Jiyoul Lee

High-mobility and low-power thin-film transistors based on multilayer MoS2 crystals

Unlike graphene, the existence of bandgaps (1-2 eV) in the layered semiconductor molybdenum disulphide, combined with mobility enhancement by dielectric engineering, offers an attractive possibility of using single-layer molybdenum disulphide field-effect transistors in low-power switching devices. However, the complicated process of fabricating single-layer molybdenum disulphide with an additional high-k dielectric layer may significantly limit its compatibility with commercial fabrication. Here we show the first comprehensive investigation of process-friendly multilayer molybdenum disulphide field-effect transistors to demonstrate a compelling case for their applications in thin-film transistors. Our multilayer molybdenum disulphide field-effect transistors exhibited high mobilities (>100 cm(2) V(-1) s(-1)), near-ideal subthreshold swings (~70 mV per decade) and robust current saturation over a large voltage window. With simulations based on Shockleys long-channel transistor model and calculations of scattering mechanisms, these results provide potentially important implications in the fabrication of high-resolution large-area displays and further scientific investigation of various physical properties expected in other layered semiconductors.

Printable ion-gel gate dielectrics for low-voltage polymer thin-film transistors on plastic.

An important strategy for realizing flexible electronics is to use solution-processable materials that can be directly printed and integrated into high-performance electronic components on plastic. Although examples of functional inks based on metallic, semiconducting and insulating materials have been developed, enhanced printability and performance is still a challenge. Printable high-capacitance dielectrics that serve as gate insulators in organic thin-film transistors are a particular priority. Solid polymer electrolytes (a salt dissolved in a polymer matrix) have been investigated for this purpose, but they suffer from slow polarization response, limiting transistor speed to less than 100 Hz. Here, we demonstrate that an emerging class of polymer electrolytes known as ion gels can serve as printable, high-capacitance gate insulators in organic thin-film transistors. The specific capacitance exceeds that of conventional ceramic or polymeric gate dielectrics, enabling transistor operation at low voltages with kilohertz switching frequencies.

Correlation between photoelectric and optical absorption spectra of thermally evaporated pentacene films

We have measured the spectral photoresponse of Al/pentacene Schottky junction photodiodes and optical absorption spectra of pentacene films thermally evaporated on glass. The photoelectric response exhibited the genuine highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) transition at 1.97 eV and interband absorption peaks at 2.3 and 2.5 eV. These peaks are also identified in the optical absorption spectra, but they are dominated by additional strong exciton peaks at 1.82 and 2.1 eV. By comparing these complementary measurements, we determine the HOMO–LUMO gap energy of 1.97 eV and the fundamental exciton binding energy of 0.15 eV for thin solid pentacene.

High carrier densities achieved at low voltages in ambipolar PbSe nanocrystal thin-film transistors

Efficient transport of both electrons and holes with high carrier densities is a requirement for obtaining light-emitting transistors from films of colloidal semiconductor nanocrystals. Such devices offer an approach to efficient electrically pumped nanocrystal lasers with tunable emission. Here, we report a low-voltage ambipolar thin-film transistor that features high carrier mobility and high induced carrier density by combining a PbSe nanocrystal film with a high-capacitance ion-gel gate dielectric layer ( approximately 22 and approximately 9 microF/cm2 for electron and hole accumulation, respectively). At operation voltages below 2.5 V, electron and hole densities higher than approximately 10(14) carriers/cm2 could be achieved in the PbSe nanocrystal film, which corresponds to approximately 3 electrons or holes per particle. Carrier mobilities were also dependent on charge density and were as high as 0.4 and 0.02 cm2/(V s) for electrons and holes, respectively.

