Jin-Hyuk Bae

Surface modification of a ferroelectric polymer insulator for low-voltage readable nonvolatile memory in an organic field-effect transistor

We demonstrate that the sequential surface modification of a ferroelectric polymer insulator plays an essential role in both the enhancement of the carrier mobility and the shift in the turn-on voltage (Von) in an organic ferroelectric field-effect transistor (FeFET) for nonvolatile memory. The surface of a ferroelectric polymer insulator, poly(vinylidene fluoride-trifluoroethylene), is physicochemically modified by the successive treatments of ultraviolet-ozone (UVO) and CF4 plasma to understand how the surface morphology and the hydrophobicity affect the grain size, the mobility, and Von in the FeFET. In a pentacene-based FeFET, the CF4 plasma irradiation leads to the mobility enhancement by a factor of about 5 as well as the shift in Von toward a positive voltage direction while the UVO treatment results in only the shift in Von toward a negative voltage direction. It is found that the sequence of the two successive treatments is critical for tailoring interfacial interactions between the ferroelectric...

Structural origin of the mobility enhancement in a pentacene thin-film transistor with a photocrosslinking insulator

We present the underlying mechanism for the mobility enhancement in a pentacene thin-film transistor (TFT) with a photocrosslinking polymeric insulator, poly(vinyl cinnamate) (PVCi). Experimental results for the optical anisotropy, x-ray diffraction, and microscopic layer-by-layer coverage of the pentacene film on the photocrosslinked PVCi layer, exposed to a linearly polarized ultraviolet (LPUV) light, clearly demonstrate the importance of the structural packing of pentacene grains rather than the directional alignment of the pentacene molecules for the mobility enhancement. The packing density of pentacene grains is directly related to the number of photocrosslinking sites of the PVCi insulator produced by the LPUV. It is found that the mobility in the pentacene TFT is linearly proportional to the number of photocrosslinked sites of the PVCi insulator serving as interaction sites for the layer-by-layer coverage of the pentacene molecules with no preferred orientation.

Importance of the Functional Group Density of a Polymeric Gate Insulator for Organic Thin-Film-Transistors

We describe the importance of the functional group density of polymeric insulators (PI) for organic thin-film-transistors (OTFTs) in terms of the insulator processing temperature and the exposure of a linearly polarized ultraviolet (LPUV) light. The PI layers processed at lower temperatures than the boiling temperature (Tb) of the solvent have higher densities of functional groups than those processed above Tb. The carrier mobility in the pentacene OTFT processed below Tb increases at least by a factor of three with maintaining other electrical properties such as the threshold voltage and the current on/off ratio. Our results suggest that the preferential alignment of the pentacene molecules is not the main physical mechanism for the mobility enhancement. From the mobility anisotropy resulting from the polarization of the LPUV, the packing density of the pentacene molecules on the PI layer, dictated primarily by the density of functional groups, is found to play a critical role on the magnitude of the mobility.

Effects of Interfacial Charge Depletion in Organic Thin-Film Transistors with Polymeric Dielectrics on Electrical Stability

We investigated the electrical stabilities of two types of pentacene-based organic thin-film transistors (OTFTs) with two different polymeric dielectrics: polystyrene (PS) and poly(4-vinyl phenol) (PVP), in terms of the interfacial charge depletion. Under a short-term bias stress condition, the OTFT with the PVP layer showed a substantial increase in the drain current and a positive shift of the threshold voltage, while the PS layer case exhibited no change. Furthermore, a significant increase in the off-state current was observed in the OTFT with the PVP layer which has a hydroxyl group. In the presence of the interfacial hydroxyl group in PVP, the holes are not fully depleted during repetitive operation of the OTFT with the PVP layer and a large positive gate voltage in the off-state regime is needed to effectively refresh the electrical characteristics. It is suggested that the depletion-limited holes at the interface, i.e., interfacial charge depletion, between the PVP layer and the pentacene layer play a critical role on the electrical stability during operation of the OTFT.

FABRICATION OF ORGANIC THIN-FILM TRANSISTORS BASED ON HIGH DIELECTRIC NANOCOMPOSITE INSULATORS

Organic thin-film-transistors (OTFTs) with solution-processed high dielectric gate insulators were fabricated. The gate insulators were made of high dielectric TiO2 nanoparticles dispersed uniformly in a polymer matrix of nylon 6 which is known to align liquid crystal molecules. It was found that the nanocomposite insulator in a pentacene-based OTFT reduces the operating voltage but it produces substantial current leakage. A buffer layer of polyvinylphenol on the top of the nanocomposite layer was found to block the leakage current and to increase significantly the carrier mobility as well as the current on-off ratio.

