Can Organic Conjugated Polymers, Despite Their Broad Excitonic Linewidth, Serve as Materials for Strong Plasmon-Exciton Coupling?

Conjugated polymers are organic molecular semiconductors that are unique materials for next-generation thin-film optoelectronic devices. Because of their low dielectric constant, conjugated polymers support highly localized Frenkel excitons, which have binding energies of over a few hundred meV. These Frenkel excitons are particularly interesting for coupling with light because of their stability at room-temperature, imbuing the polymers with large absorption and emission cross-sections. This makes conjugated polymers particularly suitable for flexible photovoltaic and light-emitting devices, and have potential for exotic physics such as Bose-Einstein condensation and low-threshold lasing of exciton-polaritons.<br/><br/>When considering plasmon-exciton interactions, the strong coupling regime can be achieved when the rate of energy exchange between the plasmon and exciton exceeds the decay rates of the individual oscillators. When this occurs, the two oscillators hybridize to form new light-matter quasiparticles, called plasmon-exciton polaritons, which have two distinct energies separate from the initial oscillators. This is typically observed when the splitting energy between the two polariton modes exceeds the linewidth of the individual oscillators. Therefore, conventionally, it has been assumed that to achieve strong plasmon-exciton coupling, it is necessary to have narrow plasmon and exciton linewidths.<br/><br/>Plasmonic nanostructures have been employed as an architecture for enhancing light-trapping in conjugated polymer-based solar cells, leading to up to 2-fold enhancement in device efficiencies [1]. When incorporating plasmonic nanostructures into conjugated polymer devices, there have previously been signatures of strong light-matter interactions observed, such as strong splitting in the scattered-light spectra [2,3] and enhanced absorption at the red-edge of the polymer’s absorption band. But, is it possible for conjugated polymers, with their broad excitonic linewidths, to serve as materials for strong plasmon-exciton coupling?<br/><br/>Here, we explore two systems to investigate the nature of light-matter interactions between surface plasmons and excitons in conjugated polymers: 1) conjugated polymer-coated Ag plasmonic metasurfaces [2,3] and 2) Ag-conjugated polymer core-shell nanoparticles [4]. We employ dark-field scattered light spectroscopy, finite-difference time-domain simulations, and analytical Mie theory calculations to evaluate the spectroscopic nature of plasmon-exciton coupling. We investigate a range of different conjugated polymers, including P3HT, PCDTBT, and PTB7, as well as model conjugated polymer systems based on vibrationally-dressed Lorentzian oscillators, modified by Frank-Condon factors. We show that conjugated polymer-coated Ag plasmonic metasurfaces exhibited a splitting energy of &gt;1700 meV, far exceeding the linewidths of either the plasmonic metasurface or the conjugated polymer. We predict that for Ag-conjugated polymer core-shell nanoparticles, there exist many conditions to achieve strong coupling by varying different parameters of the Lorentzian oscillators. Finally, we demonstrate the application of these strongly coupled plasmon-exciton polaritons to improve the absorption and carrier generation in hybrid organic-2D mixed-dimensional photovoltaics [5].<br/><br/>1) C. E. Petoukhoff <i>et al.,</i> <i>J. Photon. Energy </i><b>5:057002</b> (2015).<br/>2) C. T. Nemes, D. K. Vijapurapu, <i>et al.,</i> <i>J. Nanopart. Res. </i><b>15:1801</b> (2013).<br/>3) C. E. Petoukhoff and D. M. O’Carroll, <i>Nature Commun. </i><b>6:7899</b> (2015) 1-13.<br/>4) C.E. Petoukhoff <i>et al</i>., <i>Polymers. </i><b>12:2141</b> (2020) 1-19.<br/>5) C.E. Petoukhoff, <i>et al</i>. <i>ACS Nano. </i><b>10</b> (2016) 9899-9908.