Charge transport studies in fluorene – Dithieno[3,2-b:2′,3′-d]pyrrole oligomer using time-of-flight photoconductivity method

Charge mobility characteristics of a newly synthesised 2,6-bis[2-(9,9-dihexyl-9H-fluorene)]-N-(4-hexylphenyl)-dithieno[3,2-b:2′,3′-d]pyrrole oligomer (DTP-FLU) was studied as a function of electric field and temperature using time-of-flight photoconductivity measurement. It is found that the DTP-FLU oligomer is a hole transporting material with a hole mobility of 7.7 × 10^?6 cm²/Vs at an applied electric field of 2.9 × 10⁵ V/cm at 298 K. The dependence of hole mobility with applied electric field and temperature is studied in detail by analyzing the experimental results using the Bassler’s Gaussian disorder model and Correlated disorder model. The energetic disorder parameter (σ) = 100 meV, mobility pre-factor (μ_∞) = 6.1 × 10^?4 cm²/Vs and positional disorder parameter (Σ) = 2.4 were extracted using Gaussian disorder model. The film morphology and photophysical properties of this new oligomer are also studied in detail.     

Two-dimensional π-conjugated molecules based-on 2,6,9,10-tetra(prop-1-yn-1-yl)anthracene and their application to solution-processed photovoltaic cells

Novel two-dimensional π-conjugated molecules, i.e., 5′,5″′-((9,10-bis((4-hexylphenyl)ethynyl)anthracene-2,6-diyl)bis(ethyne-2,1-diyl))bis(5-hexyl-2,2′-bithiophene) (2,6-HBT) and 5′,5″′-((2,6-bis((4-hexylphenyl)ethynyl)anthracene-9,10-diyl)bis(ethyne-2,1-diyl))bis(5-hexyl-2,2′-bithiophene) (9,10-THB) were successfully synthesized and utilized as highly soluble p-type organic semiconductors for organic thin film transistors (TFTs) and solar cells. From the TFTs, the high hole mobility of the pristine film of 9,10-THB was measured to be 0.07 cm² V⁻¹ s⁻¹ (I_on/off = 10⁶–10⁷), which is mainly attributed to slip-stacked charge-transport behavior in J-aggregation-induced crystallites. Further, a solution-processed solar cell made of 9,10-THB and PC₆₁BM exhibited very promising and reproducible power conversion efficiencies of 3.30% and 2.53% with composition 1:1 and 1:2 w/w ratio, respectively.     

A study of charge transport in a novel electroluminescent poly(phenylene vinylene-co-fluorenylene vinylene) based π-conjugated polymer

The charge transport properties in a novel electroluminescent poly{[2-(4′,5′-bis(3″-methylbutoxy)-2′-p-methoxy-phenyl)phenyl-1,4-phenylene vinylene]-co-(9,9-dioctyl-2,7-fluorenylene vinylene)} (BPPPV-PF) have been studied using a time-of-flight (TOF) photoconductivity technique. The TOF transients for holes were recorded over a range of temperatures (207–300 K) and electric fields (1.5 × 10⁵–6.1 × 10⁵ V/cm). The hole transport in this polymer was weakly dispersive in nature with a mobility at 300 K of 5 × 10⁻⁵ cm²/V s at 2.5 × 10⁵ V/cm. This increased to 8.4 × 10⁻⁵ cm²/V s at 6.1 × 10⁵ V/cm. The temperature and field dependence of charge mobility has been analyzed using the disorder formalisms (Bässler’s Gaussian disorder model (GDM) and correlated disorder model (CDM)). The fit with Gaussian disorder (GDM) model yielded the mobility pre-factor μ_∞ = 1.2 × 10⁻³ cm²/V s, energetic disorder parameter σ = 82 meV and positional disorder parameter Σ = 1.73. The average inter-site separation (a = 7 Å) and the charge localization length (L = 3.6 Å) was estimated by assuming the CDM type charge transport. The microscopic charge transport parameters derived for this polymer are almost identical to the reported values for fully conjugated polymers with high chemical purity. The results presented indicate that the charge transport parameters can be controlled and optimized for organic optoelectronic applications.     

