Synthesis, characterization and photovoltaic properties of poly(cyclopentadithiophene-alt-isoindigo)
Journal
Polymer Chemistry
Journal Volume
4
Journal Issue
20
Pages
5351-5360
Date Issued
2013
Author(s)
Abstract
Isoindigo based conducting polymers have attracted extensive interest for polymer solar cell application since isoindigo is a green material and renewable from plants. We have synthesized four soluble low band gap isoindigo based polymers (PCI) with cyclopentadithiophene (CPDT) as the donor unit and isoindigo (I) as the acceptor unit, decorated with two kinds of alkyl side chains, octyl (8) and 2-ethylhexyl (e), via the Stille cross-coupling reaction denoted as PC8I8, PC8Ie, PCeI8 and PCeIe. By changing the side chain of copolymers from linear (PC8I8) to branched (PCeIe), the λmax of absorption is blue shifted from 1.37 to 1.48 eV and the HOMO level is lowered from -5.24 to -5.45 eV. The changes are due to the twist coplanarity of the polymer backbone. The density functional theory calculation revealed that the dihedral angle of copolymers has been increased from 14° to 20°. The properties of PC8Ie and PCeI8 lie between those of PC8I8 and PCeIe. The type of the side chain plays a major role in determining the photovoltaic performance of copolymers. The branched side chain improves the solubility of the polymer and increases the effective phase separation between the copolymer and PCBM. This results in favorable nanomorphology of the active layer. Thus, PCeIe with branched side chains on both donor and acceptor units exhibits the best photovoltaic properties with a Voc of 0.80 eV, Jsc of 11.6 mA cm -2 and fill factor of 43.0% and power conversion efficiency of 4.0%. The power conversion efficiency of this type of polymer could be further improved by optimizing the fabrication conditions and interlayer modification. This study offers a useful guideline for the molecular design of high efficiency isoindigo-based polymer solar cells. © 2013 The Royal Society of Chemistry.
SDGs
Other Subjects
Conducting polymers; Conversion efficiency; Copolymers; Nanostructured materials; Phase separation; Solar cells; Branched side chains; Donor and acceptor; Photovoltaic performance; Photovoltaic property; Polymer backbones; Polymer Solar Cells; Power conversion efficiencies; Stille cross-coupling; Chains
Type
journal article