Coarse-grained simulations of the solution-phase self-assembly of poly(3-hexylthiophene) nanostructures

Abstract

Under certain conditions the conjugated polymer poly(3-hexylthiophene) (P3HT) self-assembles into high-aspect-ratio nanostructures (known as nanofibres, nanowires, or nanoribbons) when cooled below its solubility limit in a marginal solvent such as anisole. Such nanostructures are potentially beneficial for organic photovoltaic device performance. In this work, Langevin dynamics simulations of a coarse-grained model of P3HT in implicit anisole solvent are used to study the self-assembly of P3HT nanostructures for polymer chain lengths and concentrations used experimentally to prepare P3HT nanofibres. The coarse-grained model is parametrised to match the local structure and dynamics of an atomistic model with explicit solvent. Nanofibres are also prepared experimentally and characterised by atomic force microscopy and UV-vis spectroscopy. The simulations match the experimental phase behaviour of P3HT in anisole, showing aggregation of P3HT at 293 and 308 K but not at 323 or 353 K. Single-chain simulations at 293 K reveal two distinct nano-scale aggregate morphologies: hairpins and helices. Hairpin aggregates, which are the precursors of nanofibres, are slightly favoured energetically at 293 K for nuclei of the critical size of ≈80 monomers for aggregation. Consequently, chains in multi-chain aggregates adopt the hairpin morphology exclusively in simulations at experimental concentrations at 293 K. The simulated aggregate sizes match experimentally measured nanofibre widths. An estimate of the shift in UV-vis absorption of P3HT due to the change in conjugation length with aggregation in the simulations agrees reasonably well with experiment and shows that most of the spectral red shift that occurs with nanofibre formation is due to increased planarisation of the P3HT chains. In addition to providing insight into the mechanisms of nanofibre formation, the simulations resolve details of the molecular-level organisation of chains in P3HT nanofibres hitherto inaccessible by experiment.

Molecular-Scale Understanding of Cohesion and Fracture in P3HT:Fullerene Blends

“…24 P3HT was modeled using modified OPLS-AA parameters. [44][45] Simulations of P3HT:fullerene mixtures were carried out with 50-monomer P3HT chain lengths (8.32 kDa) at weight-percent's of P3HT of 25% (11 P3HT oligomers and either 328 ICMA or 302 PCBM), 50% (23 P3HT oligomers and either 229 ICMA or 210 PCBM molecules), and 75% (34 P3HT oligomers and either 117 ICMA or 104 PCBM molecules). P3HT/fullerene bilayer simulations using the same 50-monomer P3HT model were carried out with 48 P3HT oligomers and 400 fullerenes.…”

Energy transfer in pendant perylene diimide copolymers

“…The large Stokes shift and the increased width of the new broad emission feature relative to the free SM suggests that this effect is due to exciton-transfer and subsequent emission from lower energy chromophore-aggregate states. 48,49 The pure acceptor polymer AP20:0:1 has an effective acceptor concentration of 0.054 M or 29.51 wt%. AP20:01 also showed a bathochromic shift in emission in the solid state, though to a lesser extent than SP20:1:0 ( Figure 6 (b)).…”

The (Non-)Local Density of States of Electronic Excitations in Organic Semiconductors

“…moieties are in this case insufficient as they disregard the anisotropic character of the π-π interactions that drive biaxial backbone ordering [67][68][69][70][71].…”