Exciton Dynamics in Donor-Acceptor-donor Molecules: Impact of Molecular Structure and Self-Assembly on the Exciton Lifetime

Organic semiconductor materials such as planar conjugated small molecules are of great interest to the photovoltaic community [1, 2]. In thin films, the exciton and charge carrier dynamics, which are crucial to photovoltaic device operation, depend in a non-trivial way on the organic molecular structure and on the molecular organization in the solid state. [3, 4] Several recent studies have established that the exciton diffusion strongly depends on the crystalline order of the organic films. [5, 6] Our work presents the study of the exciton dynamics in an innovative class of molecular semiconductors able to present high crystalline order. This family of molecules has a “dumbbell-shaped” structure based on triazatruxene units that act as a π-stacking platform. [7] Such molecules with different side-chain have been found to self-assemble into various crystalline and liquid crystalline phases. We have studied exciton dynamics of molecular with different side-chain: in solution and in thin films for different solid state phases. In solution, the lifetime corresponds to the reference value that can be obtained without intermolecular interaction. In thin films, we measured the exciton lifetime for different molecular structures in order to correlate the exciton diffusion length with the intermolecular stacks. The results reveal a significant increase in exciton lifetime with structural order. [1] Y. Lin et al., Chem. Soc. Rev. 2012, 41, 4245. [2] S. D. Collins et al., Adv. Energy Mater. 2017, 7, 1602242. [3] J. A. Bartelt et al., Adv. Energy Mater. 2013 , 3 , 364. [4] J. T. Bloking et al., Adv. Energy Mater. 2014, 4, 201301426. [5] R. R. Lunt et al., Adv. Mater., 2010, 22, 11, 1233-1236. [6] A. T. Haedler et al., Nature, 2015, 523,196–199. [7] I. Bulut et al., J. Mater. Chem. A, 2015, 3, 6620.