Sheng-Han Li

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 ...

Integration of organic light-emitting diode and organic transistor via a tandem structure

A high-performance organic active matrix pixel was fabricated by using a metal oxide (V2O5) coupling layer that effectively integrates an organic light-emitting diode (OLED) on top of an organic field-effect transistor (OFET). The field-effect mobility of the OFET approached 0.5cm2V−1s−1 and the ON/OFF current ratio was >103. The brightness of the OLED was on the order of 2000cd∕m2, with an efficiency above 3.3cd∕A. The present work describes in detail a methodology for sizing and stacking an OFET in bottom-emitting active matrix pixel circuits. The confinement of pixel dimension ensures the uniformity of light emission. The material for coupling layer can be tailored to achieve maximum device efficiency. A unique active matrix pixel circuit is proposed that renders both the OFET and OLED their individual performance after integration.

Vertical organic light emitting transistor

The authors demonstrate a vertical organic light emitting transistor achieved by stacking a capacitor on top of an organic light emitting diode (OLED). This unique device has dual functions, emitting light as an OLED and switching current as a transistor. When the capacitor is under bias, the storage charges on the thin electrode shared by two cells modulate the charge injection of the OLED active cell, hence controlling the current flow and subsequently tuning the light emission. Due to the vertical integration, this device can be operated at low voltage, which provides a solution for OLED display applications.

Achieving ambipolar vertical organic transistors via nanoscale interface modification

Organic field-effect transistors have been the subject of much recent inquiry due to their unique properties. Here, the authors report an ambipolar vertical organic field-effect transistor, which consists of a capacitor cell vertically stacked with an organic active cell, separated by a thin source electrode. By inserting a nanoscale transition-metal-oxide layer at the source/organic interface, the authors fabricated the organic ambipolar transistors with low working voltage and high current output. The thin transition-metal oxide and partial oxidization metal grains form a unique nanostructure that balances the injection barrier height of two types of carriers at the source/organic contact.

Solution-processed poly(3-hexylthiophene) vertical organic transistor

The fabrication and operation of a solution-processed vertical organic transistor are now demonstrated. The vertical structure provides a large cross section and a short channel length to counter the inherent limitations of the organic materials. The operation of a vertical organic transistor relies on a transition metal oxide layer, V2O5, to lower the carrier injection barrier at the organic/metal interface. The effect of the oxide thickness was examined to verify the role of transition metal oxide in device operation. By studying the device performance at different temperatures and in solvent environments, an operating mechanism that occurs via an ion drift and doping process was proposed. The drift direction of the dissolved Li+ ion can be controlled by altering the gate voltage bias in order to change the carrier injection barrier.

Organic single-crystal complementary inverter

The authors demonstrate the operation of an organic single-crystal complementary circuit in the form of a simple inverter. The device is constructed from a high mobility p-type organic single-crystal transistor of tetramethylpentacene (TMPC) and a n-type single-crystal transistor of N,N′-di[2,4-difluorophenyl]-3,4,9,10-perylenetetracarboxylic diimide (PTCDI). Field-effect mobilities of up to 1.0cm2∕Vs are reported for TMPC devices, while a mobility of 0.006cm2∕Vs is reported for a n-type PTCDI single-crystal device. Considering that organic single-crystal inverters have not yet been explored, they are representative of potential candidates for use in high-performance complementary circuits.

Stacked metal cathode for high-contrast-ratio polymeric light-emitting devices

An extremely high optical absorbing film made of alternating aluminum–silver layers was used as cathode in polymeric light-emitting devices (PLEDs). Physical properties of the cathodes were characterized by I–V measurement, atomic force microscopy, and x-ray photoemission spectroscopy. As a result of the slow evaporation rate, each pair of the aluminum–silver layer was shown to be in the form of aluminum–aluminum oxide nanoclusters embedded in an amorphous charge conducting network of silver. The nanoclusters helped to absorb and scatter the ambient light effectively. The use of four alternating layers structure in conventional PLEDs demonstrated 126% enhancement of contrast under 1000lx ambient illumination. The I–V characteristics of the black cathode PLEDs remained intact when compared with reference PLEDs. This technology offers precise control of the cathode quality in terms of its reflectivity and conductivity.

Controlling optical properties of electrodes with stacked metallic thin films for polymeric light-emitting diodes and displays

A semi-transparent metallic film and a high optical absorbing film were constructed with stacking metallic films. Both films were used as cathodes for polymeric light-emitting diodes (PLEDs). The semi-transparent film was made of gold/aluminum/gold thin multilayers with its optical transparency of the device reaches as high as /spl sim/70% in the visible region without capping layer, and the electrical sheet resistance reduces below 10 /spl Omega//square. During illumination of the PLED, there was approximately 47% of light emitting from the top of the cathode surface, and 53% of light from the ITO side. The high optical absorbing film, also refer to as the black cathode, was constructed with four alternating layers of aluminum-silver, each aluminum or silver layer is 4 nm thick. The PLED with this black cathode demonstrated 126% enhancement of contrast under 1000 lx ambient illumination. The physical properties of these two cathodes were characterized by current-voltage measurement and atomic force microscopy. Ultraviolet-visible transmission spectroscopy and X-ray photoemission spectroscopy were also used to characterize the semi-transparent cathode and the black cathode respectively. For polymer light-emitting device, it is believed that morphology modification at each interface of the cathode plays a crucial role in determining the optical properties and conductivity of the over cathode.

A photoelectron spectroscopy study of tunable charge injection barrier between metal/organic interface

Photoelectron spectroscopy has been used to investigate a tunable charge injection barrier at the metal/organic interface. Results in this study show that the morphology of the Al electrode in the indium tin oxide (ITO)∕LiF∕Al/pentacene structure plays a critical role. When the sample is biased across ITO and Al electrodes, shifts in the binding energies of certain core-level electrons are observed on the surface of the discontinuous thin Al electrode. In contrast, no such shifts are observed on the thick Al electrode. Further studies indicate that applying a voltage bias changes the energy alignment between the discontinuous thin Al electrode and the pentacene layer deposited on it.

Spatially localized band-gap renormalization and band-filling effects in three growth-interrupted multiple asymmetric coupled narrow quantum wells

For the first time to our knowledge, we have observed a large excitonic linewidth broadening at low temperatures in three growth-interrupted asymmetric coupled GaAs/Al0.3Ga0.7As narrow quantum wells as the irradiance increases. We attribute this broadening to the decrease of the exciton binding energy that results from spatially localized band-gap renormalization. We also observed the stepwise saturation of the photoluminescence emission peaks as irradiance increases. We attribute this saturation to the result of spatially localized band filling. Based on time-resolved photoluminescence measurements, we have determined the nature of the recombination processes. We have also determined the exciton densities. In both undoped and modulation-doped samples, the small interface island area that results from growth interruption allows us to generate a large carrier density in the islands; both band-gap renormalization and band-filling effects become stronger even at low irradiances. When the temperature is higher than the transition temperature, the free-carrier recombination dominates the photoluminescence spectrum. The band-gap renormalization then results in a red shift of the photoluminescence emissions.