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Measurement of charge carrier mobility and photoinduced charge separation in liquid crystalline semi-conductors

Measurement of key physical parameters of liquid crystalline semiconductors:

- Charge carrier mobility
-- p-type materials: Charge carrier mobilities have been measured in four hexabenzocoronene (HBC) derivatives using pulse radiolysis time resolved microwave conductivity. Three are crystalline (K-phase) at room temperature and display high charge carrier mobilities (ca. 0.5 cm²/Vs), undergoing a transition to a columnar liquid crystalline phase (Dh) at ca 100°C above which the mobility drops by half. On cooling the D K transition is reversible with a hysteresis of ca 20°C. Neither side chain branching nor chirality significantly affected the measured mobilities, but purity of the sample was determined to be very important. The fourth compound maintained liquid crystallinity at all temperatures, and has a mobility of ca 0.2 cm²/Vs at room temperature, which increases gradually with temperature with no transition to a value close to the previous three compounds at the highest temperature.

-- n-type materials: Freshly prepared hexaazatrinaphthalene derivatives display a moderate charge mobility of ca 0.1cm²/Vs at room temperature, decreasing by a factor of ~2 at the transition to the Dh phase. On cooling back to room temperature the mobility remains low indicating that an irreversible phase transition has occurred.
Investigation of several perylene-diimide (PEDI) derivatives shows a large variation in mobility dependent on the alkyl chain substituents. For the derivative used in the HBC composites, a value of 0.01cm²/Vs at room temperature was found.

Conclusions: All HBC derivatives are capable of supporting rapid charge transport at RT. Chirality of the branched alkyl side chains does not have a pronounced positive influence on the mobility contrary to expectations. Purity is an important factor. The mobilities in the n-type components are in general substantially lower than for the HBCs.

- Photo-induced charge separation:
Photo-induced charge separation has been studied in thin, spin coated films of ca 50/50 mixtures of a perylenediimide derivative and two different HBCderivatives using flash-photolysis time resolved microwave conductivity. Charge separation in the blends is greatly enhanced compared with that for the separate components. This is attributed to electron transfer from HBC to PEDI with the efficiency being approximately equal for excitation of either component. Evidence is found for direct photo-induced charge separation at long wavelengths, which is attributed to absorption in regions of the composite in which HBC and PEDI are in intimate contact. The efficiency of charge separation decreases dramatically with increasing light intensity. This is attributed to exciton-exciton annihilation within the separate HBC and PEDI domains competing with exciton diffusion to the interfacial region where charge transfer takes place. Charge separation in the HBC derivatives can vary by a factor of about four. Room temperature liquid crystalline natures in HBC derivatives appears to have a positive influence on overall photocurrent generation which compensates for the higher mobility and charge separation efficiency found for crystalline derivatives.

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Reported by

Delft University of Technology
2600 AA Delft