A solar thermophotovoltaic (STPV) device converts solar radiation to electricity. In contrast to a conventional solar photovoltaic (PV) cell, an intermediate absorber/emitter in an STPV device allows conversion of the broad solar spectrum to a narrow-band spectrum tailored near the bandgap of the PV cell. In practice, this conversion process is difficult due to the high temperature operation of the absorber/emitter pair. It is therefore of great importance to understand the energy transport of the device and the effect of various surface properties and geometries on this energy transport. In this work, we use a validated model that describes the radiative heat transfer between the components of a planar STPV device to investigate design parameters in the conversion process. We show how the best performing solar absorber surface for a particular geometry may not necessarily be the best for a different geometry. Similarly, we demonstrate that the most efficient emitter surface does not typically lead to the highest power densities in the PV cell. These counter-intuitive results are discussed and can provide into the design of high efficiency STPV devices.