Bipolar Transport Research Articles

Synthesis and characterization of novel phosphorescent host materials based on triphenylpyridine derivatives

Three novel phosphorescent host materials, 4′,4‴-(4-phenylpyridine-2,6-diyl)bis(N,N-diphenyl-[1,1′-biphenyl]-4-amine) (P1), 2,6-bis(4′-(naphthalen-1-yl)-[1,1′-biphenyl]-4-yl)-4-phenylpyridine (P2), and 2,6-bis(3′,5′-dimethyl-[1,1′-biphenyl]-4-yl)-4-phenylpyridine (P3), based on triphenylpyridine with a symmetrical molecular conformation were designed and synthesized from 2,6-bis(4-bromophenyl)-4-phenylpyridine (3) reacting with [4-(diphenylamino)phenyl]boronic acid (A1), [4-(naphthalen-1-yl)phenyl]boronic acid (A2), and (3,5-dimethylphenyl)boronic acid (A3) through Suzuki coupling reaction, respectively. The compounds obtained were characterized by elemental analysis, 1H NMR, and 13C NMR spectroscopy. The thermal properties of these materials were studied by thermogravimetric analysis and differential scanning calorimetry measurements, which showed that they exhibited excellent thermal stability with glass transition temperatures over 94 °C. Their photophysical properties were also investigated, and the results indicated that all compounds possessed considerably high triplet energy level greater than 2.6 eV; they also emitted blue fluorescence in different solvents. Finally, the results of the density function theory calculations revealed that the compound P1 showed the well-separated frontier molecular orbital, which indicated that it possessed bipolar carrier transport ability for holes and electrons, whereas the compound P2 and P3 exhibited greater overlap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital.

Bipolar Thermoelectrical Transport of SnSe Nanoplate in Low Temperature**Supported by the National Basic Research Program of China under Grant No. 2016YFA0200800, the Strategic Priority Research Program of Chinese Academy of Sciences under Grant No. XDB30000000, the Strategic Priority Research Program of Chinese Academy of Sciences under Grant No. XDB07030100, the Sinopec Innovation Scheme (A-381), and the Rise.Sinopec Fund under Grant

Bulk SnSe is an excellent thermoelectrical material with the highest figure-of-merit value of ZT = 2.8, making it promising in applications. Temperature-dependent electrical and thermoelectrical properties of SnSe nanoplates are studied at low temperature. Conductivity drops and rises again as temperature is lowered. The Seebeck coefficient is positive at room temperature and becomes negative at low temperature. The change of the sign of the Seebeck coefficient indicates influence of bipolar transport of the semiconductive SnSe nanoplate. The bipolar transport is caused by the Fermi energy changing with temperature due to different contributions from donors and acceptors at different temperatures.

Efficient aggregation-induced delayed fluorescent materials based on bipolar carrier transport materials for the fabrication of high-performance nondoped OLEDs with very small efficiency roll-off

The integration of bipolar carrier transport materials with an aggregation-induced delayed fluorescence unit generates robust luminescent materials, which lead to efficient nondoped OLEDs with high efficiency stability.

Modulation in structural and electronic properties of 2D Ga2O3 by chemical passivation

The dangling bonds on the surfaces of 2D Ga2O3 can be effectively passivated by hydrofluorination, making 2D Ga2O3 with ultra-high carrier mobility and bipolar transport property.

Magnetic bipolar transistor based on ZnO/NiO/Si heterostructure using pulsed laser deposition

Oxide semiconductors are promising candidates for next generation electronics. In this work, magnetic bipolar transistor was fabricated by growing thin films of p-NiO and n-ZnO on n-type silicon wafer by pulsed laser deposition technique with an in-situ annealing at 670° C in the presence of oxygen. The structural characterization of these films was done by X-ray diffraction and Raman spectroscopy and magnetic properties were studied by vibrating sample magnetometer (VSM). I-V characteristic of fabricated transistor was tested in common emitter configuration with DC biasing. Junction parameters such as ideality factor, series resistance, and transistor parameters like q-point were determined by using conventional transistor output characteristics. The diode and transistor showed an increase in current with the externally applied magnetic field due to the presence of Nickel or Oxygen vacancies in NiO attributing to spin polarized bipolar transport. Therefore the current amplification in these devices can be controlled by spin; making it attractive for spintronic applications.

