A large increase in the quantum efficiency (QE) and open-circuit voltage Voc of GaInAsSb thermophotovoltaic (TPV) devices is obtained by the use of an AlGaAsSb window layer compared with devices without a window layer. The TPV structure, grown on GaSb substrates by organometallic vapor phase epitaxy or molecular beam epitaxy, consists of a 1-μm-thick n-GaInAsSb base layer, a 3-μm-thick p-GaInAsSb emitter layer, a 100-nm-thick AlGaAsSb window layer, and a 25-nm-thick GaSb contacting layer. The band-gap energy of the lattice-matched GaInAsSb is 0.53–0.55 eV. The peak internal QE of the TPV cells with the window is >90%, compared with less than 60% for those without the window. At a short-circuit current density of ∼ 1000 mA/cm2, Voc of ∼ 300 meV is obtained for cells with the window layer, compared with less than 220 meV without the window layer. These increases are attributed to a substantial decrease in the surface recombination velocity with the window layer. Based on a standard calculation, the electron diffusion length in the p-GaInAsSb layer is at least 5 μm. © 1997 American Institute of Physics.