Won-Tae Park | Dongguk University (original) (raw)

Papers by Won-Tae Park

ACS applied materials & interfaces, Jan 30, 2016

Charge transport in carbon nanotube network transistors strongly depends on the properties of the... more Charge transport in carbon nanotube network transistors strongly depends on the properties of the gate dielectric that is in direct contact with the semiconducting carbon nanotubes. In this work, we investigate the dielectric effects on charge transport in polymer-sorted semiconducting single-walled carbon nanotube field-effect transistors (s-SWNT-FETs) by using three different polymer insulators: A low-permittivity (εr) fluoropolymer (CYTOP, εr = 1.8), poly(methyl methacrylate) (PMMA, εr = 3.3), and a high-εr ferroelectric relaxor [P(VDF-TrFE-CTFE), εr = 14.2]. The s-SWNT-FETs with polymer dielectrics show typical ambipolar charge transport with high ON/OFF ratios (up to ∼10(5)) and mobilities (hole mobility up to 6.77 cm(2) V(-1) s(-1) for CYTOP). The s-SWNT-FET with the lowest-k dielectric, CYTOP, exhibits the highest mobility owing to formation of a favorable interface for charge transport, which is confirmed by the lowest activation energies, evaluated by the fluctuation-induce...

ACS Applied Materials & Interfaces, 2017

We report on the fabrication of an organic thin-film semiconductor formed using a blend solution ... more We report on the fabrication of an organic thin-film semiconductor formed using a blend solution of soluble ambipolar small molecules and an insulating polymer binder that exhibits vertical phase separation and uniform film formation. The semiconductor thin films are produced in a single step from a mixture containing a small molecular semiconductor, namely, quinoidal biselenophene (QBS), and a binder polymer, namely, poly(2-vinylnaphthalene) (PVN). Organic field-effect transistors (OFETs) based on QBS/PVN blend semiconductor are then assembled using top-gate/bottom-contact device configuration, which achieve almost four times higher mobility than the neat QBS semiconductor. Depth profile via secondary ion mass spectrometry and atomic force microscopy images indicate that the QBS domains in the films made from the blend are evenly distributed with a smooth morphology at the bottom of the PVN layer. Bias stress test and variable-temperature measurements on QBS-based OFETs reveal that the QBS/PVN blend semiconductor remarkably reduces the number of trap sites at the gate dielectric/semiconductor interface and the activation energy in the transistor channel. This work provides a one-step solution processing technique, which makes use of soluble ambipolar small molecules to form a thin-film semiconductor for application in high-performance OFETs.

Advanced Materials, 2016

A highly conductive, air stable and scalable poly(3,4-ethylenedioxythiophene) (PEDOT): poly(4-sty... more A highly conductive, air stable and scalable poly(3,4-ethylenedioxythiophene) (PEDOT): poly(4-styrenesulfonate) (PEDOT:PSS) are prepared by using mass production ultrafiltration. By effectively removing excess PSS and various reaction impurities using repeated 100 nm pore membrane filtration, purified PEDOT:PSS exhibit conductivity as high as 2000 S cm(-1) .

Scientific Reports, 2016

The general form of interfacial contact resistance was derived for organic thin-film transistors ... more The general form of interfacial contact resistance was derived for organic thin-film transistors (OTFTs) covering various injection mechanisms. Devices with a broad range of materials for contacts, semiconductors, and dielectrics were investigated and the charge injections in staggered OTFTs was found to universally follow the proposed form in the diffusion-limited case, which is signified by the mobility-dependent injection at the metal-semiconductor interfaces. Hence, real ohmic contact can hardly ever be achieved in OTFTs with low carrier concentrations and mobility, and the injection mechanisms include thermionic emission, diffusion, and surface recombination. The non-ohmic injection in OTFTs is manifested by the generally observed hook shape of the output conductance as a function of the drain field. The combined theoretical and experimental results show that interfacial contact resistance generally decreases with carrier mobility, and the injection current is probably determined by the surface recombination rate, which can be promoted by bulk-doping, contact modifications with charge injection layers and dopant layers, and dielectric engineering with high-k dielectric materials.