Balancing charge-transporting characteristics in bipolar host materials

Bipolar host materials with balanced charge transporting property were synthesized using a solvent-less green reaction method. The characteristics of these synthesized bipolar host materials were examined by thermal and spectroscopic analysis. The energy levels of these materials were estimated from cyclic voltammograms and absorption spectra. The optimized molecular geometries and spatial distributions were obtained from molecular simulations. Moreover, the current density vs. voltage features of single-carrier devices were investigated to assess the bipolar transport characteristics of the host materials. As the length of the alkyl chain increased, the electron-transporting capability and hole-transporting ability exhibited optimum values at the octyl chain attachment. The current efficiency, power efficiency and quantum efficiency values of white organic light-emitting diodes prepared by blending blue and yellow iridium phosphors with these bipolar hosts were high with decreasing alkyl chain length. The result was remarkably similar to the trend of current density characteristics of single-carrier devices. The power efficiency of the octyl chain attached bipolar host was approximately three times higher than that of typical blended hole and electron transporting materials. This enhanced efficiency was attributed to the well-balanced charge transfer by the bipolar host material inside an emissive layer. Moreover, the morphology of the device fabricated with a blend of charge transport materials changed due to deterioration, whereas that of the device fabricated with bipolar host materials did not change after 12 h of operation at 9 V.

Theoretical Studies of Bipolar Transport in CnBTBT-FmTCNQ Donor-Acceptor Cocrystals.

The development of crystals with bipolar transport characteristics is essential for high-performance organic field effect transistor (OFET) devices. In this work, we theoretically investigated the bipolar transport behaviors in CnBTBT-FmTCNQ cocrystals. It is found that bipolar transport can be realized in C8BTBT-TCNQ and C12BTBT-TCNQ cocrystals with room-temperature electron/hole mobility up to 1.8/0.75 and 2.5/1.8 cm2 V-1 s-1, respectively. The comparable electron- and hole-transfer integrals between the nearest-neighbor molecule pairs as well as the small hole reorganization energy of the TCNQ molecule are responsible for the balanced electron and hole mobilities. Moreover, because of the π-π stacking between neighboring molecules, all cocrystals show strong anisotropic transport characteristic for both electron and hole transport with the mobility along the π-π stacking direction much larger than those along the other two directions. This work provides the possibility of high-performance OFET engineering and also enriches the OFET families with bipolar transport characteristics.

Synthesis and Semiconducting Characteristics of the BF2 Complexes of Bisbenzothiophene-Fused Azadipyrromethenes.

A novel type of dibenzothiophene [b]-fused core-expanded azaBODIPYs were obtained through an efficient post-functionalization of tetrabrominated azadipyrromethenes, using CuI-catalyzed cyclization, followed by BF2 complexation. These dyes show nearly planar skeletons, strong NIR absorption with maximum peaks up to 733 nm, and remarkable low-lying LUMO level of -4.15 eV. The field-effect transistor based on 1b exhibits bipolar transport properties, with the highest electron and hole mobilities up to 0.012 and 0.046 cm2 V-1 s-1, respectively.

Large transverse thermoelectric figure of merit in a topological Dirac semimetal

Thermoelectric (TE) conversion in conducting materials is of eminent importance for providing renewable energy and solid-state cooling. Although traditionally, the Seebeck effect plays a key role for the TE figure of merit zST, it encounters fundamental constraints hindering its conversion efficiency. Most notably, there are the charge compensation of electrons and holes that diminishes this effect, and the intertwinement of the corresponding electrical and thermal conductivities through the Wiedemann-Franz (WF) law which makes their independent optimization in zST impossible. Here, we demonstrate that in the Dirac semimetal Cd3As2 the Nernst effect, i.e., the transverse counterpart of the Seebeck effect, can generate a large TE figure of merit zNT. At room temperature, zNT = 0.5 in a small field of 2 T; it significantly surmounts its longitudinal counterpart zST for any field and further increases upon warming. A large Nernst effect is generically expected in topological semimetals, benefiting from both the bipolar transport of compensated electrons and holes and their high mobilities. In this case, heat and charge transport are orthogonal, i.e., not intertwined by the WF law anymore. More importantly, further optimization of zNT by tuning the Fermi level to the Dirac node can be anticipated due to not only the enhanced bipolar transport, but also the anomalous Nernst effect arising from a pronounced Berry curvature. A combination of the former topologically trivial and the latter nontrivial advantages promises to open a new avenue towards high-efficient transverse thermoelectricity.

Unipolar Injection and Bipolar Transport in Electroluminescent Ru-Centered Molecular Electronic Junctions

Bias-induced light emission and light-induced photocurrents were used as independent probes of charge transport in carbon-based molecular junctions containing Ru(bpy)3. The thickness, bias, and tem...

