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Synergistic Effects of Terpolymer Regioregularity on the Performance of All-Polymer Solar Cells

Kim, Sang Woo, Kim, Honggi, Lee, Jin-Woo, Lee, Changyeon, Lim, Bogyu, Lee, Jaechol, Lee, Youngu, Kim, Bumjoon J.
Macromolecules 2019 v.52 no.2 pp. 738-746
absorption, composite polymers, crystallites, electrical properties, solar cells, solar energy, synergism, thiophene
Random terpolymers with three different monomer units can provide broader light absorption than the most widely used donor–acceptor (D–A) alternating copolymers, but their electrical properties are often sacrificed by the randomly distributed monomers in the polymeric backbone that prevent efficient intermolecular π–π interactions. Here, we report the development of a regioregular terpolymer and demonstrate its importance in enhancing the power conversion efficiency (PCE) of all-polymer solar cells (all-PSCs). To investigate the impact of the monomer sequence and regioregularity in the terpolymer, we designed and synthesized two terpolymers (Ra-(D₁–A–D₂–A) random terpolymer and RR-(D₁–A–D₂–A) regioregular terpolymer) consisting of two electron-donating benzodithiophene (BDT) units with different side chains and one electron-withdrawing fluorinated thieno[3,4-b]thiophene (TT-F) unit. As a reference polymer, we also synthesized the D₁–A alternating copolymer. The RR-(D₁–A–D₂–A) film exhibited stronger π–π stacking and a larger crystallite size than the D₁–A and Ra-(D₁–A–D₂–A) films, resulting in 1 order of magnitude higher hole mobility than those of the other polymers. When blended with the P(NDI2HD–DTAN) polymer acceptor, the RR-(D₁–A–D₂–A)-based all-PSC yielded an outstanding PCE of 6.13%, which was superior to those of the D₁–A-based all-PSCs (4.81%) and Ra-(D₁–A–D₂–A)-based all-PSCs (4.93%). These findings indicate that the synthesis of the regioregular terpolymer is a promising design strategy for the development of high-performance all-PSCs with improved optical and electrical properties.