Hole Transport Behavior Research Articles

Conjugated Polythiophene Frameworks as a Hole‐Selective Layer on Ta3N5 Photoanode for High‐Performance Solar Water Oxidation

AbstractDiscovering a competent charge transport layer promoting charge separation in photoelectrodes is a perpetual pursuit in photoelectrochemical (PEC) water splitting to achieve high solar‐to‐hydrogen (STH) conversion efficiency. Here, a conjugated polythiophene framework (CPF‐TTB) on Ta3N5 is elaborately electropolymerized, substantiating the hole transport behavior in their heterojunction. Tailored band structures of the CPF‐TTB/Ta3N5 reinforce the separation of photogenerated carriers, elevating a fill factor of the photoanode modified with a cocatalyst. The enhanced hole extraction enables the NiFeOx/CPF‐TTB/Ta3N5/TiN photoanode to generate a remarkable water oxidation photocurrent density of 9.12 mA cm−2 at 1.23 V versus the reversible hydrogen electrode. A tandem device combining the photoanode with a perovskite/Si solar cell implements an unbiased solar water splitting with a STH conversion efficiency of 6.26% under parallel illumination mode. This study provides novel strategies in interface engineering for metal nitride‐based photoelectrodes, suggesting a promise of the organic–inorganic hybrid photoelectrode for high‐efficiency PEC water splitting.

Salt-Assisted Selective Growth of H-phase Monolayer VSe2 with Apparent Hole Transport Behavior

Vanadium diselenide (VSe2) exhibits versatile electronic and magnetic properties in the trigonal prismatic (H-) and octahedral (T-) phases. Compared to the metallic T-phase, the H-phase with a tunable semiconductor property is predicted to be a ferrovalley material with spontaneous valley polarization. Herein we report an epitaxial growth of the monolayer 2D VSe2 on a mica substrate via the chemical vapor deposition (CVD) method by introducing salt in the precursor. Our first-principles calculations suggest that the monolayer H-phase VSe2 with a large lateral size is thermodynamically favorable. The honeycomb-like structure and the broken symmetry are directly observed by spherical aberration-corrected scanning transmission electron microscopy (STEM) and confirmed by giant second harmonic generation (SHG) intensity. The p-type transport behavior is further evidenced by the temperature-dependent resistance and field-effect device study. The present work introduces a new phase-stable 2D transition metal dichalcogenide, opening the prospect of novel electronic and spintronics device design.

Solution Processable Benzotrithiophene (BTT)-Based Organic Semiconductors: Recent Advances and Review.

Conjugated polymers and small molecules (SMs) with fused electron-rich and electron-deficient building blocks are promising materials for low-cost organic electronic devices. Benzotrithiophene (BTT) is one such electron-rich hybrid building block composed of three fused thiophene moieties with an extended π-system that has been widely used to synthesize a variety of electronic materials. Additionally, BTT has a planar and sulfur-rich core with a number of distinct advantages, including structural diversity, tunable electro-optical properties and exceptional hole-transport behavior. So far, four BTT-based isomers have been synthesized on a gram scale from seven isomeric structures, three of which are symmetric (BTT1-3) and one of which is asymmetric (BTT5), for use in a variety of optoelectronic applications. However, no report summarizing the progress of BTT-based semiconductors for electronic applications is available. The current review presents an overview of the recent developments in BTT-based monomers, polymers and SMs, as well as their applications in energy harvesting. Additionally, recent advances on charge transport devices, most notably organic solar cells (OSCs), organic thin field-effect transistors (OTFTs), and perovskite solar cells (Pero-SCs) are also surveyed and summarized. It is anticipatedthat this comprehensive review will stimulate further research and development of future BTT-based electronic materials, particularly for low-cost and high-performance organic electronic devices.

Understanding the Role of Surface States on Mesoporous NiO Films.

