Charge Carrier Transport Characteristics Research Articles

Enhanced photocatalytic hydrogen evolution using dual templates of sulfur-doped carbon nitride

The band structure of semiconducting photocatalysts plays a crucial role in determining their charge carrier transport characteristics and photocatalytic activity. In this work, a simple method to modulate the band structures of carbon nitride by infusing dopants and porous materials is experimentally demonstrated. A combination of trithiocyanuric acid and ammonium chloride to calcinate in an air environment, sulfur dopants and a significant amount of porosity are infused into carbon nitride concurrently. The sulfur-doped and porous carbon nitride material shows remarkable performance in photocatalytic hydrogen production, achieving a maximum hydrogen evolution rate of 4516.84 μmol g−1 h−1. Furthermore, infusion of sulfur dopants and abundant porous into carbon nitride allows for substantial and continuous tuning of the conduction and valence band positions. The band structures modulation enhanced the optical absorption in visible light region, making it suitable for efficient solar energy conversion. The findings of this work are feasible to pave a foundation for future clean energy technology.

Enhanced Interface Charge Carrier Transport of SnO2/CeO2 via Oxygen Vacancy Synergized Heterojunction for Triethylamine Sensing Property.

Efficient charge carrier transport characteristics are critical to achieving the excellent performance of metal-oxide semiconductor gas sensors. Herein, SnO2/CeO2 heterojunction layered nanosheets with abundant oxygen vacancies were successfully synthesized through a simple solvothermal assisted high-temperature calcination method. The synergistic effect of oxygen vacancies and heterojunctions promoting the charge carrier transport properties at the SnO2/CeO2 interface for the enhanced sensing properties of triethylamine (TEA) was highlighted. As a result, the optimized SnO2/CeO2 exhibits improved gas sensing performance at 173 °C to 50 ppm of TEA. These include high response (205), excellent selectivity, low detection limit, and good long-term stability. This enhanced gas sensing property of SnO2/CeO2 is mainly attributed to the fact that the heterojunction and oxygen vacancies act as dual active sites synergistically inducing electron transfer, thereby effectively modulating the transport properties of the interfacial charge carriers, and thus facilitate the surface reactions efficiently. In this work, the dual-engineering strategy of synergistic interaction of heterojunction and oxygen vacancies can provide new perspectives for the design of advanced gas sensing materials.

Enhancement of electrical properties by CuS–PVP nanocomposite on the ZnO/polymer heterojunction

AbstractA nanocomposite matrix of covellite–polyvinylpyrrolidone (CuS–PVP) was synthesized through a solution processing method and applied via spin coating onto hydrothermally grown zinc oxide (ZnO) nanorods. Characterization techniques, including X‐ray powder diffraction and scanning electron microscopy, confirmed the successful decoration of chain‐like CuS–PVP polymer composites onto hexagonal ZnO nanorods. The UV‐vis spectroscopy revealed an absorption peak at 372 nm, indicating excitonic absorption of ZnO, and a reduction in the band gap from 3.11 to 2.59 eV upon CuS–PVP polymer decoration, enhancing light harvesting efficiency. Photoluminescence spectra demonstrated the lowest charge carrier recombination rate in the ZnO/CuS–PVP sample, promoting efficient charge separation and transportation, supported by Fourier‐transform infrared analysis indicating chemical bonding. Subsequently, the impact of CuS–PVP as an insertion layer in the ZnO/poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS) inorganic–organic heterojunction was investigated. Favorable band alignment between ZnO, CuS–PVP, and PEDOT:PSS improved conduction, increasing current from 0.3394 to 1.14 mA for D2 (poly‐ethyleneterephthalate/Indium tin oxide/ZnO/PEDOT:PSS/Al) and D1 (poly‐ethyleneterephthalate/Indium tin oxide/ZnO/CuS–PVP/PEDOT:PSS/Al) at a +2 V bias. Transient photo‐responses (J–t) at 0 V bias revealed decreased rise and fall times from 0.382 and 1.154 s to 0.301 and 0.229 s for D2 and D1 devices, respectively. This novel approach provides control over charge carrier transport characteristics at the interface, potentially benefiting self‐powered photo‐detection in optoelectronic devices.

