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Resistive switching behavior of poly (4-vinylphenol):graphene oxide (PVP:GO) composite and ultrathin HfO x hybrid bilayer was explored for developing high-performance flexible resistive random access memory (RRAM) devices. These devices, fabricated with a PVP:GO equivolume solution spin-coated and atomic layer deposited ultrathin HfO x as an active bilayer, exhibited excellent bipolar switching behavior with set and reset voltages as low as 0.6 and −1.46 V, ${I}_{ \mathrm{\scriptscriptstyle ON}}/{I}_{ \mathrm{\scriptscriptstyle OFF}}$ of >105, and ac and dc switching endurance of over 1400 and 800 cycles, respectively. The same device configuration realized over a flexible polyethylene terephthalate (PET) substrate exhibited a memory window of >103 even after undergoing large mechanical strain (correspoding to a 5-mm bending radius). In addition, after 150 cumulative cycles of consecutive tensile and compressive strain at a 5-mm bending radius, flexible RRAMs demonstrated a clear memory window of $4\times 10^{{3}}$ for 104 s. Overall, the incorporation of GO into the PVP solution resulted in achieving better control over conductive filament (CF) growth and, therefore, improved repeatability and reliability. This article indicates that the strategy of incorporating composite and organic–inorganic active bilayer can lead toward the development of high-performance flexible RRAMs.

Organic FETs (OFETs) with bilayer dielectric poly(4-vinylphenol) (PVP)/HfO2 and polyvinyl alcohol (PVA)/HfO2 were fabricated on a flexible low-cost Kapton tape substrate with TIPS -pentacene:polystyrene as an active layer. The devices exhibited average and maximum field-induced mobility of 0.25(±0.10) and 0.37 cm $^{{{2}}}\text{V}^{{-{1}}}\,\,\text{s}^{{-{1}}}$ for PVP/HfO2 devices, and 0.35(±0.18) and 0.69 cm $^{{{2}}}\text{V}^{{-{1}}}\,\,\text{s}^{{-{1}}}$ for PVA/HfO2 devices, respectively, with low threshold voltage and ${I}_{ \mathrm{\scriptscriptstyle ON}}$ to ${I}_{ \mathrm{\scriptscriptstyle OFF}}$ ratio approaching $10^{{{4}}}$ for both types of devices. Overlapping transfer multiple scans with very less performance variation under ambient conditions were observed. The devices were found to be fairly stable even when electrical stress was applied after a period of 5 months. Devices exhibited small degradation in ${I} _{{\text {DS}}}$ and small $\Delta {V} _{{\text {TH}}}$ with high degree of reliability and small performance spreads under combined effects of ambient exposure over a period of 5 months and successive electrical stress. In addition, inverter based on these OFETs with external load was also demonstrated.

In this work, a combination of a thin layer of Copper (II) phthalocyanine (CuPc) and TIPS-pentacene crystal was used to enhance the photo-sensing spectrum of photo- sensitive organic field-effect transistors (photo-OFETs). In TIPS-pentacene/CuPc bi-layer devices, the different conduction path not only reduced the total number of traps but also enhanced the sensing spectrum. It was found that, though the TIPS-pentacene was able to sense only blue and green colour well, and CuPc was able to sense only red and green colour well; the TIPS-pentacene/CuPc bi-layer were able to sense all three colors, enhancing the range of sensing. On illumination, bi-layer TIPS-pentacene/CuPc OFETs exhibited higher values of shift in threshold voltage (|ΔVTH|) compared to neat TIPS-pentacene and bare CuPc OFETs, due to change in the spatial distribution in the trap state density and associated pattern of trapping of photo-generated charge carriers. It was also found that the values of photo-responsivity (Rmax) and photo-detection time for the bi-layer OFETs were in between that for OFETs with TIPS-pentacene or CuPc.

Physics behind the improvement in breakdown voltage of AlGaN/GaN HEMTs with carbon-doping of GaN buffer is discussed. Modeling of carbon as acceptor traps and self-compensating acceptor/donor traps is discussed with respect to their impact on avalanche breakdown. Impact of carbon behaving as a donor as well as acceptor traps on electric field relaxation and avalanche generation is discussed in detail to establish the true nature of carbon in GaN that delays the avalanche action. This understanding of the behavior of carbon-doping in GaN buffer is then utilized to discuss design parameters related to carbon doped buffer. Design parameters such as undoped channel thickness and relative trap concentration induced by carbon-doping are discussed with respect to the performance metrics of breakdown voltage, leakage current, sheet charge density, and dynamic ON-resistance.

In part I of this paper, we developed physical insights into the role and impact of acceptor and donor traps—resulting from C-doping in GaN buffer—on avalanche breakdown in AlGaN/GaN HEMT devices. It was found that the donor traps are mandatory to explain the breakdown voltage improvement. In this paper, silicon doping is proposed and explored as an alternative to independently engineer donor trap concentration and profile. Keeping in mind the acceptor and donor trap relative concentration requirement for achieving higher breakdown buffer, as depicted in part I of this paper, silicon & carbon codoping of GaN buffer is proposed and explored in this paper. The proposed improvement in breakdown voltage is supported by physical insight into the avalanche phenomena and role of acceptor/donor traps. GaN buffer design parameters and their impact on breakdown voltage as well as leakage current are presented. Finally, a modified Si-doping profile in the GaN buffer is proposed to lower the C-doping concentration near GaN channel to mitigate the adverse effects of acceptor traps in GaN buffer.

