An Organic Field-Effect Transistor Emits Ambipolar Light
Anne L. Fischer
A new light-emitting field-effect transistor suggests the possibility of electrically pumped organic lasers, potentially prompting investigators to consider novel approaches to multifunctional field-effect devices. A group of researchers from the Institute of Nanostructured Materials of the Consiglio Nazionale delle Ricerche (CNR) in Bologna, Italy, and from IBM Zurich Research GmbH in Switzerland has reported the observation of light emission from a transistor with ambipolar current characteristics.
The structure of the light-emitting field-effect transistor consists of a coevaporated thin film of hole-transporting and electron-transporting materials.
Describing the transport of electrons and holes, ambipolar transport usually is asymmetric because the mobilities of the carriers are not matched. For maximum efficiency, the transistor structure allows the electron-hole balance to be matched in accordance with the carriers' respective mobilities. If the electron-hole balance cannot be tuned, there is an imbalance in charge carriers that do not recombine and that therefore do not contribute to light emission, reducing efficiency.
Higher charge mobility
The team was aiming for an ambipolar transistor structure because it allows higher (field effect) charge mobility than the bulk charge mobility of organic LEDs as well as the tuning of the electron-hole balance by the gate voltage. A high emission quantum efficiency requires the efficient generation of excitons by recombination of electrons and holes, and in the case of unipolar transport, the minority charge carriers limit that recombination. In addition, the higher charge mobility prevents exciton quenching by interaction with free charges as well as photon re-absorption by charged species.
Constance Rost and colleagues at IBM first investigated ambipolar organic field-effect transistors that comprised a bilayer structure of an electron and a hole transporting material. They observed no light emission from these devices because of the choice of materials and the architecture. And because of the formation of the channels in the different materials, a bilayer structure can have a relatively small interface at which electrons and holes can recombine, compared with the bulk.
Joint activity with the CNR led the researchers to increase the active interface area as well as to adjust electron and hole mobility by mixing two materials. Challenges included maintaining good transport properties for both materials in a coevaporated film and finding materials with appropriate band alignment. The electron transport material had sufficient transport properties; however, coevaporation affects the mobility of the hole-transport material. Also, a fluorescent material with a smaller bandgap was required.
The field-effect transistors are based on a coevaporated film of α-T5 as the hole-transporting and P13 as the electron-transporting material. A heavily doped, N-type silicon wafer with aluminum back contact acted as a gate electrode and substrate. The gate insulator is made of a thermally grown 150-nm-thick SiO2 layer.
The researchers measured the transistor output and transfer characteristics using an Agilent 4155C semiconductor parameter analyzer, and the photocurrent, employing a Hamamatsu S1336 photodiode. Their results showed that the organic field-effect transistor exhibits ambipolar conduction over a wide range of bias conditions.
The new device integrates the functionality of LEDs and thin-film transistors. This may help with the development of future devices and improve the understanding of the fundamentals of device physics. The scientists' next step is to explore the potential of the electroluminescent ambipolar field-effect device for the development of an organic electrically pumped laser by improving its electrical and optical properties through the integration of organic heterostructures and photonic nanostructures.
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