Illumination-Induced Changes in PcVO Pigment Films

Absorption spectra and the structure of films of the organic pigment vanadyl phthalocyanine (PcVO) with a thickness less than 100 nm are studied before and after prolonged exposure to sunlight or LED sources in the near-UV, visible, and near-IR regions (368 nm, 535 nm and 866 nm, respectively). While the phase structure of the films is insensitive to sunlight, the spectral profiles are characteristically changed by narrow-band illumination whose integral intensity is one order of magnitude smaller but is applied in the regions close to the main optical maxima (Q-band and Soret band). The observed changes indicate a partial polymorphic transition in the layer. Direct structural methods (XRD, scanning electron microscopy, optical interference microscopy) are much less informative in the case of films with such thicknesses, which are most often used in organic optoelectronics devices.