Semiconducting phase separation

The speed of p- and n-type doping and that of p-n junction formation depend on the ionic conductivity of the solid electrolyte. Because of the generally nonpolar characteristics of luminescent polymers like PPV, and the polar characteristics of solid electrolytes, the two components within the electroactive layer will phase separate. Thus, the speed of the electrochemical doping and the local densities of electrochemically generated p- and n-type carriers will depend on the diffusion of the counterions from the electrolyte into the luminescent semiconducting polymer. As a result, the response time and the characteristic performance of the LEC device will highly depend on the ionic conductivity of the solid electrolyte and the morphology and microstructure of the composite. 

Only after viewing the membrane as a thin film semiconductive phase can one begin to seriously evaluate its potentialities. It is a multidimensional problem, and in the chlor-alkali cells the water transport is controlled by brine concentration while caustic strength controls the cathode efficiency. The membrane provides a low energy pathway for the phase change and separation process. 

Here in this chapter, studies on molecular blends of liquid crystalline materials, especially discotic liquid crystalline blends as semiconductor are overviewed in terms of both macroscopic miscibility and nanoscopic phase separation. Discotic liquid crystal has been extensively studied as one-dimentional semiconducting materials in recent years [12-14]. In particular, organic thin film photovoltaics is an interesting research field of its application. We see blended molecular systems... 

The presence of phase separation should always be considered a possibility. Morral (1968) has shown that the thermodynamic tendency for phase separation increases as the number of components increases. Possible exceptions are at compositions with simple stoichiometric ratios, e.g. As2Sc3. While there is extensive experimental evidence documenting phase separation in oxide systems (Levin (1970)), similar experimental evidence in the non-oxide semiconducting glasses is still very meager (Myers and Berkes (1972)). 

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. 

Based on the above analysis and interpretation, we can tentatively propose the morphological criteria for enhanced electrical properties (i) semiconducting phase in composite should be morphologically continuous (ii) in order to supply enough tv o-phase interfacial area, the phase separation of SP/IP composite should be on the nanometer scale. These criteria can be used to conduct the further optimization of SP/IP composite for electronic application. 

Fig. 1. Subsolidus T-x phase diagram of the LaHj (metallic)-LaH3 (semiconducting) stem based on differential scanning calorimetry and X-ray dif action measurements. Instead of the originally assumed continuous solid solution with cubic Fm3m structure, the metal to semicondurtor transition proceeds via at least 9 phases separated by miscibility gaps. After Condcr et al. (1991).

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