Jae Bon Koo

Hysteresis of pentacene thin-film transistors and inverters with cross-linked poly(4-vinylphenol) gate dielectrics

The authors report the effects of hydroxyl groups (OH bonds) on the electrical reliabilities of pentacene organic thin-film transistors (OTFTs) with poly-4-vinylphenol (PVP) gate dielectrics. PVP gate dielectric films mixed with different concentrations of methylated poly(melamine-co-formaldehyde) (MMF) were fabricated, and experiments on the hysteresis behavior of the OTFT device were conducted. Pentacene TFTs with the PVP (MMF 0wt.%) exhibited a large hysteresis, while in the PVP (MMF 125wt.%), nearly no hysteresis was observed. Large hysteresis observed in OTFT devices was confirmed to be strongly related to the hydroxyl groups existing inside of the polymeric dielectrics and could reduced by the decrease of OH group.

Stretchable and Transparent Electrodes using Hybrid Structures of Graphene–Metal Nanotrough Networks with High Performances and Ultimate Uniformity

Transparent electrodes that can maintain their electrical and optical properties stably against large mechanical deformations are essential in numerous applications of flexible and wearable electronics. In this paper, we report a comprehensive analysis of the electrical, optical, and mechanical properties of hybrid nanostructures based on graphene and metal nanotrough networks as stretchable and transparent electrodes. Compared to the single material of graphene or the nanotrough, the formation of this hybrid can improve the uniformity of sheet resistance significantly, that is, a very low sheet resistance (1 Ω/sq) with a standard deviation of less than ±0.1 Ω/sq, high transparency (91% in the visible light regime), and superb stretchability (80% in tensile strain). The successful demonstration of skin-attachable, flexible, and transparent arrays of oxide semiconductor transistors fabricated using hybrid electrodes suggests substantial promise for the next generation of electronic devices.

Hysteresis-free organic field-effect transistors and inverters using photocrosslinkable poly(vinyl cinnamate) as a gate dielectric

We have fabricated organic field-effect transistors (OFETs) and inverters using photocrosslinkable poly(vinyl cinnamate) (PVCN) as a gate dielectric. The photocrosslinked PVCN dielectric film has superior insulating properties and does not require thermal curing. The high water resistance of the dielectric, which arises because PVCN is hydroxyl group-free, means that the devices were found to be hysteresis-free in all operations. The OFETs with the PVCN dielectric were found to exhibit a carrier mobility of 0.51cm2∕Vs, an on/off ratio of 106, and a subthreshold swing of 0.913V/decade. An organic inverter consisting of two OFETs exhibited a high inverter gain of 17.9.

Organic Nano-Floating-Gate Memory with Polymer:[6,6]-Phenyl-C61 Butyric Acid Methyl Ester Composite Films

Here, we report on a pentacene-based, nonvolatile transistor memory device with poly(4-vinyl phenol) (PVP):[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) nano-composite films as the charge storage site. Incorporation of PCBM molecules into PVP dielectric materials as charge storage sites for electrons resulted in a reversible shift in the threshold voltage (VTh) and reliable memory characteristics. The characteristics of the pentacene memory device were as follows: a relatively high field-effect mobility (µFET) (0.2–0.3 cm2 V-1 s-1) with a large memory window (ca. 20 V), a high on/off ratio (~104) during writing and erasing with application of an operating gate voltage of 60 V for a short duration time (~1 ms), and a retention time of about 40 h.

Organic Thin-Film Transistors with Short Channel Length Fabricated by Reverse Offset Printing

We report on the fabrication of organic thin-film transistors (OTFTs) with a reverse-offset-printed Ag metal source/drain (S/D) electrode pattern. The printed electrodes had a channel length of less than 5 μm and resistivity of 3 × 10−6 Ω cm. The OTFTs were fabricated from regioregular poly(3-hexylthiophene) as the semiconductor and poly(methyl methacrylate) as the gate insulator. The transfer and output characteristics of a top-gate OTFT with a channel length of 5 μm were evaluated. Here we discuss in detail the technological challenges encountered with reverse offset printing and the failure modes.

