We report, for the first time, the growth and characterization of lattice-mismatched Al0.88In0.12As on GaAs (1 0 0) as part of a program to develop a high-efficiency, triple-junction space solar cell. The expected practical efficiency of this cell is 31%. The proposed triple-junction cell consists of a 2.1 -eV Al0.88In0.12As top cell, a 1.6 -eV In0.48Ga0.52As0.23P0.77 middle cell, and a 1.2 -eV In0.13Ga0.87As bottom cell. The Al0.88In0.12As layers were grown on GaAs(100) wafers cut 6 degrees off-axis. using metal organic precursors trimethylaluminum, trimethylindium and the hydride arsine in a horizontal, reduced-pressure metal organic vapor-phase epitaxy (MOVPE) reactor. Crystalline quality was evaluated through triple-axis X-ray diffractometry. Reciprocal lattice mapping showed that the Al0.88In0.12As layers were relaxed to a large extent, and were of high crystalline quality. Doping was performed using silane (n-type) and diethylzinc (p-type). Secondary ion mass spectrometry (SIMS) and electrochemical C-V profiling showed that it was possible to obtain abrupt doping profiles to the levels required for solar cell fabrication. Details of these results and those of dislocation density measurement using transmission electron microscopy (TEM) are described in this paper.