Surface states of mesoporous NiO semiconductor films have particular properties differing from the bulk and are able to dramatically influence the interfacial electron transfer and adsorption of chemical species. To achieve a better performance of NiO-based p-type dye-sensitized solar cells (p-DSCs), the function of the surface states has to be understood. In this paper, we applied a modified atomic layer deposition procedure that is able to passivate 72% of the surface states on NiO by depositing a monolayer of Al2O3. This provides us with representative control samples to study the functions of the surface states on NiO films. A main conclusion is that surface states, rather than the bulk, are mainly responsible for the conductivity in mesoporous NiO films. Furthermore, surface states significantly affect dye regeneration (with I–/I3– as redox couple) and hole transport in NiO-based p-DSCs. A new dye regeneration mechanism is proposed in which electrons are transferred from reduced dye molecules to intra-bandgap states, and then to I3– species. The intra-bandgap states here act as catalysts to assist I3– reduction. A more complete mechanism is suggested to understand the particular hole transport behavior in p-DSCs, in which the hole transport time is independent of light intensity. This is ascribed to the percolation hole hopping on the surface states. When the concentration of surface states was significantly reduced, the light-independent charge transport behavior in pristine NiO-based p-DSCs transformed into having an exponential dependence on light intensity, similar to that observed in TiO2-based n-type DSCs. These conclusions on the function of surface states provide new insight into the electronic properties of mesoporous NiO films.

Intramolecular hydrogen-bonding effects on structural and electronic properties of pyrrole-phenylene derivatives: a DFT study

Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations are employed to evaluate structural, electronic, charge-transport and optical properties of pyrrole-phenylene copolymer ((Py-co-Ph)4) and its derivatives. The distorted coplanar structure is found for the pristine pyrrole-phenylene copolymer, while the functionalized structures with –OCH3, –OH, and –F exhibit the highly coplanar structures due to larger π-orbital overlap along neighboring backbones and strong intramolecular hydrogen bonding interaction. A good linear correlation between HOMO and LUMO as a functional resonance effect is observed. A better hole transport behavior is obtained from –OCH3, –N(CH3)2, –OH, –CN, and –F functionalizations compared to the parent. According to the optical properties, –CN, –COOH, and –NO2 substitutions reveal the absorption peaks covering the solar region, which are characterized as the potential solar materials.

Resonant tunneling and hole transport behavior in low noise silicon tri-gate junctionless single hole transistor

The fabrication of p-type silicon junctionless tri-gate transistors and their temperature dependent transport studies are reported in this work. The fabricated transistors have shown a good transfer characteristic down to a low temperature of ∼ 80 K with an ON/OFF ratio of 106. The threshold voltage and the subthreshold slope were found to be dependent on temperature. In particular, the threshold voltage and the flat band voltage have positive slopes of 2.24 and 1.19 mV K−1, respectively, with temperature. Channel resistance was found to be increasing with decreasing temperature. The devices have shown a typical 1/f noise behavior in the frequency range of (1–50) Hz and 1/f2 type behavior in the frequency range of (50–100 Hz). At a temperature of 4.2 K, current vs. gate voltage characteristic at a fixed source drain bias shows clear coulomb peaks with different intervals for different gate bias voltages and the observed spikes were consistent within the sub-bands. We relate this to the single hole tunneling, mediated by the charged acceptors available in the channel region. Coupling strength of the dopants was also studied.

Tunable electronic properties of multilayer InSe by alloy engineering for high performance self-powered photodetector

