A study of effects of electrode contacts on performance of organic-based light-emitting field-effect transistors

Herein is presented a comparative performance analysis of heterojunction organic-based light-emitting field-effect transistors (OLEFETs) with symmetric (Au only) and asymmetric (Au and LiF/Al) electrode contacts. The devices had a top source–drain contact with long-channel geometry and were produced by sequentially depositing p-type pentacene and n-type N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (P13) using a neutral cluster beam deposition apparatus. The spectroscopic, structural and morphological properties of the organic thin films were examined using photoluminescence (PL) spectroscopy, X-ray diffraction (XRD) method, laser scanning confocal and atomic force microscopy (LSCM, AFM). Based upon the growth of high-quality, well-packed crystalline thin films, the devices demonstrated ambipolar field-effect characteristics, stress-free operational stability, and light emission under ambient conditions. Various device parameters were derived from the fits of the observed characteristics. The hole mobilities were nearly equal irrespective of the electrode contacts, whereas the electron mobilities of the transistors with LiF/Al drain electrodes were higher due to the low injection barrier. For the OLEFETs with symmetric electrodes, electroluminescence (EL) occurred only in the vicinity of the hole-injecting electrode, whereas for the OLEFETs with asymmetric electrodes, the emission occurred in the vicinity of both hole- and electron-injecting electrodes. By tuning the carrier injection and transport through high- and low-work function metals, the hole-electron recombination sites could be controlled. The operating conduction and light emission mechanism are discussed with the aid of EL images obtained using a charge-coupled device (CCD) camera.