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.

Resonance Raman spectroscopic imaging is introduced as a physical probe to identify and spatially map morphology-dependent variations of intra- and interchain interactions and order in poly-3-hexylthiophene (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) photovoltaic blend thin films. Absorption spectra and C=C symmetric stretching Raman modes of P3HT/PCBM blend films show contributions from two distinct species that are assigned as aggregated and unaggregated P3HT chains with characteristic Raman frequencies of approximately 1450 (I(C=C)(agg)) and approximately 1470 cm(-1) (I(C=C)(un)), respectively. Upon thermal annealing of blend films, the relative concentrations of I(C=C)(agg) and I(C=C)(un) species (R = I(C=C)(agg)/I(C=C)(un)) changes on average from 0.79 +/- 0.20 (as-cast) to 2.45 +/- 0.77 (annealed). It is proposed that R values report on the relative densities of states (DOS) of aggregated and unaggregated species, and resonance Raman imaging is then used to spatially map morphology-dependent variations of R values and uncover subclassifications of these species. From both R and frequency dispersion resonance Raman images of I(C=C)(agg) and I(C=C)(un) species, four distinct types of P3HT chains are identified and mapped in annealed P3HT/PCBM blend thin films: (i) highly aggregated/crystalline, (ii) partially aggregated, (iii) interfacial, and (iv) unaggregated/PCBM rich. The change in aggregation upon annealing is attributed to an increase in planarity of the P3HT chains that is determined from the ratios of C=C/C-C symmetric stretching mode intensities.

The doping efficiencies of regioregular (r-Re) and regiorandom (r-Ra) poly(3-hexylthiophene) (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) were studied in solution using electron paramagnetic resonance (EPR), (19)F NMR, optical absorption, and Raman spectroscopy. EPR spectra of doped r-Re P3HT possess significantly larger amounts of paramagnetic species than r-Ra P3HT with similar F4-TCNQ loadings (∼0.1% up to 50%), which is confirmed by corresponding optical absorption spectra. (19)F NMR also show a rapid disappearance of the pristine F4-TCNQ signal when small amounts of r-Re P3HT are added due to minority paramagnetic species acting as efficient spin relaxation channels. Raman spectra of both P3HT variants indicate strong interactions with F4-TCNQ, however, the presence of free charges is only detected in r-Re samples owing to its ability to aggregate and adopt ordered conformations allowing for delocalization of hole charges after initial contact with the dopant.

TitleThe effect of 2, 3,5,6-tetrafluoro-7,7,8,8- The effect of the strong electron acceptor, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 -TCNQ), on poly(3-hexylthiophene) (P3HT) aggregates is studied. aggregates is attributed to efficient charge separation stemming owing to the more polarizable nature of chains comprising the aggregate p-stack.

This paper is focused on how the spectroscopic properties of conjugated polymers evolve in the size range between single polymer chains and the bulk material. The measurements used single-particle spectroscopy techniques and include both static and dynamic measurements. The main observation of this work is that the spectroscopic properties of MEH-PPV evolve rapidly as a function of nanoparticle size and achieve bulk-like properties for nanoparticles greater than 10 nm in size. Nanoparticles were assembled by a reprecipitation technique and characterized by fluorescence emission spectroscopy. The physical origin of the size-dependent spectroscopic properties is assigned to the distance dependence of four main processes: electronic energy transfer between blue and red sites, triplet-triplet annihilation, singlet exciton quenching by triplets, and singlet exciton quenching by hole polarons.

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.

Despite intense, long-term interest in organic semiconductors from both an applied and fundamental perspective, key aspects of the electronic properties of these materials remain poorly defined. A particularly challenging problem is the molecular nature of positive charge carriers, that is, holes or oxidized species in organics. Here, the unique ability of single-molecule spectroelectrochemistry (SMS-EC) to unravel complex electrochemical process in heterogeneous media is used to study the oxidation of nanoparticles of the conjugated polymer poly(9,9-dioctylfluorene-co-benzothiadiazole). A reversible hole-injection charging process has been observed that occurs primarily by initial injection of shallow (untrapped) holes, but soon after the injection, a small fraction of the holes becomes deeply trapped. Good agreement between experimental data and simulations strongly supports the presence of deep traps in the studied nanoparticles and highlights the ability of SMS-EC to study energetics and dynamics of deep traps in organic materials at the nanoscale.

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