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Negative field-dependent charge mobility in crystalline organic semiconductors with delocalized transport

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Charge-carrier mobility has been investigated by time-of-flight (TOF) transient photocurrent in a lateral transport configuration in highly crystalline thin films of 2,7-dioctyl[1]benzothieno [3,2-b][1] benzothiophene (C8-BTBT) grown by a zone-casting alignment technique. High TOF mobility has been revealed that it is consistent with the delocalized nature of the charge transport in this material, yet it featured a positive temperature dependence at \( T \ge 295\,{\text{K}} \). Moreover, the mobility was surprisingly found to decrease with electric field in the high-temperature region. These observations are not compatible with the conventional band-transport mechanism. We have elaborated an analytic model based on effective-medium approximation to rationalize the puzzling findings. The model considers the delocalized charge transport within the energy landscape formed by long-range transport band-edge variations in imperfect organic crystalline materials and accounts for the field-dependent effective dimensionality of charge transport percolative paths. The results of the model calculations are found to be in good agreement with experimental data.

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The authors gratefully acknowledge Nippon Kayaku Co. for providing C8-BTBT for this study. This research was supported by the Slovenian Research Agency and the Belgian FWO in the framework of the international collaboration project ORSIC-HIMA (Project No. N1-0024/G0B5914 N), by the European Research Council via Grant No. 320680 (EPOS CRYSTALLI), and by the Volkswagen Foundation through the project “Understanding the dependence of charge transport on morphology in organic semiconductor films”.

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Correspondence to Andrey Kadashchuk.

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Kadashchuk, A., Janneck, R., Tong, F. et al. Negative field-dependent charge mobility in crystalline organic semiconductors with delocalized transport. Chem. Pap. 72, 1685–1695 (2018).

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