In this study, the effect of atmospheric hydrogen plasma treatment on the in-plane conductivity of solution-processed zinc oxide (ZnO) in various environments is reported. The hydrogen-plasma-treated and untreated ZnO films exhibited ohmic behavior with room-temperature in-plane conductivity in a vacuum. When the untreated ZnO film was exposed to a dry oxygen environment, the conductivity rapidly decreased, and an oscillating current was observed. In certain cases, the thin film reversibly ‘switched’ between the high- and low-conductivity states. In contrast, the conductivity of the hydrogen-plasma-treated ZnO film remained nearly constant under different ambient conditions. We infer that hydrogen acts as a shallow donor, increasing the carrier concentration and generating oxygen vacancies by eliminating the surface contamination layer. Hence, atmospheric hydrogen plasma treatment could play a crucial role in stabilizing the conductivity of ZnO films.

Evolution-in-Materio is a computational paradigm in which an algorithm reconfigures a material's properties to achieve a specific computational function. This article addresses the question of how successful and well performing Evolution-in-Materio processors can be designed through the selection of nanomaterials and an evolutionary algorithm for a target application. A physical model of a nanomaterial network is developed which allows for both randomness, and the possibility of Ohmic and non-Ohmic conduction, that are characteristic of such materials. These differing networks are then exploited by differential evolution, which optimises several configuration parameters (e.g., configuration voltages, weights, etc.), to solve different classification problems. We show that ideal nanomaterial choice depends upon problem complexity, with more complex problems being favoured by complex voltage dependence of conductivity and vice versa. Furthermore, we highlight how intrinsic nanomaterial electrical properties can be exploited by differing configuration parameters, clarifying the role and limitations of these techniques. These findings provide guidance for the rational design of nanomaterials and algorithms for future Evolution-in-Materio processors.

We report on the electrical behaviour of thin films of bovine brain microtubules (MTs). For samples in both their dried and hydrated states, the measured currents reveal a power law dependence on the applied DC voltage. We attribute this to the injection of space-charge from the metallic electrode(s). The MTs are thought to form a complex electrical network, which can be manipulated with an applied voltage. This feature has been exploited to undertake some experiments on the use of the MT mesh as a medium for computation. We show that it is possible to evolve MT films into binary classifiers following an evolution in materio approach. The accuracy of the system is, on average, similar to that of early carbon nanotube classifiers developed using the same methodology.

Organic photovoltaic (OPV) devices offer the ability to tune the electronic and optical properties of the active layer by selection of a wide range of molecules; however, their power conversion efficiencies currently lag other competing photovoltaic technologies. One method to enhance their performance is to add a further active material into the absorber layer, resulting in a ternary OPV. However, selecting appropriate ternary blend components to yield an improvement in performance is challenging due to the multitude of materials properties and physical processes that ternary blends can display. Here, we perform a systematic set of experiments on OPV ternary blends incorporating either of the donor polymers P3HT or PTB7 and the fullerene acceptors PCBM and ICBA. Some combinations of ternary blends are shown to outperform the reference binaries in terms of open-circuit voltage or short-circuit current; however, this was not observed for all combinations. Improvements in internal quantum efficiency of the order of 25% were observed for PTB7-based ternaries compared to the reference binary, which is attributed to a reduction in charge recombination. All blends showed some improvement in open-circuit voltage with addition of ICBA due to alloying of the fullerene components, but to differing degrees which is argued to be due to molecular morphology. These findings demonstrate that the benefits one can obtain using a ternary OPV approach vary depending on the materials system to an extent that depends upon the ternary blend morphology.

In order to realise the potential of organic photovoltaic devices (OPVs) to provide cheap, scalable access to renewable energy, it is necessary to improve their lifetime and cost of encapsulation. The aim of this work is to achieve these aims by blending the donor and acceptor with the commodity polymer, PMMA, to form a ternary blend device with enhanced lifetime. We find that ternary OPV devices prepared in this manner can have up to double the lifetime of the binary control devices to an extent that depends upon the PMMA morphology and the processing additives used. Further, we find that the initial performance of ternary OPVs may decrease (in the case of PTB7-based blends) or slightly increase (in the case of P3HT-based blends) when compared to their binary counterparts, which is hypothesised to be due to donor compatibility with the PMMA. These findings suggest that this approach can be employed in other OPV blend systems and give design rules to maximise the positive impact on device lifetime.

