Physics colloquium-“How Driving Force & Charge Transfer Distance Control Free Carrier Generation at Donor/Acceptor Interfaces”

Obadiah Reed

NREL and CU Boulder

Abstract: Charge separation in organic photovoltaics appears to follow several mechanisms depending on the molecular details of the system in question. In this talk I present evidence from photoinduced absorption-detected magnetic resonance (PADMR) that charge transfer between phthalocyanine family donor molecules and a fullerene acceptor results in a driving force dependent distribution of charge-transfer distances, and that this behavior correlates well with the free charge generation yield measured via time-resolved microwave conductivity. We show that the highest driving force sample possesses both the lowest free charge yield and the shortest average charge transfer distance, as evinced by the relative strength of the magnetic dipole and isotropic exchange coupling. Conversely, the sample that displays optimal free charge yield at an intermediate driving force exhibits the weakest exchange coupling suggesting that the average charge transfer distance is substantially larger. These results are consistent with a charge separation model wherein a distribution of charge transfer distances governs whether the product state is free mobile carriers or a bound charge-transfer state. We suggest this as a parallel pathway that can allow free charge generation without the need for engineering specific intermolecular orientations, or necessarily including large amounts of disorder to drive charge separation.

Bio: Obadiah Reid earned a PhD in Physical Chemistry from the University of Washington in 2010. He joined NREL in 2010 and also held a research position at CU Boulder since 2014. His research interests include charge generation, transport, and recombination processes in organic semiconductors; development of new analytical tools and methods; and the convergent application of experimental and theoretical approaches.

All lectures held in CTLM102 unless otherwise specified.

Pre-seminar snacks will be offered in CoorsTek 140 from 3:30pm-4:00pm.