A large increase in the quantum efficiency (QE) and open-circuit voltageVocof 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-&mgr;m-thickn-GaInAsSb base layer, a 3-&mgr;m-thickp-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&percent;,compared with less than 60&percent; for those without the window. At a short-circuit current density of∼1000 mA/cm2,Vocof∼300 meVis 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 thep-GaInAsSb layer is at least 5 &mgr;m. ©1997 American Institute of Physics.