Chain-Shu Hsu

Conjugated polymer nanostructures for organic solar cell applications

Recently, there has been tremendous progress in the development of polymer-based organic solar cells. Polymer-based solar cells have attracted a great deal of attention because they have the potential to be efficient, inexpensive, and solution processable. New materials, nanostructures, device designs, and processing methods have been developed to achieve high device efficiencies. This review focuses on the fabrication techniques of conjugated polymer nanostructures and their applications for organic solar cells. We will first introduce the fundamental knowledge of organic solar cells and emphasize the importance of nanostructures. Then we will discuss different strategies for fabricating conjugated polymer nanostructures, including topics such as polymer nanowires, nanoparticles, block copolymers, layer-by-layer deposition, nanoimprint lithography, template methods, nanoelectrodes, and porous inorganic materials. The effects of the nanostructures on the device performance will also be presented. Efficiencies higher than 10% are expected for polymer-based solar cells by using new materials and techniques.

Efficient white light emission in conjugated polymer homojunctions

We study polymer light-emitting diodes with a homojunction, i.e., junction between two layers with the same host material. One layer is poly (9,9-dioctylfluorene-2,7-diyl) (PFO) host blended with a small amount of poly (2-methoxy-5 (2-ethyl-hexyloxy)-1,4-phenylene vinylene), another layer is either pure PFO or PFO host blended with green-emitting polyfluorene copolymers. Such homojunction devices are solution processed and show efficient white light emission. The peak luminance 3000cd∕m2 is reached at 10V with Internationale de L’Eclairage (CIE) coordinate (0.34, 0.34).

General method to solution-process multilayer polymer light-emitting diodes

An intermediate liquid buffer layer is introduced to overcome the dissolution problem of solution-processed multilayer conjugated polymer light-emitting diodes. This method can be applied to arbitrary combinations of polymers with no restriction on solvents. As an example, a hole-blocking layer is successfully spin coated on the common p-type emissive polymer layers. One green- and two blue-emitting polymers are chosen as the emissive layers. The electron-hole balance and efficiency are significantly improved by the addition of hole-blocking layer. The electroluminescence efficiency can be increased up to nine times, while the luminance up to seven times. In particular, 1.5cd∕A is obtained for deep blue emission from poly(9,9-dioctyl-fluorene) with 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene spin coated as the hole-blocking material.

Nano Approach Investigation of the Conduction Mechanism in Polyaniline Nanofibers

A nanotechnological approach is applied to measurements of the electric field dependence of resistance under a high electric field while in low voltage. With this technique, the conduction mechanism on a mesoscopic scale is explored in a single, nonagglomerated nanofiber. Polyaniline nanofibers are prepared by vigorous mixing of aniline and oxidation agent ammonium persulfate in acid solution. They exhibit a uniform nanoscale morphology rather than agglomeration as that produced via conventional chemical oxidation. The as-synthesized polyaniline nanofibers are doped (dedoped) with a HCl acid (NH(3) base), and their temperature behaviors of resistances follow an exponential function with an exponent of T(-1/2). To measure the conduction mechanism in a single nanofiber, the dielectrophoresis technique is implemented to position nanofibers on top of two electrodes with a nanogap of 100-600 nm, patterned by electron-beam lithography. After the devices are irradiated by electron beam to reduce contact resistances, their temperature behaviors and electric field dependences are unveiled. The experimental results agree well with the theoretical model of charging energy limited tunneling. Other theoretical models such as Efros-Shklovskii and Motts one-dimensional hopping conduction are excluded after comparisons and arguments. Through fitting, the size of the conductive grain, separation distance between two grains, and charging energy per grain in a single polyaniline nanofiber are estimated to be about 4.9 nm, 2.8 nm, and 78 meV, respectively. The nanotechnological approach, where the nanogap and the dielectrophoresis technique are used for single nanofiber device fabrication, is applied for determination of mesoscopic charge transport in a polyaniline conducting polymer.