ACS applied materials & interfaces, Jan 13, 2016

The threshold voltage and onset voltage for p-channel and n-channel regimes of solution-processed... more The threshold voltage and onset voltage for p-channel and n-channel regimes of solution-processed ambipolar organic transistors with top-gate/bottom-contact (TG/BC) geometry were effectively tuned by gate buffer layers in between the gate electrode and the dielectric. The work function of a pristine Al gate electrode (-4.1 eV) was modified by cesium carbonate and vanadium oxide to -2.1 and -5.1 eV, respectively, which could control the flat-band voltage, leading to a remarkable shift of transfer curves in both negative and positive gate voltage directions without any side effects. One important feature is that the mobility of transistors is not very sensitive to the gate buffer layer. This method is simple but useful for electronic devices where the threshold voltage should be precisely controlled, such as ambipolar circuits, memory devices, and light-emitting device applications.

Advanced materials (Deerfield Beach, Fla.), 2016

A precise control over the film thickness is a vital requirement for achievement of high performa... more A precise control over the film thickness is a vital requirement for achievement of high performance in thin-film electronic devices. On page 2752, Y.-Y. Noh and co-workers develop an effective way to deposit a large-area and uniform ultrathin polymer film with a molecular-level precision via a simple wire-wound bar-coating method for high-performance organic transistors and gas sensors.

Journal of the American Chemical Society, Jan 6, 2016

Charge carriers typically move faster in crystalline regions than in amorphous regions in conjuga... more Charge carriers typically move faster in crystalline regions than in amorphous regions in conjugated polymers because polymer chains adopt a regular arrangement resulting in a high degree of π-π stacking in crystalline regions. In contrast, the random polymer chain orientation in amorphous regions hinders connectivity between conjugated backbones; thus, it hinders charge carrier delocalization. Various studies have attempted to enhance charge carrier transport by increasing crystallinity. However, these approaches are inevitably limited by the semicrystalline nature of conjugated polymers. Moreover, high-crystallinity conjugated polymers have proven inadequate for soft electronics applications because of their poor mechanical resilience. Increasing the polymer chain connectivity by forming localized aggregates via π-orbital overlap between several conjugated backbones in amorphous regions provides a more effective approach to efficient charge carrier transport. A simple strategy rel...

Advanced Materials, 2016

An effective approach to deposit a uniform ultrathin polymer film over a large area with molecula... more An effective approach to deposit a uniform ultrathin polymer film over a large area with molecular level precision is demonstrated by the simple wire-wound bar-coating method. The bar-coated ultrathin films not only exhibit high transparency of up to 90% in the visible wavelength range but also high charge carrier mobility with a high degree of percolation through the uniformly covered polymer nanofibrils. They are capable of realizing highly sensitive multigas sensors and represent the first successful report of ethylene detection using a sensor based on organic field-effect transistors.

Advanced materials (Deerfield Beach, Fla.), Jan 25, 2015

The universal role of high-k fluorinated dielectrics in assisting the carrier transport in transi... more The universal role of high-k fluorinated dielectrics in assisting the carrier transport in transistors for a broad range of printable semiconductor is explored. These results present general rules for how to design dielectric materials and achieve devices with a high carrier concentration, low disorder, reliable operation, and robust properties.

Organic solar cells (OSCs) based on polymers and small molecules have seen a tremendous increase ... more Organic solar cells (OSCs) based on polymers and small molecules have seen a tremendous increase in interest during the past few years. Signifi cant progress in this fi eld seeded the prospect for a cost-effective and easy-to-fabricate photovoltaic technology-typical advantages claimed for organic (opto-)electronic devices. Very recently, certifi ed cell effi ciencies in excess of 7% have been reported for polymer based cells. For large-scale and high-throughput production of OSCs, liquid processing of the functional layers is desirable. Aside from the active organic layers, inter-layers are typically required to facilitate the extraction of the photo-generated charges. Specifi cally, on the anode side, polyethylene dioxythiophene:polystyrenesulfonate (PEDOT:PSS) is regularly used. However, PEDOT:PSS is burdened with structural and electrical inhomogeneity and has been demonstrated to be an origin of limited device lifetime. Particularly, the aqueous PEDOT:PSS dispersion and the acidic nature can cause substantial degradation. Very recently, transition metal-oxides (TMOs) such as molybdenum-, vanadium-, or tungsten-oxide (MoO 3 , V 2 O 5 , and WO 3 ) with high work functions (WFs) of up to 6.9 eV have been shown to be promising alternatives to PEDOT:PSS. TMOs have also been used as constituents of the connecting architecture in stacked organic light-emitting diodes and organic tandem solar cells. The unique energetics of these TMOs has so far been predominantly accessible for fi lms thermally evaporated in high-vacuum.

