Flexible, high mobility short-channel organic thin film transistors and logic circuits based on 4H-21DNTT.

The low mobility and large contact resistance in organic thin-film transistors (OTFTs) are the two major limiting factors in the development of high-performance organic logic circuits. Here, solution-processed high-performance OTFTs and circuits are reported with a polymeric gate dielectric and 6,6 bis (trans-4-butylcyclohexyl)-dinaphtho[2,1-b:2,1-f]thieno[3,2-b]thiophene (4H-21DNTT) for the organic semiconducting layer. By optimizing and controlling the fabrication conditions, a high saturation mobility of 8.8 cm2 V-1 s-1 was demonstrated as well as large on/off ratios (> 106) for relatively short channel lengths of 15 µm and an average carrier mobility of 10.5 cm2 V-1 s-1 for long channel length OTFTs (> 50 µm). The pseudo-CMOS inverter circuit with a channel length of 15 µm exhibited sharp switching characteristics with a high signal gain of 31.5 at a supply voltage of 20 V. In addition to the inverter circuit, NAND logic circuits were further investigated, which also exhibited remarkable logic characteristics, with a high gain, an operating frequency of 5 kHz, and a short propagation delay of 22.1 µs. The uniform and reproducible performance of 4H-21DNTT OTFTs show potential for large-area, low-cost real-world applications on industry-compatible bottom-contact substrates.

Reflectance-Based Organic Pulse Meter Sensor for Wireless Monitoring of Photoplethysmogram Signal.

This paper compares the structural design of two organic biosensors that minimize power consumption in wireless photoplethysmogram (PPG) waveform monitoring. Both devices were fabricated on the same substrate with a red organic light-emitting diode (OLED) and an organic photodiode (OPD). Both were designed with a circular OLED at the center of the device surrounded by OPD. One device had an OLED area of 0.06 cm2, while the other device had half the area. The gap distance between the OLED and OPD was 1.65 mm for the first device and 2 mm for the second. Both devices had an OPD area of 0.16 cm2. We compared the power consumption and signal-to-noise ratio (SNR) of both devices and evaluated the PPG signal, which was successfully collected from a fingertip. The reflectance-based organic pulse meter operated successfully and at a low power consumption of 8 µW at 18 dB SNR. The device sent the PPG waveforms, via Bluetooth low energy (BLE), to a PC host at a maximum rate of 256 kbps data throughput. In the end, the proposed reflectance-based organic pulse meter reduced power consumption and improved long-term PPG wireless monitoring.

Comparative Design Study for Power Reduction in Organic Optoelectronic Pulse Meter Sensor.

This paper demonstrated a new design structure for minimizing the power consumption of a pulse meter. Monolithic devices composed of a red (625 nm) organic light-emitting diode (OLED) and an organic photodiode (OPD) were fabricated on the same substrate. Two organic devices were designed differently. One had a circle-shaped OLED in the center of the device and was surrounded by the OPD, while the other had the opposite structure. The external quantum efficiency (EQE) of the OLED and the OPD were 7% and 37%, respectively. We evaluated and compared the signal-to-noise ratio (SNR) of the photoplethysmogram (PPG) signal on different parts of the body and successfully acquired clear PPG signals at those positions, where the best signal was obtained from the fingertip at a SNR of about 62 dB. The proposed organic pulse meter sensor was operated successfully with a power consumption of 0.1 mW. Eventually, the proposed organic biosensor reduced the power consumption and improved the capability of the pulse meter for long-term use.

Investigating the Influence of Alkyl Chain Length in Poly(3-alkylthiophene)s Over the Thin Film Morphology by Optical and Electrical Characterization.