Photolithography-Based Patterning of Liquid Metal Interconnects for Monolithically Integrated Stretchable Circuits

We demonstrate a new patterning technique for gallium-based liquid metals on flat substrates, which can provide both high pattern resolution (∼20 μm) and alignment precision as required for highly integrated circuits. In a very similar manner as in the patterning of solid metal films by photolithography and lift-off processes, the liquid metal layer painted over the whole substrate area can be selectively removed by dissolving the underlying photoresist layer, leaving behind robust liquid patterns as defined by the photolithography. This quick and simple method makes it possible to integrate fine-scale interconnects with preformed devices precisely, which is indispensable for realizing monolithically integrated stretchable circuits. As a way for constructing stretchable integrated circuits, we propose a hybrid configuration composed of rigid device regions and liquid interconnects, which is constructed on a rigid substrate first but highly stretchable after being transferred onto an elastomeric substrate. This new method can be useful in various applications requiring both high-resolution and precisely aligned patterning of gallium-based liquid metals.

Top-gate staggered poly(3,3″′-dialkyl-quarterthiophene) organic thin-film transistors with reverse-offset-printed silver source/drain electrodes

Here, we report on high-performance top-gated poly(3,3″′-dialkyl-quarterthiophene) (PQT-12) organic thin-film transistors (OTFTs) with reverse-offset-printed (ROP) silver (Ag) source/drain (S/D) electrodes. OTFT devices with ROP S/D electrodes using Ag nanopaste show higher performance (∼0.01 cm2/Vs) than those fabricated by vacuum electron beam evaporation with conventional photolithography and a standard lift-off process (∼1 × 10−3 cm2/Vs). This dissimilarity is attributed to the higher work function (−4.9 eV) of the ROP Ag electrode due to AgO formation on the Ag surface during thermal annealing. This results in a low interfacial hole injection energy barrier between the S/D electrodes and the PQT-12 semiconductor.

Device Characteristics of Pentacene Dual-Gate Organic Thin-Film Transistor

We fabricated pentacene organic thin-film transistors (OTFTs) with a dual-gate structure, in which 300-nm-thick thermally grown SiO2 and 500-nm-thick parylene were used as a bottom-gate and a top-gate dielectric, respectively. The threshold voltage (Vth) of the dual-gate OTFT changed systematically with the application of voltage bias to the top-gate electrode. When voltage bias from -20 to 20 V was applied to the top-gate electrode, Vth changed from 9.4 to -9.3 V. The range of Vth shift in the dual-gate OTFT with a thin 10 nm pentacene layer was much wider than that with a thick 500 nm pentacene layer. This shift of Vth due to the body effect allows the change from an enhancement- to a depletion-mode transistor, which is beneficial for the fabrication of organic circuits.

Pentacene-thin film transistors with ZrO2 gate dielectric layers deposited by plasma-enhanced atomic layer deposition

The effect of the dielectric constant and surface roughness of gate dielectrics on the electrical performance of pentacene-thin-film transistor (TFT) was investigated using high-K ZrO 2 films deposited by plasma-enhanced atomic layer deposition. The dielectric constant of ZrO 2 (κ ZrO 2 ) was in the range of 15.6-33.0, and the surface roughness was increased with κ ZrO 2 in the presently reported devices. Threshold voltage and subthreshold swing were effectively reduced with increasing κ ZrO 2 and were remarkably low, with values of -0.42 V and 0.15 V/dec, respectively, when κ ZrO 2 = 33.0. To the contrary, the carrier mobility was determined by the surface roughness of ZrO 2 gate dielectrics.

Stretchable Organic Thin-Film Transistors Fabricated on Wavy-Dimensional Elastomer Substrates Using Stiff-Island Structures

Stretchable organic thin-film transistors (OTFTs) were fabricated on the polydimethysiloxane (PDMS) elastomer substrates by employing the wavy-dimensional and polyimide stiff-island structures. A low-temperature solution process was also designed to obtain high strain profiles. The endurable maximum strains were estimated to be 2.28, 9.70, and 9.32% for the OTFTs formed on the flat, 1D-, and 2D-wavy PDMS elastomers, respectively. The field-effect mobilities were obtained to be 5 ~ 7 × 10-4 cm2 V-1 s-1 for all devices and they did not exhibit any degradation under the stretchable conditions before the fracture. The results suggest that the proposed methodologies were quite suitable for high-performance stretchable OTFTs.