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How do disorder, reorganization, and localization influence the hole mobility in conjugated copolymers?

Journal of the American Chemical Society (2013-01-10)
Sebastian T Hoffmann, Frank Jaiser, Anna Hayer, Heinz Bässler, Thomas Unger, Stavros Athanasopoulos, Dieter Neher, Anna Köhler
ABSTRACT

In order to unravel the intricate interplay between disorder effects, molecular reorganization, and charge carrier localization, a comprehensive study was conducted on hole transport in a series of conjugated alternating phenanthrene indenofluorene copolymers. Each polymer in the series contained one further comonomer comprising monoamines, diamines, or amine-free structures, whose influence on the electronic, optical, and charge transport properties was studied. The series covered a wide range of highest occupied molecular orbital (HOMO) energies as determined by cyclovoltammetry. The mobility, inferred from time-of-flight (ToF) experiments as a function of temperature and electric field, was found to depend exponentially on the HOMO energy. Since possible origins for this effect include energetic disorder, polaronic effects, and wave function localization, the relevant parameters were determined using a range of methods. Disorder and molecular reorganization were established first by an analysis of absorption and emission measurements and second by an analysis of the ToF measurements. In addition, density functional theory calculations were carried out to determine how localized or delocalized holes on a polymer chain are and to compare calculated reorganization energies with those that have been inferred from optical spectra. In summary, we conclude that molecular reorganization has little effect on the hole mobility in this system while both disorder effects and hole localization in systems with low-lying HOMOs are predominant. In particular, as the energetic disorder is comparable for the copolymers, the absolute value of the hole mobility at room temperature is determined by the hole localization associated with the triarylamine moieties.

MATERIALS
Product Number
Brand
Product Description

Supelco
Phenanthrene solution, certified reference material, 5000 μg/mL in methanol
Sigma-Aldrich
Phenanthrene, 98%
Sigma-Aldrich
Phenanthrene, sublimed grade, ≥99.5%
Supelco
Phenanthrene, analytical standard, for environmental analysis
Supelco
Phenanthrene, certified reference material, TraceCERT®