Photoexcitation dynamics in regioregular and regiorandom polythiophene films

Journal ArticleUsing a variety of optical probes techniques we studied the photoexcitation dynamics in thin films of poly-3-hexyl thiophene with regioregular order that forms lamellae structures with increased interchain interaction, as well as regiorandom order that keeps a chainlike morphology. In regiorandom films we found that intrachain excitons with correlated induced absorption and stimulated emission bands are the primary excitations; they give rise to a moderately strong photoluminescence band. In regioregular films, on the contrary, we found that the primary excitations are excitons with a much larger interchain component; this results in lack of stimulated emission, vanishing intersystem crossing, and very weak photoluminescence band. In regioregular films we also measured photogenerated geminate polaron pairs with ultrafast dynamics that are precursor to long-lived polaron excitations

Ultrafast dynamics of excitons and solitons in disubstituted polyacetylene

Journal ArticleWe studied the ultrafast photoexcitation dynamics in disubstituted polyacetylene (DPA). We found two distinctively different relaxation channels for the photogenerated excitons; ionic and covalent pathways. In DPA films the photogenerated odd-parity (Bu) excitons that are responsible for the high photoluminescence quantum efficiency follow the ionic relaxation pathway. During the hot exciton thermalization process, however, a fraction of the Bu excitons undergo a phonon-assisted transition to the covalent 2Ag state, which consequently decomposes into two neutral soliton-antisoliton pairs that are subject to ultrafast recombination or dissociate into stable neutral solitons. In DPA solutions the ionic channel remains unchanged, however, the covalent channel becomes ineffective due to the different hot exciton thermalization pathways

Triplet Exciton Generation in Bulk-Heterojunction Solar Cells based on Endohedral Fullerenes

Organic bulk-heterojunctions (BHJ) and solar cells containing the trimetallic nitride endohedral fullerene 1-[3-(2-ethyl)hexoxy carbonyl]propyl-1-phenyl-Lu3N@C80 (Lu3N@C80-PCBEH) show an open circuit voltage (VOC) 0.3 V higher than similar devices with [6,6]-phenyl-C[61]-butyric acid methyl ester (PC61BM). To fully exploit the potential of this acceptor molecule with respect to the power conversion efficiency (PCE) of solar cells, the short circuit current (JSC) should be improved to become competitive with the state of the art solar cells. Here, we address factors influencing the JSC in blends containing the high voltage absorber Lu3N@C80-PCBEH in view of both photogeneration but also transport and extraction of charge carriers. We apply optical, charge carrier extraction, morphology, and spin-sensitive techniques. In blends containing Lu3N@C80-PCBEH, we found 2 times weaker photoluminescence quenching, remainders of interchain excitons, and, most remarkably, triplet excitons formed on the polymer chain, which were absent in the reference P3HT:PC61BM blends. We show that electron back transfer to the triplet state along with the lower exciton dissociation yield due to intramolecular charge transfer in Lu3N@C80-PCBEH are responsible for the reduced photocurrent

Photochemical Charge Separation in Poly(3-hexylthiophene) (P3HT) Films Observed with Surface Photovoltage Spectroscopy

Surface photovoltage spectroscopy (SPS) was used to probe photon induced charge separation in thin films of regioregular and regiorandom poly(3-hexylthiophene) (P3HT) as a function of excitation energy. Both positive and negative photovoltage signals were observed under sub-band-gap (<2.0 eV) and super-band-gap (>2.0 eV) excitation of the polymer. The dependence of the spectra on substrate work function, thermal annealing, film thickness, and illumination intensity was investigated, allowing the identification of interface, charge transfer (CT), and band-gap states in the amorphous and crystalline regions of the polymer films. The ability to probe these states in polymer films will aid the development and optimization of organic electronic devices such as photovoltaics (OPVs), light-emitting diodes (OLEDs), and field effect transistors (OFETs). The direction and size of the observed photovoltage features can be explained using the depleted semiconductor model. © 2013 American Chemical Society