Charge Recombination in Conjugated Polymer/Fullerene Blended Films Studied by Transient Absorption Spectroscopy

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Abstract

The recombination dynamics of long-lived photogenerated charge carriers are investigated by transient absorption spectroscopy (TAS) on micro- to millisecond time scales in a blend of poly[2-methoxy-5-(3‘,7‘-dimethyloctyloxy)-1-4-phenylene vinylene], (MDMO-PPV) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C61 (PCBM) at room temperature. In this work we focus on the physical origins of these recombination dynamics. Studies are conducted as a function of blend composition (including consideration of pristine MDMO-PPV films), solvent, temperature, and the effect of white light bias. We conclude that following low-intensity excitation of the MDMO-PPV/PCBM blend, MDMO-PPV polarons are trapped in an inhomogeneous distribution of localized states (∼1017 cm-3) above the mobility edge for the polymer valence band. The recombination dynamics of these polarons with PCBM anions are limited by the thermally activated detrapping of these polarons, being rather insensitive to both PCBM anion dynamics and MDMO-PPV/PCBM phase segregation. Steady-state white light illumination results in the continuous occupancy of the tail of these localized states, causing an acceleration of the recombination dynamics. These observations are discussed in terms of their relevance to device function.

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