Understanding Morphology-Dependent Polymer Aggregation Properties and Photocurrent Generation in Polythiophene/Fullerene Solar Cells of Variable Compositions

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Abstract

We fabricate poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) thin film solar cells of variable weight/weight (w/w) compositions (i.e., 1:1 to 1:4) to systematically perturb polymer packing (aggregation) properties and assess their impact on local electronic structure and photocurrent generation efficiency. A combination of absorption spectroscopy and resonance Raman spectroscopic and photocurrent imaging techniques are used to quantify and spatially map morphology-dependent cofacial, π−π aggregated P3HT chains and correlate these structures to local photocurrent production characteristics. On average, increasing the PCBM weight fraction results in blue shifts and broadening for absorption and Raman spectra in the dominant P3HT C═C stretching mode region (∼1450−1470 cm−1), whereas symmetric stretching C—C modes show decreased intensities and red shifts. P3HT/PCBM absorption spectra are fitted near the resolved P3HT onset region using a weakly coupled H-aggregate model that reveals decreases in the relative amounts of aggregated/unaggregated P3HT chains as well as interchain exciton coupling in the aggregated component. Raman bands of P3HT C═C modes can likewise be decomposed into contributions from both aggregated (IC═Cagg.) and unaggregated (IC═Cun.) chains, and like absorption spectra, IC═Cagg./IC═Cun. values decrease with increased PCBM content. Combined Raman and photocurrent imaging studies of 1:1 P3HT/PCBM devices reveal that most aggregated (ordered) P3HT chains reside primarily outside PCBM-rich regions, but this, surprisingly, reverses for >1:1 PCBM w/w loadings where all aggregated P3HT chains reside within PCBM-rich regions. This effect is attributed to a change in the type of P3HT aggregation from inter- to primarily intrachain (or self-aggregated) that is supported by decreases in the interchain exciton coupling parameter from absorption fits as well as Raman C—C and C═C (agg.) frequency maps. The results reveal not only the importance of the polymer aggregation state but also its spatial location in the film that together have a large impact on charge transport properties and material performance.

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