Organic Electronics

We demonstrate high-performance flexible polymer OFETs with P-29-DPP-SVS in various geometries. T... more We demonstrate high-performance flexible polymer OFETs with P-29-DPP-SVS in various geometries. The mobilities of TG/BC OFETs are approximately 3.48 ± 0.93 cm2/V s on a glass substrate and 2.98 ± 0.19 cm2/V s on a PEN substrate. The flexible P-29-DPP-SVS OFETs exhibit excellent ambient and mechanical stabilities under a continuous bending stress of 1200 times at an R = 8.3 mm. In particular, the variation of μFET, VTh and leakage current was very negligible (below 10%) after continuous bending stress. The BG/TC P-29-DPP-SVS OFETs on a PEN substrate applies to flexible NH3 gas sensors. As the concentration of NH3 increased, the channel resistance of P-29-DPP-SVS OFETs increased approximately 100 times from ∼107 to ∼109 Ω at VSD = −5 V and VGS = −5 V.

Chemical Communications

A π-conjugated polyazine containing diketopyrrolopyrrole (DPP) moiety, PDBTAZ, is synthesized thr... more A π-conjugated polyazine containing diketopyrrolopyrrole (DPP) moiety, PDBTAZ, is synthesized through a simple condensation polymerization. PDBTAZ is found to be a high-performance ambipolar semiconductor in organic thin film transistors (OTFTs), showing an electron mobility of up to 0.41 cm(2) V(-1) s(-1) and a hole mobility of up to 0.36 cm(2) V(-1) s(-1).

Advanced materials (Deerfield Beach, Fla.), Jan 26, 2015

Ultrathin and dense metal oxide gate di-electric layers are reported by a simple printing of AlOx... more Ultrathin and dense metal oxide gate di-electric layers are reported by a simple printing of AlOx and HfOx sol-gel precursors. Large-area printed indium gallium zinc oxide (IGZO) thin-film transistor arrays, which exhibit mobilities >5 cm(2) V(-1) s(-1) and gate leakage current of 10(-9) A cm(-2) at a very low operation voltage of 2 V, are demonstrated by continuous simple bar-coated processes.

Advanced materials (Deerfield Beach, Fla.), 2015

Ultra-thin and dense sol-gel metal oxide films for gate dielectric layers in large-area printed m... more Ultra-thin and dense sol-gel metal oxide films for gate dielectric layers in large-area printed metal oxide and organic thin-film transistors are reported by a simple wire bar coating method. Y.-Y. Noh, M.-H. Yoon, and co-workers describe this method on page 5043.

Chemical Communications, 2012

A π-conjugated polyazine containing diketopyrrolopyrrole (DPP) moiety, PDBTAZ, is synthesized thr... more A π-conjugated polyazine containing diketopyrrolopyrrole (DPP) moiety, PDBTAZ, is synthesized through a simple condensation polymerization. PDBTAZ is found to be a high-performance ambipolar semiconductor in organic thin film transistors (OTFTs), showing an electron mobility of up to 0.41 cm(2) V(-1) s(-1) and a hole mobility of up to 0.36 cm(2) V(-1) s(-1).

