The success of organic photovoltaic (OPV) devices is directly related to the ability of excitations to propagate through the semiconductor and for excitons to dissociate into free charge carriers before they recombine. Some interfaces have been shown to aid in efficient exciton dissociation while others, like grain boundaries, can lead to fast recombination. Understanding the processes that occur at interfaces in these types of materials is essential for efficient device design. In this thesis, we investigate the role of two types of interfaces on excitons in the model OPV system C₆₀/Au(111). First, we develop a two-stage epitaxial growth method for well-ordered thin films of C₆₀ on Au(111). Compared to the traditional single-stage growth methods, our recipe consistently generates thin films with an extremely high degree of long-range orientational order. Next, the electronic structure at the C₆₀-Au interface are investigated using angle-resolved photoemission spectroscopy (ARPES). The Fermi surface reveals a partially filled hybridized Au-C₆₀ LUMO state dispersing across the Fermi level. The dynamics and dispersion of the lowest lying charge transfer excitonic state in thin films of C₆₀/Au(111) are investigated by time-resolved ARPES. We find a momentum-dependent reduction in the lifetime of the excitonic state that appears to be correlated with the position of the interfacial states. The dispersion shows a combination of electronic interfacial states and excitonic signatures. Together, the data implies exciton dissociation is a momentum-dependent process at the C₆₀-Au interface. Finally, we investigate the effect of a small amount of additional rotational domains present in the film - even when barely measurable via low-energy electron diffraction and ARPES - on the lifetime of the excitonic state. Using scanning tunnelling microscopy and spectroscopy, we find the grain boundaries are expected to act as exciton funnels, increasing the local exciton density at the grain boundaries and therefore the films' susceptibility to exciton-exciton annihilation. These results highlight the importance of an extremely high degree of order required to measure intrinsic exciton lifetimes, reinforcing the importance of our two-stage growth recipe. Moreover, the results suggest that disorder induced exciton quenching may play a role in limiting the efficiency of organic optoelectronics.
Rysa Greenwood (Thu,) studied this question.