Structure and Electronic Gap State Density at Organic Semiconductor Interfaces

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Although several theories have been proposed to determine the electronic level alignment at the interface of two organic semiconductors (OSC), for example the polaron state model or the induced density of states model, so far no satisfying and complete theory for predicting the level offsets has been developed. We will conduct a comprehensive study of the electronic level alignment of organic-organic (small molecule) heterostructures using complementary techniques to understand the relationship between structural defects and electronic structure. The innovative combination of high-resolution X-ray scattering and high sensitivity UPS allows the detailed description of structural defects and electronic states located in the gap region, which are believed to play an essential role in the level alignment. In particular, we will prepare a series of organic-organic heterostructures with suitable material combinations with different electronic (ionization energy, energy gap) and structural (molecular conformation, crystal structure) properties. By variation of the growth parameters (substrate, growth rate, temperature) we will tune the structural properties of the heterostructure, which will in return modify the electronic properties. As the final result we aim for the direct relationship between coherently scattering material volume, that is the essentially defect free volume, versus the HOMO-level offset between the two OSCs.