Chih-Wei Chu

Transition metal oxides as the buffer layer for polymer photovoltaic cells

Polymer-based photovoltaic cells have been fabricated by inserting a thin, transparent, transition metal oxide layer between the transparent anode (indium tin oxide) and the polymer layer. Two different transition metal oxides, namely vanadium oxide and molybdenum oxide, were used and the device performance was compared. The surface of the oxide films and the interface between the polymer and the oxide was studied with the help of atomic force microscopy. The effect of the thickness of the oxide layer on electrical characteristics of the device was also studied and optimized thickness was achieved to give high power conversion efficiency of 3.3% under simulated AM1.5G illumination of 100mW∕cm2.

Epitaxial growth of a monolayer WSe2-MoS2 lateral p-n junction with an atomically sharp interface

Electronic junctions on edge Two-dimensional materials such as graphene are attractive materials for making smaller transistors because they are inherently nanoscale and can carry high currents. However, graphene has no band gap and the transistors are “leaky”; that is, they are hard to turn off. Related transition metal dichalcogenides (TMDCs) such as molybdenum sulfide have band gaps. Transistors based on these materials can have high ratios of “on” to “off” currents. However, it is often difficult to make a good voltage-biased (p-n) junction between different TMDC materials. Li et al. succeeded in making p-n heterojunctions between two of these materials, molybdenum sulfide and tungsten selenide. They did this not by stacking the layers, which make a weak junction, but by growing molybdenum sulfide on the edge of a triangle of tungsten selenide with an atomically sharp boundary Science, this issue p. 524 The regrowth of the second transition metal dichalcogenide monolayer by edge epitaxy creates a lateral p-n heterojunction. Two-dimensional transition metal dichalcogenides (TMDCs) such as molybdenum sulfide MoS2 and tungsten sulfide WSe2 have potential applications in electronics because they exhibit high on-off current ratios and distinctive electro-optical properties. Spatially connected TMDC lateral heterojunctions are key components for constructing monolayer p-n rectifying diodes, light-emitting diodes, photovoltaic devices, and bipolar junction transistors. However, such structures are not readily prepared via the layer-stacking techniques, and direct growth favors the thermodynamically preferred TMDC alloys. We report the two-step epitaxial growth of lateral WSe2-MoS2 heterojunction, where the edge of WSe2 induces the epitaxial MoS2 growth despite a large lattice mismatch. The epitaxial growth process offers a controllable method to obtain lateral heterojunction with an atomically sharp interface.

High-performance organic thin-film transistors with metal oxide/metal bilayer electrode

We demonstrate bilayer source-drain (S-D) electrodes for organic thin film transistors (OTFT). The bilayer consists of a transition metal oxide (MoO3,WO3, or V2O5) layer and a metal layer. The metal oxide layer, directly contacting the organic semiconducting layer, serves as the charge-injection layer. The overcoated metal layer is responsible for the conduction of charge carriers. We found that the metal oxide layer coupled between pentacene and metal layers played an important role in improving the field-effect transistor characteristics of OTFTs. Devices with the bilayer S-D electrodes showed enhanced hole-injection compared to those with only metal electrode. High field-effect mobility of 0.4cm2∕Vs and on/off current ratios of 104 were obtained in the pentacene based TFTs using the bilayer S-D electrodes at a gate bias of −40V. The improvement is attributed to the reduction in the contact barrier and the prevention of metal diffusion into the organic layer and/or unfavorable chemical reaction between ...

Organic thin-film transistors with nanocomposite dielectric gate insulator

High-performance organic thin-film transistors (OTFTs) with a nanoparticle composite dielectric layer have been demonstrated. The dielectric layer consists of cross-linked poly-4-vinylphenol (PVP) and high-dielectric titanium dioxide (TiO2) nanoparticles. Because of the nanosize of TiO2, it disperses well in the organic solvent, which makes it possible to use solution-processable methods to prepare the dielectric layer. OTFTs with pentacene as the semiconducting layers have been demonstrated; it was found that the OTFTs with the nanocomposite dielectric layer have higher field-induced current than that of conventional devices because the dielectric constant of the gate insulator is increased. This finding opens an interesting direction for the preparation of high-performance OTFTs without complicated sputtering of high-κ dielectric materials.

Control of the nanoscale crystallinity and phase separation in polymer solar cells

Grazing-incidence x-ray diffraction and atomic force microscopy were performed on bulk heterojunction regioregular poly(3-hexylthiophene) (RR-P3HT) [6,6]-phenyl-C71-butyric acid methyl esters spin-cast films with different film processing conditions to correlate the crystalline nanostructure of P3HT with the corresponding solar cell performance. The increase in long wavelength absorption for solvent annealed films is related to highly conjugated crystal structure of RR-P3HT phase-separated in the active layer. Upon thermal annealing, the solvent annealed 50-nm-thick device shows high solar cell performance with fill factor up to 73% and power conversion efficiency of 3.80%.

Surfactant-free water-processable photoconductive all-carbon composite.

