Balanced Ambipolar Organic Field-Effect Transistors by Polymer Preaggregation

Ambipolar blend-based organic electrochemical transistors and inverters

CMOS-like circuits in bioelectronics translate biological to electronic signals using organic electrochemical transistors (OECTs) based on organic mixed ionic-electronic conductors (OMIECs). Ambipolar OECTs can reduce the complexity of circuit fabrication, and in bioelectronics have the major advantage of detecting both cations and anions in one device, which further expands the prospects for diagnosis and sensing. Ambipolar OMIECs however, are scarce, limited by intricate materials design and complex synthesis. Here we demonstrate that judicious selection of p- and n-type materials for blend-based OMIECs offers a simple and tunable approach for the fabrication of ambipolar OECTs and corresponding circuits. These OECTs show high transconductance and excellent stability over multiple alternating polarity cycles, with ON/OFF ratios exceeding 10³ and high gains in corresponding inverters. This work presents a simple and versatile new paradigm for the fabrication of ambipolar OMIECs and circuits with little constraints on materials design and synthesis and numerous possibilities for tunability and optimization towards higher performing bioelectronic applications.

Ambipolar organic electrochemical transistors simplify bioelectronics circuitry but are challenging due to complicated material design and synthesis. Here, the authors demonstrate that p- and n-type blends offer a simple and tuneable approach for the fabrication of ambipolar devices and circuits.

Pyridine-Containing Donor-Acceptor Diarylnitroxides: Noncovalent Stabilization of the Redox States

A series of new pyridyl- or 2-pyridyloxide-containing donor-acceptor diarylnitroxides was obtained and characterized; high stability of the ortho-2-pyridyl-containing diarylnitroxides was determined by kinetic measurements (τ_1/2 =1733 h in benzene). Comparative voltammetric study of new nitroxides and their analog in which the Py replaces the Ph group revealed both through-bond and through-space stabilization of the NO redox states with the pyridyl/ 2-pyridyloxide moiety, providing reversibility of both oxidation and reduction processes. Adaptive conformational behavior of new pyridyl/pyridyloxide containing nitroxides upon one-electron oxidation and reduction was confirmed by DFT calculations. Stimuli-responsive conformational changes allow switching on/off dispersion and electrostatic interactions within the molecule and increase stability of the redox states. Spectroelectrochemical measurements provided experimental evidence for reversibility of the through-space stabilization of the oxidized state of the nitroxides with the neighboring pyridine lone pair.

The Quinonoid Zwitterion Interlayer for the Improvement of Charge Carrier Mobility in Organic Field-Effect Transistors

The interface between the semiconductor and the dielectric layer plays a crucial role in organic field-effect transistors (OFETs) because it is at the interface that charge carriers are accumulated and transported. In this study, four zwitterionic benzoquinonemonoimine dyes featuring alkyl and aryl N-substituents were used to cover the dielectric layers in OFET structures. The best interlayer material, containing aliphatic side groups, increased charge carrier mobility in the measured systems. This improvement can be explained by the reduction in the number of the charge carrier trapping sites at the dielectric active layer interface from 10¹⁴ eV⁻¹ cm⁻² to 2 × 10¹³ eV⁻¹ cm⁻². The density of the traps was one order of magnitude lower compared to the unmodified transistors. This resulted in an increase in charge carrier mobility in the tested poly [2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno [3,2-b]thiophene)] (DPPDTT)-based transistors to 5.4 × 10⁻¹ cm² V⁻¹ s⁻¹.

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.

High performance p-channel and ambipolar OFETs based on imidazo[4,5-f]-1,10-phenanthroline-triarylamines

A series of phenanthroline functionalized triarylamines (TAA) has been designed and synthesised to evaluate their OFET characteristics. Solution processed OFET devices have exhibited p-channel/ambipolar behaviour with respect to the substituents, in particular methoxyphenyl substitution resulted with highest mobility (μ h) up to 1.1 cm2 V-1 s-1 with good I on/off (106) ratio. These compounds can be potentially utilized for the fabrication of electronic devices.

Organic-based inverters: basic concepts, materials, novel architectures and applications

The review article covers the materials and techniques employed to fabricate organic-based inverter circuits and highlights their novel architectures, ground-breaking performances and potential applications.

