Pump-Probe X-ray Photoemission Reveals Light-Induced Carrier Accumulation in Organic Heterojunctions

The energy level alignment in the heterojunction formed by tetracene and copper phthalocyanine grown on the Ag(111) substrate has been studied for two different sequences of layer stacking. Filled and empty molecular levels as well as charge transfer states have been characterized by combining ultraviolet and two-photon photoemission spectroscopies. It is shown that the layer in contact with the substrate determines the molecular arrangements of the whole system, thus inducing different interface dipoles at the heterojunction, depending on the stacking sequence. Such dipoles strongly influence the exciton dynamics and charge separation at the interface. The accumulation of the charge carriers in the heterojunction has been observed by measuring the transient shift of core level photoemission lines, which arises after the resonant excitation of either of the two molecular constituents.