Photoconductivity action spectrum

There appears to be an intrinsic carrier generation process at ca. 3 eV. An absorption maximum at ca. 2 eV has no counterpart in the photoconduction action spectrum, and a maximum in photoconduction at ca. 1.7 eV also has no counterpart in absorbance. 

Intrinsic e"/h+ pair generation depends on the width of the forbidden energy gap and therefore takes place at high photon energies. The gain is in general very low (10 6 < G < 10-4) and with similar values for electrons and holes. Moreover, the photoconductivity action spectrum does not generally resemble the absorption spectrum. 

The photoconductivity action spectrum generally resembles the solid absorption spectrum. 

Kroeze JE, Savenije TJ, Vermeulen MJW, Warman JM. (2003) Contactless determination of the photoconductivity action spectrum, exciton diffusion length and charge separation efficiency in polythiophene-sensitized TiOj bilayers./Rfc yr Chem B 107 7696-7705. 

The photogeneration of charged excitations in oligothiophene thin films was only studied for a-6T in which the threshold for photoexcitation of charges has been located at around 2.2 eV by a comparison of the one photon excitation curve for radiative recombination and the photoconduction action spectrum [257]. 

Figure 9.13. Photoconductivity-action spectrum and absorption of a Cfio film- (Rqjrinted from ref 37 1993, with permission from Springer-Verlag.)...

Figure 9.17. Photoconductance-action spectrum of an oxygen-free Cao film (top curve) and for increasing levels of oxygen contamination. (Reproduced by permission of the Ainerican Physical Society from ref 43.

In systems in which the charge-transfer excitation band differs from the action spectrum of photoconductivity, the doping effect may be due to a change of recombination path that results in an enhancement of carrier liefetime (e.g. holes in merocyanines and phthalocyanines). (Details on the mechanism are given in 10,11,74).)... 

The ratio of peak photocurrent to dark current was as high as 300 at 300 °K and 600 at 120 °K. The action spectrum of photoconductivity of this and other studied polydiacetylenes24 is significantly shifted to shorter wavelengths from the absorption spectrum. 

Horhold found that the action spectrum of phenyl-substituted polyarylene-vinylenes shifts substantially to longer wavelengths when the photoconductor probe is simultaneously irradiated by UV light from another source31. The action spectrum of photoconductivity of the polymer subjected to UV radiation is the same as the action spectrum of photoconductivity of the cation radical of the same polymer... 

Fig. 12. Action spectrum of photoconductivity of sensitized PVK. (1) PVK + methylene blue. (2) PVK + methylene blue + TCNE124)...

Fig. 21. a. Action spectra for photopolymerization of neat (triangles) and partially polymerized (circles) multilayer systems. AOD is the change in optical density at a given irradiation dose b Action spectrum of intrinsic photoconductivity of a fully polymerized multilayer assembly from Ref. 

Crystals of poly[2,4-hexadiyne-l,6-diol bis(p-toluenesulphonate)] [59, 60] are highly photoconductive, with high carrier mobilities, near 20m V s even at low electric fields. This corresponds to drift velocities of about 2.2 x 10 m s i.e. close to the velocity of sound in the material. It is interesting to note that the spectral dependence of photoconductivity (the so called action spectrum) in this and other polyacetylenes does not follow the absorption spectrum, but is shifted towards shorter wavelengths. Charge-carrier generation follows the Onsager one-dimensional model, at least for electric fields up to 10 V m [60, 61]. 

Figure 13 Photocurrent measurement of a polymer bundle prepared on a mica surface (a) optical micrograph of the experimental setup for photoconduction measurement of a single stripe of polymer 21 (b) apparent photocurrent generation at various applied voltages irradiated with green light (510 540 nm) (c) photocurrent action spectrum of a single polymer bundle.

Dissociation of an excited singlet state in conjugated polymers is a process of particular interest because the semiconductor model postulated that the primary optically excited species is a well separated electron-hole state already. This would imply that the exciton binding energy is of the order kT only [23]. However, it was observed that the action spectrum of photoconduction in systems like poly-phenylenevinylene (PPV) and the laddertype poly-phenylene (LPPP) is flat within Si <— So transition and increases steeply until the excess energy, relative to the... 

Figure 9.13 presents an example for the action spectrum of intrinsic photoconduction, i.e., a plot of the number of charge carries produced pw absorbed photon, for a Jt-conjugated polydiacetylene crystal (DCH). There is no photoresponse at the... 

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