Organic Field-Effect Transistors with a Bilayer Gate Dielectric Comprising an Oxide Nanolaminate Grown by Atomic Layer Deposition.

We report on top-gate OFETs with a bilayer gate dielectric comprising an Al2O3 /HfO2 nanolaminate layer grown by atomic layer deposition and an amorphous fluoro-polymer layer (CYTOP). Top-gate OFETs display average carrier mobility values of 0.9 ± 0.2 cm2/(V s) and threshold voltage values of -1.9 ± 0.5 V and high operational and environmental stability under different environmental conditions such as damp air at 50 °C (80% relative humidity) and prolonged immersion in water at a temperature up to 95 °C.

Self-(Un)rolling Biopolymer Microstructures: Rings, Tubules, and Helical Tubules from the Same Material.

We have demonstrated the facile formation of reversible and fast self-rolling biopolymer microstructures from sandwiched active-passive, silk-on-silk materials. Both experimental and modeling results confirmed that the shape of individual sheets effectively controls biaxial stresses within these sheets, which can self-roll into distinct 3D structures including microscopic rings, tubules, and helical tubules. This is a unique example of tailoring self-rolled 3D geometries through shape design without changing the inner morphology of active bimorph biomaterials. In contrast to traditional organic-soluble synthetic materials, we utilized a biocompatible and biodegradable biopolymer that underwent a facile aqueous layer-by-layer (LbL) assembly process for the fabrication of 2D films. The resulting films can undergo reversible pH-triggered rolling/unrolling, with a variety of 3D structures forming from biopolymer structures that have identical morphology and composition.

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.

Tetracyano isoindigo small molecules and their use in n-channel organic field-effect transistors.

N,N'-Dihexyl-6,6'-dicyanoisoindigo, N,N'-didecyl-5,5',6,6'-tetracyanoisoindigo, N,N'-dihexyl-5,5',6,6'-tetracyanoisoindigo, and N,N'-dihexyl-5,5',6,6'-tetracyanothienoisoindigo have been synthesised in moderate yields by the reaction of corresponding di and tetrabromo species with CuCN, with microwave heating leading to higher yields and fewer side products for the tetrasubstituted species. Di- and tetracyano substitution anodically shifts the molecular reduction potential relative to the unsubstituted cores by ca. 0.4 and 0.8 V, respectively, with the resultant values for the tetracyano derivatives (-0.58 to -0.67 V vs. FeCp2(+/0)) suggesting the possibility of air-stable electron transport. All the synthesised cyano derivatives operate in n-channel OFETs, while the tetrabromothienoisoindigo derivative functions in a p-channel transistor. The tetracyanothienoisoindigo derivative exhibits the highest field-effect electron mobility values - up to 0.04 and 0.09 cm(2) V(-1) s(-1) in spin-coated and inkjet-printed devices respectively - and OFETs incorporating this compound have been shown to operate in air without significant degradation of their mobility values in the saturation regime.

Organic photovoltaic cells with stable top metal electrodes modified with polyethylenimine.

Efficient organic photovoltaic cells (OPV) often contain highly reactive low-work-function calcium electron-collecting electrodes. In this work, efficient OPV are demonstrated in which calcium electrodes were avoided by depositing a thin layer of the amine-containing nonconjugated polymer, polyethylenimine (PEIE), between the photoactive organic semiconductor layer and stable metal electrodes such as aluminum, silver, or gold. Devices with structure ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/poly(3-hexylthiophene):indene-C60-bis-adduct (P3HT:ICBA)/PEIE/Al demonstrated overall photovoltaic device performance comparable to devices containing calcium electron-collecting electrodes, ITO/PEDOT:PSS/P3HT:ICBA/Ca/Al, with open-circuit voltage of 775±6 mV, short-circuit current density of 9.1±0.5 mA cm(-2), fill factor of 0.65±0.01, and power conversion efficiency of 4.6±0.3%, averaged over 5 devices at 1 sun.

Systematic reliability study of top-gate p- and n-channel organic field-effect transistors.

We report on a systematic investigation on the performance and stability of p-channel and n-channel top-gate OFETs, with a CYTOP/Al2O3 bilayer gate dielectric, exposed to controlled dry oxygen and humid atmospheres. Despite the severe conditions of environmental exposure, p-channel and n-channel top-gate OFETs show only minor changes of their performance parameters without undergoing irreversible damage. When correlated with the conditions of environmental exposure, these changes provide new insight into the possible physical mechanisms in the presence of oxygen and water. Photoexcited charge collection spectroscopy experiments provided further evidence of oxygen and water effects on OFETs. Top-gate OFETs also display outstanding durability, even when exposed to oxygen plasma and subsequent immersion in water or operated under aqueous media. These remarkable properties arise as a consequence of the use of relatively air stable organic semiconductors and proper engineering of the OFET structure.

