Qianfei Xu

Nonvolatile electrical bistability of organic/metal-nanocluster/organic system

Two-terminal electrical bistable devices have been fabricated using a sandwich structure of organic/metal/organic as the active medium, sandwiched between two external electrodes. The nonvolatile electrical bistability of these devices can be controlled using a positive and a negative electrical bias alternatively. A forward bias may switch the device to a high-conductance state, while a reverse bias is required to restore it to a low-conductance state. In this letter, a model to explain this electrical bistability is proposed. It is found that the bistability is very sensitive to the nanostructure of the middle metal layer. For obtaining the devices with well-controlled bistability, the middle metal layer is incorporated with metal nanoclusters separated by thin oxide layers. These nanoclusters behave as the charge storage elements, which enable the nonvolatile electrical bistability when biased to a sufficiently high voltage. This mechanism is supported by the experimental data obtained from UV–visible ...

Efficient single-layer “twistacene”-doped polymer white light-emitting diodes

Bright, efficient, and stable white polymer light-emitting diodes based on blue polyfluorene doped by a “twistacene,” 6, 8, 15, 17-tetraphenyl-1.18, 4.5, 9.10, 13.14-tetrabenzoheptacene (3) (TBH), are demonstrated. In “twistacene” the terminal pyrene moieties serve two functions: (i) to stabilize the inherently unstable heptacene and (ii) to enable the oligoacene to be a strongly fluorescent molecule. As a result, efficient and very bright white polymer light-emitting diodes are obtained. The maximum luminance of the devices exceeds 20000cd∕m2. The maximum luminous efficiency is 3.55cd∕A at 4228cd∕m2 while the maximum power efficiency is 1.6lm∕W at 310cd∕m2. The device obtains a stable white balance by a combination of energy transfer from the blue polyfluorene to TBH by 1% TBH doping plus the host emission. The device emission color is not a function of bias current, which is ideal for various applications, from lighting to the backlight for liquid crystal displays.

Organic nonvolatile memory by controlling the dynamic copper-ion concentration within organic layer

Copper (Cu) migration into semiconductor materials like silicon is a well-known and troublesome phenomenon often causing adverse effect on devices. Generally a diffusion barrier layer is added to prevent Cu metallization. We demonstrate an organic nonvolatile memory device by controlling the Cu-ion (Cu+) concentration within the organic layer. When the Cu+ concentration is high enough, the device exhibits a high conductive state due to the metallization effect. When the Cu+ concentration is low, the device displays a low conductance state. These two states differ in their electrical conductivity by more than seven orders of magnitude and can be precisely switched by controlling the Cu+ concentration through the application of external biases. The retention time of both states can be more than several months, and the device is promising for flash memory application. Discussions about the device operation mechanism are provided.

Ultrahigh efficiency green polymer light-emitting diodes by nanoscale interface modification

We report highly efficient green polymer light-emitting diodes (PLEDs) achieved by introducing a nanoscale interfacial layer, made of calcium (2) acetylacetonate [Ca(acac)2], between the aluminum cathode and the green polyfluorene polymer. Ca(acac)2 is solution processible, therefore it is ideal for the fabrication of PLEDs. It is believed that the Ca(acac)2 layer plays multiple roles in enhancing the device performance. Firstly, it enhances the injection of electrons, which are the minority carriers in our green polyfluorene PLEDs. Secondly, it provides a buffer layer, preventing the quenching of luminescence from the aluminum electrode. Thirdly, it behaves as a hole-blocking layer, and subsequently enhances exciton formation. Based on Ca(acac)2/aluminum cathode, we obtained device efficiency as high as 28 cd/A at 2650 cd/m2 brightness, which is an improvement by a more than a factor of 3 over devices using calcium/aluminum as the cathode.

Polymer Optoelectronic Devices with High-Conductivity Poly(3,4-Ethylenedioxythiophene) Anodes

Abstract The conductivity of poly(3,4‐ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) film can be enhanced by more than two orders in magnitude by adding a compound with two or more polar groups, such as ethylene glycol (EG), meso‐erythritol (IUPAC name: 1,2,3,4‐tetrahydroxybutane), or 2‐nitroethanol, into the PEDOT:PSS aqueous solution. The mechanism of the increase in conductivity for PEDOT:PSS has been studied using Raman spectroscopy and atomic force microscope (AFM). Here we propose that the change in conductivity is due to the conformational change of PEDOT chains in the film. In untreated PEDOT:PSS films, coil, linear, or expanded‐coil conformations of the PEDOT chains may be present. In treated PEDOT:PSS films, the linear or expanded‐coil conformations may becomes the dominant form for PEDOT chains. This conformational change results in the enhancement of charge‐carrier mobility in the film and leads to enhanced conductivity. The high‐conductivity PEDOT:PSS film is ideal as the electrode for polymer optoelectronic devices. In this article, we report on the fabrication of polymer light‐emitting diodes (PLEDs) and photovoltaic cells (PVs) made using a highly conductive form of PEDOT:PSS as anode, and we demonstrate its performance relative to that of similar device using indium‐tin oxide (ITO) as the anode.

Enhanced efficiency of plastic photovoltaic devices by blending with ionic solid electrolytes

One of the major technology bottlenecks of polymer photovoltaic cells is the low photoinduced current, due to the low carrier mobility and short exciton migration distance. In this letter we demonstrated that the electric current for polymer PV cells can be significantly enhanced by adding a small amount of ionic solid electrolyte. Heterojunction polymer photovoltaic devices, consisting of poly[2-methoxy-5-(2′-ethyl-hexyoxy)-1,4-phenylene vinylene] (MEH-PPV) C60 and/or methanofullerene([6,6]-phenyl C61-butyric acid methyl ester) (PCBM) as the active materials, were fabricated. It has been found that the power efficiency of the organic was enhanced by blending ionic solid electrolyte, such as polyethylene oxide into the active layer. It is believed that the optimized polymer morphology, the improved electrical conductivity, and the in situ photodoping of MEH-PPV contribute to this enhancement of photovoltaic efficiency.

Experimental study on thickness-related electrical characteristics in organic/metal-nanocluster/organic systems

Organic bistable devices with the trilayer structure, organic/metal-nanocluster/organic, interposed between two electrodes have been systematically studied by varying the thickness of the organic layers and the metal-nanocluster layer. Devices fabricated in this fashion exhibit either electrical bistability or current step, depending on the thickness of the metal-nanocluster layer. Electrical bistable devices have been studied by fixing the metal-nanocluster layer thickness at 20 nm and changing the organic-layer thickness from 20 to 60 nm. Device injection current at the on state shows an exponential decrease with an increasing organic-layer thickness, suggesting that the electron transmission probability of the devices decreases with an increasing thickness of the organic layer. This is in agreement with theoretical calculations based on the single-band Hubbard model. The evolution of the electrical current step is observed for devices fabricated by fixing the organic-layer thickness at 50 nm and changi...

Nanofabrication module integrated with optical aligner

In this article, we describe a simple module that can be integrated with a commercial optical aligner for nanoimprint lithography or optical lithography. The module provides a convenient low-cost technique to transform an optical aligner for microfabrication into a nanofabrication machine. This combination enables the creation of nanoscale features and alignment of multiple-layer lithographic patterns with submicron accuracy within one instrument. Imprinting of 30 nm half-pitch lines has been demonstrated by the module, as well as submicron alignment. The module has also been used to fabricate micro- and nanoscale patterns simultaneously by the combination of optical and imprint lithography.