Abstract
In this work, four donor–acceptor copolymers, PFDTBTDI-DMO, PFDTBTDI-8, PDBSDTBTDI-DMO, and PDBSDTBTDI-8, based on alternating 2,7-fluorene or 2,7-dibenzosilole flanked by thienyl units, as electron-donor moieties and benzothiadiazole dicarboxylic imide (BTDI) as electron-accepting units, have been designed and synthesized for photovoltaic applications. All polymers were synthesized in good yields via Suzuki polymerization. The impact of attaching two different alkyl chains (3,7-dimethyloctyl vs. n-octyl) to the BTDI units upon the solubilities, molecular weights, optical and electrochemical properties, and thermal and structural properties of the resulting polymers was investigated. PFDTBTDI-8 has the highest number average molecular weight (Mn = 24,900 g·mol−1) among all polymers prepared. Dibenzosilole-based polymers have slightly lower optical band gaps relative to their fluorene-based analogues. All polymers displayed deep-lying HOMO levels. Their HOMO energy levels are unaffected by the nature of either the alkyl substituents or the donor moieties. Similarly, the LUMO levels are almost identical for all polymers. All polymers exhibit excellent thermal stability with Td exceeding 350 °C. X-ray powder diffraction (XRD) studies have shown that all polymers have an amorphous nature in the solid state.
Highlights
Solution-processable polymer solar cells (PSCs) have received substantial consideration as a renewable energy source due to their benefits such as being flexible devices, light weight, low costs, and easy fabrication [1,2,3,4]
In 2013, Wang et al synthesized two novel copolymers based on DTBTDI and benzodithiophene units which were used for solar cell applications [23]
We reported the synthesis of four fluorene and dibenzosilole-based copolymers by copolymerizing 2,7-fluorene and 2,7-dibenzosilole (DBS) with both 4,7-di(5-bromo-thien-2-yl)2,1,3-benzothiadiazole-5,6-N-(3,7-dimethyloctyl)dicarboxylic imide (M1) and 4,7-di(5-bromo-thien2-yl)-2,1,3-benzothiadiazole-5,6-N-octyl-dicarboxylic imide (M2) and yielded PFDTBTDI-dimethyloctyl side chains (DMO), PFDTBTDI-8, PDBSDTBTDI-DMO, and PDBSDTBTDI-8, respectively
Summary
Solution-processable polymer solar cells (PSCs) have received substantial consideration as a renewable energy source due to their benefits such as being flexible devices, light weight, low costs, and easy fabrication [1,2,3,4]. A new acceptor moiety 4,7-di-2-thienyl-2,1,3-benzothiadiazole-5,6-N-alkyl-dicarboxylic imide (DTBTDI), which consists of the dicarboxylic imide group fused to the BT unit has been developed [20,23,24] This acceptor has been shown as a stronger electron-deficient unit compared to both DTBT and DTffBT analogues. In 2013, Wang et al synthesized two novel copolymers based on DTBTDI and benzodithiophene units which were used for solar cell applications [23] These polymers were prepared by the Stille polymerization between dibrominated DTBTDI and distannylated benzodithiophene monomers. Fluorene unit is one of the promising donor building blocks for solar cell applications owing to the low HOMO energy levels and acceptable hole mobilities of PF derivatives, which provide high. The prepared polymers were characterized by UV-vis and cyclic voltammetry to determine their energy band gaps
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