Modified structure of two-dimensional polythiophene derivatives by incorporating electron-deficient units into terthiophene-vinylene conjugated side chains and the polymer backbone: synthesis, optoelectronic and self-assembly properties, and photovoltaic application

Abstract

Molecular engineering on the conjugated side chains of two-dimensional (2D) conjugated polymers was conducted and its effect on the optical, electronic, self-assembly and photovoltaic properties was investigated. A new monomer, M2, was prepared by capping (E)-3′-(2-(2,5-dibromothiophen-3-yl)vinyl)-4,4′′-bis(2-ethylhexyl)-2,2′:5′,2′′-terthiophene, M1, with two heptanoyl groups, and then coupled with 5,5′-bis(trimethylstannyl)-2,2′-bithiophene via microwave-assisted Stille polymerization to produce a series of polythiophene derivatives with terthiophene-vinylene conjugated side chains, TTV–PTs. Copolymer P2 shows a down-shifted HOMO energy level, enhanced solubility, and red-shifted absorption, as compared with P1; however, the bulky side chains significantly disrupt the coplanarity of thiophene rings in the polymer backbone and the ability to self-assemble into an ordered structure. The GIXRD measurements reveal that the original crystallinity of P1 can be recovered by simply inserting a few 2,1,3-benzothiadiazole units into the polythiophene main chain in P2 through a random copolymerization route to yield a terpolymer, P3, which possesses excellent crystallinity, thereby causing a three-fold increment in hole mobility. Furthermore, the P1/PC₆₁BM, P2/PC₆₁BM, and P3/PC₆₁BM solar devices exhibit power conversion efficiencies of 3.89%, 1.52%, and 4.17%, respectively, under AM1.5G illumination with an intensity of 100 mW cm⁻².