Solid-state junctions

The different types of quinones active in photosynthesis are being used as electron acceptors in solar cells. The compounds such as Fd and NADP could also be used as electron/proton acceptors in the photoelectrochemical cells. Several researchers have attempted the same approach with a combination of two or more solid-state junctions or semiconductor-electrolyte junctions using bulk materials and powders. Here, the semiconductors can be chosen to carry out either oxygen- or hydrogen-evolving photocatalysis based on the semiconductor electronic band structure. 

While the application of photovoltaic cells has been dominated by solid-state junction devices principally made from silicon, recent work in this field offers the prospect of efficient solar-energy conversion by novel methods. 

There are several methods to investigate the charge states of a semiconductor electrode, for example high-frequency resistometry (HFR) [Otl]. Below a transistor-like set-up, as shown in the inset of Fig. 3.2, will be discussed because it shows in an exemplary way the similarities and differences of solid-state junctions and liquid junctions. 

Although most solid-state junctions depend on single-crystal materials to avoid massive electron-hole recombination, photoelectrodes having reasonable effi-... 

In the model presented here, only recombination in the bulk of the semiconductor has been considered. It is well known from solid state junction (minority carrier devices) that also recombination within the space charge layer can taken place. In this case, a quality factor n, ranging between 1 and 2, is introduced. Then Eq. (45) has to be replaced by... 

In the theory of non-equilibrium processes at solid state junction and also semiconductor-liquid interfaces, as developed in the previous section, frequently quasi-Fermi levels have been used for the description of minority carrier reactions [90, 91], A concept for a quantitative analysis for reactions at n- and p-type electrodes has been derived [92, 93], using the usual definition of a quasi-Fermi level (Eqs. (3a) and (3b)). Taking a valence band process as an example, the quasi-Fermi level concept can be illustrated as follows ... 

Models of i-V data for sensitized electrodes invariably begin with Gartner s treatment developed for solid-state junctions [28] that was later extended to photo-electrochemical cells by Butler [29]. In Gartner s model, light absorption by the semiconductor itself yields a photocurrent that contains a drift component asso-... 

The leitmotifs of these devices include bespoke dye sensitisers, space-quantised nanoscale structures that enable hot carrier or multiple exciton generation, molecular and solid-state junction architectures that lead to efficient exciton dissociation and charge separation, and charge collection by percolation through porous or mesoscale phases. Another common theme underlying the devices discussed in this book is the... 

Fig. 24. A prison of energy bands in absence and in presence of intercalated species visualizes the possibilities of electrochemical preparative techniques for solid state junctions visuahzes...

Reagent grade chemicals were utilized. Electrodeposition was carried out in a conventional three-electrode cell under potentiostatic control with a mercury sulfate electrode as reference (in the following, all potentials are quoted versus this reference) and a platinum wire as counter electrode. Solutions were stirred and deoxygenated by bubbling nitrogen. Backside ohmic contacts of Si samples were achieved with an InGa eutectic. The i-U characteristics of the solid state junctions were measured in air. 

APE is a very general phenomenon that has been observed for solid-state junctions for n- and / -type semiconductors alike, and for a wide variety of semiconductor materials. 

The status of the dye-sensitized device as the only verified alternative to solid-state junction devices has already been discussed. It must be recognized that the solid-state devices, particularly the silicon p-n junction cells benefit from forty years of industrial and development experience, the technology transfer from the silicon-based electronics industry, and even the widespread availability of high-quality silicon at low cost resulting from the expansion of that industry. The procedures for high-yield fabrication of silicon devices, both crystalline and amorphous, are well understood, with costing well established on the basis of decades of solid industrial experience. For dye-sensitized... 

In an electrochemical context, the situation is somewhat simplified since the value of Ef for an electrode surface can be controlled (with the aid of a stable reference electrode and an electrical double layer, which serves to screen electrons that reside in the electrode such that the energy level of a molecular orbital in solution is not impacted by the applied potential). Thus, in electrochemical studies, the value of 4> can be reduced to zero, enabling electrons to be removed from the HOMO orbital of solution-based molecules (oxidation), or to be added to the LUMO orbital (reduction). However, in the context of a solid-state molecular electronic junction, the value of a nonresonant interfacial barrier remains essentially static at different voltages. We discuss some of the reasons in more detail below that it cannot be assumed that their energy levels in a completed junction are set by the properties of the individual, isolated components. In any case, the presence of an interfacial barrier in a solid-state junction is a critical parameter in determining the electron transport properties of the device, regardless of the specific mechanism in operation. Thus, the appearance of Figure 10.12 needs to be taken in the context of the specific mechanism(s) under study. 

Most robust and efficient sensitizers are based on the metal-to-ligand charge transfer (MLCT) excited states of polypyridyl Ru compounds, which have been well reviewed [254, 255]. Dye-sensitized nanocrystalline electrochemical photovoltaic system has recently become a validated and credible competitor to solid-state junction devices for the conversion of solar energy into electricity [453-455]. More details of some applications can be found in the Sect. 1.7.3.5 and Vol. 6 [456]. 

Electropolymerized semiconducting polymer films also form various solid-state junctions. An ITO/polythienylene (77, PT)/A1 Schottky junction type cell gave = 1.07 V and = 1.35 pA cm The poly(/>-phenylene) 24, PPP)/n-Si junction showed a conversion efficiency of 3.2% at a light intensity of 7 mW cm ... 

Despite the formal similarity of Eq. 5 with rectifying solid state junctions, it should be noted that although the current jo is related to... 

FIGURE 3.38 (a) An example of a molecular solid state junction used for the determination... 

Rabache, V., J. Chaste, P. Petit, M. L. Della Rocca, P. Martin, J. C. Lacroix, R. L. McCreery, and P. Lafarge. Direct observation of large quantum interference effect in anthraquinone solid-state junctions. J. Am. Chem. Soc. 135, 2013 10218-10221. 

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