Aging induced traps in organic semiconductors

Trap states in organic single layer devices have been detected by the method of thermally stimulated currents (TSC). These devices have been exposed to potentially harmful atmospheres of oxygen and humid nitrogen in order to investigate the influence of ambient atmosphere on the trap spectrum of materials used for organic light-emitting diodes. Discrete traps with activation energies of 105 and 140 meV have been found in α-NPD (N,N′-di(1-naphthyl)-N,N′-diphenylbenzidin). In 1-NaphDATA (4,4′,4″-Tris(N-2-naphthyl)-N-phenylamino-triphenylamin) discrete trap levels with activation energies of 100 and 235 meV are present. The exposure to humidity forms a deep hole trap of 0.5 eV in 1-NaphDATA, whereas in α-NPD no new trap was detected. The influence of the so generated traps on the charge transport is demonstrated by I–V characteristics.     

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.     

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.     

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.     

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.     

Electron spin resonance of thin films of organic light-emitting material tris(8-hydroxyquinoline) aluminum doped by magnesium

We have successfully observed electron spin resonance (ESR) signals of radical anions in thin films of tris(8-hydroxyquinoline) aluminum (Alq₃), a compound widely used as electron transporting and luminescent layers in organic light-emitting diodes. To obtain definitely defined radical-anion states in Alq₃, we doped Alq₃ with Mg by co-evaporating these materials. The obtained g value and peak-to-peak ESR linewidth ΔH_pp of Alq₃ radical anions are 2.0030 and 2.19 mT, respectively. Theoretical g value and hyperfine interactions were calculated by density functional theory method, which are in good agreement with the experimental results. A quantitative evaluation of doping concentration was performed. We confirmed that doped charges are localized at deep trapping sites by the lineshape analysis and temperature dependence of the ESR signals. Morphological investigation using transmission electron microscopy clarified that the co-evaporated Mg atoms form clusters.     

Spiro[fluorene-7,9′-benzofluorene] host and dopant materials for blue light-emitting electroluminescence device

New spiro-type 5-biphenyl-spiro[fluorene-7,9′-benzofluorene] (BH-1BP) and 5-diphenyl amine-spiro[fluorene-7,9′-benzofluorene] (BH-1DPA) were synthesized for use as blue organic light-emitting host and dopant materials, respectively. Their optical properties, including their UV absorption, photoluminescence and energy levels, were measured and blue OLEDs were made from them. The structure of the blue device is ITO/DNTPD/α-NPD/BH-1BP:5% dopant/Alq₃ or ET4/Al–LiF. Here, α-NPD is used as the hole transport layer, DNTPD as the hole injection layer, BH-1DPA or diphenyl-[4-(2-[1,1;4,1]terphenyl-4-yl-vinyl)-phenyl]-amine (BD-1) as the blue dopant materials, Alq₃ or ET4 as the transporting layer and Al as the cathode. The blue devices doped with 5% BH-1DPA and BD-1 show blue EL emissions at 444–448 nm at 7 V. A high efficiency of 3.28 cd/A and the CIE coordinates (0.14, 0.11) at 7 V can be achieved from the devices composed of BH-1BP:5% BD-1/ET4.     

Thermally stimulated processes in heterocyclic materials suitable for heterolayer organic light emitting diodes

We studied ultraviolet photoelectron spectroscopy (UPS) and thermally stimulated processes in two types of phenylquinoxalines, and one type of starburst trisphenylquinoxaline in order to characterize the valence band structure and the localized states. An ionization potential of about 6.0 eV was determined for each material using UPS. Thermally stimulated discharge currents (TSDC) gave evidence of dipolar and/or charge transport mechanisms. We assigned each TSDC peak to the different elementary processes using in addition dielectric spectroscopy (DES), and from thermally stimulated luminescence (TSL) curves, performed for the first time on these materials, we estimated the mean trap depths.     

