Cheng-Yin Wang

Stable low-voltage operation top-gate organic field-effect transistors on cellulose nanocrystal substrates.

We report on the performance and the characterization of top-gate organic field-effect transistors (OFETs), comprising a bilayer gate dielectric of CYTOP/Al2O3 and a solution-processed semiconductor layer made of a blend of TIPS-pentacene:PTAA, fabricated on recyclable cellulose nanocrystal-glycerol (CNC/glycerol) substrates. These OFETs exhibit low operating voltage, low threshold voltage, an average field-effect mobility of 0.11 cm(2)/(V s), and good shelf and operational stability in ambient conditions. To improve the operational stability in ambient a passivation layer of Al2O3 is grown by atomic layer deposition (ALD) directly onto the CNC/glycerol substrates. This layer protects the organic semiconductor layer from moisture and other chemicals that can either permeate through or diffuse out of the substrate.

Pyrrole[3,2-d:4,5-d′]bisthiazole-bridged bis(naphthalene diimide)s as electron-transport materials

A series of materials with 2,6-disubstituted-N-alkyl-pyrrole[3,2-d:4,5-d′]bisthiazole (PBTz) with triisopropylsilyl- (TIPS), bromo- and naphthalene diimide (NDI) groups were synthesized. The electronic properties of 2,6-bis-TIPS- and 2,6-dibromo-N-hexyl-PBTz were studied by cyclic voltammetry and by density functional theory (DFT) calculations, and their solid-state packing was examined by the single crystal X-ray structural analysis. DFT calculations and the electrochemical data revealed that this core is both a weak donor and a weak acceptor. Small molecules with bis(NDI)-substituted N-alkyl-PBTz architecture were studied by differential pulse voltammetry, UV-vis absorption spectroscopy, and differential scanning calorimetry, and their electrical properties were examined in n-channel organic field-effect transistors using solution-processed films. The electron mobility value μe as high as 0.13 cm2 V−1 s−1 with a Ion/Ioff ratio of 5 × 105 and threshold voltage Vth = 4.9 V was observed for PBTz-bridged bis(naphthalene diimide) with hexyl chains on pyrrole and NDI nitrogen atoms, while the material with longer dodecyl groups showed μe up to 0.19 cm2 V−1 s−1 with a Ion/Ioff ratio of 7 × 104 and Vth = 7.9 V in a 1 : 1 polystyrene matrix. Finally, compounds with electron-withdrawing acetyl groups at position 6 of the NDI units were examined by electrochemistry and in OFET configurations.

A Study on Reducing Contact Resistance in Solution-Processed Organic Field-Effect Transistors

We report on the reduction of contact resistance in solution-processed TIPS-pentacene (6,13-bis(triisopropylsilylethynyl)pentacene) and PTAA (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]) top-gate bottom-contact organic field-effect transistors (OFETs) by using different contact-modification strategies. The study compares the contact resistance values in devices that comprise Au source/drain electrodes either treated with 2,3,4,5,6-pentafluorothiophenol (PFBT), or modified with an evaporated thin layer of the metal-organic molecular dopant molybdenum tris-[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd)3), or modified with a thin layer of the oxide MoO3. An improved performance is observed in devices modified with Mo(tfd)3 or MoO3 as compared to devices in which Au electrodes are modified with PFBT. We discuss the origin of the decrease in contact resistance in terms of increase of the work function of the modified Au electrodes, Fermi-level pinning effects, and decrease of bulk resistance by electrically doping the organic semiconductor films in the vicinity of the source/drain electrodes.

Engineering the mechanical properties of ultrabarrier films grown by atomic layer deposition for the encapsulation of printed electronics

Direct deposition of barrier films by atomic layer deposition (ALD) onto printed electronics presents a promising method for packaging devices. Films made by ALD have been shown to possess desired ultrabarrier properties, but face challenges when directly grown onto surfaces with varying composition and topography. Challenges include differing nucleation and growth rates across the surface, stress concentrations from topography and coefficient of thermal expansion mismatch, elastic constant mismatch, and particle contamination that may impact the performance of the ALD barrier. In such cases, a polymer smoothing layer may be needed to coat the surface prior to ALD barrier film deposition. We present the impact of architecture on the performance of aluminum oxide (Al2O3)/hafnium oxide (HfO2) ALD nanolaminate barrier films deposited on fluorinated polymer layer using an optical calcium (Ca) test under damp heat. It is found that with increasing polymer thickness, the barrier films with residual tensile stre...

