Fabrication of Alternating Copolymers Based on Cyclopentadithiophene-Benzothiadiazole Dicarboxylic Imide with Reduced Optical Band Gap: Synthesis, Optical, Electrochemical, Thermal, and Structural Properties.

Ary R Murad,Rebar T Abdulwahid,Mohamad A Brza,Ahmed Iraqi,Salah R Saeed,Shujahadeen B Aziz,Sozan N Abdullah

doi:10.3390/polym13010063

Abstract

A series of alternating copolymers containing cyclopentadithiophene (CPDT) flanked by thienyl moieties as electron-donor units and benzothiadiazole dicarboxylic imide (BTDI) as electron-acceptor units were designed and synthesized for solar cell applications. Different solubilizing side chains, including 2-ethylhexyl chains and n-octyl chains were attached to CPDT units, whereas 3,7-dimethyloctyl chains and n-octyl chains were anchored to the BTDI moieties. The impact of these substituents on the solubilities, molecular weights, optical and electrochemical properties, and thermal and structural properties of the resulting polymers was investigated. PCPDTDTBTDI-EH, DMO was synthesized via Suzuki polymerization, whereas PCPDTDTBTDI-8, DMO, and PCPDTDTBTDI-EH, 8 were prepared through direct arylation polymerization. PCPDTDTBTDI-8, DMO has the highest number average molecular weight (Mn = 17,400 g mol−1) among all polymers prepared. The PCPDTDTBTDI-8, DMO and PCPDTDTBTDI-8, 8 which have n-octyl substituents on their CPDT units have comparable optical band gaps (Eg ~ 1.3 eV), which are around 0.1 eV lower than PCPDTDTBTDI-EH, DMO analogues that have 2-ethylhexyl substituents on their CPDT units. The polymers have their HOMO levels between −5.10 and −5.22 eV with PCPDTDTBTDI-EH, DMO having the deepest highest occupied molecular orbital (HOMO) energy level. The lowest unoccupied molecular orbital (LUMO) levels of the polymers are between −3.4 and −3.5 eV. All polymers exhibit good thermal stability with decomposition temperatures surpassing 350 °C. Powder X-ray diffraction (XRD) studies have shown that all polymers have the amorphous nature in solid state.

Highlights

Cyclopenta [2,1-b:3,4-b0 ]dithiophene (CPDT) derivatives are analogous materials to fluorene derivatives, where two thiophenes rather than two phenyl groups are bridged by a carbon atom
The results indicate that substituting 2-ethylhexyl chains by n-octyl chains on CPDT units would lead to lower Eg of the polymers, while changing 3,7-dimethyloctyl chains by n-octyl chains on benzothiadiazole dicarboxylic imide (BTDI) moieties has a minimal effect on the Eg of the polymers
Four novel low band gap alternating copolymers including cyclopentadithiophene (CPDT) flanked by thienyl units as electron donor moieties and benzothiadiazole dicarboxylic imide (BTDI) as electron acceptor units were synthesized via two different palladium catalysed cross coupling polymerizations

Summary

Introduction

Cyclopenta [2,1-b:3,4-b0 ]dithiophene (CPDT) derivatives are analogous materials to fluorene derivatives, where two thiophenes rather than two phenyl groups are bridged by a carbon atom. PCPDTBT is an alternating copolymer comprising the CPDT as the donor moiety and 2,1,3-benzothiadiazole (BT) as an acceptor unit, which was first synthesized using Stille polymerization by Brabec et al and used in photovoltaic cells [4]. It has an optical band gap (Eg ) of around 1.4 eV with a broad absorption up to 850 nm.

Methods

Results

Conclusion