A uniform ultrathin polymer film is deposited over a large area with molecularlevel precision by the simple wire-wound bar-coating method. The bar-coated ultrathin films not only exhibit high transparency of up to 90% in the visible wavelength range but also high charge carrier mobility with a high degree of percolation through the uniformly covered polymer nanofibrils. They are capable of realizing highly sensitive multigas sensors and represent the first successful report of ethylene detection using a sensor based on organic field-effect transistors.

Understanding the sensing mechanism in organic chemical sensors is essential for improving the sensing performance such as detection limit, sensitivity, and other response/recovery time, selectivity, and reversibility for real applications. Here, we report a highly sensitive printed ammonia (NH) gas sensor based on organic thin film transistors (OTFTs) with fluorinated difluorobenzothiadiazole-dithienosilole polymer (PDFDT). These sensors detected NH down to 1 ppm with high sensitivity (up to 56%) using bar-coated ultrathin (<4 nm) PDFDT layers without using any receptor additives. The sensing mechanism was confirmed by cyclic voltammetry, hydrogen/fluorine nuclear magnetic resonance, and UV/visible absorption spectroscopy. PDFDT-NH interactions comprise hydrogen bonds and electrostatic interactions between the PDFDT polymer backbone and NH gas molecules, thus lowering the highest occupied molecular orbital levels, leading to hole trapping in the OTFT sensors. Additionally, density functional theory calculations show that gaseous NH molecules are captured via cooperation of fluorine atoms and dithienosilole units in PDFDT. We verified that incorporation of functional groups that interact with a specific gas molecule in a conjugated polymer is a promising strategy for producing high-performance printed OTFT gas sensors.

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Although n-type transparent conductors have been commercialized with high optical transmittance and electrical conductivity, the realization of their p-type counterparts has been a challenging problem. Here, we report the synthesis of a highly conductive transparent p-type sulfur-doped CuI (CuI:S) thin film using a liquid-iodination method with a thiol additive. The CuI:S film shows a remarkably high electrical conductivity of 511 S cm–1 with an optical transmittance of greater than 80%. Furthermore, additional hole doping of CuI:S with H2O2 treatment improves the electrical conductivity to 596 S cm–1. Consequently, CuI:S exhibits a record-high figure of merit (FOM) value of 63,000 M Ω–1 (73,000 M Ω–1 with H2O2 treatment), which is ∼370% (∼430% with H2O2 treatment) higher than the previously reported record-high FOM value. The highly conducting CuI:S electrode is successfully applied as transparent conducting electrodes of the organic light-emitting diode and transparent p-type thin-film transistor. The liquid-iodination chemical method with unconventional control of the reaction parameters can be generalized to produce high-quality metal halide thin films, allowing them to be applicable for transparent electronics and optoelectronics.

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Organic semiconductors (OSCs) are promising sensing materials for printed flexible gas sensors. However, OSCs are unstable in the humid air, which limits the realization of gas sensors for multiple usages. In this paper, we report a facile and effective way to improve the air stability of an OSC film to realize multiple reversibly used printed gas sensors by adding molecular additives. The tetracyanoquinodimethane (TCNQ) or 4-aminobenzonitrile (ABN) additives effectively prevent adsorption of moisture from the air on the OSC layer, thereby providing a stable gas sensor operation. The organic field-effect transistor (OFET)-based indacenodithiophene-co-benzothiadiazole with TCNQ or ABN shows highly reliable ammonia (NH 3 ) gas sensing up to 10 ppm in air, with 23.14% sensitivity, and the gas sensor signal can recover up to 100%. In particular, the stability of gas detection is greatly improved by the additives, which can be performed in the air for 16 days. The result indicates that the elimination of moisture trapped in OSCs with molecule additives is critical in the improvement of device air/operational stabilities and the achievement of highperformance OFET-based gas sensors.