Highly Reliable Organic Field-Effect Transistors with Molecular Additives for a High-Performance Printed Gas Sensor.

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 (NH3) 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 high-performance OFET-based gas sensors.

Towards efficient and stable perovskite solar cells employing non-hygroscopic F4-TCNQ doped TFB as the hole-transporting material.

Designing an efficient and stable hole transport layer (HTL) material is one of the essential ways to improve the performance of organic-inorganic perovskite solar cells (PSCs). Herein, for the first time, an efficient model of a hole transport material (HTM) is demonstrated by optimized doping of a conjugated polymer TFB (poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl)diphenylamine)]) with a non-hygroscopic p-type dopant F4-TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) for high-efficiency PSCs. The PSC with the F4-TCNQ doped TFB exhibits the best power conversion efficiency (PCE) of 17.46%, which surpasses that of the reference devices, i.e., 16.64 (LiTFSI + TBP-doped Spiro-OMeTAD as the HTM) and 11.01% (LiTFSI + TBP-doped TFB as the HTM). F4-TCNQ doped TFB was believed to favor efficient charge and energy transfer between the perovskite and the hole transport layer and to reduce charge recombination as evidenced by steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) analysis. Moreover, the hydrophobic nature of F4-TCNQ contributed to enhancing the stability of the device under ambient conditions with a RH of 45%. The device reported herein retained ca. 80% of its initial efficiency after 10 days, significantly superior to both LiTFSI + TBP-doped Spiro-OMeTAD (ca. 30%) and LiTFSI + TBP-doped TFB (ca. 10%) based counterparts. This simple yet novel strategy paves the way for demonstrating a promising route for a wide range of highly efficient solar cells and other photovoltaic applications.

Spontaneous Doping at the Polymer-Polymer Interface for High-Performance Organic Transistors.

Low- k amorphous fluorinated polymers such as poly(perfluoroalkenylvinyl ether) (CYTOP) have widely been used as gate dielectrics for organic field-effect transistors (OFETs) because of their strong hydrophobicity to prevent the penetration of moisture and other contaminants and their perfect solvent orthogonality with organic semiconductors. Here, we report a new functionality of the fluorinated low- k polymer dielectrics, which is spontaneous p doping at the dielectric-semiconductor interface in OFETs. This functionality makes the ambipolar charge transport a unipolar p type. In the OFETs based on indacenodithiophene- co-benzothiadiazole and diketopyrrolopyrrole-thieno[3,2- b]thiophene, the charge transport is obviously ambipolar when paired with common polymer dielectrics such as poly(methyl methacrylate); however, it is perfectly modulated to the unipolar p type by applying the fluorinated dielectrics of CYTOP and poly(tetrafluoroethylene) (Teflon). We propose that this modulation of charge transport results from the rearrangement of C-F bonds at the interface between the fluorine-containing dielectrics and the conjugated polymer semiconductors by proper thermal annealing. These well-aligned dipole moments lead to an abrupt downshift of the Fermi level of the semiconductor toward the highest occupied molecular orbitals near the dielectric-semiconductor interface, which provides a p-doping effect on the channel transport and results in unipolar p-type characteristics in the composed OFETs. This study reveals a new functionality of the fluorinated dielectrics for future organic electronics.

Design of New Isoindigo-Based Copolymer for Ambipolar Organic Field-Effect Transistors.

We report the synthesis of a new conjugated polymer composed of isoindigo (IID) and 2,3-bis[thiophenyl-2-yl]thiophene acrylonitrile (CNTVT) subunits for high-performance n-type organic field-effect transistors (OFETs). To realize high electron mobility for the IID-based conjugated polymer, an electron-withdrawing nitrile group is incorporated into the vinylene unit, thereby shifting the energy of the lowest unoccupied molecular orbital for efficient electron injection from Au electrodes without disrupting the backbone planarity. Uniaxially aligned IID24-CNTVT-conjugated polymer films for efficient intramolecular charge transport are achieved by off-center spin-coating from preaggregated solutions. To obtain its stable preaggregation in solution, a binary solvent system (a mixture of good and bad solvents) chosen with the assistance of Hansen solubility parameter simulation is used. Through this process, highly aligned IID24-CNTVT films are obtained by off-center spin coating from a solvent mixture of 9:1 dichlorobenzene/2-methoxyethanol as the good and bad solvents, respectively. The properties of the aligned IID24-CNTVT films are characterized with various analytical techniques, including UV-visible absorption spectroscopy, angle-resolved near-edge X-ray absorption fine structure spectroscopy, and grazing-incidence wide-angle X-ray scattering. Top-gate/bottom-contact OFETs with IID24-CNTVT films aligned in the direction of charge transport exhibit a high-electron field-effect mobility of 0.83 ± 0.13 cm2/V·s.

Toward High Conductivity of Electrospun Indium Tin Oxide Nanofibers with Fiber Morphology Dependent Surface Coverage: Postannealing and Polymer Ratio Effects.

High electrical conductivity of metal oxide thin films needs uniform surface coverage, which has been the issue for the thin films based on electrospun nanofibers (NFs) that have advantage over the sputtered/spin-coated films with respect to large surface area and mechanical flexibility. Herein, we investigated a reduction in the sheet resistance of electrospun indium tin oxide (ITO) NF films with improved surface coverage. We found that the surface coverage depends significantly on the electrospinnable polymer concentration in the precursor solutions, especially after post-hot-plate annealing following the infrared radiation furnace treatment. The postannealing process increases crystallinity and oxygen vacancies. However, with a higher PVP content, it makes the surface of ITO NFs more prominently rough as a result of the formation of larger sphere-shaped ITO particles on the NF surface, which gives rise to poor surface coverage. A less poly(vinylpyrrolidone) (PVP) content in ITO NF films by electrospinning for short deposition times was found to improve surface coverage even after postannealing. The sheet resistance notably decreases, down to as low as 350 Ω/sq, with a high transmittance of over 90%. Our study provides an understanding on how to achieve high electrical conductivity of ITO NF films with high surface coverage, which can be utilized for the optoelectronic and sensing applications.