HOMO-HOMO Electron Transfer: An Elegant Strategy for p-Type Doping of Polymer Semiconductors toward Thermoelectric Applications.

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 HOMOOSC-1 to HOMOOSC-2 . A shift of work function toward the HOMOOSC-1 upon doping is confirmed by ultraviolet photoelectron spectroscopy (UPS). Detailed X-ray photoelectron spectroscopy (XPS) and UV-vis-NIR absorption studies confirm HOMOOSC-1 to HOMOOSC-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 µW m-1  K-2 at a Seebeck coefficient of 140 µV K-1 for a very low doping ratio of 4 mol%. Also, the concept of HOMOOSC-1 to HOMOOSC-2 electron transfer is a highly efficient, stable and generic way to p-dope other conjugated polymers.