In this report, we have demonstrated the optical non-volatile memory characteristics using CuPc OFET. The memory operation was comprehensively demonstrated with different programming conditions. It was found that the programming of CuPc OFET with an electric pulse at the gate terminal under UV-light photo-illumination compared to other programming conditions, could substantially increase the memory window due to massive charge trapping in the polymer electret layer, which causes shift in the device transfer characteristics from low-conduction state (“OFF state”, or logic 0) to high conduction state (“ON state”, or logic 1) at VGS = 0V. From device operation at −50V, a memory window of greater than 45V could be achieved by applying a programming voltage of +70 V at the gate terminal under UV-light photo-illumination. Moreover, it was completely erased by applying −100 V at the gate terminal in dark.

Photo-response of solution processed flexible TIPS-pentacene organic field-effect transistors is evaluated under illumination with visible light of red, green, and blue colors having minimum wavelengths of 620, 520, and 460 nm. For −10 V operation, pristine photo-OFETs exhibited average field-effect mobility of 0.11 cm $^{2}\text{V}^{-1}\text{s}^{-1}$ , near zero threshold voltage, and current ON–OFF ratio of ~105. These photo-OFETs exhibited prevalent photovoltaic characteristics with shift in the threshold voltage upon illumination, which was found to increase with rising intensity, illumination time, and gate bias during illumination. For low-voltage operation at −5 V, maximum current modulation of $4\times 10^{4}$ and 102, and photo-responsivity of 17 mA/W and 35 mA/W, respectively, were achieved for blue (intensity of 1.7 mW/cm2) and green (intensity of 0.4 mW/cm2) light illuminations. However, these photo-OFETs did not show significant response to red light. A fast dynamic response to periodic pulses of illumination was also observed. For a gate bias of +10 V and illumination time of 500 s, maximum current modulation of 105 was achieved for blue light illumination.

In this work, we have explored the effect of TIPSpentacene: polystyrene (PS) blend on electrical performance and photo-response of solution processed organic field-effect transistors (OFETs). It was observed that though the blend OFETs show better electrical performance, the photo-responsivity (R) of neat TIPS-pentacene OFETs was significantly better. This study was performed for different color lights such as blue, red, and green, as well as for ultraviolet light (UV). R was highest for blue color (~2600 mA/W for neat, ~230 mA/W for blend), and smaller for other cases. Low R value in blend devices is attributed to less trapping of photo-generated charge carriers during photo-illumination compared to their neat counterparts.

We report on the fabrication of bottom gate top contact organic field effect transistors with hybrid gate dielectric and TIPS-Pentacene:PS blend as active layer on PowerCoat™ HD 230 paper substrate. The hybrid dielectric layer leads the devices operated at relatively low voltage of −10 V with avg. and maximum field effect mobility of $0.52(\pm 0.16)$ and 0.78 cm2 V−1 S−1 respectively, with near zero threshold voltage and on-off current ratio approaching ~104. The devices have shown excellent operational stability when tested with DC bias stress of $\mathrm{V}_{\mathrm{DS}}=\mathrm{V}_{\mathrm{GS}}=-10\ \mathrm{V}$ for 1 h and minuscule changes in electrical characteristics were observed with continuous multiple transfer cycle scans. In addition, we have demonstrated a resistive load inverter circuit with varying the load resistance with these paper OFETs.

Effect of UV irradiation $( \lambda _{Peak} \quad =365$ nm) on the electrical performance of the OFETs with CuPc as active layer is demonstrated. The devices have shown stable electrical performance under dark with avg. and max. field effect mobility of $( 1.0 \pm 0.3 ) \times 10 ^{-2}$ and $1.4 \times 10 ^{-2}$ cm $^{2}\, \mathrm {V}^{-1}\, \mathrm {s}^{-1}$ respectively. Under UV illumination there found a shift in threshold voltage and change in ON current along with fast response and recovery time of 0.4 s and 2 s respectively with periodic UV ON-OFF pulses.

The crucial role of molecular weight of the polymer dielectric in regulating the electrical performance and photo-response of TIPS-pentacene OFETs has been explored using poly(vinyl alcohol) (PVA) as the polymer dielectric layer. With increasing molecular weight of PVA, a degradation in the electrical and photo-response was observed. Atomic-force microscopy (AFM), Fourier-transform infrared spectroscopy (FTIR) and X-ray Diffraction (XRD) studies revealed that with increase in PVA molecular weight, polarizability of PVA dielectric layer and thus strength of the OFET channel reduced due to decreased hydroxyl group concentration oriented perpendicular to the PVA-semiconductor interface. Further, due to rising surface roughness of underlying PVA film with its molecular weight, macroscopic irregularities and therefore trap density in the active layer grown over it also increases. Therefore, both of these molecular weight dependent factors influence overall device performance and photo-response of OFETs in a critical way.

This paper for the first time presents a comprehensive computational modeling approach for AlGaN/GaN high electron mobility transistors. Impact of the polarization charge at different material interfaces on the energy band profile as well as parasitic charge across the epitaxial stack is modeled and studied. Furthermore, impact of surface and bulk traps on two-dimensional electron gas, device characteristics, and gate leakage is accounted in this paper. For the first time, surface states modeled as donor type traps were correlated with gate leakage. Moreover, a new approach to accurately model the forward gate leakage in Schottky gate devices is proposed. Finally, impact of lattice and carrier heating is studied, while highlighting the relevance of carrier heating, lattice heating, and bulk traps over the device characteristics. In addition to this, modeling strategy for other critical aspects like parasitic charges, quantum effects, S/D Schottky contacts, and high field effects is presented.

