Organic transistors double up as efficient OLEDs

March 21, 2017 // By Julien Happich
Publishing their results in the ACS Photonics Letter under the title "Highly Efficient Red Organic Light-Emitting Transistors (OLETs) on High‑k Dielectric", a team of Italian researchers has been exploring the use of a high-k polymer as the gate dielectric for the design of a novel type of Organic Light-Emitting device, doubling up as an organic transistor.

As for other electronic devices based on organic materials (OLEDs, solar cells, plastic memories), Organic Light-Emitting Transistors (OLETs) could help reduce overall device fabrication costs with simpler manufacturing processes while adding mechanical flexibility. But since OLETs behave both as a thin-film transistor and at the same time as a light emitter (under an appropriate bias), they could be particularly suited to the design of flat panel displays with a simplified structure both at the backplane level and for the organic stack itself (requiring fewer organic layers than typically needed for OLEDs).

The red OLET in the ON-state.

In their paper, the researchers compared various tri-layer red OLET stacks only distinguished by the type of organic dielectric used between the gate and the active materials. In a stack comprising top source and drain electrodes on top of an n-type organic semiconductor (for the electron-transport layer), a recombination layer for the light emission and a p-type hole-transport layer, the gate electrode (ITO on a glass substrate) was separated from the active materials by a high-k dielectric or a low-k dielectric layer.

Schematic representation of the trilayer OLET device.

The researchers opted for a tri-layer architecture (with the n-type and p-type semiconductors separated by a recombination layer) with the hope to drastically remove exciton-quenching effects due to interaction with charges. The separated layers also allowed ambipolar charge transport, maximizing exciton recombination through electron−hole balance, they noted.

Due to the specific host−guest matrix system they used for the recombination layer, the OLET they designed emitted at around 626nm in the visible spectrum (red).