Thin GaAs photovoltaic heterostructures are grown by MOCVD with various p-GaAsbase thicknesses. The total n/p absorbing thickness is varied systematically.Output voltages up to ∼1.155V were obtained for individual n/pjunctions at an average illumination intensity of ∼8W/cm2.Novel phototransducer devices are then achieved with a vertical epitaxialheterostructure architecture, monolithically integrating 5 or more such thin n/pjunctions. Around the design wavelength, the stacked heterostructure design isyielding an optimal external quantum efficiency approaching unity divided by thenumber of junctions. The modeled and measured conversion efficiencies areexceeding 60%. The photocarrier extraction properties are simulated fordifferent junction thicknesses using a model based on a 3-dimensional (3D)radially-symmetric TCAD implementation of the heterostructures. The studyclearly demonstrates that for such thin n/p junctions the photocarrierextraction can still be efficient due to the operation at reduced currentdensities and higher voltages in heterostructures enhancing electrical powerextraction. With the supplementary add-on of a window layer with a reduced sheetresistance for the stacked structure, we demonstrate the possible efficientoperation of phototransducers for optical inputs exceeding 150 W/cm2,even for the case of devices designed without gridlines.