Photocurrent quantum yield

Figure 2. Relative photocurrent vj. temperature for the CdS.Te (100 ppm) photoelectrode of Figure 1 in 1M OH /IM S2" electrolyte excited in identical geometries with equivalent intensities (ein/s) of 514.5 nm (0) and 501.7 nm (O) light at +0.7 V vs. Ag (PRE). The scale is such that the 25°C, 501.7 nm photocurrent has been arbitrarily set to 100 and corresponds to a current density of 0.38 mA/ cm2 and a photocurrent quantum yield of 0.66.

Calculated from eq.2 where 4>x is the photocurrent quantum yield, and r is the ratio of emission quantum yields between open circuit and the potential where 4>x is measured. 

Surface state densities of the order of 10 cm are commonplace for semiconductor electrodes of the sort considered in previous sections of this chapter. These translate to equivalent volume densities of 10 cm for nanocrystalline films. Such high densities enhance light absorption by trapped electrons in surface states, giving rise to photochromic and electrochromic efiects [297-299] (see below). Unusually high photocurrent quantum yields are also observed with sub-band-gap light with these photoelectrode materials. Corresponding sub-band-gap phenomena are rather weak and difficult to detect with single crystal counterparts. 

A plot of the photocurrent quantum yield versus excitation wavelength is termed the photoaction or photocurrent action spectrum. These spectra are obtained at short-circuit in a two-electrode arrangement or with an external bias in a three-electrode configuration. The photocurrent quantum yield is defined as the number of electrons measured in the external circuit divided by the number of absorbed photons. It is experimentally difficult to calculate the number of absorbed photons and corrections for scattered or transmitted light often appear to be fudge factors that increase the uncertainty of the absolute photocurrent yield. Therefore, the incident photocurrent yield is often reported which represents a lower limit of the true photocurrent quantum yield. 

Clark and Sutin 63] reported that the quantum yield for electron injection from Ru[4,7-(CH3)2-phen]3 + to rutile Ti02 was unity at pH 1 but dropped to zero at higher pH. At that time, a quantum yield of unity had little precedence. Spitler and Calvin had measured adsorption isotherms and sensitized photocurrent quantum yields for Rose Bengal/Ti02 and Rhodamine B/ZnO [53] and reported surface coverage-independent values of less then 0.03. The results suggested that only a... 

Figure 1.9 Photocurrent (quantum yield Q) as a function of the Ti02 formation potential Up, film thickness, respectively.

Photocurrent Spectra Further correlation with the electronic structure of the films can be obtained by photocurrent measurements. The results are shown in Figure 1.32, with grains 1 and 2 as examples. As indicated from the photocurrent spectra (Figure 1.32a), the photocurrent quantum yield above the bandgap of 5.0 eVis... 

Spray painted (spray deposited) titanium dioxide coatings were sensitized [205] with chemically deposited cadmium selenide thin films the structural, optical, and pho-toelectrochemical characterization of these composite films indicate the importance of thermal treatments in improving the photocurrent quantum yields. Up to 400 °C, the effect of air annealing is to shift the onset of absorption to longer wavelengths and improve the photocurrent substantially. 

The photoconductive properties of P26, P29, and P30 were studied by the single-layer photocells in the sandwich-type structure of ITO/Pt polyyne/Al (ITO = indium tin oxide).57,58 These polymers show moderate photoconductivity. A photocurrent quantum yield of 0.01% was estimated in most cases, which does not vary much with variation of the central fluorene ring. 

It is clear that the photocurrent quantum yield of a flat semiconductor electrode will depend on the relative magnitudes of the width of the retrieval layer, dsc + L, and the penetration depth of the light, (1/a). An expression for the flux of photogenerated minority carriers arriving at the surface, g, was derived originally by... 

High photocurrent quantum yields have also been obtained with porous, nanocrystalline CdS, CdSe, Ti02, and ZnO electrodes [142-144]. In these systems, bulk recombination is negligible due to the small size of the structural units (L > size). Surface recombination is prevented by effective scavenging of the photogenerated holes by a reducing agent interpenetrated in the nanoporous system. 

The photocurrent is due to direct and surface state mediated transfer of holes (processes 14 and 17 respectively). The competitive processes which determine the photocurrent quantum yield are summarised in Fig. 8. 

At the present state of the art, however, the entire held of nanochemistry is still at a level of basic research. Despite the relatively high photocurrent quantum yields, solar applications did not arise until now, since the Q-particle doped electrodes decompose under illumination within several days or weeks. Keeping in mind, however, the complicated manner in which the photochemistry and photophysics of Q-particles is determined by the interplay of particle size and surface chemistry, it appears reasonable that a lot of detailed investigations have to be carried out to reliably judge the potential of Q-particle materials. 

The ability of porous photoelectrochemical systems to separate effectively electrons and holes is widely known since the presentation of the dye-sensitized particulate Ti02 solar cell [16, 105, 130-137]. In this system, the photocurrent quantum yield (the number of electrons counted in the external circuit as photocurrent divided by the number of absorbed photons) is close to unity. This means that electron-hole pair recombination is essentially absent. Efficient separation of photogenerated electrons and holes was demonstrated with several other photoelectrochemical systems [105, 130-137]. Photovoltaic devices based on permeated hole-conducting and... 

Fig. 18 Comparison of the photocurrent quantum yield [Q = jpH/e(l — versus the wavelength of the incident light, measured with a nonporous n-type single-crystal electrode of CaP (a), and with a macroporous GaP electrode (b). With a nonporous electrode the quantum yield is very low for light absorbed in the indirect transition hv < 2.7 eV). In contrast, for a porous electrode the quantum yield is unity for light of energy above the gap [hv > 2.2 eW) (from Ref. 139).

Naturally, film formation from fullerene-porphyrin dyads is attractive for photocurrent generation. The group of Sereno has reported the use of dyad 173 (Figure 13.92) in the spin-coating preparation of Sn02-supported films. Using this dyad, in which the distance separating the donor and the acceptor is estimated at 14 A, photocurrent quantum yields of 20% are observed in the presence of hydroquinone as a sacrificial donor. 

Figure 8.21 The dependence of the photocurrent quantum yield on the photon energy of a polished n-GaP electrode (dashed curve) and the same electrode after porous etching (solid curve) 16Ccm", 10VvsSCEin0.5M HjSOj solution at 1 V vs SCE (after [76]).

Fig. 47 Photocurrent quantum yields vs. spacer length (n = number of methylene groups). The solid curve is only a guide to the eye. The data were taken from [ 142]...

---