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Abstract: Graphene field-effect transistor structures were used to investigate the role of molecular alignment on charge transport properties of heterostructures comprising a single-layer graphene and variable thickness of N,N′-bis(n-octyl)-(1,7&1,6)-dicyanoperylene-3,4:9,10-bisdicarboximide (PDI8-CN2) - an n-type organic semiconductor. Our atomic force microscopy data show that under selected growth conditions PDI8-CN2 grows in a layer-by-layer fashion up to a second monolayer. The first layer comprises flat-lying molecules, whereas the molecules in the second layer orient themselves in an upright orientation. Transconductance measurements show that the flat-lying molecules have little effect on the position of the Fermi level in graphene. Upright oriented molecules in the second layer instead, have a strong effect as to neutralize native p-type doping of graphene and cause a shift of charge-neutrality level towards the Dirac point. We interpret such behavior in terms of different orientation of the surface dipole on layers with different molecular orientations. At the same time the overall mobility of the charge carriers reaches values exceeding 3000 cm2/Vs.
Keywords: organski polprevodniki, grafen, transport električnega naboja
Published in RUNG: 13.10.2020; Views: 2609; Downloads: 0
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Abstract: The electric-charge transport in organic semiconductors is essentially different to the transport in ordered inorganic crystals. The reason is in thelocalization of the energy states, which act as charge-carrier transport channels between molecules. Consequently, the determination of the basic transport parameters in organic materials is inherently more involved than in their inorganic counterparts. The analytical methods that are used to describe charge transport in inorganic materials are unsuitable, since they are based on the extended electronic energy structure. We report here on the simulation of charge transport in organic semiconductor thin films. The simulation is based on the Monte Carlo method and describes the charge-carrier transport within the framework of carrier hopping between the sites. We employed a Gaussian energy distribution of the hopping sites with disorder elements. The degree of disorder is characterized by the width of the Gaussian distribution and is measured in eV units. The results of the transport simulation in 3,4,9,10-perylenedianhydride tetracarboxylic acid (PTCDA) show that the photogenerated charge-carrier current depends on the film thickness, temperature and disorder degree. The simulated photocurrents have the same amplitude in thick films as in the thin films, but the overall shape of the I(t) curve is more dispersive in thin films. The charge-carrier mobility decreases with the increasing degree of disorder at a given temperature. The simulation of the photogenerated positive charge carriers current matches with the time-of-flight experiment in a glass/ITO/PTCDA(600 nm)/In heterostructure at room temperature and an applied bias voltage of 8 V.
Keywords: neurejeni kristali, metoda Monte Carlo, organski polprevodniki, transport naboja, PTCDA, tranzientne meritve
Published in RUNG: 10.07.2015; Views: 5870; Downloads: 23
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