The carrier transport of carefully purified regioregular poly(3-hexylthiophene) films was studied using time-of-flight photocurrent measurements. The authors find balanced ambipolar transport with a room-temp. mobility for holes of 3 * 10-4 cm2 V-1 s-1 and for electrons of 1.5 * 10-4 cm2 V-1 s-1 at elec. fields >=105 V/cm. The transport is relatively field independent and weakly temp. dependent, pointing to a high degree of chem. regioregularity and purity. These factors make poly(3-hexylthiophene) attractive for use in a range of electronic applications.
This thesis presents the feasibility of liquid metal in reconfigurable antenna applications. Unlike conventional reconfigurable antennas, liquid metal possesses the ability to reconfigure an antenna aperture in new ways largely undiscovered. In this work, the design, analysis and measurement of two liquid metal reconfigurable antennas are carried out bringing together research on materials, microfluidics and electromagnetics. In addition, a like for like comparison of applying liquid metal and PIN diode in pattern reconfigurable antennas is presented. On the RF characterisation, test fixtures are designed, analysed and measured to evaluate liquid metal and copper up to 67 GHz, and PIN diode up to 6 GHz. While various liquid metal antennas exist, the current state of the art has mainly implemented liquid metal in point-like contacts or limited confined areas. On how liquid metal can be used to connect/disconnect large areas of metalisation and achieve radiation performance not possible by using conventional switches is demonstrated in a frequency bandwidth reconfigurable antenna. The antenna results in a 2 dB gain and 24% efficiency enhancement. The second antenna presents a frequency tunable patch antenna formed from liquid metal. The antenna reconfigures its resonance in a continuous manner. A measured total usable spectrum of 73% is achieved. In the like for like pattern reconfigurable antennas comparison, the liquid metal shows no effect on resonance whereas PIN diodes cause 27% resonance shift. The liquid metal antenna shows up to 1.4 dB gain and 13% efficiency higher than that of the PIN diode antenna. In the RF characterisation, the diode shows up to 6.5 dB higher insertion loss than liquid metal. Liquid metal measurements up to 67 GHz show identical behaviour as of copper. Results conclude that liquid metal has a high feasibility in reconfigurable antenna applications and its RF performance is as of a typical conductor.
Silver Nanowires (Ag NWs) are a heavily researched material, mostly as a replace-ment for the industry standard Indium Tin Oxide (ITO). Ag NWs are sought after be-cause they can be used on flexible substrates and be solution deposited. One main drawback of silver NWs is the film failure during either electrostatic discharge (ESD) tests or direct Joule heating, at currents significantly lower than ITO can handle. In this work the up scaling of the spray deposition of Ag NWs and the current induced failure that is observed during film use in electronic devices are investigated. Both of these areas are ones of the few stopping Ag NWs from being adopted by the industry. As part of this project, a holistic approach is taken, where a machine for the deposition was designed and tuned to improve the efficiency of the process, then using that tool process devel-opment was undertaken using various dispersions and finally the current induced failure of the deposited films was investigated. This way a complete understanding was achieved of the spray process and of silver nanowires as a material.
Using a spray deposition tool, built and commissioned during the project, the indus-try requirements of less than 10% uniformity variation, optical transmission over 90% and haze bellow 1% were achieved for 150 mm width at 100 Ω/sq. Furthermore, better understanding of the spray pattern and nozzle overlap was achieved using experimental and simulation data. Based on this tool and the knowledge acquired from its design, a series of customer machines were designed for various applications. The original tool was upgraded to an airbrush-based system, in order to facilitate for R&D work such as process development and sample preparation for small scale application tests. Various nanowire dispersions were tested from various suppliers using this upgraded tool with success. Using the fine-tuned process, films were deposited for solar cells, liquid crystal displays, light emitting diodes and heaters, with great preliminary results. Finally, the deposition of nanowires on 3D surfaces was achieved with great success.
To understand the current induced failures, the failure mechanism and the maximum current density that the nanowires can withstand needs to be identified. To do that, small scale tests were undertaken and then the films were imaged using SEM and AFM. From these measurements it was found that the maximum current density (18-260 MA/cm2), at which the NW films fail, is much lower than what has been observed in lit-erature (18-260 MA/cm2). As part of this work it was found that for a 100 Ω/sq film, the failure occurred at a current density of 7 MA/cm2. This is because of the Joule heating of the nanowires, which in the case of dense networks initiates in the junctions. This is be-cause the junction resistance dominates over the resistance of the individual wires, since it is an order of magnitude higher. It was found that denser nanowire networks (lower sheet resistance), fail at lower current densities. This is because the number and the prox-imity of the junctions increase, resulting in quicker heating and failure. Furthermore, when network is consisted of multiple layers of nanowires, the heat dissipates at a slow-er rate, since the top nanowires have just air for the heat to be transferred to, in contrast with the bottom layer wires, which dissipate heat through direct contact with the glass substrates (better heat conductor than air). The failure is always in a line, perpendicular to the current flow. This is happening because the film failure originates from a weak spot of the sample. This weak spot is on an axis perpendicular to the current flow where there is a less dense patch of the film. When one junction fails, more current has to pass through a smaller number of nanowires, resulting in a chain reaction. Various treatments and NW hybrid materials are proposed in order to decrease the succeptability to the cur-rent induced failure resistance of the films. Two of them are the incorporation of gra-phene on the junctions, which acts as a heat sink and the introduction of a PEDOT:PSS film, which acts as a larger area for the heat to dissipate through and also, creates more channels for the current to flow through.
Recently, the RF/microwave electronic technology evolved with the consideration of plastic and organic substrates. Such a technology offers two-folded benefits: in one side for lowering the fabrication cost and in another side for the possibility to bend electronic devices. Such a technology is particularly interesting for the implementation of antenna system. This paper is dealing with the design of flexible microstrip antenna 1:2 array. Theoretical approach on the typically symmetrical antenna 1:2 array is proposed. The design methodology of microstrip antenna combined with 1:2 T-power divider (T-PWD) is described. Based on the transmission line theory, the S-parameter model of the antenna system with non-standard reference load is established. Then, the microstrip antenna passive system is theoretical analysed in function of the physical dimensions of the designed structure. The feasibility of the flexible antenna passive system is investigated with the proof-of-concept (POC) designed on Kapton substrate. The POC prototype consisted of microstrip antenna 1:2 array is designed to operate at about 5.8 GHz. Comparisons between the full wave simulated and measured return losses were performed. Then, simulated radiation pattern highlights the efficiency of the fabricated prototype of passive antenna array.