Synthesis,photoluminescent and electroluminescent properties of a novel europium(III) complex involving both hole- and electron-transporting functional groups

A novel europium(III) complex involving a carbazole fragment as hole-transporting group and an oxadiazole fragment as electron-transporting group was synthesized and used as red light-emitting material in organic light-emitting diodes (OLEDs). The complex is amorphous, and exhibits high glass transition temperature (T_g = 157 °C) and high thermal stability with a 5% weight loss temperature of 367 °C. Two devices, device 1: ITO/NPB (40 nm)/Eu(III) complex (30 nm)/Alq₃ (30 nm)/LiF (1 nm)/Al (100 nm) and device 2: ITO/NPB (40 nm)/3% Eu(III) complex: CBP (30 nm)/BCP (10 nm)/Alq₃ (30 nm)/LiF (1 nm)/Al (100 nm), were fabricated, where NPB is N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine, Alq₃ is tris(8-hydroxyquinoline) Al(III), CBP is 4,4′-bis(carbazole-9-yl)-biphenyl, and BCP is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, respectively. In contrast with device 1, owing to less self-quenching and better charge confinement, device 2 shows improved performances: the maximum luminance of device 2 was dramatically increased from 199 to 1845 cd/m², the maximum current efficiency was increased from 0.69 to 2.62 cd/A, the turn-on voltage was decreased from 9.5 to 5.5 V, and higher color purity was attained.     

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.     

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.     

Anomalous room temperature magnetoresistance in organic semiconductors

We report the substantial change in the large room temperature (∼8% at 100 Oe, up to 15% at 1000 Oe) magnetoresistance of thin organic semiconductor films of tris-(8-hydroxyquinoline) aluminum (Alq₃) upon doping with PtOEP and Ir(ppy)₃ complexes. The origin of magnetic field effects on charge transport properties of organic semiconductors until now has remained obscure. We propose a model for the anomalous magnetoresistance and its change with doping based on the charge transport in these semiconductors being electron–hole (e–h) recombination limited. The process of e–h recombination includes formation of correlated e–h pairs and the subsequent annihilation of e–h pairs with different rates for the singlet and triplet spin states. The e–h pairs may also dissociate back into free charge carriers. We suggest that a magnetic field controls spin interconversion of e–h pairs. In the absence of field the singlet mixes with the entire triplet manifold by hyperfine interaction. The magnetic field lifts the triplet degeneracy, and for strong field, the mixing remains only between the singlet and the T₀ component of the triplet, thus changing the e–h recombination rate and hence the current. The experimental results are consistent with the model.     

Site occupancy and mechanisms of thermally stimulated luminescence in Ca9Ln(PO4)7 (Ln = lanthanide)

The structural properties, X-ray-excited radioluminescence and thermally stimulated luminescence (TSL) of Ca₉Ln(PO₄)₇ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) were investigated. (La–Nd) occupied mainly the largest M3 site, whereas the smallest lanthanides (Yb, Lu) adopted the smallest octahedral M5 site. The intermediate-sized ions (Sm–Tm) occupied the medium-sized sites M1 and M2. Only the Ln³⁺ ions with the highest Ln^3+/4+ ionization energy (Ln = Pr, Nd, Tb, Dy) were able to trap holes and become luminescent centers for thermally stimulated Ln³⁺ 4f–4f emission. For low Ln^3+/4+ ionization potential (Ln = La, Gd, Yb, Lu), no hole trapping at Ln³⁺ was possible and Mn²⁺ impurities at the M5 sites were the effective hole traps and luminescent centers for thermally stimulated Mn²⁺ 3d–3d red emission. Very broad TSL peaks were attributed to thermally assisted detrapping of electrons from positive traps, possibly Ln³⁺ at Ca²⁺ sites, tunneling back towards the hole trap/luminescent center. Ca₉Ln(PO₄)₇ compounds doped with Ln = Tb or Lu were found to be efficient X-ray-excited long-lasting phosphorescence materials, emitting in the green and in the red, respectively.     

