Charged Polaron Polaritons in an Organic Semiconductor Microcavity

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Charged Polaron Polaritons in an Organic Semiconductor Microcavity

Chiao-Yu Cheng, Rijul Dhanker, Christopher L. Gray, Sukrit Mukhopadhyay, Eric R. Kennehan, John B. Asbury, Anatoliy Sokolov, and Noel C. Giebink

Phys. Rev. Lett. 120, 017402 – Published 3 January 2018

Abstract

We report strong coupling between light and polaron optical excitations in a doped organic semiconductor microcavity at room temperature. Codepositing ${MoO}_{3}$ and the hole transport material 4, $4^{'}$ -cyclohexylidenebis[ $N$ , $N$ -bis(4-methylphenyl)benzenamine] introduces a large hole density with a narrow linewidth optical transition centered at 1.8 eV and an absorption coefficient exceeding $10^{4} {cm}^{- 1}$ . Coupling this transition to a Fabry-Pérot cavity mode yields upper and lower polaron polariton branches that are clearly resolved in angle-dependent reflectivity with a vacuum Rabi splitting $ℏ Ω_{R} > 0.3 eV$ . This result establishes a path to electrically control polaritons in organic semiconductors and may lead to increased polariton-polariton Coulombic interactions that lower the threshold for nonlinear phenomena such as polariton condensation and lasing.

Received 15 August 2017
Revised 27 September 2017

DOI:https://doi.org/10.1103/PhysRevLett.120.017402

Physics Subject Headings (PhySH)

Charge Optical & microwave phenomena Optoelectronics Polaritons Polarons

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Chiao-Yu Cheng¹, Rijul Dhanker¹, Christopher L. Gray², Sukrit Mukhopadhyay³, Eric R. Kennehan², John B. Asbury², Anatoliy Sokolov³, and Noel C. Giebink^1,*

¹Department of Electrical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
²Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
³The Dow Chemical Company, 1776 Building, Midland, Michigan 48674, USA

^*ncg2@psu.edu

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Vol. 120, Iss. 1 — 5 January 2018

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Images

Figure 1
Schematic showing the difference between a traditional organic semiconductor exciton polariton (a) and a polaron polariton (b). The former involves a Frenkel exciton transition between the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively), whereas the latter involves a radical cation (or anion; not shown) transition, in this example, between the HOMO and a lower-lying molecular orbital.
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Figure 2
(a) Molecular structure of TAPC together with the optical transitions calculated for its cation using the B3LYP functional. (b) Molar absorption coefficient of neutral TAPC (black line) and its cation ( ${TAPC}^{+}$ , red line). The solution consists of $50 μ M$ TAPC dissolved in dichloromethane with a $0.5 M$ tetrabutylammonium tetrafluoroborate ( ${TBABF}_{4}$ ) electrolyte. The cation was created by electrochemically oxidizing TAPC for 40 min at 0.7 V with respect to a $Ag / {Ag}^{+}$ reference electrode in acetonitrile containing $0.01 M {AgNO}_{3}$ and $0.1 M {TBABF}_{4}$ ; the inset shows the associated cyclic voltammetry scan. (c) Complex refractive index optical constants $\tilde{n} = n + i k$ , determined via variable angle spectroscopic ellipsometry for 40-nm-thick films of TAPC coevaporated with increasing concentrations of ${MoO}_{3}$ on a glass substrate. In (b) and (c), the ${TAPC}^{+}$ transitions in the 1.5–2.2 eV spectral range (purple arrows) are associated with excitation from low-lying orbitals into the partially occupied HOMO, whereas that at approximately 3.4 eV (green arrow) involves excitation from the HOMO into the LUMO as illustrated by the corresponding color arrows in (a).
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Figure 3
Angle-dependent reﬂectivity spectra of microcavities containing (a) 10 wt % ${MoO}_{3}$ :TAPC and (b) 30 wt % ${MoO}_{3}$ :TAPC. The spectra (blue lines) are acquired at incidence angles ranging from 20° to 75° in 5° increments and are offset vertically from one another for clarity. The dashed black lines indicate the simulated reflectivity at each incidence angle based on the associated ${MoO}_{3}$ :TAPC complex refractive index dispersions displayed in Fig. 2. The solid red lines are drawn as a guide to show the anticrossing behavior. The vertical dashed green line at 1.77 eV designates the bare polaron transition energy, and the purple reflectivity spectra at the bottom of each plot are the angle-dependent reflectivities simulated for the bare cavity.
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Figure 4
(a) Polariton dispersion for the 30 wt % ${MoO}_{3}$ :TAPC cavity extracted from the reﬂectivity data of Fig. 3. The green solid line indicates the uncoupled polaron transition energy, the black dashed line denotes the bare cavity mode dispersion, and the red solid lines are a ﬁt from the coupled oscillator model described in the text. (b) Linear fit (red line) to the dependence of the Rabi splitting on the square root of the polaron absorbance determined for the 10, 20, and 30 wt % ${MoO}_{3}$ :TAPC microcavities.
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