Hamilton R, Bailey C, Duffy W, Heeney M, Shkunov M, Sparrowe D, Tierney S, McCulloch I, Kline RJ, DeLongchamp DM, Chabinyc M (2006)The influence of molecular weight on the microstructure and thin film transistor characteristics of pBTTT polymers., In: Bao Z, Gundlach DJ, (eds.), ORGANIC FIELD-EFFECT TRANSISTORS V6336 McGlashon A, Whitehead KS, Bradley DDC, Campbell AJ, Zhang W, Heeney M, Smilgies DM, Shkunov M, Genevicius K, Amassian A, Malliaras GG, Amassian A Spectroscopic and morphological investigation of conjugated photopolymerisable quinquethiophene liquid crystals, In: Current Applied Physics
3′-methyl-(5,5′′-bis[3-ethyl-3-(6-phenyl-hexyloxymethyl)-oxetane])-2,2′:5′,2′′-terthiophene (5T(Me)Ox) is a solution processable small molecule semiconductor displaying smectic-C and nematic liquid crystal phases. The pendant oxetane group can be polymerized in situ in the presence of a suitable photoacid at concentrations ≥1% by weight. Spin-coated films of pure 5T(Me)Ox and 5T(Me)Ox doped with the soluble photoacid were characterized by absorption and photoluminescent spectroscopy. Thick pristine films showed absorption and emission from a crystalline phase. Thin monolayer (<5 nm) films, as well as thicker photoacid doped films, instead showed absorption from an H-aggregate phase and emission from an excimer. Optical microscopy showed a significant change in film structure upon addition of the photoacid; large and well-orientated crystals being replaced by much smaller domains which appear to vary in thickness. Grazing Incidence Wide Angle X-Ray Scattering (GIWAXS) was used to characterize the packing and orientation of molecules in the crystalline and doped samples. The results are consistent with the photoacid doped samples forming layers of H-aggregate phase monolayer sheets parallel to the substrate where the photoacid inhibits the transition into the three-dimensionally ordered crystalline phase. Field-effect transistors and light emitting diodes were constructed incorporating 5T(Me)Ox as the active layer. Pure 5T(Me)Ox field-effect transistors showed good, p-type device characteristics, but the morphological changes upon doping result in a loss of transistor action. In the diodes, curing through melting and exposure to UV light followed by photoacid removal resulted in an increase in current density but a decrease in light emission. These results indicate that the presence of the photoacid (≥1% by weight) can have a dramatic effect on the structure, morphology and device performance of ordered, photopatternable materials for organic electronics. © 2011 Elsevier B.V. All rights reserved.
Shkunov M, McCulloch I, Zhang W, Heeney M, Bailey C, Giles M, Graham D, Shkunov M, Sparrowe D, Tierney S (2003)Polymerisable liquid crystalline organic semiconductors and their fabrication in organic field effect transistors, In: Journal of Materials Chemistry
Semiconducting nanowires (NWs) are becoming essential nano-building blocks for advanced devices from sensors to energy harvesters, however their full technology penetration requires large scale materials synthesis together with efficient NW assembly methods. We demonstrate a scalable one-step solution process for the direct selection, collection and ordered assembly of silicon NWs with desired electrical properties from a poly-disperse collection of NWs obtained from a Supercritical Fluid-Liquid-Solid growth process. Dielectrophoresis (DEP) combined with impedance spectroscopy provides a selection mechanism at high signal frequencies (>500 kHz) to isolate NWs with the highest conductivity and lowest defect density. The technique allows simultaneous control of five key parameters in NW assembly: selection of electrical properties, control of NW length, placement in pre-defined electrode areas, highly preferential orientation along the device channel and control of NWs deposition density from few to hundreds per device. Direct correlation between DEP signal frequency and deposited NWs conductivity is directly confirmed by field-effect transistor and conducting-AFM data. Fabricated NW transistor devices demonstrate excellent performance with up to 1.6 mA current, 106-107 on/off ratio and hole-mobility of 50 cm2 V-1 s-1.
In order to achieve high performance, the design of devices for large-area electronics needs to be optimized despite material or fabrication shortcomings. In numerous emerging technologies thin-film transistor (TFT) performance is hindered by contact effects. Here, we show that contact effects can be used constructively to create devices with performance characteristics unachievable by conventional transistor designs. Source-gated transistors (SGTs) are not designed with increasing transistor speed, mobility or sub-threshold slope in mind, but rather with improving certain aspects critical for real-world large area electronics such as stability, uniformity, power efficiency and gain. SGTs can achieve considerably lower saturation voltage and power dissipation compared to conventional devices driven at the same current; higher output impedance for over two orders of magnitude higher intrinsic gain; improved bias stress stability in amorphous materials; higher resilience to processing variations; current virtually independent of source-drain gap, source-gate overlap and semiconductor thickness variations. Applications such as amplifiers and drivers for sensors and actuators, low cost large area analog or digital circuits could greatly benefit from incorporating the SGT architecture.
Fabrication of display products by low cost printing technologies such as ink jet, gravure offset lithography and flexography requires solution processable semiconductors for the backplane electronics. The products will typically be of lower performance than polysilicon transistors, but comparable to amorphous silicon. A range of prototypes are under development, including rollable electrophoretic displays, AMLCD's, (active matrix liquid crystal displays), and flexible OLED (organic light emitting diode) displays. Organic semiconductors that offer both electrical performance and stability with respect to storage and operation under ambient conditions are required. This work describes the development of reactive mesogen semiconductors, which can polymerise within mesophase temperatures, "freezing in" the order in crosslinked domains. These crosslinked domains offer mechanical stability and are inert to solvent exposure in further processing steps. Reactive mesogens containing conjugated aromatic cores, designed to facilitate charge transport and provide good oxidative stability, were prepared and their liquid crystalline properties evaluated. Both time-of-flight and field effect transistor devices were prepared and their electrical characterisation reported. © 2006 SID.
© 2014 IEEE.This paper characterizes coplanar waveguide (CPW) lines formed by ink-jet printed conductors on flexible Kapton substrates at frequencies up to 60 GHz. It is shown that the lines have losses of approximately 1.5 dB/mm but that this relatively high loss is predominantly due to the loss tangent of the substrates and not the lower conductivity of the silver inks used in this implementation. On an Alumina sample the loss is shown to be 0.8 dB/mm for the same ink-jet printed lines. CST simulation has been used to verify the calculated results.
Conductive patterns of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)/multi-walled carbon nanotube (MWCNT) composites were deposited on glass substrates using a drop on demand (DOD) inkjet printer, with the concentration of CNT varied from 0.01 wt% to 0.05 wt%. We show that by increasing the concentration of the nanotubes in the ink, percolated networks of well distributed carbon nanotubes in the printed samples can be achieved. Moreover, the orientation of the nanotubes in the printed sample can be controlled using a novel simple approach. The impact of the nanotube alignment on the conduction properties of inkjet printed nano-hybrid materials is studied and shown in this Letter. Samples with aligned nanotubes show a 53% enhanced conductivity in comparison with the randomly oriented nanotubes. The results show that the electrical performance of the nano-composite can be improved further by controlling the dispersion and orientation of the nano-filler in the printed samples. Carbon nanotubes orientation control in the printed PEDOT:PSS/MWCNT nano-composite samples. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Single-crystal semiconductors have been at the forefront of scientific interest for more than 70 years, serving as the backbone of electronic devices. Inorganic single crystals are typically grown from a melt using time-consuming and energy-intensive processes. Organic semiconductor single crystals, however, can be grown using solution-based methods at room temperature in air, opening up the possibility of large-scale production of inexpensive electronics targeting applications ranging from field-effect transistors and light-emitting diodes to medical X-ray detectors. Here we demonstrate a low-cost, scalable spray-printing process to fabricate high-quality organic single crystals, based on various semiconducting small molecules on virtually any substrate by combining the advantages of antisolvent crystallization and solution shearing. The crystals’ size, shape and orientation are controlled by the sheer force generated by the spray droplets’ impact onto the antisolvent’s surface. This method demonstrates the feasibility of a spray-on single-crystal organic electronics.
