Electrical characterizations of a pentacene-based thin film transistor under optical excitation

Université de Limoges, Faculté des Sciences et Techniques, CNRS, UMR 6172, Institut de Recherche XLIM, Département MINACOM, 123 avenue Albert Thomas, 87060 Limoges, France

Corresponding author: bruno.lucas@unilim.fr

Received: 20 February 2007
Revised: 10 July 2007
Accepted: 12 September 2007
Published online: 7 December 2007

Abstract

Light effect on electrical properties of a pentacene-based transistor was studied. We showed that under optical excitation, the phenomenon of photoconductivity can modify both carrier density and mobility; these changes depend on the wavelength, on the carrier photogeneration rate and on the material absorption coefficient. At first, we examined structural, optical and electrical properties of pentacene films; a thickness of 50 nm is the good compromise to obtain optimal performances. Then we observed the effect of illumination on the drain current in the accumulation regime: at 365 nm, we obtained a maximum ratio of photocurrent to dark current (I_{dsillumination}/I_dsdark) of about 2 × 10³ when the gate is turned off, mobility and threshold voltage values around 0.02 cm² V⁻¹ s⁻¹ and 7.5 V respectively. Moreover the dynamics study (off-state) showed sufficiently fast response times with large current gains to envisage applications in the area of light activated memory components. Finally, we examined the performances of a new component based on the coupling of an organic light emitting diode (with an emission peak at 550 nm) as an input unit with an organic transistor as an output unit. This last study enabled to underscore an increase in mobility without modifying the carrier density for low carrier photogeneration rates.