Functional solid additive modified PEDOT:PSS as an anode buffer layer for enhanced photovoltaic performance and stability in polymer solar cells

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is most commonly used as an anode buffer layer in bulk-heterojunction (BHJ) polymer solar cells (PSCs). However, its hygroscopic and acidic nature contributes to the insufficient electrical conductivity, air stability and restricted photovoltaic (PV) performance for the fabricated PSCs. In this study, a new multifunctional additive, 2,3-dihydroxypyridine (DOH), has been used in the PEDOT: PSS buffer layer to obtain modified properties for PEDOT: PSS@DOH and achieve high PV performances. The electrical conductivity of PEDOT:PSS@DOH films was markedly improved compared with that of PEDOT:PSS. The PEDOT:PSS@DOH film exhibited excellent optical characteristics, appropriate work function alignment, and good surface properties in BHJ-PSCs. When a poly(3-hexylthiohpene):[6,6]-phenyl C61-butyric acid methyl ester blend system was applied as the photoactive layer, the power conversion efficiency of the resulting PSCs with PEDOT:PSS@DOH(1.0%) reached 3.49%, outperforming pristine PEDOT:PSS, exhibiting a power conversion enhancement of 20%. The device fabricated using PEDOT:PSS@DOH (1.0 wt%) also exhibited improved thermal and air stability. Our results also confirm that DOH, a basic pyridine derivative, facilitates adequate hydrogen bonding interactions with the sulfonic acid groups of PSS, induces the conformational transformation of PEDOT chains and contributes to the phase separation between PEDOT and PSS chains.

Efficient exciton generation in atomic passivated CdSe/ZnS quantum dots light-emitting devices

We demonstrate the first-ever surface modification of green CdSe/ZnS quantum dots (QDs) using bromide anions (Br-) in cetyl trimethylammonium bromide (CTAB). The Br- ions reduced the interparticle spacing between the QDs and induced an effective charge balance in QD light-emitting devices (QLEDs). The fabricated QLEDs exhibited efficient charge injection because of the reduced emission quenching effect and their enhanced thin film morphology. As a result, they exhibited a maximum luminance of 71,000 cd/m2 and an external current efficiency of 6.4 cd/A, both significantly better than those of their counterparts with oleic acid surface ligands. In addition, the lifetime of the Br- treated QD based QLEDs is significantly improved due to ionic passivation at the QDs surface.

Photo-assisted molecular engineering in solution-processed organic thin-film transistors with a blended semiconductor for high mobility anisotropy

This paper reports a viable method for enhancing mobility anisotropy in solution-processed organic thin-film transistors (TFTs) by irradiating linearly polarized ultraviolet (LPUV) light onto a 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene)/poly (vinyl cinnamate) (PVCi) blended semiconductor film. The needle-like crystalline domains in the blended semiconductor film were aligned along the polarization direction of the LPUV light, suggesting that TIPS-pentacene molecules are oriented anisotropically due to an intermolecular interaction and aggregation with the photo-aligned PVCi molecules. The mobility anisotropy reached up to about 50, which is the highest value achieved in solution-processed organic TFTs with a polymeric insulator.

Reduction in Contact Resistance of Pentacene Thin-Film Transistors by Formation of an Organo-Metal Hybrid Interlayer

We propose a viable method of reducing the contact resistance of organic electronic devices by the formation of an organo-metal hybrid interlayer between a metal electrode and an organic semiconducting layer. The hybrid interlayer is produced by simultaneous deposition of the same materials as the electrode and the organic semiconductor, without any extra buffer layer. In such a hybrid interlayer, the metal clusters are uniformly dispersed on entangled organic surfaces so that they contribute significantly to the reduction of the contact resistance. The contact resistance is reduced by about a factor of seven. A simple theoretical model, based on the carrier accumulation and tunneling effects, well describes the essential features of the contact resistance as a function of the thickness of the hybrid interlayer.

Topography-Guided Spreading and Drying of 6,13-bis(triisopropylsilylethynyl)-pentacene Solution on a Polymer Insulator for the Field-Effect Mobility Enhancement

We report on the enhancement of the field-effect mobility of solution-processed 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) by unidirectional topography (UT) of an inkjet-printed polymer insulator. The UT leads to anisotropic spreading and drying of the TIPS-pentacene droplet and enables to spontaneously develop the ordered structures during the solvent evaporation. The mobility of the UT-dictated TIPS-pentacene film (0.202 ± 0.012 cm2/Vs) is found to increase by more than a factor of two compared to that of the isotropic case (0.090 ± 0.032 cm2/Vs). The structural arrangement of the TIPS-pentacene molecules in relation to the mobility enhancement is described within an anisotropic wetting formalism. Our UT-based approach to the mobility enhancement is easily applicable to different classes of soluble organic field-effect transistors by adjusting the geometrical parameters such as the height, the width, and the periodicity of the UT of an inkjet-printed insulator.