According to Equations 3.6 and 3.22, the open-circuit voltage increases logarithmically with short-circuit current ISC, which is directly proportional to irradiance G. Hence according to Equation 3.23, the voltage factor SF increases with rising irradiance G, as does (according to Equation 3.12) the idealized fill factor
Figure 3.32 Field test of concentrator cells. Here, sunlight is concentrated on relatively small solar cells using lens systems. To attain a reasonable energy yield, concentrator cells should always be tracked biaxially (Courtesy of DOE/NREL)
FFg thus, according to Equation 3.20, theoretical efficiency also increases. Hence optical concentration of sunlight can somewhat optimize solar cell efficiency still further. However, to do this it is necessary to increase doping so that the series resistance of such a concentrator cell remains sufficiently low. In addition, inasmuch as the absolute value of non-converted radiant power increases as irradiance rises, it is essential that such solar cells be cooled. Moreover, only direct beam radiation can be used for the optical concentration of sunlight.
Owing to the preponderance of diffuse radiation in Central Europe, the use of concentrator solar cells (see e. g. Figures 3.32 and 3.33) is not particularly worthwhile. According to [3.11], magnitude 232 sunlight concentration and 28.8% efficiency were attained for a small lab concentrator cell (0.05 m2) made of GaAs. The record for small silicon concentrator cells (AZ — 1m2) is 27.6% efficiency with magnitude 92 concentration.