Dynamics of Photoinduced Charge Transfer

So far the discussion has centered on the steady state aspects of carrier generation and collection at semiconductor-electrolyte interfaces. As with their metal electrode counterparts, a wealth of information can be gleaned from perturbation-response type of measurements. An important difference, however, lies in the vastly different timescale windows that are accessible in the two cases. The critical RC time-constant of the cell in a transient experiment is given by 

Rjn is the measurement resistor (across which the current or photocurrent is measured) and Rg is the electrolyte resistance. The term C is the capacitance, which, in the metal case, is the Helmholtz layer capacitance, Ch- (Once again, the Gouy region is ignored here.) For semiconductor-electrolyte interfaces, we have seen that two layers are involved in a series circuit configuration with corresponding capacitances of Csc and Ch (Fig. 6). Because Ch Csc, C Csc- This assumption is usually justified because Ch — 10 F cm and Csc = 1Q- — 10 F cm . If the composite resistance (/ m + R i) is 100 ohm, then tceii for metal electrodes is 10 s and that for the semiconductor case is 

What are the processes important in a dynamic interrogation of the semiconductor-electrolyte interface  

diffusion of minority carriers from the field-free region to the space charge layer edge, 

carrier recombination via surface states or via traps in the space charge layer. 

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In our opinion, the interesting photoresponses described by Dvorak et al. were incorrectly interpreted by the spurious definition of the photoinduced charge transfer impedance [157]. Formally, the impedance under illumination is determined by the AC admittance under constant illumination associated with a sinusoidal potential perturbation, i.e., under short-circuit conditions. From a simple phenomenological model, the dynamics of photoinduced charge transfer affect the charge distribution across the interface, thus according to the frequency of potential perturbation, the time constants associated with the various rate constants can be obtained [156,159-163]. It can be concluded from the magnitude of the photoeffects observed in the systems studied by Dvorak et al., that the impedance of the system is mostly determined by the time constant. 

Lewis FD, Letsinger RL, Wasielewski MR (2001) Dynamics of photoinduced charge transfer and hole transport in synthetic DNA hairpins. Acc Chem Res 34 159-170 Li Z, Cai Z, Sevilla MD (2001) Investigation of proton transfer within DNA base pair anion and cation radicals by density functional theory (DFT).J Phys Chem B 105 10115-10123 Li Z, Cai Z, Sevilla MD (2002) DFT calculations on the electron affinities of nucleic acid bases dealing with negative electron affinities. J Phys Chem A 106 1596-1603 Lillicrap SC, Fielden EM (1969) Luminescence kinetics following pulse irradiation. II. DNA. J Chem Phys 51 3503-3511... 

Lewis FD, Letsinger RL, Wasielewski MR. Dynamics of photoinduced charge transfer and hole transport in synthetic DNA hairpins. Acc Chem Res 2001 34 159-70. 

Dynamics of photoinduced charge transfer and hole transport in synthetic DNA hairpins 01ACR159. 

Fig. 1.20. Time resolution of photoinduced charge transfer in MDMO-PPV/PCBM composites. AT/T dynamics for pure MDMO-PPV (continuous line) and MDMO-PPV/PCBM ( ) at probe wavelengths of 580 nm and 700 nm. Dotted lines are single exponential fits to the PA of the composites...

Fig. 1.19. Quenching of the coherent vibrational oscillations of MDMO-PPV upon photoinduced charge transfer. The AT/T dynamics for pure MDMO-PPV (continuous line) and for MDMO-PPV/PCBM (1 3 wt. ratio) (dashed line), excited by a sub-10-fs pulse, was recorded at the probe wavelength of 610 nm. The inset shows the Fourier transform of the oscillatory component of the MDMO-PPV signal, the nonresonant Raman spectrum of MDMO-PPV (excitation 1064 nm) and the resonant Raman spectrum of an MDMO-PPV/PCBM sample (excitation 457 nm). For the resonant Raman spectrum of MDMO-PPV, it was necessary to quench the strong background luminescence by adding PCBM...

Electron Transfer Reactions and Exciolexes - Photoinduced electron transfer is one of the most important areas of research. A review of photoinduced electron transfer and electron acceptor complexes usefully surveys the subject . Details of the mechanisms can be obtained by very short time resolution spectroscopy. Dynamic solvent effects on intramolecular electron-transfer involve solvent fluctuations. Time resolved ps emission spectroscopy has been used to examine the kinetics of intramolecular charge transfer in bis(4-aminophenyl)sulphone in ethanol as a function of temperature in this respect 2. it has... 

We have demonstrated in two independent studies that electrolyte effects can have a strong influence on the energetics of a charge-separated species and on the dynamics of intramolecular electron transfer in moderately polar media. In the case of weakly exoergic ET, the ion dynamics can effectively control the transfer rate. The developed spectroscopic probes indicate that association with ions can stabilize a photoinduced charge-separated species by as much as 1 eV. Further investigation of electrolyte effects in photoinduced electron transfer, particularly in the "inverted region, is needed. 

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