The application of an In0.7Al0.3As window layer material on the properties of lattice-mismatched (LMM) single-junction (SJ) In0.69Ga0.31As thermophotovoltaic (TPV) cells grown by solid source molecular beam epitaxy (MBE) was studied as an alternative to conventional InAsP window layers. For these 0.60 eV bandgap SJ devices, high performance was achieved, displaying open-circuit voltage of 355 mV, power density of 0.532 W/cm(2) and a fill factor of 66.5% measured at a current density value of 2.26 A/cm(2). The measured internal quantum efficiencies were close to similar to 100% at wavelengths between approximately 1.2-1.4 mm, suggestive that low carrier recombination rates in the vicinity of this alternative In(0.70)Al(0.30)AS/In0.69Ga0.31As window/emitter interface has been achieved. The quantum efficiency data coupled with the device parameters show that InAlAs can be successfully substituted for conventional InAsP window layers in metamorphic TPV devices. By using an interface consisting solely of III-As alloys for the window/emitter material pairing as opposed to the conventional InAsP/InGaAs window/emitter design, this approach avoids the problematic III-P/III-As interface transition for MBE growth of TPV devices that has been shown to impact carrier collection efficiency [Hudait MK, Lin Y, Palmisiano MN, Ringel SA. 0.6-ev band gap In0.69Ga0.31As thermophotovoltaic devices grown on InAsyP1-y step-graded buffers by molecular beam epitaxy. IEEE Electron Dev Lett 2003:24(9):538-40]. (C) 2008 Elsevier Ltd. All rights reserved.