Conjugated polymers semiconducting properties

Polymers. The Tt-conjugated polymers used in semiconducting appHcations are usually insulating, with semiconducting or metallic properties induced by doping (see Flectrically conductive polymers). Most of the polymers of this type can be prepared by standard methods. The increasing use of polymers in devices in the last decade has led to a great deal of study to improve the processabiUty of thin films of commonly used polymers. 

The introduction of bulky side chains that contain adamantyl groups to poly(p-phenylenevinylene) (PPV), a semiconducting conjugated polymer, decreases the number of interchain interactions. This action will reduce the aggregation quenching and polymer photoluminescence properties would be improved [93]. 

The simplest polymer with a conjugated backbone is polyacetylene. Its structure is similar to that of the saturated polymer polyethylene, but has one of the hydrogen atoms removed from each carbon of the polyethylene chain. Each carbon atom in the polyacetylene chain thus has one excess electron which is not involved in the basic chemical binding. And if the separation of the carbon were constant, polyacetylene would conduct along the chain in other words it would behave like a metal in one dimension. But unfortunately this is not true as the free electrons tend to get localized in shorter double bonds. Conjugated polymers can at best be expected to display semiconducting properties. 

Although this very elementary description of conjugated polymers in terms of energy band theory explains how conjugation can lead to semiconducting and metallic properties, it is incomplete for three important reasons ... 

The science and technology of conducting polymers are inherently interdisciplinary they fall at the intersection of three established disciplines chemistry, physics and materials science. These macromolecular materials are synthesized by the methods of organic chemistry, and their electronic structure and electronic properties fall within the domain of condensed matter physics. Efficient processing of conjugated polymer materials into useful forms requires input from materials science (or, more specifically, from polymer science). In the following sections, we address many aspects of the interdisciplinary science of semiconducting and metallic polymers. 

Conjugated polymers in their undoped, semiconducting state are electron donors upon photoexcitation (electrons promoted to the antibonding it band). The idea of using this property in conjunction with a molecular electron acceptor... 

In the spirit of the goal of this review, we focus on those aspects of the science of conjugated polymers that make them unique as NLO materials i.e. on the role of bond relaxation in the excited state (soliton and polaron formation) in the NLO response of conjugated polymers. As emphasized in Section IV, when photoexcited, bond relaxation in the excited state leads to the formation of electronic states within the energy gap of the semiconductor. These gap states change the optical properties of the polymer (photoinduced absorption). In this sense, semiconducting polymers are inherently nonlinear in their optical response. This process is shown schematically in Fig. VE-1. 

Unlike (SN), most polymers correspond to closed-shell systems where all the electrons are paired. Such a configuration leads to insulating or semiconducting properties as noted previously. Polyacetylenes and related conjugated polymers, for example, have conductivities that classify them as semiconductors. The carbon atom in polyacetylene is sp hybridized, which leaves one p electron out of the bond-forming hybrid orbitals. In principle, such a structure might be expected to give rise to extended electronic states formed by overlap of the p (tt) electrons and thus provide a basis for metallic behavior in polymers. 

Self-organization in many solution-processed, semiconducting conjugated polymers results in complex microstructures in which ordered microcrystaUine domains are embedded in an amorphous matrix [20,21]. This has important consequences for electrical properties of these materials Charge transport is usually limited by the most difficult hopping processes and is therefore dominated by the disordered matrix, resulting in low charge-carrier mobilities (<10 cm V s ) [22]. 

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