Photoeffect

Illumination with light having a wavelength larger than the band gap of silicon generates a photocurrent under an anodic potential on an n-Si electrode but has essm-tially no effect on p-Si, as would be expected from the basic theories of semiconductor electrochemistry. However, the photocurrent may not be sustained because of the formation of an oxide film, which passivates the silicon surface to various degrees depending on the electrolyte composition. In solutions without fluoride species, the photocurrent is only a transient phenomenon before the formation of the oxide film. In fluoride solutions, in which oxide film is dissolved, a sustained photocurrent can be obtained. 

The anodic photocurrent of n-Si in aqueous solutions in the absmce of fluoride decays very rapidly due to the formation of an oxide film. It decays less rapidly in the presence of a reducing agent which can compete efficiently for the holes from the valence band and slow down the rate of oxidation. Ferrate ions and iodine ions have been found to compete favorably with the oxidation of the silicon surface. On the other hand, in solutions containing a small amount of fluoride ions insufficient to completely remove the oxide film, the photocurrent exhibits a fast decay. According to Matsumura and Morrison, the rapid decay of the photocurrent in solutions with a small amount of fluoride is due to the catalytic effect of fluoride ions at the Si/SiO interface. 

The photocurrent in nonfluoride solutions is affected by the amount of preanodic current passed through the sample as shown in Fig. 5.10. It is also seen that the photocurrent onset potential is shifted to more anodic values with formation of an oxide film and the amount of shift is related to the thickness of the film. This shift is due to the potential drop across a growing oxide layer and is one of the reasons for the difference between the photocurrent onset potential and the flatband potential.  

FIGURE 5.10. (a) Photocurrent voltage curves in IM NH4CI and IM NH4F at pH 4.S (b) photocurrent transients at +0.5 V, Data in NH4CI also show the effect of oxide layer formation as a function of the amount of coulombs having passed before the experiment. (Reprinted with permission from Gerischer and Lubke. 1987 Wiley-VCH.) 

The current multiplication due to the presence of fluoride depends on the pH of the solution, as shown in Fig. 5.11. It decreases with increasing pH and almost disappears above pH 7. At high pH, formation of oxide prevents direct reaction of silicon with fluoride species, and the pH value at which this occurs decreases with increasing light intensity. 

---

Extended X-ray absorption fine structure (EXAFS) measurements based on the photoeffect caused by collision of an inner shell electron with an X-ray photon of sufficient energy may also be used. The spectrum, starting from the absorption edge, exhibits a sinusoidal fine structure caused by interferences between the outgoing and the backscattered waves of the photoelectron which is the product of the collision. Since the intensity of the backscattering decreases rapidly over the distances to the next neighbor atoms, information about the chemical surroundings of the excited atom can be deduced. 

Although the conductivity change Aa [relation (8)] of microwave conductivity measurements and the Ac of electrochemical measurements [relation (1)] are typically not identical (owing to the theoretically accessible frequency dependence of the quantities involved), the analogy between relations (1) and (8) shows that similar parameters are addressed by (photo)electrochemical and photoinduced microwave conductivity measurements. This includes the dynamics of charge carriers and dipoles, photoeffects, flat band and capacitive behavior, and the effect of surface states. 

Figure 7 shows an example of a space-resolved microwave conductivity measurement of the semiconducting surface of a natural pyrite (FeS2) sample (from Murgul, Turkey). The overflow of the PMC signal (white color) was adjusted to a level that shows the patterns of distribution of low photoeffects (dark areas). Figure 8 shows a similar image in which,... 

Figure 11. Dynamic microwave conductivity-potential curves taken with a ZnO single crystal and shown for two potential sweep velocities (a) and (b) and a corresponding dynamic (photo)current-potential curve (bottom). The dark effects and photoeffects are indicated for the two cases. Curves 1 and 2 correspond to (a) and (b) respectively.

Sugfmoto Y, Peter LM (1995) Photoeffects during cathodic electrodeposition of CdTe. J Electroanal Chem 386 183-188... 

Arico AS, Antonucci V, Antonucci PL, Cocke DL, Russo U, Giordano N (1990) Photoeffects at the polycrystaUine pyrrhotite-electrolyte interface. Sol Energy Mater 20 323-340... 

Within the potential range where Ru(bpy)3 remains in the aqueous phase, photocurrent responses are clearly observed with a slow rising time of the order of 10 s as shown in Fig. 14(a). According to the convention employed by these authors, positive currents correspond to the transfer of a negative charge from water to DCE. No photoresponses were observed in the absence of either the dye in the aqueous phase or TCNQ in DCE. Further analysis of the interfacial behavior of the product TCNQ revealed that the ion transfer occurred outside of the polarizable window [cf. Fig. 14(d)], confirming that these photoresponses are not affected by coupled ion-transfer processes. An earlier report also showed photoeffects for the photoreduction of the viologen under similar conditions [131]. 

