Elsevier

Tetrahedron

Volume 70, Issue 12, 25 March 2014, Pages 2141-2150
Tetrahedron

N-Alkyl- and N-aryl-dithieno[3,2-b:2′,3′-d]pyrrole-containing organic dyes for efficient dye-sensitized solar cells

https://doi.org/10.1016/j.tet.2014.02.002Get rights and content

Abstract

Two new organic sensitizers, 2-cyano-3-(6-(4-(diphenylamino)phenyl)-4-(2-ethylhexyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-yl)acrylic acid and 2-cyano-3-(6-(4-(diphenylamino)phenyl)-4-(4-(hexyloxy)phenyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-yl)acrylic acid, consisting of electron donating (triphenylamine) and electron accepting (cyanoacrylic acid) functionalities linked by two different rigidified π-spacers, N-alkyl- and N-aryl-dithieno[3,2-b:2′,3′-d]pyrrole, were designed, synthesized and applied for dye-sensitized solar cells, respectively. The materials were successfully synthesized through Knoevenagel condensation reactions. Ultraviolet–visible absorption spectra revealed that the use of either of rigidified π-spacer resulted in similar charge transfer transition, however, enhanced spectral response was observed when compared with an oligothiophene analogue. In terms of their photovoltaic performance, new dyes outperformed the reference bithiophene sensitizer when tested with nitrile-based and ionic liquid-based electrolytes.

Graphical abstract

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Two new organic sensitizers, 1 and 2, consisting of an electron donating (triphenylamine) and an electron accepting (cyanoacrylic acid) functionality linked by two different rigidified π-spacers were applied for dye-sensitized solar cells. UV–Vis absorption spectra of 1 and 2 revealed enhanced spectral response when compared with an oligothiophene analogue, and they yielded 6.1% and 6.2% power conversion efficiencies, respectively.

Introduction

Over the past two decades, dye-sensitized solar cells (DSCs)1 have been the target of intensive research due to their capability to convert sunlight to electricity and a certified conversion efficiency of 11.1% has been reported by Y. Chiba et al.2 One of the key material challenges in the DSC research is the development of efficient photosensitizer that can be produced at low cost. Traditionally, functional ruthenium(II)–polypyridyl complexes such as dye N3 [Ru{(4,4′-CO2H)2bipy}2(NCS)2] and the doubly deprotonated analogue of N3, dye N719 [(Bu4N)2[Ru{(4,4′-CO2H)2bipy}2(NCS)2]],3 developed by Grätzel et al. are the most successful dyes with promising device characteristics. However, such complexes contain expensive ruthenium metal and require complex synthesis and tricky purification steps. On the other hand, continuous efforts for the design and development of organic sensitizers have helped to gradually narrow the performance gap between organic and ruthenium-based sensitizers. Metal-free organic donor–acceptor dyes have achieved conversion efficiencies over 10% and the porphyrin sensitizers rival the ruthenium-based sensitizers with conversion efficiencies beyond 12%,4 thus paying tributes to the advancement recently achieved in this field.

Key advantages with organic sensitizers (dyes) are that they have facile synthetic strategies and high absorption coefficients when compared to the dominant ruthenium-based dyes. One successful strategy to improve the performance of DSCs has been the design of typical ‘donor–acceptor’ (D–π-A) modular organic dyes featuring triarylamine or its subsequent derivatives as donor fragments and cyanoacrylic acid as an acceptor fragment through the use of a central π-conjugated linker. This strategy with the variation of π-conjugated segments has attracted much attention and has led to a huge array of dyes that display promising energy conversion efficiencies.1(d), 5 Such D–π-A modular structures allow intramolecular charge transfer (ICT) transitions that broaden the absorption spectrum and narrow the optical band gap. There is, therefore, considerable synthetic interest in exploring new π-conjugated linkers and in making molecules for efficient DSCs. Reports of DSC devices using such π-conjugated linkers have recently emerged that have utilized thieno[3,2-b]thiophene,6 benzodiathiazole,7 3,4-ethylenedioxythiophene,8 dithieno[3,2-b:2′,3′-d]thiophene,9 dithieno[3,2-b:2′,3′-d]pyrrole,10 4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b’]dithiophene,11 phenoxazine12 and fused bis-thiazole.13 While this progress is encouraging, considerable scope still exists to develop new sensitizers that possess broad and efficient optical absorption, narrow band-gaps and adequate solubility for DSC fabrication.