Multilayer indium selenide (InSe) is a good candidate for high performance electronic and optoelectronic devices. The electrical performance of InSe is effectively regulated by dielectric layers, contact electrodes and surface doping. However, as a powerful tool to tune properties of materials, alloy engineering is absent for multilayer InSe. In this letter, for the first time, we investigate the electrical property of InSe1-xTex alloys and optoelectronic property of InSe-InSe0.82Te0.18p-n heterojunction. The electrical transport properties of InSe1-xTex alloys strongly depend on the content of Te composition. With the ratio of Te/Se increasing, the n-type electron transport behavior of InSe gradually transfers to the p-type hole transport behavior of InSe0.82Te0.18. The p-n InSe-InSe0.82Te0.18 heterojunction shows a rectification effect and a self-powered photodetection. The self-powered photodetector (SPPD) has a broad photodetection range from visible light (400 nm) to near-infrared (NIR) light (1000 nm). The responsivity (R) of SPPD is 14.1 mA/W under illuminated by NIR light (900 nm) at zero bias, which is comparable to some of the 2D heterojunctions NIR photodetectors measured with an external bias. The SPPD also shows a stable and fast response to NIR light (900 nm). This work demonstrates that the electrical transport properties of InSe1-xTex alloys significantly rely on the ratio of Te/Se and suggests that InSe-InSe1-xTex p-n heterojunction has a excellent potential for application in the self-powered optoelectronic device.

Largely Suppressed Magneto-Thermal Conductivity and Enhanced Magneto-Thermoelectric Properties in PtSn4.

Highly conductive topological semimetals with exotic electronic structures offer fertile ground for the investigation of the electrical and thermal transport behavior of quasiparticles. Here, we find that the layer-structured Dirac semimetal PtSn4 exhibits a largely suppressed thermal conductivity under a magnetic field. At low temperatures, a dramatic decrease in the thermal conductivity of PtSn4 by more than two orders of magnitude is obtained at 9 T. Moreover, PtSn4 shows both strong longitudinal and transverse thermoelectric responses under a magnetic field. Large power factor and Nernst power factor of approximately 80–100 μW·cm−1·K−2 are obtained around 15 K in various magnetic fields. As a result, the thermoelectric figure of merit zT is strongly enhanced by more than 30 times, compared to that without a magnetic field. This work provides a paradigm for the decoupling of the electron and hole transport behavior of highly conductive topological semimetals and is helpful for developing topological semimetals for thermoelectric energy conversion.

Optimization of Hole Injection and Transport Layers for High-Performance Quantum-Dot Light-Emitting Diodes

High-luminance, efficient quantum-dot light-emitting diodes (QLEDs) have been achieved by optimizing the balance between the hole injection layer (HIL) and the hole transport layer (HTL). Different concentrations of vanadium oxide (V2O5) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(4-sec-butylphenyl)diphenylamine)] (TFB) solutions were used to form the efficient HIL and HTL, respectively, for the QLEDs. The hole injection and transport behavior was characterized by using hole-only devices (HODs). The QLEDs, which were prepared with 0.5 wt.% of V2O5 and 0.1 wt.% of TFB as HIL and HTL, respectively, showed a maximum current efficiency of 2.27 cd·A−1 and a maximum luminance of 71,260 cd·m−2. Moreover, the turn-on voltage of the device was as low as 2.2 V due to the efficient carrier injection and transport. The results provide useful information for fabricating high-performance QLEDs.

Cytochrome C-decorated graphene field-effect transistor for highly sensitive hydrogen peroxide detection

High-level in vivo reactive oxygen species (ROS) can damage many biomolecules via oxidative stress and play vital roles in the pathogenesis of several bodily disorders. Therefore, fast and sensitive monitoring strategies for trace ROS, such as hydrogen peroxide (H2O2), are of great significance. Herein, we present a highly sensitive field-effect transistor (FET) sensor based on single-layer graphene for trace hydrogen peroxide (H2O2) detection. Graphene and cytochrome c (Cyt c) were employed as the conductive substrate material and biomolecular receptor for H2O2 detection, respectively. High-efficiency charge transfer can be achieved by reliable electrical contact across the Cyt c/underlying graphene interface. The Cyt c/graphene FET platform exhibited hole-transport behavior with high conductivity and high sensitivity toward H2O2 with a detection limit of 100fM and rapid response time (<1s). Moreover, our sensor platform was able to specifically discriminate H2O2 from a series of interfering substances, such as dopamine, ascorbic acid, glucose, uric acid and glutamate. This result, therefore, demonstrates that the proposed Cyt c/single-layer graphene FET sensor could facilitate the high-efficiency charge transfer between the redox center of the Cyt c/graphene interface, indicating a promising application in future trace H2O2 or free radical biosensors.

