John K. Grey

Resonance Raman overtones reveal vibrational displacements and dynamics of crystalline and amorphous poly(3-hexylthiophene) chains in fullerene blends

Resonance Raman spectra of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester blend thin films display progressions of overtone and combination bands (up to two harmonics) involving the dominant symmetric C=C backbone stretching mode of P3HT that encode excited state vibrational displacements and dynamics information. Contributions from both crystalline (aggregated) and amorphous (unaggregated) P3HT domains are resolved and intensities are analyzed using the time-dependent theory of spectroscopy. Raman spectra, excitation profiles, and absorption spectra are simulated with the same parameters using a single electronic state description for each P3HT form. Time-dependent wavepacket overlaps expose vibrational coherence on sub-100 fs timescales, which is usually difficult to extract from conventional ultrafast pump-probe spectra and transients of polymer∕fullerene blends. The results demonstrate the potential of simpler CW resonance Raman approaches to uncover excited state geometry changes and early vibrational dynamics from distinct morphological forms in polymer∕fullerene blends.

Resonance Raman studies of excited state structural displacements of conjugated polymers in donor/acceptor charge transfer complexes

Resonance Raman spectra of poly(2-methoxy-5-(3-7-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) and small molecule acceptor blend charge transfer (CT) complexes reveal long and detailed progressions of overtone and combination bands. These features are sensitive to the specific MDMO-PPV/acceptor interactions and enable quantitative calculations of vibrational mode specific displacements of the polymer CT complex.

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

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.

Observation of the missing mode effect in a poly-phenylenevinylene derivative: Effect of solvent, chain packing, and composition

Optical emission spectra of poly[2-methoxy-5-[3(),7()-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) in dilute solutions exhibit a vibronic progression interval (∼1225u2002cm(-1)) that does not correspond to any ground state vibrational mode frequency. This phenomenon is assigned as the missing mode effect (MIME) in which five key displaced polymer backbone vibrational modes in the range of 800-1600u2002cm(-1) contribute to the MIME interval. Emission spectra are calculated by analytically solving the time-dependent Schrödinger equation using estimates of mode-specific vibrational displacements determined independently from preresonance Raman intensities. Emission spectra of MDMO-PPV thin films and nanoparticles are measured and lineshapes show an increase of the MIME frequency to ∼1340u2002cm(-1) in addition to changes in vibronic intensity distributions and energies. Composite blend thin films consisting of MDMO-PPV and a fullerene derivative (1:1 w/w) exhibit a substantially larger MIME interval (∼1450u2002cm(-1)) that arises from an increase in polymer chain planarity. This structural change is most apparent from large decreases of the excited state displacement of an out-of-plane C-H bending mode (961u2002cm(-1)) that becomes forbidden in the planar structure.

Light trapping enhancement in thin film solar cells by breaking symmetry in nanostructures

We experimentally demonstrate highly efficient light-trapping structures that is achieved by breaking the symmetry in inverted nanopyramids on c-Si. The fabrication of these structures is cost-effective and scalable. Our optical measurement for the structures on 10-μm-thick c-Si cells shows the Shockley-Queisser efficiency of 27.9%. We further fabricate plasmonic metal structures on the symmetry-breaking inverted nanopyramids. When a light-absorbing polymer layer is deposited on top of the plasmonic structures, we observe that the plasmonic light trapping exceeds the Lambertian limit. The remarkable light trapping increases the short circuit current by 2.5 times. We expect the symmetry-breaking structures to be broadly applicable to thin-film solar cells.

Symmetry-breaking nanostructures for enhanced light-trapping in thin film solar cells

We introduce a manufacturable method to break the symmetry in inverted nanopyramids on c-Si. This method broadly enhances light trapping and would increase the efficiency from 25 to 26.4% for thick c-Si cells. We further use the nanopyramids as a template to deposit plasmonic metal structures and demonstrate enhanced light absorption in organic solar cells. The enhancement exceeds 100% in some cases by concentrating the plasmonic bands tuned to the polymer absorption. The result agrees well with our measured surface plasmon polariton band structures. We expect our approach to be broadly applicable to thin-film solar cells.

Exciton and polaron interactions in self-assembled conjugated polymer aggregates

We study exciton coupling and interconversion between neutral and charged states of different spin in pi-stacked conjugated polymer aggregates. Rigorous self-assembly approaches are used to prepare aggregate nanofibers that permit reliable control of polymer chain conformational and packing (intra- and interchain) order within these structures. Exciton coupling can be tuned between the H- and J-aggregate limits, which has important implications for determining the fates of excitons and polarons. Single molecule intensity modulation spectroscopy was performed on individual nanofibers and large quenching depths of emissive singlet excitons by triplets are found in J-aggregate type structures. We propose that high intrachain order leads to exciton delocalization that effectively lowers singlet-triplet energy splittings thus increasing triplet yields. Exciton-polaron and polaron-polaron interactions are next investigated in both H- and J-type nanofibers where polarons are injected by charge transfer doping. We find that the enhanced intrachain order of J-aggregates enables efficient intrachain polaron transport and leads to significantly larger doping efficiencies than less ordered H-aggregates. As polaron densities increase, signatures of spin-spin interactions between polarons on adjacent chains become appreciable leading to the formation of a spinless bipolaron. Overall, these studies demonstrate the potential for controlling and directing exciton and polaron interactions via tuning of subtle intra- and interchain ordering characteristics of aggregates, which could benefit various polymeric optoelectronic applications.

Spectroscopic and electrical imaging of disordered polymeric solar cells: understanding aggregation effects on material performance

The manner in which polymer chains pack and organize in thin film structures is crucial to maximizing the efficiency of charge and energy transport processes in solar cell devices. We use new spectroscopic and electrical imaging tools to spatially map and correlate local structure (chain conformation, packing, morphology) to local photocurrent generation efficiency. Both Raman and photoluminescence approaches are used that provide unique insights into important structural attributes and how they vary with film morphology. Simultaneous electrical measurements are then used to establish the roles of specific structural features to photocurrent production.