Electrical contact considerations for diamond electron emission diodes

The demonstration of diamond devices has substantiated the superior capability of diamond in high power electronics that relied on the preparation of p-type and n-type diamond through boron and phosphorus doping, respectively, and the growth of high purity intrinsic diamond. We present an approach for electrical contacts to homoepitaxial, phosphorus doped, n-type, diamond that utilizes an interfacial layer of highly nitrogen doped, nanostructured carbon grown by plasma enhanced CVD (PECVD). This contact strategy was utilized in a pin diamond diode for electron source applications. The pin-nano-carbon structure was prepared on HPHT type IIb (111) oriented substrates with intrinsic, n-type, and nano-carbon layer grown in dedicated PECVD systems. The nanostructured nitrogen doped carbon layer was synthesized under argon addition to promote re-nucleation. Diodes from this pin-nano-carbon structure were prepared by lithography and mesa-etched devices contacted by Ti/Pt/Au metallurgy. Final processing in a hydrogen plasma established negative electron affinity properties for electron emission. Electrical characterization of the diodes commenced in vacuum after annealing at ~600 °C for 15 min and observation of exciton light emission indicated bipolar transport. At a forward bias of 14 V a current of 0.1A was measured and at 17 V its increase to 0.5A corresponded to a current density >1500 A/cm2. Compared to conventional pin diodes, the introduction of the nano-carbon layer enhanced the diode and electron emission current by more than an order of magnitude. This was attributed to the reduced contact resistivity of 5.5 × 10−3 Ω cm2 at room temperature. Light emission and diode operation at temperatures >750 °C indicated superior stability of the electrical contact. The n-type layer was characterized by SIMS indicating a phosphorus incorporation of ~2 × 1019cm-3 and for the nano-carbon layer a nitrogen incorporation of ~5 × 1020cm−3. Addressing contact limitations to n-type diamond through the growth of moderately phosphorus doped epilayers followed by highly nitrogen doped nano-carbon layers could provide a preferred approach for electronic devices that could also be extended to (100) surfaces.

Fast self-trapping of light in photorefractive semiconductors

The paper presents an analysis of fast self-trapping of light as part of high-intensity pulse propagation in photorefractive gallium arsenide. Temporal evolution of picosecond pulses has been analysed for various material parameter values, using numerical techniques which take into consideration bipolar carrier transport and the hot-electron effect. A strong laser beam self-bending effect, which can be used for switching optical signals, has been demonstrated, and the conditions of its occurrence have been discussed.

P605Correlation of left atrial phasic transport function and arrhythmogenic substrate in patients with atrial fibrillation: cardiac magnetic resonance feature tracking and bipolar voltage mapping

Abstract Background Bipolar voltage mapping is a widely accepted approach to identify areas of arrhythmogenic substrate in patients presenting for atrial fibrillation (AF) ablation. However, until now little is known about the correlation of left atrial (LA) bipolar voltage distribution and LA transport function. Purpose To study the impact of LA low voltage zones (LVZ) on LA transport function. Methods 107 consecutive patients presenting for ablation of symptomatic AF (34 paroxysmal AF, 73 persistent AF) were prospectively enrolled. Each patient underwent cardiac magnetic resonance imaging (CMR) within 24 hours prior to the ablation procedure. 59 patients were in sinus rhythm (SR) and 48 in AF. LA phasic indexed volumes (LAVi) and ejection fractions were calculated using biplane area length formula. In addition LA phasic strains and strain rates were analyzed using dedicated software (Figure 1A & B). LA bipolar voltage mapping was performed prior to beginning of ablation in sinus rhythm using a 3-dimensional mapping system and LVZ were defined as areas of bipolar voltage <0.5mV. Results LVZ were present in 47 patients (23 in SR). The area of LVZ was 14.6cm2 (5.3–34.0). For patients in AF at the time of CMR only elevated minimal and maximal LAVi (p=0.001 and p=0.002 respectively) but no LA functional parameter was predictive for the occurrence of LVZ. In contrast for patients in SR all LA phasic volumes (endsystolic, pre atrial contraction and enddiastolic LAVi) and LA function parameters (passive, active and total ejection fraction (EF), reservoir, conduit and booster pump strains and strain rates) were predictive for the occurrence of LVZ. After clustered and pooled multivariate logistic regression only impaired booster pump strain rate was still predictive for occurrence of LVZ (OR 0.974, 95% CI 0.950–0.998, p=0.036). In addition Pearson correlation analysis revealed a strong link between LA booster pump functional parameters and cm2 expansion of LVZ areas: LA active EF, LA booster pump strain and SR (r=−0.42, p=0.044; r=−0.47, p=0.024; r=−0.65, p=0.001 [Figure 1C] respectively). Conclusion For patients in SR LA transport function is closely linked to the occurrence of LA LVZ and outperforms LA volumetric measurements for the prediction of LA LVZ. Furthermore LA booster pump function parameters show robust correlation to the extension of LA LVZ.