Phase separation induced high carrier mobility and electrical stability are achieved in organic field-effect transistors using TIPS-pentacene:polystyrene blends. Rigid Si/SiO 2 substrate was especially chosen to explore the phase separation. A vertical phase separation between TIPS-pentacene and polystyrene as confirmed from scanning electron microscopic image, evetually leads to excellent carrier mobility in polymer blend devices compared to that of neat TIPS-pentacene. Maximum hole mobility improved from 0.2 cm 2 V −1 s −1 for neat TIPS-pentacene on SiO 2 to 2.6 cm 2 V −1 s −1 for TIPS-pentacene blends with PS, with average value of 1.5 cm 2 V −1 s −1 . Apart from higher mobility, TIPS-pentacene:PS blend devices also showed much lower decay in drain current (∼30%) during a constant bias-stress of 2 h, compared to neat devices (∼80%). Interestingly, This decay was fully recovered for blend devices under rest conditions. The corresponding shift in the threshold voltage due to bias-stress was lower for TIPS-pentacene:PS device due to better quality of interface as confirmed by lower values of density of interface traps and higher trapping time. High electrical stability in TIPS-pentacene:polystyrene blend devices was also supported by repeatabiliiy studies, which exhibited nearly unchanged device characteristics.

High-performance solution processed flexible organic field-effect transistors with 6,13(bis-triisopropylsily-lethynyl) pentacene and polystyrene blend are demonstrated with high electro-mechanical stability. For −5 V operation, field-effect mobility up to 1.1 cm $^{2}~\text{V}^{\mathrm {-1}}~\text{s}^{\mathrm {-1}}$ and threshold voltage as low as −0.1 V were obtained with high current on–off ratios of $\sim 10^{5}$ due to high quality dielectric–semiconductor interface developed during solvent evaporation. Stable electrical characteristics were achieved with increasing duration of mechanical strain, and after multiple cycles of tensile and compressive strain. Drain current decay of 10%, very large trapping time of $\sim 10^{8}$ s, and a very small threshold voltage shift of 0.3 V were observed during bias stress of 1 h, signifying low charge carrier trapping and a high quality of dielectric–semiconductor interface, which was retained largely after two days of continuous tensile strain. Moreover, after 100 cycles of tensile and compressive strain, the corresponding shift in threshold voltage due to bias stress was still $\sim 0.5$ V. Overall, a high performance and stability were demonstrated under collective effects of mechanical and electrical stress.

Flexible organic field-effect transistors (OFETs) with TIPS-pentacene: polystyrene (PS) blend are demonstrated to exhibit enhanced mobility and significantly improved electrical stability compared to neat TIPS-pentacene on poly(4-vinylphenol) (PVP) dielectric (bi-layer OFETs), along with high mechanical stability. Due to merit of high quality dielectric-semiconductor interface, pristine TIPS-pentacene: PS blend OFETs exhibited maximum mobility of 0.93 cm2 V−1 s−1 with average of 0.44(±0.25) cm2 V−1 s−1 compared to 0.14(±0.10) cm2 V−1 s−1 for bi-layer OFETs with high current on-off ratios on the order 105 for both. Both types of devices exhibited high electrical stability upon bending with increasing magnitude of strain or its duration up to 5 days. However, significant differences in electrical stability of devices were observed upon applying constant bias-stress for 40 min to 1 h. Pristine blend devices exhibited outstanding electrical stability with very low drain current decay of

The effect of structural dissimilarity of the additive solvent from the main solvent on the properties of the solution and resulting crystals for solution processed organic field-effect transistors (OFETs) is comprehensively studied. Organic semiconductor TIPS-Pentacene was explored with toluene as the main solvent and benzene, cyclohexane, and hexane as additives for making semiconductor solutions. The solution formation with various solvents is explained by Hansen’s solubility theory, according to which the binary solvent with higher structural dissimilarity between constituent solvents is a weaker solvent of the organic semiconductor; which eventually supports better molecular aggregation and crystallinity in the resulting crystallites. As investigated by XRD, the degree of crystalline order systematically increased with rising dissimilarity in component solvents; integrated intensity ratio rising from 0.52 for toluene to 0.54, 0.69 and 0.72 respectively for toluene/benzene, toluene/cyclohexane, and toluene/hexane. The variation in degree of crystallinity is also reflected in electrical properties of OFETs, where the field-effect mobility improved from 0.05 cm2/(V.s) for toluene solvent to higher than 0.1 cm2/(V.s) for toluene/hexane.

Flexible organic field-effect transistors with high electrical stability upon bending are demonstrated on indium tin oxide coated polyethylene terephthalate substrates with TIPS-Pentacene semiconductor crystals formed by drop casting on a hybrid gate dielectric consisting hafnium dioxide grown by atomic layer deposition and spin coated poly(4-vinylphenol). Fabricated devices exhibited excellent p-channel characteristics with field-effect mobility up to 0.12 cm2/Vs with high current on/off ratio >104 and low threshold voltage of −0.2 V. Device performance was slightly affected by mechanical strain applied by bending for 5 min with radius varying from 12.5 mm to as low as 5.0 mm; and a high stability in performance was demonstrated upon applying constant tensile strain for more than 48 h at bending radius of 5.0 mm. It was found that strain induced changes in the device performance primarily occur due to increase in dielectric surface roughness; and the semiconductor-dielectric interface uniformity is influenced more with magnitude of strain rather than its duration.

