RESEARCH ARTICLE

Intermolecular coupling enhanced thermopower in single-molecule diketopyrrolopyrrole junctions

¹ State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
² Department of Physics, Lancaster University, Lancaster LA1 4YB, UK
³ State Key Laboratory of Applied Organic Chemistry (SKLAOC), College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China
⁴ Innovation Laboratory for Sciences and Technologies of Energy Materials of Fujian Province (IKKEM), Xiamen 361005, China
⁵ Beijing National Laboratory for Molecular Sciences, Beijing 100190, China
⁶ Department of Physics, Faculty of Science, Taibah University, Madinah, Saudi Arabia

^* Corresponding authors (emails: liuzt@lzu.edu.cn (Zitong Liu); c.lambert@lancaster.ac.uk (Colin J. Lambert); whong@xmu.edu.cn (Wenjing Hong))

Received: 15 May 2022
Revised: 11 October 2022
Accepted: 11 October 2022

Abstract

Sorting out organic molecules with high thermopower is essential for understanding molecular thermoelectrics. The intermolecular coupling offers a unique chance to enhance the thermopower by tuning the bandgap structure of molecular devices, but the investigation of intermolecular coupling in bulk materials remains challenging. Herein, we investigated the thermopower of diketopyrrolopyrrole (DPP) cored single-molecule junctions with different coupling strengths by varying the packing density of the self-assembled monolayers (SAM) using a customized scanning tunneling microscope break junction (STM-BJ) technique. We found that the thermopower of DPP molecules could be enhanced up to one order of magnitude with increasing packing density, suggesting that the thermopower increases with larger neighboring intermolecular interactions. The combined density functional theory (DFT) calculations revealed that the closely-packed configuration brings stronger intermolecular coupling and then reduces the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap, leading to an enhanced thermopower. Our findings offer a new strategy for developing organic thermoelectric devices with high thermopower.