Channel width effect for organic thin film transistors using TIPS-pentacene employed as a dopant of poly-triarylamine

The effects of the physical channel width on the characteristics of organic thin film transistors (OTFTs), made with 6,13-bis(triisopropyl-silylethynyl)-pentacene (TIPS-pentacene) embedded into poly-triarylamine (PTAA, hole conductor within an active channel), have been examined in this paper. The devices are estimated by measuring the drain-source current (I_DS) for different contact metals such as Au and Ag, at fixed gate and drain voltages. The results show that the threshold voltage (V_T) and I_DS increase with increasing channel width. Furthermore, it has been observed that the field effect mobility is dependent on V_T, which is influenced by the channel width. The OTFTs, produced using Au and Ag contacts, exhibited the highest values of mobility in the saturation regime, namely 5.44 × 10^?2 and 1.33 × 10^?2 cm²/Vs, respectively.     

All-conjugated diblock copolymer of poly(3-hexylthiophene)-block-poly(3-phenoxymethylthiophene) for field-effect transistor and photovoltaic applications

The electronic properties, morphology and optoelectronic device characteristics of conjugated diblock copolythiophene, poly(3-hexylthiophene)-block -poly(3-phenoxymethylthiophene) (P3HT-b-P3PT), are firstly reported. The polymer properties and structures were explored through different solvent mixtures of chloroform (CHCl₃), dichlorobenzene (DCB), and CHCl₃:DCB (1:1 ratio). The absorption maximum (λ_max) of P3HT-b-P3PT prepared from DCB was around 554 nm with a shoulder peak indicative for the highly crystalline structure around 604 nm while that from CHCl₃ was 516 nm without the clear shoulder peak. The field-effect hole mobility of P3HT-b-P3PT increased from ～6.0 × 10^?3, ～8.0 × 10^?3 to ～2.0 × 10^?2 cm² V^?1 s^?1 as the DCB content in the solvent mixture enhanced. The AFM images suggested that the highly volatile CHCl₃ processing solvent led to the amorphous structure, on the other hand, less volatile DCB resulted in the largely crystalline structure of the P3HT-b-P3PT. Such difference on the polymer structure and hole mobility led to the varied power conversion efficiency (PCE) of the photovoltaic cells fabricated from the blend of P3HT-b-P3PT/[6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) (1:1, w/w): 1.88 (CHCl₃), 2.13 (CHCl₃:DCB (1:1)), and 2.60% (DCB). The PCBM blend ratio also significantly affected the surface structure and the solar cell performance. The PCE of polymer/PCBM could be improved to 2.80% while the ratio of polymer to PCBM went to 1:0.7. The present study suggested that the surface structures and optoelectronic device characteristics of conjugated diblock copolymers could be easily manipulated by the processing solvent, the block segment characteristic, and blend composition.     

Organic light-emitting diodes based on an ambipolar single crystal

Slice-like organic single crystals of 1,4-bis(2-cyano-2-phenylethenyl)benzene (BCPEB) are grown by the physical vapor transport (PVT) method, and exhibit a very high photoluminescence quantum efficiency (Φ_PL) of 75%. The ambipolar behavior of BCPEB single crystals are confirmed using the time of flight technique. The high efficiency and balanced (μ_h = 0.059 cm²/Vs and μ_e = 0.070 cm²/Vs) carriers’ mobility imply that the BCPEB single crystal is a promising light-emitting layer in the diodes structure. Intense green electroluminescence (EL) from a diode has been successfully demonstrated at an applied electric field of 2 × 10⁵ V/cm.     

