High-Performance Vertical Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) are promising transducers for biointerfacing due to their high transconductance, biocompatibility, and availability in a variety of form factors. Most OECTs reported to date, however, utilize rather large channels, limiting the transistor performance and resulting in a low transistor density. This is typically a consequence of limitations associated with traditional fabrication methods and with 2D substrates. Here, the fabrication and characterization of OECTs with vertically stacked contacts, which overcome these limitations, is reported. The resulting vertical transistors exhibit a reduced footprint, increased intrinsic transconductance of up to 57 mS, and a geometry-normalized transconductance of 814 S m^-1 . The fabrication process is straightforward and compatible with sensitive organic materials, and allows exceptional control over the transistor channel length. This novel 3D fabrication method is particularly suited for applications where high density is needed, such as in implantable devices.

Interplay between Vacuum-Grown Monolayers of Alkylphosphonic Acids and the Performance of Organic Transistors Based on Dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene

Monolayers of six alkylphosphonic acids ranging from C8 to C18 were prepared by vacuum evaporation and incorporated into low-voltage organic field-effect transistors based on dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT). Similar to solution-assembled monolayers, the molecular order for vacuum-deposited monolayers improved with increasing length of the aliphatic tail. At the same time, Fourier transform infrared (FTIR) measurements suggested lower molecular coverage for longer phosphonic acids. The comparison of FTIR and vibration frequencies calculated by density functional theory indicated that monodentate bonding does not occur for any phosphonic acid. All monolayers exhibited low surface energy of ∼17.5 mJ/m(2) with a dominating Lifshitz-van der Waals component. Their surface roughness was comparable, while the nanomechanical properties were varied but not correlated to the length of the molecule. However, large improvement in transistor performance was observed with increasing length of the aliphatic tail. Upon going from C8 to C18, the mean threshold voltage decreased from -1.37 to -1.24 V, the field-effect mobility increased from 0.03 to 0.33 cm(2)/(V·s), the off-current decreased from ∼8 × 10(-13) to ∼3 × 10(-13) A, and for transistors with L = 30 μm the on-current increased from ∼3 × 10(-8) to ∼2 × 10(-6) A, and the on/off-current ratio increased from ∼3 × 10(4) to ∼4 × 10(6). Similarly, transistors with longer phosphonic acids exhibited much better air and bias-stress stability. The achieved transistor performance opens up a completely "dry" fabrication route for ultrathin dielectrics and low-voltage organic transistors.

Conducting nanofibers and organogels derived from the self-assembly of tetrathiafulvalene-appended dipeptides

We demonstrate the nonaqueous self-assembly of a low-molecular-mass organic gelator based on an electroactive p-type tetrathiafulvalene (TTF)-dipeptide bioconjugate. We show that a TTF moiety appended with diphenylalanine amide derivative (TTF-FF-NH2) self-assembles into one-dimensional nanofibers that further lead to the formation of self-supporting organogels in chloroform and ethyl acetate. Upon doping of the gels with electron acceptors (TCNQ/iodine vapor), stable two-component charge transfer gels are produced in chloroform and ethyl acetate. These gels are characterized by various spectroscopy (UV-vis-NIR, FTIR, and CD), microscopy (AFM and TEM), rheology, and cyclic voltammetry techniques. Furthermore, conductivity measurements performed on TTF-FF-NH2 xerogel nanofiber networks formed between gold electrodes on a glass surface indicate that these nanofibers show a remarkable enhancement in the conductivity after doping with TCNQ.

Effect of heat treatment in aluminium oxide preparation by UV/ozone oxidation for organic thin-film transistors

Effect of heat treatment in aluminium oxide (AlO(x)) preparation employing UV/ozone exposure of thermally-evaporated aluminium is reported. AlO(x) is combined with 1-octylphosphonic acid to form a gate dielectric in low-voltage organic thin-film transistors based on pentacene. For short UV/ozone exposure times the 100 degrees C-heating step that immediately follows UV/ozone oxidation of aluminium leads to a decrease in the transistor threshold voltage of up to 8% and - fourfold reduction in the gate dielectric current density. Transistors with AlO(x) prepared by 60-minute UV/ozone oxidation do not exhibit such behaviour. These results are explained in terms of reduced density of charged oxygen vacancies in the UV/ozone oxidized AlO(x).

Via-Hole Addressed TFT and Process for Large-Area A-Si:H Electronics

We demonstrate a new technology for RC gate delay reduction, by fabricating an array of amorphous silicon thin-film transistors (a-Si:H TFTs) on a thin glass substrate provided with via holes. All gates are connected through via holes to a metal line that is run on the back side of the substrate. We opened via holes with a diameter of 35 to 50 μm in 50 μm glass foil. For the first time, all TFT pattern definition steps used a process which employs electrophotographic toner masks.

Direct Writing and Lift-Off Patterning of Copper Lines at 200°C Maximum Process Temperature

We adapted a new technique for depositing copper lines, at a maximum process temperature of 200°C. The technique is based on the decomposition of copper hexanoate by UV light, followed by annealing in H2 [1]. A copper film resistivity of 8 μΩcm is obtained. We patterned this copper metallization on Corning 7059 glass substrates by three different techniques, including exposure through a shadow mask, lift-off of xerographic toner, and direct writing.

a-Si:H TFTs Made on Polyimide Foil by PE-CVD at 150°C

We have fabricated high-performance amorphous silicon thin-film transistors (a-Si:H TFTs) on 2 mil. (51 µm) thick polyimide foil substrates. The TFT structure was deposited by r.f.-excited plasma enhanced chemical vapor deposition (PECVD). All TFT layers, including the gate silicon nitride, the undoped, and the n+ amorphous silicon were deposited at a substrate temperature of 150°C. The transistors have inverted-staggered back-channel etch structure. The TFT off-current is ∼ 10−12 A, the on-off current ratio is > 107, the threshold voltage is 3.5 V, the sub-threshold slope is ∼ 0.5V/decade, and the linear-regime mobility is ∼ 0.5 cm2V−1s−1. We compare the mechanical behavior of a thin film on a stiff and on a compliant substrate. The thin film stress can be reduced to one half by changing from a stiff to a compliant substrate. A new equation is developed for the radius of curvature of thin films on compliant substrates.