ACS Applied Materials & Interfaces, 2014

In order to determine the effects of actual &... more In order to determine the effects of actual 'chalcogen atoms' on semiconducting properties for application in a variety of optoelectronic devices, a class of donor (D)-acceptor (A) polymer semiconductors, namely PBDP-Fu, PBDP-Th, and PBDP-Se, containing the recently formulated benzodipyrrolidone (BDP) accepting unit and furan (Fu), thiophene (Th), or selenophene (Se) as a donating unit has been synthesized, characterized, and used in an active layer of organic field-effect transistors (OFETs). With the LUMO levels being comparatively consistent for all three polymers (-3.58 to -3.60 eV) due to the dominant BDP contribution to the polymer backbone, the HOMO energies are somewhat sensitive to the structurally distinctive feature of the donor counits used. Utilizing a combination of X-ray diffraction (XRD) and atomic force microscopy (AFM), it is apparent that further crystalline domains occur with edge-on orientation for the polymers (PBDP-Th and PBDP-Se) with relatively heavier chalcogen atoms such as Th and Se, compared with PBDP-Fu which has a rather amorphous nature. Investigation of their OFET performance indicates that all the polymers show well balanced ambipolar operations. The desirable morphological structures of both the PBDP-Th and PBDP-Se result in higher mobilities in OFETs than those of PBDP-Fu. In particular, 200 °C annealed PBDP-Se OFETs results in ambipolarity being mobile for both holes of up to 1.7 × 10(-2) cm(2)/V·s and electrodes of up to 1.9 × 10(-2) cm(2)/V·s. In addition, OFETs with PBDP-Th show nearly equivalent charge carrier mobilities for both holes (μ(h) = 1.2 × 10(-2) cm(2)/V·s) and electrons (μ(e) = 1.1 × 10(-2) cm(2)/V·s). Consequently, we systematically demonstrate how the manipulation of existing heteroaromatics can modulate the electronic properties of conjugated D-A polymers, elucidating structure-property relationships that are desirable for the rational design of next generation materials.

ACS applied materials & interfaces, Jan 27, 2014

We report on a technique using mixed self-assembled monolayers (SAMs) to finely regulate ambipola... more We report on a technique using mixed self-assembled monolayers (SAMs) to finely regulate ambipolar charge injection in polymer organic field-effect transistors. Differing from the other works that employ single SAM specifically for efficient charge injection in p-type and n-type transistors, we blend two different SAMs of alkyl- and perfluoroalkyl thiols at different ratios and apply them to ambipolar OFETs and inverter. Thanks to the utilization of ambipolar semiconductor and one SAM mixture, the device and circuit fabrications are facile with only one step for semiconductor deposition and another for SAM treatment. This is much simpler with respect to the conventional scheme for the unipolar-device-based complementary circuitry that demands separate deposition and processing for individual p-channel and n-channel transistors. Our results show that the mixed-SAM treatments not only improve ambipolar charge injection manifesting as higher hole- and electron-mobility and smaller thre...

Molecular Crystals and Liquid Crystals, 2014

ABSTRACT We report ambipolar organic field effect transistors (OFETs) and complementary inverters... more ABSTRACT We report ambipolar organic field effect transistors (OFETs) and complementary inverters based on polyazine containing diketopyrrolopyrrole (PBTDAZ). PDBTAZ OFETs showed well-balanced electron and hole mobilities of 0.13 and 0.12 cm2/(V·s) at a low annealing temperature of 100oC using bottom-contact, top-gate geometry with a CYTOPTM gate dielectric layer. The mobilities were improved to 0.56 cm2/(V·s) by thermal annealing (150°C). The ambipolar complementary-inverter-based PDBTAZ OFETs showed ideal Vinv near half of VDD with a high gain of 15∼20 due the well-balanced electron and hole mobilities.

Advanced Functional Materials, 2014

ABSTRACT Despite extensive progress in organic field-effect transistors, there are still far fewe... more ABSTRACT Despite extensive progress in organic field-effect transistors, there are still far fewer reliable, high-mobility n-type polymers than p-type polymers. It is demonstrated that by using dopants at a critical doping molar ratio (MR), performance of n-type polymer poly[[N,N9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)] (P(NDI2DO-T2)) field-effect transistors (FETs) can be significantly improved and simultaneously optimized in mobility, on–off ratio, crystallinity, injection, and reliability. In particular, when using the organic dopant bis(cyclopentadienyl)–cobalt(II) (cobaltocene, CoCp2) at a low concentration (0.05 wt%), the FET mobility is increased from 0.34 to 0.72 cm2 V–1 s–1, and the threshold voltage was decreased from 32.7 to 8.8 V. The relationship between the MR of dopants and electrical characteristics as well as the evolution in polymer crystallinity revealed by synchrotron X-ray diffractions are systematically investigated. Deviating from previous discoveries, it is found that mobility increases first and then decreases drastically beyond a critical value of MR. Meanwhile, the intensity and width of the main peak of in-plane X-ray diffraction start to decrease at the same critical MR. Thus, the mobility decrease is correlated with the disturbed in-plane crystallinity of the conjugated polymer, for both organic and inorganic dopants. The method provides a simple and efficient approach to employing dopants to optimize the electrical performance and microstructure of P(NDI2DO-T2).

