Water- and alcohol-soluble cationic phenanthroline derivatives as efficient cathode interfacial layers for bulk-heterojunction polymer solar cells

Abstract

In recent years, cathode interfacial layers (CILs), as versatile functional layers, have been extensively investigated for use in organic electronics. Many excellent CILs have been designed and prepared, and these have provided a distinct improvement in device performance. In this study, two bathophenanthroline-based cationic CILs (Bphen-Et and Bphen-Pr) were synthesized via a simple intramolecular cyclization reaction with 1,2-dibromoethane and 1,3-dibromopropane, respectively, for use in polymer solar cells (PSCs), and their chemical, thermal, photophysical, electrochemical, and photovoltaic properties were characterized. Bphen-Et and Bphen-Pr exhibited high thermal stability, and their glass transition temperatures exceeded 100 °C. Additionally, Bphen-Et has a glass transition temperature above 181 °C. The electrochemical characterization shows that Bphen-Et and Bphen-Pr have very deep highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, where the HOMOs and LUMOs of Bphen-Et and Bphen-Pr are −6.63 and −6.67 and −4.16 and −4.18 eV, respectively. These very low HOMOs and LUMOs enhance the photovoltaic performance and decrease interfacial energy loss. The PSCs based on the poly[4,8-bis(2-ethylhexyloxyl)benzo[1,2-b:4,5-b′]dithio-phene-2,6-diyl-alt-ethylhexyl-3-fluorothithieno[3,4-b]thiophene-2-carboxylate-4,6-diyl] (PTB7): [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) system with Bphen-Et and Bphen-Pr CILs exhibit simultaneous enhancement in open-circuit voltage, short-circuit current density, and fill factor, and their power conversion efficiency increases from 3.98% to 8.05%, relative to that of the bare Al device. This type of cationic aromatic compound shows promise as a candidate CIL for use in PSCs.