Photoexcitation under illumination

Figure 6.26 The principles of photoelectrochemical storage cells. L is the photoexcited electrode which, under illumination, provides a current i and builds up photoproducts. These can recombine in the dark, again producing an electric current. St =...

Generally there are two different methods for measuring excited state spectra in the ms time regime. Typically, IR PIA spectra are not recorded with the lock-in technique but by referencing several hundred accumulated single beam spectra (by FTIR spectrometer) under illumination and in the dark, while UV/VIS PIA uses a lock-in detector to filter out signal changes due to photoexcitation. 

Photoexcited charge-transfer reactions for which the back charge transfer is rather slow, shifting the equilibrium of the system to the product side under illumination. This type of reaction is called a photoredox reaction [89,90], and a typical example is the reaction of the photoexcited state of thionine (dye) with Fe as an electron acceptor shifting the equilibrium to the product side (the color of the aqueous solution is violet in the dark and colorless under illumination, and this color change is reversible). For this kind of reaction couple, when two electrodes are dipped in the solution of the reaction couple, a photopotential and photocurrent can be induced by introducing an asymmetric factor in the cell. Such an as)mimetric factor is, for instance, two different electrodes or different illumination of the electrodes (e.g. illumination at one electrode while the other electrode is kept in the dark). 

C60 has been used to produce solvent-cast and LB films with interesting photoelec-trochemical behavior. A study of solvent-cast films of C60 on Pt rotating disc electrodes (RDEs) under various illumination conditions was reported [284]. Iodide was used as the solution-phase rednctant. The open-circuit potential shifted by 74 mV per decade of illumination intensity from a continuous wave (cw) argon-ion laser. The photocurrent versus power was measured at -0.26 V under chopped illumination (14-Hz frequency, vs. SCE) up to 30 mW cm and was close to linear. The photoexcitation spectrum (photocurrent versus wavelength) was measured at 0.02 V (vs. SCE) from 400 to 800 mn and found to be... 

Under light illumination, semiconductor electrodes absorb the energy of photons to produce excited electrons and holes in the conduction and valence bands. Compared with photoelectrons in metals, photoexcited electrons and holes in semiconductors are relatively stable so that the photo-effect on electrode reactions manifests itself more distinctly with semiconductor electrodes than with metal electrodes. 

The surface properties of ultradisperse semiconductor CdS which are determined in during its preparation, were shown to make a decisive influence on the regularities of interfacial transfer of photoexcited electron. In this section, we consider the effect of surface properties of ultradisperse CdS on the regularities of photoreduction of various substances under stationary illumination of CdS. 

Steady state photoelectrochemical behaviour of colloidal CdS For the purposes of the studies reported here, the photocurrent was taken to be the total current recorded at the ORDE from an illuminated colloidal dispersion of CdS minus the current recorded under identical condition from the same dispersion in the dark. In both studies, the photocurrents generated by CdS particles illuminated at the ORDE exhibited a wavelength dependence (action spectrum) identical to the absorption spectrum of colloidal and bulk CdS [166,168], unambiguously indicating that the observed photocurrent is due entirely to ultra-band gap photoexcited conduction band electrons. However, it should be noted that, unless stated otherwise (e.g. the action spectrum experiments), the particle suspensions of both studies were usually irradiated with white light from a 250 W quartz iodine projector lamp to maximise the photocurrents observed. 

Illumination by a suitable laser radiation in the visible generates conjugational defects, whose vibrational spectra have been recorded [42]. Under photoexcitation, PA becomes a photoconductor with a fast decay time. The nature of the photoexcited species is not yet identified, even if theories have been proposed. In the first, elementary, step of the excitation, within the conduction band a very short-lived species is generated, which decays rapidly into other states with longer lifetimes. 

Evaluation of an SC photovoltaic performance is usually accomplished with a set of tests. Those involve measurements of the current-voltage (I-V) curves for the cell in the dark and under light illumination, fluorescent decay upon photoexcitation, determination of the incident photon-to-electron-conversion efficiency (TPCE), etc. 

Suspended semiconductor nanoparticles also differ from high-aspect-ratio semiconductor photoelectrodes in another aspect. High-aspect-ratio nanowire/macroporous photoelectrodes in low-level injection are connected to current collectors and are thus intended to operate at a specific power point (i.e., at a particular combination of current-potential values that maximizes the product of the quasi-Fermi-level splitting and the net photocurrent). In contrast, a suspended semiconductor nanoparticle functions without any external contacts at precisely open-circuit conditions. That is, at the operational conditions, photoexcited semiconductor nanoparticles suspended in a solution pass no net current, that is, 0=0, and their quasi-Fermi levels are offset by the maximum value possible under the operative illumination and recombination conditions. 

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