The photophysics of the three transition metal phthalocyanines NiPc, CuPc and ZnPc in their β-phase has been studied using complementary experimental techniques: photoluminescence, optical absorption and electronenergy-loss spectroscopy. We demonstrate that the strongly different photoluminescence signal can be understood based on the different exciton dispersions, and that tiny differences in the structure and thus the intermolecular orbital overlap is able to model these differences. Our results clearly indicate a significant admixture of charge-transfer excitations to the lowest exciton state. They additionally demonstrate the need of a detailed microscopic understanding of organic semiconductors for their application potential.

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With the aim of pushing the knowledge and understanding on mixed films of organic semiconductors forward, blends of ZnPC and F8ZnPC in different ratios are manufactured. The films have a polycrystalline structure, as indicated by electron diffraction profiles and infrared-spectroscopy. Photoluminescence data show completely different spectra for the blends, compared to the pure materials, which can be ascribed to the suppressing of excimer formation and the appearance of a new charge-transfer excitation between the two different molecules in the blends. This new excitation can also be seen in optical absorption. Momentum dependent measurements of the electronic excitations by electron energy-loss spectroscopy confirm the localized character of the new charge-transfer excitation in the blends. Our experimental data help understand the important issue of donor/acceptor coupling in organic semiconductors.