Photocurrent intensity

Salvador [100] introduced a non-equilibrium thermodynamic approach taking entropy into account, which is not present in the conventional Gerischer model, formulating a dependence between the charge transfer mechanism at a semiconductor-electrolyte interface under illumination and the physical properties thermodynamically defining the irreversible photoelectrochemical system properties. The force of the resulting photoelectrochemical reactions are described in terms of photocurrent intensity, photoelectochemical activity, and interfacial charge transfer... 

Figure 5.51 Applied potential dependence on the photocurrent intensities of RUC7VCeS / Au (O), RuCsVCgS/Au ( ), RuC17S/Au ( ) and RuC13S/Au ( ) electrodes Xex, 470 nm [TEOA] = 5 x 10-2 M [NaC104] = 0.1 M. Reprinted from Thin Solid Films, 350, Y. Koide,N. Terasaki,T. Akiyama and S. Yamada, Effects of spacer-chain length on the photoelectrochemical responses of mono-layer assemblies with ruthenium tris(2,2 -bipyridine)-viologen linked disulfides, 223-227, Copyright (1999), with permission from Elsevier Science...

Under short-circuit conditions the cathodic photocurrent was observed for the photoelectrochemical cell the quantum yield was ca 0.5 % [309, 318, 332]. The photocurrent intensity in the free-base porphyrin-Ceo system was enhanced fivefold compared with that in the corresponding free-base porphyrin monomer system. Such enhancement of the photocurrent demonstrates that C6o acts an effective mediator in a sequential electron-transfer process. The photocurrent intensity in the free-base porphyrin-Ceo cell was ca one order of magnitude larger than that in the zinc porphyrin-Cfio cell [309, 318, 332]. Two different electron-transport mecha-... 

In the presence of redox couples confined to the hydrophobic liquid phase, photoinduced heterogeneous electron transfer can be effectively monitored by photoelectrochemical techniques under potentiostatic conditions. The photocurrent responses are uniquely related to specifically adsorbed porphyrins, as demonstrated by the photocurrent anisotropy to the angle of polarisation of the incident illumination (Section 4.3). Systematic studies of the photocurrent intensity as a function of the formal potential of the redox couple and the Galvani potential difference revealed that the dynamics of electron transfer are determined by the distance separating the redox species at the interface. Other processes including decay of the electronically excited state, back electron transfer, porphyrin regeneration and coupled ion transfer play important role on the dynamics of the photocurrent responses. 

A film of electropolymerized Ru(bpy)3 pendant polymer containing pyrrole groups 48) gave a cathodic photocurrent when irradiated in the presence of 4-methyl-benzenediazonium tetrafluoroborate (4-CH3QH4N BF4) in acetonitrileThe beneficial effect on the photocurrent intensity effected by increasing film thickness is rapidly cancelled by the resulting electron diffusion limitation. 

The quantitative measurements are based on the decrease in the photocurrent intensity of the phthalocyanine-sensitized graphene-CdS nanocomposite, which is caused by an increase in the steric hindrance due to the formation of the immunocomplex antibody-antigen on the surface (Fig. 13b). The feasibility of the proposed immunosensor for clinical application was evaluated in the diagnosis of prostate cancer by analyzing real samples from three healthy males and three patients. The results showed that the developed method and the enzyme-linked... 

Figure C3.2.4. Plot of the log of photocurrent against number of methyl units in a alkylsilane based monolayer self-assembled on a n silicon electrode. The electrode is immersed in a solution witli an electron donor. Best fits of experimental data collected at different light intensities ( ) 0.3 mW cm ( ) 0.05 mW cm. From [10].

Photodiodes produce an electric field as a result of pn transitions. On illumination a photocurrent flows that is strictly proportional to the radiation intensity. Photodiodes are sensitive and free from inertia. They are, thus, suitable for rapid measurement [1, 59] they have, therefore, been employed for the construction of diode array detectors. 

Figure 15-21. Shon circuit current (closed circles) and photocurrent al -1 V bias (open circles) as a function of light intensity for the 1T0/ MEH-FFV/Cho/Au device (reproduced by pennission of the American Institute of Physics from Ref.

Figure 15-21 shows the dependence of the short circuit current and the photocurrent at -IV (reverse) bias as a function of the illumination intensity (514.5 nrn) the data show no indication of saturation al light intensities up to ca. 1 W/cm2. 

Stationary microwave electrochemical measurements can be performed like stationary photoelectrochemical measurements simultaneously with the dynamic plot of photocurrents as a function of the voltage. The reflected photoinduced microwave power is recorded. A simultaneous plot of both photocurrents and microwave conductivity makes sense because the technique allows, as we will see, the determination of interfacial rate constants, flatband potential measurements, and the determination of a variety of interfacial and solid-state parameters. The accuracy increases when the photocurrent and the microwave conductivity are simultaneously determined for the same system. As in ordinary photoelectrochemistry, many parameters (light intensity, concentration of redox systems, temperature, the rotation speed of an electrode, or the pretreatment of an electrode) may be changed to obtain additional information. 

