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Authors
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Janus, K. ; Chlebosz, D. ; Janke, A. ; Goldeman, W. ; Kiersnowski, A.
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Title
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Contributions of polymer chain length, aggregation and crystallinity degrees in a model of charge carrier transport in ultrathin polymer films
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Date
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27.01.2023
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Number
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61502
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Abstract
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Herein we report a systematic study on the quantitative relationships between molecular structure, crystallinity, aggregation, and charge carrier transport in films of poly(3-hexylthiophene-co-thiophenes) (P3HTTs) with thickness ranging from 5.7 to 8.0 nm. The P3HTTs contained 10, 20, or 30 mol % of thiophene monomer units. Our UV–Vis, X-ray diffraction, and atomic force microscopy (AFM) studies indicated that the variable thiophene content in the P3HTTs as well as the solution aging prior to film casting permitted a control over aggregation density (Xa), crystallinity degree (Xc), crystal fragmentation, and overall film morphology. Increasing the thiophene content in P3HTT caused a linear increase in the Xa and a linear drop in the Xc as well as crystal sizes. Aging the solution prior to casting resulted in a change of film morphology from granular to fibrillar and a noticeable increase in both Xa and Xc. The hole mobility determined from field-effect transistor characteristics grew nonlinearly with increasing thiophene content in P3HTT and was boosted by the aggregation-induced increase in Xa or Xc. For the 7.5 nm-thin P3HTT film (30 mol % of thiophene), the µh was as high as 0.017 cm2 × V–1 × s–1. In order to explain the observed µh variation, we have employed a model of charge transport through a series circuit, where crystals, aggregates, and the amorphous phase were considered units with different resistance to charge carrier transport. A systematic analysis of the experimental dataset revealed a quantitative correlation of the µh with the polymer end-to-end distance, interaggregate distance, and crystallinity index. Also, the correlation expressed with a single equation derived in this work enabled estimation of the minimum mobility of the amorphous phase and maximum mobility of the crystal phase in the quasi-two-dimensional films. Our original findings suggest that in quantitative considerations of charge transport in the ultrathin polymer films, the morphology should be considered secondary to molar mass and phase composition.
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Publisher
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Macromolecules
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Wikidata
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Citation
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Macromolecules 56 (2023) 964-973
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DOI
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https://doi.org/10.1021/ACS.MACROMOL.2C01998
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Tags
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