Applied Physics Letters, 2014

ACS applied materials & interfaces, Jan 30, 2016

Charge transport in carbon nanotube network transistors strongly depends on the properties of the... more Charge transport in carbon nanotube network transistors strongly depends on the properties of the gate dielectric that is in direct contact with the semiconducting carbon nanotubes. In this work, we investigate the dielectric effects on charge transport in polymer-sorted semiconducting single-walled carbon nanotube field-effect transistors (s-SWNT-FETs) by using three different polymer insulators: A low-permittivity (εr) fluoropolymer (CYTOP, εr = 1.8), poly(methyl methacrylate) (PMMA, εr = 3.3), and a high-εr ferroelectric relaxor [P(VDF-TrFE-CTFE), εr = 14.2]. The s-SWNT-FETs with polymer dielectrics show typical ambipolar charge transport with high ON/OFF ratios (up to ∼10(5)) and mobilities (hole mobility up to 6.77 cm(2) V(-1) s(-1) for CYTOP). The s-SWNT-FET with the lowest-k dielectric, CYTOP, exhibits the highest mobility owing to formation of a favorable interface for charge transport, which is confirmed by the lowest activation energies, evaluated by the fluctuation-induce...

ACS Applied Materials & Interfaces, 2017

We report on the fabrication of an organic thin-film semiconductor formed using a blend solution ... more We report on the fabrication of an organic thin-film semiconductor formed using a blend solution of soluble ambipolar small molecules and an insulating polymer binder that exhibits vertical phase separation and uniform film formation. The semiconductor thin films are produced in a single step from a mixture containing a small molecular semiconductor, namely, quinoidal biselenophene (QBS), and a binder polymer, namely, poly(2-vinylnaphthalene) (PVN). Organic field-effect transistors (OFETs) based on QBS/PVN blend semiconductor are then assembled using top-gate/bottom-contact device configuration, which achieve almost four times higher mobility than the neat QBS semiconductor. Depth profile via secondary ion mass spectrometry and atomic force microscopy images indicate that the QBS domains in the films made from the blend are evenly distributed with a smooth morphology at the bottom of the PVN layer. Bias stress test and variable-temperature measurements on QBS-based OFETs reveal that the QBS/PVN blend semiconductor remarkably reduces the number of trap sites at the gate dielectric/semiconductor interface and the activation energy in the transistor channel. This work provides a one-step solution processing technique, which makes use of soluble ambipolar small molecules to form a thin-film semiconductor for application in high-performance OFETs.

Advanced Materials, 2016

A highly conductive, air stable and scalable poly(3,4-ethylenedioxythiophene) (PEDOT): poly(4-sty... more A highly conductive, air stable and scalable poly(3,4-ethylenedioxythiophene) (PEDOT): poly(4-styrenesulfonate) (PEDOT:PSS) are prepared by using mass production ultrafiltration. By effectively removing excess PSS and various reaction impurities using repeated 100 nm pore membrane filtration, purified PEDOT:PSS exhibit conductivity as high as 2000 S cm(-1) .

Scientific Reports, 2016

The general form of interfacial contact resistance was derived for organic thin-film transistors ... more The general form of interfacial contact resistance was derived for organic thin-film transistors (OTFTs) covering various injection mechanisms. Devices with a broad range of materials for contacts, semiconductors, and dielectrics were investigated and the charge injections in staggered OTFTs was found to universally follow the proposed form in the diffusion-limited case, which is signified by the mobility-dependent injection at the metal-semiconductor interfaces. Hence, real ohmic contact can hardly ever be achieved in OTFTs with low carrier concentrations and mobility, and the injection mechanisms include thermionic emission, diffusion, and surface recombination. The non-ohmic injection in OTFTs is manifested by the generally observed hook shape of the output conductance as a function of the drain field. The combined theoretical and experimental results show that interfacial contact resistance generally decreases with carrier mobility, and the injection current is probably determined by the surface recombination rate, which can be promoted by bulk-doping, contact modifications with charge injection layers and dopant layers, and dielectric engineering with high-k dielectric materials.

