P-n photoelectrolysis cell

Fornaiini L, Nozik AJ, Parkinson BA (1984) The energetics of p/n photoelectrolysis cells. J Phys Chem 88 3238-3243... 

Nozik AJ (1976) p-n photoelectrolysis cell. Appl Phys Lett 29 150-153... 

Fujishima A, Honda K (1972) Electrochemical photolysis of water at a semiconductor electrode. Nature 238 37-38 Nozik AJ (1976) p-n photoelectrolysis cell. Appl Phys Lett 29 150-153... 

The Energetics of p/n Photoelectrolysis Cells The interfacial aspects of combining both photoanodes and photocathodes discussed using both theory and experiment. 67... 

With PECs made of n-type and p-type semiconductors in contact with an electrolyte, the requirement of an external bias can be eliminated [11]. A first example of these p-n photoelectrolysis cells was reported by Nozik in 1976 [12] with an n-Ti02/p-GaP heterotype device whose efficiency at zero bias was 0.25%. The energy diagram of such a cell, also called photochemical diode, is... 

Hydrogen Evolving Solar Cells Principles in the design of semiconductor electrodes, surface modification strategies, p-n junction cells, and photoelectrolysis by suspended semiconductor particles, discussed. 66... 

The p-n photoelectrolysis approach,60 on the other hand, simply combines a n-type semiconductor photoanode and a p-type semiconductor photocathode in an electrolysis cell (Fig. 2c). The pros and cons of this twin-photosystem approach (which mimicks plant photosynthesis) were enumerated earlier in this Chapter (see Section 2). Table 16 provides a compilation of the semiconductor photocathode and photoanode combinations that have been examined. Reference 67 may also be con suited in this regard for combinations involving n WSe2, n MoSe2, n WS2, n TiCH, p InP, p GaP and p Si semiconductor electrodes. 

The photoelectrolysis of H2O can be performed in cells being very similar to those applied for the production of electricity. They differ only insofar as no additional redox couple is used in a photoelectrolysis cell. The energy scheme of corresponding systems, semiconductor/liquid/Pt, is illustrated in Fig. 9, the upper scheme for an n-type, the lower for a p-type electrode. In the case of an n-type electrode the hole created by light excitation must react with H2O resulting in 02-formation whereas at the counter electrode H2 is produced. The electrolyte can be described by two redox potentials, E°(H20/H2) and E (H20/02) which differ by 1.23 eV. At equilibrium (left side of Fig. 9) the electrochemical potential (Fermi level) is constant in the whole system and it occurs in the electrolyte somewhere between the two standard energies E°(H20/H2) and E°(H20/02). The exact position depends on the relative concentrations of H2 and O2. Illuminating the n-type electrode the electrons are driven toward the bulk of the semiconductor and reach the counter electrode via the external circuit at which they are consumed for Hj-evolution whereas the holes are dir tly... 

Photoelectrolysis cells of the p-n variety can be constructed using the same semiconductor material, one doped p-type and the other n-type or with two different (n-and p-) semiconductors. A photoelectrolysis cell reported by Leygraf et al. (1982),... 

Photoelectrolysis of Water with Ti02-Covered Solar-Cell Electrodes A hybrid structure, involving a p-n junction Si cell coated with a Ti02 film by CVD, is studied. 231... 

Table 16. Photoelectrolysis cells using n-type semiconductor photoanodes and p-type semiconductor photocathodes.

However, the last few years have also seen a growing awareness of the problems inherent in using the semiconductor-electrolyte interface as a means of solar-energy conversion. Very long-term stability may not be possible in aqueous electrolytes and no oxide material has been identified that has properties suitable for use as a photoanode in a photoelectrolysis cell. Highly efficient photovoltaic cells are known, both in aqueous and non-aqueous solutions, but it is far from clear that the additional engineering complexity, over and above that required for the dry p-n junction photovoltaic device, will ever allow the "wet photovoltaic cells to be competitive. These, and other problems, have led to something of a pause in the flood of papers on semiconductor electrochemistry in the last two years and the current review is therefore timely. I have tried to indicate what is, and is not, known at present and where future lines of development may lie. Individual semiconductors are not treated in detail, but it is hoped that most of the theoretical strands apparent in the last few years are discussed. 

A variety of other systems have been suggested and some were also tested such as cells containing an n-semiconductor anode and a p-type cathode, photoelectrolysis cells integrated with photovoltaic configurations, and tandem and cascade type of cells (Nozik and Memming, 1996). 

In photoelectrolysis cells, the concentrations of electrons and holes are changed from their dark equilibrium values due to creation of electron-hole pairs by absorption of photons. Since thermal equilibration with lattice vibrations is much faster than the electron-hole recombination of electrons, electrons and holes can be considered to be in thermal equilibrium with the lattice, even though they are metastable states. Therefore, Fermi-Dirac statistics can still be used by defining quasi-Fermi levels ( Ep and pEp) for electrons and holes in terms of their steady-state concentrations under illumination, n and p. ... 

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