Influence of temperature and irradiance on triple-junction solar subcells

Lattice matched triple-junction solar cells based on the materials Ga0.50In0.50P, Ga0.99In0.01As, and Ge are widely used in space and concentrator applications. In this study, the three subcells of this structure are investigated individually using component solar cells. The investigation covers a broad range of operating conditions, namely temperatures T from 5 to 170 °C and concentration ratios C from 1 to 3000. The temperature dependence of the external quantum efficiencies is analyzed. Further, the I-V-characteristics are analyzed in light of varying temperature and irradiance. The influence of temperature on current-matching is discussed. Special emphasis is given to the Ge bottom cell I-V-curve which collapses at high temperatures. Increasing irradiance counteracts this effect, however series resistance becomes influential. It is shown that at high temperatures the Ge cell can contribute negatively to the power output of the triple-junction device. The limit temperature, defined as the temperature where the open circuit voltage VOC of a cell vanishes, is determined from extrapolation of the measurements using an analytical model of VOC(T,C) and the influence of irradiance is discussed. It is shown, that for the Ge bottom cell the limit temperature lies in the range of operating conditions for space missions close to the sun and approaches operating conditions in CPVT applications.