other hand, in the case of Au electrode, only positive polarons signals are clearly observed and PCBM signals are nearly undetected. These different behaviors of FI-ESR signals between Al and Au top electrodes are discussed in relation to the difference of work functions.     

Low voltage organic light emitting diode using p–i–n structure

Efficient n-type doping has been achieved by doping Liq in electron transport material Alq₃. Detailed investigation of current density–voltage characteristics of electron only devices with different doping concentrations of Liq in Alq₃ has been performed. An increase in current density by two orders of magnitude has been achieved with 33 wt% of Liq doped in Alq₃. Organic light emitting diode with p–i–n structure was fabricated using F₄-TCNQ doped α-NPD as hole transport layer, Ir(ppy)₃ doped CBP as emitting layer and 33 wt% Liq doped Alq₃ as electron transport layer. Comparison of OLEDs fabricated using undoped Alq₃ and 33 wt% Liq doped Alq₃ as electron transport layer shows reduction in turn on voltage from 5 to 2.5 V and enhancement of power efficiency from 5.8 to 10.6 lm/W at 5 V.     

Photo-induced charge separation process in (PCBM-C120O)/(M3EH-PPV) blend solid film studied by means of X and K-bands ESR at 77 and 120 K

The new fullerene dimer adduct PCBM-C₁₂₀O was studied as an electron acceptor by light-induced ESR (LESR) when blended with the electron donor-conjugated polymer M3EH-PPV in a thin solid film. ESR parameters for the PCBM-C₁₂₀O anion radical and the M3EH-PPV polaron (triaxial g_x,y,z factors) were determined by X- and K-band ESR techniques at temperatures of 77 and 120 K. It was found that isotropic g₀ of PCBM-C₁₂₀O anion radical ESR spectra obtained in this blend under illumination is close to g-factors of electrochemically reduced (C₁₂₀-O)^? and does not coincide with the analogous parameter of (C₁₂₀-O)^2? (reported in literature). Spin relaxation parameters (T₁ and T₂) of the PCBM-C₁₂₀O anion radical and M3EH-PPV positive polaron are similar to T₁, T₂ for light-induced ion radicals in the P3HT/PCBM blend.     

Efficient multilayer organic light emitting diode

Some organic light emitting diodes with Alq₃ as emission layer, multilayer composed of m-MTDATA, NPB, TPD or SA as hole transport layer and lithium doped Alq₃ as electron injection assistant layer have been investigated. The current voltage characteristics and the electroluminescent (EL) output voltage characteristics have been investigated. It is found that the m-MTDATA is suitable as a hole injection layer close to the hole injection electrode ITO, NPB is suitable in contact with the emission layer, SA and TPD can be inserted between them to form an energy ladder structure, with which the efficiency of device has been increased. When thin lithium doped Alq₃ layer is inserted between Alq₃ and the aluminium electrode, the driving voltage of the devices has been clearly decreased while the current density and the EL output increased.     

Spin and charge distribution in hexaperylene hexafluorophosphate, (C20H12)6PF6

The 6:1 radical cation salt of perylene (PE) and hexafluorophosphate is an organic semiconductor with one unpaired electron spin per formula unit. The crystal structure consists of three nearly orthogonal (PE) dimers, which are grouped around the PF₆⁻ anion. Anisotropy and temperature dependence of the g tensor and linewidth of the electron spin resonance (ESR) at 9.5 GHz were studied, in order to determine the origin of the observed ESR line narrowing and the distribution of the radical spin density over the different PE molecules within the unit cell of (PE)₆PF₆.     

ESR studies of photogenerated polarons in regioregular poly(3-alkylthiophene)–fullerene composite

Electron spin resonance (ESR) studies of photogenerated polarons in composite of regioregular poly(3-octylthiophene) (PAT8) and fullerene (C₆₀) using variable photoexcitation energy up to 4.1 eV are reported. Two transient light-induced ESR (LESR) signals of photogenerated positive and negative polarons on PAT8 (g₁=2.002) and C₆₀ (g₂=1.999), respectively, are observed below 200 K, which are caused by photoinduced electron transfer between PAT8 and C₆₀. A remarkable enhancement of the LESR signals in excitation spectrum at around 1.8 eV is observed compared with the case of pure regioregular PAT8, which is consistent with the enhancement of the photoconductivity at around 1.8 eV where optically forbidden transition of C₆₀ occurs. Spin–lattice relaxation time of the polaron spins of PAT8 is obtained considering inhomogeneous broadening of the ESR linewidth due to the hyperfine interactions, which is at least approximately 18 times longer than that of C₆₀. Prompt and persistent LESR signal components are observed by transient response of the LESR signal. The prompt contribution shows a monotonic increase with increasing excitation-light intensity, while the persistent contribution is found to be excitation intensity independent due to deep traps of the photogenerated polarons.     

