Ultrasensitive Multi-Functional Flexible Sensors Based on Organic Field-Effect Transistors with Polymer-Dispersed Liquid Crystal Sensing Layers.

Ultrasensitive flexible sensors with multi-sensing functions are required for various applications in flexible electronics era. Here we demonstrate flexible polymer-dispersed liquid crystal (PDLC)-integrated-organic field-effect transistors (OFETs) (PDLC-i-OFETs), which sensitively respond to various stimulations including weak gas (air) flow, direct physical touch, light, and heat. The flexible PDLC-i-OFETs were fabricated by spin-coating the poly(methyl methacrylate) (PMMA)-dispersed 4,4'-pentyl-cyanobiphenyl (5CB) layers on the poly(3-hexylthiophene) (P3HT) channel layers of OFETs with 200 µm-thick poly(ethylene naphthalate) (PEN) substrates. The flexible PDLC-i-OFET devices could sense very weak nitrogen gas flow (0.3 sccm), which cannot be felt by human skins, and stably responded to direct physical touches (0.6~4.8 g load). In addition, the present devices showed very sensitive photoresponses to a visible light and exhibited excellent heat-sensing characteristics at a temperature of 25~70 °C. In particular, the present flexible PDLC-i-OFET devices could sense two different stimulations at the same time, indicative of promising multi-sensing capabilities.

Influence of Weak Base Addition to Hole-Collecting Buffer Layers in Polymer:Fullerene Solar Cells.

We report the effect of weak base addition to acidic polymer hole-collecting layers in normal-type polymer:fullerene solar cells. Varying amounts of the weak base aniline (AN) were added to solutions of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The acidity of the aniline-added PEDOT:PSS solutions gradually decreased from pH = 1.74 (AN = 0 mol% ) to pH = 4.24 (AN = 1.8 mol %). The electrical conductivity of the PEDOT:PSS-AN films did not change much with the pH value, while the ratio of conductivity between out-of-plane and in-plane directions was dependent on the pH of solutions. The highest power conversion efficiency (PCE) was obtained at pH = 2.52, even though all devices with the PEDOT:PSS-AN layers exhibited better PCE than those with the pristine PEDOT:PSS layers. Atomic force microscopy investigation revealed that the size of PEDOT:PSS domains became smaller as the pH increased. The stability test for 100 h illumination under one sun condition disclosed that the PCE decay was relatively slower for the devices with the PEDOT:PSS-AN layers than for those with pristine PEDOT:PSS layers.

Flexible Thermal Sensors Based on Organic Field-Effect Transistors with Polymeric Channel/Gate-Insulating and Light-Blocking Layers.

Here, we report flexible thermal sensors based on organic field-effect transistors (OFETs) that are fabricated using polymeric channel and gate-insulating layers on flexible polymer film substrates. Poly(3-hexylthiophene) and poly(methyl methacrylate) were used as the channel and gate-insulating layers, respectively, whereas indium-tin oxide-coated poly(ethylene naphthalate) films (thickness = 130 µm) were employed as the flexible substrates. Aluminum-coated polymer films were attached on top of the channel parts in the flexible OFETs to block any influence by light illumination. The present flexible OFET-based thermal sensors exhibited typical p-type transistor characteristics at a temperature range of 25-100 °C, while the hole mobility of devices was linearly increased with the temperature. The drain current could be amplified at various temperatures by adjusting the gate and drain voltages. In particular, stable sensing performances were measured during the repeated approaching/retreating cycle with a heat source. The flexible OFET thermal sensors attached on human fingers could sense heat from human fingers as well as from approaching objects.

Strong Composition Effects in All-Polymer Phototransistors with Bulk Heterojunction Layers of p-type and n-type Conjugated Polymers.

