Journal
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 131, Issue 28, Pages 9654-9662Publisher
AMER CHEMICAL SOC
DOI: 10.1021/ja900636z
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- UNM startup funds
- Oak Ridge Associated Universities
- Petroleum Research Fund
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Resonance Raman spectroscopic imaging is introduced as a physical probe to identify and spatially map morphology-dependent variations of intra- and interchain interactions and order in poly-3-hexylthiophene (P3HT) and [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) photovoltaic blend thin films. Absorption spectra and C=C symmetric stretching Raman modes of P3HT/PCBM blend films show contributions from two distinct species that are assigned as aggregated and unaggregated P3HT chains with characteristic Raman frequencies of similar to 1450 (I-C=C(agg)) and similar to 1470 cm(-1) (I-C=C(un)), respectively. Upon thermal annealing of blend films, the relative concentrations of I-C=C(agg) and I-C=C(un) species (R = I-C=C(agg)/I-C=C(un)) changes on average from 0.79 +/- 0.20 (as-cast) to 2.45 +/- 0.77 (annealed). It is proposed that R values report on the relative densities of states (DOS) of aggregated and unaggregated species, and resonance Raman imaging is then used to spatially map morphology-dependent variations of R values and uncover subclassifications of these species. From both Rand frequency dispersion resonance Raman images of I-C=C(agg) and I-C=C(un) species, four distinct types of P3HT chains are identified and mapped in annealed P3HT/PCBM blend thin films: (i) highly aggregated/crystalline, (ii) partially aggregated, (iii) interfacial, and (iv) unaggregated/PCBM rich. The change in aggregation upon annealing is attributed to an increase in planarity of the P3HT chains that is determined from the ratios of C=C/C-C symmetric stretching mode intensities.
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