Carrier mobility in doped Sb<sub>2</sub>Se<sub>3</sub> based on deformation potential theory

Antimony selenide (Sb<sub>2</sub>Se<sub>3</sub>) is an element-rich, cost-effective, and non-toxic material used as an absorber layer in solar cells. The performance of solar cells is significantly influenced by the transport characteristics of charge carriers. However, these characteristics in Sb<sub>2</sub>Se<sub>3</sub> have not been well understood. In this work, through density functional theory and deformation potential theory, we investigate the hole transport properties of pure Sb<sub>2</sub>Se<sub>3</sub> and As-, Bi-doped Sb<sub>2</sub>Se<sub>3</sub>. The incorporation of as element and Bi element does not introduce additional impurity levels within the band gap. However, the band gaps are reduced in both As-Sb<sub>2</sub>Se3 and Bi-Sb<sub>2</sub>Se<sub>3</sub> due to the band shifts of energy levels. This phenomenon is primarily attributed to the interactions between the unoccupied 4p and 6p states of the doping atoms and the unoccupied 4p states of Se atoms, as well as the unoccupied 5p states of Sb atoms. In this study, we calculate and analyze three key parameters affecting mobility: effective mass, deformation potential, and elastic constants. The results indicate that effective mass has the greatest influence on mobility, with Bi-Sb<sub>2</sub>Se<sub>3</sub> exhibiting the highest average mobility. The average effective mass is highest in As-Sb<sub>2</sub>Se<sub>3</sub> and lowest in Bi-Sb<sub>2</sub>Se<sub>3</sub>. The elastic constants of the As- and Bi-doped Sb<sub>2</sub>Se<sub>3</sub> structures show minimal differences compared with that of the intrinsic Sb<sub>2</sub>Se<sub>3</sub> structure. By comparing the intrinsic, As-doped, and Bi-doped Sb<sub>2</sub>Se<sub>3</sub>, it is evident that doping has a minor influence on deformation potential energy along various directions. The study reveals that the hole mobility in Sb<sub>2</sub>Se<sub>3</sub> displays significant anisotropy, with higher mobilities observed in the <i>x</i>-direction and the <i>y</i>-direction than in the <i>z</i>-direction. This discrepancy is attributed to stronger covalent bonding primarily in the <i>x</i>- and <i>y</i>-direction, while in the <i>z</i>-direction weaker van der Waals forces is dominant. The directions with enhanced charge carrier transport capability contribute to efficient transfer and collection of photo-generated charge carriers. Therefore, our research theoretically underscores the significance of controlling the growth of antimony selenide along specific directions.

Fabrication of UV Photodetectors Based on Photoelectrochemically Etched Nanoporous Silicon: Effect of Etchants Ratio

Despite several attempts to enhance the electrical and charge carrier transport characteristics of porous silicon (PSi), the requisite conditions for optimally synthesizing n-PSi with appealing optoelectronic properties are yet to be achieved. Therefore, this research explores the effect of the chemical ratio of precursor materials (HF:C2H6O:H2O2) on the surface morphology, crystalline structure, and optical and electric properties of PSi. The PSi was produced by photoelectrochemical etching followed by anodization of the n-type Si under light illumination. The properties of the as-prepared PSi were studied by means of microscopic and spectroscopic techniques. The HF:C2H6O:H2O2 chemical ratio was optimized at 2 : 1 : 1. A metal–semiconductor–metal (MSM) ultraviolet photodetector (Pt/n-PSi/Pt) was fabricated, which exhibited high performances under UV light (365 nm) illumination. The photodetector was shown to be highly stable and reliable with a rapid rise time of 0.56 s at a bias voltage of +5 V. The MSM photodetector displayed responsivity (Rp) of 9.17 A/m at 365 nm, which significantly exceeds the values reported for TiC/porous Si/Si in some contemporary research. The photodetector fabricated from n-PSi, synthesized at an optimum chemical ratio (2 : 1 : 1) exhibited the best photodetection performance, possibly due to the high porosity and defect-free state of the n-PSi thin films.

