Molecular Packing-Dependent Exciton and Polariton Dynamics in Anthradithiophene Organic Crystals
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.471
Molecular Packing-Dependent Exciton and Polariton Dynamics in Anthradithiophene Organic Crystals Jonathan D B Van Schenck1, Gregory Giesbers1, Akash Kannegulla1, Li-Jing Cheng1, John E. Anthony2, Oksana Ostroverkhova1 1
Oregon State University, Corvallis, OR, United States.
2
University of Kentucky, Lexington, KY, United States.
Abstract
Polarization-dependent absorption spectra of two functionalized derivatives of fluorinated anthradithiophene, diF TES-ADT and diF TDMS-ADT, were studied in the crystal phase using a Holstein-like Hamiltonian. For both molecules, the primary contribution to the lowest energy absorption was found to be the S 0-S1 excitonic transition perturbed by an intermolecular coupling of 15 meV for both TES and TDMS. A secondary contribution, consistent with that from charge-transfer states, was also found. Additionally, absorption spectra were analysed when crystals were placed inside of optical microcavities formed by two metal mirrors. Cavities exhibited a primary absorption peak determined to be an enhanced absorption from the lowest-energy S0-S1 transition.
INTRODUCTION Organic semiconductors (OSC) have long been an enticing alternative to inorganic SC because of the relative ease of processing and lower costs. Developments in the field have made OSCs closer to wider commercialization of their applications, enabled by demonstrations of charge carrier mobilities over 10 cm2/Vs and power conversion efficiencies exceeding 10% [1]. Yet there are still open questions about the nature of excitonic states in OSCs, specifically how intermolecular interactions affect the excitonic states in the solid state, depending on the molecular packing [2]. Organic polaritonics, where OSC excitonic states are coupled to photon modes in an optical cavity, has also recently received much attention due to their promise of high performance photonic devices such as low-threshold polariton lasers [3]. However, the detailed structure of organic polariton states is not well understood, especially in organic crystals, due to the complicated interaction between excitonic, vibronic and lightmatter coupling [4]. In this paper, we present an analysis of how intermolecular coupling affects the excitonic structure of two model molecular crystal systems and discuss its implications for optical properties of these materials in polaritonic devices. We focus on two functionalized derivatives of anthradithiophene (ADT): diF-TES-ADT (TES) and diF-
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TDMS-ADT (TDMS) (Fig. 1(a)-(c)). Both TES and TDMS crystals have a triclinic ). TDMS has its two longest crystal structure and one molecule per unit cell ( crystal axes approximately equal (a = 7.2 Å, b = 11.3 Å, c = 11.9 Å) leading to a “slipstack” packing scheme. TES has its two shortes
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