Ultranarrow Bandwidth Organic Photodiodes by Exchange Narrowing in Merocyanine H‐ and J‐Aggregate Excitonic Systems

AbstractUltranarrowband organic photodiodes (OPDs) are demonstrated for thin film solid state materials composed of tightly packed dipolar merocyanine dyes. For these dyes the packing arrangement can be controlled by the bulkiness of the donor substituent, leading to either strong H‐ or strong J‐type exciton coupling in the interesting blue (H‐aggregate) and NIR (J‐aggregate) spectral ranges. Both bands are shown to arise from one single exciton band according to fluorescence measurements and are not just a mere consequence of different polymorphs within the same thin film. By fabrication of organic thin‐film transistors, these dyes are demonstrated to exhibit hole transport behavior in spin‐coated thin films. Moreover, when used as organic photodiodes in planar heterojunctions with C60 fullerene, they show wavelength‐selective photocurrents in the solid state with maximum external quantum efficiencies of up to 11% and ultranarrow bandwidths down to 30 nm. Thereby, narrowing the linewidths of optoelectronic functional materials by exciton coupling provides a powerful approach to produce ultranarrowband organic photodiodes.

Naphthalene Diimide Ammonium Directed Single-Crystalline Perovskites with “Atypical” Ambipolar Charge Transport Signatures in Two-Dimensional Limit

A single crystal of a lead–iodine-based 2D perovskite having naphthalene diimide ammonium (NDIA) molecules as organic layers was developed, and the charge transport property was studied using field-effect transistors (FETs) measurements. Structure determination reveals the layered alternative stacking of lead iodide sheets and NDIA bilayers. The presence of NDIA promoted the lead iodide octahedron to form the unique three-point co-planar [Pb3I10]4– unit, which then connected into the 2D network in a corner-sharing manner. The NDIA cations closely stacked into 1D chains within the bilayers that were being sandwiched between the inorganic layers. FET characteristics of the single crystal obtained at room temperature demonstrate VDS-dependent electron and hole transport behavior with mobilities reaching up to more than 5 × 10–3 cm2 V–1 s–1. The 1D stack of NDIAs contributes greatly to the performance improvement for both the charge transport and the stability.

The ambipolar transport behavior of WSe2 transistors and its analogue circuits

Tungsten diselenide (WSe2) has many excellent properties and provides superb potential in applications of valley-based electronics, spin-electronics, and optoelectronics. To facilitate the digital and analog application of WSe2 in CMOS, it is essential to understand the underlying ambipolar hole and electron transport behavior. Herein, the electric field screening of WSe2 with a thickness range of 1–40 layers is systemically studied by electrostatic force microscopy in combination with non-linear Thomas–Fermi theory to interpret the experimental results. The ambipolar transport behavior of 1–40 layers of WSe2 transistors is systematically investigated with varied temperature from 300 to 5 K. The thickness-dependent transport properties (carrier mobility and Schottky barrier) are discussed. Furthermore, the surface potential of WSe2 as a function of gate voltage is performed under Kelvin probe force microscopy to directly investigate its ambipolar behavior. The results show that the Fermi level will upshift by 100 meV when WSe2 transmits from an insulator to an n-type semiconductor and downshift by 340 meV when WSe2 transmits from an insulator to a p-type semiconductor. Finally, the ambipolar WSe2 transistor-based analog circuit exhibits phase-control by gate voltage in an analog inverter, which demonstrates practical application in 2D communication electronics.

