Dr. Liping Ma Profile

Dr. Liping Ma

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Proceedings Article | 22 September 2005 Paper

Vertical organic transistors

Liping Ma, Shenghan Li, Yang Yang

Proceedings Volume 5940, 59400V (2005) https://doi.org/10.1117/12.618583

KEYWORDS: Electrodes, Capacitors, Organic light emitting diodes, Transistors, Capacitance, Field effect transistors, Dielectrics, Humidity, Semiconductors, Copper

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A capacitor that basically consists of two parallel metal plates with dielectric materials in between, can store charge at the two inner surfaces of the metal plates when a voltage bias is applied. As the thickness of the metal-plate electrodes decrease to nanometers range, we find an interesting physical phenomenon, i.e., the stored charges can modify the physical properties of the semiconductor layer coated on top of it. This discovery leads us to demonstrate a whole-new concept field-effect transistor, a vertical organic transistor with a novel device structure by stacking gate-source-drain vertically. This vertical stack consists of an active cell (drain/organics/source) on top of a capacitor cell (source/dielectrics/gate). When the gate (capacitor) is biased, charges are stored in the capacitor cell. As the thickness of the source electrode is thin enough, typically in the nanometer range, the active cell can also sense the stored charges within the capacitor cell, and, subsequently, modulate the charge injection from the source into the organics. This unique device structure provides an extremely short "channel length" and large channel conduction area. As a result, we achieved organic transistors with low working voltage (less than 5 V), high current output (up to 10 mA or 4 A/cm²), and high ON/OFF ratio (up to 4×10⁶). This device solves two long-standing issues with organic transistors, high working voltages and low current output. This novel device with its enhanced operating characteristics opens new directions for organic transistors and their applications. The device operation mechanism is different from traditional field effect transistors, where we proposed an injection-controlled mechanism, which can basically explain the observed electrical phenomenon of our vertical transistors.

Proceedings Article | 17 November 2003 Paper

Ohmic contact for high-performance organic electronic devices

Liping Ma, Yang Yang

Proceedings Volume 5217, (2003) https://doi.org/10.1117/12.505108

KEYWORDS: Diodes, Copper, Aluminum, Heat treatments, Electrodes, Metals, Interfaces, Organic electronics, Annealing, Organic materials

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A high-performance organic diode is demonstrated by using C₆₀ sandwiched between a cathode and an anode using metals with different diffusivity and donor ability. In this manuscript,copper (Cu)and aluminum (Al)are selected as the cathode and anode, respectively. C₆₀ is used as the organic electron-acceptor for its high stability and high carrier mobility. The as-prepared diode shows poor performance.However,after heat treatment, the Cu/C₆₀ interface becomes an Ohmic contact through Cu diffusion and charge-transfer processes,allowing highly efficient electron injection from the Cu electrode. On the other hand, a rectified C₆₀/Al contact is formed, prohibiting efficient electron injection from the Al electrode into C₆₀. Hence,a high-performance organic diode is formed through a heat treatment process, not by the selection of metals with different work functions. Due to the high mobility of C₆₀, the device shows megahertz frequency response, and it can also handle rather high current density (363 A/cm² at 2.4V). This opens the way for the formation of high-performance organic electronic devices.

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