Dual-gate organic phototransistor with high-gain and linear photoresponse.

The conversion of light into electrical signal in a photodetector is a crucial process for a wide range of technological applications. Here we report a new device concept of dual-gate phototransistor that combines the operation of photodiodes and phototransistors to simultaneously enable high-gain and linear photoresponse without requiring external circuitry. In an oppositely biased, dual-gate transistor based on a solution-processed organic heterojunction layer, we find that the presence of both n- and p-type channels enables both photogenerated electrons and holes to efficiently separate and transport in the same semiconducting layer. This operation enables effective control of trap carrier density that leads to linear photoresponse with high photoconductive gain and a significant reduction of electrical noise. As we demonstrate using a large-area, 8 × 8 imaging array of dual-gate phototransistors, this device concept is promising for high-performance and scalable photodetectors with tunable dynamic range.

Ultraflexible Near-Infrared Organic Photodetectors for Conformal Photoplethysmogram Sensors.

Flexible organic optoelectronic devices simultaneously targeting mechanical conformability and fast responsivity in the near-infrared (IR) region are a prerequisite to expand the capabilities of practical optical science and engineering for on-skin optoelectronic applications. Here, an ultraflexible near-IR responsive skin-conformal photoplethysmogram sensor based on a bulk heterojunction photovoltaic active layer containing regioregular polyindacenodithiophene-pyridyl[2,1,3]thiadiazole-cyclopentadithiophene (PIPCP) is reported. The ultrathin (3 µm thick) photodetector exhibits unprecedented operational stability under severe mechanical deformation at a bending radius of less than 3 µm, even after more than 103 bending cycles. Deliberate optimization of the physical dimensions of the active layer used in the device enables precise on/off switching and high device yield simultaneously. The response frequency over 1 kHz under mechanically deformed conditions facilitates conformal electronic sensors at the machine/human interface. Finally, a mechanically stretchable, flexible, and skin-conformal photoplethysmogram (PPG) device with higher sensitivity than those of rigid devices is demonstrated, through conformal adherence to the flexuous surface of a fingerprint.

Ultraflexible Transparent Oxide/Metal/Oxide Stack Electrode with Low Sheet Resistance for Electrophysiological Measurements.

Flexible, transparent electrodes are a crucial component for future implantable and wearable systems. For practical applications, conductivity and flexibility should be further improved to prevent signal attenuation, heat generation, and disconnection. Herein, we fabricate an ultraflexible transparent electrode with low sheet resistance (8.6 Ω/sq) using an indium-tin-oxide/Au/indium-tin-oxide (ITO) multilayer on a 1 µm thick parylene substrate. The electrodes were foldable and when compared to pristine ITO displayed greater mechanical robustness. Applicability for large-area applications was confirmed through electrochemical impedance measurements, and the compatibility of electrode arrays for in vivo applications was demonstrated with an optogenetic experiment. As a result of the ultraflexible transparent electrode's excellent conformity to soft tissue, voltage signals induced by light stimulation directly below the electrode were successfully recorded on the moving muscle.

High Sensitivity Tuning of Work Function of Self-Assembled Monolayers Modified Electrodes Using Vacuum Ultraviolet Treatment.

We demonstrate systematic work function tuning of thiol-based SAM-modified gold electrodes with high controllability and sensitivity as high as 0.05 eV using vacuum ultraviolet technique (VUV). Under different irradiation times, both work function and wettability of the metal surface is modified. Fine tuning of the electrode work function is demonstrated by observable changes in the reverse current of a polymer Schottky diode. Additionally, the change in SAM chemical functionality validates the work function changes of VUV-irradiated electrodes. Our selective work function patterning on a single Au electrode via VUV could also reduce the required fabrication steps for more complex circuits.

Printable elastic conductors by in situ formation of silver nanoparticles from silver flakes.

Printable elastic conductors promise large-area stretchable sensor/actuator networks for healthcare, wearables and robotics. Elastomers with metal nanoparticles are one of the best approaches to achieve high performance, but large-area utilization is limited by difficulties in their processability. Here we report a printable elastic conductor containing Ag nanoparticles that are formed in situ, solely by mixing micrometre-sized Ag flakes, fluorine rubbers, and surfactant. Our printable elastic composites exhibit conductivity higher than 4,000 S cm-1 (highest value: 6,168 S cm-1) at 0% strain, and 935 S cm-1 when stretched up to 400%. Ag nanoparticle formation is influenced by the surfactant, heating processes, and elastomer molecular weight, resulting in a drastic improvement of conductivity. Fully printed sensor networks for stretchable robots are demonstrated, sensing pressure and temperature accurately, even when stretched over 250%.

Ultraflexible organic photonic skin.

Thin-film electronics intimately laminated onto the skin imperceptibly equip the human body with electronic components for health-monitoring and information technologies. When electronic devices are worn, the mechanical flexibility and/or stretchability of thin-film devices helps to minimize the stress and discomfort associated with wear because of their conformability and softness. For industrial applications, it is important to fabricate wearable devices using processing methods that maximize throughput and minimize cost. We demonstrate ultraflexible and conformable three-color, highly efficient polymer light-emitting diodes (PLEDs) and organic photodetectors (OPDs) to realize optoelectronic skins (oe-skins) that introduce multiple electronic functionalities such as sensing and displays on the surface of human skin. The total thickness of the devices, including the substrate and encapsulation layer, is only 3 µm, which is one order of magnitude thinner than the epidermal layer of human skin. By integrating green and red PLEDs with OPDs, we fabricate an ultraflexible reflective pulse oximeter. The device unobtrusively measures the oxygen concentration of blood when laminated on a finger. On-skin seven-segment digital displays and color indicators can visualize data directly on the body.

