SPATIALLY-RESOLVING NANOSCOPIC STRUCTURE AND EXCITONIC-CHARGE-TRANSFER QUENCHING IN MOLECULAR SEMICONDUCTOR HETEROJUNCTIONS

Near-field scanning optical microscopy (NSOM) and scanning force micro scopy have been employed to spatially resolve the complex morphologies , spectroscopy, and charge transfer induced fluorescence quenching eff iciencies of a (perylene phenethylimide)/(titanyl phthalocyanine) bila yer (PPEI/TiOPc). The PPEI/TiOPc bilayer is a typical example of a n-l ike/p-like molecular semiconductor heterojunction, which is a common c omponent in photocells, LEDs, and other devices. NSOM-polarized fluore scence and transmission data - and separate bulk X-ray diffraction and spectroscopic measurements - on PPEI/TiOPc bilayers and PPEI and TiOP c single layers has lend to a nanoscopic and mesoscopic picture of how vacuum deposition and subsequent solvent-vapor-annealing controls the local structure of these films. The layers and bilayers are highly or ganized, containing localized crystalline regions which are preferenti ally oriented relative to the substrate and PPEI/TiOPc interface. In h ighly annealed bilayers, only a small fraction of the area of the inte rface makes good contact between the bilayers, and the contact regions are less than 100 nm(2) in most cases. The consequences of the observ ed morphology on the charge separation efficiencies at the interface i s examined. It is shown that exciton migration both perpendicular and parallel to the molecular interface are involved in the charge separat ion mechanism. Extended methylene chloride, solvent-vapor-annealing of PPEI films produces long needle-like PPEI crystals with a range of si zes, as follows: width (50-200 nm), length (1000-2000 nm), and height (50-200 nm). Annealing of the TiOPc yields nanocrystallites that are p referentially oriented relative to the interface with a height in the range of 10-100 nm and widths in the range of < 10 nm to 30 nm.