pn-Heterojunction effects of perylene tetracarboxylic diimide derivatives on pentacene field-effect transistor

Temperature Sensors Based on Organic Field-Effect Transistors

The rapid growth of wearable electronics, Internet of Things, smart packaging, and advanced healthcare technologies demand a large number of flexible, thin, lightweight, and ultralow-cost sensors. The accurate and precise determination of temperature in a narrow range (~0–50 °C) around ambient temperatures and near-body temperatures is critical for most of these applications. Temperature sensors based on organic field-effect transistors (OFETs) have the advantages of low manufacturing cost, excellent mechanical flexibility, easy integration with other devices, low cross-sensitivity, and multi-stimuli detectability and, therefore, are very suitable for the above applications. This article provides a timely overview of research progress in the development of OFET-based temperature sensors. First, the working mechanism of OFETs, the fundamental theories of charge transport in organic semiconductors, and common types of OFET temperature sensors based on the sensing element are briefly introduced. Next, notable advances in the development of OFET temperature sensors using small-molecule and polymer semiconductors are discussed separately. Finally, the progress of OFET temperature sensors is summarized, and the challenges associated with OFET temperature sensors and the perspectives of research directions in this field are presented.

Balanced Ambipolar Charge Transport in Phenacene/Perylene Heterojunction-Based Organic Field-Effect Transistors

Electronic devices relying on the combination of different conjugated organic materials are considerably appealing for their potential use in many applications such as photovoltaics, light emission, and digital/analog circuitry. In this study, the electrical response of field-effect transistors achieved through the evaporation of picene and PDIF-CN₂ molecules, two well-known organic semiconductors with remarkable charge transport properties, was investigated. With the main goal to get a balanced ambipolar response, various device configurations bearing double-layer, triple-layer, and codeposited active channels were analyzed. The most suitable choices for the layer deposition processes, the related characteristic parameters, and the electrode position were identified to this purpose. In this way, ambipolar organic field-effect transistors exhibiting balanced mobility values exceeding 0.1 cm² V^-1 s^-1 for both electrons and holes were obtained. These experimental results highlight also how the combination between picene and PDIF-CN₂ layers allows tuning the threshold voltages of the p-type response. Scanning Kelvin probe microscopy (SKPM) images acquired on picene/PDIF-CN₂ heterojunctions suggest the presence of an interface dipole between the two organic layers. This feature is related to the partial accumulation of space charge at the interface being enhanced when the electrons are depleted in the underlayer.

Picene and PTCDI based solution processable ambipolar OFETs

Facile and efficient solution-processed bottom gate top contact organic field-effect transistor was fabricated by employing the active layer of picene (donor, D) and N,N'-di(dodecyl)-perylene-3,4,9,10-tetracarboxylic diimide (acceptor, A). Balanced hole (0.12 cm2/Vs) and electron (0.10 cm2/Vs) mobility with Ion/off of 104 ratio were obtained for 1:1 ratio of D/A blend. On increasing the ratio of either D or A, the charge carrier mobility and Ion/off ratio improved than that of the pristine molecules. Maximum hole (µmax,h) and electron mobilities (µmax,e) were achieved up to 0.44 cm2/Vs for 3:1 and 0.25 cm2/Vs for 1:3, (D/A) respectively. This improvement is due to the donor phase function as the trap center for minority holes and decreased trap density of the dielectric layer, and vice versa. High ionization potential (- 5.71 eV) of 3:1 and lower electron affinity of (- 3.09 eV) of 1:3 supports the fine tuning of frontier molecular orbitals in the blend. The additional peak formed for the blends at high negative potential of - 1.3 V in cyclic voltammetry supports the molecular level electronic interactions of D and A. Thermal studies supported the high thermal stability of D/A blends and SEM analysis of thin films indicated their efficient molecular packing. Quasi-π-π stacking owing to the large π conjugated plane and the crystallinity of the films are well proved by GIXRD. DFT calculations also supported the electronic distribution of the molecules. The electron density of states (DOS) of pristine D and A molecules specifies the non-negligible interaction coupling among the molecules. This D/A pair has unlimited prospective for plentiful electronic applications in non-volatile memory devices, inverters and logic circuits.

Transparent and Colorless Polyimides Containing Multiple Trifluoromethyl Groups as Gate Insulators for Flexible Organic Transistors with Superior Electrical Stability

A suitable insulating polymer material that is compatible with the fabrication process of organic transistors and has excellent electrical properties is critically required for the next-generation flexible organic electronics. In this study, using a one-step polymerization method, we synthesized two different solution-processable polyimides (PIs) incorporated with abundant trifluoromethyl groups. Not only were the two resulting PIs-termed 6FDA-6FDAM-PI and 6FDA-TFMB-PI-well soluble in organic solvents, but also they showed transparent and colorless optical properties. The fluorinated PI films showed smooth surface topographies and surface energy values that were appropriate for their use in bottom-gate organic transistors. Organic transistors separately fabricated with 6FDA-6FDAM-PI and 6FDA-TFMB-PI as the gate insulators showed excellent device performance and electrical stability under various testing conditions, especially for pentacene-based devices. The excellent performance of the devices with fluorinated PIs was attributed to the enhanced microstructure of the organic semiconductor and the fluorine-rich characteristic of the underlying gate insulator. Furthermore, organic complementary circuits including the basic logic gates of NOT, NOR, and NAND were demonstrated using these devices.

Photoconductance from Exciton Binding in Molecular Junctions

We report on a theoretical analysis and experimental verification of a mechanism for photoconductance, the change in conductance upon illumination, in symmetric single-molecule junctions. We demonstrate that photoconductance at resonant illumination arises due to the Coulomb interaction between the electrons and holes in the molecular bridge, so-called exciton-binding. Using a scanning tunneling microscopy break junction technique, we measure the conductance histograms of perylene tetracarboxylic diimide (PTCDI) molecules attached to Au-electrodes, in the dark and under illumination, and show a significant and reversible change in conductance, as expected from the theory. Finally, we show how our description of the photoconductance leads to a simple design principle for enhancing the performance of molecular switches.