Resonance Raman Spectroscopic- and Photocurrent Imaging of Polythiophene/Fullerene Solar Cells

Resonance Raman−photocurrent imaging (RRPI) is introduced to spatially map the morphology-dependent polymer aggregation state to local photocurrent generation efficiency in poly-(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) blend thin film photovoltaic devices. The C═C symmetric stretching mode of P3HT is decomposed into contributions from aggregated ( I C═C agg, ∼1450 cm −1 ) and unaggregated ( I C═C un, ∼1470 cm −1 ) chains, and the ratios of their integrated intensities ( I C═C agg / I C═C un, R ) is used as a reporter for the local P3HT aggregation state. Maps of R values and photocurrents are generated for both as-cast and annealed P3HT/PCBM devices that permit direct spatial correlations between the P3HT aggregation state and local photocurrent generation efficiency. Regions of increased P3HT aggregation are observed at both P3HT/PCBM interfaces and in P3HT-rich areas that result in decreased photocurrent generation. Voltage-dependent RRPI studies are also performed at several applied bias levels that reveal distinct changes in photocurrents due to morphology-dependent charge mobility characteristics.