ACS applied materials & interfaces, Jan 13, 2016

The threshold voltage and onset voltage for p-channel and n-channel regimes of solution-processed... more The threshold voltage and onset voltage for p-channel and n-channel regimes of solution-processed ambipolar organic transistors with top-gate/bottom-contact (TG/BC) geometry were effectively tuned by gate buffer layers in between the gate electrode and the dielectric. The work function of a pristine Al gate electrode (-4.1 eV) was modified by cesium carbonate and vanadium oxide to -2.1 and -5.1 eV, respectively, which could control the flat-band voltage, leading to a remarkable shift of transfer curves in both negative and positive gate voltage directions without any side effects. One important feature is that the mobility of transistors is not very sensitive to the gate buffer layer. This method is simple but useful for electronic devices where the threshold voltage should be precisely controlled, such as ambipolar circuits, memory devices, and light-emitting device applications.

Advanced materials (Deerfield Beach, Fla.), 2016

A precise control over the film thickness is a vital requirement for achievement of high performa... more A precise control over the film thickness is a vital requirement for achievement of high performance in thin-film electronic devices. On page 2752, Y.-Y. Noh and co-workers develop an effective way to deposit a large-area and uniform ultrathin polymer film with a molecular-level precision via a simple wire-wound bar-coating method for high-performance organic transistors and gas sensors.

Journal of the American Chemical Society, Jan 6, 2016

Charge carriers typically move faster in crystalline regions than in amorphous regions in conjuga... more Charge carriers typically move faster in crystalline regions than in amorphous regions in conjugated polymers because polymer chains adopt a regular arrangement resulting in a high degree of π-π stacking in crystalline regions. In contrast, the random polymer chain orientation in amorphous regions hinders connectivity between conjugated backbones; thus, it hinders charge carrier delocalization. Various studies have attempted to enhance charge carrier transport by increasing crystallinity. However, these approaches are inevitably limited by the semicrystalline nature of conjugated polymers. Moreover, high-crystallinity conjugated polymers have proven inadequate for soft electronics applications because of their poor mechanical resilience. Increasing the polymer chain connectivity by forming localized aggregates via π-orbital overlap between several conjugated backbones in amorphous regions provides a more effective approach to efficient charge carrier transport. A simple strategy rel...

Advanced Materials, 2016

An effective approach to deposit a uniform ultrathin polymer film over a large area with molecula... more An effective approach to deposit a uniform ultrathin polymer film over a large area with molecular level precision is demonstrated by the simple wire-wound bar-coating method. The bar-coated ultrathin films not only exhibit high transparency of up to 90% in the visible wavelength range but also high charge carrier mobility with a high degree of percolation through the uniformly covered polymer nanofibrils. They are capable of realizing highly sensitive multigas sensors and represent the first successful report of ethylene detection using a sensor based on organic field-effect transistors.

Advanced materials (Deerfield Beach, Fla.), Jan 25, 2015

The universal role of high-k fluorinated dielectrics in assisting the carrier transport in transi... more The universal role of high-k fluorinated dielectrics in assisting the carrier transport in transistors for a broad range of printable semiconductor is explored. These results present general rules for how to design dielectric materials and achieve devices with a high carrier concentration, low disorder, reliable operation, and robust properties.

Organic solar cells (OSCs) based on polymers and small molecules have seen a tremendous increase ... more Organic solar cells (OSCs) based on polymers and small molecules have seen a tremendous increase in interest during the past few years. Signifi cant progress in this fi eld seeded the prospect for a cost-effective and easy-to-fabricate photovoltaic technology-typical advantages claimed for organic (opto-)electronic devices. Very recently, certifi ed cell effi ciencies in excess of 7% have been reported for polymer based cells. For large-scale and high-throughput production of OSCs, liquid processing of the functional layers is desirable. Aside from the active organic layers, inter-layers are typically required to facilitate the extraction of the photo-generated charges. Specifi cally, on the anode side, polyethylene dioxythiophene:polystyrenesulfonate (PEDOT:PSS) is regularly used. However, PEDOT:PSS is burdened with structural and electrical inhomogeneity and has been demonstrated to be an origin of limited device lifetime. Particularly, the aqueous PEDOT:PSS dispersion and the acidic nature can cause substantial degradation. Very recently, transition metal-oxides (TMOs) such as molybdenum-, vanadium-, or tungsten-oxide (MoO 3 , V 2 O 5 , and WO 3 ) with high work functions (WFs) of up to 6.9 eV have been shown to be promising alternatives to PEDOT:PSS. TMOs have also been used as constituents of the connecting architecture in stacked organic light-emitting diodes and organic tandem solar cells. The unique energetics of these TMOs has so far been predominantly accessible for fi lms thermally evaporated in high-vacuum.