We are also interested in exploring the D–π-A module for organic sensitizers and in this paper, we report the facile synthesis and characterization of the optical, electrochemical and photovoltaic properties of two new organic sensitizers 2-cyano-3-(6-(4-(diphenylamino)phenyl)-4-(2-ethylhexyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-yl)acrylic acid (coded as 1) and 2-cyano-3-(6-(4-(diphenylamino)phenyl)-4-(4-(hexyloxy)phenyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-yl)acrylic acid (coded as 2). These organic dyes feature N-alkyl- and N-aryl-dithieno[3,2-b:2′,3′-d]pyrrole derivates and were synthesized via the Knoevenagel condensation of an appropriate aldehyde with active methylene group of the cyanoacrylic acid acceptor unit and their chemical structures were confirmed by 1H NMR spectroscopy and mass spectrometry. We further set out to examine the effect of changing the chemistry of the central part of molecule will have on the DSC performance. New dyes 1 and 2 were synthesized along with a reference dye (3) such that all the materials have triphenylamine as a common donor and cyanoacrylic acid as an acceptor component. It is anticipated that these small variations within the structure of a target material can cause significant differences in photovoltaic properties.14, 15, 16 All the molecular structures are shown in Fig. 1.

Section snippets

Synthesis and characterization

The new materials were designed by using dithieno[3,2-b:2′,3′-d]pyrrole as a central π-spacer, where alkyl and aryl substitutions were allocated on the nitrogen atom of pyrrole ring, thus generating 1 and 2, respectively. Dyes 1 and 2 were synthesized by reacting the aldehyde precursors, 6-(4-(diphenylamino)phenyl-4-(2-ethylhexyl))-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-carbaldehyde and 6-(4-(diphenylamino)phenyl-4-(4-(hexyloxy)phenyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-carbaldehyde, at reflux with

Conclusions

We have reported the design, synthesis and characterization of two new organic dyes, 1 and 2, for use as photosensitizers in DSC devices. The dyes are bestowed with N-alkyl- and N-aryl-dithieno[3,2-b:2′,3′-d]pyrrole units (1 and 2, respectively) as central π-spacers, of which latter is the first example of such modification. Synthesis of 1 and 2 has been achieved in good yields and the dyes have been analytically characterized. On the basis of UV–vis absorption and cyclic voltammetry

Reagents and methods

All the reagents and chemicals used, unless otherwise specified, were purchased from Sigma–Aldrich Co. The solvents used for various reactions were obtained from Merck Speciality Chemicals (Sydney, Australia) and were used as such. 4-(2-Ethylhexyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrole was purchased from Luminescence Technology Corporation, Taiwan and was used as such. Unless otherwise specified, all 1H and 13C NMR spectra were recorded using a Bruker AV400 spectrometer at 400 MHz and 100.6 MHz,

Acknowledgements

A.G. would like to acknowledge the Flexible Electronics Theme of CSIRO Future Manufacturing Flagship, Monash University, Clayton and RMIT University, Melbourne Australia for providing equipment support. A.G. would further like to thank Dr. Jo Cosgriff and Dr. Carl Braybrook from Materials Science and Engineering, CSIRO Clayton VIC 3169 for MS analysis. S.V.B. acknowledges financial support from the Australian Research Council under a Future Fellowship Scheme (FT110100152) and the School of

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    Present address: CSIRO Materials Science and Engineering, Bag 10, Clayton, 3169 Victoria, Australia.

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