Improving solution-processed n-type organic field-effect transistors by transfer-printed metal/semiconductor and semiconductor/semiconductor heterojunctions

Solution-processed n-type organic field effect transistors (OFETs) are in need of proper metal contact for improving injection and mobility, as well as balanced hole mobility for building logic circuit units. We address the two distinct problems by a simple technique of transfer-printing. Transfer-printed Au contacts on a terrylene-based semiconductor (TDI) significantly reduced the inverse subthreshold slope by 5.6 V/dec and enhanced the linear mobility by over 5 times compared to evaporated Au contacts. Hence, devices with a high-work-function metal (Au) are comparable with those with low-work-function metals (Al and Ca), indicating a fundamental advantage of transfer-printed electrodes in electron injection. We also transfer-printed a poly(3-hexylthiophene) (P3HT) layer onto TDI to construct a double-channel ambipolar transistor by a solution process for the first time. The transistor exhibits balanced hole and electron mobility (3.0 × 10⁻³ and 2.8 × 10⁻³ cm² V⁻¹ s⁻¹) even in a coplanar structure with symmetric Au electrodes. The technique is especially useful for reaching intrinsic mobility of new materials, and enables significant enlargement of the material tanks for solution-processed functional heterojunction OFETs.     

Study on mobility enhancement in MOVPE-grown AlGaN/AlN/GaN HEMT structures using a thin AlN interfacial layer

Al_0.26Ga_0.74N/AlN/GaN high-electron-mobility transistor (HEMT) structures with AlN interfacial layers of various thicknesses were grown on 100-mm-diameter sapphire substrates by metalorganic vapor phase epitaxy, and their structural and electrical properties were characterized. A sample with an optimum AlN layer thickness of 1.0 nm showed a highly enhanced Hall mobility (μ_Hall) of 1770 cm²/Vs with a low sheet resistance (ρ_s) of 365 Ω/sq. (2DEG density n_s = 1.0 × 10¹³/cm²) at room temperature compared with those of a sample without the AlN interfacial layer (μ_Hall = 1287 cm²/Vs, ρ_s = 539 Ω/sq., and n_s = 0.9 × 10¹³/cm²). Electron transport properties in AlGaN/AlN/GaN structures were theoretically studied, and the calculated results indicated that the insertion of an AlN layer into the AlGaN/GaN heterointerface can significantly enhance the 2DEG mobility due to the reduction of alloy disorder scattering. HEMTs were successfully fabricated and characterized. It was confirmed that AlGaN/AlN/GaN HEMTs with the optimum AlN layer thickness show superior DC properties compared with conventional AlGaN/GaN HEMTs.     

Self-catalyzed molecular beam epitaxy growth and their optoelectronic properties of vertical GaAs nanowires on Si(111)

Self-assembled GaAs nanowires were grown by molecular beam epitaxy (MBE) on un-pretreated Si(111) substrates under different As₄/Ga flux ratios (V/III ratios). It has been found that the fraction of vertical wires would be nearly 100% when the As₄/Ga ratio arrives 90. The transmission electron microscopy (TEM) and micro-photoluminescence (PL) spectra results have indicated that the GaAs nanowires grown under a larger V/III ratio (90) have a pure ZB structure. Field-effect transistors (FET) based on single nanowire were fabricated with GaAs nanowires grown under the larger V/III ratio (90). The characteristics of the FET reveal a hole concentration of 3.919×10¹⁷ cm⁻³ and a hole mobility of 0.417 cm² V⁻¹s⁻¹. Photodetectors based on single nanowire and multiple nanowires structure with a metal-semiconductor-metal (MSM) electrode configuration have been proposed and demonstrated. All the photodetectors operating at room temperature exhibit good photoconductive performance, excellent stability, reproducibility and superior peak responsivity (87.67 A/W under 5 V for single nanowire photodetector).     

Effective mobility in amorphous organic transistors: Influence of the width of the density of states

The temperature dependence of poly(3-hexylthiophene-2,5-diyl) (P3HT)/polystyrene (PS) blend organic transistor current/voltage (I–V) characteristics has been experimentally and theoretically studied. The planar transistors, realized by drop casting the P3HT/PS ink, exhibit high mobilities (over 5 × 10⁻³ cm² V⁻¹ s⁻¹) and good overall characteristics. A transistor model accounting for transport mechanisms in disordered organic materials was used to fit the measured characteristics. Using a single set of parameters, the measured effective mobility versus gate bias, either increasing or decreasing with the gate bias depending on temperature, is well reproduced over a wide temperature range (130–343 K). A Gaussian density of states width of 0.045 eV was determined for this P3HT/PS blend. The transistor I–V characteristics are very well described considering disordered material properties within a self-consistent transistor model.     