The new salt κ-ET2[Hg(SCN)2I]: crystal structure and physical properties

The structure, resistivity and ESR of a new organic conductor κ-ET₂[Hg(SCN)₂I](ET=bis(ethylenedithio) tetrathiafulvalene) are studied. Its main crystallographic parameters are found to be: M-1213.2, a-38.03(1), b= 11.80(1), c = 8.329(9) Å; γ=98.1(2)°; V-3700(2) ų; space group P2₁Ib; Z = 4. It is shown that the radical cations are packed according to the κ-type in the organic sheets, and the anions form polymerized chains. Two different radical cation layers with different amounts of shortened S···S contacts and distinct interaction between anionic and cationic sheets are found. ESR linewidth is found to be 9-11G (300 K), which is substantially narrower than a typical linewidth for ET-based radical cation salts of the κ-type. Based on temperature dependences of the resistance anisotropy and ESR parameters, we suggest that a structure rearrangement with electron localization may take place around 50 K. The comparison of crystal structure and properties of the title compound with other salts of the family ET₂[Hg(SCN)_3−nX_n], where X=Cl, Br, and n = 1.2, is camed out.     

Organic light-emitting devices with triphenylphosphine oxide layer

We have developed organic light-emitting devices using triphenylphosphine oxide (Ph₃PO) layers. The operating voltage of device is substantially reduced by using a Ph₃PO layer. For example, the required voltages for a current density of 20 mA/cm² are 3.5 and 9.7 V for the devices with Ph₃PO and Alq₃ layers, respectively. Good electron transporting property of Ph₃PO results in a high luminance of 1000 cd/m² at a low driving voltage of 4.1 V in a device with a structure of ITO/2-TNATA (15 nm)/α-NPD:rubrene (1%, 10 nm)/α-NPD (30 nm)/Ph₃PO (60 nm)/LiF (0.5 nm)/Al.     

LESR study on PPV–PPE/PCBM composites for organic photovoltaics

Photoinduced charge carrier generation and charge transfer in the new conjugated poly(phenylene vinylene)–poly(phenylene ethynylene) (PPV–PPE) copolymers mixed with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) were studied by LESR technique at 77 K. The effective charge separation has been verified by two ESR signals belonging to positive polarons on the polymer chains and to radical anions of PCBM molecules, respectively. The relaxation times T₁ and T₂ of the anion radical of PCBM were estimated by anisotropic linewidth analysis and microwave power saturation studies. The low temperature recombination of long living charge carrier pairs after cessation of photoexcitation was studied and described by a tunnel model in which the recombination time depends on the distances between charge carrier pairs. Results on PPV–PPE/PCBM were compared with those of a poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO–PPV)/PCBM composite.     

Fabrication and properties of p-type K doped Zn1−xMgxO thin film

A series of K doped Zn_1−xMg_xO thin films have been prepared by pulsed laser deposition (PLD). Hall-effect measurements indicate that the films exhibit stable p-type behavior with duration of at least six months. The band gap of the K doped Zn_1−xMg_xO films undergoes a blueshift due to the Mg incorporation. However, photoluminescence (PL) results reveal that the crystallinity decreased with the increasing of Mg content. The fabricated K doped p-type Zn_0.95Mg_0.05O thin film exhibits good electrical properties, with resistivity of 15.21 Ω cm and hole concentration of 5.54 × 10¹⁸ cm⁻³. Furthermore, a simple ZnO-based p-n heterojunction was prepared by deposition of a K-doped p-type Zn_0.95Mg_0.05O layer on Ga-doped n-type ZnO thin film with low resistivity. The p-n diode heterostructure exhibits typical rectification behavior of p-n junctions.     

A study of alkali metal triphenylides in the solid state

In the search for new organic superconductors based upon aromatic radical anions, recent theoretical predictions indicated that polycyclic aromatic compounds could be some interesting candidate. Among them triphenylene (Tp) seemed to be an interesting candidate. Triphenylides containing one, two or three alkali metal atoms (K, Rb, Cs) per Tp were synthesized. Only K salts had a semi-conductor like conductivity with a gap of 0.16 eV, all the others were insulators. Very weak LFMA signal was obtained for KTp at 4 K. A signal at g=4 was observed in the ESR spectrum of K₂Tp and Rb₂Tp. Infrared and Raman spectra of the solids are reported.     