We report the effect of varying the semiconductor:polymer blend ratio on the electrical performance and bias-stress stability of the organic field effect transistors (OFETs) with TIPS-Pentacene and polystyrene as semiconductor and polymer combination. Device performance was found to improve with increasing polymer content in the solution. Devices with 1:3 TIPS-Pentacene:polystyrene blend outperformed the other counterparts with high performance, and demonstrated least performance variation with regular 100 transfer measurement cycles. In addition, 1:3 blend devices have shown the least normalized drain current decay of $\sim$ 8.5 % with the bias stress condition of V DS = V GS = -30 V for 2 h, as compared to the 1:1, 3:1 and neat devices with the decay of 15 %, 37 % and 71 % respectively. Additionally, a better recovery of electrical characteristics was observed in blend devices with larger polymer fractions from the deteriorating effects of gate bias stress.

In this work, a TCAD based approach is presented to model electrical behavior of TIPS-Pentacene: polymer blend organic field-effect transistors (OFETs). Stress induced and temperature induced effects are also captured, thereby accurately modeling the device behavior under different operating conditions. The framework is validated against experimental results and is then used to estimate the effect of temperature on characteristics of inverter circuit using this device.

Effect of thermal annealing on the performance of flexible OFETs was investigated. Annealing at low temperatures up to 50 °C enhanced the molecular ordering and granular arrangement leading to improved device performance, however, a permanent degradation was induced at high temperature annealing due to loss in crystallinity and crack generation. An annealed device exhibited a larger decay in drain current than a pristine device under constant bias stress. Under combined effects of mechanical and thermal stress, effect of thermal stress was found to be dominant at higher annealing temperatures, resulting in monotonic performance degradation.

Inverter circuits based on neat TIPS-pentacene and TIPS-pentacene: polystyrene blend organic field-effect transistors (OFETs) with external resistive load has been studied. Resistive loads of 28 ΜΩ, 40 ΜΩ, 68 ΜΩ and 108 ΜΩ were used to study the inverter transfer characteristics. Transfer characteristics of both types of inverters are improved with increasing load resistance. Inverter with a representative OFET with neat TIPS-pentacene shows a DC voltage gain of −6.9, swing of 40 V, and static power dissipation of 9.5 μW with load of 108 ΜΩ and 50 V power supply. Similarly, inverter with a representative TIPS-pentacene: polymer blend OFET shows a DC voltage gain of −8.7, swing of 44 V, and static power dissipation of 10.2 μW with the same load. For both neat TIPS-pentacene and TIPS-pentacene: polymer blend inverters, maximum DC gain and voltage swing are achieved with highest value of resistive load. However, switching thresholds near V DD /2 are obtained with moderate resistive loads.

Organic field effect transistors (OFETs) have been widely explored for their popular applications in flat panel displays and various kinds of affordable sensors. The overall performance of the device depends on the quality of semiconductor-dielectric interface, and degree of crystalline order in the active layer, close to the above said interface. In the special league of solution processed OFETs, semicondutor: polymer blends have been reported as one of the best possible ways to achieve an intimate, uniform and defect free semiconductor-dielectric interface in a single step. In addition, degree of crystalline order can be further ameliorated through solvent vapor annealing (SVA), which essentially controls the evaporation time of the solvent.

We report on high-mobility top-gate organic field-effect transistors (OFETs) and complementary-like inverters fabricated with a solution-processed molecular bis(naphthalene diimide)-dithienopyrrole derivative as the channel semiconductor and a CYTOP/Al2O3 bilayer as the gate dielectric. The OFETs showed ambipolar behavior with average electron and hole mobility values of 1.2 and 0.01 cm2 V−1 s−1, respectively. Complementary-like inverters fabricated with two ambipolar OFETs showed hysteresis-free voltage transfer characteristics with negligible variations of switching threshold voltages and yielded very high DC gain values of more than 90 V/V (up to 122 V/V) at a supply voltage of 25 V.

A series of conjugated polymers has been synthesized by Sonogashira coupling of N,N′-bis(2-octyldodecyl)-2,6-dibromonaphthalene-1,4,5,8-bis(dicarboximide) and four para-diethynylbenzene derivatives: 1,4-diethynyl-2,5-dihexadecyloxybenzene, 1,4-diethynyl-2,5-bis(2-octyldodecyloxy) benzene, 1,4-bis(2-ethylhexyl)-2,5-diethynylbenzene, 1,4-diethynyl-2,5-bis(trifluoromethyl)benzene. The polymers display absorption maxima at wavelengths ranging from 530 nm to 654 nm with molar absorptivities ranging from ca. 4 to 7 × 104 M−1 cm−1. The peak reduction potentials, determined by differential pulse voltammetry, for polymer films varied from −0.93 to −1.14 V vs. ferrocenium/ferrocene with the trifluoromethyl-substituted derivative being the most readily reduced. All four polymers exhibited electron transport characteristics in bottom-gate/top-contact field-effect transistors, showing average electron mobility values ranging from 1.4 × 10−4 to 3.7 × 10−3 cm2 V−1 s−1.

Performance of pentacene organic field-effect transistors (OFETs) is significantly improved by treatment of SiO2 with octyltrichlorosilane (OTS-8) compared to octadecyltrichlorosilane (OTS-18). The average hole mobility in these OFETs is increased from 0.4 to 0.8 cm2/Vs when treating the dielectric with OTS-8 versus OTS-18 treated devices. The atomic force microscope (AFM) images show that the OTS-8 treated surface produces much larger grains of pentacene (∼500 nm) compared to OTS-18 (∼100 nm). X-ray diffraction (XRD) results confirmed that the pentacene on OTS-8 is more crystalline compared to the pentacene on OTS-18, resulting in higher hole mobility.

