The coupling of resonant modes between two surfaces is important in near-field heat transfer and near-field thermophotovoltaic (TPV) systems. Recently, coupled-mode theory (CMT) has been developed for the analysis and optimal design of TPV systems. In this paper, we use CMT to analyze the "emitter-vacuum-PV cell" configuration, and quantitatively show that how the emitter of a nanostructure can drastically improves the near-field TPV device performance. The key feature of the nanostructure is the additional geometry-induced resonant mode, whose energy is lower than the original surface plasmon polariton (SPP) resonant frequency and much closer to the bandgap of the PV cell. Specifically, we show that with a simple grating structure, the generated power density of a TPV cell is increased from 13 W/cm2 to 34 W/cm2 when the PV cell is fixed at 300 K and the emitter 1000 K. The increase is over 20 times higher when both planar and grating emitters are at a lower temperature of 500 K.