The electronic device properties of semiconductor heterojunctions are dominated by the band lineup and by electronic interface states at the phase boundary. These parameters are also of crucial importance in thin film heterojunction solar cells. Unfortunately, the relative effects of principally different physical mechanisms, which lead to the formation of interface states and which govern the band alignment, are still a matter of debate as different mechanism may be operative simultaneously. In order to elucidate interface properties on a molecular level we have studied a number of different heterointerfaces by surface science techniques. In van der Waals-epitaxy heterointerfaces (2D/2D) the band lineup is close to the electron affinity rule (no interface dipole) corrected by a small quantum dipole contribution due to vitual gap states. In quasi van der Waals-epitaxy heterointerfaces (3D/2D) a strong interface dipole is found, which is related to the polarity of the polar contact surface (structural dipole). In solar cell related heterojunctions (3D/3D; e. g. CIS/CdS, CdTe/CdS, TiO₂/CdTe) the interface dipole is governed by electronic interfaces states (virtual gap states). In some solar cell devices the band linep is evidently changed by the preparation procedure empirically optimized for better performance (empirical interface engineering). These changes are not yet understood in their origin.

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