Elsevier

Polymer

Volume 51, Issue 2, 21 January 2010, Pages 390-396
Polymer

Low-bandgap poly(4H-cyclopenta[def]phenanthrene) derivatives with 4,7-dithienyl-2,1,3-benzothiadiazole unit for photovoltaic cells

https://doi.org/10.1016/j.polymer.2009.12.009Get rights and content

Abstract

A series of conjugated polymer bearing 4H-cyclopenta[def]phenanthrene (CPP) unit have been synthesized and was evaluated in bulk heterojunction solar cell. The alternating copolymers with CPP unit were incorporated with 4,7-dithienyl-2,1,3-benzothiadiazole (DTBT) unit by Suzuki conditions. The newly synthesized copolymers, poly(2,6-((4,4-bis(2-ethylhexyl)-4H-cyclopenta[def]phenanthrene))-alt-(4,7-((2-thienyl)-2,1,3-benzothiadiazole))) (PCPP-DTBT), and poly(2,6-(4,4-bis(4-((2-ethylhexyl)oxy)phenyl)-4H-cyclopenta[def]phenanthrene)-alt-(4,7-((2-thienyl)-2,1,3-benzothiadiazole))) (PBEHPCPP-DTBT), contain dialkyl and bis(alkoxyphenyl) groups in the CPP unit, respectively. The HOMO–LUMO energy bandgaps of these materials, estimated from UV–vis spectroscopy and cyclic voltammetry (CV), were 2.00 eV for PCPP-DTBT and 1.80 eV for PBEHPCPP-DTBT. Bulk heterojunction solar cells based on the blends of the polymers with [6,6]phenyl-C71-butyric acid methyl ester (PC71BM) gave power conversion efficiencies as 1.00% for PCPP-DTBT and 1.12% for PBEHPCPP-DTBT under AM 1.5, 100 mW/cm2.

Introduction

The most representative configuration of polymer solar cells is bulk heterojunction device which is composed of a blend of an electron-donating material (n-type), and an electron-accepting material (p-type) such as (6,6)-phenyl C61-butyric acid methyl ester or (6,6)-phenyl C71-butyric acid methyl ester (PCxBM). Many new conjugated polymers with a small band gaps are being researched for polymer solar cells (PSCs) to cover the long-wavelength region for the improvement of total photovoltaic current [1], [2], [3], [4], [5], [6].

Several examples have been known to show that utilization of the donor and acceptor functionalities in conjugate backbone of the alternative copolymer structures is an efficient way to decrease the band gaps. In the donor/acceptor (D–A) combinations, 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT) unit has been used effectively as an acceptor, and copolymerized with many kinds of donor segments, such as fullerene [7], [8], silafluorene [9], [10], carbazole [11], dithienosilole [12], and cyclopenta[2,1-b:3,4-b]dithiophene [13]. When these D–A types of copolymers were applied to photovoltaic solar cells (PSCs), the power conversion efficiencies (PCEs) in the range of 0.18%–5.4% have been reported [14], [15].

Recently, we reported new blue-emitting polymers, dialkyl substituted poly(2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[def]phenanthrene)) (PCPP) [16], [17], and bis(alkoxyphenyl) substituted poly(2,6-(4,4-bis(4-((2-ethylhexyl)oxy)phenyl)-4H-cyclopenta[def]phenanthrene)) (BEHPCPP) [18] with a rigid backbone, which generates stabilized and efficient blue electroluminescence without exhibiting any peak in the long-wavelength region even after prolonged annealing or operation of the devices in air. In addition to the stability of the CPP unit, it's combination with DTBT unit can generate the larger difference between the donor HOMO level and the acceptor LUMO level of the copolymer, which could be an essential key for the enhanced open circuit voltage (VOC) of the PSCs [19].

In this paper, we report the synthesis and photovoltaic properties of new CPP-based conjugated copolymers for solar cell, poly(2,6-((4,4-bis(2-ethylhexyl)-4H-cyclopenta[def]phenanthrene))-alt-(4,7-((2-thienyl)-2,1,3-benzothiadiazole))) (PCPP-DTBT), and poly(2,6-(4,4-bis(4-((2-ethylhexyl)oxy)phenyl)-4H-cyclopenta[def]phenanthrene)-alt-(4,7-((2-thienyl)-2,1,3-benzothiadiazole))) (PBEHPCPP-DTBT).

Section snippets

Instruments

IR spectra were recorded on a Perkin–Elmer 16F PC FTIR spectrometer with samples prepared as KBr pellet. 1H and 13C NMR spectra were recorded with a Varian Gemini-300 (300 MHz) spectrometer and chemical shifts were recorded in ppm units with TMS as the internal standard. Flash column chromatography was performed with Merck silica gel 60 (particle size 230–400 mesh ASTM) with ethyl acetate/hexane or methanol/methylene chloride gradients unless otherwise indicated. Analytical thin layer

Synthesis and characterization of polymers

The general synthetic routes towards the monomer and polymers are outlined in Scheme 1. Commercially available thiophene (1) was lithiated using n-buthyllithium and then treated with tributylchlorostanne to afford tributyl(2-thienyl)stannane (2), which was coupled with commercially available 4,7-dibromobenzo-2,1,3-thiadiazole (3) in THF by Stille coupling reaction to give 4,7-di-2-thienyl-2,1,3-benzothiadiazole (4). Compound 4 was brominated using N-bromosuccinimide to generate

Conclusion

In conclusion, the D–A type of alternating copolymers of PCPP-DTBT and PBEHPCPP-DTBT were synthesized by Suzuki coupling polymerization and characterized. The CPP unit was combined with DTBT unit to generate the larger difference between the donor HOMO level and the acceptor LUMO level of the copolymer, which could be an essential key for the enhanced open circuit voltage (VOC) of the PSCs. Both of conjugated polymers showed significant absorbance up to around 700 nm region. The HOMO–LUMO

Acknowledgement

This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2008-314-C00200).

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