Organic Electronics

We demonstrate high-performance flexible polymer OFETs with P-29-DPP-SVS in various geometries. T... more We demonstrate high-performance flexible polymer OFETs with P-29-DPP-SVS in various geometries. The mobilities of TG/BC OFETs are approximately 3.48 ± 0.93 cm2/V s on a glass substrate and 2.98 ± 0.19 cm2/V s on a PEN substrate. The flexible P-29-DPP-SVS OFETs exhibit excellent ambient and mechanical stabilities under a continuous bending stress of 1200 times at an R = 8.3 mm. In particular, the variation of μFET, VTh and leakage current was very negligible (below 10%) after continuous bending stress. The BG/TC P-29-DPP-SVS OFETs on a PEN substrate applies to flexible NH3 gas sensors. As the concentration of NH3 increased, the channel resistance of P-29-DPP-SVS OFETs increased approximately 100 times from ∼107 to ∼109 Ω at VSD = −5 V and VGS = −5 V.

Chemical Communications

A π-conjugated polyazine containing diketopyrrolopyrrole (DPP) moiety, PDBTAZ, is synthesized thr... more A π-conjugated polyazine containing diketopyrrolopyrrole (DPP) moiety, PDBTAZ, is synthesized through a simple condensation polymerization. PDBTAZ is found to be a high-performance ambipolar semiconductor in organic thin film transistors (OTFTs), showing an electron mobility of up to 0.41 cm(2) V(-1) s(-1) and a hole mobility of up to 0.36 cm(2) V(-1) s(-1).

Advanced materials (Deerfield Beach, Fla.), Jan 26, 2015

Ultrathin and dense metal oxide gate di-electric layers are reported by a simple printing of AlOx... more Ultrathin and dense metal oxide gate di-electric layers are reported by a simple printing of AlOx and HfOx sol-gel precursors. Large-area printed indium gallium zinc oxide (IGZO) thin-film transistor arrays, which exhibit mobilities >5 cm(2) V(-1) s(-1) and gate leakage current of 10(-9) A cm(-2) at a very low operation voltage of 2 V, are demonstrated by continuous simple bar-coated processes.

Advanced materials (Deerfield Beach, Fla.), 2015

Ultra-thin and dense sol-gel metal oxide films for gate dielectric layers in large-area printed m... more Ultra-thin and dense sol-gel metal oxide films for gate dielectric layers in large-area printed metal oxide and organic thin-film transistors are reported by a simple wire bar coating method. Y.-Y. Noh, M.-H. Yoon, and co-workers describe this method on page 5043.

Chemical Communications, 2012

A π-conjugated polyazine containing diketopyrrolopyrrole (DPP) moiety, PDBTAZ, is synthesized thr... more A π-conjugated polyazine containing diketopyrrolopyrrole (DPP) moiety, PDBTAZ, is synthesized through a simple condensation polymerization. PDBTAZ is found to be a high-performance ambipolar semiconductor in organic thin film transistors (OTFTs), showing an electron mobility of up to 0.41 cm(2) V(-1) s(-1) and a hole mobility of up to 0.36 cm(2) V(-1) s(-1).