Polymer hot-carrier transistor with low bandgap emitter

Vertical polymer hot-carrier transistor using the low bandgap material poly(3-hexylthiophene) as both the emitter and the collector are studied. The common emitter current gain is shown to depend on the LiF thickness and the emitter thickness, with maximal value at 31. Current density as high as 31mA∕cm2 is achieved when collector voltage is −10V. For the device using blend of poly(3-hexylthiophene) and high bandgap polymer poly(9-vinylcarbazole) as the emitter, the current density rises sharply to 428mA∕cm2. The brightness of 3000cd∕m2 is obtained as a polymer light-emitting diode is driven by the transistor with the same area. The transistor can be operated at 100kHz.

Increasing organic vertical carrier mobility for the application of high speed bilayered organic photodetector

The direct influence of the vertical carrier mobility on the frequency response of bilayered organic photodiodes (PDs) is investigated for the first time. With fullerene as the acceptor material, changing vertical hole mobility from 2.3×10−5 to 2.8×10−4u2002cm2/Vu2009s increases PD bandwidth from 10 to 80 MHz under a 4 V operation. The influence of deposition rate on vertical hole mobility of pentacene film is also discussed. Our results facilitate the application of bilayered organic PDs on the detection of very-high-frequency optical signals.

Polymer photodetector with voltage-adjustable photocurrent spectrum

Polymer photodetectors with voltage-adjustable photoresponse from visible to near infrared range are demonstrated. Poly(3-hexylthiophene) and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) blend is used as the active layer. The photoresponse can be continuously adjusted by the thickness of the active layer as well as the applied voltage bias. The thickness of the active layer is varied from 250 nm to 16.2u2002μm. The mechanism for the photoresponse adjusted by the thickness can be attributed to the absorption of the photons in the infrared range by thick PCBM layer. The mechanism for the photoresponse adjusted by the applied bias can be attributed to the carrier recombination reduction when the applied bias increases. The adjustable photodetector also has high operating speed up to 10 kHz.

Integration of polymer light-emitting diode and polymer waveguide on Si substrate

We integrate a polymer light-emitting diode (PLED) and a polymer waveguide on a Si substrate. The light emitted from the PLED is coupled to the waveguide by a diffuser and a reflection layer with coupling efficiency about 1%. There is no delay nor distortion between PLED emission and the light propagation in the waveguide. Good direct modulation characteristics of the waveguide output are demonstrated up to 200kHz. The device structure and processes are based on easy spin coating and are compatible to Si technology.

Polymer Infrared Proximity Sensor Array

A near-infrared proximity sensor array is achieved by integrating a polymer light-emitting diode and a polymer photodetector (PD). A green emission is converted into deep red peaked at 670 nm by the inorganic phosphor Intematix R670 with quantum efficiency of over 20%. A bandpass filter is used to select a spectral tail of phosphor luminescence with a wavelength above 700 nm. The emissive polymer is green polyfluorene. The infrared PD contains a thick film of a blend of poly(3-hexylthiophene) and (6,6)-phenyl-C61-butyric acid methyl ester up to a thickness of 8 m. Position of a moving object at a distance of 10 cm is detected in real time by the array with dynamic images displayed on the computer screen.

Effect of gate metal on polymer transistor with glass substrate

Poly(3-hexylthiophene) (P3HT) field-effect transistors (FETs) are fabricated on glass substrates with SiO2 as a gate dielectric over the gate. Indium tin oxide (ITO), Al, and Cr are employed as gate metals. For spin-coated FET, the mobility increases from 10−4–10−5cm2∕Vs for ITO and Al gates to 10−2cm2∕Vs for Cr gate. After O2 plasma treatment, the SiO2 roughness can be made as low as 0.7nm. The mobility is further improved up to 0.3cm2∕Vs by dip-coating P3HT. Crossed rods such as morphology can be observed in dip-coated FET with high mobility, indicating high degree of self-assembly facilitated by the flat SiO2 surface over Cr gate.