Non-functionalized soluble diketopyrrolopyrrole: Simplest p-channel core for organic field-effect transistors

We report the synthesis, characterization and behavior in field-effect transistors of non-functionalized soluble diketopyrrolopyrrole (DPP) core with only a solubilizing alkyl chain (i.e. –C₁₆H₃₃ or –C₁₈H₃₇) as the simplest p-channel semiconductor. The characteristics were evaluated by UV–vis and fluorescence spectroscopy, X-ray diffraction, cyclic voltammetry (CV), thermal analysis, atomic force microscopy (AFM) and density functional theory (DFT) calculation. For top-contact field-effect transistors, two types of active layers were prepared either by a solution process (as a 1D-microwire) or thermal vacuum deposition (as a thin-film) on a cross-linked poly(4-vinylphenol) gate dielectric. All the devices showed typical p-channel behavior with dominant hole transports. The device made with 1D-microwiress of DPP-R18 showed field-effect mobility in the saturation region of 1.42 × 10^?2 cm²/V s with I_ON/I_OFF of 1.82 × 10³. These findings suggest that the non-functionalized soluble DPP core itself without any further functionalization could also be used as a p-channel semiconductor for low-cost organic electronic devices.     

Benzopyrazine-fused tetracene derivatives: Thin-film formation at the crystalline mesophase for solution-processed hole transporting devices

Benzopyrazine-fused tetracene (TBPy) and its disulfide (TBPyS) bearing alkoxy groups (OCH₃, OC₈H₁₇) were designed and synthesized to obtain π-expanded tetracene derivatives. These derivatives are featured with long-wavelength light absorption property (λ_onset: up to 820 nm), photooxidative stability (half-lives (τ_1/2): 11 times longer than tetracene), and solubility for solution process. The methoxy compounds (TBPy-C1 and TBPyS-C1) were used for single-crystal X-ray crystallographic analysis and single-crystal organic field-effect transistor (OFET) devices showing relationship between packing structures and hole mobilities. The octyloxy compounds (TBPy-C8 and TBPyS-C8) were investigated on solution-processed thin-film formation and hole transport property in thin-film OFET devices. Crystalline mesophase of TBPy-C8 and TBPyS-C8 was characterized by differential scanning calorimetry analysis showing endothermic peaks at 98 and 198 °C on its second heating process and exothermic peaks at 177 and 76 °C on its second cooling process for TBPyS-C8, and played crucial roles in thin-films formation. Hole mobility of 1.7 × 10^?2 cm²/V s (with V_th = ?30 V and I_ON/I_OFF = 10⁴) was obtained for the thin-film OFET device of TBPyS-C8.     

D–A copolymer with high ambipolar mobilities based on dithienothiophene and diketopyrrolopyrrole for polymer solar cells and organic field-effect transistors

Donor–acceptor (D–A) type conjugated polymers have been developed to absorb longer wavelength light in polymer solar cells (PSCs) and to achieve a high charge carrier mobility in organic field-effect transistors (OFETs). PDTDP, containing dithienothiophene (DTT) as the electron donor and diketopyrrolopyrrole (DPP) as the electron acceptor, was synthesized by stille polycondensation in order to achieve the advantages of D–A type conjugated polymers. The polymer showed optical band gaps of 1.44 and 1.42 eV in solution and in film, respectively, and a HOMO level of 5.09 eV. PDTDP and PC₇₁BM blends with 1,8-diiodooctane (DIO) exhibited improved performance in PSCs with a power conversion efficiency (PCE) of 4.45% under AM 1.5G irradiation. By investigating transmission electron microscopy (TEM), atomic force microscopy (AFM), and the light intensity dependence of J_SC and V_OC, we conclude that DIO acts as a processing additive that helps to form a nanoscale phase separation between donor and acceptor, resulting in an enhancement of μ_h and μ_e, which affects the J_SC, EQE, and PCE of PSCs. The charge carrier mobilities of PDTDP in OFETs were also investigated at various annealing temperatures and the polymer exhibited the highest hole and electron mobilities of 2.53 cm² V⁻¹ s⁻¹ at 250 °C and 0.36 cm² V⁻¹ s⁻¹ at 310 °C, respectively. XRD and AFM results demonstrated that the thermal annealing temperature had a critical effect on the changes in the crystallinity and morphology of the polymer. The low-voltage device was fabricated using high-k dielectric, P(VDF-TrFE) and P(VDF-TrFE-CTFE), and the carrier mobility of PDTDP was reached 0.1 cm² V⁻¹ s⁻¹ at V_d = −5 V. PDTDP complementary inverters were fabricated, and the high ambipolar characteristics of the polymer resulted in an output voltage gain of more than 25.     