Optimum channel thickness in pentacene-based thin-film transistors

We report on the influence of pentacene channel thickness on the field-effect hole mobility in pentacene-based thin-film transistors (TFTs) that employ the top-contact mode for the source/drain electrodes. Our pentacene channel layers were deposited in the thickness range of 16–90 nm by thermal evaporation on 450 nm thick Al2O3+x dielectric films. The TFTs with increasingly thinner pentacene layers displayed correspondingly higher hole mobility, but an optimum thickness was determined to be about 30 nm because the TFTs with pentacene layers thinner than 30 nm exhibited high leakage current in the off-state bias regime. After a proper chemical treatment was performed onto the Al2O3+x gate dielectric, our optimized TFT with a 30 nm thick pentacene channel exhibited high mobility of ∼0.2 cm2/V s with an on/off current ratio of 105.

Pentacene thin-film transistors with Al2O3+x gate dielectric films deposited on indium-tin-oxide glass

We report on the fabrication of pentacene thin-film transistors (TFTs) with Al2O3+x films as the gate dielectric that has been deposited on indium-tin-oxide glass by rf magnetron sputtering at room temperature. Although the Al2O3+x was expected to show lower capacitance and breakdown field than stoichiometric Al2O3, our pentacene TFTs with optimized thin Al2O3+x gate dielectric exhibited a moderately high field mobility of 0.14 cm2/V s, an outstanding subthreshold slope of 0.88 V/dec, and an on/off ratio over 106. Our work demonstrates that RT-deposited Al2O3+x is a promising gate dielectric material for organic TFTs.

Comparative study of the photoresponse from tetracene-based and pentacene-based thin-film transistors

We report on the photoresponse from tetracene-based and pentacene-based thin-film transistors (TFTs) with semitransparent NiOx source/drain electrodes and SiO2∕p+-Si substrate. Both organic TFTs have been fabricated with identical channel thickness and device geometry. Compared with pentacene-based TFTs, the tetracene-TFT exhibited superior potentials as a photodetector in the visible and ultraviolet range although it showed a field mobility (μ=0.003cm2∕Vs) which is two orders of magnitude lower than that of the pentacene-based TFT (μ=∼0.3cm2∕Vs). The tetracene-TFT displayed a high photo-to-dark current ratio (Iph∕Idark) of 3×103, while that of the pentacene-TFT was only ∼10.

Effects of substrate temperature on the device properties of pentacene-based thin film transistors using Al2O3+x gate dielectric

We report on the electrical properties of pentacene-based organic thin film transistors (OTFTs), where the active pentacene channel layers have been deposited at fixed deposition rate of 1 A/s on a 250-nm-thick Al2O3+x gate dielectric film at various substrate temperatures: room temperature, 60 °C, and 90 °C. The grain size of the pentacene layer was found to increase with the substrate temperature, accompanied by a phase transition. The highest saturation current of 20 μA (under a gate bias of −40 V) was obtained with a high field-effect hole mobility of ∼0.21 cm2/V s from an OTFT prepared at 90 °C despite a relatively low on/off current ratio of 2×105. It is concluded that the pentacene channel deposited at a high substrate temperature contains not only large grains but also high density traps.

Flexible semitransparent pentacene thin-film transistors with polymer dielectric layers and NiOx electrodes

We have fabricated the flexible semitransparent pentacene-based thin-film transistors (TFTs) with poly-4-vinylphenol (PVP) dielectric layers which were deposited by spin coating on a thermostable plastic substrate with a conductive film. For the source∕drain (S∕D) electrodes of our flexible pentacene TFTs both Au and semitransparent NiOx have been tested. It was found that NiOx was better matched to the pentacene channel for the S∕D contacts than Au. Our flexible pentacene TFTs with semitransparent NiOx contacts exhibited mobility of ∼0.24cm2∕Vs higher than that achieved with Au contacts (∼0.14cm2∕Vs) and also demonstrated a higher initial drain current.

Reliable and Uniform Thin‐Film Transistor Arrays Based on Inkjet‐Printed Polymer Semiconductors for Full Color Reflective Displays

Stable uniform performance inkjet-printed polymer transistor arrays, which allow demonstration of flexible full-color displays, were achieved by new ambient processable conjugated copolymer semiconductor, and OTFT devices incorporating this material showed high mobility values>1.0 cm2 V(-1) s(-1). Bias-stress stability of the devices was improved with a channel-passivation layer, which suppresses the density of trap states at the channel interface.