Dual-junction Ga0.5In0.5P/GaAs solar cells on Ge substrates have rapidly gone from small, high-efficiency laboratory cells, to large-area, high-efficiency cells manufactured at spectrolab in high volume. Over 500,000 of these dual-junction (DJ) cells with 27-cm(2) area have been produced, with average AMO load point efficiency of 21.4%. The next step in the evolution of this type of multijunction solar cell has been taken, with the development of triple-junction (TJ) Ga0.5In0.5P/GaAs/Ge cells. The addition of the germanium third junction, plus several significant improvements in the device structure, have led to a measured efficiency of 27.0% (AMO, 28 degreesC) at Spectrolab on large-area (> 30 cm(2)) TJ cells. The TJ cell is now in production at Spectrolab. Ga0.5In0.5P/GaAs/Ge cells are viable not only for non-concentrating space applications, but also for terrestrial and space concentrator systems. Efficiencies up to 32.3% at 47 suns under the terrestrial AM1.5D spectrum have been achieved. (C) 2001 Published by Elsevier Science B.V. All rights reserved.