The most intriguing property of organic semiconductors for optoelectronic applications is their chemical tunability which yields high potential to tailor and control their properties according to their functional purpose in the device. However, the device relevant properties of organic films such as the energy level alignment of the molecular transport levels or the optical generation of (free) charge carriers are not only determined by the intrinsic properties of the individual molecules. They are also severely influenced by dielectric screening mechanisms as well as by the polarizability of the organic material and hence by the collective properties of the organic solid.
In this work, we present new insight into the ultrafast collective response of organic semiconductor films after optical excitation. The combination of time- and angle resolved photoemission with a fs-XUV light source allows us to simultaneously follow the transient evolution of the occupied as well as of the unoccupied band structure after fs-optical excitation with visible light. As model system, we investigate thin films of the prototypical organic molecule C60 on the Ag(111) surface. The unoccupied part of the C60 band structure reveals the well-known quasi-particle dynamics of the exciton formation and its decay in C60. Most interestingly, we observe transient changes of the linewidth of all occupied molecular orbitals upon optical excitation which can be directly linked to the characteristic timescales of the exciton decay of C60. These observations are attributed to a collective transient polarization of the molecular film caused by the exciton formation at distinct C60 sites.
Benjamin Stadtmueller, "Optical excitations in organic materials: Ultrafast exciton dynamics and their collective response in organic thin films (Conference Presentation)," Proc. SPIE 10530, Ultrafast Phenomena and Nanophotonics XXII, 1053010 (Presented at SPIE OPTO: January 31, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2286246.5751513304001.
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