Absorption semiconducting polymers

All systems presented in this section show lasing only in the optical pumping mode. There is much interest in electrically pumped devices, but for molecular glasses the difficulties in achieving high excitation densities and low absorption due to charge carriers and electrodes have yet to be overcome. This problem and the related semiconducting polymer lasers that are based on the same principles will not be covered here, but are treated in recent reviews [214-216]. 

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. 

Fig. 4.3 Absorption and emission spectra of a number of semiconducting polymers with different molecular structures. The emission colors span the visible spectrum.

Poly(3,4-ethylenedioxythiophene) (PEDOT) is such a low-bandgap semiconducting polymer [1106,1195,1196] the bandgap of PEDOT is below 1.5 eV in the undoped state. After doping, this material exhibits reduced absorption in the visible the oscillator strength shifts from around 1.5 eV (lowest ir-ir transition) to be-... 

As a new form of carbon, fullerene ( 50) is an excellent electron acceptor capable of taking on as many as sfac electrons. Therefore, fullerene can easily form charge-transfer salts with a variety of alkaline metal donors [1257]. Sariciftci et al. [1170] found photoinduced electron transfer from semiconducting polymers onto fullerene, with interesting and unique photophysical properties in those composites. In such cases, conjugated polymers act as electron donors upon photoexcitation (electrons promoted to the antibonding TT band), and then fullerene absorbs photoexcited electrons in the LUMO level. As a result, new absorption occurs in the photoexcitation spectrum of those composites, assigning to the allowed HOMO (Tig)-LUMO (Tju) transitions of [1258]. Once... 

Thus, the non-linear absorption of the conjugated polymer/C6o composite films is resonantly enhanced through population of the photoexcited states within the energy gap of the semiconducting polymer. The yield... 

Xerography has driven much of the research into conjugated-side-chain semiconducting polymers. The most studied photoconductive polymer is poly(N-vinylcarbazole) (1 PNVC). " PNVC in its pure form is a hole conductor with a dark conductivity of the order of 10" Scm". The photoresponse is limited to the absorption profile of the carbazole moiety. Both properties are modified when molecular species are added, so that impurities can mask the intrinsic properties of PNVC, e.g. purification leads to lower quantum efficiency. 

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