Time-resolved photoconductivity

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. 

Despite the extensive photoconductivity data, the nature of photoexcitations in poly(phenylenevinylene), PPV, and its soluble derivatives has remained controversial. In part, the controversy arises from the conflict between the results obtained with fast time-resolved photoconductivity and those obtained by the more familiar steady-state photoconductivity the latter indicate a strong T-dependence for the np product. Recent experiments have resolved the apparent conflict [203]. The idea is rather simple If the sample is sufficiently thin that the photocarriers can be swept out before a significant fraction fall into traps, the steady-state photocurrent will provide information similar to that obtained at short times by transient photoconductivity (in the latter, sample thickness is not important since pre-trapping and trap dominated transport are separated in the time domain). 

Time-Resolved Photoconduction in the Polymer of the Bis(p-toluenesulfonate) of 2,4-Hexadiyne-l,6-diol... 

Recently, Baumann et al.(43) have measured time-resolved photoconductivity in PDA-TS-6 crystals as well as polyacetylene excited by 25 ps pulses of a Nd YAG laser (ftU) = 2.3 eV). The response time of the detector was 200 ps. The transient signal shown in fig.5 reveals a fast initial peak with instrument-limited pulse-shape followed by a slower decaying tail. The field dependence of the peak height (fig.6) parallels that of the carrier generation process and is in accord with what Donovan and Wilson have found on a 20 ns time resolution. The quantum efficiency associated with the fast photocurrent peak is 1.5x10 times the dc-quantum efficiency measured at hu) = 2.7 eV. 

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... 

Quenching of the Intersystem Crossing to the Triplet State 521 Photoinduced 1RAV Studies 522 Time-Resolved Photoinduced Studies 524 Sensitization of Photoconductivity 525 Magnetic Properties 526... 

J. Tauc, Time-Resolved Spectroscopy of Electronic Relaxation Processes P.E. Vanier, IR-Induced Quenching and Enhancement of Photoconductivity and Photoluminescence... 

The photocyanation and the photohydrolysis of 4-chloroanisole and 4-fluoroanisole have been studied with time-resolved spectroscopy and by measurement of the photoconductivity in anhydrous and aqueous acetonitrile and tert-butyl alcohol solutions632. The mechanism is depicted in equation 167. The transient species which have been... 

A3.5 Time-resolved photoluminescence studies of GaN A3.6 Persistent photoconductivity in GaN A3.7 Electrical transport in wurtzite and zincblende GaN A3.8 Characterisation of III-V nitrides by capacitance transient spectroscopy... 

Donor acceptor charge transfer complex based photoreceptors continue to be described in the literature and studied using modern spectroscopic techniques but none has been commercialized. For example, the photoconducting charge transfer complex between poly(V-epoxypropylcarbazole) and TNF has been studied with transient absorption and time-resolved fluorescence. On the basis of Monte Carlo simulations, the results were interpreted in terms of a heterogeneity of charge transfer complexes with different radiative probabilities and a distribution of initial charge pair separation distances [30c]. 

The similarity of the temperature dependence of steady-state photocurrent in thin films to that of the transient photoconductivity in the sub-ns regime implies that, in thin films, carrier sweep-out occurs prior to deep trapping. The weak residual T-dependence above 80 K in the thinnest sample is again similar to that observed in sub-ns time-resolved experiments. In the time domain, this corresponds to the temperature dependent tail characteristic of the transient photocurrent [199]. This weak T-dependence arises from the effect of shallow traps with multiple release and retrapping during carrier sweep-out. Note that the... 

Transient terahertz spectroscopy Time-resolved terahertz (THz) spectroscopy (TRTS) has been used to measure the transient photoconductivity of injected electrons in dye-sensitised titanium oxide with subpicosecond time resolution (Beard et al, 2002 Turner et al, 2002). Terahertz probes cover the far-infrared (10-600 cm or 0.3-20 THz) region of the spectrum and measure frequency-dependent photoconductivity. The sample is excited by an ultrafast optical pulse to initiate electron injection and subsequently probed with a THz pulse. In many THz detection schemes, the time-dependent electric field 6 f) of the THz probe pulse is measured by free-space electro-optic sampling (Beard et al, 2002). Both the amplitude and the phase of the electric field can be determined, from which the complex conductivity of the injected electrons can be obtained. Fitting the complex conductivity allows the determination of carrier concentration and mobility. The time evolution of these quantities can be determined by varying the delay time between the optical pump and THz probe pulses. The advantage of this technique is that it provides detailed information on the dynamics of the injected electrons in the semiconductor and complements the time-resolved fluorescence and transient absorption techniques, which often focus on the dynamics of the adsorbates. A similar technique, time-resolved microwave conductivity, has been used to study injection kinetics in dye-sensitised nanocrystalline thin films (Fessenden and Kamat, 1995). However, its time resolution is limited to longer than 1 ns. 

The overall diagram of evolution of the excited states and reactive intermediates of a photoinitiating system working through its triplet state can be depicted in Scheme 10.2 [249]. Various time resolved laser techniques (absorption spectroscopy in the nanosecond and picosecond timescales), photothermal methods (thermal lens spectrometry and laser-induced photocalorimetry), photoconductivity, laser-induced step scan FTIR vibrational spectroscopy, CIDEP-ESR and CIDNP-NMR) as well as quantum mechanical calculations (performed at high level of theory) provide unique kinetic and thermodynamical data on the processes that govern the overall efficiency of PIS. 

Keeping all these things in mind, it is intriguing to address the following two questions. (1) Why do all-frans-/3-carotene crystals give rise to photoconductivity that extends to the near-infrared region (2) Is it possible to find metastable states in dA -trans- -carotene crystals that corresponds to the solitons in fran -polyacetylene In order to answer these questions, we have applied photoinduced and time-resolved absorption spectroscopies to the aW-trans-/3-carotene single crystals (Hashimoto et al., 1998). 

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