Donor-π-acceptor conjugated polymers form the material basis for high power conversion efficiencies in organic solar cells. Large dipole moment change upon photoexcitation via intramolecular charge transfer in donor-π-acceptor backbone is conjectured to facilitate efficient charge-carrier generation. However, the primary structural changes that drive ultrafast charge transfer step have remained elusive thereby limiting a rational structure-function correlation for such copolymers. Here we use structure-sensitive femtosecond stimulated Raman spectroscopy to demonstrate that π-bridge torsion forms the primary reaction coordinate for intramolecular charge transfer in donor-π-acceptor copolymers. Resonance-selective Raman snapshots of exciton relaxation reveal rich vibrational dynamics of the bridge modes associated with backbone planarization within 400 fs, leading to hot intramolecular charge transfer state formation while subsequent cooling dynamics of backbone-centric modes probe the charge transfer relaxation. Our work establishes a phenomenological gating role of bridge torsions in determining the fundamental timescale and energy of photogenerated carriers, and therefore opens up dynamics-based guidelines for fabricating energy-efficient organic photovoltaics.

A perylene (donor−dimer)−acceptor− (donor−dimer) pentamer array is synthesized to investigate the competition between excimer formation and Forster resonance energy transfer. Using time-resolved fluorescence, we show that, upon excitation, the isolated perylene dimer forms an excimer with a time constant of 4.3 ns. However, in the pentamer array, when either of two constituent dimers donate their energy to the acceptor fluorophore, the excimer energy trap is eliminated. The pentamer macromolecule shows broad absorption and reduced self-absorption, at some cost to fluorescence quantum yield.

Luminescent solar concentrators based on molecular dyes are a promising approach to light collection.

Power conversion efficiency of a solar cell is a complex parameter which usually hides the molecular details of the charge generation process. For rationally tailoring the overall device efficiency of the dye-sensitized solar cell, detailed molecular understanding of photoinduced reactions at the dye-TiO 2 interface has to be achieved. Recently, near-IR absorbing diketopyrrolopyrrole-based (DPP) low bandgap polymeric dyes with enhanced photo-stabilities has been used for TiO 2 sensitization with moderate efficiencies. To improve the reported device performances, a critical analysis of the polymer-TiO 2 interaction and electron transfer dynamics is imperative. Employing a combination of time-resolved optical measurements complemented by low temperature EPR and steady-state Raman spectroscopy on polymer-TiO 2 conjugates, we provide direct evidence for photoinduced electron injection from the TDPP-BBT polymer singlet state into TiO 2 through the C=O group of the DPP-core. A detailed excited state description of the electron transfer process in films reveals instrument response function (IRF) limited (<110 fs) charge injection from a minor polymer fraction followed by a picosecond recombination. The major fraction of photo-excited polymers however, does not show injection indicating pronounced ground state heterogeneity induced due to non-specific polymer-TiO 2 interactions.Our work therefore underscores the importance of gathering molecular-level insight into the competitive pathways of ultrafast charge generation along with probing the chemical heterogeneity at the nanoscale within the polymer-TiO 2 films for optimizing photovoltaic device efficiencies.

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