Nicolas Bérubé

Direct observation of ultrafast long-range charge separation at polymer–fullerene heterojunctions

In polymeric semiconductors, charge carriers are polarons, which means that the excess charge deforms the molecular structure of the polymer chain that hosts it. This results in distinctive signatures in the vibrational modes of the polymer. Here, we probe polaron photogeneration dynamics at polymer:fullerene heterojunctions by monitoring its time-resolved resonance-Raman spectrum following ultrafast photoexcitation. We conclude that polarons emerge within 300 fs. Surprisingly, further structural evolution on ≲ 50-ps timescales is modest, indicating that the polymer conformation hosting nascent polarons is not significantly different from that near equilibrium. We interpret this as suggestive that charges are free from their mutual Coulomb potential because we would expect rich vibrational dynamics associated with charge-pair relaxation. We address current debates on the photocarrier generation mechanism at molecular heterojunctions, and our work is, to our knowledge, the first direct probe of molecular conformation dynamics during this fundamentally important process in these materials.

Two-dimensional spatial coherence of excitons in semicrystalline polymeric semiconductors: Effect of molecular weight

The electronic properties of macromolecular semiconductor thin films depend profoundly on their solid-state microstructure, which in turn is governed, among other things, by the processing conditions selected and the polymer chemical nature and molecular weight. Specifically, low-molecular-weight materials form crystalline domains of cofacially

Resonance Raman spectroscopy and imaging of push–pull conjugated polymer–fullerene blends

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Large electronic bandwidth in solution-processable pyrene crystals: The role of close-packed crystal structure

-stacked molecules, while the usually entangled nature of higher molecular-weight polymers leads to microstructures comprised of molecularly ordered crystallites interconnected by amorphous regions. Here, we examine the interplay between extended exciton states delocalized along the polymer backbones and across polymer chains within the

Designing Polymers for Photovoltaic Applications Using ab Initio Calculations

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Low Band Gap Polymers Design Approach Based on a Mix of Aromatic and Quinoid Structures

-stack, depending on the structural development with molecular weight. We combine optical spectroscopies, thermal probes, and theoretical modeling, focusing on neat poly(3-hexylthiophene) (P3HT), one of the most extensively studied polymer semiconductors, of weight-average molecular weight of 3-450\,kg/mol. The spatial coherence within the chain is significantly reduced (by nearly 30\%). These observations give valuable structural information; they suggest that the macromolecules in aggregated regions of high-molecular-weight P3HT adopt a more planar conformation compared to low-molecular-weight materials. This results in the observed increase in intrachain exciton coherence. In contrast, shorter chains seem to lead to torsionally more disordered architectures. A rigorous, fundamental description of primary photoexcitations in

Thiocarbonyl Substitution in 1,4-Dithioketopyrrolopyrrole and Thienopyrroledithione Derivatives: An Experimental and Theoretical Study

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New way of polymer design for organic solar cells using the quinoid structure

-conjugated polymers is hence developed: two-dimensional excitons are defined by the chain-length dependent molecular arrangement and interconnectivity of the conjugated macromolecules, leading to interplay between intramolecular and intermolecular spatial coherence.

Organic solar cells: How can the theory guide the experience?

Blends of alternating ‘push–pull’ donor/acceptor (d/a) co-polymers with soluble fullerenes as active materials have shown promise for increasing power conversion efficiencies in organic photovoltaic (OPV) devices. We investigate morphology-dependent optical and electronic properties of poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) blended with [6,6]-phenyl C61 butyric acid methyl ester (PCBM) using electronic absorption and resonance Raman spectroscopies. Selective resonance excitation spanning the entire PCPDTBT absorption envelope (∼400–900 nm) was used to probe via Raman spectroscopy the degree of composition- and conformation-dependent charge transfer character along the polymer backbone. Raman intensities of characteristic PCPDTBT backbone donor/acceptor moieties vary with excitation wavelength. We perform density functional theory (DFT) calculations to assign Raman-active vibrational modes and correlate polymer backbone conformations to the degree of intra-chain donor/acceptor charge transfer character. We find the best agreement between experimental and simulated spectra for planarized PCPDTBT backbone consistent with strong charge transfer character along the backbone, which also gives rise to a new red-shifted absorption band appearing in PCBM blends. Resonance Raman and photocurrent imaging experiments were next used to spatially map morphology-dependent vibrational signatures of PCPDTBT donor/acceptor moieties within functioning solar cell devices. Solvent additives were applied using 1,8 octanedithiol (ODT) to modify PCPDTBT:PCBM morphologies and compared to as-cast blends. Raman and photocurrent images indicate a well-mixed morphology that we propose induces planarization of the PCPDTBT backbone.

G0W0 implementation using Lanczos algorithm and Sternheimer equation

We examine the interdependence of structural and electronic properties of two substituted pyrene crystals by means of combined spectroscopic probes and density-functional theory calculations. Substituted pyrenes are useful model systems to unravel the interplay of crystal structure and electronic properties in organic semiconductors. To study the effect of steric encumbrance on the crystalline arrangement of two 1,3,6,8-tetraalkynylpyrene derivatives, one features linear n-hexyl side groups while the other contains branched trimethylsilyl groups. Both derivatives form triclinic crystal structures when grown from solution, but the electronic dispersion behavior is significantly different due to differences in π-π overlap along the π-stacking axis. Both systems display dispersion of around 0.45 eV in the valence band, suggesting a high intrinsic hole mobility. However, the direction of the dispersion is different: it is primarily along the π-stacking axis in the trimethylsilyl-substituted derivative, but less aligned with this crystal axis in the hexyl-substituted molecule. This is a direct consequence of the differences in co-facial π electron overlap revealed by the crystallographic studies. We find that photophysical defects, ascribed to excimer-like states, point to the importance of localized trap states.