Organic transistors double up as efficient OLEDs
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).
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.
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).
In their experiments, OLETs were fabricated on a 450nm-thick high-k polymer (P(VDF-TrFE-CFE) with a relative electrical permittivity value of 27-30) for the gate dielectric, as well as on PMMA (relative electrical permittivity 3), respectively.
The dielectric layer is either PMMA
(reference polymer platform) or a high-k polymer
(P(VDF-TrFE-CFE)). The active region is
constituted by a blend of TCTA and Ir(piq)3 (20%)
sandwiched between (bottom) p- and (top) n-type
organic semiconductor layers.
They found that while a VGS sweep up to -20V could hardly turn the PMMA-OLET On (with very little drain−source current and no light emission), the high-k-based OLET could be driven at much lower bias with a light output corresponding to approximately 17μW and an overall External Quantum Efficiency reaching 4% at a gate bias of -3V. In fact, the PMMA-OLET only reached its full ON-state at -100V.
To explain their findings, the researchers compare their tri-layer OLET device as a stack of active regions forming two parallel organic TFTs of opposite polarization.
“During the p-type ID−VG sweep, light is emitted in two voltage regions: the first one in which only the p-type OTFT is operating and the second region where both OTFTs are in their ON-state, with balanced charge carrier densities”, they write.
Another benefit from the lower bias (when using a high-k polymer for the gate dielectric) is the lower electric fields involved, with lower stress on the device, which promotes a longer lifetime with respect to their PMMA counterparts.
The research was carried out by consultant company Etc S.r.l. on behalf of advanced functional materials company SAES Getters – www.saesgetters.com
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