Semiconducting polymers optical properties

The polysdanes are normally electrical insulators, but on doping with AsF or SbF they exhibit electrical conductivity up to the levels of good semiconductors (qv) (98,124). Conductivities up to 0.5 (H-cm) have been measured. However, the doped polymers are sensitive to air and moisture thereby making them unattractive for practical use. In addition to semiconducting behavior, polysilanes exhibit photoconductivity and appear suitable for electrophotography (qv) (125—127). Polysdanes have also been found to exhibit nonlinear optical properties (94,128). 

More General Treatments of Electron Correlation in Polymers.—The introduction of excitonic states was just a simple example to show how one can go beyond the HF approximation to obtain correlated electron-hole pairs, whose energy level(s) may fall into the forbidden gaps in HF theory, and form the basis for interpretation of optical phenomena in semiconducting polymers. The schemes described until now for investigation of certain types of correlation effects (the DODS method for ground-state properties and the exciton-picture for excited states) are relatively simple from both the conceptual and computational points of view and they have been actually used at the ab initio level. It is evident, on the other hand, that further efforts are needed in polymer electronic structure calculations if we want to reach the level of sophistication in correlation studies on polymers which is quite general nowadays in molecular quantum mechanics. 

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. 

Some aspects of computational quantum chemistry applied to the analysis of the electronic structure of polymers are reviewed in connection with the timely trends observed in their electrical and optical properties. The paper is organized as follows after an introduction (Section 36.1), the basic theory of the quantum chemical methodologies as applied to periodic chains is summarized (Section 36.2). Several fields of applications are then presented photoelectron spectra (Section 36.3), conducting and semiconducting conjugated polymers (Section 36.4), hnear and non-linear optical properties (Section 36.5) and the role of charge transfer in organic chains (Section 36.6). Possible developments for the near future are also sketched. 

Recently, there has been a great deal of Interest In semiconducting organic polymers, particularly polyacetylene ((CH) ), as electronic materials for applications where low cost and large area are important. This report first discusses the potential of organic polymer semiconductors to meet the electronic, physical and economic constraints Imposed by the photovoltaic application. Then, recent results on the structural, electrical, and optical properties of one candidate material, polyacrylonitrile (PAN), are presented. Areas for further Investigation are Indicated. 

Cornil, D.A. dos Santos, D. Beljonne, Z. Shuai, ).-L. Bredas, Gas Phase to Solid State Evolution of the Electronic and Optical Properties of Conjugated Chains A Theoretical Investigation, in G. Hadziioan-nou, P. F. van Hutten (eds.). Semiconducting Polymers, Wiley-VCH, Weinheim (2000), p. 235. 

---