One-dimensional position-sensitive superheated-liquid-droplet in-phantom neutron dosimeter

The one‐dimensional (1D) position‐sensitive superheated‐liquid‐droplet in‐phantom neutron dosimeter incorporating a sensitive volume emulsion has been fabricated, prepared, and tested. The 1D position‐sensitive superheated‐liquid‐droplet dosimeter (SLDD) is fabricated from a 3/8‐in.‐o.d., 1/4‐in.‐i.d., 20‐cm‐long, PlexiglasTM‐walled tube filled with a mixture of superheated‐liquid FreonTM droplets and host medium glycerol solution. Washer‐shaped piezoelectric acoustic transducers are positioned at both ends of the tube; they determine the number and positions of the acoustic events when the superheated‐liquid droplets evaporate upon neutron irradiation. The SLDD is sensitive to a wide range of neutron energy, from thermal (0.0253 eV) up to 10 MeV and higher. The SLDD is irradiated with the 137Cs and 60Co γ sources, as well as a 252Cf neutron source to test for its radiation response and spatial resolution. The SLDD based on the Freon‐134a superheated‐liquid droplets operating at 20 °C and 1 atm is found t...

Experimental investigation of defect-assisted and intrinsic water vapor permeation through ultrabarrier films.

In the development of ultrabarrier films for packaging electronics, the effective water vapor transmission rate is a combination of permeation through pinhole defects and the intrinsic permeation through the actual barrier film. While it is possible to measure the effective permeation rate through barriers, it is important to develop a better understanding of the contribution from defects to the overall effective barrier performance. Here, we demonstrate a method to investigate independently defect-assisted permeation and intrinsic permeation rates by observing the degradation of a calcium layer encapsulated with a hybrid barrier film, that is, prepared using atomic layer deposition (ALD) and plasma enhanced deposition (PECVD). The results are rationalized using an analytical diffusion model to calculate the permeation rate as a function of spatial position within the barrier. It was observed that a barrier film consisting of a PECVD SiN(x) layer combined with an ALD Al2O3/HfO(x) nanolaminate resulted in a defect-assisted water vapor transmission rate (WVTR) of 4.84 × 10(-5) g/m(2) day and intrinsic WVTR of 1.41 × 10(-4) g/m(2) day at 50 °C/85% RH. Due to the low defect density of the tested barrier film, the defect-assisted WVTR was found to be three times lower than the intrinsic WVTR, and an effective (or total) WVTR value was 1.89 × 10(-4) g/m(2) day. Thus, improvements of the barrier performance should focus on reducing the number of defects while also improving the intrinsic barrier performance of the hybrid layer.

Stable organic thin-film transistors

Organic thin-film transistors exhibit an unprecedented level of reliability, bringing them closer to commercialization. Organic thin-film transistors (OTFTs) can be fabricated at moderate temperatures and through cost-effective solution-based processes on a wide range of low-cost flexible and deformable substrates. Although the charge mobility of state-of-the-art OTFTs is superior to that of amorphous silicon and approaches that of amorphous oxide thin-film transistors (TFTs), their operational stability generally remains inferior and a point of concern for their commercial deployment. We report on an exhaustive characterization of OTFTs with an ultrathin bilayer gate dielectric comprising the amorphous fluoropolymer CYTOP and an Al2O3:HfO2 nanolaminate. Threshold voltage shifts measured at room temperature over time periods up to 5.9 × 105 s do not vary monotonically and remain below 0.2 V in microcrystalline OTFTs (μc-OTFTs) with field-effect carrier mobility values up to 1.6 cm2 V−1 s−1. Modeling of these shifts as a function of time with a double stretched-exponential (DSE) function suggests that two compensating aging mechanisms are at play and responsible for this high stability. The measured threshold voltage shifts at temperatures up to 75°C represent at least a one-order-of-magnitude improvement in the operational stability over previous reports, bringing OTFT technologies to a performance level comparable to that reported in the scientific literature for other commercial TFTs technologies.