Temperature-dependent hole transport for pentacene thin-film transistors with a SiO2 gate dielectric modified by (NH4)2Sx treatment

The effect of a SiO2 gate dielectric modified by (NH4)2Sx treatment on the temperature-dependent hole transport behavior for pentacene-based organic thin-film transistors (OTFTs) is studied. (NH4)2Sx treatment leads to the formation of SSi bonds (i.e., the formation of a sulfurated layer) on the SiO2 surface that serves to reduce the SiO2 capacitance, increasing the value of the hole mobility in OTFTs. The temperature-dependent hole transport is dominated by the multiple trapping model. It is shown that (NH4)2Sx treatment leads to a reduction in the activation energy, resulting from the formation of a sulfurated layer at the pentacene/SiO2 interface that serves to suppress the pentacene-SiO2 interaction. (NH4)2Sx treatment provides an opportunity to realize the stable and reliable carrier conduction behavior for OTFTs.

Efficiency Improvement of Deep‐Ultraviolet Light Emitting Diodes with Gradient Electron Blocking Layers

In this paper, a gradient electron blocking layer (GEBL) is introduced to improve efficiency of deep‐ultraviolet light‐emitting diodes (DUV‐LEDs). Various structures of DUV‐LEDs are simulated to determine the energy band diagram variation and carrier injection mechanism resulting from the insertion of the GEBL. The simulation results show the improved electron and hole transport behavior in AlGaN multi quantum wells (MQWs). Especially, the injection efficiency of holes is improved with the increasing number of GEBL steps, which lead to the enhancement of internal quantum efficiency (IQE). The DUV‐LED structures with GEBL are grown in a high‐temperature metal‐organic chemical vapor deposition (HT‐MOCVD) system. Electroluminescence (EL) spectra show that the emission intensity at a wavelength of 280 nm from a DUV‐LED with a 12‐steps EBL is about 2.3 times that from a DUV‐LED with a single EBL.

Diamine-cored tetrastilbene compounds as solution-processable hole transport materials for stable organic light emitting diodes

A series of diamine-cored tetrastilbene (DTS) derivatives bearing various aliphatic and aromatic substituents was designed and synthesized for use as solution-processed hole transport layers (HTLs) in organic light emitting diodes (OLEDs). The chemical structures of the DTS derivatives were strategically designed to increase solubility in organic solvents as well as to avoid self-crystallization, and thus ensure a stable morphology under Joule heating while maintaining efficient hole transport capabilities. The five DTS derivatives, composed of different conjugation structures, yielded reasonably good hole transport behavior with a marginal charge carrier mobility of ∼10−5 cm2V−1s−1, which is similar to that of vacuum-deposited N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB). Due to the high glass transition temperatures of the DTS derivatives, this satisfactory charge transport behavior and smooth surface morphology were conserved up to 180 °C. Green OLEDs were prepared using tris-(8-hydroxyquinoline) aluminum (Alq3):C545T as the emission layer, and the OLED performances of the solution-processed DTS HTLs and the vacuum-deposited NPB HTL were compared. A high luminance efficiency of 11.5 cd A−1 was obtained for one solution-processed DTS HTL, which exceeds that of the NPB HTL (10.01 cd A−1). Furthermore, the DTS HTLs enabled a stable OLED operation, with double the accelerated half-life of the NPB-based device.

Solution-processed field-effect transistors based on polyfluorene –cesium lead halide nanocrystals composite films with small hysteresis of output and transfer characteristics

We have designed and investigated electrical and optical properties of solution-processed organic field-effect transistors (OFETs) based on conjugated polymer PFO and perovskite –cesium lead halide nanocrystals (CsPbI3) composite films. It was shown that OFETs based on PFO:CsPbI3 films exhibit current-voltage (I-V) characteristics of OFETs with dominant hole transport and saturation current behavior at temperatures 200–300 K. It was found that PFO:CsPbI3 OFETs have a negligible hysteresis of output and transfer characteristics especially at temperatures below 250 K. The values of the hole mobility estimated from I-Vs of PFO:CsPbI3 OFETs were found to be ∼2.4 10−1 cm2/Vs and ∼1.9 10−1 cm2/Vs in saturation and low fields regimes respectively at 300 K; the hole mobility dropped down to ∼6 10−3 cm2/Vs and 2.8 10−3 cm2/Vs respectively at 200 K, and then down to 5.5 10−5 cm2/Vs at 100 K (in low field regime), which is characteristic of hopping conduction. The effect of sensitivity to light and light-emitting effect were found under application of negative source-drain and gate pulse voltages to PFO:CsPbI3 OFETs at 300 K. The mechanism of charge carrier transport in OFETs based on PFO:CsPbI3 hybrid films is discussed.