Zn-doped CdS/CdSe as efficient strategy to enhance the photovoltaic performance of quantum dot sensitized solar cells

Metal ion doping is a suitable approach for optimizing quantum dots (QDs), which can change the recombination dynamics and charge separation of QDs via creating electronic states within their band gap, and improve their optical and electrical properties. In this work, the transition metal Zn-doped CdS and CdSe QDs are performed by SILAR (successive ionic layer adsorption and reaction) method. And the sensitized solar cells (QDSSCs) based on the Zn-doped CdS and CdSe QDs are assembled. The effect of Zn-doping concentration on the optical properties and charge carrier transport characteristics of photoanodes is investigated. Based on these QDs sensitized TiO2 photoanodes, the assembled QDSSCs with Zn-dopant of a molar concentration of 1.6 mol % exhibited a maximum power conversion efficiency (PCE) of 5.59 %, which is increased by 26.76 % in comparison with that of the QDSSC (4.41%) without Zn-doping. The significantly enhanced PCE of QDSSCs was attributed to improved current density and open-circuit voltage, particularly arising from enhanced light absorbance by narrowing the band gap with the midgap states and upshift of Fermi level promoted by the high electrons concentration due to the ionization of Zn impurities. In addition, the Zn-doping contributes to attenuation of the interfacial charge recombination rate and prolongs the electron lifetime, resulting in more efficient charge collection in QDSSCs.

Synthesis, properties, and charge transport characteristics of the conjugated copolymers containing the azaisoindigo and benzothiadiazole units

We report two novel azaisoindigo and benzothiadiazole-containing ternary copolymers PAZDTBT and PAZDTBTF. The thermal, optical and electrochemical properties, conjugated backbone planarity, film morphology and molecular packing mode, as well as charge transport characteristics of two copolymers, were studied systematically through either experimental evaluations or theoretical modellings. The copolymer PAZDTBTF containing fluorine atoms on benzothiadiazole units owns an energy level of the lowest unoccupied molecular orbital (LUMO) (−3.56 eV) similar to that of non-fluorinated copolymer PAZDTBT (−3.53 eV). Both ternary copolymers PAZDTBT and PAZDTBTF exhibited ambipolar charge carrier transport characteristics. When compared with the averaged hole/electron mobilities of 0.377 and 0.092 cm2 V−1 s−1 for PAZDTBT, the polymer PAZDTBTF has an enhanced electron mobility of 0.126 cm2 V−1 s−1, though its hole mobility of 0.358 cm2 V−1 s−1 was a little bit lower than that of PAZDTBT. Our work demonstrates the great potentials of ternary π-conjugated system with two different acceptor units in fine-tuning the optoelectronic properties of polymer semiconductors.

Hole-transport comparison between solution-processed and vacuum-deposited organic semiconductors

Charge transport in the amorphous organic small molecules α-NPD (N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine) and Spiro-TAD (2,2′,7,7′-tetrakis(N,N-diphenylamino)-9,9-spirobifluorene) is investigated in solution-processed films and compared to charge transport in vacuum-deposited films of the same molecule. By optimizing the solution-deposition conditions, such as solvent and concentration, equal charge-transport parameters for solution-processed and vacuum-deposited films are demonstrated. Modeling of the charge carrier transport characteristics was performed by drift-diffusion simulations. The dependence of the charge carrier mobility on temperature, carrier density, and electric field was found to be the same for vacuum deposition and solution processing. In both material processing cases, hole mobilities of 4 × 10−8 m2 V−1 s−1 for spiro-TAD and 0.9 × 10−8 m2 V−1 s−1 for α-NPD are obtained, demonstrating that solution processing can be a viable alternative to vacuum deposition in terms of charge transport.

Defect-enriched tunability of electronic and charge-carrier transport characteristics of 2D borocarbonitride (BCN) monolayers from ab initio calculations.