This paper studies the influence of alkyl-chain length in poly(3-alkylthiophene)s over the morphology of thin films and electrical parameters of the devices based on it. Regioregular poly(3-hexylthiophene) and poly(3-octylthiophene) were chosen as the semiconducting materials for the study. The morphological variations were studied by absorption spectroscopy, photoluminescence spectroscopy and X-ray diffraction study. The absorption and photoluminescence showed decreased coplanarity of main chain in poly(3-octylthiophene) over poly(3-hexylthiophene) and which was later confirmed using X-ray diffraction studies which clearly showed increased interchain spacing in case of poly(3-octylthiophene). The schottky diodes fabricated using these materials showed decreased mobility in poly(3-octylthiophene) based diodes as measured by space-charge limiting current method and photo-induced charge carrier extraction by linearly increasing voltage technique. Moreover, we observed a negative field dependence of mobility at room temperature in both the devices and attributed this to the presence of dominant positional disorder in poly(3-alkylthiophene)s. Furthermore, the photocurrent dependence on electric field too showed inferior mobility of poly(3-octylthiophene) based diodes.

Influence of Parametric Variations on Hydrothermal Growth of ZnO Nanostructures for Hybrid Polymer/ZnO Based Photodetector.

Dumbbell and flower like ZnO nano-crystals were grown via hydrothermal process. The as-prepared dumbbells, with length of 0.8-10 µm and edge length of 0.3-0.8 µm possess a hexagonal structure, while flowers with lengths ranging from 1-6 µm with hexagonal structure have been synthesized. The effect of temperature, solution concentration and growth time on the size and shapes of the ZnO nanostructures has been studied using Field emission scanning electron microscope (FESEM) and X-ray diffractometer (XRD). Further the optical properties of nanostructures were investigated by Photoluminescence (PL) spectroscopy, which shows emission in UV and visible regions. From Diffused reflectance spectroscopic analysis (DRA) it was observed that ZnO nanodumbbells and nanoflowers have a direct band gap of 3.27 eV and 3.25 eV respectively. The I-V plot showed dependence of current values under dark and illumination over the annealing temperature during the growth stage. Thus we report a control over the shape and dimension of nanostructures by varying various parameters having implications for (opto)electronic devices.

Effect of Addition of KI on the Hydrothermal Growth of ZnO Nanostructures Towards Hybrid Optoelectronic Device Applications.

We report the structural and optoelectronic properties of Zinc oxide (ZnO) nanostructures prepared by hydrothermal method. The morphological, structural and optical properties of the grown ZnO nanostructures were investigated using X-ray diffraction (XRD), scanning electron microscope (SEM) and photoluminescence spectroscopy (PL) respectively. Upon addition of relatively small amount of KI during the in-situ hydrothermal growth the nanorods were formed, further increasing the concentration led to increased diameter of these nanorods and finally at relatively higher concentration of KI, ZnO nanosheets were formed. Later these structures were used to fabricate bi-layer ZnO/P3HT based hybrid photodiode. Subsequent hybrid photodiode measurement with ZnO nanorods and ZnO nanosheets indicated that the nanosheets exhibited improved photodiode response. Compared to the ZnO nanorod/P3HT devices, the optimized photodiode with the dense ZnO nanosheets/P3HT have shown significant increase in the rectification ratio and the photosenstivity from 3.21 to 1420 and from 5.85 to 1330 respectively. The enhanced photodiode response of bi-layered devices consisting of ZnO nanosheets indicated that optimizing the shape and size of ZnO nanostructures had a significant influence on the overall photocurrent and the observed results have been explained on the basis of reduction in the defect density with pronounced absorption in the UV region, thus leading to improved transmission of light in the visible range through these layers.

Two Dimensional Optoelectronic Simulation Based Comparison of Top and Bottom Contact Organic Phototransistors.

This paper deals with two-dimensional optoelectronic simulation based study of organic phototransistors. The top and bottom contact device structures were simulated to compare their characteristics under dark and under light illumination conditions. It was observed that the performance of these phototransistors can be tuned by shining a monochromatic light of given intensity. Further, the photosensitivity of the top and bottom contact phototransistors was compared and it was concluded that the difference in their photocurrent can be attributed to their different carrier extraction areas, which also leads to the difference in their contact resistances. Our results clearly indicate that the top contact structure has higher photosensitivity compared to the bottom contact structure. Furthermore, we simulated the dependence of photosensitivity on gate voltage, light intensity and channel length of the two structures and found the presence of both photoconductive and photovoltaic effects governing performance of organic phototransistors. Finally by simply scaling the device channel length contact resistance in the bottom and top contact organic photo transistors were estimated under different light illumination conditions.