Controlling the pore size in conjugated polymer films via crystallization-driven phase separation

A wide range of possible applications in sensors and optoelectronic devices have focused considerable attention on porous membranes made of semi-conducting polymers. In this study, porous films of poly(3-hexylthiophene) (P3HT) were conveniently constructed through spin-coating of solutions of a blend of P3HT and polyethylene glycol (PEG). Pores were formed by phase separation driven simultaneously by incompatibility and crystallization. The influence of the polymer concentration (c), molecular weight (Mn) and spin-coating temperature (Tsp) on the pore size and structure was investigated. With increasing c from 0.5 to 5.0 wt%, the pore diameter (d) varied from ≈1.3 μm to ≈38 μm. Similarly, we observed a substantial increase of d with increasing Mn of PEG, while changing Mn of P3HT did not affect d. Micron- and nano-scale pores coexisted in porous P3HT films. While incompatibility of P3HT and PEG caused the formation of nano-pores, micron-scale pores resulted from crystallization in the PEG-rich domains by forcing PEG molecules to diffuse from the surrounding PEG-P3HT blend region to the crystal growth front.

Fully rubbery integrated electronics from high effective mobility intrinsically stretchable semiconductors

An intrinsically stretchable rubbery semiconductor with high mobility is critical to the realization of high-performance stretchable electronics and integrated devices for many applications where large mechanical deformation or stretching is involved. Here, we report fully rubbery integrated electronics from a rubbery semiconductor with a high effective mobility, obtained by introducing metallic carbon nanotubes into a rubbery semiconductor composite. This enhancement in effective carrier mobility is enabled by providing fast paths and, therefore, a shortened carrier transport distance. Transistors and their arrays fully based on intrinsically stretchable electronic materials were developed, and they retained electrical performances without substantial loss when subjected to 50% stretching. Fully rubbery integrated electronics and logic gates were developed, and they also functioned reliably upon mechanical stretching. A rubbery active matrix based elastic tactile sensing skin to map physical touch was demonstrated to illustrate one of the applications.

Self-Assembled PCBM Nanosheets: A Facile Route to Electronic Layer-on-Layer Heterostructures

We report on the self-assembly of semicrystalline [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) nanosheets at the interface between a hydrophobic solvent and water, and utilize this opportunity for the realization of electronically active organic/organic molecular heterostructures. The self-assembled PCBM nanosheets can feature a lateral size of >1 cm² and be transferred from the water surface to both hydrophobic and hydrophilic surfaces using facile transfer techniques. We employ a transferred single PCBM nanosheet as the active material in a field-effect transistor (FET) and verify semiconductor function by a measured electron mobility of 1.2 × 10^-2 cm² V^-1 s^-1 and an on-off ratio of ∼1 × 10⁴. We further fabricate a planar organic/organic heterostructure with the p-type organic semiconductor poly(3-hexylthiophene-2,5-diyl) as the bottom layer and the n-type PCBM nanosheet as the top layer and demonstrate ambipolar FET operation with an electron mobility of 8.7 × 10^-4 cm² V^-1 s^-1 and a hole mobility of 3.1 × 10^-4 cm² V^-1 s^-1.

Realizing Large-Area Arrays of Semiconducting Fullerene Nanostructures with Direct Laser Interference Patterning

We present a laser interference patterning method for the facile fabrication of large-area and high-contrast arrays of semiconducting fullerene nanostructures, which does not rely on a tedious application of sacrificial photoresists or photomasks. A solution-deposited phenyl-C₆₁-butyric acid methyl ester (PCBM) fullerene thin film is exposed to a spatially modulated illumination intensity, as realized by a two-beam laser interference. The PCBM molecules exposed to strong intensity are photochemically transformed into a low-solubility dimeric state, so that the nontransformed PCBM molecules can be selectively removed in a subsequent solution-based development step. Following brief exposure to green laser light (λ = 532 nm, t = 5 s, p = 0.17 W cm^-2) in the designed two-beam interference setup, and a 1 min development in a tuned acetone-chloroform solution, we realize well-defined and ordered PCBM nanostripe patterns with a fwhm line width of ∼200 nm and a repetition rate of ∼2.900 lines mm^-1 over a large area of 1 cm². We demonstrate that a desired high contrast is effectuated because the initial PCBM-dimer transformation rate is dependent on the square of the illumination intensity. The semiconducting functionality of the patterned fullerene is verified in a field-effect transistor experiment, where a typical PCBM nanostripe featured an electron mobility of 5.3 × 10^-3 cm² V^-1 s^-1 and an on/off ratio of 3 × 10³.

Enhancing ambipolar carrier transport of black phosphorus field-effect transistors with Ni–P alloy contacts

High-performance ambipolar black phosphorus field-effect transistors with low-resistance ohmic contacts were achieved via low-temperature vacuum annealing.