We report the development of highly efficient and stable solution-processed organic light-emitting transistors (OLETs) that combine a polymer heterostructure with the transparent high-k dielectric poly(vinylidenefluoride(0.62)-trifluoroethylene(0.31)-chlorotrifluoroethylene(0.7)) (P(VDF-TrFE-CTFE)). The polymer heterostructure comprises of poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']-dithiophen-2-yl)-alt-[1,2,5]thiadiazolo[3,4-c]pyridine] (PCDTPT) and Super Yellow as charge-transporting and light-emitting layers, respectively. Device characterization shows that the use of P(VDF-TrFE-CTFE) leads to larger channel currents (approximate to 2 mA) and lower operating voltages (-35 V) than for previously reported polymer based OLETs. Furthermore, the combined transparency of the dielectric and gate electrode, results in efficient bottom emission with external quantum efficiency of approximate to 0.88% at a luminance L >= 2000 cd m(-2). Importantly, the resulting OLETs exhibit excellent shelf life and operational stability. The present work represents a significant step forward in the pursuit of all-solution-processed OLET technology for lighting and display applications.

This paper reports on the experimental investigation of metal-insulator-metal (MIM) diodes based on alkyltrichlorosilane self-assembled monolayers (SAMs) with different alkyl chain lengths. The insulating SAM is sandwiched between two metal contacts, Pt and Ti, with different work functions. The electronic properties of the MIM diodes can be tuned by controlling the alkyl chain length of the SAMs to address different constraints in high speed electronics applications. Data fitting of the tunneling current through the MIM junctions using the Fowler-Nordheim model suggests that the device operation is influenced by the barrier heights of the diodes and thicknesses of the SAMs. The current-voltage characteristics achieved in MIM diodes based on alkyltrichlorosilane SAMs make them promising candidates for high speed electronics applications.

A photolithographic process for the fabrication of short channel solution-processed zinc oxide transistors (ZnO TFTs) was optimized. To avoid damage to the ZnO film by the photolithography, a bottom gate, bottom contact (BG-BC) device structure was adopted. A perhydropolysilazane (PHPS) precursor, thermally annealed and then treated in oxygen plasma, was used as the gate insulator on an aluminum (Al) gate electrode. Al source and drain electrodes, with a minimum channel length of 5 μm were successfully defined using photolithography. A mobility of 1.5×10−2 cm2/Vs, on/off ratio of 106 and good contact between the source and drain (S/D) and semiconductor were achieved for solution-processed ZnO TFTs having various channel lengths, showing no degradation of device properties. The relatively low mobility was attributed to increased roughness of the gate insulator resulting from development during the photolithographic process. These results suggest that short-channel solution-processed ZnO TFTs can be fabricated by adopting optimised photolithographic processes.

We report the development of low operating voltages in inorganic-organic hybrid light-emitting transistors (HLETs) based on a solution-processed ZrO x gate dielectric and a hybrid multilayer channel consisting of the heterojunction In2O3/ZnO and the organic polymer "Super Yellow" acting as n- and p-channel/emissive layers, respectively. Resulting HLETs operate at the lowest voltages reported to-date (<10 V) and combine high electron mobility (22 cm2/(V s)) with appreciable current on/off ratios (≈103) and an external quantum efficiency of 2 × 10-2% at 700 cd/m2. The charge injection, transport, and recombination mechanisms within this HLET architecture are discussed, and prospects for further performance enhancement are considered.

We report on the lifetime of unencapsulated organic photovoltaic diodes (OPVs) based on a ternary blend of poly(3-hexylthiophene) (P3HT), phenyl-C-61-butyric acid methyl ester (PCBM) and a soft insulating polymer, poly(methyl methacrylate) (PMMA) as compared to reference binary P3HT:PCBM OPVs. The performance of ternary devices was shown to decay more slowly than that of their binary counterparts to an extent that depends on the relative humidity (RH). The power conversion efficiency of ternary OPVs when stored in a low humidity environment (1% RH) decayed to 80% of their initial value after 200 h, almost double that of the reference binary OPVs. AFM measurements suggest that the PMMA forms pillars within the P3HT:PCBM matrix. It is proposed that the PMMA absorbs water in the active layer, and in doing so, slows the rate of deep trap formation that would otherwise lead to enhanced Shockley-Read-Hall recombination.

