The Shockley-Queisser limit is known to set the uppermost limit on conversion efficiency for conventional single-bandgap solar cells. In past years different approaches were suggested in order to overcome this limit. Yet, apart for multi-junction cells that are inherently complicated devices, none were realized successfully. Photon-enhanced thermionic emission (PETE) conversion is similar to photovoltaic conversion, but relies on emission of photo-generated electrons across a vacuum gap between two surfaces having different work functions. Heat produced in PV cells is a loss of energy, and an increase in temperature decreases the cell efficiency. On the contrary, in PETE converters, heat produced by thermalization and non-radiative recombination increases the cathode temperature and with it the rate of electron emission. As a result, the conversion efficiency of PETE devices is not restricted by the Shockley Queisser limit that corresponds to the bandgap of the absorbing material. In this work we analyze the ideal performance of PETE devices in comparison to ideal PV cells. The efficiency of PETE devices is shown to increase with the flux concentration and cathode temperature reaching efficiencies near 45% at concentration of 1000 suns. The effect of spectral splitting to PETE devices with different bandgaps is also discussed, showing that spectral splitting is not necessarily beneficial.