Heterojunctions between different graphitic nanostructures, including fullerenes, carbon nanotubes and graphene-based sheets, have attracted significant interest for light to electrical energy conversion. Because of their poor solubility, fabrication of such all-carbon nanocomposites typically involves covalently linking the individual constituents or the extensive surface functionalization to improve their solvent processability for mixing. However, such strategies often deteriorate or contaminate the functional carbon surfaces. Here we report that fullerenes, pristine single walled carbon nanotubes, and graphene oxide sheets can be conveniently coassembled in water to yield a stable colloidal dispersion for thin film processing. After thermal reduction of graphene oxide, a solvent-resistant photoconductive hybrid of fullerene-nanotube-graphene was obtained with on-off ratio of nearly 6 orders of magnitude. Photovoltaic devices made with the all-carbon hybrid as the active layer and an additional fullerene block layer showed unprecedented photovoltaic responses among all known all-carbon-based materials with an open circuit voltage of 0.59 V and a power conversion efficiency of 0.21%. The ease of making such surfactant-free, water-processed, carbon thin films could lead to their wide applications in organic optoelectronic devices.

Layer-by-Layer Graphene/ TCNQ Stacked Films as Conducting Anodes for Organic Solar Cells

Large-area graphene grown by chemical vapor deposition (CVD) is a promising candidate for transparent conducting electrode applications in flexible optoelectronic devices such as light-emitting diodes or organic solar cells. However, the power conversion efficiency (PCE) of the polymer photovoltaic devices using a pristine CVD graphene anode is still not appealing due to its much lower conductivity than that of conventional indium tin oxide. We report a layer-by-layer molecular doping process on graphene for forming sandwiched graphene/tetracyanoquinodimethane (TCNQ)/graphene stacked films for polymer solar cell anodes, where the TCNQ molecules (as p-dopants) were securely embedded between two graphene layers. Poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM) bulk heterojunction polymer solar cells based on these multilayered graphene/TCNQ anodes are fabricated and characterized. The P3HT/PCBM device with an anode structure composed of two TCNQ layers sandwiched by three CVD graphene layers shows optimum PCE (∼2.58%), which makes the proposed anode film quite attractive for next-generation flexible devices demanding high conductivity and transparency.

A highly stable nonbiofouling surface with well-packed grafted zwitterionic polysulfobetaine for plasma protein repulsion.

An ideal nonbiofouling surface for biomedical applications requires both high-efficient antifouling characteristics in relation to biological components and long-term material stability from biological systems. In this study we demonstrate the performance and stability of an antifouling surface with grafted zwitterionic sulfobetaine methacrylate (SBMA). The SBMA was grafted from a bromide-covered gold surface via surface-initiated atom transfer radical polymerization to form well-packed polymer brushes. Plasma protein adsorption on poly(sulfobetaine methacrylate) (polySBMA) grafted surfaces was measured with a surface plasmon resonance sensor. It is revealed that an excellent stable nonbiofouling surface with grafted polySBMA can be performed with a cycling test of the adsorption of three model proteins in a wide range of various salt types, buffer compositions, solution pH levels, and temperatures. This work also demonstrates the adsorption of plasma proteins and the adhesion of platelets from human blood plasma on the polySBMA grafted surface. It was found that the polySBMA grafted surface effectively reduces the plasma protein adsorption from platelet-poor plasma solution to a level superior to that of adsorption on a surface terminated with tetra(ethylene glycol). The adhesion and activation of platelets from platelet-rich plasma solution were not observed on the polySBMA grafted surface. This work further concludes that a surface with good hemocompatibility can be achieved by the well-packed surface-grafted polySBMA brushes.

Electric-field-induced charge transfer between gold nanoparticle and capping 2-naphthalenethiol and organic memory cells

An electrical transition, induced by a high electric field, was observed in a device consisting of a polystyrene film containing 2-naphthalenethiol-capped Au nanoparticles sandwiched between two Al electrodes. The current through the device changed from a charge-injection-controlled current to a space-charge-limited current. The latter current is higher than the former by more than three orders of magnitude at 2 V. Asymmetrical current-voltage curve was observed along the two polarity directions for the device after the transition. This transition is attributed to an electric-field-induced charge transfer between the Au nanoparticle and the capping 2-naphthalenethiol. The device exhibits good stability in the high conductivity state, so it can be used as a write-once-read-many times electronic memory.

Effective connecting architecture for tandem organic light-emitting devices

An effective connecting structure for tandem organic light-emitting devices is reported. The connecting structure consists of a thin metal layer as the common electrode, a hole-injection layer containing MoO3 on one side of the common electrode, and an electron-injection layer involving Cs2CO3 on the other side. Such a connecting structure permits efficient opposite hole and electron injection into two adjacent emitting units and gives tandem devices superior electrical and optical performances. Furthermore, the present connecting structure involves no sputtering or handling of reactive metals during device fabrication and can be prepared purely by thermal evaporation, thus rendering device processing more feasible.