Photoconductivity of PPV

The photoconductivity of PPV prepared by the precursor route has been studied by several groups [142-145]. The polymer has a photoconductivity threshold at 506 nm that coincides well with the absorption edge [145]. Measurements of the transient photocurrent indicate a dispersive type of transport. The current is predominantly carried by holes... 

There has also been much research into the photoconductive properties of PPV and derivatives [188-190]. This work has direct application for use in photodiodes [191], photovoltaic cells [109,192,193], optocouplers [113], and electrophotography [194]. It was discovered that the photoconductivity of PPV derivatives can be enhanced by several orders of magnitude through the use of dopants such as C60 compounds. [189,190,195]. 

Figure 15-8. Spectral response of the steady state photoconductivity of MEH-PPV alone and MEH-PPV/Cjo for several concentrations at 300 K and a biasing field of 104 V/cm (reproduced by permission of Elsevier Science from Ref. (18)].

Moreover, the remarkable sensitization of the photoconductivity of MEH-PPV by C o implies that the initial photo-excitations are mobile charged polarons (see... 

Figure 10.54 Comparison of (a) UV-vis absorption and (b) photoluminescence (PL) spectra (lex= 360 nm) ofCPNT, bilayer nanotube I ( 4 nm thick PPV layer), bilayer nanotube II ( 18 nm thick PPV layer), and PPV nanotube (PNT). The samples (1.0 mg) were dispersed in chloroform (5 ml) by uItrasonication. The PL intensities were normalized against the corresponding absorption value at an excitation wavelength of 360 nm. (Reprinted with permission from Advanced Materials, Photoconductivity of Single-Bilayer Nanotubes Consisting of Poly(p-phenylenevinylene) (PPV) and Carbonized-PPV Layers by K. Kim, B. H. Kim, S.-H. Joo etal, 17, 4, 464-468. Copyright (2005) Wiley-VCH)...

In this paper we report on the photoconductivity of fully oriented Durham/Graz-polyacetylene and of fully converted, highly oriented polyphenylenevinylene (PPV) films. Though the two materials are different in several aspects - e.g. polyacetylene is adegenerate ground state system whereas PPV is the non-degenerate one, which has important consequences for the kind of possible excitations - they both show similar behaviour, when looking at the transient photocurrent response. 

Figure 8.29. The transient photoconductivity of MEH-PPV and MEH-PPV/Ceo for various concentrations. (Reproduced by permission of Elsevier Science from ref 58.)...

Another interesting applications area for fullerenes is based on materials that can be fabricated using fullerene-doped polymers. Polyvinylcarbazole (PVK) and other selected polymers, such as poly(paraphcnylene-vinylene) (PPV) and phenylmethylpolysilane (PMPS), doped with a mixture of Cgo and C70 have been reported to exhibit exceptionally good photoconductive properties [206, 207, 208] which may lead to the development of future polymeric photoconductive materials. Small concentrations of fullerenes (e.g., by weight) lead to charge transfer of the photo-excited electrons in the polymer to the fullerenes, thereby promoting the conduction of mobile holes in the polymer [209]. Fullerene-doped polymers also have significant potential for use in applications, such as photo-diodes, photo-voltaic devices and as photo-refractive materials. 

The use of interpenetrating donor-acceptor heterojunctions, such as PPVs/C60 composites, polymer/CdS composites, and interpenetrating polymer networks, substantially improves photoconductivity, and thus the quantum efficiency, of polymer-based photo-voltaics. In these devices, an exciton is photogenerated in the active material, diffuses toward the donor-acceptor interface, and dissociates via charge transfer across the interface. The internal electric field set up by the difference between the electrode energy levels, along with the donor-acceptor morphology, controls the quantum efficiency of the PV cell (Fig. 51). 

The situation improved considerably when the active layer was formed by a network of two (donor and acceptor) interpenetrating polymers or small molecules [34,119,121-123,35], For example Buckminsterfullerene C6o mixed with MEH-PPV is very effective in dissociating the exciton created by the incident light. C6o acts as an acceptor and the polymer, as a donor. The transfer of electron from MEH-PPV to fullerene occurs because fullerene has a larger electron affinity. The hole is left at the MEH-PPV because it has small ionization potential. The exciton dissociation results in quenching of the PL by factor up to 104 and in increasing the photoconductivity considerably. The transfer rate... 

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