Highly efficient organic light-emitting diodes using novel hole-transporting materials

A new class of tetraminobiphenyl derivatives, which contains a 3,3′,5,5′-tetraminobiphenyl core, has been synthesized and examined as a hole-transporting material for organic light-emitting diodes. We fabricated the organic light-emitting diode (OLED) cells with tetraminobiphenyl derivatives as hole-injecting layer, hole-transporting layer and hole-injecting and transporting layer for green device with tris(8-quinolinolato)aluminum (Alq₃) doped with 1% of Coumarin 545T (C545T) as green emitting layer. Tetraminobiphenyls were found to be useful as a novel hole-transporting material. The electroluminescent device with the 3,3′,5,5′-tetrakis(p-tolyldiamino)biphenyl (TTAB) as a hole-transporting layer was more efficient than that with the analogous triarylamine, N,N′-di(naphthalene-1-yl)-N,N′-diphenyl benzidine (NPB). The external luminous efficiency of the device IV having TTAB as one hole-injecting and transporting layer can reach 14.55 cd/A, which is higher than the standard device I (11.66 cd/A) using two layers, a hole-injecting layer and a hole-transporting layer.     

A blue organic emitting diode derived from new styrylamine type dopant materials

We have designed and synthesized new dopant materials based on the styrylamine moiety, 4-[(1,2-diphenyl)-4′-(N,N-diphenyl-4-vinylbenzenamine)]biphenyl (4) and 4-[(1,2-diphenyl)-4′-(N,N-diphenyl-4-vinylbenzenamine)]terphenyl (8). Blue OLEDs were obtained from new styrylamine dopant materials and compared with those of blue dopant bis[4-(di-p-N,N-diphenylamino)styryl]stilbene (DSA-Ph) and diphenyl[4-(2-terphenyl vinyl)phenyl]amine (R-BD). The ITO/DNTPD/NPB/MADN:dopant/Alq₃/Al-LiF device obtained from 4 shows blue EL spectrum at 469 nm and high efficiency 3.02 cd/A at 7 V. 8 also shows blue EL spectrum around λ_max = 468 nm, efficiency of 3.51 cd/A and a current density of 25.94 mA/cm² (855.7 cd/m²) at 7 V.     

Transient electroluminescence in multilayer organic light-emitting diodes: experiment and theory

We have investigated a multilayer organic light-emitting diode (OLED) with 1,3,5-tris(N,N-bis-(4,5-methoxy-phenyl)-aminophenyl)-benzene (TAPB) acting as hole transporting layer (HTL) and tris(8-hydroxy-quinolinolato) aluminium (Alq₃) as electron transporting layer (ETL). Positive charge carriers in the HTL were detected optically as a function of the applied bias. Furthermore, we investigated the DC-characteristics of current and brightness as well as the onset behaviour of the electroluminescence (EL) as a function of the applied bias. An analytical model is developed to describe the observed carrier concentrations as well as the current–voltage characteristics and the transient EL measurements quantitatively.     

Determination of trap levels in CZT:In by thermally stimulated current spectroscopy

Many defects in semi-insulating (SI) cadmium zinc telluride (Cd_1-xZn_xTe or CZT) ingots grown by the melt methods act as trapping centers to introduce deep levels in the band gap, which has strong effects on CZT detection properties. The thermally stimulated current (TSC) spectroscopy was used to measure these traps, and the initial rise method and the simultaneous multiple peaks analysis (SIMPA) method were introduced to characterize trap levels in SI-CZT:In. The results show that there is a larger error in the determination for the trap peaks with the initial rise method due to the interference of overlapping peaks, while the SIMPA method demonstrates a better performance in resolving these overlapping peaks simultaneously for a full characterization of trap levels. On this basis, a theoretical SIMPA fitting, which is composed of ten trap levels and a deep donor level E_DD dominating the dark current in SI-CZT:In, is achieved. Furthermore, the reason of high resistivity in CZT:In was explained by the relationship between E_DD level and Fermi level.     

