  Phthalocyanines in combination with C$_{60}$ are benchmark materials for
organic solar cells. Here we have studied the morphology and electronic
properties of co-deposited mixtures (blends) of these materials forming a bulk
heterojunction as a function of the concentration of the two constituents. For
a concentration of 1:1 of CuPc:C$_{60}$ a phase separation into about 100 nm
size domains is observed, which results in electronic properties similar to
layered systems. For low C$_{60}$ concentrations (10:1 CuPc:C$_{60}$) the
morphology, as indicated by Low-Energy Electron Microscopy (LEEM) images,
suggests a growth mode characterized by (amorphous) domains of CuPC, whereby
the domain boundaries are decorated with C$_{60}$. Despite of these markedly
different growth modes, the electronic properties of the heterojunction films
are essentially unchanged.