Development of inexpensive and efficient photo- and electro-catalysts is vital for clean energy applications. Electronic and structural properties can be tuned by the introduction of defects to achieve the desirable electrocatalytic activity. Using first-principles molecular dynamics simulations, the structural, dynamical, and electronic properties of 2D borocarbonitride (h-BCN) sheets have been investigated, highlighting how anti-site defects in B and N doped graphene significantly influence the bandgap, and thereby open up new avenues to tune the chemical behavior of the 2D sheets. In the present work, all of the monolayers investigated display direct bandgaps, which reduce from 0.99 eV to 0.24 eV with increasing number of anti-site defects. The present results for the electronic structure and findings for bandgap engineering open up applications of BCN monolayers in optoelectronic devices and solar cells. The influence of the anti-site distribution of B and N atoms on the ultra-high hole/electron mobility and conductivity is discussed based on density functional theory coupled with the Boltzmann transport equation. The BCN defect monolayer is predicted to have carrier mobilities three times higher than that of the pristine sheet. The present results demonstrate that BN doped graphene monolayers are likely to be useful in the next-generation 2D field-effect transistors.

Investigation on charge-carrier transport characteristics of transformer oil-based nanofluids

Adding nanoparticles to traditional transformer oil can improve their heat exchange properties as well as enhance their dielectric withstand characteristics. Investigations into the charge-carrier transport characteristics of these transformer oil-based nanofluids (TNFs) can explain their improved insulating characteristics, and clarify the mechanisms dictating these modifications. In this study, we measured the conduction current and velocity field of TNFs under high electric field excitation for the first time, for analysis of charge-carrier transport. We describe different transport processes according to the magnitude of the applied electric field, characterized as ohmic, tunneling, and space charge limited current (SCLC) stages. In the ohmic stage, characterized by very low electric field strengths, the increase in conduction current is due to the increase in the number of carriers from the addition of nanoparticles. In the tunneling stage (medium-to-high electric field strengths), the predominant charge carriers in the TNF change from ions and colloidal particles, to electrons emitted from the electrode. The thickness of the Schottky barrier at the metal-liquid interface increases on the addition of nanoparticles, which reduces the number of electrons that pass through to the interface region. The field strength required for electron transmission is enhanced, and the insulation strength is improved. In the SCLC stage (very high electric fields), the carrier mobility is reduced because of the larger trap density of TNFs and the electrical discharge is suppressed.

Temperature dependent diode and photovoltaic characteristics of graphene-GaN heterojunction

Understanding the charge carrier transport characteristics at the graphene-GaN interface is of significant importance for the fabrication of efficient photoresponsive devices. Here, we report on the temperature dependent diode and photovoltaic characteristics of a graphene/n-GaN heterostructure based Schottky junction. The graphene/n-GaN heterojunction showed rectifying diode characteristics and photovoltaic action with photoresponsivity in the ultra-violet wavelength. The current-voltage characteristics of the graphene/n-GaN heterojunction device were investigated under dark and light illumination with changes in temperature. Under dark conditions, an increase in the forward bias current as well as saturation current was observed, and a decrease in the device ideality factor was obtained with an increase in temperature. Under illumination of light, a decrease in the open circuit voltage (Voc) and an increase in the short circuit current density (Jsc) was obtained with an increase in temperature. The increase in saturation current and carrier recombination with the increase in temperature leads to a reduction in Voc, while the photo-generated carrier increases in the heterojunction interface at higher temperatures contributing to the increase in Jsc. The observed temperature dependent device characteristics of the graphene/n-GaN heterojunction can be significant to understand the junction behavior and photovoltaic action.

Ambipolar tetrafluorodiphenylethene-based donor–acceptor copolymers: synthesis, properties, backbone conformation and fluorine-induced conformational locks

Tetrafluorodiphenylethene-based donor–acceptor copolymers with multiple conformational locks exhibit excellent ambipolar charge carrier transport characteristics.

