Blend film charge transport

Fig. 38 Time-of-flight (TOP) transients for P3HT PCBM blend films of various compositions. It can be clearly seen that the blending ratio of 1 1 yields nondispersive charge transport and similar mobilities (transition times) for both the electrons and the holes. (Reprinted with permission from [189], 2005, American Institute of Physics)...

Babel, A., Wind, J.D. and Jenekhe, S.A., Ambipolar charge transport in air-stable polymer blend thin-film transistors, Adv. Func. Mater, 14, 891-898, 2004. 

On the other hand, miscibility is, in one sense, one of the characteristic properties in liquid crystals. Miscibility in liquid crystals is a well-known macroscopic property where one can see two mesogens exhibiting a thermodynamically identical liquid crystalline phase are mixed to show its phase at arbitrary component ratio. This has already been applied to liquid crystals for LCDs to control some properties such as temperature range of nematic phase. A diversity of functional properties such as temperature range, anisotropic electrical permittivity, viscosity etc. can be controlled in nematic blends and non-mesogenic molecules also can be a component which contributes to the resultant properties as they behave like a solute in liquid solution. However, charge transport property has not yet been well studied in terms of molecular blends with liquid crystalline materials, while thin film organic photovoltaics have been so extensively studied in recent years as molecular blends. 

Bulk heterojunction active layer in organic thin film solar cells is a typical example of mixture for functional blends. The mixture of p- and n-types of semiconductors have to form an appropriate geometry of two compounds to make effective charge transport paths for hole and electron as well as effective interfaces at which charges are generated with less efficient recombination. In addition, excitons generated by photon abosorption have to migrate for a certain distance to reach to the interface and it also needs an appropriate path. 

Tuladhar, S.M., D. Poplavskyy, S.A. Choulis, J.R. Durrant, D.D.C. Bradley, and J. Nelson. Ambipolar charge transport in films of methanofiillerene and poly(phenylenevinylene)/methanofiiUerene blends. Adv Futic Mater 15 1171. 

As discussed in Section 7.2, charge transport in TFTs occurs between the source and drain electrodes, which are usually in the same plane. Therefore, a continuous semiconducting layer at the channel region is needed for TFT operation. Controlling blends to vertically phase separate to form stratified structure presents an effective way to keep the connectivity of semiconducting layer in blended films. In the next section, phase behavior in polymer blend films will be reviewed followed by examples of semiconducting/insuiating polymer blends with vertical stratified structure. 

The lack of the bipolar transport in binary blends of polymers for most of the compositions shows that controlling the thin-film morphology is challenging. Therefore, the key issue is to realize an interpenetrating and bicontinuous networks of binary polymer blends in order to establish ambipolar charge transport. 

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