Photopotentials, open circuit

The photopotentials of dye-gas-metal systems have reached an order of 0.1 mV 55> and those of alkali metal-aromatic junctions 0.2 to 1.0 V 53). Whereas the photo-emf of noble metal-aromatic junctions were of the order 1—15 mV 53>, thin tetracene films sandwiched between two different evaporated metal electrodes (Au, Al) showed photovoltaic effects with an open-circuit photovoltage up to... 

The general solution of the system of transport equations for electrons and holes permits the photopotential of an open circuit to be calculated. The assumption that the total potential change due to illumination occurs in the space-charge region of a semiconductor, i.e., equilibrium value of , and that the exchange currents and... 

It follows from Eq. (36) that open circuit is negative if a depletion layer is formed near the surface. A negative value of (ppb corresponds to a decrease in sc and can clearly be explained as an unbending of the bands under illumination. 

In addition to the use of open-circuit photopotentials, the variation in interfacial capacitance with electrode potential can be utilized to determine the flatband potential as well as the semiconductor dopant concentration. A discussion of the capacitance-potential response of the semiconductor-electrolyte interface is beyond the scope of this text. The reader is referred to Reference 7 for a more complete discussion of this subject. 

The experiments were performed with single crystal (111) p-Si electrodes with a resistivity of about 5.5 ohm cm non-aqueous electrolytes were used consisting of absolute methanol containing tetramethylammonium chloride (TMAC) or acetonitrile containing tetraethyl ammonium perchlorate (TEAP). The flat-band potentials or p-Si in the two electrolytes were determined from Mott-Schottky plots (in the dark) in the depletion range of the p-Si electrode, from open-circuit photopotential measurements, and from the values of electrode potential at which anodic photocurrent is first observed in n-type Si electrodes. These three methods all yielded consistent flat-band potential values for p-Si of + 0.05V (vs SCE)... 

Below, we discuss one more (photoelectrochemical) method for determination of the flat-band potential [40, 172]. The flat-band potential can be determined (i) as the photocurrent onset potential Eomsi (ii) from the dependence of electrode open-circuit photopotential Eocph on light intensity J, as the limiting value of Eoc at a sufficiently high J and (iii) by extrapolating, to zero photocurrent, the potential dependence of the photocurrent jvh squared (see Section 7). These methods are based on the concept... 

An alternative technique for the measurement of sc is to study the photopotential. The theory of this is discussed in some detail in Sect. 7, but the essential features of the measurement are shown in Fig. 16. After equilibration in the dark, when the potential of the electrode at open circuit becomes equal to the redox potential Vredox, the light is turned on and the electrode potential changes at open circuit in such a way that the bands become flat. There are many problems with this technique and it is considerably less reliable than a properly conducted a.c. experiment, but it may give a reasonably accurate picture if surface recombination is small (vide infra). Some results for p-GaAs in aqueous solution are shown in Fig. 17 and the S values derived are of the order 0.7, though the dispersion apparent in Fig. 17 makes a quantitative interpretation difficult. 

The current measured at Vught is the sum of electron and hole currents i = ip + h-At the open-circuit potential condition the net current is zero, that is, ip = -in and the photopotential is called the open-circuit photopotential, Voc- When there is very little surface recombination and the electrode reaction is sufficiently fast so that the concentration of the reagents at the surface remains equal to their equilibrium values, the relation between open-circuit photopotential and light intensity for an n-type material can be expressed for a by... 

Figure 10.10 Current-voltage curves for electrochemical storage processes, A or B. Ua and (/b are the redox potentials for the respective storage processes. Process A may be charged by the photodriven current-voltage curve P, whereas process B may not. In the photodriven I-V curve P, is the voltage corresponding to the point of maximum power, Pn ), and 4c and Pph are the short-circuit current and open-circuit photopotential respectively.

Figure 13. Dependence of the absolute value of the open-circuit photopotential [(pp l couple equilibrium potential WSe trode (solid line, p-type dashed line, n-type) in acetonitrile solutions. Redox couple (figures in parentheses are charge numbers of the ox and red components) (1) anthracene (0/-1) (2) phtalonitrile (0/ - 1) (3) nitrobenzene (0/ - 1) (4) 2,2 -bipyridyl complex of ruthenium (+2/+1) (5) azobenzene (0/ -1) (6)...

D. Laser and A. J. Bard, Semiconductor electrodes VIII. Digital simulation of open-circuit photopotentials, J. Electrochem. Soc. 123 (1976) 1833-1837. 

Studied cell characteristics Polycrystalline photoelectrode, Pt counter electrodes, cell configuration is similar to Fig. 4, ,edox of O2, H+/H2O couple = 1.23 V vs NHE at pH = 1, redox Ag/Ag+= 0.80 V VS NHE, normal hydrogen electrode illumination 500 W Hg lamp, conversion efficiency = 1%, photopotential = 0.28 V vs NHE, open-circuit voltage of the charged cell = 0.28 V, and short-circuit current = 0.3 mA cm . ... 

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