Magnetic field effects on the conductivity of organic bipolar and unipolar devices at room temperature

Magnetic field effects on the conductivity of different types of organic devices: undoped and dye doped aluminium (III) 8-hydroxyquinoline (Alq₃)-based organic light emitting diodes (OLEDs), electron-only Alq₃-based diodes, and a hole-only N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD)-based diode were studied at room temperature. Only negative magnetoresistance (MR) was observed for the Alq₃-based devices. The addition of a rubrene dye in Alq₃-based OLEDs quenches the MR by a factor of 5. The α-NPD hole-only device showed only positive MR. Our results are discussed with respect to the actual models for MR in organic semiconductors. Our results are in good agreement with the bipolaron model.     

Inhomogeneous transport property of Alq3 thin films: Local order or phase separation?

Steady-state current–voltage (I–V) and impedance–voltage (Z–V) measurements were performed on in situ (UHV) prepared metal (Ag, Al)/Alq₃/indium-tin oxide (ITO) devices after exposure to air. When increasing the positive bias on the top metal electrode to a relatively well-defined critical value, a transition from semiconducting to semi- or even insulating behavior of the contacted Alq₃ thin film is observed by means of I–V measurements. The final insulating state remains stable when applying negative bias to the Ag electrode. In the case of the Al electrode, there is a voltammetric current wave under a well-defined negative bias indicating a redox reaction of mobile ions at the Al electrode.The Z–V measurements reveal a peculiar feature of ac transport through the Alq₃ thin films, namely the equivalent series capacitance is equal to its parallel counterpart in the frequency range from 100 to 1 MHz and amounts to only a fraction (0.3–0.5) of the expected geometrical capacitance of the device. An equivalent electrical circuit has been developed, based on the existence of two parallel transport paths: an insulating (amorphous) Alq₃-phase shunted by a semiconducting (semi-insulating) one, both running into the impedance of the back contact. The equivalent circuit model composed exclusively of frequency independent elements is useful for predicting the maximum frequency for retaining the full geometrical capacitance. Even though the model is capable of describing the bias dependence of the impedance correctly, it does not shine light on the nature of the (ordered) phase or domain responsible for the dielectric loss. The possibility of local order connected with dipole–dipole interaction in the metal/Alq₃ interface zone is discussed. In any case, the ordered portion of the organic material seems to form the huge interface dipole of about 1 eV with Ag or Al [M.A. Baldo, S.R. Forrest, Phys. Rev. B 64 (2001) 085201], the direction of the dipole promoting electron injection to Alq₃. Then the semiconductor-to-insulator transition could be initiated by a damage of the interface dipole under a critical positive dc bias of the metal, preventing the flow of both dc and the real component of low-frequency ac current. The transition is not accompanied by any significant change in the impedance of the back contact common to both phases.     

Organic light-emitting device using new distyrylarylene host materials

Novel distyrylarylene-based blue host materials, 4,4′-bis(1,2-diphenylvinyl)-p-terphenyl (2), 4,4′-bis[1-phenyl-2-(p-tolyl)vinyl]-p-terphenyl (3) and 4,4′-bis[1-phenyl-2-(4,4′-biphenyl)vinyl]-p-terphenyl (4) were successfully synthesized using 4,4′-bisbenzoyl-p-terphenyl (1) through Wittig–Hornor reaction and a blue organic light-emitting diode (OLED) was made from them. The structure of the blue device is ITO/DNTPD/NPB/Host:DSA-Ph/Alq₃/Al-LiF. Here, NPB is used as a hole transport layer, DNTPD as the hole injection layer, DSA-Ph as the blue dopant, Alq₃ as the electron transport layer and Al as the cathode. The blue device doped with 5%-DSA-Ph shows a blue EL spectrum at 463 nm and a high efficiency of 4.14–5.13 cd/A.     

Interfacial charges in organic hetero-layer light emitting diodes probed by capacitance–voltage measurements

The bias dependent capacitance of organic hetero-layer light emitting diodes (LEDs) based on N,N′-diphenyl-N,N′-bis(1-naphtyl)-1,1-biphenyl-4,4 diamine (NPB) and tris(8-hydroxyquinoline)aluminium (Alq₃) shows below the built-in voltage, a step-like change from a value corresponding to the total organic layer thickness to a higher value given by the Alq₃ layer thickness. The bias and frequency dependent behaviour of the capacitance can be explained by the presence of negative charges with a density of −6×10¹¹ e/cm² at the NPB–Alq₃ interface. This leads to an inhomogeneous potential distribution inside the device with a discontinuity of the electric field at the organic–organic interface.     