The combination of photocurrent measurements with photoinduced microwave conductivity measurements yields, as we have seen [Eqs. (11), (12), and (13)], the interfacial rate constants for minority carrier reactions (kn sr) as well as the surface concentration of photoinduced minority carriers (Aps) (and a series of solid-state parameters of the electrode material). Since light intensity modulation spectroscopy measurements give information on kinetic constants of electrode processes, a combination of this technique with light intensity-modulated microwave measurements should lead to information on kinetic mechanisms, especially very fast ones, which would not be accessible with conventional electrochemical techniques owing to RC restraints. Also, more specific kinetic information may become accessible for example, a distinction between different recombination processes. Potential-modulation MC techniques may, in parallel with potential-modulation electrochemical impedance measurements, provide more detailed information relevant for the interpretation and measurement of interfacial capacitance (see later discus-... 

Figure 18. (a) PMC potential curves and (b) photocurrent curves for n-silicon in contact with a 50 mM Fe(CN)j /4" aqueous electrolyte for different light intensities. Scan rate 20 mV. The light intensity is varied from 100 mW cm-2 (top curves), to 50,20, and 10 mW cm-2 (bottom curves).23... 

Figure 21. (a) PMC potential and (b) cathodic photocurrent-potential curves for a p-Si (111) electrode (resistivity, 10 ft cm). Electrolyte, 1 M NH4F light intensity 1 mW cm-2. Sweep toward negative potentials. 

Figure 23. Influence of light pulsing frequency on peak height and peak position of n-WSe2 in contact with 50 mMFe2+/3+ (5 mM H2SO4). Pulsing frequencies between 11 and 110 cps are compared forthe PMC and photocurrent curves (hght intensity, 50 mW cm-2).

It is interesting to note that independent, direct calculations of the PMC transients by Ramakrishna and Rangarajan (the time-dependent generation term considered in the transport equation and solved by Laplace transformation) have yielded an analogous inverse root dependence of the PMC transient lifetime on the electrode potential.37 This shows that our simple derivation from stationary equations is sufficiently reliable. It is interesting that these authors do not discuss a lifetime maximum for their formula, such as that observed near the onset of photocurrents (Fig. 22). Their complicated formula may still contain this information for certain parameter constellations, but it is applicable only for moderate flash intensities. 

Combination of Intensity-Modulated Photocurrent and Microwave Spectroscopy... 

Therefore intensity-modulated photocurrent Spectroscopy has been developed by Peter and co-workers as a tool for the analysis of photocurrent responses in the frequency domain.42,43 An optoacoustic coupler is... 

Intensity-modulated photocurrent spectroscopy has been used in combination with microwave reflectivity measurements to investigate hydrogen evolution at a p-type silicon45 and an n-type silicon.46 The measurement of amplitude and phase under harmonic generation of excess carriers, performed by Otaredian47 on silicon wafers in an attempt to separate bulk and surface recombination, should also be mentioned here. 

Preliminary measurements with space-resolved PMC techniques have shown that PMC images can be obtained from nanostructured dye sensitization cells. They showed a chaotic distribution of PMC intensities that indicate that local inhomogeneities in the preparation of the nanostructured layer affect photoinduced electron injection. A comparison of photocurrent maps taken at different electrode potentials with corresponding PMC maps promises new insight into the function of this unconventional solar cell type. 

Photocells The basic construction of a photocell is illustrated in Figure 17. A photocurrent flows when the photocathode is illuminated, this is proportional to the intensity of illumination if the supply potential has been chosen to be higher than the saturation potential. A minimal potential is required between the photocathode and the anode in order to be able to collect the electrons that are emitted. The sensitivity is independent of frequency up to 10 Hz. The temperature sensitivity of evacuated photocells is very small. The dark current (see below) is ca. 10 " A[l]. 

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... 

A thorough insight into the comparative photoelectrochemical-photocorrosion behavior of CdX crystals has been motivated by the study of an unusual phenomenon consisting of oscillation of photocurrent with a period of about 1 Hz, which was observed at an n-type CdTe semiconductor electrode in a cesium sulfide solution [83], The oscillating behavior lasted for about 2 h and could be explained by the existence of a Te layer of variable width. The dependence of the oscillation features on potential, temperature, and light intensity was reported. Most striking was the non-linear behavior of the system as a function of light intensity. A comparison of CdTe to other related systems (CdS, CdSe) and solution compositions was performed. 

The prototype DSCs used liquid electrolytes, typically L/I2 in an organic solvent such as propylene carbonate. The electron generation/collection problem in this cell has been discussed analytically with the help of intensity-modulated photocurrent and photovoltage spectroscopy [314]. A particularly challenging issue has been the replacement of the liquid electrolyte with a solid charge-transport material... 

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