Theory-Driven Insight into the Crystal Packing of Trialkylsilylethynyl Pentacenes

The functionalization of oligoacenes and similar π-conjugated chromophores with trialkylsilylethynyl groups has proven to be a versatile means to enhance solubility and solution processability and engineer solid-state packing arrangements to produce organic semiconductors that demonstrate outstanding charge-carrier transport characteristics. While a general, empirical-based geometric model has been developed and implemented to direct the solid-state packing arrangements of these molecular materials, there exist numerous examples where the model falters. Here, we employ electronic structure methods to probe the noncovalent, intermolecular interactions of two closely related systems that result in two very different crystal packing configurations: triisopropylsilylethynyl (TIPS) pentacene and its triethylsilylethynyl (TES) analog. The quantum-chemical evaluation details how the slightly larger electron density contained within the volume of the TIPS moiety with respect to TES is in part responsible for the ...

Molecular organization in MAPLE-deposited conjugated polymer thin films and the implications for carrier transport characteristics

The morphological structure of poly(3-hexylthiophene) (P3HT) thin films deposited by both Matrix Assisted Pulsed Laser Evaporation (MAPLE) and solution spin-casting methods are investigated. The MAPLE samples possessed a higher degree of disorder, with random orientations of polymer crystallites along the side-chain stacking, π–π stacking, and conjugated backbone directions. Moreover, the average molecular orientations and relative degrees of crystallinity of MAPLE-deposited polymer films are insensitive to the chemistries of the substrates onto which they were deposited; this is in stark contrast to the films prepared by the conventional spin-casting technique. Despite the seemingly unfavorable molecular orientations and the highly disordered morphologies, the in-plane charge carrier transport characteristics of the MAPLE samples are comparable to those of spin-cast samples, exhibiting similar transport activation energies (56 vs. 54 meV) to those reported in the literature for high mobility polymers. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 39–48

Dramatically enhanced performances and ideally controlled nano-morphology via co-solvent processing in low bandgap polymer solar cells

The device performance of photovoltaics with a polymer:fullerene bulk heterojunction (BHJ) structure, consisting of DT-PDPP2T-TT donor polymer and poly(3-hexylthiophene):[6,6]phenyl-C61-butyric acid methyl ester (PC61BM) acceptor compound, was investigated as a function of co-solvent composition. An enhancement of the photocurrent density and fill factor is observed in diodes made by spin-coating with chloroform mixed with ortho-dichlorobenzene, which allows a significantly higher device efficiency of 5.55% compared to diodes made from neat chloroform (efficiency = 3.61%). To clarify the role of the co-solvent, we investigated the nanoscale morphology with AFM, TEM and 2D-GIWAXS techniques and also the free-charge carrier mobility via space-charge limited current theory. We obtained the result that, under such supersaturated conditions, co-solvents induce increased polymer crystalline aggregation into a 3D phase structure and boost charge-carrier transport characteristics. This provides a rational basis for the development of ideally-controlled BHJ films that yield efficient DT-PDPP2T-TT:PCBM solar cells. Therefore, carefully selecting solvent mixtures provides an approach toward efficient low bandgap polymer solar cells.

Molecular Engineering of Nonhalogenated Solution-Processable Bithiazole-Based Electron-Transport Polymeric Semiconductors

The electron deficiency and trans-planar conformation of bithiazole is potentially beneficial for the electron-transport performance of organic semiconductors. However, the incorporation of bithiazole into polymers through a facile synthetic strategy remains a challenge. Herein, 2,2′-bithiazole was synthesized in one step and copolymerized with dithienyldiketopyrrolopyrrole to afford poly(dithienyldiketopyrrolopyrrole-bithiazole), PDBTz. PDBTz exhibited electron mobility reaching 0.3 cm2 V–1 s–1 in organic field-effect transistor (OFET) configuration; this contrasts with a recently discussed isoelectronic conjugated polymer comprising an electron-rich bithiophene and dithienyldiketopyrrolopyrrole, which displays merely hole-transport characteristics. This inversion of charge-carrier transport characteristics confirms the significant potential for bithiazole in the development of electron-transport semiconducting materials. Branched 5-decylheptacyl side chains were incorporated into PDBTz to enhance polyme...