Self-forming electrode modification in organic field-effect transistors

We report on self-forming electrode modification by mixing 2,3,4,5,6-pentafluorothiophenol (PFBT) directly into the solution of the organic semiconductor prior to film formation on top of existing metal electrodes. During the formation of the semiconductor layer from the mixed solution, PFBT chemisorbs on the underlying source/drain electrodes and modifies their electronic properties. The modification of evaporated silver, gold, or printed silver electrodes with PFBT is analyzed by X-ray photoelectron spectroscopy. The use of this self-forming electrode modification is applied to solution-processed p-channel top-gate 6,13-bis(triisopropylsilylethynyl)pentacene/poly[bis(4-phenyl) (2,4,6-trimethylphenyl) amine] organic field-effect transistors (OFETs) that comprise bare silver or gold source/drain electrodes. The proposed new method simplifies device fabrication while yielding OFETs with a performance level that is comparable to that of reference devices in which the metal electrodes are modified with PFBT prior to the fabrication of the semiconductor layer.

Langmuir–Blodgett Thin Films of Diketopyrrolopyrrole-Based Amphiphiles

We report on two π-conjugated donor-acceptor-donor (D-A-D) molecules of amphiphilic nature, aiming to promote intermolecular ordering and carrier mobility in organic electronic devices. Diketopyrrolopyrrole was selected as the acceptor moiety that was disubstituted with nonpolar and polar functional groups, thereby providing the amphiphilic structures. This structural design resulted in materials with a strong intermolecular order in the solid state, which was confirmed by differential scanning calorimetry and polarized optical microscopy. Langmuir-Blodgett (LB) films of ordered mono- and multilayers were transferred onto glass and silicon substrates, with layer quality, coverage, and intermolecular order controlled by layer compression pressure on the LB trough. Organic field-effect transistors and organic photovoltaics devices with active layers consisting of the amphiphilic conjugated D-A-D-type molecules were constructed to demonstrate that the LB technique is an effective layer-by-layer deposition approach to fabricate self-assembled, ordered thin films.

Highly stable organic field-effect transistors with engineered gate dielectrics(Conference Presentation)

Organic field-effect transistors (OFETs) have the potential to lead to low-cost flexible displays, wearable electronics, and sensors. While recent efforts have focused greatly on improving the maximum charge mobility that can be achieved in such devices, studies about the stability and reliability of such high performance devices are relatively scarce. In this talk, we will discuss the results of recent studies aimed at improving the stability of OFETs under operation and their shelf lifetime. In particular, we will focus on device architectures where the gate dielectric is engineered to act simultaneously as an environmental barrier layer. In the past, our group had demonstrated solution-processed top-gate OFETs using TIPS-pentacene and PTAA blends as a semiconductor layer with a bilayer gate dielectric layer of CYTOP/Al2O3, where the oxide layer was fabricated by atomic layer deposition, ALD. Such devices displayed high operational stability with little degradation after 20,000 on/off scan cycles or continuous operation (24 h), and high environmental stability when kept in air for more than 2 years, with unchanged carrier mobility. Using this stable device geometry, simple circuits and sensors operating in aqueous conditions were demonstrated. However, the Al2O3 layer was found to degrade due to corrosion under prolonged exposure in aqueous solutions. In this talk, we will report on the use of a nanolaminate (NL) composed of Al2O3 and HfO2 by ALD to replace the Al2O3 single layer in the bilayer gate dielectric use in top-gate OFETs. Such OFETs were found to operate under harsh condition such as immersion in water at 95 °C. This work was funded by the Department of Energy (DOE) through the Bay Area Photovoltaics Consortium (BAPVC) under Award Number DE-EE0004946.