Electrical properties and spectroscopic studies of HClO4-doped poly(p-diethynylbenzene)

Electrical properties of poly(p-diethynylbenzene) (PDEB), chemically doped with perchloric acid (HClO₄), were investigated as a function of dopant concentration and relative humidity. The doped polymer was characterized by X-ray photoelectron spectroscopy (XPS), UV—Vis, IR, electron spin resonance (ESR) and scanning electron microscopy (SEM). A possible structure of the HClO₄-doped PDEB was proposed on the basis of spectroscopic studies. In the presence of humidity, HClO₄-doped PDEB follows the proton conduction mechanism.     

Energetic trap distributions in organic semiconductors

The electronic trap distributions of the vapor deposited electron transport material Alq₃ [tris(8-(hydroxyquinoline) aluminum] and hole transport material 1-NaphDATA [4,4′,4″-tris(N-(1-naphthyl)-N-phenylamino)-triphenylamine] have been analyzed by the fractional TSC method (thermally stimulated current) and the thermally stimulated luminescence (TL) technique. The obtained trap distributions can be described by a Gaussian distribution in case of Alq₃ and two discrete trap levels in case of 1-NaphDATA. In the case of Alq₃, a distinction between the trapped charge carrier polarity was possible. A correlation between the I–V curves of electron- and hole-only devices and the respective trap distributions suggests that the trap distribution is responsible for the shape of the observed current–voltage characteristics. It will be shown that the observed trap states cannot be explained by intrinsic tail states of regular HOMO and LUMO levels.     

Doping competition of anions during the electropolymerization of pyrrole in aqueous solutions

The doping competition of anions in the electropolymerization of pyrrole was studied in aqueous solutions containing two kinds of anions with the same concentration of 0.1 mol dm^-3. The composition analysis of the polypyrrole (PPy) films prepared shows that the doping amount of p-toluene sulfonate (TsO^-) prevails over that of perchlorate, chloride and nitrate anions. The conductivity of the PPy film is close to the higher value of the two PPy films doped with a single anion. When polymerization is performed in the solution containing TsO^- and NO₃^-, TsO^-is mainly doped at Structure I, while NO₃^- is mainly at Structure II of the PPy film.     

A study of electroless copper-phosphorus coatings with the addition of silicon carbide (SiC) and graphite (Cg) particles

Copper composite coating with graphite (C_g) and/or silicon carbide (SiC) particles were deposited by electroless plating. The surface morphology of the coatings that were analysed using scanning electron microscopy (SEM) showed that Cu particles were uniformly distributed. The obtained coating thickness was approximately ± 5 μm. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) techniques were used to characterise the structure and to study the phase transition of the coatings, respectively. Phases such as Cu, Cu₂O, Cu₃P, Cu₃Si, SiC and C_g were observed from X-ray diffraction patterns and the presence of Cu₂O, Cu₃P and Cu₃Si was confirmed by differential scanning calorimetry (DSC) studies. The results demonstrated that SiC and C_g particles have little influence on the phase transition of the coating. The hardness and wear resistance of Cu-P composite coatings were improved with the incorporation of SiC particles. The friction coefficient of Cu-P composite coatings decreased with the incorporation of C_g particles. Atomic force microscopy (AFM) results of coatings showed that the roughness of the coatings increased with the incorporation of SiC to the Cu-P coatings and decreased with the incorporation of C_g. Cu-P-C_g-SiC composite coatings showed a moderate roughness, hardness between Cu-P-SiC and Cu-P-C_g coatings, had low friction and good anti-wear properties. The anti corrosion resistance of the electroless Cu-P composite coatings on carbon steel were studied in 3.5% NaCl and 1 M HCl solutions by the potentiodynamic polarisation technique. The study revealed that the corrosion resistance increased with the incorporation of SiC particles in the Cu-P and Cu-P-C_g matrix but reduced with the incorporation of graphite.     

Synthesis of electrochemically and thermally stable amorphous hole-transporting carbazole dendronized fluorene

A novel amorphous hole-transporting carbazole dendrimer, 2,7-bis[3,6-di(carbazol-9-yl)carbazol-9-yl]-9,9-bis-n-hexylfluorene (G2CF), was synthesized by a divergent approach involving bromination and Ullmann coupling reactions. G2CF showed UV–vis absorption bands at 304 and 332 nm in chloroform solution and the photoluminescence spectra showed a maximum peak at 373 nm in a bluish-purple region. Differential scanning calorimetry (DSC) and cyclic voltammetry (CV) analysis revealed G2CF was an electrochemically and thermally stable amorphous material with a high glass transition temperature (T_g) of 237 °C. The organic light-emitting device having the structure of ITO/G2CF/Alq₃/LiF:Al exhibited a bright green emission with a maximum luminescence of 11,000 cd/m² at 16 V and a turn-on voltage of 5.4 V.