HOMO–HOMO Electron Transfer: An Elegant Strategy for p‐Type Doping of Polymer Semiconductors toward Thermoelectric Applications
Mahima Goel, Marie Siegert, Gert Krauss, John Mohanraj, Adrian Hochgesang, C. David Heinrich, Martina Fried, Jens Pflaum, Mukundan Thelakkat
Abstract
Abstract Unlike the conventional p‐doping of organic semiconductors (OSCs) using acceptors, here, an efficient doping concept for diketopyrrolopyrrole‐based polymer PDPP[T] 2 ‐EDOT (OSC‐1) is presented using an oxidized p‐type semiconductor, Spiro‐OMeTAD(TFSI) 2 (OSC‐2), exploiting electron transfer from HOMO OSC‐1 to HOMO OSC‐2 . A shift of work function toward the HOMO OSC‐1 upon doping is confirmed by ultraviolet photoelectron spectroscopy (UPS). Detailed X‐ray photoelectron spectroscopy (XPS) and UV–vis–NIR absorption studies confirm HOMO OSC‐1 to HOMO OSC‐2 electron transfer. The reduction products of Spiro‐OMeTAD(TFSI) 2 to Spiro‐OMeTAD(TFSI) and Spiro‐OMeTAD is also confirmed and their relative amounts in doped samples is determined. Mott–Schottky analysis shows two orders of magnitude increase in free charge carrier density and one order of magnitude increase in the charge carrier mobility. The conductivity increases considerably by four orders of magnitude to a maximum of 10 S m −1 for a very low doping ratio of 8 mol%. The doped polymer films exhibit high thermal and ambient stability resulting in a maximum power factor of 0.07 µ W m −1 K −2 at a Seebeck coefficient of 140 µ V K −1 for a very low doping ratio of 4 mol%. Also, the concept of HOMO OSC‐1 to HOMO OSC‐2 electron transfer is a highly efficient, stable and generic way to p‐dope other conjugated polymers.