One of the fundamental design paradigms in organic photovoltaic (OPV) device engineering holds that charge separation requires diffusion of excitons to an interface. This basic concept has driven an enormous materials science engineering effort focused on construction of n- and p-type domains sizes commensurate with a nominal exciton diffusion length (on the time scale of the radiative lifetime) of order 10 nm. Here, we show that polarized optical excitation of isolated pristine crystalline nanowires of a small molecule n-type organic semiconductor, 7,8,15,16-tetraazaterrylene (TAT), generates a significant population of charge-separated polaron pairs along the crystallographic <b>a</b>-(π-stack) axis (π-stack direction). Charge separation was signaled by pronounced power-law photoluminescence decay polarized along the same axis. In the transverse direction, we observed exponential decay associated with relaxed Frenkel excitons localized on TAT monomers. We propose that this effect derives from directional, structure specific charge-transfer interactions between adjacent TAT molecules in the crystalline assembly, and demonstrates the potential for charge-separation without the requirement for transport of excitons to an interface. The strongly directional response to excitation polarization gives new insight into the nature of polaron-pair generation, separation and anisotropic transport within the boundaries of single crystals, and offers the prospect for new OPV design paradigms based on single-crystal donor domains.

Directional charge separation in isolated 7,8,15,16-tetraazaterrylene (TAT) crystalline nanowires

251st National Meeting (2016)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.