Existing inorganic materials for radiation sensors suffer from several drawbacks, including their inability to cover large curved areas, lack of tissue equivalence toxicity, and mechanical inflexibility. As an alternative to inorganics, poly(triarylamine) (PTAA) diodes have been evaluated for their suitability for detecting radiation via the direct creation of X-ray induced photocurrents. A single layer of PTAA is deposited on indium tin oxide (ITO) substrates, with top electrodes selected from Al, Au, Ni, and Pd. The choice of metal electrode has a pronounced effect on the performance of the device; there is a direct correlation between the diode rectification factor and the metal-PTAA barrier height. A diode with an Al contact shows the highest quality of rectifying junction, and it produces a high X-ray photocurrent (several nA) that is stable during continuous exposure to 50 kV Mo K alpha X-radiation over long time scales, combined with a high signal-to-noise ratio with fast response times of less than 0.25 s. Diodes with a low band gap, 'Ohmic' contact, such as ITO/PTAA/Au, show a slow transient response. This result can be explained by the build-up of space charge at the metal-PTAA interface, caused by a high level of charge injection due to X-ray-induced carriers. These data provide new insights into the optimum selection of metals for Schottky contacts on organic materials, with wider applications in light sensors and photovoltaic devices.
Masuda T, Frolov SV, Shkunov M, Vardeny ZV, Hamaguchi M, Yoshino K, Hirohata M, Hidayat R, Tada K, Sada T, Teraguchi M (1997)Optical properties and electroluminescence characteristics of polyacetylene derivatives dependent on substituent and layer structure, In: Synthetic Metals911-3pp. 283-287
The solution-based assembly of field-effect transistors using nanowire inks, processed at low temperatures, offers an enormous potential for low power applications envisioned for the “Internet of Things,” including power management sensor circuits and electronics for in vivo bioimplants. Such low-temperature assembly, however, yields substantial contact potential barriers, with limited capacity for high current applications. In this study, the Schottky effect in a specific transistor configuration is utilized to achieve much reduced power consumption, with low saturation voltages (≈1 V), with relatively thick 230-nm SiO2 dielectrics. These source-gated transistors (SGTs) employ solution-deposited silicon nanowire arrays. A range of metal electrode work functions are investigated as device contacts and SGT operation is realized only in the structures with high source contact barriers. Such devices show very early drain current pinch-off, abruptly saturating at low drain voltages. The authors show that drain-current modulation is achieved via the gate field acting on the source barrier and lowering it through image force effects. Activation energy measurements reveal gate-induced source barrier lowering of ≈3 meV V−1. Numerical simulations show excellent correlation with the experimental data. These features, coupled with flat current saturation characteristics, are ideal for a range of low power applications, including wearable electronics and autonomous systems.
Heeney M, Bailey C, Genevicius K, Macdonald I, Shkunov M, Sparrowe D, Tierney S, Wagner R, Zhang WM, Chabinyc ML, Kline RJ, Mcgehee MD, Toney MF, Mcculloch I (2006)Liquid-crystalline semiconducting polymers with high charge-carrier mobility, In: NATURE MATERIALS5(4)pp. 328-333
NATURE PUBLISHING GROUP
Printing of highly conductive tracks at low cost is of primary importance for the emerging field of flexible, plastic, and large-area electronics. Commonly, this is achieved by printing of metallic conductive inks, often based on Ag or Cu nanoparticles dispersed in organic solvents. The solvents, which must be safely removed, have particular storage and handling requirements, thus increasing the process costs. By using water-based inks containing micron-sized silver flakes, both material and process costs can be reduced, making these inks attractive for industrial applications. However, the sintering of flake inks requires higher temperatures than nano-sized inks owing to the particles’ smaller surface area-to-volume ratio, meaning that when cured thermally the conductivity of many flake inks is lower than nanoparticle alternatives. This problem can be addressed by the application of visible light photonic curing; however, the substrate must be protected and so process parameters must be defined for each material/substrate combination. Here, we report results of a large-scale trial of photonic curing of aqueous flake silver inks on poly(ethylene terephthalate) substrates in an industrial setting. The resistivity of printed patterns after an optimized photocuring regime matched those reported for typical nanoparticle inks; on the order of 100 μΩ cm depending on substrate and geometry. Scanning electron microscopy revealed evidence for structural changes within the printed films consistent with localized melting and necking between adjacent particles, leading to an improved percolation network. Furthermore, in the large-scale industrial trial employing screen-printed silver lines, the manufacturing yield of conductive lines was increased from 44% untreated to 80% after photocuring and reached 100% when photocuring was combined with thermal curing. We believe this to be the first reported observation of an increase in the yield of printed electronic structures following photocuring. We propose a crack-healing mechanism to explain these increases in yield and conductivity. We further report on the effects of the photonic curing on the mechanical bending stability of the printed conductors and discuss their suitability for wearable applications.
Silicon nanowires (Si NW) are ideal candidates for low-cost solution processed field effect transistors (FETs) due to the ability of nanowires to be dispersed in solvents, and demonstrated high charge carrier mobility. The interface between the nanowire and the dielectric plays a crucial role in the FET characteristics, and can be responsible for unwanted effects such as current hysteresis during device operation. Thus, optimal nanowire- dielectric interface is required for low-hysteresis FET performance. Here we show that NW FET hysteresis mostly depends on the nature of the dielectric material by directly comparing device characteristics of dual gate Si NW FETs with bottom SiO2 gate dielectric and top hydrophobic fluoropolymer gate dielectric. As the transistor semiconducting nanowire channel is identical in both tops and bottom operational regimes, the performance differences originate from the nature of the nanowire-dielectric interface. Thus, very high 30 volt hysteresis is observed for forward and reverse gate bias scans with SiO2 interface; however, hysteresis is significantly reduced to 6 volt for the fluoropolymer dielectric interface. The differences in hysteresis are ascribed to the polar OH- groups present at SiO2/Si nanowire interface, and mostly absent at fluoropolymer/Si nanowire interface. We further demonstrate that high density of charge traps for bottom gate SiO2 interface (1× 1013cm-2) is reduced by over an order of magnitude for top-fluoropolymer gate interface (7.5 × 1011 cm-2), therefore highlighting the advantage of hydrophobic polymer gate dielectrics for nanowire field-effect transistor applications.
A physical description of low-field behavior of a Schottky source-gated transistor (SGT) is outlined where carriers crossing the source barrier by thermionic emission are restricted by JFET action in the pinch-off region at the drain end of the source. This mode of operation leads to transistor characteristics with low saturation voltage and high output impedance without the need for field relief at the edge of the Schottky source barrier and explains many characteristics of SGT observed experimentally. 2-D device simulations with and without barrier lowering due to the Schottky effect show that the transistors can be designed so that the current is independent of source length and thickness variations in the semiconductor. This feature together with the fact that the current in an SGT is independent of source-drain separation hypothesizes the fabrication of uniform current sources and other large-area analog circuit blocks with repeatable performance even in imprecise technologies such as high-speed printing.