Improved electron injection into Alq3 based OLEDs using a thin lithium carbonate buffer layer

A cathode buffer layer of lithium carbonate (Li₂CO₃) we used to improve the electro-optical properties of organic light-emitting diodes (OLEDs). Li₂CO₃ layers with various thicknesses were prepared by thermally evaporating Li₂CO₃ powders. When a 1-nm-thick Li₂CO₃ layer was inserted between the aluminum (Al) cathodes and tris(8-hydroxyquinolinato)aluminum (Alq₃) electron-transporting layers, device properties such as the turn-on voltage, the maximum luminance, and the device efficiency were improved, becoming better than and comparable to those of devices with LiF and Cs₂CO₃ buffer layers. The surface of the Alq₃ film became smoother after the Li₂CO₃ layer was deposited. The reaction mechanisms between Li₂CO₃ and Alq₃ were also investigated. X-ray and ultraviolet photoelectron spectroscopy results show that some electrons transfer from Li₂CO₃ into Alq₃, which increases the electron concentration in Alq₃ films and moves the Fermi level close to the lowest unoccupied molecular orbital (LUMO) of Alq₃. Thus, the electron injection efficiency was enhanced due to a lower electron injection barrier, which improves the charge carrier balance in OLEDs and leads to better device efficiency.     

Determination of trap states in ladder type polymers

Electrically active defects have a profound impact on the semiconducting properties of conjugated polymer systems. Defect-induced thermally stimulated current (TSC), technique is used to probe the trap levels in these polymers. Detailed TSC investigations in oriented and as cast films of poly(benzimadazobenzophenanthroline) BBL, a ladder-type, high-temperature conjugated polymer are carried out. We estimate the energetics and the density of the trap states in BBL using this technique. Results obtained from complimentary experiments such as long-lived photocurrent studies are correlated with that obtained from TSC. We also demonstrate the applicability of this approach to characterise trap states in other conjugated polymer systems such as poly-3-octylthiophene, P3OT and poly(2-methoxy-5-(2′-ethyl-hexoxy)-1,4-para-phenylenevinylene) (MEHPPV).     

Theoretical studies on electronic and electron blocking properties of iridium complexes with phenylpyrazolato ligands

The tris(1-phenylpyrazolato,N,C2′)iridium(III) Ir(ppz)₃, (fac-Ir(ppz)₃, 1; mer-Ir(ppz)₃, 2) and iridium(III)bis(1-phenylpyrazolato,N,C2′) (2,2,6,6-tetramethyl-3,5-heptane-dionato-O,O) ppz₂Ir(dpm) (C-cis,N-trans-ppz₂Ir(dpm), 3; C-cis,N-cis-ppz₂Ir(dpm), 4) have been investigated theoretically to explore their electronic structures, spectroscopic and electron blocking properties. A detailed comparison of the electronic structure characteristics of the two isomers has been addressed for pointing out differences in absorption and emission properties. The geometries and electronic structures are investigated at B3LYP and CIS levels for ground and excited states, respectively. At the TD-DFT and PCM levels, 1–4 give rise to absorptions at 329, 346, 355 and 347 nm, respectively, and phosphorescent emissions at 377, 461 and 405 nm for 1–3, respectively. The transitions of 1–2 are attributed to [d(Ir) + π(phenyl)] → [π*(pyrazolyl)] charge transition, whereas 3–4 are related to [d(Ir) + π(phenyl)] → [π*(pyrazolyl) + π*(dpm)]. The reorganization energies computed for hole (λ_hole) except 2 are smaller than that of N,N′-diphenyl-N,N′-bis(1,1′-biphenyl)-4,4′-diamine which is a typical hole transport material. Fac-Ir(ppz)₃ is the most efficient electron blocking material among the four complexes.