2,2′-(Benzo[c][1,2,5]thiadiazol-4,7-diyl)-4,4′-dialkyl-bis(4H-dithieno[3,2-b:2′,3′-d]pyrrole) (DTP-BTD-DTP) donor–acceptor–donor (D-A-D) and 4-alkyl-2,6-bis(benzo[c][1,2,5]thiadiazol-4-yl)-4H-dithieno[3,2-b:2′,3′-d]pyrrole (BTD-DTP-BTD) acceptor–donor–acceptor (A-D-A) triads, with or without additional alkylation in the DTP 6- or BTD 7-positions, respectively, have been synthesized using Stille coupling reactions, characterized using UV–vis absorption spectroscopy and electrochemistry, modeled using density functional theory calculations, and used as charge-transport materials in field-effect transistors. The choice of alkyl substitution pattern has only minor effects on the optical and redox behavior but can be used to modify the thermal properties and solubility of these compounds. The D-A-D and A-D-A triads show long-wavelength absorption maxima at 566–588 and 517–521 nm, respectively, in solution. These transitions are attributed to excitation from a delocalized HOMO to a BTD-localized LUMO and, accor...

We report on vertically stacked complementary inverters implemented with a solution-processed [6,6]-phenyl c 61 butyric acid methyl ester (PCBM) n -channel thin-film transistor (TFT) fabricated on top of a 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) and poly(triarylamine) (PTAA) blend p -channel TFT. With a shared common gate electrode positioned between two dielectric layers, bottom-contact p - and top-contact n -channel TFTs showed saturation mobility values of 0.25 and 0.004 cm 2 /V s and threshold voltages of −3.9, and 0.3 V, respectively. The inverter yielded a gain value of −24 V/V with a switching threshold voltage value of 3.3 V at a supply voltage of 7 V. This demonstration of the use of solution-processed semiconductors in a vertically stacked complementary inverter geometry is a step forward towards the development of low-cost complementary electronics.

The contact resistance in pentacene organic field-effect transistors (OFETs) is found to be significantly reduced by selectively doping the organic semiconductor region beneath the source/drain electrodes. A 10 nm co-evaporated (1:1 ratio) layer of molybdenum tris-[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] and pentacene was deposited under the metal electrodes for this purpose. The width-normalized contact resistance (varying channel lengths of 25–200 μm used for the study) in contact-doped devices was lowered significantly (0.5 kΩ-cm) in comparison to reference devices (3.4 kΩ-cm) in the accumulation regime (VGS = −30 V). Doping of the contacts did not affect the stability of the devices under continuous bias stress significantly.

A series of highly soluble donor–acceptor (D–A) copolymers containing N-(3,4,5-tri-n-decyloxyphenyl)-dithieno[3,2-b:2′,3′-d]pyrrole (DTP) or N-(2-decyltetradecyl)-dithieno[3,2-b:2′,3′-d]pyrrole (DTP′) as donor and three different acceptors, 4,7-dithien-2-yl-[2,1,3]-benzothiadiazole, 4,9-dithien-2-yl-6,7-di-n-hexyl-[1,2,5]thiadiazolo[3,4-g]quinoxaline and 4,8-dithien-2-yl-2λ4δ2-benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole (BThX, X = BTD, TQHx2, BBT, respectively) were synthesized by Stille coupling polymerizations. The optical and electrochemical properties of these copolymers were investigated, along with their use in field-effect transistors and photovoltaic devices. The band gaps (eV) estimated from UV-vis-NIR spectra and electrochemical measurements of the copolymers varied from ca. 1.5–0.5 eV, and were consistent with quantum-chemical estimates extrapolated using density functional theory. Oxidative and reductive spectroelectrochemistry of the copolymers indicated they can be both p-doped and n-doped, and three to four differently colored redox states of the polymers can be accessed through electrochemical oxidation or reduction. The DTP-BThBTD and DTP-BThTQHx2 copolymers exhibited average field-effect hole mobilities of 1.2 × 10−4 and 2.2 × 10−3 cm2/(Vs), respectively. DTP-BThBBT exhibited ambipolar field-effect characteristics and showed hole and electron mobilities of 1.2 × 10−3 and 5.8 × 10−4 cm2/(Vs), respectively. Bulk heterojunction photovoltaic devices made from blends of the copolymers with 3′-phenyl-3′H-cyclopropa[1,9](C60-Ih)[5,6]fullerene-3′-butanoic acid methyl ester (PCBM) (1:3 weight ratio) exhibited average power conversion efficiencies as high as 1.3% under simulated irradiance of 75 mW/cm2.

Low-voltage pentacene organic field-effect transistors (OFETs) with different gate dielectric interfaces are studied and their performance in terms of electrical properties and operational stability is compared. Overall high electrical performance is demonstrated at low voltage by using a 100 nm-thick high-κ gate dielectric layer of aluminum oxide (Al2O3) fabricated by atomic layer deposition (ALD) and modified with hydroxyl-free low-κ polymers like polystyrene (PS), divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) (Cyclotene™, Dow Chemicals), and as well as with the widely used octadecyl-trichlorosilane (OTS). Devices with PS and BCB dielectric surfaces exhibit almost similar electrical performance with high field-effect mobilities, low subthreshold voltages, and high on/off current ratios. The higher mobility in pentacene transistors with PS can be correlated to the better structural ordering of pentacene films, as demonstrated by atomic force microscopy (AFM) images and X-ray diffraction (XRD). The devices with PS show good electrical stability under bias stress conditions (VGS = VDS = −10 V for 1 h), resulting in a negligible drop (∼2%) in saturation current (IDS) in comparison to that in devices with OTS (∼12%), and to a very high decay (∼30%) for the devices with BCB.

