Sequentially Processed P3HT/CN6-CP•−NBu4+ Films: Interfacial or Bulk Doping?

Abstract

Derivatives of the hexacyano-[3]-radialene anion radical (CN6-CP•−) emerge as a promising new family of p-dopants having a doping strength comparable to that of archetypical dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4TCNQ). Here, mixed solution (MxS) and sequential processing (SqP) doping methods are compared by using a model semiconductor poly(3-hexylthiophene) (P3HT) and the dopant CN6-CP•−NBu4 + (NBu4 + = tetrabutylammonium). MxS films show a moderate yet thickness-independent conductivity of ≈0.1 S cm−1. For the SqP case, the highest conductivity value of ≈6 S cm−1 is achieved for the thinnest (1.5–3 nm) films whereas conductivity drops two orders of magnitudes for 100 times thicker films. These results are explained in terms of an interfacial doping mechanism realized in the SqP films, where only layers close to the P3HT/dopant interface are doped efficiently, whereas internal P3HT layers remain essentially undoped. This structure is in agreement with transmission electron microscopy, atomic force microscopy, and Kelvin probe force microscopy results. The temperature-dependent conductivity measurements reveal a lower activation energy for charge carriers in SqP samples than in MxS films (79 meV vs 110 meV), which could be a reason for their superior conductivity. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim