Won Min Yun

Hysteresis-free pentacene field-effect transistors and inverters containing poly(4-vinyl phenol-co-methyl methacrylate) gate dielectrics

The influence of hydroxyl groups on the hysteresis of pentacene field-effect transistors (FETs) and metal-insulator-semiconductor diodes containing poly(4-vinyl phenol) and poly(4-vinyl phenol-co-methyl methacrylate) (PVP-PMMA) gate dielectrics was investigated. The electrical characteristics and Fourier transform infrared spectroscopy measurements show that hysteresis is intimately related to the presence of free OH groups in the polymer gate dielectrics. The methyl methacryl moieties in PVP-PMMA minimize residual water in the polymer and form hydrogen bonds with the hydroxyl groups, thus reducing the number of free OH species. Therefore, pentacene FETs and inverters using PVP-PMMA gate dielectrics exhibit high, hysteresis-free performances.

Hysteresis behaviour of low-voltage organic field-effect transistors employing high dielectric constant polymer gate dielectrics

Here, we report on the fabrication of low-voltage-operating pentacene-based organic field-effect transistors (OFETs) that utilize crosslinked cyanoethylated poly(vinyl alcohol) (CR-V) gate dielectrics. The crosslinked CR-V-based OFET could be operated successfully at low voltages (below 4 V), but abnormal behaviour during device operation, such as uncertainty in the field-effect mobility (μ) and hysteresis, was induced by the slow polarization of moieties embedded in the gate dielectric (e.g. polar functionalities, ionic impurities, water and solvent molecules). In an effort to improve the stability of OFET operation, we measured the dependence of μ and hysteresis on dielectric thickness, CR-V crosslinking conditions and sweep rate of the gate bias. The influence of the CR-V surface properties on μ, hysteresis, and the structural and morphological features of the pentacene layer grown on the gate dielectric was characterized and compared with the properties of pentacene grown on a polystyrene surface.

Polyaniline‐Based Conducting Polymer Compositions with a High Work Function for Hole‐Injection Layers in Organic Light‐Emitting Diodes: Formation of Ohmic Contacts

It is a great challenge to develop solution-processed, polymeric hole-injection layers (HILs) that perform better than small molecular layers for realizing high-performance small-molecule organic light-emitting diodes (SM-OLEDs). We have greatly improved the injection efficiency and the current efficiency of SM-OLEDs by introducing conducting polymer compositions composed of polyaniline doped with polystyrene sulfonate and perfluorinated ionomer (PFI) as the HIL. During single spin-coating of conducting polymer compositions, the PFI layer was self-organized at the surface and greatly increased the film work function. It enhanced hole-injection efficiency and current efficiency by introducing a nearly ohmic contact and improving electron blocking. Our results demonstrate that solution-processed polyaniline HILs with tunable work functions are good candidates for reducing process costs and improving OLED performance.

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.

Thermally evaporated SiO thin films as a versatile interlayer for plasma-based OLED passivation.

Silicon monoxide (SiO) thin films were introduced as an efficient interlayer for achieving plasma-based organic light-emitting diode (OLED) surface passivation. The SiO thin films could be consecutively formed via thermal evaporation, without breaking the vacuum, after deposition of the OLED cathode. The plasma resistivity and UV-blocking characteristics of the SiO interlayer protected the OLED devices against electrical and optical degradation during the plasma-enhanced atomic layer deposition (PEALD) and plasma-enhanced chemical vapor deposition (PECVD) passivation processes. In addition, the nonconformal deposition and hydroxyl group-rich surface characteristics of the SiO thin films yielded enhanced surface pinhole coverage and a higher initial film density in the subsequently deposited PEALD-based Al2O3 barrier film. As a result, the OLEDs with a SiO/Al2O3 bilayer passivation layer displayed a remarkably increased device shelf life compared to devices prepared using Al2O3-only passivation. A MOCON test showed that the water vapor transmission rate (WVTR) of the SiO/Al2O3 bilayer film was 0.0033 g/(m(2) day), 2.3 times lower than the rate of a single Al2O3 barrier film. The results of our study demonstrated the multipurpose role of a SiO interlayer in plasma-based OLED passivation. The layer acted as a damage-free protective layer for the underlying OLED devices and an assistant layer to improve the upper barrier film performance.

Solvent-free solution processed passivation layer for improved long-term stability of organic field-effect transistors

In an effort to realize organic field-effect transistors (OFETs) that are stable over long periods of time, we have designed an organic–inorganic hybrid passivation material (TGD622t) prepared via a non-hydrolytic sol–gel process that does not require the use of solvents. Fourier-transform infrared spectroscopy, atomic force microscopy, and UV-visible spectroscopy demonstrated the high density and low porosity of the organic–inorganic hybrid transparent TGD622t film after low-temperature curing (below 100 °C). The dense TGD622t passivation layer, which exhibited a water vapor transmission rate (WVTR) of 0.434 g m−2 per day, effectively protected the poly[9,9-dioctylfluorenyl-2,7-diyl]-co-(bithiophene)]-based OFETs from humidity and oxygen in ambient air, resulting in a much more robust OFET performance with long-term stability relative to the operation of unpassivated devices.

Solution‐Processed Organic Photovoltaic Cells with Anthracene Derivatives

Solution-processed small-molecule bulk heterojunction photovoltaic cells are fabricated by using [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) as electron acceptor and triisopropylsilylethynyl anthracene (TIPSAnt) derivatives substituted with naphthalene (TIPSAntNa) and bithiophene (TIPSAntBT) as electron donors. In contrast to TIPS-pentacene, the TIPSAnt derivatives are not susceptible to Diels-Alder reactions with PCBM when processed in solution, as confirmed by UV/Vis measurements. Photoluminescence quenching measurements show exciton diffusion lengths of 5 and 3 nm for TIPSAntBT and TIPSAntNa, respectively. Blending TIPSAntBT and TIPSAntNa with PCBM (1:1, 1:2, 1:3, and 1:4 weight ratios) produces films that possess adequate hole and electron mobilities. The morphological changes that result from varying the blending ratio range from obvious phase-segregated crystalline domains at a 1:1 ratio to homogeneous, nearly amorphous phases at a 1:4 ratio. Bulk heterojunction solar cells prepared by using a TIPSAntBT:PCBM blend reach power conversion efficiencies as high as 1.4 %.

Photopatternable Poly(4-styrene sulfonic acid)-Wrapped MWNT Thin-Film Source/Drain Electrodes for Use in Organic Field-Effect Transistors

We describe the cross-linking of poly(4-styrene-sulfonic acid) (PSS) by exposure to ultraviolet (UV) light (λ = 255 nm) under a vacuum. Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) showed that the photo-crosslinking of PSS resulted from coupling between radicals that were generated in the polymer chains by UV excitation. The photo-cross-linkable characteristics of PSS were employed to fabricate solution-processable, photopatternable, and conductive PSS-wrapped multiwalled carbon nanotube (MWNT) composite thin films by wrapping MWNTs with PSS in water. During photo-cross-linking, the work function of the PSS-wrapped MWNTs decreased from 4.83 to 4.53 eV following cleavage of a significant number of sulfonic acid groups. Despite the decreased work function of the PSS-wrapped MWNTs, the photopatterned PSS-wrapped MWNTs produced good source/drain electrodes for OFETs, yielding a mobility (0.134 ± 0.056 cm²/(V s)) for the TIPS-PEN field-effect transistors fabricated using PSS-wrapped MWNTs as source/drain electrodes that was higher than the mobility of gold-based transistors (0.011 ± 0.004 cm²/(V s)).

High Tgcyclic olefin copolymer/Al2O3 bilayer gate dielectrics for flexible organic complementary circuits with low-voltage and air-stable operation

Here we present solution-processed ultrathin cyclic olefin copolymer (COC)/Al2O3 bilayer gate dielectrics for low-voltage and flexible N,N′-ditridecyl perylene diimide (PTCDI-C13)-based n-type organic field-effect transistors (OFETs) and their complementary circuits. The PTCDI-C13 thin films grown on the COC/Al2O3 bilayer gate dielectrics formed large and flat grains with thermal treatment, resulting in high OFET performance, and stability in an ambient air atmosphere. Despite the high glass transition temperature of the COC, the COC thin films showed good mechanical flexibility with the application of bending strain, and OFETs with bilayer gate dielectrics showed stable operation up to a strain of 1.0%. Complementary inverters based on the PTCDI-C13 and pentacene OFETs with bilayer dielectrics functioned at a low voltage of 5 V, and exhibited a high gain of 63.

All-organic solution-processed two-terminal transistors fabricated using the photoinduced p-channels

All-organic solution-processed two-terminal transistors were fabricated by replacing the “field-induced p-channel” of an organic field-effect transistor with a “photoinduced p-channel.” A simple device structure—containing solution-processed 2,6-di(naphthalene-2-yl)-9,10-bis(triisopropylsilylethynyl)anthracene single crystals as the active layer (on a plastic substrate) and poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) as source and drain electrodes—enabled high-performance photosensor (and even transistor) behavior, showing pseudo-output and transfer curves with an on/off ratio of 5×102. We explain this photogenerated p-channel effect using Helfrich’s theory, which describes the photodetrapping behavior of a space-charge-limited current under illumination.