Dresden 2011 – wissenschaftliches Programm
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O: Fachverband Oberflächenphysik
O 73: [DS] Organic Electronics and Photovoltaics III (jointly with CPP, HL, and O)
O 73.6: Vortrag
Donnerstag, 17. März 2011, 15:15–15:30, GER 38
Spatially resolved photoresponse of pentacene thin film transistors: slow component by trap release — •Christian Westermeier, Matthias Fiebig, and Bert Nickel — Department für Physik und CeNS, Ludwig-Maximilians-Universität München
Organic thin film transistors (OTFTs) have witnessed continuous improvement over the past years and become suited for widespread application. Small organic molecules, such as pentacene, are often used for OTFTs, since their ordered thin-film structures result in high charge carrier mobilities. Although pentacene has attracted high interest of research, the electronic transport and photoresponse mechanisms and their relation to the morphology and trap densities of the polycrystalline film are not well understood.
We perform spatially and time resolved photoresponse measurements of pentacene TFTs using a laser scanning setup for local illumination with varying laser frequencies and a photon energy of 1.96 eV. The excitation corresponds to the upper Davydov component of the S1 state of pentacene. Besides the absorption in the 50 nm pentacene film, a significant fraction of the laser light is absorbed in the Si wafer upon reflection. Since the excitons in pentacene decay mostly radiationless, both contributions result in local heating. We argue that heat assisted detrapping results in the release of holes from trap states, thus the effective charge carrier mobility increases locally. These bolometric effects are associated with a slow contribution to the photoresponse on a millisecond timescale and with an inhomogeneous structure presumably corresponding to the trap density distribution in pentacene.