A new ambipolar blue emitter for NTSC standard blue organic light-emitting device

A novel blue emitter, In2Bt, featured with a rigid and coplanar distyryl-p-phenylene backbone flattened by two different bridging atoms (i.e. carbon and sulfur) exhibits high thermal and morphological stability (T_g ～ 192 °C) and ambipolar charge carrier mobilities in the range of 10^?4 ～ 10^?5 cm² V^?1 s^?1. OLED device: ITO/PEDOT:PSS (300 Å)/α-NPD (200 Å)/TCTA (100 Å)/In2Bt (200 Å)/TPBI (500 Å)/LiF (5 Å)/Al (1500 Å) utilized In2Bt as an emitter gave a maximum brightness as high as 8000 cd m^?2 (12 V) and saturated-blue emission with CIE chromaticity coordinates of (0.16, 0.08), which is very close to the National Television Standards Committee (NTSC) standard blue gives an enlarged palette of colors for color displays.     

In0.75Ga0.25As channel layers with record mobility exceeding 12,000 cm2/Vs for use in high-κ dielectric NMOSFETs

In_0.75Ga_0.25As channel layers with a record mobility exceeding 12,000 cm²/Vs for use in high-κ dielectric NMOSFETs have been fabricated. The device structures which have been grown by molecular beam epitaxy on 3″ semi-insulating InP substrate comprise a 10 nm strained In_0.75Ga_0.25As channel layer and a high-κ oxide based dielectric layer (κ ≅ 20). Electron mobilities of 12,033 and 7,042 cm²/Vs have been measured for sheet carrier concentrations n_s of 2.5 × 10¹² and 6 × 10¹² cm⁻², respectively.     

Highly efficient green phosphorescent organic light emitting diodes with simple structure

A series of simple structures is investigated for realization of the highly efficient green phosphorescent organic light emitting diodes with relatively low voltage operation. All the devices were fabricated with mixed host system by using 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and 1,3,5-tri(p-pyrid-3-yl-phenyl)benzene (TpPyPB) which were known to be hole and electron type host materials due to their great hole and electron mobilities [μ_h(TAPC): 1 × 10^?2 cm²/V s and μ_e(TpPyPB): 7.9 × 10^?3 cm²/V s] [1]. The optimized device with thin TAPC (5–10 nm) as an anode buffer layer showed relatively high current and power efficiency with low roll-off characteristic up to 10,000 cd/m². The performances of the devices; with buffer layer were compared to those of simple devices with single layer and three layers. Very interestingly, the double layer device with TAPC buffer layer showed better current and power efficiency behavior compared to that of three layer device with both hole and electron buffer layers (TAPC, TpPyPB, respectively).     

p-Doped p-phenylenediamine-substituted fluorenes for organic electroluminescent devices

Two novel p-phenylenediamine-substituted fluorenes have been designed and synthesized. Their applications as hole injection materials in organic electroluminescent devices were investigated. These materials show a high glass transition temperature and a good hole-transporting ability. It has been demonstrated that the 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) doped p-phenylene-diamine-substituted fluorenes, in which F4-TCNQ acts as p-type dopant, are highly conducting with a good hole-transporting property. The organic light emitting devices (OLEDs) utilizing these F4-TCNQ-doped materials as a hole injection layer were fabricated and investigated. The pure Alq₃-based OLED device shows a current efficiency of 5.2 cd/A at the current density of 20 mA/cm² and the operation lifetime is 1500 h with driving voltage increasing only about 0.7 mV/h. The device performance and stability of this hole injection material meet the benchmarks for the commercial requirements for OLED materials.     

