Polymers as Photoexcited Donors

Several attempts to use organic polymeric semiconductors as the active component in photovoltaic devices have been reported during the last two decades. Interest in the photovoltaic properties of conjugated polymers like polyacetylene, various derivatives of polythiophenes and polylp ra-phenylcnc vinylenejs arose 

Furthermore, the positions of the absorption maxima of the single components in the composite are not shifted compared to the pristine materials. There is no indication of states below the ti-ti- gap of the conjugated polymer that might arise from grormd state interactions such as doping. 

15 Conjugated Polymer Based Plastic Solar Cells 

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Semiconducting, conjugated polymers as photoexcited electron donors... 

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... 

MEH-PPV. Upon adding Ceo, the triplet signal for the 1.35 eV PIA band is completely quenched. Instead a strong spin =1/2 signal dominates, indicated charged polarons as photoexcitation on the polymer donor (Figure 8.13 [59]). This confirms that the photoinduced electron transfer occurs on a time scale sufficiently fast to quench the intersystem crossing to the triplet state. 

Polysilanes are cr-conjugated polymers composed of Si-Si skeletons and organic pendant groups. They are insulators with filled intramolecular valence bands and empty intramolecular conduction bands. However, because of strong cr conjugation, they have rather narrow band gaps of less than 4 eV [24,25] and are converted to conductors by photoexcitation or by doping electron donors or acceptors. Recently they have attracted much attention because of their potential utility as one-dimensional conductors, nonlinear optical materials, and electroluminescent materials [26-28]. 

Photodiodes utilize principally the photophysical process of semiconductors. The most typical juctions to attain photoinduced charge separation are shown in Fig. 27 a c. If a photoexcited compound (P) is arranged with donor and/or acceptor on an electrode as shown in Fig. 25 (d), it must work as a kind of photodiode based on new principle of photochemical reaction. A polymer film must be most promising to construct such photoconversion element. 

When an electron is added (by doping with donors or by photoexcitation) to the NDGS semiconductor polymer, it does not go into the conduction band as is the case in a conventional semiconductor. It deforms the polymer chain as shown in Fig. 2.4. The actual... 

Different ir-conjugated polymers have been examined as donors and exhibit electron transfer to C o upon photoexcitation. The acceptor side is equally... 

We have studied pMEA, pMAEA and pDRlM in comparison, in order to determine what is the importance of Rau s classification on the photoinduced birefringence. These three polymers have increasing dipole moments, and their comparison clearly indicate that the pseudostilbene -type azobenzenes are the best candidates for photoinduced orientation. Their absorbance in the visible range of the spectrum allows the use of lower power lasers (514 nm), the coincidental absorbances of the cis and trans isomers allows photoexcitation of both trans-cis and cis-trans isomerization processes. Both are necessary for orientation, and the lower the polarity of the azobenzene, the slower the cis-trans thermal isomerization process. The levels, rates and stabilities of the photoinduced birefringence, all are hi er for pDRlM in comparison with the other two, as is the efficiency of the process. Almost all our research is concentrated on the donor-acceptor substituted azobenzenes. 

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