A new class of X-ray sensor – in which there is a blend of poly(triarylamine) (PTAA) and 6,13-bis(triisopropylsilylethynyl) (TIPS)-pentacene in the active layer of a diode structure – has been developed. The crystalline pentacene provides a fast route for charge carriers and leads to enhanced performance of the sensor. The first time-of-flight charge-carrier mobility measurement of this blend is reported. The mobility of PTAA and TIPS-pentacene in a 1:25 molar ratio was found to be 2.2 × 10−5 cm2 V−1 s−1 (averaged for field strengths between 3 × 104 and 4 × 105 V cm−1), which is about 17 times higher than that obtained in PTAA over the same range of field strengths. This higher mobility is correlated with a fourfold increase in the X-ray detection sensitivity in the PTAA:TIPS-pentacene devices.
Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of nanowires. DEP is used for nanowire analysis, characterization, and for solution-based fabrication of semiconducting devices. The method works by applying an alternating electric field between metallic electrodes. The nanowire formulation is then dropped onto the electrodes which are on an inclined surface to create a flow of the formulation using gravity. The nanowires then align along the gradient of the electric field and in the direction of the liquid flow. The frequency of the field can be adjusted to select nanowires with superior conductivity and lower trap density. In this work, flow-assisted DEP is used to create nanowire field effect transistors. Flow-assisted DEP has several advantages: it allows selection of nanowire electrical properties; control of nanowire length; placement of nanowires in specific areas; control of orientation of nanowires; and control of nanowire density in the device. The technique can be expanded to many other applications such as gas sensors and microwave switches. The technique is efficient, quick, reproducible, and it uses a minimal amount of dilute solution making it ideal for the testing of novel nanomaterials. Wafer scale assembly of nanowire devices can also be achieved using this technique, allowing large numbers of samples for testing and large-area electronic applications.
In this paper, we propose a robust microwave characterization of inkjet printed components on flexible substrates, which aim at measuring the material properties of silver nanoparticle inks and the supporting dielectric spacer employed during measurements. Starting with propagation constant extracted from multiline thru-reflect-line calibration with coplanar waveguide (CPW) standards and then proceeding with finite element modeling of CPWs, the proposed technique can dynamically produce an interpolated search space by automatic driving of simulation tools. In the final stage, the algorithm utilizes a least-square optimization routine to minimize the deviation between model and measurements. Our technique significantly reduces the computing resources and is able to extract the material parameters using even a nominal ink profile. Characteristic impedances for CPWs are extracted using series resistor measurements from 10 MHz to 20 GHz. It is also shown that the proposed characterization methodology is able to detect any changes in material properties induced by changes in fabrication parameters such as sintering temperature. Ink conductivities of approximately 2.973×10^7 S/m and spacer dielectric constant of 1.78 were obtained for the inkjet printed CPWs on PET. In addition, the inkjet printed CPWs sintered at 170°C and 220°C on Kapton had conductivities of 0.187×10^7 and 0.201×10^7 S/m respectively. We verified our technique by measuring the material parameters with conventional approach.
McCulloch I, Bailey C, Genevicius K, Heeney M, Shkunov M, Sparrowe D, Tierney S, Zhang W, Baldwin R, Kreouzis T, Andreasen JW, Breiby DW, Nielsen MM (2006)Designing solution-processable air-stable liquid crystalline crosslinkable semiconductors, In: PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES364(1847)pp. 2779-2787
Solution-processable single-crystalline inorganic semiconducting nanowires are excellent building blocks for printable electronics requiring high performance of semiconducting components. Excellent charge carrier mobilities of crystalline nanowires combined with solvent-based nanowire processing open up possibilities for low-cost nanowire electronics targeting a variety of applications ranging from flexible circuits to chemical and biological sensors. Nanowire field-effect transistors are key devices for most of such applications. Recent developments in controllable nanowire positioning and orientation on the substrates and electrical property selection provide the necessary technological breakthroughs enabling the fabrications of reproducible nanowire transistors. In this chapter, we discuss the nanowire assembly methods and high-spatial-resolution scanning probe microscopy techniques towards scalable fabrication of high-performance printable nanowire field-effect transistors.
Frolov SV, Shkunov M, Valy Vardeny Z, Tada K, Hidayat R, Hirohata M, Teraguchi M, Masuda T, Yoshino K (1997)Spectral narrowing of emission in di-substituted polyacetylene, In: Japanese Journal of Applied Physics, Part 2: Letters369 A/Bpp. L1268-L1271
Solution processed field-effect transistors based on single crystalline silicon nanowires (Si NWs) with metal Schottky contacts are demonstrated. The semiconducting layer was deposited from a nanowire ink formulation at room temperature. The devices with 230nm thick SiO2 gate insulating layers show excellent output current-voltage characteristics with early saturation voltages under 2 volts, constant saturation current and exceptionally low dependence of saturation voltage with the gate field. Operational principles of these devices are markedly different from traditional ohmic-contact field-effect transistors (FETs), and are explained using the source-gated transistor (SGT) concept in which the semiconductor under the reverse biased Schottky source barrier is depleted leading to low voltage pinch-off and saturation of drain current. Device parameters including activation energy are extracted at different temperatures and gate voltages to estimate the Schottky barrier height for different electrode materials to establish transistor performance - barrier height relationships. Numerical simulations are performed using 2D thin-film approximation of the device structures at various Schottky barrier heights. Without any adjustable parameters and only assuming low p-doping of the transistor channel, the modelled data show exceptionally good correlation with the measured data. From both experimental and simulation results, it is concluded that source-barrier controlled nanowire transistors have excellent potential advantages compared with a standard FET including mitigation of short-channel effects, insensitivity in device operating currents to device channel length variation, higher on/off ratios, higher gain, lower power consumption and higher operational speed for solution processable and printable nanowire electronics.
The present work focuses on nanowire (NW) applications as semiconducting elements in solution processable field-effect transistors (FETs) targeting large-area low-cost electronics. We address one of the main challenges related to NW deposition and alignment by using dielectrophoresis (DEP) to select multiple ZnO nanowires with the correct length, and to attract, orientate and position them in predefined substrate locations. High-performance top-gate ZnO NW FETs are demonstrated on glass substrates with organic gate dielectric layers and surround source-drain contacts. Such devices are hybrids, in which inorganic multiple single-crystal ZnO NWs and organic gate dielectric are synergic in a single system. Current-voltage (I-V) measurements of a representative hybrid device demonstrate excellent device performance with high on/off ratio of 10^7, steep subthreshold swing (s-s) of 400 mV/dec and high electron mobility of 35 cm2 V-1 s-1 in N2 ambient. Stable device operation is demonstrated after 3 months of air exposure, where similar device parameters are extracted including on/off ratio of 4x10^6, s-s 500 mV/dec and field-effect mobility of 28 cm2 V-1 s-1. These results demonstrate that DEP can be used to assemble multiples of NWs from solvent formulations to enable low-temperature hybrid transistor fabrication for large-area inexpensive electronics.
We have successfully prepared mono- and bi-functionalized multiwall carbon nanotubes (MWCNT) with thiophene, amine and thiophene-amine groups. The dispersion of nanotubes has been enhanced and stable optimized dispersions in organic solvents were obtained. These functionalized nanotubes have been successfully incorporated into bulk heterojunction (BHJ) organic photovoltaic (OPV) cells with a poly (3-hexyl thiophene) (P3HT) and [6, 6]-phenyl-C(61)-butyric acid methyl ester (PCBM) photoactive blended layer. The incorporation of MWCNT with different functional groups, in the active layer, results in different cell performance with respect to a reference cell. A maximum power conversion efficiency of 2.5% is achieved with the inclusion of thiophene functionalized nanotubes. This improvement in the device performance is attributed to an extension of the exciton dissociation volume and charge transport properties through the nanotube percolation network in P3HT/CNT, PCBM/CNT or both phases. This is believed to be due to more efficient dispersion of the functionalized nanotubes within the photoactive composite layer.