Solution-processed n-type organic field-effect transistors (OFETs) based on the fullerene derivative {6}-1-(3-(2- thienylethoxycarbonyl)-propyl)-{5}-l-phenyl-[5,6]-C61 (TEPP) and phenyl-C61-butyric acid methyl ester (PCBM) in a multiring source/drain structure are reported. Devices with TEPP show high electron mobility up to 7.8 x 10-2 cm2/Vs in the saturation regime for bottom-contact OFETs with Au S/D electrodes with a solution-processed fullerene derivative. The ON/OFF ratios reported in this letter, which are in the range of 105 -106, are among the highest values reported for such devices. This mobility is always higher compared to PCBM devices prepared in identical conditions. The mobility of TEPP and PCBM increased with increasing temperatures in the range of 100-300 K with activation energy of 78 and 113 meV, respectively, which suggests that the thermally activated hopping of electrons is dominant in TEPP.

A comprehensive study for crystal alignment of TIPS-Pentacene by an off-center spin coating has been presented. In a normal center spin coating method, the centrifugal force on the grains is multidirectional which results into crystals aligning in multiple directions depending on the location on the sample. However in an off-center spin coating method where the samples are kept at a distance away from the center, a unidirectional centrifugal force in the direction of the sample causes all the grains to align nearly in the direction of this force. The spin speed was the paramount factor influencing the overall grain size, whereas the overall alignment depends on the off-center distance. All the films in these experiments were found to have very large crystal sizes, ranging up to 5 microns. In addition, films formed by off-center method were found to be more crystalline than the center spin coated film and show a metastable crystalline polymorph.

Top-gate bottom-contact field-effect transistors based on solution-processed films of molecules in which two naphthalene-1,8:4,5-bis(dicarboxdiimide)s are bridged by thieno[3,2-b]thiophene, dithien...

In this paper, the influence of gate-source and drain-source bias on the bias-stress stability and lifetime of pentacene-based low-voltage (−3 V) organic thin-film transistors (TFTs) built on plastic substrate has been investigated. The 10%-current-decay lifetime is used for analyzing the influence of applied bias on the bias-stress stability of TFTs, and to compare various biasing conditions. Our results show a 3 to 4 times higher 10%-current-decay lifetime when magnitude of gate-source and drain-source voltage are equal and less than 2.5 V during bias stress, compared to that when drain-source voltage is kept at −3.0 V.

Organic thin-film transistors (TFTs) have potential as pixel drivers in flexible active-matrix organic light-emitting diode displays [1]. Hence it is essential to analyze the charge-carrier injection and extraction dynamics of organic TFTs to gain a better understanding of the trapping and detrapping at the TFT interfaces. From the current-voltage characteristics of the TFTs, many important parameters can be extracted, such as carrier mobility, threshold voltage, on/off ratio, subthreshold slope and transconductance. But to quantitatively evaluate the trapping and detrapping dynamics, displacement current measurements on two-terminal long-channel capacitors (LCCs) are far more useful [2, 3]. The cross-section and the layout of an LCC are schematically shown in Fig. 1. Unlike a TFT, an LCC has only one contact, so that carriers are injected into and extracted from the semiconductor through the same contact. To increase the signal-to-noise ratio, a very large channel length (up to 6 cm) is employed. While Liang et al. have performed displacement current measurements on pentacene-based LCCs [2,3], we report here on displacement current measurements on LCCs based on four different organic semiconductors: pentacene, dinaptho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT), 2,9-didecyl-DNTT (C 10 -DNTT) and diphenyl-DNTT (DPh-DNTT). In TFTs, these semiconductors show hole mobilities ranging from 1 to 7 cm 2 /Vs. The goal of the displacement current measurements reported here is to study how the choice of the semiconductor affects the trapping and detrapping dynamics in organic TFTs.

The presence of NaCl in the nutrient solution promoted nitrate uptake in parent Anabaena sp. PCC 7120, mutants SP7 (defective in nitrate reductase activity) and SP17 (partially defective in nitrate reductase activity), but not in the mutant SP9 (defective in nitrate transport and reduction). Nitrate reductase activity of the parent and mutant SP17 increased with increasing concentration of nitrate in saline medium, while mutants SP7 and SP9 did not respond to the altered salinity. Although Na+ was not required for nitrate reductase activity, its presence in the nutrient solution enhanced nitrate reduction. Complete removal of Na+ from the nutrient solution markedly reduced nitrogenase activity in all the strains, while raising the concentration of NaCl to 50 mmol l-1 or above, was equally toxic to nitrogenase activity. External NaCl at 200 mmol l-1 brought down the nitrogenase activity to the same residual level as observed without Na+.

