Significantly Reduced Thermal-Activation Energy for Hole Transport via Simple Donor Engineering: Understanding the Role of Molecular Parameters for Thermoelectric Behaviors

Abstract

Thermal activation energy for charge transfer (Eact,σ) plays a crucial role in determining the electrical properties of organic semiconductors, which are largely dominated by the Coulomb binding energy (Ecoul,ICTC) or static energy disorder (σICTC) of the formed integer charge transfer complexes at low or high doping concentration, respectively. Herein, we provide two typical donor-acceptor type polymers with distinct donors to disclose the role of molecular parameters in response for their corresponding thermoelectric (TE) behaviors. Noticeably, both the Ecoul,ICTC and σICTC of the polymers can be effectively restrained by varying the initial carbazole (CZ) donor to the dithieno[3,2-b:2',3'-d]pyrrole (DTP) moiety, which contributes to the remarkably decreased Eact,σ values of the PDTP-DPP than that of PCZ-DPP. Accordingly, the optimized power factors (PF) for PDTP-DPP (10.8 μW m-1 K-2) is almost 5 times higher than the primary PCZ-DPP (1.8 μW m-1 K-2) at ambient condition. In addition, a further improved PF over 85.5 μW m-1 K-2 can be achieved by PDTP-DPP at 488 K due to the synergy of thermal-induced dedoping and thermal-activated semiconducting behavior. Ultraviolet photoelectron and X-ray photoelectron spectroscopy measurements confirm the lower thermal activation energy for efficient p-doping of PDTP-DPP than that of PCZ-DPP.