Evolution-in-Materio uses evolutionary algorithms (EA) to exploit the physical properties of unconfigured, physically rich materials, in effect transforming them into information processors. The potential of this technique for machine learning problems is explored here. Results are obtained from a mixture of single walled carbon nanotubes and liquid crystals (SWCNT/LC). The complex nature of the voltage/current relationship of this material presents a potential for adaptation. Here, it is used as a computational medium evolved by two derivative-free, population-based stochastic search algorithms, particle swarm optimisation (PSO) and differential evolution (DE). The computational problem considered is data classification. A custom made electronic motherboard for interacting with the material has been developed, which allows the application of control signals on the material body. Starting with a simple binary classification problem of separable data, the material is trained with an error minimisation objective for both algorithms. Subsequently, the solution, defined as the combination of the material itself and optimal inputs, is verified and results are reported. The evolution process based on EAs has the capacity to evolve the material to a state where data classification can be performed. PSO outperforms DE in terms of results’ reproducibility due to the smoother, as opposed to more noisy, inputs applied on the material.

A ZnO transistor with carrier mobility of 3 cm2 V−1 s−1 using a SiO2 insulator formed at low-temperature (180 °C) from solution-processed perhydropolysilazane.

We report on the optimization of the plasma treatment conditions for a solution-processed silicon dioxide gate insulator for application in zinc oxide thin film transistors (TFTs). The SiO2 layer was formed by spin coating a perhydropolysilazane (PHPS) precursor. This thin film was subsequently thermally annealed, followed by exposure to an oxygen plasma, to form an insulating (leakage current density of ∼10(-7) A/cm(2)) SiO2 layer. Optimized ZnO TFTs (40 W plasma treatment of the gate insulator for 10 s) possessed a carrier mobility of 3.2 cm(2)/(V s), an on/off ratio of ∼10(7), a threshold voltage of -1.3 V, and a subthreshold swing of 0.2 V/decade. In addition, long-term exposure (150 min) of the pre-annealed PHPS to the oxygen plasma enabled the maximum processing temperature to be reduced from 180 to 150 °C. The resulting ZnO TFT exhibited a carrier mobility of 1.3 cm(2)/(V s) and on/off ratio of ∼10(7).

The effects of hydrogen plasma treatment on the active layer of top-contact zinc oxide thin film transistors are reported. The transfer characteristics of the reference devices exhibited large hysteresis effects and an increasing positive threshold voltage (VTH) shift on repeated measurements. In contrast, following the plasma processing, the corresponding characteristics of the transistors exhibited negligible hysteresis and a very small VTH shift; the devices also possessed higher field effect carrier mobility values. These results were attributed to the presence of functional groups in the vicinity of the semiconductor/gate insulator interface, which prevents the formation of an effective channel.

Solution-based oxide gate dielectric layers are prepared from metal nitrates using UV-assisted annealing at 150 °C. The leakage current densities of ZrO2 and Al2O3 dielectrics are less than about 10-9 A/cm2 at 1 MV/cm and comparable to those formed by annealing at higher temperatures. High dielectric constants and the low leakage current behavior of the dielectric layers provide excellent ZnO TFT performance, with a field effect mobility of 1.37 cm2/V.s and an off-current density of 10-12 A/cm2. This low fabrication temperature process is compatible with future plastic electronics technology. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

We report the electrical behavior of undoped zinc oxide thin-film transistors (TFTs) fabricated by low-temperature chemical spray pyrolysis. An aerosol system utilizing aerodynamic focusing was used to deposit the ZnO. Polycrystalline films were subsequently formed by annealing at the relatively low temperature of 140°C. The saturation mobility of the TFTs was 2 cm 2 /Vs, which is the highest reported for undoped ZnO TFTs manufactured below 150°C. The devices also had an on/off ratio of 10 4 and a threshold voltage of −3.5 V. These values were found to depend reversibly on measurement conditions.

We report on the effects of device processing conditions, and of changing the electrode materials, on the switching and negative differential resistance (NDR) behaviour of metal/organic thin film/metal structures. The organic material was an ambipolar molecule containing both electron transporting (oxadiazole) and hole transporting (carbazole) chemical groups. Switching and NDR effects are observed for device architectures with both electrodes consisting of aluminium; optimized switching behaviour is achieved for structures incorporating gold nanoparticles. If one of the Al electrodes is replaced by a higher work function metal or coated with an electron-blocking layer, switching and NDR are no longer observed. The results are consistent with a model based on the creation and destruction of Al filaments within the thin organic layer.