Bandgap optimum

FIG. 58. (a) Refractive index 2.07cV- Ih) cubic bandgap E. (c) hydrogen content Ch. and (d) microstnicture parameter R as function of frequency at optimum pressures (see Fig. 54) at three power densities. 

Where ria and r c are, respectively, the anodic and cathodic overpotentials. Considering all these losses an optimum bandgap of 2.0 to 2.25 eV is required for the materials used as photoelectrodes for water photoelectrolysis. In practical cases, a reasonable value of overall solar efficiency is 10% for single bandgap devices involving two photons and 16% for dual photosystem devices involving 4 photons [102,103,110,111]. 

Annealed Films. For many years, the CdSe photoelectrode— polysulphide electrolyte PEC— was probably the most studied system in PEC research. The bandgap of (bulk—see later) CdSe is ca. 1.7 eV, which is close to the theoretical optimum of 1.5 eV for photovoltaic cells in general and in relative terms, that system was fairly stable in terms of self-oxidation of the semiconductor fihn in the electrolyte by the photogenerated holes. 

In Refs. 10 and 11, aqueous NaiSiOs was added to SbCls in glacial acetic acid (SbCls hydrolyzes in water unless complexed or the solution is moderately acidic or strongly alkaline). A pH of ca. 3 was optimum below 2.5, adhesion was poor above 4, basic antimony salts precipitated. The solution was kept below room temperature to prevent rapid bulk precipitation. No XRD pattern was found for the as-deposited film, which was presumed to be amorphous. Annealing at 170°C crystallized the film, at least partly. The bandgap of the as-deposited film was reported to be 2.48 eV and that of the annealed film 1.76 eV. Photoconductivity was exhibited by the annealed film but not by the as-deposited one. 

Only one example of Sn-Se has been reported [110]. Films were deposited from a room-temperature selenosulphate solution of SnCli complexed with triethanolamine and added NaOH. Polyvinylpyrollidone (PVP) was also added and in general slowed down the deposition. At an optimum concentration of PVP, a maximum terminal thickness was obtained (although no comparison with films deposited from PVP-free solutions was given). No XRD pattern was observed for the as-deposited films heating in an inert atmophere at ca. 330°C gave the pattern of SnSe. The bandgap was 0.95 eV (indirect). The films were n-type, with a resistivity of ca. 10 O-cm... 

In spite of the described findings it is generally believed that - at least on module level - reproducibility and production-yield profit from inclusion of i-ZnO. It is obvious that the influence of localized flaws in the absorber film, such as pin-holes, is more severe if those are directly in contact with the highly doped contact layer [14]. Similarly, if the device properties are slightly inhomogeneous on a microscopic scale, the i-ZnO may be beneficial in terms of performance. Such inhomogeneities could be caused, e.g., by lateral bandgap fluctuations in the absorber film. In this case, the optimum resistivity of the i-ZnO will depend on the amount of fluctuations present [15]. 

This semiconductor has the optimum bandgap for sunlight conversion (1.42 eV) and so, like Si, is commonly used for solid-state photovoltaic elements. In our case the complete water decomposition for hydrogen and oxygen does not take place. 

The semiconductor electrode most studied in photoelectrolytic cells has been n-Ti02, and in photogalvanic cells n-Sn02. Because the bandgap energies are 3.0 eV and 3.5 eV respectively, they are not optimum semiconductors as they only make use of about 5 per cent of the solar energy. For this reason there has been research into other semiconductors, for example cadmium sulphide. In all cases the efficiency is fairly low. 

As already discussed, many a times, undoped CPs (those which are semiconducting in nature with optimum bandgap and optoelectronic properties)... 

When such a map has been constructed, it can be used to select semiconductors of different bandgap, but the same lattice constant, so that the lattice matching which is so desirable in a heterojunction can be made between a specified wide bandgap window semiconductor and a series of photovoltaically active alloy semiconductors. For example. Fig. 10 shows the iso-lattice constant line with a lattice constant value equal to that of CdS (5.82 eV). The bandgaps of semiconductors which have this lattice constant range from about 1.06 eV to 1.80 eV this range makes the Cu-Ag-In-S-Se alloy system an attractive possible source of materials to be used in solar cells of tandem systems. A description of experiments on this and other ternary alloy systems is the subject of another section of this chapter. However, before we return to that matter we shall first consider optimum design of individual cells for incorporation into the system. 

The L2 and L2.5 also produce significant gains, even at bandgaps below the optimum value, but when the threshold is > 2.5 g the PCE efficiency gain at 1 sun is marginal. Therefore, for 1-sun applications it is critical to optimize the MEG process to approach the case as closely as possible. 

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