Semiconducting polymer X-radiation detectors are a completely new family of low-cost radiation detectors with potential application as beam monitors or dosimeters. These detectors are easy to process, mechanically flexible, relatively inexpensive, and able to cover large areas. However, their x-ray photocurrents are typically low as, being composed of elements of low atomic number (Z), they attenuate x-rays weakly. Here, the addition of high-Z nanoparticles is used to increase the x-ray attenuation without sacrificing the attractive properties of the host polymer. Two types of nanoparticles (NPs) are compared: metallic tantalum and electrically insulating bismuth oxide. The detection sensitivity of 5 µm thick semiconducting poly([9,9-dioctylfluorenyl-2,7-diyl]-co-bithiophene) diodes containing tantalum NPs is four times greater than that for the analogous NP-free devices; it is approximately double that of diodes containing an equal volume of bismuth oxide NPs. The x-ray induced photocurrent output of the diodes increases with an increased concentration of NPs. However, contrary to the results of theoretical x-ray attenuation calculations, the experimental current output is higher for the lower-Z tantalum diodes than the bismuth oxide diodes, at the same concentration of NP loading. This result is likely due to the higher tantalum NP electrical conductivity, which increases charge transport through the semiconducting polymer, leading to increased diode conductivity.
The use of high quality semiconducting nanomaterials for advanced device applications has been hampered by the unavoidable growth variability of electrical properties of one-dimensional nanomaterials, such as nanowires and nanotubes, thus highlighting the need for the characterization of efficient semiconducting nanomaterials. In this study, we demonstrate a low-cost, industrially scalable dielectrophoretic (DEP) nanowire assembly method for the rapid analysis of the electrical properties of inorganic single crystalline nanowires, by identifying key features in the DEP frequency response spectrum from 1 kHz to 20 MHz in just 60 s. Nanowires dispersed in anisole were characterized using a three-dimensional DEP chip (3DEP), and the resultant spectrum demonstrated a sharp change in nanowire response to DEP signal in 1–20 MHz frequency range. The 3DEP analysis, directly confirmed by field-effect transistor data, indicates that nanowires of higher quality are collected at high DEP signal frequency range above 10 MHz, whereas lower quality nanowires, with two orders of magnitude lower current per nanowire, are collected at lower DEP signal frequencies. These results show that the 3DEP platform can be used as a very efficient characterization tool of the electrical properties of rod-shaped nanoparticles to enable dielectrophoretic selective deposition of nanomaterials with superior conductivity properties.
This paper presents the modelling of a coplanarwaveguide bottom-gated FET switch using indium-arsenide nanowires. The nanowires have been included on the switch using dielectrophoresis, which is a solution processable technique. This is a necessary first step towards developing a fully printable switch on a flexible substrate, for low cost microwave devices, built using additive manufacturing methods. The measured S-parameters show the switching capabilities of the device with an insertion loss of 9 dB, when the switch is open (gate voltage ≥ 60 V). The development of a distributed circuit model that matches the measured data is described, alongside the calculated network parameters used to represent the coplanar-waveguide and the nanowires. The model fits the measured results within 8%, making it suitable for inclusion in a CAD based circuit simulator.
This paper describes microwave characterization of coplanar waveguide (CPW) lines formed by ink-jet printed technology on flexible polyethylene terephthalate (PET) substrates. The reel-to-reel printing process uses inkjet printing as a precursor for 2μm copper plating, which allows significantly lowered resistances as compared to traditional inks. A multiline TRL calibration technique has been used to characterize the propagation constant and reflection coefficient of the CPW lines. With the aid of four sets of measurements at two identical labs, it is shown that the fabricated samples have contact repeatability, permitting redundant multiline calibrations.
Fabrication techniques such as laser patterning offer excellent potential for low cost and large area device fabrication. Conductive polymers can be used to replace expensive metallic inks such as silver and gold nanoparticles for printing technology. Electrical conductivity of the polymers can be improved by blending with carbon nanotubes. In this work, formulations of acid functionalized multiwalled carbon nanotubes (f-MWCNTs) and poly(ethylenedioxythiophene) [PEDOT]:polystyrene sulphonate [PSS] were processed, and thin films were prepared on plastic substrates. Conductivity of PEDOT:PSS increased almost four orders of magnitude after adding f-MWCNTs. Work function of PEDOT:PSS/f-MWCNTs films was ∼0.5 eV higher as compared to the work function of pure PEDOT:PSS films, determined by Kelvin probe method. Field-effect transistors source–drain electrodes were prepared on PET plastic substrates where PEDOT:PSS/f-MWCNTs were patterned using laser ablation at 44 mJ/pulse energy to define 36 μm electrode separation. Silicon nanowires were deposited using dielectrophoresis alignment technique to bridge laser patterned electrodes. Top-gated nanowire field effect transistors were completed by depositing parylene C as polymer gate dielectric and gold as the top-gate electrode. Transistor characteristics showed p-type conduction with excellent gate electrode coupling, with an ON/OFF ratio of ∼200. Thereby, we demonstrate the feasibility of using high workfunction, printable PEDOT:PSS/f-MWCNTs composite inks for laser patterned source/drain electrodes for nanowire transistors on flexible substrates.
Adamopoulos G, Bashir A, Thomas S, Gillin WP, Georgakopoulos S, Shkunov M, Baklar MA, Stingelin N, Maher RC, Cohen LF, Bradley DDC, Anthopoulos TD (2010)Spray-Deposited Li-Doped ZnO Transistors with Electron Mobility Exceeding 50 cm(2)/Vs, In: ADVANCED MATERIALS22(42)pp. 4764-+
WILEY-V C H VERLAG GMBH
Crouch DJ, Skabara PJ, Lohr JE, McDouall JJW, Heeney M, McCulloch I, Sparrowe D, Shkunov M, Coles SJ, Horton PN, Hursthouse MB (2005)Thiophene and selenophene copolymers incorporating fluorinated phenylene units in the main chain: Synthesis, characterization, and application in organic field-effect transistors, In: CHEMISTRY OF MATERIALS17(26)pp. 6567-6578
AMER CHEMICAL SOC
This paper presents a low temperature, solution-based processing method of highly transparent, sparse networks of carbon nanotubes via annealing process that dramatically improves the conductivity of thin films of octadecylamine functionalized highly soluble single-wall carbon nanotubes by up to five orders of magnitude. This increase in conductivity obtained at low temperatures allows for the creation of transparent conducting carbon nanotube (CNT) films via printed deposition of contacts for photovoltaic, light emitting, and display devices. An increase in films conductivity has been shown with process temperatures of 200°C at normal atmospheric pressure. The dependence between the sheet resistance of CNT layers and the annealing parameters is analyzed together with Raman and FTIR data, suggesting a relationship between the loss of octadecylamine functional groups along with the healing of CNT defects during the annealing process and the dramatic conductivity improvement of CNT layers.
