Alan K. Thomas

Spatially Resolving Ordered and Disordered Conformers and Photocurrent Generation in Intercalated Conjugated Polymer/Fullerene Blend Solar Cells.

Resonance Raman spectroscopy was used to identify ordered and disordered conformers of poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) blended with the electron acceptor [6,6]-phenyl C61 butyric acid methyl ester (PCBM) in bulk heterojunction (BHJ) solar cells where PCBM intercalates into PBTTT side groups. We show that the PBTTT thiophene ring symmetric C=C stretching mode consists of contributions from ordered (ℏωC=C = 1489 cm–1, fwhm ∼ 15 cm–1) and disordered (ℏωC=C = 1500 cm–1, fwhm ∼ 25 cm–1) components and their relative amounts are sensitive to PCBM loading, annealing and excitation energy. The 1500 cm–1 PBTTT component originates from twisted thiophene rings and disordered side groups due to PCBM intercalation in a mixed kinetic phase and thermal annealing promotes ordering of PBTTT chains from the formation of bimolecular PBTTT/PCBM crystals. Density functional theory (DFT) Raman simulations of PBTTT monomers support these assignments. Resonance Raman images of annealed PBTTT/PCBM model solar cells confirm that ordered PBTTT chains are most concentrated in PCBM-rich bimolecular crystals and corresponding intensity modulated photocurrent spectroscopy (IMPS) and imaging measurements show increased nongeminate charge recombination at the boundaries of ordered/disordered regions.

High intrachain order promotes triplet formation from recombination of long-lived polarons in poly(3-hexylthiophene) J-aggregate nanofibers.

Photoluminescence (PL) of single poly(3-hexylthiophene) (P3HT) J-aggregate nanofibers (NFs) exhibits strong quenching under intensity-modulated pulsed excitation. Initial PL intensities (I(0)) decay to steady-state levels (ISS) typically within ∼ 1-10 μs, and large quenching depths (I(0)/I(SS) >2) are observed for ∼ 70% of these NFs. Similar studies of polymorphic, H-aggregate type P3HT NFs show much smaller PL quenching depths (I(0)/I(SS) ≤ 1.2). P3HT chains in J-type NF π-stacks possess high intrachain order, which has been shown previously to promote the formation of long-lived, delocalized polarons. We propose that these species recombine nongeminately to triplets on time scales of >1 ns. The identity of triplets as the dominant PL quenchers was confirmed by subjecting NFs to oxygen, resulting in an instantaneous loss of triplet PL quenching (I(0)/I(SS) ∼ 1). The lower intrachain order in H-type NFs, similar to P3HT thin-film aggregates, localizes excitons and polarons, leading to efficient geminate recombination that suppresses triplet formation at longer time scales. Our results demonstrate the promise of self-assembly strategies to control intrachain ordering within multichromophoric polymeric aggregate assemblies to tune exciton coupling and interconversion processes between different spin states.

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.