Characterization of solution processed,p-doped films using hole-only devices and organic field-effect transistors

We report a solution processed, p-doped film consisting of the organic materials 4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine (MTDATA) as the electron donor and 2-(3-(adamantan-1-yl)propyl)-3,5,6-trifluorotetracyanoquinodimethane (F3TCNQ-Adl) as the electron acceptor. UV–vis–NIR absorption spectra identified the presence of a charge transfer complex between the donor and acceptor in the doped films. Field-effect transistors were used to characterize charge transport properties of the films, yielding mobility values. Upon doping, mobility increased and then slightly decreased while carrier concentration increased by two orders of magnitude, which in tandem leads to conductivity increasing from 4 × 10^?10 S/cm when undoped to 2 × 10^?7 S/cm at 30 mol% F3TCNQ-Adl. The hole density was calculated based on mobility values extracted from OFET data and conductivity values extracted from bulk I–V data for the MTDATA: x mol% F3TCNQ-Sdl films. These films were then shown to function as the hole injection/hole transport layer in a phosphorescent blue OLED.     

Synthesis and semiconducting properties of Na2MnPO4F. Application to degradation of Rhodamine B under UV-light

Na₂MnPO₄F is synthesized by hydrothermal route at 453 K and the physical properties and photo-electrochemical characterizations are reported. The compound crystallizes in a monoclinic system (SG: P 2₁/n) with the lattice constants: a=13.7132 Å, b=5.3461 Å, c=13.7079 Å, β=119.97°. The UV–visible spectroscopy shows an indirect optical transition at 2.68 eV; a further direct transition occurs at 3.70 eV, due to the charge transfer O²⁻: 2p → Mn²⁺: e_g. The thermal variation of the electrical conductivity is characteristic of a semiconducting behavior with activation energy of 39 meV and an electron mobility (µ_318 K=5.56×10⁻⁴ cm² V⁻¹ s⁻¹), thermally activated. The flat band potential (+0.47 V_SCE) indicates that the valence band derives mainly from O²⁻: 2p orbital with a small admixture of F⁻ character while the conduction band is made up of Mn²⁺: t_2g orbital. The electrochemical impedance spectroscopy shows the contribution of both the bulk and grains boundaries. The photocatalytic performance of Na₂MnPO₄F for the degradation of Rhodamine B (RhB) is demonstrated on the basis of the energy diagram. 88% of the initial concentration is degraded under UV light and the oxidation follows a first order kinetic with a rate constant of 0.516 h⁻¹. Neither adsorption nor photolysis is observed. The photoactivity results from the electron transition from the hybridized band (O²⁻, F⁻) to the Mn²⁺: e_g orbital, occurring in the UV region. The catalyst was subjected to three successive photocatalytic cycles, thus proving its long term stability.     

Impact of the gate stack on the electrical performances of 3D multi-channel MOSFET (MCFET) on SOI

In this study, we integrate and compare the electrical performances of metal/high-K embedded gates in 3D multi-channel CMOSFETs (MCFETs) on SOI. The electrical characteristics of embedded gates obtained by filling cavities with TiN/HfO₂, TiN/SiO₂ or N⁺ poly-Si/SiO₂ are compared to a planar reference. In particular, we investigate electron and hole mobility behaviours (300 K down to 20 K) in embedded and planar structures, the gate leakage current and the negative bias temperature instability (NBTI). Despite a lower mobility, TiN/HfO₂ gate stack demonstrates the best I_ON/I_OFF compromise and exhibits NBTI life time higher than 10 years up to 1.3 V.