The morphology of thin films of N,N′-dimethyl-3,4,9,10-perylenetetracarboxylic acid diimide (DMe-PTCDI), vacuum deposited onto quartz substrates, kept at various temperatures (Ts) between 300 and 450 K, was investigated by the Atomic Force Microscopy. The samples were polycrystalline, with no appreciable amount of amorphous phase. Below Ts=330 K, the dominating forms are oval crystallites, whereas above this temperature ribbon-like crystallites prevail. Optical spectra of the films in the visible spectral region were measured and correlated with the morphology of the samples. Details of the spectra can be explained assuming the effect of two types of interactions in the films, related to the crystal structure. Photovoltage spectra of indium tin oxide/DMe-PTCDI structures were measured on samples deposited at various temperatures. The diffusion lengths of excitons, estimated from a combination of the optical and photovoltage spectra, vary with Ts. For Frenkel excitons, the exciton diffusion length amounts to (25±5) nm for the films deposited at 300 K and (55±5) nm for the films deposited at 370 K, whereas for charge-transfer excitons the respective values amount to (10±5) nm for the films deposited at 300 K and (25±5) nm for the films deposited at 370 K.