Tse a Ng

A skin-inspired organic digital mechanoreceptor

Sensing the force digitally Our skin provides us with a flexible waterproof barrier, but it also contains a sensor array that feels the world around us. This array provides feedback and helps us to avoid a hot object or increase the strength of our grip on an object that may be slipping away. Tee et al. describe an approach to simulate the mechanoreceptors of human skin, using pressure-sensitive foils and printed ring oscillators (see the Perspective by Anikeeva and Koppes). The sensor successfully converted pressure into a digital response in a pressure range comparable to that found in a human grip. Science, this issue p. 313; see also p. 274 An artificial skin based on flexible printed organic circuits and pressure sensors mimics the ability to sense physical force. [Also see Perspective by Anikeeva and Koppes] Human skin relies on cutaneous receptors that output digital signals for tactile sensing in which the intensity of stimulation is converted to a series of voltage pulses. We present a power-efficient skin-inspired mechanoreceptor with a flexible organic transistor circuit that transduces pressure into digital frequency signals directly. The output frequency ranges between 0 and 200 hertz, with a sublinear response to increasing force stimuli that mimics slow-adapting skin mechanoreceptors. The output of the sensors was further used to stimulate optogenetically engineered mouse somatosensory neurons of mouse cortex in vitro, achieving stimulated pulses in accordance with pressure levels. This work represents a step toward the design and use of large-area organic electronic skins with neural-integrated touch feedback for replacement limbs.

Flexible image sensor array with bulk heterojunction organic photodiode

Thick organic bulk heterojunction photodiodes with low dark current 1V∕μm is sufficient to achieve >75% charge collection in films of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] and [6,6]-phenyl-C61-butyric acid methyl ester blends up to 4μm thick, and the rate of photocurrent decay is reduced at saturation fields. The integration of a 4μm thick sensor layer onto a flexible amorphous silicon thin-film transistor backplane gave an image sensor array with 35% external quantum efficiency and noise equivalent power of 30pW∕cm2 at reverse bias voltage of −4V.

Scalable printed electronics: an organic decoder addressing ferroelectric non-volatile memory

Scalable circuits of organic logic and memory are realized using all-additive printing processes. A 3-bit organic complementary decoder is fabricated and used to read and write non-volatile, rewritable ferroelectric memory. The decoder-memory array is patterned by inkjet and gravure printing on flexible plastics. Simulation models for the organic transistors are developed, enabling circuit designs tolerant of the variations in printed devices. We explain the key design rules in fabrication of complex printed circuits and elucidate the performance requirements of materials and devices for reliable organic digital logic.

A flexible high potential printed battery for powering printed electronics

Mechanically flexible arrays of alkaline electrochemical cells fabricated using stencil printing onto fibrous substrates are shown to provide the necessary performance characteristics for driving ink-jet printed circuits. Due to the dimensions and material set currently required for reliable low-temperature print processing of electronic devices, a battery potential greater than that sourced by single cells is typically needed. The developed battery is a series interconnected array of 10 low resistance Zn-MnO2 alkaline cells, giving an open circuit potential of 14 V. This flexible battery is used to power an ink-jet printed 5-stage complementary ring oscillator based on organic semiconductors.

Sol-gel solution-deposited InGaZnO thin film transistors.

Thin film transistors (TFTs) fabricated by solution processing of sol-gel oxide semiconductor precursors in the group In-Ga-Zn are described. The TFT mobility varies over a wide range depending on the precursor materials, the composition, and the processing variables, with the highest mobility being about 30 cm(2)/(V s) for IZO and 20 cm(2)/(V s) for IGZO. The positive dark bias stress effect decreases markedly as the mobility increases and the high mobility devices are quite stable. The negative bias illumination stress effect is also weaker in the higher mobility TFTs, and some different characteristic properties are observed. The TFT mobility, threshold voltage, and bias stress properties are discussed in terms of the formation of self-compensated donor and acceptor states, based on the chemistry and thermodynamics of the sol-gel process.

Highly sensitive tactile sensors integrated with organic transistors

This paper presents a highly sensitive capacitive pressure sensor composed of a polymer dielectric film with a nano-needle structure. The nano-needle polymer films were prepared by facile fabrication methods including breath figures formation followed by stamping. The pressure sensitivity of the sensor reached 1.76 kPa−1 in the low pressure range (<1 kPa), which is comparable to the sensitivity of human skin. Analysis of the geometries and densities effect was shown, and the nano-needle film showed better sensitivity in comparison to films with hemispherical or conical structures. The pressure sensors were integrated with printed organic thin film transistors to enable flexible, large-area tactile sensing applications.

Comparing the kinetics of bias stress in organic field-effect transistors with different dielectric interfaces

The kinetics of trap formation and dissociation is investigated in organic field-effect transistors to understand how the dielectric surface affects device stability. Devices with surface treatment of octadecyl-trichlorosilane (OTS) show faster trapping rates than an untreated oxide dielectric. Trap release is also quicker from the OTS interface compared to the untreated interface, thus implying that traps have lower barriers in the OTS interface than in the untreated interface. Images of trap distribution are obtained by electric force microscopy for polyfluorenethiophene transistors, but the traps in polythiophene transistors could not be imaged due to shielding charges.

Low temperature a-Si:H photodiodes and flexible image sensor arrays patterned by digital lithography

Hydrogenated amorphous silicon-based image sensor arrays were fabricated on polyethylene naphthalate substrates, with photodiodes optimized for process temperatures of 150°C. An optimal i-layer thickness was determined to minimize carrier recombination and to maintain sufficient light absorption and acceptable leakage current. Patterning of the thin-film transistor backplane was accomplished using ink-jet printed etch masks. A flexible image sensor is demonstrated with 75dots∕in. resolution over 180×180pixels and with sensitivity of 1.2pW∕cm2.

Gate bias stress effects due to polymer gate dielectrics in organic thin-film transistors

The operational stability of organic thin-film transistors (OTFTs) comprising bilayer polymer dielectric of poly(methylsilsesquioxane) (pMSSQ) and either the epoxy resin SU-8 or poly(4-vinyl phenol) was examined. Although not in direct contact with the semiconductor materials, the bottom dielectric layer did affect OTFT stability through water ion movement or charge injection inside the bottom dielectrics. In the comparison between our best polymer dielectric pMSSQ/SU-8 to the silicon oxide dielectric, the result emphasized that, at equal initial charge concentration, polymer dielectrics did not alleviate threshold-voltage shift but did maintain more stable current due to the lower gate capacitance than silicon oxide.

Batch fabrication and characterization of ultrasensitive cantilevers with submicron magnetic tips

We have batch fabricated ultrasensitive silicon cantilevers with integrated submicron magnetic tips and have characterized both their mechanical and magnetic properties. Cantilevers with spring constants as small as 10−5 N/m were fabricated, with quality factors in the range of 2.5–3.5×104 and a force sensitivity as good as 64×10−18 N Hz−1/2 at room temperature in vacuum. Cantilever spring constants were measured by observing thermomechanical position fluctuations with a fiber optic interferometer, while resonance frequencies and quality factors were inferred from cantilever ring down transients. Polycrystalline nickel tips as small as 1.2 μm×0.4 μm×0.2 μm were fabricated on the cantilevers by electron beam lithography, thermal evaporation, and lift-off. Tip magnetic moments were inferred from the shift of the cantilever frequency versus magnetic field and show a 0.60±0.12 T saturation magnetization, indicating that less than 28 nm of oxide forms on the tips during processing. Force sensitivity was demons...