EP02.03.02 : Charged Polaron Polaritons and Bipolarons in Organic Devices

4:00 PM–4:30 PM Apr 3, 2018

PCC North, 200 Level, Room 222 BC

Noel Giebink1

1, The Pennsylvania State University, University Park, Pennsylvania, United States

Exciton-photon polaritons that emerge in the strong light-matter coupling regime have been studied extensively in optical microcavities using a variety of organic and inorganic semiconductors. Being comprised of charge neutral excitons and photons, the resulting polaritons also carry no charge and therefore their motion cannot be manipulated directly with applied electric fields. The first half of this talk will focus on strong coupling between light and charge-carrying polaron optical excitations in an organic semiconductor at room temperature. We show that a radical cation transition of hole-doped TAPC can be strongly coupled to the optical field in a planar microcavity to yield polaron polariton states with a vacuum Rabi splitting >0.3 eV. The resulting polaron polaritons are unique to organic semiconductors and may lead to increased Coulombic polariton-polariton interaction that reduces the threshold for phenomena such as parametric amplification and Bose-Einstein condensation as well as providing a pathway to exploit charged polaritons in practical optoelectronic devices.

The second half of the talk will focus on bipolaron states, in which two electrons or two holes occupy a single molecule or conjugated polymer segment. These states have a long history that dates back to the early work in conducting polymers; however, they are considered unlikely to form in organic semiconductor devices such as LEDs, photovoltaics, and transistors because of the strong on-site Coulomb repulsion energy penalty, known as the Hubbard energy. Here, we use charge modulation spectroscopy to directly reveal a bipolaron sheet density >1010 cm-2 at the interface between an indium tin oxide anode and the common small molecule organic semiconductor TPD. We find that the magnetocurrent response of hole-only devices correlates closely with changes in the bipolaron concentration, supporting the bipolaron model of unipolar organic magnetoresistance and suggesting that it may be more of an interface than a bulk phenomenon. These results are understood on the basis of a quantitative interface energy level alignment model, which indicates that bipolarons are generally expected to be significant near contacts in the Fermi level pinning regime and thus may be more prevalent in organic semiconductor devices than previously thought.