Young Paul R., Stolojan Vlad, King Simon, Ng Calvin C H, Devabhaktuni Vijaya, Shkunov Maxim, Tan Hoe H, Jagadish Chennupati, Aaen Peter, Caroff Philippe, Mirkhaydarov Bobur, Votsi Haris, Sahu Abhishek (2019)Solution‐Processed InAs Nanowire Transistors as Microwave Switches, In: Advanced Electronic Materials5(1)1800323
The feasibility of using self‐assembled InAs nanowire bottom‐gated field‐effect transistors as radio‐frequency and microwave switches by direct integration into a transmission line is demonstrated. This proof of concept is demonstrated as a coplanar waveguide (CPW) microwave transmission line, where the nanowires function as a tunable impedance in the CPW through gate biasing. The key to this switching capability is the high‐performance, low impedance InAs nanowire transistor behavior with field‐effect mobility of ≈300 cm2 V−1 s−1, on/off ratio of 103, and resistance modulation from only 50 Ω in the full accumulation mode, to ≈50 kΩ when the nanowires are depleted of charge carriers. The gate biasing of the nanowires within the CPW results in a switching behavior, exhibited by a ≈10 dB change in the transmission coefficient, S21, between the on/off switching states, over 5–33 GHz. This frequency range covers both the microwave and millimeter‐wave bands dedicated to Internet of things and 5G applications. Demonstration of these switches creates opportunities for a new class of devices for microwave applications based on solution‐processed semiconducting nanowires.
Hybrid field-effect-transistors (FETs) with germanium nanowire (NW) arrays and organic gate dielectric are presented. The nanowire deposition steps are fully compatible with printed electronics route. NW FETs demonstrate good performance with On/Off ratios of ~10 and hole mobilities of ∼13 cm /Vs in both nitrogen and air atmosphere. These results suggest that the hybrid nanowire FETs could be used in large area inexpensive electronics. © 2011 Materials Research Society.
Mixed halide Perovskite solar cells are commonly produced by depositing PbCl2 and CH3NH3I from a common solvent followed by thermal annealing, which in an up-scaled manufacturing process is likely to take place under ambient conditions. However, it has been reported that, similar to the effects of thermal annealing, ambient humidity also affects the crystallisation behaviour and subsequent growth of the Perovskite films. This implies that both of these factors must be accounted for in solar cell production. In this work, we report for the first time the correlation between the annealing time, relative humidity and device performance for inverted, mixed halide CH3NH3PbI(3-x)Clx Perovskite solar cells with active area ≈1 cm2. We find a trade-off between ambient humidity and the required annealing time to produce efficient solar cells, with low humidities needing longer annealing times and vice-versa. At around 20% RH, device performance depends relatively weakly on annealing time, but at higher (30 - 40% RH) or lower (0 - 15 % RH) humidities it is very sensitive. Processing in humid environments is shown to lead to the growth of both larger Perovskite grains and larger voids; similar to the effect of thermal annealing, which also leads to grain growth. Therefore, samples which are annealed for too long under high humidity show loss of performance due to low open circuit voltage caused by an increased number of shunt paths. Based on these results it is clear that humidity and annealing time are closely interrelated and both are important factors affecting the performance of Perovskite solar cells. The findings of this work opens a route for reduced annealing times to be employed by control of humidity; critical in roll-to-roll manufacture where low manufacturing time is preferred for cost reductions.
The present invention provides a gas detector (100) for detecting a volatile organic compound (VOC) gas. The gas detector comprises at least one transducer (10) comprising at least one nanowire (20) comprising an arene compound (22) to capture a VOC gas. An electronic characteristic (e.g. threshold voltage of a FET transudcer) of the transducer changes when a VOC gas is captured by the arene compound. The present invention also provides a mobile device; a nanowire; a nanowire matrix; a transducer; a use of a gas detector; a method of detecting a VOC gas; and a method of manufacturing a gas detector.
Heeney M, Bailey C, Duffy W, McCulloch I, Tierney S, Shkunov M, Sparrowe D, Zhang W (2006)Light absorbing polymers incorporating 3,6-dialkylthieno[3,2-b]thiophene for photovoltaic applications, In: ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY232pp. 177-177
AMER CHEMICAL SOC
Mixed halide Perovskite solar cells (PSCs) are commonly produced by depositing PbCl2 and CH3NH3I from a common solvent followed by thermal annealing, which in an up-scaled manufacturing process is likely to take place under ambient conditions. However, it has been reported that, similar to the effects of thermal annealing, ambient humidity also affects the crystallisation behaviour and subsequent growth of the Perovskite films. This implies that both of these factors must be accounted for in solar cell production. In this work, we report for the first time the correlation between the annealing time, relative humidity (RH) and device performance for inverted, mixed halide CH3NH3PbI(3−x)Cl x PSCs with active area ≈1 cm2. We find a trade-off between ambient humidity and the required annealing time to produce efficient solar cells, with low humidities needing longer annealing times and vice-versa. At around 20% RH, device performance weakly depends on annealing time, but at higher (30%–40% RH) or lower (0%–15% RH) humidities it is very sensitive. Processing in humid environments is shown to lead to the growth of both larger Perovskite grains and larger voids; similar to the effect of thermal annealing, which also leads to grain growth. Therefore, samples which are annealed for too long under high humidity show loss of performance due to low open circuit voltage caused by an increased number of shunt paths. Based on these results it is clear that humidity and annealing time are closely interrelated and both are important factors affecting the performance of PSCs. The findings of this work open a route for reduced annealing times to be employed by control of humidity; critical in roll-to-roll manufacture where low manufacturing time is preferred for cost reductions.
In this letter, we demonstrate a solution-based method for a one-step deposition and surface passivation of the as-grown silicon nanowires (Si NWs). Using N,N-dimethylformamide (DMF) as a mild oxidizing agent, the NWs' surface traps density was reduced by over 2 orders of magnitude from 1×10(13) cm(-2) in pristine NWs to 3.7×10(10) cm(-2) in DMF-treated NWs, leading to a dramatic hysteresis reduction in NW field-effect transistors (FETs) from up to 32 V to a near-zero hysteresis. The change of the polyphenylsilane NW shell stoichiometric composition was confirmed by X-ray photoelectron spectroscopy analysis showing a 35% increase in fully oxidized Si4+ species for DMF-treated NWs compared to dry NW powder. Additionally, a shell oxidation effect induced by DMF resulted is a more stable NW FET performance with steady transistor currents and only 1.5 V hysteresis after 1000 h of air exposure
Flexible radiation dosimeters have been produced incorporating thick films (>1 μm) of the semiconducting polymer poly([9,9-dioctylfluorenyl-2,7-diyl]-co-bithiophene). Diode structures produced on aluminium-metallised poly(imide) substrates, and with gold top contacts, have been examined with respect to their electrical properties. The results suggest that a Schottky conduction mechanism occurs in the reverse biased diode, with a barrier to charge injection at the aluminium electrode. Optical absorption/emission spectra reveal a band gap of 2.48 eV for the polymer. The diodes have been used for direct charge detection of 17 keV X-rays, generated by a molybdenum source. Using operating voltages of -10 and -50 V respectively, sensitivities of 54 and 158 nC/mGy/cm3 have been achieved. Increasing the operating voltage shows that the diodes are stable up to approximately -200 V without significant increase in the dark current of the device (<0.2 nA).
