Two-dimensional spatial coherence of excitons in semicrystalline polymeric semiconductors: Effect of molecular weight

Abstract

The electronic properties of macromolecular semiconductor thin films depend profoundly on their solid-state microstructure, which in turn is governed, among other things, by the processing conditions selected and the polymer's chemical nature and molecular weight. Specifically, lowmolecular-weight materials form crystalline domains of cofacially π-stacked molecules, while the usually entangled nature of higher molecular-weight polymers leads to microstructures comprised of molecularly ordered crystallites interconnected by amorphous regions. Here, we examine the interplay between extended exciton states delocalized along the polymer backbones and across polymer chains within the π-stack, depending on the structural development with molecular weight.Such two-dimensional excitations can be considered as Frenkel excitons in the limit of weak intersite coupling. We combine optical spectroscopies, thermal probes, and theoretical modeling, focusing on neat poly(3-hexylthiophene) (P3HT) -one of the most extensively studied polymer semiconductors -of weight-average molecular weight (M w ) of 3-450 kg/mol. In thin-film structures of high-molecular-weight materials (M w > 50 kg/mol), a balance of intramolecular and intermolecular excitonic coupling results in high exciton coherence lengths along chains (~4 thiophene units), with interchain coherence limited to ~2.5 chains. In contrast, for structures of low-M w P3HT (<40 kg/mol), the interchain exciton coherence is dominant (~20% higher than in architectures formed by high-molecular-weight materials). In addition, the spatial coherence within the chain is significantly reduced (by nearly 30%). These observations give valuable structural information; they suggest that the macromolecules in aggregated regions of high-molecular-weight P3HT adopt a more planar conformation compared to low-molecular-weight materials. This results in the observed increase in intrachain exciton coherence. In contrast, shorter chains seem to lead to torsionally more disordered architectures. A rigorous, fundamental description of primary photoexcitations in π-conjugated polymers is hence developed: two-dimensional excitons are defined by the chain-length dependent molecular arrangement and interconnectivity of the conjugated macromolecules, leading to interplay between intramolecular and intermolecular spatial coherence.

Excitonic coupling dominates the homogeneous photoluminescence excitation linewidth in semicrystalline polymeric semiconductors

“…This PL excitation linewidth is roughly an order of magnitude broader than the lower limit in the single-chain PL spectra reported by Thiessen et al [6], who investigated P3HT with M w that is near the transition in polymer microstructure from a nonentangled, paraffinic to an entangled, semicrystalline, two-phase morphology in bulk films [5,12]. In that work, while the emission energy of individual P3HT chains varied substantially over much of the visible range, the PL spectral line shape was found to be nearly invariant over measurement of hundreds of different chains.…”

“…Excitonic coupling effects, both along and between chains, are incorporated by an intrachain hopping integral t, and an interchain hopping integral J . Typically, one finds that J /t ∼ 0.1 within a hybrid HJ aggregate model [4,5]. For an isolated chain, the homogeneous linewidth scales as [11] …”

“…Regioregular poly(3-hexylthiophene) (P3HT) is amongst several materials that have provided a platform to develop an understanding of such interplay, since its absorption and photoluminescence (PL) spectral line shape can be analyzed using well-established concepts pertaining to molecular aggregates. This allows one to extract intricate information on the magnitude of excitonic coupling, the extent of energetic disorder, and the amount of correlation in the disordered energy landscape [4,5]. However, extensive PL measurements on isolated P3HT chains at low temperature reveal that the energy of the PL spectrum can vary over a large spectral range throughout the visible spectrum [6].…”

“…Indeed, mixed quantum-classical atomistic simulations demonstrate that energetic disorder of single P3HT chains arises through the interplay of excited-state delocalization and electron-hole polarization, governed by intrachain torsional disorder, and giving rise to a heterogeneous distribution of chromophore energies [7]. Intriguingly, while the PL origin emission spanned a broad spectral range at 4 K, its linewidth only varied from ∼10-30 meV and the spectral line shape was essentially invariant [6], in contrast to the broad * carlos.silva@umontreal.ca line shapes with very strong microstructure dependence in bulk films [5]. The single-chain PL data at low temperature reported in Ref.…”

Charge transfer from delocalized excited states in a bulk heterojunction material

“…Although ultrafast time resolved photoluminescence lineshape measurements can address the size of a single excited state Eigenfunction, 52,72 the presence of coherences between excited states would result in wavefunctions with a significantly greater extent. This is a critical distinction as several observations of excited state coherences have been made.…”