Fundamental theories of piezotronics and piezo-phototronics

Wurtzite structured mateirals such as ZnO, GaN, InN and CdS simultaneously exhibit piezoelectric, semiconducting and photoexcitation properties. The piezotronic effect is to use the inner crystal potential generated by piezoelectric polarization charges for controlling/tuning the charge carrier transport characteristics in these materials. The piezo-phototronic effect is about the use of piezoelectric charges to tube the generation, transport, separation and/or recombination of charge carriers at p-n junction. This article reviews the fundamental theories of piezotronics and piezo-phototronics, forming their basis for electromechancial devices, sensors and energy sciences. Starting from the basic equations for piezoelectricity, semiconductor and photoexcitation, analytical equations for describing the strain-tuned device current were derived. Through analytical calculations and numerical simulations, it was confirmed that the piezoelectric polarization charges can act in the form of inner-crystal charges in the depletion region, resulting in a change in Schottky barrier height, depletion region shift and/or formation of a charge channel, which can be used effectively to enhance the efficiency of LED, solar cell and photon detectors.

Anisotropic assembly of conjugated polymer nanocrystallites for enhanced charge transport.

The anisotropic assembly of P3HT nanocrystallites into longer nanofibrillar structures was demonstrated via sequential UV irradiation after ultrasonication to the pristine polymer solutions. The morphology of resultant films was studied by atomic force microscopy (AFM), and quantitative analysis of intra- and intermolecular ordering of polymer chains was performed by means of static absorption spectroscopy and quantitative modeling. Consequently, the approach to treat the precursor solution enhanced intra- and intermolecular ordering and reduced the incidence of grain boundaries within P3HT films, which contributed to the excellent charge carrier transport characteristics of the corresponding films (μ ≈ 12.0 × 10(-2) cm(2) V(-1) s(-1) for 96% RR P3HT).

Effect of light incidence angle on optical absorption characteristics of low bandgap polymer-based bulk heterojunction organic solar cells

The bulk heterojunction organic solar cell based on thieno[3,4-b]thiophene/benzodithiophene (PTB7) is one of an alternative candidate for traditional silicon-based solar cells owing to its advantages of ease of manufacture, low cost, and flexibility. Currently, many research studies of these devices focus on power conversion efficiency (PCE) enhancement with only normal sunlight incidence. In this study, we have experimentally verified that PCE markedly decreased from 5.51 to 3.47% as incidence angle was changed from 0 to 60°. Using the finite-difference time-domain method, we found that the degeneration of optical absorption is caused by the decreased electrical field intensity in the photoactive layer over the entire wavelength range due to the optical interference profile change. In addition, we confirmed that a higher incidence angle also results in unbalanced charge carrier transport characteristics, resulting in further decrease in solar cell efficiency.

A coaxial structure of multiwall carbon nanotubes on vertically aligned Si nanorods and its intrinsic characteristics

We present a unique process for fabricating silicon nanorods wrapped with a graphitic material on a silicon substrate by the chemical vapor deposition method with no metal catalyst, as well as characterization of their intrinsic properties. First, well-ordered silicon nanorod axes were grown via an electroless metal deposition method, followed by chemical vapor deposition to wrap the axes with a carbon nanotube. Interestingly, the use of ethanol treatment before chemical vapor deposition prevents the formation of SiOx layers, which may be necessary as seed layers for carbon nanotube growth. Since this method for carbon nanotube growth does not involve a metal catalyst, the intrinsic properties of the Si NRs were well characterized. A few characterization methods (XPS, Raman spectroscopy, and EELS) were carried out to prove that the Si NRs were completely surrounded by CNTs. In addition, the conductance results (by terahertz time-domain spectroscopy) show that the charge carrier transport characteristics of the multiwall carbon nanotubes are found mainly in the outermost shell, and that the Si NR surface was well passivated by the multiwall carbon nanotube structure. This coaxial structure, which does not require a metal catalyst, represents a significant step forward for realizing applications for carbon nanotube devices.