Organic heterojunctions of layered perylene and phthalocyanine dyes: characterization with UV-photoelectron spectroscopy and luminescence quenching

We present here the characterization of organic/organic' heterojunctions created from either of two perylene dyes, perylenetetracarboxylicdianhydride (PTCDA) or the bisimide derivative perylenetetracarboxylicdianhydride-N,N'-bis (butyl)imide (C4-PTCDI), and two chloro-metallated donor phthalocyanines (ClAlPc or ClInPc). The perylene dyes were selected to create thin films with the core of the perylene dye parallel to the substrate plane (PTCDA) or nearly vertical to the substrate plane, with layer planes defined by the butyl substituents (C4-PTCDI). We compare the frontier orbital offsets revealed by UV-photoelectron spectroscopy, and quenching of luminescence of the perylene dyes, as a function of Pc coverage. The ionization potentials (IPs) of the Pc layers, the degree to which interface dipoles are formed at the Pc/perylene dye interface, and the degree of quenching of the perylene luminescence are affected by the structure of the Pc/perylene interface. Pc/PTCDA heterojunctions show significant interface dipoles and higher IPs for the first-deposited Pc layers compared to Pc/C4-PTCDI heterojunctions, which show negligible interface dipoles and lower overall IP values for initial Pc layers. Luminescence of the selectively excited perylene layers is quenched by the addition of even submonolayer coverages of Pc. This quenching process occurs as a result of both energy transfer (perylene to Pc) and charge transfer (Pc to perylene). Luminescence from monomeric and aggregated ClAlPc and ClInPc monolayers is seen on C4-PTCDI films, whereas only luminescence from the aggregated forms of these Pcs is seen on PTCDA films. These studies reveal aspects of organic heterojunction energetics which may have important implications for organic solar cell design.