Growth of the cyanobacterium Anabaena sp. PCC 7120 and its nitrate assimilation-defective mutants was inversely proportional to the NaCl concentration in the medium. Presence of nitrate in the saline medium protected the growth of the parent but not of the mutant strains from salt toxicity. On the other hand, ammonium nitrogen protected the growth of all the strains from salt toxicity. However, the effect was less than that of nitrate. An altered sodium transport system was evident in the mutant strains and was most marked in mutant SP9. The cellular sodium concentration in parent and mutant strains also varied. Although mutant SP9 exhibited the lowest level of cellular sodium, it was as sensitive to salt toxicity as other strains. It is assumed that merely the presence of a toxic level of NaCl in the ambient environment is sufficient to damage the structural and functional components of the plasma membrane.

This paper reports the use of hot-wire chemical vapour deposited (HWCVD) Silicon nitride as a passivation layer for Organic Field Effect Transistors (OFETs). Firstly, the degradation study of the OFETs is done with time. A thin (10–20 nm) layer of silicon nitride is deposited on the OFETs, at a low temperature (

The c group of Gram-negative gliding bacteria, has a long history of cosmopolitan occurrence. It has great biodiversity despite the absence of sexual reproduction. This wide biodiversity may be reflected in the wide spectrum of its secondary metabolites. These cyanobacterial secondary metabolites are biosynthesized by a variety of routes, notably by non-ribosomal peptide synthetase or polyketide synthetase systems, and show a wide range of biological activities including anticancer, antibacterial, antiviral and protease inhibition activities. This high degree of chemical diversity in cyanobacterial secondary metabolites may thus constitute a prolific source of new entities leading to the development of new pharmaceuticals.

Multi-frequency transconductance technique is successfully applied in this work for the first time for interface characterization of OFETs. Standard charge pumping measurements are used on silicon MOSFETs for the validation of MFT technique. The method is implemented on pentacene as well as the P3HT based OFETs with SiO2 as the gate dielectric. Our results show interface state densities in the range of 1012/cm2/eV for both the samples. The P3HT films are also shown to have additional trap centres which respond to frequencies above 100 kHz. Our results therefore clearly indicate that the MFT technique is indeed a highly useful technique for interface characterization of OFETs.

Pentacene organic field effect transistors are fabricated using polymethyl methacrylate (PMMA) as a gate dielectric on flexible polymeric substrates like the indium tin oxide coated polyethylene terephthalate (ITO coated PET), and polyethylene napthalate (PEN) in top contact configuration. The ITO and aluminium (evaporated on PEN) act as the gate materials for the OFETs. The devices on ITO coated PET show electron mobility up to 0.07 cm2/V-s and the on/off ratios in the order of 103.

A systematic and comprehensive study on the charge-carrier injection and trapping behavior was performed using displacement current measurements in long-channel capacitors based on four promising small-molecule organic semiconductors (pentacene, DNTT, C10-DNTT and DPh-DNTT). In thin-film transistors, these semiconductors showed charge-carrier mobilities ranging from 1.0 to 7.8 cm2 V−1 s−1. The number of charges injected into and extracted from the semiconductor and the density of charges trapped in the device during each measurement were calculated from the displacement current characteristics and it was found that the density of trapped charges is very similar in all devices and of the order 1012 cm−2, despite the fact that the four semiconductors show significantly different charge-carrier mobilities. The choice of the contact metal (Au, Ag, Cu, Pd) was also found to have no significant effect on the trapping behavior.

Nitrate assimilation-defective mutants SP7, SP9, and SP17 of the cyanobacterium Anabaena sp. PCC 7120 were isolated by use of transposon mutagenesis and screened on medium containing chlorate. SP7 and SP17 represented nitrate reductase-defective nature, while mutant SP9 appeared to be a regulatory mutant exhibiting pleiotropic behavior. Kinetics of nitrate uptake system exhibited K(s) values of 31-38mgr;M for parent, SP7, and SP17 strains; however, mutant SP9 exhibited a high K(s) value of 109.5mgr;M. Defective nitrate reductase was apparent in mutant SP7 and SP9, while mutant SP17 exhibited partial defective nature. Methyl viologen-dependent NR activity in parent strain presented a biphasic nature with K(m) values of 0.13 and 2.47 mM, whereas a single K(m) value (2.96 mM) was observed for mutant SP17. Mutant SP9 was also defective in nitrite uptake and reduction. Mutant strains exhibited derepressed nitrogenase activity in the presence of nitrate, while glutamine synthetase activity remained unaltered.http://link.springer-ny. com/link/service/journals/00284/bibs/39n5p237.html/HEA

Fusarium udum, Fusarium oxysporum f. sp. ciceri and F.oxysporum f.sp.lentis cause severe economic loss to pigeonpea, chickpea and lentil crops. Forty three isolates of Trichoderma were screened for their antagonistic ability against the test pathogens. Potent strains were identified on the basis of in vitro antagonistic ability and fungicide tolerant capability. They were subjected for mutagenesis using NTG for improvement of their antagonistic potential. Two stable mutants who exhibited enhanced bioefficacy against the soilborne fungi and tolerated carbendazim and copper sulphate were subjected for protoplast fusion in order to achieve superior strain. Protoplasts were obtained from the mutants with novozyme 234 and were fused in the presence of PEG and hybrids were isolated on selective medium. Selected fast growing stable fusants were tested for fungicide tolerant capability and bioefficacy in dual culture experiments. The fusant TPTF3 exerted high degree of antagonistic ability in controlling the radial growth of test pathogens which was between 90–94% and also tolerated carbendazim and copper sulphate up to 125 μm and 120mM respectively. The fusant strain produced higher level of chitinase and â-1, 3 glucanase as compared to the mutants and wild type parent strains. Hence, in present study, superior strain with broad spectrum distinct antagonistic and fungicide tolerant abilities was developed which could be used in IPM programme for the control of soilborne diseases of pulse crops.

