In order to find an upper theoretical limit for the efficiency of *p‐n* junction solar energy converters, a limiting efficiency, called the *detailed balance limit* of efficiency, has been calculated for an ideal case in which the only recombination mechanism of hole‐electron pairs is radiative as required by the principle of detailed balance. The efficiency is also calculated for the case in which radiative recombination is only a fixed fraction *f _{c}* of the total recombination, the rest being nonradiative. Efficiencies at the matched loads have been calculated with band gap and

*f*as parameters, the sun and cell being assumed to be blackbodies with temperatures of 6000°K and 300°K, respectively. The maximum efficiency is found to be 30% for an energy gap of 1.1 ev and

_{c}*f*= 1. Actual junctions do not obey the predicted current‐voltage relationship, and reasons for the difference and its relevance to efficiency are discussed.

_{c}## REFERENCES

*A*as large as three.

*Tables of Series, Products and Integrals*(Deutscher Verlag d. Wissensch., Berlin, 1957), pp. 149, 413;

*Tables of Functions*, (Dover Publications, New York, 1945) 4th ed., pp. 269, 273.

*Electrons and Holes in Semiconductors*(D. Van Nostrand Company, Inc., Princeton, New Jersey, 1950) p. 308; the product of Eqs. (18) and (19).

*I‐V*plot by various authors, in particular W. G. Pfann and W. van Roosbroeck (See footnote 2). The results do not, however, appear to have been published in analytic form in which the matching factor

*m*is shown to be a function solely of the variable $zop\u2009=\u2009Vop/Vc\u2009=\u2009\upsilon (xg,xc,f)(xg/xc).$

**Pt. II**, Suppl. No.

*Solid State Physics*(Academic Press, Inc., New York, 1960) Vol. 2, Part 2, p. 1180.

*Structure and Properties of Thin Films*, edited by C. A. Neugebauer, J. B. Newkirk, and D. A. Vermilyea (John Wiley & Sons, Inc., New York, 1959), p. 298;