Photovoltaic solar energy conversion is investigated theoretically over a temperature range of 0–400°C using semiconductor materials with band gaps varying from 0.7 to 2.4 ev. Three cases are considered. In Case I, the junction current is the ideal current. In Case II, the junction current is the ideal plus a recombination current; and in Case III, a recombination current. The best conversion performance is obtained for the ideal current; the worst, for the recombination current. The maximum conversion efficiency occurs in materials with higher band gap as the temperature is increased. GaAs is close to the optimum material for temperatures below 200°C. Experimental measurements are presented on Si, GaAs, and CdS cells. The measurements on Si and GaAs agree with theoretical expectations as far as the gross behavior is concerned. The CdS cell behaves anomalously as if it were made from a material with band gap of 1.1 ev.
LETTERS TOBloch function for the bottom of the conduction band, normalized to one in unit volume. The pertinent point which we wish to make is that it is necessary to include the fact that U^{YN) (the square amplitude, at the nucleus, of the appropriately normalized Bloch function) is of the order of 10 2 in evaluating ^F 2 {rN) for use in hyperfine interaction calculations. When this is not done, i.e., if it is assumed that \pF 2 (rN) = p{rN), the estimated interaction, and thus the F-center resonance width, are small by several orders of magnitude.With this in mind the writer has used the results of the KS calculation together with Hartree-Fock calculations 4 for sodium and potassium to estimate the F-center absorption width. The former is used to obtain J{TN) and the latter to obtain an approximation to U(YN) in terms of the ratio of the corresponding atomic orbital at the nucleus to its value at the outer maximum. The results pertaining to the interaction at one of the six metal ion nuclei surrounding the vacancy (for the F-center ground state) are: KC1, / 2 =1.4X10 21 cm"*, w 2 fay) = 5.4X10*; NaCl, /*=2.2 X10 21 cm -3 , w 2 (^) = 3X10 2 . This leads to theoretical resonance widths, by use of the methods discussed by Kittel, of 58 oersteds and 280 oersteds, respectively. These are to be compared with experimental values of 54 and 162 oersteds. 3 The agreement is quite satisfactory.In view of these conclusions it appears that the possibly misnamed "continuum model" can give a reasonable picture of the F center. In discussing charge concentration and energy levels the point emphasized in this letter is not important; however, if detail within the unit cell is significant, a more careful consideration of the role of the cell-periodic part of the wave function is required. A more detailed discussion of the KS calculations is in preparation, and some of these points will be considered at greater length therein. In closing, the writer wishes to express his appreciation to Professor C. Kittel and Professor F. Seitz for their interest and comments on this subject.
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