Photoresponse Of Organic Field-Effect Transistors Based On Soluble Semiconductors And Dielectrics
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Photoresponse Of Organic Field-Effect Transistors Based On Soluble Semiconductors And Dielectrics Nenad Marjanović, Th. B. Singh, Serap Günes, Helmut Neugebauer and Niyazi Serdar Sariciftci Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University Linz, Altenbergerstrasse 69, A-4040 Linz, Austria
ABSTRACT Photoactive organic field-effect transistors, photOFETs, based on a conjugated polymer/fullerene blend, MDMO-PPV: PCBM (1:4), and polymeric dielectrics as polyvinylalcohol (PVA) or divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) with top source and drain electrodes were fabricated and characterized in dark and under AM1.5 illumination. With LiF/Al as top source and drain contacts the devices feature n-type transistor behavior in dark with electron mobility of 10-2 cm2/Vs. Under illumination, a large free carrier concentration from photo-induced charge transfer at the polymer/fullerene bulk heterojunction (photodoping) is created. The device performance was studied with different illumination intensities and showed to be strongly influenced by the nature of the organic dielectric/organic semiconductor interface resulting in phototransistor behavior in BCB-based photOFETs and in phototransistor or photoresistor behavior for PVA-based photOFETs. INTRODUCTION Photoresponsive organic field effect transistors (photOFETs) are interesting since the transistor properties are extended with added light sensitivity. PhotOFET arrays can be used directly as highly sensitive image sensors, but also as basis for light induced switches, for light triggered amplification and/or detection circuits etc. Among the various concepts for achieving highly efficient photoinduced charge generation, a well-known route is “the bulk heterojunction concept” which uses acceptor materials with high electron affinity (such as C60 or soluble derivatives of C60) mixed with conjugated polymers as electron donors [1]. Photoactive organic field-effect transistors have been demonstrated using various organic and polymeric semiconductors. A photoresponsivity as high as 0.5 to 1 A/W has been achieved using pristine poly (3-octylthiophene), polyfluorene and bifunctional spiro compounds [2-4]. However, photOFETs based on conjugated polymer/fullerene blends with high photoresponsivity were not realized yet. In this paper, we report investigations on photOFETs using a conjugated polymer/fullerene blend as a photoactive semiconductor layer with highly transparent polymeric dielectrics as gate-insulator. A photoresponsivity of 5 A/W is obtained under air mass (AM) 1.5 illumination intensity (100 mW/cm2), in photOFETs based on PVA as gate-insulating layer. The photOFETs fabricated with BCB gate-insulating layers show less photoresponse for the same illumination conditions compared to that of PVA. The high photoresponse of the devices are correlated with the generation of large free carrier concentrations in the bulk under illumination of this highly photoactive polymer/fullerene blend layer. The results sugge
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