New organic sensitizers using 4-(cyanomethyl)benzoic acid as an acceptor group for dye-sensitized solar cell applications
Introduction
There is continuing interest in the development of renewable energy resources, with solar energy in particular, as an alternate to fossil fuels [1]. Among different solar cells using organic materials, dye-sensitized solar cells (DSSCs) have attracted considerable attention since they were first reported by Grätzel and co-workers [2]. Until recently, record-performing DSSCs relied on inorganic ruthenium (Ru)-based sensitizers to achieve energy conversion efficiencies over 10% [3]. A record high conversion efficiency of 13.0% based on zinc–porphyrin complexes has been reported by the Grätzel group [4]. However, research over the last decade has helped to narrow the performance gap between these Ru-polypyridyl compounds and metal-free organic sensitizers, yielding a new generation of highly efficient organic dyes [5], [6] and conversion efficiency surpassing 10% has been demonstrated for a metal-free sensitizer based DSSCs [7]. The key advantages of organic sensitizers include low cost facile synthesis, higher molar extinction coefficients and tunable absorption and electrochemical properties through suitable molecular design [8]. The majority of these organic sensitizers feature a series of common design elements, the most common being a donor–acceptor structure where an electron donating building block, such as a triarylamine is linked to an electron accepting group via an oligothiophene bridge. Such a push–pull type donor-(π-spacer)-acceptor (D-π-A) system enhances light harvesting at longer wavelengths due to the intramolecular charge transfer transition and small variations within these different sections cause significant differences in photovoltaic properties [9], [10], [11]. The electron acceptor moiety in D-π-A modular organic sensitizers plays a crucial role as the dye attaches to the titania semiconductor surface through the acceptor unit and the use of cyanoacrylic acid as an acceptor functionality has been studied in particular. This is highlighted by a review of Peter Bäuerle and co-workers [5], where in excess of 95% of all dye structures discussed feature a cyanoacrylic acid group. While a vast number of novel D-A dyes have been reported, efforts towards the development of alternative attachment groups have been very limited [12], [13], [14].
In this report we use two design concepts for these D-π-A organic dyes and study their effect on the optical and photovoltaic properties, when applied to a reference dye structure. The first concept extends the use of 4-(cyanomethyl)benzoic acid as an attachment group [15] while the second modification involves the replacement of the bridging para-phenyl group in the triaryl donor unit with a thiophene group [16], [17], [18]. The use of 4-(cyanomethyl)benzoic acid acceptor may provide better performance as it can exert a red-shift of the absorption spectrum due to the extension of the π-electron system. Replacement of the phenyl group with thiophene can further show significant spectral red-shift, higher extinction coefficient and narrower optical band gaps [19]. Additionally, the lower aromaticity of thiophene compared to phenyl can result in superior charge delocalization across the chromophore [20], [21]. Therefore, with the combined use of directly linked oligothiophenes and 4-(cyanomethyl)benzoic acid acceptor group, we can expect superior DSSC performance. It is important to mention that this work is the first examination of these two concepts that have been applied collectively. Fig. 1 shows the dye structures investigated in this work.
Section snippets
Materials
All reagents and chemicals used, unless otherwise specified, were purchased from Sigma-Aldrich Co. The solvents used for reactions were obtained from Merck Specialty Chemicals (Sydney, Australia) and were used as received.
Spectroscopic measurements
Unless otherwise specified, all 1H and 13C NMR spectra were recorded using a Bruker AV400 spectrometer at 400 MHz and 100.6 MHz, respectively, or a Bruker AV200 spectrometer at 200 MHz and 50 MHz, respectively. Chemical shifts (δ) are measured in parts per million. Thin Layer
Design concept and materials characterization
The aim of this study was to investigate the use of 4-(cyanomethyl)benzoic acid acceptor group in-conjunction with oligothiophene π-spacers. This investigation will help to explore the range of central π-spacers, such as oligothiophenes (this study) and bridged thiophenes (previous study [15]), to be studied with 4-(cyanomethyl)benzoic acid acceptor functionality. It was further thought to carry out a side by side comparison by replacing the bridging para-phenyl group with thiophene
Conclusions
In conclusion, we have demonstrated that incorporating both the design features, (1) the replacement of the conventional cyanoacrylic acid group with a new 4-(cyanomethyl)benzoic acid acceptor group and (2) the replacement of the bridging para-phenyl group in the triaryl donor unit with a thiophene group, in a novel DSSC sensitizer result in superior light-harvesting, enhanced photocurrent density and overall device performance, and longer electron lifetimes for nitrile-based and ionic
Acknowledgements
A.G. would like to acknowledge the Flexible Electronics Theme of CSIRO Future Manufacturing Flagship and Monash University, Clayton 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. A.B. thanks Commonwealth Scientific and Industrial Research Organisation for support through the Julius Career Award. The use of the NCI National Facility supercomputers at
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