A series of dihalo- and bis-aroyl-substituted benzo[1,2-b:6,5-b′]dithiophene-4,5-diones were synthesized, and their electronic, electrochemical, and electrical properties investigated. Synthetic strategies to increase (i) the conjugation length of the base molecular structure – through introduction of thiophene units bearing electronically neutral substituents (hydrogen or alkyl groups) or strong electron-withdrawing pentafluorobenzoyl group(s) – and (ii) the electron affinity – by moving to a benzo[1,2-d:4,3-d′]bis(thiazole)-4,5-dione structure – were developed. Molecular packing in the single crystal was studied by single-crystal X-ray structural analysis, and this information was subsequently used in the determination of the electronic band structures, densities of states (DOS), effective transfer integrals, and effective charge-carrier masses via density functional theory (DFT) methods. The charge-carrier transport properties of the benzo[1,2-b:6,5-b′]dithiophene-4,5-dione and benzo[1,2-d:4,3-d′]bis(thiazole)-4,5-dione derivatives were investigated through the fabrication and characterization of organic field-effect transistors (OFETs) via both solution-processed and vacuum-deposited films. 2,7-Bis-pentafluorobenzoyl-benzo[1,2-b:6,5-b′]dithiophene-4,5-dione (10a) exhibited field-effect behavior with an average electron mobility μe = 4.4 (±1.7) × 10−4 cm2 V−1 s−1 when the active layer was vacuum-deposited, and a larger μe= 6.9 × 10−3 cm2 V−1 s−1 when the active layer was solution-processed. These results are in stark contrast with the DFT-determined electronic band structure and effective mass, which indicate that the material possesses good intrinsic charge-carrier transport characteristics. The combined results reveal the importance of thin-film processing and that further processing refinements could lead to improved device performance. Only one material with benzo[1,2-d:4,3-d′]bis(thiazole)-4,5-dione core, 2,7-bis-(4-n-hexyl-thiophene-2-yl)-benzo[1,2-d:4,3-d′]bis(thiazole)-4,5-dione (19d), showed average μe = 8.2 × 10−5 cm2 V−1 s−1 in OFET with solution-processed active layer. Unexpectedly, measurable hole transport was observed for 2,7-bis-(5-n-nonyl-thiophen-2-yl)-benzo[1,2-b:6,5-b′]dithiophene-4,5-dione (19b) (μh = 8.5 × 10−5 cm2 V−1 s−1) and 2,6-bis-(thiophen-2-yl)-3,5-di-n-hexyl-4H-cyclopenta[1,2-b:5,4-b′]dithiophen-4-one (30a) (μh = 3.7 × 10−4 cm2 V−1 s−1).

Perylene-3,4,9,10-tetracarboxylic diimides (PDI) have been linked to norbornene through either an imide or a “bay” position. These PDI-functionalised norbornenes are readily polymerised using a ruthenium ring-opening metathesis polymerisation (ROMP) initiator. In one case the polymerisation is shown to be well controlled with a linear dependence of molecular weight on the monomer/initiator ratio. For the polymers with PDI–norbornene linkages through the imide nitrogen, UV-vis spectroscopy suggests significant PDI aggregation both in dilute solution and the solid state, a powder X-ray diffraction peak attributable to π-stacking is seen, and weak n-channel field-effect transistor behavior (electron mobilities

Soluble alternating copolymers of N-(3,4,5-tri-n-dodecyloxyphenyl)dithieno[3,2-b:2′,3′-d]pyrrole donor groups and 2,3-di-n-decylquinoxaline, 2,3-di-n-decylpyrido[3,4-b]pyrazine, 2,3,6,7-tetrakis(n-decyloxy)benzo[a,c]phenazine, or 2,3,6,7-tetrakis(n-decyloxy)dibenzo[f,h]pyrido[4,3-b]quinoxaline acceptors were synthesised using Stille coupling reactions. Experimental absorption maxima in THF range from 645 to 770 nm. These optical data, along with the results of quantum-chemical calculations and electrochemical measurements, show that, as expected, the pyridopyrazine moiety acts as a stronger acceptor than quinoxaline and that the extended species benzophenazine and dibenzopyridoquinoxaline are stronger acceptors than quinoxaline and pyridopyrazine, respectively. Modest average hole mobilities of up to ca. 3.0 × 10−4 cm2 V−1s−1 were obtained in field-effect transistors. Bulk heterojunction photovoltaic devices made from blends of the benzo[a,c]phenazine-based polymer with 3′-phenyl-3′H-cyclopropa[1,9](C60-Ih)[5,6]fullerene-3′-butanoic acid methyl ester (1 : 3 weight ratio) exhibited average power conversion efficiencies up to 1.4%.

High performance solution-processed n-channel organic field-effect transistors based on [6,6]-phenyl C61 butyric acid methyl ester with low operating voltages (3 V) are demonstrated using a high-k hafnium dioxide gate dielectric grown by atomic layer deposition. Devices exhibit excellent n-channel performance with electron mobility values up to 0.14 cm2/V s, threshold voltages of ∼0.3 V, current on/off ratios >105, and very low values of subthreshold slope (∼140 mV/decade).

Low-voltage pentacene organic field-effect transistors are demonstrated (operating voltage of −3 V) with high-κ hafnium dioxide gate dielectrics grown by atomic layer deposition at 200 °C. A high hole mobility of 0.39 cm2/V s with low threshold voltage (