Resonance Chemical Imaging of Polythiophene/Fullerene Photovoltaic Thin Films: Mapping Morphology-Dependent Aggregated and Unaggregated CC Species

• Published in: Journal of the American Chemical Society Vol. 131; no. 28; pp. 9654 - 9662
• Main Authors: Gao, Yongqian, Grey, John K
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 22.07.2009
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja900636z

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-C61-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 ∼1450 (I CC agg) and ∼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 R and 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.