Stable organic field-effect transistors for continuous and nondestructive sensing of chemical and biologically relevant molecules in aqueous environment.

The use of organic field-effect transistors (OFETs) as sensors in aqueous media has gained increased attention for environmental monitoring and medical diagnostics. However, stable operation of OFETs in aqueous media is particularly challenging because of electrolytic hydrolysis of water, high ionic conduction through the analyte, and irreversible damage of organic semiconductors when exposed to water. To date, OFET sensors have shown the capability of label-free sensing of various chemical/biological species, but they could only be used once because their operational stability and lifetime while operating in aqueous environments has been poor, and their response times typically slow. Here, we report on OFETs with unprecedented water stability. These OFETs are suitable for the implementation of reusable chemical/biological sensors because they primarily respond to charged species diluted in an aqueous media by rapidly shifting their threshold voltage. These OFET sensors present stable current baselines and saturated signals which are ideal for detection of low concentration of small or large molecules that alter the pH of an aqueous environment. The overall response of these OFET sensors paves the way for the development of continuous chemical/biological nondestructive sensor applications in aqueous media.

Recyclable organic solar cells on cellulose nanocrystal substrates.

Solar energy is potentially the largest source of renewable energy at our disposal, but significant advances are required to make photovoltaic technologies economically viable and, from a life-cycle perspective, environmentally friendly, and consequently scalable. Cellulose nanomaterials are emerging high-value nanoparticles extracted from plants that are abundant, renewable, and sustainable. Here, we report on the first demonstration of efficient polymer solar cells fabricated on optically transparent cellulose nanocrystal (CNC) substrates. The solar cells fabricated on the CNC substrates display good rectification in the dark and reach a power conversion efficiency of 2.7%. In addition, we demonstrate that these solar cells can be easily separated and recycled into their major components using low-energy processes at room temperature, opening the door for a truly recyclable solar cell technology. Efficient and easily recyclable organic solar cells on CNC substrates are expected to be an attractive technology for sustainable, scalable, and environmentally-friendly energy production.

Stable solution-processed molecular n-channel organic field-effect transistors.

A new solution-processable small-molecule containing electron-poor naphthalene diimide and tetrazine moieties has been synthesized. The optimized spin-coated n-channel OFETs on glass substrate shows electron mobility value up to 0.15 cm(2) V(-1) s(-1) . Inkjet-printed OFETs are fabricated in ambient atmosphere on flexible plastic substrates, which exhibits an electron mobility value up to 0.17 cm(2) V(-1) s(-1) and also shows excellent environmental and operational stability.

A universal method to produce low-work function electrodes for organic electronics.

Organic and printed electronics technologies require conductors with a work function that is sufficiently low to facilitate the transport of electrons in and out of various optoelectronic devices. We show that surface modifiers based on polymers containing simple aliphatic amine groups substantially reduce the work function of conductors including metals, transparent conductive metal oxides, conducting polymers, and graphene. The reduction arises from physisorption of the neutral polymer, which turns the modified conductors into efficient electron-selective electrodes in organic optoelectronic devices. These polymer surface modifiers are processed in air from solution, providing an appealing alternative to chemically reactive low-work function metals. Their use can pave the way to simplified manufacturing of low-cost and large-area organic electronic technologies.

Inverted polymer solar cells with amorphous indium zinc oxide as the electron-collecting electrode.

We report on the fabrication and performance of polymer-based inverted solar cells utilizing amorphous indium zinc oxide (a-IZO) as the electron-collecting electrode. Amorphous IZO films of 200 nm thickness were deposited by room temperature sputtering in a high-purity argon atmosphere. The films possessed a high optical transmittance in the visible region (≥ 80%), a low resistivity (3.3 × 10⁻4 Ωcm), a low surface roughness (root mean square = 0.68 nm), and a low work function (4.46 ± 0.02 eV). Inverted solar cells with the structure a-IZO/P3HT: PCBM/PEDOT:PSS/Ag exhibited a power conversion efficiency of 3% estimated for AM 1.5G, 100 mW/cm² illumination.

AFM studies of nanoparticle deposition via electrospray ionization.

Electrospray ionization was used to deposit CdSe nanoparticles on graphite and InP substrates. The charge transferred to the substrate via the deposition was correlated to the number of particles found on the substrate by AFM observation. A charge per nanoparticle was determined from depositions of various lengths of time, which is approximately 1 electron per nanoparticle. This value was found to be consistent for concentrations of nanoparticle solutions, which differed by an order of magnitude. Such a value may be used for predicting the density of nanoparticles deposited on a substrate during the deposition. It was found that above a critical concentration, the deposition process produced large clumps of particles rather than individual particles. At lower concentrations, individual particles deposited on the graphite substrates were found to be mobile, arranging themselves on step edges of the graphite.