Optical excitations in poly(2,5-thienylene vinylene)

Hybrid-state emission in a polythienylenevinylene derivative with an electron deficient moiety

The photoluminescence (PL) of a novel imide-substituted poly(3-thienylenevinylene) derivative (imidePTV) was studied in film and solution. PL quantum efficiency was measured to be more than two orders of magnitude larger than its nonluminescent counterpart, namely, alkyl-substituted PTV and was interpreted as evidence for a near degeneracy of optically allowed 1(1)Bu and optically forbidden 2(1)Ag excitonic states. As a result, coexistence of 2(1)Ag and 1(1)Bu emissions was observed, and the predominance was found to be sensitive to temperature and morphological environment. PL of solutions in solvents of higher polarity and polarizability and from low-temperature films was dominated by the transition from the dipole allowed 1(1)Bu state. On the other hand, the PL spectra of films at high temperature and solutions in solvents of low polarity and polarizability were primarily from the 2(1)Ag state that obtains a finite transmission moment from an asymmetric perturbation mixing with the 1(1)Bu.

Effect of the side-chain-distribution density on the single-conjugated-polymer-chain conformation

The spatial arrangement of the side chains of conjugated polymer backbones has critical effects on the morphology and electronic and photophysical properties of the corresponding bulk films. The effect of the side-chain-distribution density on the conformation at the isolated single-polymer-chain level was investigated with regiorandom (rra-) poly(3-hexylthiophene) (P3HT) and poly(3-hexyl-2,5-thienylene vinylene) (P3HTV). Although pure P3HTV films are known to have low fluorescence quantum efficiencies, we observed a considerable increase in fluorescence intensity by dispersing P3HTV in poly(methyl methacrylate) (PMMA), which enabled a single-molecule spectroscopy investigation. With single-molecule fluorescence excitation polarization spectroscopy, we found that rra-P3HTV single molecules form highly ordered conformations. In contrast, rra-P3HT single molecules, display a wide variety of different conformations from isotropic to highly ordered, were observed. The experimental results are supported by extensive molecular dynamics simulations, which reveal that the reduced side-chain-distribution density, that is, the spaced-out side-chain substitution pattern, in rra-P3HTV favors more ordered conformations compared to rra-P3HT. Our results demonstrate that the distribution of side chains strongly affects the polymer-chain conformation, even at the single-molecule level, an aspect that has important implications when interpreting the macroscopic interchain packing structure exhibited by bulk polymer films.

Conjugated polymers for high-efficiency organic photovoltaics

In this review we summarize the most recent developments in conjugated polymers for high-efficiency organic photovoltaic devices. We focus on correlations of polymer chemical structures with properties, which may guide rational structural design and evaluation of photovoltaic materials.

Incorporation of fused tetrathiafulvalenes (TTFs) into polythiophene architectures: varying the electroactive dominance of the TTF species in hybrid systems

A novel polythienylenevinylene (PTV) and two new polythiophenes (PTs), featuring fused tetrathiafulvalene (TTF) units, have been prepared and characterized by ultraviolet-visible (UV-vis) and electron paramagnetic resonance (EPR) spectroelectrochemistry. All polymers undergo two sequential, reversible oxidation processes in solution. Structures in which the TTF species is directly linked to the polymer backbone (2 and 4) display redox behavior which is dictated by the fulvalene system. Once the TTF is spatially removed from the polymer chain by a nonconjugated link (polymer 3), the electroactivity of both TTF and polythiophene moieties can be detected. Computational studies confirm the delocalization of charge over both electroactive centers (TTF and PT) and the existence of a triplet dication intermediate. PTV 4 has a low band gap (1.44 eV), is soluble in common organic solvents, and is stable under ambient conditions. Organic solar cells of polymer 4:[6,6]-phenyl-C(61) butyric acid methyl ester (PCBM) have been fabricated. Under illumination, a photovoltaic effect is observed with a power conversion efficiency of 0.13% under AM1.5 solar simulated light. The onset of photocurrent at 850 nm is consistent with the onset of the pi-pi absorption band of the polymer. Remarkably, UV-vis spectroelectrochemistry of polymer 4 reveals that the conjugated polymer chain remains unchanged during the oxidation of the polymer.