Nanofibers (NFs) of poly-3-hexylthiophene (P3HT) assembled in toluene exhibit single-chain J-aggregate character. Absorption, fluorescence emission, and Raman spectroscopy of dilute NF dispersions demonstrate that P3HT chains possess long-range intrachain order (planarity) that suppresses interchain exciton coupling. We demonstrate that a delicate interplay exists between intrachain order and interchain coupling as revealed through the emission 0–0/0–1 vibronic intensity ratios. Lowering temperature and application of pressure induces minor perturbations in the NF packing, which destroys J-aggregate character and partially restores predominant interchain interactions (i.e., H-aggregate behavior). The fact that π–π stacked P3HT chains can exhibit both H- and J-aggregate behavior opens up new possibilities for controlling electronic coupling through noncovalent stacking interactions.

Nanofibers (NFs) of the prototype conjugated polymer, poly(3-hexylthiophene) (P3HT), displaying H- and J-aggregate character are studied using temperature- and pressure-dependent photoluminescence (PL) spectroscopy. Single J-aggregate NF spectra show a decrease of the 0-0/0-1 vibronic intensity ratio from ~2.0 at 300 K to ~1.3 at 4 K. Temperature-dependent PL line shape parameters (i.e., 0-0 energies and 0-0/0-1 intensity ratios) undergo an abrupt change in the range of ~110-130 K suggesting a change in NF chain packing. Pressure-dependent PL lifetimes also show increased contributions from an instrument-limited decay component which is attributed to greater torsional disorder of the P3HT backbone upon decreasing NF volume. It is proposed that the P3HT alkyl side groups change their packing arrangement from a type I to type II configuration causing a decrease in J-aggregate character (lower intrachain order) in both temperature- and pressure-dependent PL spectra. Chain packing dependent exciton and polaron relaxation and recombination dynamics in NF aggregates are next studied using transient absorption spectroscopy (TAS). TAS data reveal faster polaron recombination dynamics in H-type P3HT NFs indicative of interchain delocalization whereas J-type NFs exhibit delayed recombination suggesting that polarons (in addition to excitons) are more delocalized along individual chains. Both time-resolved and steady-state spectra confirm that excitons and polarons in J-type NFs are predominantly intrachain in nature that can acquire interchain character with small structural (chain packing) perturbations.

Recent synthetic work has realized a novel (n-type) small-molecule acceptor, 7,8,15,16-tetra-aza-terrylene (TAT), single-crystals of which can be grown oriented along the c-axis crystallographic direction, and over-coated with pentacene to form a highly ordered donor/acceptor interface for use in organic photovoltaic devices. However, characterization of single TAT crystals reveals highly variable emission spectra and excited state dynamics - properties which strongly influence photovoltaic performance. Through the use of single-crystal widefield imaging, photoluminescence spectroscopy, time correlated single photon counting, and resonant Raman studies, we conclude that this variability is a result of long-lived low-energy trap-emission from packing defects. Interestingly, we also discovered that TAT crystals whose width exceeds ∼200 nm begin acting as waveguides and optical microcavity resonators for their own photoluminescence. Several strategies are proposed for leveraging the size-dependant optical properties of TAT pillars to further enhance device performance using this active layer design.

We report the synthesis of novel azulene-substituted methacrylate polymers by free radical polymerization, in which the azulene moieties represent hydrophobic dipoles strung pendant to the polymer backbone and impart unique electronic properties to the polymers. Tunable optoelectronic properties were realized by adjusting the azulene density, ranging from homopolymers (having one azulene group per repeat unit) to copolymers in which the azulene density was diluted with other pendant groups. Treating these polymers with organic acids revealed optical and excitonic behavior that depended critically on the azulene density along the polymer chain. Copolymers of azulene with zwitterionic methacrylates proved useful as cathode modification layers in bulk-heterojunction solar cells, where the relative azulene content affected the device metrics and the power conversion efficiency reached 7.9%.

We fabricate poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) thin film solar cells of variable weight/weight (w/w) compositions (i.e., 1:1 to 1:4) to systematically perturb polymer packing (aggregation) properties and assess their impact on local electronic structure and photocurrent generation efficiency. A combination of absorption spectroscopy and resonance Raman spectroscopic and photocurrent imaging techniques are used to quantify and spatially map morphology-dependent cofacial, π-π aggregated P3HT chains and correlate these structures to local photocurrent production characteristics. On average, increasing the PCBM weight fraction results in blue shifts and broadening for absorption and Raman spectra in the dominant P3HT CdC stretching mode region (∼1450-1470 cm -1 ), whereas symmetric stretching CsC modes show decreased intensities and red shifts. P3HT/PCBM absorption spectra are fitted near the resolved P3HT onset region using a weakly coupled H-aggregate model that reveals decreases in the relative amounts of aggregated/unaggregated P3HT chains as well as interchain exciton coupling in the aggregated component. Raman bands of P3HT CdC modes can likewise be decomposed into contributions from both aggregated (I CdC agg. ) and unaggregated (I CdC un. ) chains, and like absorption spectra, I CdC agg. /I CdC un. values decrease with increased PCBM content. Combined Raman and photocurrent imaging studies of 1:1 P3HT/PCBM devices reveal that most aggregated (ordered) P3HT chains reside primarily outside PCBM-rich regions, but this, surprisingly, reverses for >1:1 PCBM w/w loadings where all aggregated P3HT chains reside within PCBMrich regions. This effect is attributed to a change in the type of P3HT aggregation from inter-to primarily intrachain (or self-aggregated) that is supported by decreases in the interchain exciton coupling parameter from absorption fits as well as Raman CsC and CdC (agg.) frequency maps. The results reveal not only the importance of the polymer aggregation state but also its spatial location in the film that together have a large impact on charge transport properties and material performance.