Probing Exciton Dynamics and Dispersion in C₆₀ Using TR-ARPES

In organic semiconductors, photoexcitation generates strongly bound excitons which recombine quickly (generally 10’s – 100’s of femtoseconds). To achieve efficient charge separation in these materials it is necessary to drive the dissociation of the exciton prior to recombination. This is one of the limiting factors for organic photovoltaic (OPV) devices today. The mechanisms that result in efficient charge separation in these materials are still not well understood. Therefore, to better advise device design it is necessary to gain a deeper understanding of both the dynamics and delocalization of excitonic states that occur post- photoexcitation with femtosecond resolution.<br/><br/>While the ultrafast response of organic semiconductors have been studied using all-optical techniques for a number of years [1], a lack of momentum resolution has limited understanding of their delocalization and symmetry. Time-resolved, angle-resolved photoemission spectroscopy (TR-ARPES) is a powerful technique that allows a user to visualize the energy-momentum landscape of a material and monitor how the material responds to photoexcitation with femtosecond resolution. Recent developments in TR-ARPES (or similar techniques) are enabling investigation of organic semiconductors because of its capability to directly map the momentum dependent dispersion of the excitonic states with high temporal resolution [2-4].<br/><br/>Here, I show the capability to probe the fate of excitonic states in C₆₀, a prototypical OPV material, using TR-ARPES pumped with 3.1eV light. We have been able to track the dynamics of the excitonic states which decay into each other with lifetimes ranging from hundreds of femtoseconds to tens of picoseconds. Constant energy contours show momentum-space structure of each state across the first Brillouin zone which disperses energy. With this improved angular resolution, we aim to access the extent of delocalization of the excitons in the C₆₀lattice.<br/><br/>[1] Askat E. Jailaubekov, X-Y. Zhu, et al., Nat. Mater., <b>12</b>, 66-73 (2013)<br/>[2] Sebastian Emmerich, Benamin Stadtmuller, Martin Aeschlimann et al., J. Phys. Chem., <b>124</b>, 23579-23587 (2020)<br/>[3] Kiana Baumgartner, Markus Scholz, et al., Nat. Comms., <b>13</b>, 2741 (2022)<br/>[4] Bennecke Wiebke, Mathias Stefan et al., arXiv:2303.13904v1 (2023)