The Photovoltaic (PV)−thermoelectric (TE) hybrid system has been demonstrated to have a broad prospect for the utilization of full−spectrum solar energy. In this application form, the performance, including photonic, thermal, and electric properties, should be addressed through careful design to enhance the power conversion efficiency (PCE) of solar energy. In this work, overall coupled photonic−thermal−electric design is carried out for a GaAs−based PV−TE hybrid system. Optimal sizes and material properties enable GaAs solar absorber to harvest about 91% full−spectrum photons and allocate different photons properly by giving high−grade photons above the bandgap to GaAs solar cells and the spectral low−grade energy to bottom NIR solar absorber, respectively. With this photonic structure, the calculated thermal and electric performance indicates that high concentration ratios (>100) are needed to get a lower temperature coefficient and a larger hot−side temperature of TE devices for higher efficiency of the PV−TE hybrid system. The results suggest that this coupled photonic−thermal−electric calculation method could be applied to various types of solar cells employed by PV−TE hybrid systems, and also provide analytical method for other full−spectrum solar harvesting applications.