X-ray diffraction investigations show, that in the vacuum evaporated layers of 2,9-di(pent-3yl)-anthra[2,1,9-def:6,5,10-d'e'f]diisoquinoline-1,3,8,10-tetrone (PTCDI) the PTCDl molecules may have edge-on or face-on arrangement. For the samples with different molecular arrangement the dependences of the photogeneration quantum yield on electric field were determined by using the photoinduced surface potential decay technique. The obtained results were analysed using the Onsager's model of photogeneration. The clear influence of the molecular order on the photogeneration efficiency has been found. (c) 2007 Elsevier B.V. All rights reserved.