Diazaisoindigo bithiophene and terthiophene copolymers for application in field‐effect transistors and solar cells

ABSTRACTTwo donor–acceptor conjugated polymers with azaisoindigo as acceptor units and bithiophene and terthiophene as donor units have been synthesized by Stille polymerization. These two polymers have been successfully applied in field‐effect transistors and polymer solar cells. By changing the donor component of the conjugated polymer backbone from bithiophene to terthiophene, the density of thiophene in the backbone is increased, manifesting as a decrease in both ionization potential and in electron affinity. Therefore, the charge transport in field‐effect transistors switches from ambipolar to predominantly hole transport behavior. PAIIDTT exhibits hole mobility up to 0.40 cm2/Vs and electron mobility of 0.02 cm2/Vs, whereas PAIIDTTT exhibits hole mobility of 0.62 cm2/Vs. Polymer solar cells were fabricated based on these two polymers as donors with PC61BM and PC71BM as acceptor where PAIIDTT shows a modest efficiency of 2.57% with a very low energy loss of 0.55 eV, while PAIIDTTT shows a higher efficiency of 6.16% with a higher energy loss of 0.74 eV. Our results suggest that azaisoindgo is a useful building block for the development of efficient polymer solar cells with further improvement possibility by tuning the alternative units on the polymer backbone. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 2691–2699

Achieving Ultrahigh Carrier Mobility in Two-Dimensional Hole Gas of Black Phosphorus.

We demonstrate that a field-effect transistor (FET) made of few-layer black phosphorus (BP) encapsulated in hexagonal boron nitride (h-BN) in vacuum exhibits a room-temperature hole mobility of 5200 cm2/(Vs), being limited just by the phonon scattering. At cryogenic temperatures, the FET mobility increases up to 45 000 cm2/(Vs), which is five times higher compared to the mobility obtained in earlier reports. The unprecedentedly clean h-BN-BP-h-BN heterostructure exhibits Shubnikov-de Haas oscillations and a quantum Hall effect with Landau level (LL) filling factors down to v = 2 in conventional laboratory magnetic fields. Moreover, carrier density independent effective mass of m* = 0.26 m0 is measured, and a Landé g-factor of g = 2.47 is reported. Furthermore, an indication for a distinct hole transport behavior with up- and down-spin orientations is found.

Synthesis and photovoltaic application of low-bandgap conjugated polymers by incorporating highly electron-deficient pyrrolo[3,4-d]pyridazine-5,7-dione units

Two donor–acceptor polymer semiconductors based on highly electron-deficient pyrrolo[3,4-d]pyridazine-5,7-dione unit were synthesized by Stille cross-coupling polymerization, and their thermal property, photophysical property, electrochemical property, microstructure, application as organic thin-film transistors, and photovoltaic property were investigated. Because of the strong electron-accepting characteristic of pyrrolo[3,4-d]pyridazine-5,7-dione, the new polymers (P1 and P2) exhibited much wider absorption, smaller bandgaps (1.70 vs 1.98 eV), and deeper LUMO levels (−3.60 vs −3.37 eV) than those of a phthalimide-based polymer PBDT-PhBT (Figure 1). The fabricated organic thin-film transistor devices exhibited hole-transport behavior, and the highest mobility of 1.14 × 10−3 cm2 V−1 s−1 was obtained. The bulk-heterojunction solar cells based on the two polymers as the electron donors and PC71BM as the electron acceptor showed a high open-circuit voltage and achieved a power conversion efficiency of 2.71% and 3.66% for polymers P1 and P2, respectively.