(Invited) Performance Evaluation of III-N Bipolar Switches Grown on GaN Substrates

The successful development of III-V nitride materials systems has led to active research of advanced electronic devices in the recent decades. III-nitride (III-N)-based devices have a bandgap engineering flexibility, large energy gap, direct band structure, and high carrier mobility when compared to Si- and SiC -based devices, providing new performance improvement opportunities for high-speed and energy-efficient electronics. Today, commercial GaN HFETs are available for microwave power amplifiers and power electronic circuits. On the other hand, the development of bipolar transistors and the related switch technologies are essential to fully exploit the potential of III-N materials systems. For instance, insulated-gate-bipolar transistors would require integrated bipolar transistor and unipolar transistor structures. The development of III-N bipolar switches in either two-terminal or three-terminal forms have therefore been actively sought. To this end, power switches that conduct the electric current in the vertical form are beneficial to achieve the ultimate Baliga’s Figure of Merit (BFOM) for switching. For GaN bipolar power switches development, low-defect-density native substrates, high-quality epitaxial growth technology, and proper device design and fabrication have become major focus areas of research. Early development of heterojunction bipolar transistors (HBTs) using GaN/InGaN heterojunctions at Georgia Tech showed promising device performance. These devices exhibited high-current gain of >100, high current drive density of >100 kA/cm2, and high d.c. power handling capability of > 3 MW/cm2. Small-signal microwave amplification with a cut-off frequency (f T) of 8GHz was reported. In this presentation, we will review the development status of III-N bipolar switches and use GaN rectifiers as a case study to discuss the device performance pertinent to the choices of GaN substrates prepared by different methods. The device structures were grown and fabricated at Georgia Tech and the device performance correlation with respect to the substrates were evaluated. High-power operation was demonstrated for homojunction GaN PIN rectifiers grown on a 2-inch bulk GaN substrate using a metalorganic chemical vapor deposition (MOCVD) reactor. The GaN substrate was prepared using a near-equilibrium ammonothermal (NEAT) method. The device fabrication employed an ion-implantation isolation, field plate, and electroplated copper electrodes. The fabricated devices have dimensions up to 1.1 mm in diameter and showed uniform V on of 3.6±0.1 V and an ideality factor of 2.08. A blocking voltage (BV) of >1.2 kV and the on-state resistance (R ON A) of 0.4 mΩ·cm2 were measured. For PIN rectifiers with a dimension of 1.1 mm in diameter, we reported a BV of 1,200 V and an on-state current drive of greater than 11 A (1 kA/cm2). These results demonstrated the feasibility of using bipolar carrier transport mechanisms and PN junctions in III-N devices for power amplifications and switching circuits.

Bipolar electronic charge transport model under trap density Gaussian spatial distribution: application to dielectric polymer interfaces

Our new approach consists of applying the Gaussian spatial distribution function to study the trapping process bipolar electronic charge transport at dielectric–dielectric as well as dielectric-electrode interfaces. Indeed, the application of the Gaussian spatial distribution function, to the traps spatial density distribution as well as to the corresponding trapping coefficients, has not been investigated so far for bipolar transport modeling in insulating and dielectric materials. In fact, this approach takes into account the non-uniformity of the trap density spatial distribution, which constitutes the main contribution that presents a more realistic aspect taking into account the existence of the inhomogeneities in the dielectric materials. Several numerical resolution techniques have been applied to the theoretical equations formulated with associated assumptions and physical conditions. The obtained results show several physical phenomena, such as charge accumulation and Maxwell–Wagner behavior. In this paper, to validate our approach, we have focused our investigations on the accumulation phenomenon of trapped charges. Our results are in good agreement with those obtained by the pulsed electro-acoustic experimental technique, for the example, epoxy–polyethylene interfaces.

From Intrinsic Bipolar Transport to the Abnormal Curves of Mobility–E1/2 in the Common Hole-Transporting Materials

Carrier transport in organic semiconductors (OSCs) plays an essential role in device performance. OSCs are generally divided into hole-transporting (p-type) and electron-transporting (n-type) mater...

