Chain Morphologies in Blue-Emitting Polyfluorenes: Impact on Light-Emitting Diodes
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0916-DD01-02
Chain Morphologies in Blue-Emitting Polyfluorenes: Impact on Light-Emitting Diodes Suchi Guha, M. Arif, and C. Volz Department of Physics, University of Missouri, Columbia, MO, 65211
ABSTRACT Dioctyl-substituted polyfluorene (PF) is especially well known for the presence of an unusual spectral feature identified as the β phase, known to originate in regions of enhanced chain planarity. Although this phase appears as a minority constituent it dominates the emission, resulting in a red shift of the luminescence. We present Raman scattering studies of poly[9,9’-(di n, octyl) fluorene] as a function of thermal cycling that establish a connection between the conformational isomers and chain morphology. Density-functional theory calculations of the vibrational spectra of single chain oligomers in conjunction with our experimental results demonstrate the incompatibility of the β phase with the overall α crystalline phase. Further, electroluminescence and photoluminescence measurements from PF-based light-emitting diodes are presented and discussed in terms of the crystalline phases and chain morphologies in the PFs. INTRODUCTION Conjugated polymers (CPs) are now a core polyfluorene R = 2-ethylhexyl constituent in numerous organic and organicC inorganic hybrid technologies [1]. Polyfluorenes R = n-octyl (PFs) have emerged as an especially attractive CP family due to their strong blue emission, high charge mobility, excellent chemical and thermal stability, and thus great prospects for device Figure 1. Schematic of polyfluorene applications [2,3]. Almost all PF derivatives utilize derivatives with two side chain solubilizing side chain substituents anchored at the functionalizations are shown in the bridging carbon atom to mould the explicit inset. molecular level properties. Use of side chains limit interchain interactions thus allowing the optical properties to be dominated by single chain excitations. These side chains give rise to a rich array of mesomorphic behavior with the appearance of a nematic liquid crystalline (n-LC) phase, which renders facile uniaxial orientation resulting in polarized light emission [4,5]. Two chemically similar but structurally different side chain moieties have been heavily studied: poly[9,9’-bis(2-ethylhexyl) fluorene] (PF2/6) and poly[9,9’-(di n, octyl) fluorene] (PF8). Substitution of the ethyl group in PF2/6 at the second carbon position of the hexyl side chain is, by construction, a chiral center (see Figure 1). PF2/6 has a limited number of conformational isomers that form a five-fold helix (5/2 or 5/1) [6]. These helices in turn self-organize into threechain unit cells resulting in well ordered semicrystalline hexagonal phase with coherence lengths exceeding 50 nm [7]. Thus, the optical properties of PF2/6 are relatively insensitive to the exact crystallographic state, thermal history, or molecular weight. PF8, on the other hand, has at least three conformational isomers that depend explicitly upon the torsional angle between the
adjacent monomer, as shown by the arro
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