Influence of the Electron Acceptor Group on the Backbone of N-(2,6-Diisopropylphenyl)-2,5-di(2-thienyl)pyrrole-Based Polymer

Abstract

Development of structurally new polymers for polymer solar cell (PSC) applications is an attractive research area due to their beneficial properties such as clean energy production, low-cost, light-weight, and roll-to-roll processing on flexible substrates.1 The power conversion efficiency (PCE) of the PSCs was finally improved to 7.0-8.6% by utilizing suitable polymers as electron donor materials, that have the appropriate energy levels with efficient light harvesting ability, and PC 71 BM as an electron accepting material.2-8 In our laboratory, we have been interested in preparing N-(2,6-diisopropylphenyl)-2,5-di(2-thienyl)pyrrole (N-aryl TPT)-based π-conjugated polymers as electron donor materials for PSC applications. Recently, we reported several N-aryl TPT-based polymers containing electron rich units such as indenofluorene (PIFTPT), 9 2,5-dioctyloxyphenylene (PPTPT), 10 3-octylthiophene (PTTPT), 10 and electron deficient units such as thiophene-benzothiadiazolethiophene (PTPTTBT), 11 benzothiadiazole (PTPTIFB and TPyTBz), 12,13 and thiadiazoloquinoxaline (TPyTDzQ) 14 for PSC applications. The N-aryl TPT-based donor (D) polymers showed maximum energy conversion efficiency of 1.35%, 10 whereas the donor-acceptor (D-A) and donor-acceptor-donor (D-A-D) polymers showed maximum PCE of 1.57%.11 The comparison studies of the N-aryl TPT-based polymers indicated that the incorporation of strong electron acceptor unit in polymer main chain significantly broadens the absorption band of the polymers and usually raises the HOMO energy levels of the polymers. Consequently, the PSCs made from the D-A or D-A-D polymers showed improved current density (J sc ) and decreased open circuit voltage (V oc ) with similar or slightly higher PCEs compared with N-aryl TPTbased donor polymers. In general, the V oc is defined as the energy difference between the valance and conduction band energy levels of the polymer and PCBM.15 In this instance, we anticipate that introducing a strong electron acceptor unit on N-aryl TPT polymer backbone might offer better photovoltaic results because the incorporation of electron acceptor group on the polymer backbone usually lower its HOMO energy level. 4,5 In this study, we wish to introduce a methylene bridged malononitrile electron acceptor group on the pyrrole ring of the known N-aryl TPT-based polymer, PIFTPT, to afford new π-conjugated polymer PIFTPCNT (Figure 1).Polymer PIFTPCNT was prepared via the procedure shown in Scheme 1. Compound 1, 2,5-bis(5-bromothiophen-2-yl)-1-(2,6-diisopropylphenyl)-1H-pyrrole, was synthesized via the known procedure.9 Compound 1 was subjected to Vilsmeier formylation reaction with DMF and POCl 3 to afford compound 2. The Knoevenagel condensation reaction of compound 2 with malononitrile in the presence of piperidine yielded monomer 3. Among the N-aryl TPT-based polymers, polymer PIFTPT showed higher V oc and FF values than other polymers. From these results, we expect that the presence of indenofluorene unit in polymer main chain enhances the carrier mobility a...

Modulation of the properties of pyrrolo[3,4-c]pyrrole-1,4-dione based polymers containing 2,5-di(2-thienyl)pyrrole derivatives with different substitutions on the pyrrole unit

“…Similar trends have also been observed for TPTbased polymers and organic dyes containing substituents on the pyrrole backbone of the TPT unit. [36][37][38] Optical studies of the This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015 new polymers suggest that the incorporation of electron withdrawing substituents such as, ester, amide and imide groups into the TPT unit alter the absorption of the final polymers significantly. The absorption maxima and optical band gaps of the polymers are summarized in Table 1.…”