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. 

I.A. Akimov, Yu.A. Cherkasov and M.I. Cherkashin, Sensibilized Photoeffect, Nauka Publ., Moscow, 1980 (in Russian)... 

Mountz, J. M. Tien, H. T., Photoeffects of pigmented lipid membranes in a micropourous filter, Photochem. Photobiol. 28, 395 -00 (1978). 

Basic properties of semiconductors and phenomena occurring at the semiconductor/electrolyte interface in the dark have already been discussed in Sections 2.4.1 and 4.5.1. The crucial effect after immersing the semiconductor electrode into an electrolyte solution is the equilibration of electrochemical potentials of electrons in both phases. In order to quantify the dark- and photoeffects at the semiconductor/electrolyte interface, a common reference level of electron energies in both phases has to be defined. 

Despite these changes in our understanding of the photoeffect, the one feature which had not been challenged was the fact that the probability of freeing the electron was still expected to increase with increasing intensity of the light. In other words, the lifetime of the quantum state was expected to decrease with growing intensity. 

Another kind of photoeffect occurs if a redox system in its ground state overlaps weakly with the bands of the electrode but has an excited state... 

In Section 7.4 we gave only a brief outline of the photoeffects caused by electron-hole generation by photons with an energy above that of... 

The fifth consequence of the theory is that the adsorptivity and catalytic activity of a semiconductor are affected by illumination. When a crystal absorbs light waves of photoelectrically active frequencies (i.e., frequencies exciting the internal photoeffect), this leads, generally speaking, to a change... 

Balko, B.A. Clarkson, K.M. (2001) The effect of doping with Ti(lV) and Sn(IV) on oxygen reduction at hematite electrodes. J. Electro-chem. Soc. 148 E85-E91 Balkwill, D. Maratea, D. Blakemore, R.P. (1980) Ultrastructure of a magnetotactic spirillum. J. Bacteriol. 141 1399-1408 Ballko, B.A. Tratnyek, P.G. (1998) Photoeffects on the reduction of carbon tetrachloride by zero-valent iron. J. Phys. Chem. B 102 1459-1465... 

Gartner WW (1959) Depletion layer photoeffects in semiconductors. Phys Rev 116 84-87... 

Ferroelectric substances such as LiNb03 or BaTi03 were found to show anomalous photovoltaic effects (APV) of the order of 103-5 V83). A thin film of a ferroelectric polymer such as PVDF also was found to show APV effects84). Voc of 2.5 x 104 V and short circuit current (Isc) of the order of nA/cm2 were reported. Although the output is very small up to now1, it could be noticed as a specific photoeffect of a thin polymer film. 

Experimental observation of photoemission currents encounters the problem of separating them from the currents of photoelectrochemical reactions of nonemission nature, which are caused by the internal photoeffect in the semiconductor (see, for example, Section 5). Photoprocesses of both the types start similarly with the interband excitation of an electron and are of threshold character with respect to the frequency of light, but the threshold quantum energy is different for these processes. Namely, the threshold of photoemission exceeds that of the internal photoeffect (and hence the threshold of ordinary photoelectrochemical reactions) by the value of the electron affinity to the semiconductor % (see Figs. 31 and 32). 

The photocurrent-voltage curve of a cell made with the I /I2 redox couple (Fig. 8) shows behavior typical of the standard DSSC. The substantial photovoltaic effect is expected from the fact that the dark current (Fig. 4) is negligible positive of about -0.5 V. On the other hand, a cell made with the FcCp2 70 redox couple shows no measurable photoeffect Its current under illumination (Fig. 8) is essentially equal to its dark current (Fig. 4). The photovoltaic effect is negligible because practically all photogenerated charge carriers recombine before they can be collected in the external circuit. In general, fast rates of reactions (4) and (5) tend to eliminate the photovoltaic effect in DSSCs. 

Now the research in the photoconductive properties of the polyconjugated materials is growing fast. Heterocycle or heteroatom-containing polymers are involved in this process due to their excellent mechanical and electric properties. The sensitized photoeffect in polyconjugated materials was first observed in 1964 [19,20] and the high significance of the increase in the photosensitivity of these compounds became apparent... 

Many authors have investigated the photoconductivity of the polydiacetylenes [142-171], The main problem discussed concerns the nature of the initial act of the photoeffect. At first, most authors considered the exciton formation to occur at the beginning with consequent dissociation on the free carriers. Then it was shown the broad band existence for directions along the chains. The unification of the excitonic and band model of the free charge carrier generation was developed [146-150],... 

---