Yu-Chiang Chao

The effect of carrier mobility in organic solar cells

The microscopic states and performance of organic solar cell are investigated theoretically to explore the effect of the carrier mobility. With Ohmic contacts between the semiconductor and the metal electrodes there are two origins of carriers in the semiconductor: the photocarriers generated by photon absorption and the dark carriers diffused from the electrodes. The power efficiency of the solar cell is limited by the recombination of a carrier with either the photocarrier or a dark carrier. Near the short-circuit condition the photocarrier recombination in the semiconductor bulk decreases as the mobility increases. Near the open-circuit condition the dark carrier recombination increases with the mobility. These two opposite effects balance with one another, resulting in an optimal mobility about 10−2 cm2/V s which gives the highest power conversion efficiency. The balance of the electron and hole mobilities are not necessary to maintain the optimal efficiency also because of the balance of the photocar...

Polymer space-charge-limited transistor

A metal grid is sandwiched between poly(3-hexylthiophene) to form a solid-state version of vacuum tube triode, where the vertical space-charge-limited current is modulated by the grid potential. The Al grid contains random submicron openings formed by a nonlithographic method. The multilayer polymer structure is made by spin coating. The operating voltage of the polymer space-charge-limited transistor is 3V, and the current gain of 506 is obtained. The characteristics of the transistor can be tuned by the diameters and the density of the openings on the grid. Similar to the vacuum tube triode, the current follows a power law voltage dependence.

Continuous blade coating for multi-layer large-area organic light-emitting diode and solar cell

A continuous roll-to-roll compatible blade-coating method for multi-layers of general organic semiconductors is developed. Dissolution of the underlying film during coating is prevented by simultaneously applying heating from the bottom and gentle hot wind from the top. The solvent is immediately expelled and reflow inhibited. This method succeeds for polymers and small molecules. Uniformity is within 10% for 5 cm by 5 cm area with a mean value of tens of nanometers for both organic light-emitting diode (OLED) and solar cell structure with little material waste. For phosphorescent OLED 25 cd/A is achieved for green, 15 cd/A for orange, and 8 cd/A for blue. For fluorescent OLED 4.3 cd/A is achieved for blue, 9 cd/A for orange, and 6.9 cd/A for white. For OLED with 2 cm by 3 cm active area, the luminance variation is within 10%. Power conversion efficiency of 4.1% is achieved for polymer solar cell, similar to spin coating using the same materials. Very-low-cost and high-throughput fabrication of efficient ...

Efficient Low‐Temperature Solution‐Processed Lead‐Free Perovskite Infrared Light‐Emitting Diodes

Lead-free perovskite infrared light-emitting diodes are achieved by using a halide perovskite CsSnI3 as an emissive layer. The film shows compact micrometer-sized grains with only a few pinholes and cracks at the grain boundaries. The device exhibits maximum radiance of 40 W sr-1 m-2 at a current density of 364.3 mA cm-2 and maximum external quantum efficiency of 3.8% at 4.5 V.

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.

Polymer hot-carrier transistor

Metal-base hot-carrier transistor with conjugated polymer emitter and collector is demonstrated. The device is fabricated by multiple spin coating with the metal base sandwiched between two polymers. A thin insulating layer of LiF is inserted between the emitter and base to enhance the hot carrier kinetic energy and reduce mutual dissolution. Using poly(9-vinylcarbazole) as the emitter, Al as the base, and poly(3-hexylthiophene) as the collector, common-emitter current gain of 25 is obtained with operation voltage as low as 5 V.

Polymer space-charge-limited transistor as a solid-state vacuum tube triode

We report the construction of a polymer space-charge-limited transistor (SCLT), a solid-state version of vacuum tube triode. The SCLT achieves a high on/off ratio of 3×105 at a low operation voltage of 1.5 V by using high quality insulators both above and below the grid base electrode. Applying a greater bias to the base increases the barrier potential, and turns off the channel current, without introducing a large parasitic leakage current. Simulation result verifies the influence of base bias on channel potential distribution. The output current density is 1.7 mA/cm2 with current gain greater than 1000.

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 2.8×10−4 cm2/V s 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.

High-frequency transport properties of spin-spray plated Ni–Zn ferrite thin films

The frequency and temperature variation of magnetoimpedance in Ni–Zn ferrite thin films fabricated by spray plating method were studied. It is observed that the frequency induces a metal–insulator crossover behavior in impedance spectra. The frequency behavior of the electrical properties of the film can be modeled by an equivalent circuit composed of resistance and capacitance. The relaxation time and activation energy of the conductivity were calculated. The result suggests that the high-frequency conductivity of the spin-spray plating of a Ni–Zn ferrite film is predominantly associated with the dielectric, rather than by magnetization, dynamics. Mechanisms underlying high-frequency transport are discussed.

Reduced hole injection barrier for achieving ultralow voltage polymer space-charge-limited transistor with a high on/off current ratio

Vertical polymer space-charge limited transistor (SCLT) operated with an ultralow voltage is demonstrated. The influence of aging effect of the oxygen plasma treated indium tin oxide electrode on the hole injection barrier and on the transistor characteristics is investigated. By reducing the hole injection barrier, the on/off ratio as high as 104 is obtained at a collector to emitter voltage as low as −0.84 V. The low operation voltage is crucial to the development of low-power large-area polymer transistor array. Inverter characteristics are also demonstrated by connecting a SCLT with a load resistor.