Semiconducting polymers have previously been used as the transduction material in x-ray dosimeters, but these devices have a rather low detection sensitivity because of the low x-ray attenuation efficiency of the organic active layer. Here, we demonstrate a way to overcome this limitation through the introduction of high density nanoparticles having a high atomic number (Z) to increase the x-ray attenuation. Specifically, bismuth oxide (Bi O ) nanoparticles (Z=83 for Bi) are added to a poly(triarylamine) (PTAA) semiconducting polymer in the active layer of an x-ray detector. Scanning electron microscopy (SEM) reveals that the Bi O nanoparticles are reasonably distributed in the PTAA active layer. The reverse bias dc currentvoltage characteristics for PTAABi O diodes (with indium tin oxide (ITO) and Al contacts) have similar leakage currents to ITO/PTAA/Al diodes. Upon irradiation with 17.5keV x-ray beams, a PTAA device containing 60wt% Bi O nanoparticles demonstrates a sensitivity increase of approximately 2.5 times compared to the plain PTAA sensor. These results indicate that the addition of high-Z nanoparticles improves the performance of the dosimeters by increasing the x-ray stopping power of the active volume of the diode. Because the Bi O has a high density, it can be used very efficiently, achieving a high weight fraction with a low volume fraction of nanoparticles. The mechanical flexibility of the polymer is not sacrificed when the inorganic nanoparticles are incorporated. © 2012 IOP Publishing Ltd.
McCulloch I, Heeney M, Chabinyc ML, DeLongchamp D, Kline RJ, Coelle M, Duffy W, Fischer D, Gundlach D, Hamadani B, Hamilton R, Richter L, Salleo A, Shkunov M, Sporrowe D, Tierney S, Zhong W (2009)Semiconducting Thienothiophene Copolymers: Design, Synthesis, Morphology, and Performance in Thin-Film Organic Transistors, In: ADVANCED MATERIALS21(10-11)pp. 1091-1109
WILEY-V C H VERLAG GMBH
Abstract cannot be made public due to confidential reasons
This project was dedicated to the development of solution-processed nanomaterials-based high-performance field-effect transistors (FETs) suitable for a new application area of printed reconfigurable antennas. The focus of research was on implementing solution processed high electron mobility InAs nanowires (NWs) as semiconducting channel in field effect transistors. The key direction of this work was the development of InAs NWs FETs with a designated high frequency waveguide geometry to enable they operation as microwave switch elements. Initially, InAs NW FETs were developed and tested in direct – current mode to allow evaluation and extraction of key transistor performance parameters such charge carrier mobility, threshold, on/off ratio, transconductance, subthreshold swing, and on-channel resistance. The InAs NW were assembled from nanowire ‘inks’ in the FETs channel via electric -field assisted assembly technique, dielectrophoresis. Nanowires were directly incorporated in FETs with bottom-gate architecture on Si/SiO2 substrates, and with top-gate architecture on quartz substrates with polymeric gate dielectrics. Current-voltage characteristics were measured both in controlled dry nitrogen atmosphere and ambient environment, and demonstrated an instability of unprotected InAs NW in ambient air. Protection of nanowire channel with Al2O3 layers has resulted in significant improvement of device stability. Optimised InAs NW FET devices demonstrated electron mobility over 1000 cm2/Vs and on-off current ratios up to 1000. Finally, a proof of principle for solution processed InAs NW field-effect transistors operating as microwave switches in 5-33GHz frequency range have been demonstrated. FET devices were implemented in co-planar waveguide (CPW) microwave transmission line geometry, providing efficient transmission or reflection of microwave signal. The FETs demonstrated high performance with transistor ON-state resistance as small as ≈50 Ω providing an excellent impedance match to that of microwave waveguide. Bringing FETs to the OFF state provided 1000 times resistance increase, resulting in FET microwave switch behaviour, characterised by ~10 dB change in scattering (S)-parameters, such as difference in transmission coefficient S21 between on/off switching states.
One of the greatest challenges in silicon photonics has been to induce light emission in silicon, with the ultimate vision is to have fully silicon-based photonics emitters or lasers which can operate by both optical and electrical pumping. Comprehensive photoluminescence (PL) and electroluminescence (EL) studies are conducted on dislocation engineering light emitting diode structures based on silicon implanted (Si:B) with Ce, Eu, and Yb rare-earth (RE) ions. The PL and EL results show very bright luminescence intensity and dramatic red shifting in luminescence peaks which shows a possible novel phenomenon of RE energy transition modification. The modification is attributed to the direct transition from Si conduction band edge to RE manifolds〖 (_^2)F〗_(7/2)^ , 〖(_^2)F〗_(5/2)^ for 〖Ce〗^(3+), 〖(_^7)F〗_j^ (j=0 to 4) for 〖Eu〗^(3+), 〖(_^2)F〗_(5/2)^ and 〖(_^2)F〗_(7/2)^ for 〖Yb〗^(3+). The emissions are shifted from the conventional lowest internal energy transition in 〖Ce〗^(3+) from around blue spectrum at ~350 nm (due to (_^2)D_(3/2)^ excited state to the 〖(_^2)F〗_(7/2)^ , 〖(_^2)F〗_(5/2)^ transitions) to ~1.35 µm in Si:B&Ce. For 〖Eu〗^(3+) the emission is shifted from around the red spectrum at ~600 nm (due to (_^5)D_0^ excited state to the 〖(_^7)F〗_j^ transitions, j=0 to 4) to ~ 1.40 µm in Si:B&Eu and for 〖Yb〗^(3+) is shifted from slightly beyond the visible region at ~980 nm (due to 〖(_^2)F〗_(5/2)^ excited state to the 〖(_^2)F〗_(7/2)^ transition) to ~ 1.43 µm in Si:B&Yb samples. The new shifting of luminescence peak into NIR region is very important for optical communication in making LEDs and lasers.
Manufacturing electronic devices by printing or coating is a key emerging technology, promising low cost and high throughput. Halide perovskites have emerged as high efficiency, solution processable photovoltaic materials, and within this thesis some of the issues relevant to their up-scaling are explored. Additionally, photo-curing – a post-processing technique with a wide range of applications in printed electronics – is investigated. Aqueous silver flake inks are a promising material for printed conductive applications, combining low cost and high conductivity. Within this thesis, photo-curing of these inks to further improve their conductivity is investigated. Photo-cured samples showed an 11x conductivity improvement compared with thermally cured samples. Furthermore, the manufacturing yield was doubled following photo-curing. These novel observations are explained, by recourse to percolation theory, by an increase in mean particle size. These results enable lower cost and increased yield in future manufacturing. Halide perovskite materials show great promise for solution processable photovoltaics. Within this thesis, the effects of ambient conditions during device processing are measured, in order to inform future up-scaled manufacturing. The chemical and morphological effects of ambient humidity in perovskite films are correlated with the annealing time used and final device performance. This work led to new insights into the combined effects of these two parameters, and a suggestion is made for reducing the annealing time. Finally, a barrier to commercialisation of perovskite solar cells is the use of toxic solvents in their fabrication. Within this thesis, a novel deposition technique is proposed, based on the synthesis of perovskite material in particulate form followed by re-dispersal in non-toxic solvents. This mitigates solvent toxicity, reduces sensitivity to ambient conditions, and in some cases enhances stability. Devices are fabricated based on this technique, and though performance remains low, a marked improvement is observed by the addition of conductive graphene flakes to the inks.