Vacuum Ultraviolet Treatment of Self-Assembled Monolayers: A Tool for Understanding Growth and Tuning Charge Transport in Organic Field-Effect Transistors.

Vacuum ultraviolet irradiation is used as a tool to systematically study the morphology, growth, and performance of small-molecule organic field-effect transistors. The surface energy can be carefully and precisely tuned by varying the dose of irradiation, allowing for the systematic study of the growth of an emerging organic semiconductor. This technique helps to methodically control the morphology and performance of organic semiconductors.

Increased mobility induced by addition of a Lewis acid to a Lewis basic conjugated polymer.

Through simple addition of a Lewis acid to a conjugated polymer bearing a Lewis basic heteroatom, the hole transport of the polymer can be effectively p-doped resulting in a two-orders increase in hole mobility. The temperature dependent hole transport of a variety of Lewis acid concentrations are explored.

A structure-property-performance investigation of perylenediimides as electron accepting materials in organic solar cells.

Solution-processed perylenediimides (PDIs) with varying peri and bay substituents are characterized in order to better understand the relationships between molecular structure, solid state order, charge transport, and photovoltaic performance. It was found that bulky bay substituents interfere with molecular packing, leading to low charge transport and photovoltaic efficiencies compared to PDIs with fewer or less disruptive substituents. We assessed the potential of PDIs as acceptors for organic photovoltaics (OPVs) by utilizing a solution-processed bilayer OPV device architecture with the donor benzoporphyrin. At AM1.5G illumination, power conversion efficiencies (PCEs) up to 2.0% are obtained for solution-processed bilayer OPVs employing PDIs as acceptors. These results demonstrate the potential of PDIs as photovoltaic acceptor materials while elucidating the relationships between molecular structure and material properties.

All-conjugated triblock polyelectrolytes.

All-conjugated triblock polyfluorenes with well-defined molecular weights and low polydispersities are synthesized via chain-growth Suzuki-Miyaura polymerization. Ionization of pendant alkylbromide chains by pyridine affords amphiphilic triblock polyelectrolytes with neutral/charged/neutral or charged/neutral/charged segments. The immiscible blocks lead to aggregation in polar and nonpolar solvents, and to complex surface morphologies depending on the polarity of the substrate. These triblock polyelectrolytes can also be used as interfacial layers in polymer light-emitting diodes to facilitate electron injection from aluminum.

DNA interlayers enhance charge injection in organic field-effect transistors.

By inserting DNA interlayers beneath the Au contact, the contact resistance of PC(70) BM field-effect transistorss is reduced by approximately 30 times at a gate bias of 20 V. The electron and hole mobilities of ambipolar diketopyrrolopyrrole transistors are increased by one order of magnitude with a reduction of the threshold voltage from 12 to 6.5 V.

Regioregular pyridal[2,1,3]thiadiazole π-conjugated copolymers.

π-Conjugated, narrow band gap copolymers containing pyridal[2,1,3]thiadiazole (PT) were synthesized via starting materials that prevent random incorporation of the PT heterocycles relative to the backbone vector. Two regioregular structures could be obtained: in one the PTs are oriented in the same direction, and in the other the orientation of the PTs alternates every other repeat unit. Compared to their regiorandom counterparts, the regioregular polymers exhibit a 2 orders of magnitude increase of the hole mobilites, from 0.005 to 0.6 cm(2) V(-1) s(-1), as determined by field-effect transistor measurements.

DNA electron injection interlayers for polymer light-emitting diodes.

Introduction of a DNA interlayer adjacent to an Al cathode in a polymer light-emitting diode leads to lower turn-on voltages, higher luminance efficiencies, and characteristics comparable to those observed using a Ba electrode. The DNA serves to improve electron injection and also functions as a hole-blocking layer. The temporal characteristics of the devices are consistent with an interfacial dipole layer adjacent to the electrode being responsible for the reduction of the electron injection barrier.

Controlling ion motion in polymer light-emitting diodes containing conjugated polyelectrolyte electron injection layers.

The properties and function of an anionic conjugated polyelectrolyte (CPE)-containing ion-conducting polyethylene oxide pendant (PF(PEO)CO(2)Na) as electron injection layers (EILs) in polymer light-emitting diodes (PLEDs) are investigated. A primary goal was to design a CPE structure that would enable acceleration of the device temporal response through facilitation of ion motion. Pristine PLEDs containing PF(PEO)CO(2)Na exhibit luminance response times on the order of tenths of seconds. This delay is attributed to the formation of ordered structures within the CPE film, as observed by atomic force microscopy. Complementary evidence is provided by electron transport measurements. The ordered structures are believed to slow ion migration within the CPE EIL and hence result in a longer temporal response time. It is possible to accelerate the response by a combination of thermal and voltage treatments that "lock" ions within the interfaces adjacent to PF(PEO)CO(2)Na. PLED devices with luminance response times of microseconds, a 10(5) fold enhancement, can therefore be achieved. Faster luminance response time opens up the application of PLEDs with CPE layers in display technologies.

Electron injection barrier reduction for organic light-emitting devices by quinacridone derivatives.

Water/alcohol-soluble quinacridone derivatives have been synthesized and utilized as an electron injection layer in organic light-emitting diodes. Initial results are very promising, as a device with a layer of Na(+)QHSO(3)(-) adjacent to an Al cathode exhibited a luminance efficiency (1.65 cd A(-1)) that was significantly enhanced relative to the efficiency (0.85 cd A(-1)) of a control device with an unstable, lower work function Ba/Al cathode.