Photorefractive diffraction gratings were studied in cells of homogeneously aligned nematic liquid crystal doped with buckminsterfullerene (C60). These Raman-Nath gratings were induced by the interference modulation of two coherent optical beams in conjunction with an applied dc field across the sample thickness. The diffraction efficiency depends on the applied dc field. Regardless of the writing-beam power, the maximal efficiency is found to occur at ∼0.8 kV cm-1. This critical field value also determines three applied-dc-field regimes that lead to three distinct behaviours of the corresponding time-evolved diffraction-intensity curves.