Evolution of photophysical and photovoltaic properties of perylene bis(phenethylimide) films upon solvent vapor annealing

A detailed study is presented of the changes in photophysical and photovoltaic properties of thin films of perylene bis(phenethylimide), PPEI, as the film structure evolved from amorphous to polycrystalline under the influence of methylene chloride vapor. A strong absorption band grew in on the low-energy side of the original spectrum as the film order increased, and the peak of the emission spectrum shifted to higher energy, decreasing the Stokes shift by 283 meV. The singlet exciton diffusion length increased by 76%, leading to enhanced quenching of the PPEI fluorescence by a surface film of titanyl phthalocyanine, TiOPc. The heterojunction barrier height in TiOPc/PPEI photovoltaic cells decreased by 88 mV after 12 min exposure to methylene chloride vapor and subsequently remained constant. The series resistance of the cells increased monotonically as the films crystallized. Photocurrents increased by a factor of 10 after brief exposure and decreased thereafter. The active depth from which photocurrent was generated increased to include the entire film thickness upon ordering. There appeared to be at least two different crystal morphologies in the ordered PPEI films; the morphology responsible for the longest wavelength absorption made up only a small fraction of the total film but contributed the highest photovoltaic efficiency. Charge carriers were apparently photogenerated directly in the bulk perylene in the high efficiency phase, while they were generated primarily at or near the illuminated interface in the lower efficiency phase. These results support the recent prediction by Kazmaier and Hoffmann (J. Am. Chem. Sec. 1994, 116, 9684-9691) that intermolecular charge transfer interactions in perylenes with the most strongly red-shifted absorption spectra should result in efficient photogeneration of charge carriers.