With the help of the Langmuir space charge theory, the performance of a photon-enhanced thermionic solar cell (PETSC) limited by space charge is discussed. It is found that space charge effects become important for the PETSC when the gap width between two electrodes is a few microns or more. At these distances, near-field heat transfer effects are negligible because the gaps are larger than the characteristic wavelength of the thermal radiation, as given by Wien's displacement law. The effects of the energy band gap, electron affinity and temperature of the cathode on the efficiency of the PETSC are analyzed in detail. The maximum efficiency of the PETSC limited by the space charge is calculated under different constraint conditions. Although the maximum efficiency obtained here is much smaller than that calculated theoretically by Schwede et al., Nature Materials 9 (2010) 762, it may be higher than those of traditional solar cells and thermionic devices as long as the gap width of two electrodes is rationally designed. (C) 2013 Elsevier B.V. All rights reserved.