Charge photoconductivity measurements

In discussing the use of photoconduction measurements for testing the photo-bleaching process, it is important to recognize that the photoconduction of dyes is due to the formation and migration of electronic charge carriers and not to impurities of photochemical decomposition. However, the following facts should not be overlooked 91>. 

The time-resolved photoconductivity measurements shown in Fig. 15 give further support for a difference in the photoinduced charge transport in the polymerized samples versus the unpolymerized samples. For the incident laser of 100 mW/cm2 and a spot size of 2.5 mm, the decay time of the photoconductivity for the unpolymerized samples is 7.4 sec, whereas the photoconductivity of the polymerized samples does not significantly drop over a 30 sec period. Also, the photoconductivity of the polymerized sample is nearly twice that of the unpolymerized samples even at the peak of the unpolymerized photoconductive response. The unnormalized values for the dark conductivity in both samples is 1.7 x 10-10 S cm-1. The photoconductivity is 5.8 x 10-11 S cm-1 for the unpolymerized sample and 1.1 x 10-10 S cm-1 for the PSLC at an optical intensity of 2 W cm-2. 

A spin nature of the effect of MF on photogeneration of free charge carriers was verified under experimental conditions of the registration of an EPR spectrum detected by photoconductivity measurements (RYDMR-Reaction Yield Detected Magnetic Resonance) of TBPDA(C6o)2- The spectrum (Fig. 4) contains two resonance peaks of negative polarity at 0.189 and 0.312 T, the peak halfwidth is 0.009 T. The second peak has a fine structure. 

Photoionization of other rare-earth ions (Eu, Sm ) has been reported by Pedrini et al. (48). They were able to detect the charge carriers in the conduction by photoconductivity measurements. 

From picosecond transient photoconductivity measurements on PPP films,22 we know that mobile charged states decay within 110 ps. In conventional routes to PPPs, defects like branched chains and large torsion angles of neighboring rings are known to occur. These defects act as shallow or deep traps for positive and negative polarons,38,39 which limit the mobility of charge carriers.40 The synthetic route toward the PPP-type ladder-polymers prevents the described defects and leads to a trap concentration of less than 1 trap per 1000 monomer units,28 whereas substi-... 

As we have seen above, absorption of radiation does not necessarily take place in the luminescent center itself, but may also occur in the host lattice. It is obvious to make a simple subdivision into two classes of optical absorption transitions, viz. those which result in free charge carriers (electrons and holes), and those which do not. Photoconductivity measurements can distinguish between these two classes. 

The photoconductivity in polyacetylene, the simplest conjugated polymer, has been the subject of intense investigations [103 106]. Transient photoconductivity measurements on a picosecond time scale have been carried out [107-112]. These ultrafast methods are a powerful tools to investigate the transport properties as well as the recombination kinetics of charged excitations. It was found [107] that the photocurrent in trans-polyacetylene consists of two components a fast component which relaxes on a picosecond time scale and for which a carrier mobility of about 1 cm V s was reported [110,111] and a slow component with earner lifetimes up to seconds. 

In turn, the interfacial charge separation process in these chemically hybridized CdS PVK nanocomposites by steady-state and picosecond time-resolved PL and photoconductivity measurements was investigated by Cheng et al. [276], The molar ratio of CdS to PVK was 1 80 and 1 34 for the PVK-lO-CdS and PVK-15-CdS samples, respectively. The average particle size of Q-CdS was estimated to be ca. 3.0nm for PVK-lO-CdS and 3.8nm for PVK-15-CdS. The photoconductivity was evaluated by the ratio of Scr/ffo where 8cr is the difference of the photocurrent of the device upon white light irradiation (cr) and the photocurrent of the same device without irradiation (co). It was found that the samples doped with CdS nanoparticles exhibited photoconductivity enhancement and shorter response time than pure PVK. Moreover, PVK-lO-CdS showed a higher photoconductivity than PVK-15-CdS although it contains less CdS. 

Friedrich D, Kunst M (2011) Analysis of charge carrier kinetics in nanoporous systems by time resolved photoconductance measurements. J Phys Chem C 115 16657-16663... 

One should note that the determination of the photogeneration quantum yield on the basis of steady-state photoconductivity measurements is difficult, because one needs to take into accormt other phenomena, like photoinjection of charge carriers from the electrode and field dependence of the charge carrier mobility. 

---