Charge carrier and exciton trapping in organic semiconductors crucially determine the performance of organic (opto-)electronic devices such as organic field-effect transistors, light-emitting diodes, or solar cells. However, the microscopic origin of the relevant traps generally remains unclear, as most spectroscopic techniques are unable to simultaneously probe the electronic and morphological structure of individual traps. Here, we employ low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) as well as tight-binding calculations derived from ab initio calculations to image the localized electronic states arising at structural defects in thin C60 films (<10 ML). The spatially and spectrally resolved STM-induced luminescence at these states reveals an enhanced radiative decay of excitons, which is interpreted in terms of the local symmetry lowering and the trapping of excitons by an X-trap. The combined mapping of the STM-induced luminescence, electronic structure, and morphology thus provides new insights into the origin and characteristics of individual exciton traps in organic semiconductors and offers new avenues to study charge carrier and exciton dynamics on molecular scales.
Keywords: C60; STM; STM-induced luminescence; X-traps.