ACS Applied Materials & Interfaces, 2014

In order to determine the effects of actual &... more In order to determine the effects of actual 'chalcogen atoms' on semiconducting properties for application in a variety of optoelectronic devices, a class of donor (D)-acceptor (A) polymer semiconductors, namely PBDP-Fu, PBDP-Th, and PBDP-Se, containing the recently formulated benzodipyrrolidone (BDP) accepting unit and furan (Fu), thiophene (Th), or selenophene (Se) as a donating unit has been synthesized, characterized, and used in an active layer of organic field-effect transistors (OFETs). With the LUMO levels being comparatively consistent for all three polymers (-3.58 to -3.60 eV) due to the dominant BDP contribution to the polymer backbone, the HOMO energies are somewhat sensitive to the structurally distinctive feature of the donor counits used. Utilizing a combination of X-ray diffraction (XRD) and atomic force microscopy (AFM), it is apparent that further crystalline domains occur with edge-on orientation for the polymers (PBDP-Th and PBDP-Se) with relatively heavier chalcogen atoms such as Th and Se, compared with PBDP-Fu which has a rather amorphous nature. Investigation of their OFET performance indicates that all the polymers show well balanced ambipolar operations. The desirable morphological structures of both the PBDP-Th and PBDP-Se result in higher mobilities in OFETs than those of PBDP-Fu. In particular, 200 °C annealed PBDP-Se OFETs results in ambipolarity being mobile for both holes of up to 1.7 × 10(-2) cm(2)/V·s and electrodes of up to 1.9 × 10(-2) cm(2)/V·s. In addition, OFETs with PBDP-Th show nearly equivalent charge carrier mobilities for both holes (μ(h) = 1.2 × 10(-2) cm(2)/V·s) and electrons (μ(e) = 1.1 × 10(-2) cm(2)/V·s). Consequently, we systematically demonstrate how the manipulation of existing heteroaromatics can modulate the electronic properties of conjugated D-A polymers, elucidating structure-property relationships that are desirable for the rational design of next generation materials.

ACS applied materials & interfaces, Jan 27, 2014

We report on a technique using mixed self-assembled monolayers (SAMs) to finely regulate ambipola... more We report on a technique using mixed self-assembled monolayers (SAMs) to finely regulate ambipolar charge injection in polymer organic field-effect transistors. Differing from the other works that employ single SAM specifically for efficient charge injection in p-type and n-type transistors, we blend two different SAMs of alkyl- and perfluoroalkyl thiols at different ratios and apply them to ambipolar OFETs and inverter. Thanks to the utilization of ambipolar semiconductor and one SAM mixture, the device and circuit fabrications are facile with only one step for semiconductor deposition and another for SAM treatment. This is much simpler with respect to the conventional scheme for the unipolar-device-based complementary circuitry that demands separate deposition and processing for individual p-channel and n-channel transistors. Our results show that the mixed-SAM treatments not only improve ambipolar charge injection manifesting as higher hole- and electron-mobility and smaller thre...

Molecular Crystals and Liquid Crystals, 2014

ABSTRACT We report ambipolar organic field effect transistors (OFETs) and complementary inverters... more ABSTRACT We report ambipolar organic field effect transistors (OFETs) and complementary inverters based on polyazine containing diketopyrrolopyrrole (PBTDAZ). PDBTAZ OFETs showed well-balanced electron and hole mobilities of 0.13 and 0.12 cm2/(V·s) at a low annealing temperature of 100oC using bottom-contact, top-gate geometry with a CYTOPTM gate dielectric layer. The mobilities were improved to 0.56 cm2/(V·s) by thermal annealing (150°C). The ambipolar complementary-inverter-based PDBTAZ OFETs showed ideal Vinv near half of VDD with a high gain of 15∼20 due the well-balanced electron and hole mobilities.

Advanced Functional Materials, 2014

ABSTRACT Despite extensive progress in organic field-effect transistors, there are still far fewe... more ABSTRACT Despite extensive progress in organic field-effect transistors, there are still far fewer reliable, high-mobility n-type polymers than p-type polymers. It is demonstrated that by using dopants at a critical doping molar ratio (MR), performance of n-type polymer poly[[N,N9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)] (P(NDI2DO-T2)) field-effect transistors (FETs) can be significantly improved and simultaneously optimized in mobility, on–off ratio, crystallinity, injection, and reliability. In particular, when using the organic dopant bis(cyclopentadienyl)–cobalt(II) (cobaltocene, CoCp2) at a low concentration (0.05 wt%), the FET mobility is increased from 0.34 to 0.72 cm2 V–1 s–1, and the threshold voltage was decreased from 32.7 to 8.8 V. The relationship between the MR of dopants and electrical characteristics as well as the evolution in polymer crystallinity revealed by synchrotron X-ray diffractions are systematically investigated. Deviating from previous discoveries, it is found that mobility increases first and then decreases drastically beyond a critical value of MR. Meanwhile, the intensity and width of the main peak of in-plane X-ray diffraction start to decrease at the same critical MR. Thus, the mobility decrease is correlated with the disturbed in-plane crystallinity of the conjugated polymer, for both organic and inorganic dopants. The method provides a simple and efficient approach to employing dopants to optimize the electrical performance and microstructure of P(NDI2DO-T2).

Applied Physics Letters, 2014