Photoconversion systems

Later experiments (4 ) were designed to determine a cell e.m.f. for the plutonium disproportionation system with a particular light source. Concentration quotients for the light and dark conditions, Qg and Qj, were determined, and an energy difference of 1.65 kcal (32 mV) was calculated by the relation -RTln C /Qd This reversible photochemical shift may be the only single-element system known at this time and certainly is the simplest such system. Even though the radioactive properties could prevent development and utilization of a plutonium photoconversion system, these studies certainly suggest that similar nonradioactive and more acceptable systems could be discovered and developed. 

Macroheterogeneous photoconversion systems can thus be constructed by utilizing solid phase of polymers. These results on the solid phase and solid-liquid interface electron relay are surely of use when designing macroheterogeneous conversion systems. The findings obtained in these studies were used to develop a new photodiode composed of polymer pendant Ru(bpy)i film as described in Section 4 of Chapter 5. 

In the classical description of nonequilibrium systems, fluxes are driven by forces [73,76,77]. Equation (8) shows that the flux of electrons (7 ) is related to the (photo)electrochemical force (VEFn) by a proportionality factor (np ). Equation (8) and the related equation for holes can be employed as a simple and powerful description of solar photoconversion systems. However, it is useful to go beyond this analysis and break V > into its component quasithermodynamic constituents, V(7 an Vp, because this helps reveal the fundamental differences between the photoconversion mechanisms of the various types of solar cells. Equation (6) can be separated into two independent electron fluxes, each driven by one of the two generalized forces, Vf7 and Vp. Equations (9a) and (9b) are expressed in the form Flux = Proportionality factor X Force ... 

Some authors have asserted that the quasi-Fermi level model requires a threshold with respect to light intensity. This problem has been discussed for photoconversion systems such as photoelectrolysis of FI2O (Gregg and Nozik, 1993 Shreve and Lewis, 1995). Since the discussion on the threshold problem has frequently led to misinterpretations, we want to clarify the situation by considering a simple charge-transfer between an n-type semiconductor and redox system, as illustrated in Fig. 2.23. The system is at equilibrium (i = 0) if the overvoltage is zero (rj = 0). Flere the quasi-Fermi levels of electrons and holes are both equal to Ap,redox (not shown). Assuming that the redox process occurs entirely via the valence band, then only the quasi-Fermi level of holes at the surface, Sp, is of interest. Anodic polarisation of the electrode in the dark produces a very small anodic current (lower i-rj curve in the centre of Fig. 2.23). As mentioned in the previous section, is practically pinned close to fip,redox (Fig. 2.23A) whereas p, differs from Ap,redox by qr]. On illumination, the anodic... 

This section focuses on charge transfer in synthetic polymer solid phases (for metal enzymes refer to Chapter 2). In many electronic devices such as electrocatalytic systems, sensors, and electrochromic display or photoconversion systems, charge transfer between redox molecules... 

Sensitization of wide band gap semiconductor photoelectrodes by coating with dye molecules has been one of the major subjects of research in photoconversion systems. The typical sensitization mechanism can be represented by Fig. 39. Ru(bpy), phthalocyanines (Pc), porphin (P), organic dyes such as Rhodamine B and mero-... 

Photoconversion Systems by Coordination, Hydrogen Bonding and Electrostatic Interactions... 

It is not yet clear if the DSSC will become the dominant type of solar cell in the future, but its ability to achieve a high photoconversion efficiency by a mechanism that is fundamentally different from conventional solar cells makes it a perfect system for exploring those features that are indispensable to all solar cells. It is now clear, for example, that a p-n junction is not a necessary require-... 

Saturated three- and four-membered heterocyclics absorb little in the readily accessible regions of the UV spectrum. Sulfur-containing rings are an exception, as can be seen in Table 9, Despite the lack of absorption of most parent compounds, there is a wealth of photochemistry of small heterocyclics. Light absorption by substituents, and energy transfer from photoexcited molecules present in the photoreactive system make photoconversion of the heterocycles practical. On the other hand, the lack of substantial absorption of their own can be exploited in the preparation of small heterocycles, by designing the system to be unsuitable for destructive energy transfer. 

The first report of direct photoconversion of water into dihydrogen and dioxygen was studied on Ti02 (n-type) electrodes by Fujishima and Honda [64]. Although the overall efficiency of the system was low, their results initiated the interest in photoelectrochemical cells for conversion of solar energy to chemical energy (or to electrical energy). 

Three spiropyrans, each molecule having a long alkyl chain located in a different relative position (l -hexadecyl, 3-decyl, and 8-dodecanoyloxymethyl), were spread as monolayers on water under irradiation and the changes of surface area and surface pressure with time were measured both when the lamp was turned on and after it was turned off. The results were especially striking observations of a reversible and an irreversible photoconversion of a surface-inactive system into a surface-active one.154... 

---