Nonexponential relaxation of photoinduced conductance in organic field effect transistors

Abstract

We report detailed studies of the slow relaxation of the photoinduced excess charge carriers in organic metal-insulator-semiconductor field effect transistors consisting of poly͑3-hexylthiophene͒ as the active layer. The relaxation process cannot be physically explained by processes, which lead to a simple or a stretchedexponential decay behavior. Models based on serial relaxation dynamics due to a hierarchy of systems with increasing spatial separation of the photo-generated negative and positive charges are used to explain the results. In order to explain the observed trend, the model is further modified by introducing a gate voltage dependent coulombic distribution manifested by the trapped negative charge carriers.Field effect transistors based on organic semiconductors and conjugated polymers have spurred tremendous interest in the development of organic/plastic electronics 1,2 and optoelectronics 3,4 due to the lower cost, facile processability, and chemical tenability. Apart from the technological implications, the metal-insulator-semiconductor field effect transistor ͑MISFET͒ geometry can also serve as a model structure to study the charge transport mechanisms and transport parameters such as field effect mobility, and semiconductorinsulator interfacial density of trap states. The selective distribution of the carrier density in the FET geometry with the large concentration of the positive charges in the channel region compared to the bulk can result in interesting situation especially upon photoexcitation. Large changes in the dark drain-source current I d or I upon photoexcitation at low-light levels was recently demonstrated in FETs based on regioregular poly͑3-octylthiophene͒. 5 It was observed that in an organic FET, which is typically a p channel and activated under an enhancement mode, the incident light intensity can act as an added control parameter. 5 The electric field distribution in the device promotes the separation of the photoinduced charge carrier ͑PCC͒. The more mobile positive charges drift towards the channel whereas the negative charges find their minimum potential in the bulk and get localized in the trap sites. Upon, switching off the photoexcitation, the photoinduced current I light (t) relaxes quite slowly with an apparent persistent behavior and the sample recovers to the dark conditions over a period of several hours. This persisting effect can lead to back-gating problems in polymer FETs. The decay mechanism in the present system depends on several aspects like defects in the insulator-semiconductor interface, bulk trap densities, diffusion rates of the PCC, and V g . We examine this decay closely as a function of the gate voltage (V g ), which largely involves the neutralization of the excess negative charges in the bulk in the background of the channel current.Slow relaxation of a PCC is known to occur in poly͑phe-nylenevinylene͒ ͑PPV͒ and poly͑alkylthiophene͒ systems. [6][7][8][9] The nonequilibrium conductance following photoexcitation in these systems was interpreted in te...

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