Effect of Chemical Doping on the Structural and Electrical Properties of Polythiophene
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Abstract
The π-conjugated organic semiconductor polythiophene (PTh) can be chemically doped to change its structure and electrical properties by many orders of magnitude. The influence of chemical doping (with iodine (I₂), ferric chloride (FeCl₃) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ) at 0, 5, 10, 15, 20 and 25 mol %) on pure polythiophene has been studied. The structural evolution has been investigated by X-ray diffraction (XRD), while the electrical transport properties have been studied using the 4-probe technique at various temperatures. Doping led to a systematic decrease of interchain d-spacing and crystallite size and a systematic increase of the room temperature electrical conductivity (from 3.1 × 10⁻⁵ S cm⁻¹ for the pristine film to 1.9 × 10¹ S cm⁻¹ for the 25 mol % I₂-doped film). The charge transport in the doped films was found to be in accordance with the three-dimensional Mott variable range hopping (VRH) model. The results confirm the existence of polaronic and bipolaronic states created by chemical doping, which control the density of carriers and their mobility. The results of this work will lead to quantitative structure–property relationships which can be applied to the design of polythiophene-based flexible electronics and related devices.
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