We report the composition effect of polymeric sensing channel layers on the performance of all-polymer phototransistors featuring bulk heterojunction (BHJ) structure of electron-donating (p-type) and electron-accepting (n-type) polymers. As an n-type component, poly(3-hexylthiopehe-co-benzothiadiazole) end-capped with 4-hexylthiophene (THBT-4ht) was synthesized via two-step reactions. A well-studied conjugated polymer, poly(3-hexylthiophene) (P3HT), was employed as a p-type polymer. The composition of BHJ (P3HT:THBT-4ht) films was studied in detail by varying the THBT-4ht contents (0, 1, 3, 5, 10, 20, 30, 40, and 100 wt %). The best charge separation in the P3HT:THBT-4ht films was measured at 30 wt % by the photoluminescence (PL) study, while the charge transport characteristics of devices were improved at the low THBT-4ht contents (<10 wt %). The photosensing experiments revealed that the photosensivity of all-polymer phototransistors was higher than that of the phototransistors with the pristine P3HT layers and strongly dependent on the BHJ composition. The highest (corrected) responsivity (RC) was achieved at 20 wt %, which can be attributable to the balance between the best charge separation and transport states, as investigated for crystal nanostructures and surface morphology by employing synchrotron-radiation grazing-incidence wide-angle X-ray scattering, high-resolution/scanning transmission electron microscopy, and atomic force microscopy.

Acidity-Controlled Conducting Polymer Films for Organic Thermoelectric Devices with Horizontal and Vertical Architectures.

Organic thermoelectric devices (OTEDs) are recognized one of the next generation energy conversion platforms because of their huge potentials for securing electricity continuously from even tiny heat sources in our daily life. The advantage of OTEDs can be attributable to the design freedom in device shapes and the low-cost fabrication by employing solution coating processes at low temperatures. As one of the major OTE materials to date, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has been used, but no study has been yet carried out on its acidity control even though the acidic components in OTEDs can seriously affect the device performance upon operation. Here we demonstrate that the addition of aniline (a weak base) can control the acidity of PEDOT:PSS and enhance the performance of OTEDs. In particular, the vertical OTEDs with aniline-doped PEDOT:PSS films (active area = 1.0 cm2) could continuously generate electricity (0.06 nW) even at low temperatures (<38 °C) when they were mounted on a desk lamp (power = 24 W).

Broadband pH-Sensing Organic Transistors with Polymeric Sensing Layers Featuring Liquid Crystal Microdomains Encapsulated by Di-Block Copolymer Chains.

We report broadband pH-sensing organic field-effect transistors (OFETs) with the polymer-dispersed liquid crystal (PDLC) sensing layers. The PDLC layers are prepared by spin-coating using ethanol solutions containing 4-cyano-4'-pentyl-biphenyl (5CB) and a diblock copolymer (PAA-b-PCBOA) that consists of LC-philic block [poly(4-cyano-biphenyl-4-oxyundecyl acrylate) (PCBOA)] and acrylic acid block [poly(acrylic acid) (PAA)]. The spin-coated sensing layers feature of 5CB microdomains (<5 µm) encapsulated by the PAA-b-PCBOA polymer chains. The resulting LC-integrated-OFETs (PDLC-i-OFETs) can detect precisely and reproducibly a wide range of pH with only small amounts (10-40 µL) of analyte solutions in both static and dynamic perfusion modes. The positive drain current change is measured for acidic solutions (pH < 7), whereas basic solutions (pH > 7) result in the negative change of drain current. The drain current trend in the present PDLC-i-OFET devices is explained by the shrinking-expanding mechanism of the PAA chains in the diblock copolymer layers.

Spin dynamics of isolated donor electrons in phosphorus-doped silicon from high-frequency electron spin resonance.

We present the spin dynamics of isolated donor electrons in phosphorus-doped silicon at low temperature and in a high magnetic field. We performed a steady-state electron spin resonance (ESR) on the sample with a dopant concentration of 6.5 × 10(16) cm(- 3) in a high field of 2.87 T (80 GHz) and at temperatures from 48 down to 1.8 K. As the temperature decreases below 16 K, the resonance spectral line changes from the usual derivative form characteristic of absorptions. Very long spin-lattice relaxation time T(1) at low temperature gives rise to rapid passage effects and results in a dramatic change in the line shape and intensity as a function of temperature. We show that the numerical analysis based on the passage effects well explains the observed spectral changes with temperature. The spin-lattice relaxation time T(1) is derived by numerical fit to the experimental data. We discuss the dynamic nuclear polarization of (31)P nuclear spins which shows up as asymmetric intensities of the hyperfine-split ESR resonance lines.