Carrier Mobility and Threshold Voltage

For the analysis of the mobility and the threshold voltage the saturation regime is used here. An analytical model based on the Shockley equation by Schmechel et al. [13] assumes that the electron and hole mobilities as determined from the linear or saturation regime are also valid in the ambipolar regime. As previously shown for the system under consideration, the mobilities determined from the saturation and ambipolar regime are in excellent agreement [27]. 

As compared with film growth at room temperature, an inereased mobility is found for the higher substrate temperature and a lurther inerease is realised by lowering the surface energy with OTS. This increase of mobility was reported for unipolar OFETs [43] and is also valid for these blends. Interestingly, for all treatments balaneed mobilities are found at about 25% C o eontent. 

At this point it is noteworthy to compare our results on the variation of eharge earrier mobilities with composition of the blends to other ambipolar mixed systems. There are on the one hand polymer-fiillerene blends used for photovoltaie applieations (in particular MDMO-PPV PCBM) [44, 45] and on 

It is remarkable, however, that despite a roughness which is larger than the film thickness in some of our films (see below), all of them show n-channel as well as p-channel transport. Obviously, there still exists a percolation path for conduction in both materials independent of phase separation and order formation. We also note that the bulk morphology does not necessarily provide precise information for organic ficld-cffcct transistors since in these devices the active channel is restricted to the first few molecular layers at the interface to the gate dielectric [48-50]. 

By modelling the molecular packing [27] a charge accumulation of about 0.012 charges per molecule is estimated. The calculated charge transfer in an ideal complex of one Cgo and one CuPc molecule was calculated to be 0.06 charge per molecule [51], 5 times higher than determined here. 

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