Commercially available organic photovoltaics (OPV) are commonly fabricated using printing and coating techniques that allow for low-cost, high throughput processing of large-area OPV devices. However, the power conversion eﬃciency (PCE) of scaled-up OPVs is often lower than that of small-area ones. This is because the deposition techniques typically being used in industry are diﬀerent to those used in research laboratories (printing/coating vs. spin-coating). Thus, detailed studies of functional materials are required to tailor the characteristics of photoactive D/A blends of OPVs in order to preserve high PCE values for scaled-up device sizes. Therefore, the aims of this thesis were to enhance the PCEs of OPV cells made using a well-known donor material (P3HT), and to develop a structured approach to fabricating large-area OPVs thus easing the transfer of fabrication procedures from laboratory to industry. To achieve the ﬁrst goal, indene-C70-bis-adduct (IC70BA) was chosen as an acceptor material for a photoactive blend with P3HT. A review of P3HT:ICBA-based solar cells indicated a signiﬁcant variation of reported device PCE values (average of 4.66±1.45%). The majority of reported device eﬃciencies were measured for OPVs with photoactive areas rarely exceeding 0.1 cm2. Therefore, a detailed study of the intrinsic characteristics of the IC70BA molecule and the morphology of the P3HT:IC70BA blends was carried out in order to design the optimal fabrication conditions for achieving higher PCEs and up-scaled device areas. Record PCEs approaching 7% were accomplished in this thesis for OPVs with photoactive areas of 0.43 cm2. This was achieved by understanding the correlation between the isomeric properties of the IC70BA molecule and the resulting D/A blend morphology depending on the fabrication conditions used. The second goal of this thesis was accomplished by designing a slot-die coating equipment that allows for the deposition of functional materials over large-areas. Diﬀerent solubilised materials were deposited in ambient conditions on glass and plastic substrates in order to fabricate OPV devices. Two diﬀerent photoactive D/A systems were used: P3HT:IC70BA and PCDTBT:PC70BM. OPV cell and module PCEs approaching 4% were achieved for devices with photoactive areas of about 35 cm2. The quality of the slot-die coated layers was investigated using LBIC, PL, and Raman mapping. This will allow for future improvements in the coating process and, therefore, increased device PCEs and operational lifetimes. In conclusion, the results obtained in this thesis show a way of fabricating eﬃcient large-area OPVs without the use of the spin-coating deposition technique. The study of materials and the development of deposition procedures allows for an accessible transfer of research outcomes from the laboratory to industry.
Sensing BTEX (Benzene, Ethylbenzene, Toluene, Xylene) pollutants is of utmost importance to reduce health risk and ensure public safety. The lack of sensitivity and selectivity of the current gas sensors and the limited number of available technologies in the field of BTEX-sensing raises the demand for the development of high-performance gas sensors for BTEX applications. The scope of this thesis is the fabrication and characterisation of high-quality field-effect transistors (FETs), with functionalised silicon nanowires (SiNWs), for the selective sensing of benzene vs. other BTEX gases. This research addresses three main challenges in SiNW FET-sensor device development: i) controllable and reproducible assembly of high-quality SiNWs for FET sensor devices using the method of dielectrophoresis (DEP), ii) almost complete elimination of harmful hysteresis effect in the SiNW FET current-voltage characteristics induced by surface states using DMF solvent, iii) selective sensing of benzene with up to ppb range of sensitivity using calixarene-derivatives. It is experimentally demonstrated that frequency-controlled DEP is a powerful tool for the selection and collection of semiconducting SiNWs with advanced electrical and morphological properties, from a poly-disperse as-synthesised NWs. The DEP assembly method also leads to a controllable and reproducible fabrication of high-quality NW-based FETs. The results highlight the superiority of DEP, performed at high signal frequencies (5-20 MHz) to selectively assemble only high-quality NWs which can respond to such high DEP frequencies. The SiNW FETs, with NWs collected at high DEP frequencies, have high mobility (≈50 cm2 V-1 s-1), low sub-threshold-swing (≈1.26 V/decade), high on-current (up to 3 mA) and high on/off ratio (106-107). The DEP NW selection is also demonstrated using an industrially scalable method, to allow establishing of NW response characteristics to different DEP frequencies in a very short time window of about 60 seconds. The choice of solvent for the dispersion of the SiNW for the DEP process demonstrates a dramatic impact on their surface trap, with DMF solvent acting as a mild oxidising agent on the NW surface shell. This surface state passivation technique resulted in the fabrication of high-quality, hysteresis-free NW FET transducers for sensor applications. Finally, the proof-of-concept SiNW FET transducer decorated with calixarene-derivative gas receptors exhibits selective detection of benzene vs. other BTEX gases up to 30 ppm concentrations, and up to sub-ppm benzene concentration. The demonstrated NW-sensors are low power and compact, and therefore can be easily mounted on a mobile device, providing instantaneous determination of hazardous gases in the surrounding atmosphere. The methodologies developed in this thesis, have a high potential to make a breakthrough in low-cost, selective gas sensors, which can be fabricated in line with printed and flexible electronic approaches.
Printable electronics offer outstanding potential for novel electronic devices that can be lightweight, flexible, transparent, large area and low cost. These next generation electronics will aid the realization of concepts such as smart cities, structural health monitoring of advanced structures, body area networks, remote medical healthcare and the internet of things. Solution processed nanomaterials hold most of the promise as viable building blocks for these future applications. A robust and reproducible approach has been developed, for high definition one-step patterning of conductive electrodes, using inkjet printing (part A, chapter 4). Controlled electrode spacing and single droplet deposition was achieved by utilising the droplet kinematic stages and overcoming the built in limitations of our experimental setup. Single and multiple silicon nanowire (NW) field effect transistors were realised in a variety of electrode configurations, demonstrating the capabilities of our technique. The controlled realisation of single NW devices enabled the characterisation of semiconducting nanostructures by correlating their electrical response with defects introduced during growth. In chapter 7 (part B) a low-cost, scalable printing process to fabricate high-quality organic semicon-ducting single crystals (OSSCs) on virtually any substrate using various types of conjugated molecules is demonstrated. By combining the advantages of antisolvent crystallization and solution shearing with spray-printing, one-step single crystal growth of various small semiconducting molecules was realised. In addition, crystal size, shape, and orientation were controlled by the sheer force generated by the im-pact of the droplets from the spray onto the antisolvent’s surface, eliminating the need for pre-deposition patterning. For enabling large scale manufacturing of printable single crystal electronics, advances towards non-destructive characterisation techniques are required. In chapter 5 (part A), the capabilities of advanced scanning probe microscopy techniques as a non-destructive alternative for evaluating the growth and device integration defects in NW based devices, was explored. Conductive Atomic Force Microscopy (c-AFM) was used for imaging critical electrical characteristics with high spatial resolution. In chapter 8 (part B), the potentials of polarised Raman spectroscopy (p-Raman) as a non-destructive approach, for characterising the anisotropy in our OSSCs were explored. By using the aforementioned crystals as reference samples, the potential of the technique is demonstrated.