Photoconductivity of fullerenes

Y Wang, Photoconductivity of fullerene-doped polymers, Nature, 356 585-587, 1992. 

Influence of weak MF on photoconductivity of fullerene C6o single crystal was studied for main optical transitions of energy 2.64, 3.07, 3.87 eV forming photoconductivity spectrum [3], For these transitions photocurrent dependences versus magnetic field induction B are shown in Fig. la. These field dependences evidence the modulation of singlet and triplet excitons concentration by weak MF [2] is reason of photocurrent sensitivity. 

Influence of EF on magnitude of photoconductivity of fullerene C6o single crystal in a weak MF can be explained in the following way. Increasing intensity of electric field causes the increase of radius of initial distance r0 between the components of electron-hole pairs and, consequently, the decrease of probability of geminate recombination. As a result, AI rises at small values of EF. At higher values of EF the probability of dissociation of pairs in states with uncorrelated spins increases, that causes nonlinear behavior of electrofield dependences of photoconductivity of fullerite C6o in MF. The distance between components of electron-hole pairs in states with uncorrelated spins was estimated as R>3.4 nm. 

Wang Y (1992) Photoconductivity of Fullerene-Doped Polymers, Nature 356 585-587... 

Photoconductivity of Fullerene-doped Polymers Y. Wang, Nature 356, 585-587 (1992). 

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. 

Heterogeneous mixing of fullerenes and fullerene derivatives with Ji-conjugated polymers has been used to produce excellent materials for photovoltaic devices [141], Upon irradiation of fullerene/polymer blends, charge transfer from the polymer to occurs, resulting in efficient photoconductivities. Better behavior of fullerene derivatives than with pristine Cgg has been observed, and attributed to the improved miscibility of the functionalized species. 

In addition to the work of Wang, Wang et al., and Hirao et al., there are many literature references to photoconductivity of the fullerenes. A partial list includes Mort et al. (1991, 1991a, 1992, 1992a. 1993), Minami (1991), Miller... 

Figure VC-3 Steady-state photoconductivity of MEH-PPY/C o composite films with different concentrations of fullerene. (Taken from ref. 181)...

With the addition of 2.7 wt% of fullerene, the photoinduced discharge rate of PVK shows a dramatic enhancement [101]. Wavelength dependence of the charge generation efficiency shows that fullerene acts as the sensitizer for the enhanced photoconductivity [101]. Figure 21 shows the field... 

The synthesis of fullerene-containing polymers is noteworthy for several reasons. On the one hand, once Cgg is attached to a polymer, most of the fullerene properties are transferred to the polymer. Thus, for instance, electroactive and photoactive polymers or polymers with nonlinear optical properties can be prepared. On the other hand, hardly processible fullerenes embedded in highly soluble polymers may become more easily amenable to further treatments. The resulting materials might eventually be used for surface coating, photoconducting devices, or to create new molecular networks. 

Photoconductivity measurements on solutions of fullerenes in alkanes gave a greater reduction of the ionization energy than expected from the van der Waals diameter of the molecule (Katoh et al., 1994). 

From the viewpoint of photochemistry, fuiierenes are good electron acceptors and many photoinduced reactions have been reported by using these fuiierenes as acceptors [16-20]. The excellent acceptor ability of fullerene is a key feature of photoconductivity for fullerene-doped polymer films such as poly(Af-vinylcarbazole) and poly(p-phenylene vinylene) [4, 21]. Furthermore, many derivatives of the fuiierenes have been synthesized due to high reactivities of fuiierenes [22]. Fullerene oligomers and polymers are interesting materials as well as pristine fuiierenes [23, 24]. 

It should be noted that under low concentrations of these photoconductive polymers (<10 mM) in solution, the electron-transfer process via the triplet excited states of fullerenes or triplet excited polymers is a major process as shown below. 

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