New concepts for light-emitting transistors

Abstract

In this study we report on new concepts to generate light emission in organic thin film transistors. The initial physical understanding of light emission from tetracene based field-effect transistors was proposed to be originated from a strong underetching of the drain and source electrodes. This underetched electrodes in combination with the evaporated tetracene is thereby believed to generate a virtual OLED at the drain electrode. Accumulated holes have to leave the gate oxide interface to reach the drain electrode by crossing the bulk of the organic semiconductor. Light then occurs by injection of electrons in a large electric field in the bulk. Today's transistors do not show the underetching anymore but are still emitting light only at the drain electrode, again supporting the initial interpretation of a defect state at the edge of the drain electrode. In this context the question how electrons can overcome a potential barrier of 2.7 eV is still open. Therefore an investigation of the gold tetracene interface by UPS and XPS techniques has been started and preliminary data indicate the unexpected result that the barrier for electrons is comparable to that for holes. In a further step the generation of an ambipolar transistor by interface doping with calcium was tried and an n-type pentacene transistor could be fabricated but the strategy failed for tetracene. Finally an electrochemical interface doping was performed by the application of Lithium triflate in PEO to a thin interface layer between gate oxide and tetracene. This leads to light emission but unfortunately also to the loss of the gate voltage influence. Based on these results a possible strategy will be presented.

Electrochemical Interface Doping in Organic Light Emitting Field Effect Transistors

“…Since in a field effect transistor charge transport occurs in the vicinity of the insulatorinterface, doping right at the interface is sufficient to control the charge transport in the channel as has been demonstrated recently for an n-type pentacene transistor. [15,16] In this paper, the concept of electrochemical volume doping is modified to an interface doping resulting in the realization of a light emitting ambipolar operating transistor. Due to the separation of the electrolyte layer and the functional organic layer, it can be applied to polymer based transistors as well as to small molecule-based transistors.…”