Suppression of bipolar transport and enhanced zT values of porous ZnSb by addition of hydrothermally synthesized Sb

ZnSb is a favourable eco-friendly, earth abundant SIand low cost thermoelectric material. We herein report a feasible approach to improve the thermoelectric performance of ZnSb by compositing with hydrothermally synthesized Sb. A series of ZnSb/Sb composites was fabricated by mixing the host material ZnSb and hydrothermally synthesized Sb followed by consolidation using cold pressing and evacuating-and-encapsulating sintering at 530 °C for 3 h. As compared to the host ZnSb, both the electrical resistivity and thermopower decrease, while both the power factor and thermal conductivity increase upon compositing with Sb. The bipolar transport of ZnSb is suppressed at temperatures up to 600 K. As a result, the ZnSb-Sb composite with 4 wt.% of hydrothermally synthesized Sb has a zT = 1.18 at 600 K, which is one of the highest values for ZnSb-based material and is about 46% enhancement as compared to the host ZnSb having zT = 0.81 at 600 K.

P‐185: Highly Efficient and Long Lifetime Bipolar Host Material for Red Phosphorescent Organic Light‐Emitting Diodes Using Benzocarbazole and Diphenyltriazine Derivatives

Red phosphorescent organic light‐emitting diodes (phOLEDs) can achieve 100% internal quantum efficiency by harvesting both singlet and triplet excitons through spin orbit coupling. To improve the external quantum efficiency (EQE) and device lifetime, many molecular designs of bipolar hosts with appropriate energy band gap and triplet energy were studied in doping system phOLEDs. Two kinds of bipolar hosts were synthesized by combining benzocarbazole hole type donor and diphenyltriazine electron type acceptor to achieve relatively low triplet energy for red phOLEDs. Two hosts with the benzocarbazole and diphenyltriazine linked through para and meta positions of the phenyl unit proved a triplet energy of 2.52 eV and bipolar carrier transport properties. The band gap of the emitters between highest energy occupied molecular orbital (HOMO) and lowest energy occupied molecular orbital (LUMO) is 2.89 eV and 2.83 eV, while the EQE was 21.7% and 20.7% respectively, as a result of doping with red emitter. It was confirmed that life time was 30 times longer than that of general 4,4′‐bis(n‐carbazolyl)‐1,1′‐biphenyl (CBP) host at 5000 cd/m2.

Benzocarbazole and diphenyltriazine derived hosts for red phosphorescent organic light-emitting diodes

A bipolar host design for red phosphorescent organic light-emitting diodes (PhOLEDs) was explored as an objective to improve the external quantum efficiency and device lifetime. Main direction of the host design was to secure low triplet energy, bipolar charge transport properties, and stability under positive and negative polarons. The objective of the host development was attained by a host design featured by low triplet energy benzocarbazole hole transport unit and diphenyltriazine electron transport unit. Two hosts with the benzocarbazole and diphenyltriazine linked through para or meta positions of the phenyl unit demonstrated a triplet energy of 2.52 eV and bipolar carrier transport properties. Based on the benzocarbazole and diphenyltriazine derived hosts, high external quantum efficiency over 23% and more than 30 times extension of the device lifetime were achieved in the red PhOLEDs.

Two novel bipolar hosts based on 1,2,4-triazole derivatives for highly efficient red phosphorescent OLEDs showing a small efficiency roll-off

Abstract Achieving high efficiency and small efficiency roll-off simultaneously for red phosphorescent organic light-emitting diodes (PhOLEDs) is still a challenge, which is largely related to the host material used in device fabrication. In this wok, we designed and synthesized two novel bipolar host materials, termed 2Cz-TAZ-2Cz and 3Cz-TAZ-3Cz in which 1,2,4-triazole was introduced at the C-2 and C-3 positions of 9-phenylcarbazole, respectively. The materials' photophysical properties were studied in detail.. It was found that 2Cz-TAZ-2Cz and 3Cz-TAZ-3Cz possess excellent thermal stability with thermal decomposition temperature (Td) of 457 and 432 °C, respectively, and that 2Cz-TAZ-2Cz has a better bipolar carrier transport compared to the typical host 4, 4′-bis(9H-carbazole-9-yl)-biphenyl (CBP). Based on the excellent performance of these materials, red phosphorescent OLEDs with 2Cz-TAZ-2Cz and 3Cz-TAZ-3Cz as hosts and bis(1-phenylisoquinoline)(acetylacetonate)iridium (III) (Ir(piq)2acac) as emitter were fabricated. The optimized 2Cz-TAZ-2Cz-based device achieved a high maximum current efficiency and external quantum efficiency (EQE) of 12.4 cd/A and 16.60%, respectively. Moreover, this device also exhibits a small efficiency roll-off, i.e., the EQE is lowered by only 12.0% and 29.4% at a brightness of 1000 cd/m2 and 10,000 cd/m2, respectively, which is superior to CBP-based device, indicating a potential far-reaching application.