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04120152

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虫号: 509283
注册: 2008-02-22
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专业: 半导体光电子器件

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For characterisation in OFETs, films of neat CuPc and C60 and their mixtures have been prepared by thermal evaporation on pre-structured Si wafers covered with thermally grown SiO2. Details of the preparation can be found in Ref. [9]. Fig. 1 shows as an example the morphology of a 1:1 mixture measured by non-contact scanning force microscopy. The image clearly demonstrates that one is not dealing with a molecular mixture of both materials but rather with a nano-phase separated structure. Note that the
height scale is more than twice the nominal film thickness of 25nm. Detailed studies [9] show that the tendency for demixing is strongest for such a 1:1 mixture and decreases towards more asymmetric mixing ratios. We have also determined the field-effect mobility from the saturation regime of OFETs for both electrons and holes as a function of the mixing ratio. For these studies we have varied the substrate treatment (oxygen plasma or silanization of the SiO2 surface) and the substrate temperature during film growth. The resulting mobilities are displayed in Fig. 2. Apparently, an exponential decrease of both μe and μh is observed upon dilution of the
corresponding conducting material with the other species. A similar decrease has been reported in the literature for an ambipolar light-emitting system of co-evaporated molecular materials [11]. Furthermore, we find that heating the substrate during evaporation in combination with silanization increases the mobilities of neat films and mixed layers by up to four orders of magnitude. Remarkably, electron mobilities exceeding 1cm /Vs can be achieved in neat C60 films. We also note that independent of
the film growth conditions balanced electron and hole mobilities are achieved at about 25% fullerene content in the mixture, which is an important prerequisite for inverters with symmetric switching behaviour [9].

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