A combination of wavelength-, time-, and polarizationresolved photoluminescence imaging on isolated P3HT nanofibers of varying molecular weight (from 10 to 65 kDa) has revealed a transition in dominant exciton coupling from primarily interchain (H-aggregation) for low molecular weight nanofibers, to predominantly intrachain (Jaggregation) coupling for high molecular weight nanofibers. Based on nanofiber width measurement from TEM imaging, the driving force for this transition appears to be folding of individual polymer chains within the lamellae, resulting in enhanced chain planarity and reduced torsional disorder.

Nanoparticles derived from π-conjugated polymers have gained widespread attention as active layer materials in various organic electronics applications. The optoelectronic, charge transfer, and charge transport properties of π-conjugated polymers are intimately connected to the polymer aggregate structure. Herein we show that the internal aggregate structure of regioregular poly(3-hexylthiophene) (P3HT) within polymer nanoparticles can be tuned by solvent composition during nanoparticle fabrication through the miniemulsion process. Using absorption spectra and single-NP photoluminescence decay properties, we show that a solvent mixture consisting of a low boiling good solvent and a high boiling marginal solvent results in polymer aggregate structure with a higher degree of uniformity and structural order. We find that the impact of solvent on the nature of P3HT aggregation within nanoparticles is different from what has been reported in thin films.

We report wavelength and time-resolved photoluminescence studies of isolated extended (1-10 μm length) poly(3-hexylthiophene) (P3HT) nanofibers (xNFs) cast on glass from suspension. The PL spectra of xNFs show multiple vibronic replicas that appear to be associated with the existence of both H- and J-type aggregates. The PL spectra of xNFs made from regioregular (rr)- (93%) and highly regioregular (hrr)-P3HT (98%) both show similarities in PL spectra suggestive of common chain packing features, as well as subtle differences that can be attributed to higher long-range order in the hrr-xNFs. Specifically, PL spectral measurements on isolated xNFs made from highly regioregular (>98%) P3HT showed a red-shifted electronic origin (≈30 meV) and increased 0-0/0-1 PL intensity ratio for the J-type species, suggestive of enhanced structural coherence length and intrachain order.

We report on a remarkable size and internal structure dependence on time- and polarization-resolved photoluminescence (PL) from individual regioregular rrP3HT (poly-3-(hexylthiophine)) nanoparticles. For the smallest particles (∼34 nm) with relatively low crystallinity (40%), the time evolution of polarization contrast is nearly stationary; for intermediate-sized particles (∼ 65 nm), depolarization occurs on a 1–2 ns time scale. The largest and most crystalline particles studied (118 nm, 70%) show a PL depolarization on a time scale of <50 ps. In every time regime, we observe P3HT nanoparticle PL dynamics that are qualitatively different from those of extended films and single-polymer chains, highlighted by intriguing differences in power law dynamics in the PL intensity at long times. This work may support the hypothesis that hierarchical assemblies of conducting polymer nanoparticles could offer a route to higher efficiency in organic photovoltaic systems.

In this Perspective, we discuss the possibility of constructing binary nanoparticle superlattices for organic photovoltaic applications and some of the interesting new photophysics emerging from preliminary studies. We summarize recent advances in nanoparticle preparation and photophysical characterization and some of the very interesting observed departures from thin-film photoluminescence dynamics. We conclude by discussing some of the challenges ahead and the possibility of new emergent physics in the assembly of polymer nanoparticles into functional devices.