Naphthalene Diimide-Based Polymers Consisting of Amino Alkyl Side Groups: Three-Component One-Pot Polymerization and Their Application in Polymer Solar Cells

Abstract

In this work, we demonstrate the microwave-assisted synthesis of naphthalene diimide-based polymers via three-component polymerization (TCP) of diynes, dialdehydes and dibenzylamine, and the applications of such polymers as cathode interfacial layers for polymer solar cells. The TCP of diynes (1a~1c), dialdehydes (2a~2b) and dibenzylamine catalyzed by InCl3 could be performed smoothly under microwave irradiation in very short reaction time, yielding soluble polymers P1~P4 with high molecular weights. The chemical structures of these resulting polymers were confirmed by nuclear magnetic resonance spectroscopy. The thermal stability, photophysical and electrochemical properties of the resulting polymers were also investigated. Besides, the effects of chemical environment of amine groups on the resulting polymers' electrode modification capability and self-doping behavior were explored by conducting scanning Kelvin probe microscopy and electron paramagnetic resonance (EPR) spectroscopy studies, respectively. It was found that the chemical environment variation of amine groups, including the decreasing electron density of the nitrogen atoms in alkylamine and the enhancing steric hindrance around the nitrogen atoms from substituent groups, can substantially influence the electrode modification capability and self-doping behavior of the resulting polymers. Moreover, quantum chemistry calculation was also conducted to qualitatively illuminate the essential distinction in chemical environment of different amine groups. It was found that the negative atomic dipole moment corrected Hirshfeld (ADCH) charge of nitrogen atoms in side chains was significantly larger than the ADCH charges of nitrogen atoms in main chains. Among all the resulting polymers, P1 can be easily dissolved in alcohol due to its amino functionalized side chain groups and thus was utilized as the cathode interlayer for polymer solar cells. The device with P1 as the cathode interlayer and PTB7-Th:PC71BM as the photoactive layer exhibits a high power conversion efficiency of 9.34%, which is much better than that of the control device without such cathode interlayer. All these results provide a guideline for the material design of amino-functionalized polymers for the optoelectronic devices. And it was also shown that the multicomponent polymerization (MCP) is an effective strategy for the synthesis of functional polymers, and may trigger broad research interests in developing effective polymerization approaches toward multi-functional polymer materials.