Small molecules containing rigidified thiophenes and a cyanopyridone acceptor unit for solution-processable bulk-heterojunction solar cells
Graphical abstract
Introduction
Bulk heterojunction (BHJ) organic solar cells have attracted a great deal of attention over the past two decades because of their promise of low-cost fabrication, ease of solution processability, light weight and potential application in flexible, large-area devices [1]. BHJ solar cells are comprised of an interpenetrating network of organic donor and acceptor domains, which is formed during their fabrication via solution processing. Traditionally, semiconducting polymers such as poly(3-hexylthiophene) (P3HT) as an electron donor material and a soluble fullerene derivative such as [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as an electron acceptor material have been used [2], [3], [4], [5]. However, recent reports of improved BHJ device performance as a result of using small molecules have attracted much attention [6], [7], [8], [9], [10]. Solution processable small molecules have a number of potential advantages over polymers such as high absorption coefficients, well defined structures, purification, high chemical stability and relatively straight-forward synthetic strategies. However, in common with polymeric structures, the design requirements of small molecules include a low optical band gap, broad absorption profile, high mobility, multiple reversible redox potentials and appropriate highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. It has been established that low band gap materials can be generated by incorporating an electron donor–π-bridge–electron acceptor (D–π–A or D–A) motif within the structure [11], [12], [13], [14], [15], [16].
Reports of BHJ devices using small organic molecules as donor components have recently emerged that have utilized squaraine [17], oligothiophene [18], merocyanine dyes [19], [20], diketopyrrolopyrrole derivatives [21], dibenzochrysene [22], and push–pull organic dyes [23], [24], [25]. Progress in the field has resulted in devices exhibiting PCEs in the excess of 9% [26], [27], [28], [29], [30]. While this progress is encouraging, considerable scope still exists to develop novel light-harvesting materials that possess broad and efficient optical absorption, deep HOMO energy levels (−5.0 to −5.5 eV) and adequate solubility for thin film formation [9], [10]. One successful strategy is the exploration of donor-acceptor molecules with a conjugated π-bridge. Such structures allow intramolecular charge transfer (ICT) transitions that broaden the absorption spectrum and narrow the optical band gap.
In our own studies of small molecule chromophores/charge transport materials based on a D–A design, we have previously shown that the use of a cyanopyridone acceptor fragment has a positive effect on the optoelectronic properties and material performance in devices [24], [31]. However, while the reference dye R1 (see Fig. 1) has almost ideal energy levels, according to the analysis by Brabec et al. [32], devices based on R1 do not give the predicted power conversion efficiencies of 8%. This is due to low currents and low fill factors. We therefore set out to examine what is the effect of changing the chemistry of the central part of the molecule on their OPV performance. We have synthesized two examples 5-((5-(4-(diphenylamino)phenyl)thieno[3,2-b]thiophene-2-yl)methylene)-1-(2-ethylhexyl)-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile (1) and 5-((6-(4-(diphenylamino)phenyl)-4-(2-ethylhexyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-yl)methylene)-1-(2-ethylhexyl)-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile (2) along with the reference dye (R1) such that all the materials have triphenylamine as a common donor and cyanopyridone as an acceptor fragment. All the molecular structures are shown in Fig. 1. In the reference dye, a non-fused bithiophene was used as the π-bridge and in this work, we examined the use of two fused thiophenes (thienothiophene) (target compound 1) and three fused rings (dithienopyrrole) (target compound 2) as alternative central cores. Our aim was to increase the absorbance of the compound at higher energy (fill in the gap around 400 nm) and we hypothesised that using more rigid cores might enhance the intermolecular interactions and therefore the fill factor of OPV devices. We know that thin films based on R1 are largely amorphous and that device performance is not improved by thermal annealing [24]. By introducing more rigid groups, we hoped to improve charge transport.
In this paper, we report the facile synthesis and characterization of the optical, electrochemical and photovoltaic properties of small molecules 1 and 2. The materials 1 and 2 were synthesized via the Knoevenagel condensation of the appropriate aldehyde with the active methylene group of the cyanopyridone acceptor and their chemical structures were confirmed by 1H NMR spectroscopy and mass spectrometry. Knoevenagel condensation of the aldehyde groups present on the oligothiophenes is an efficient way of generating a double bond between the π-bridge and an acceptor unit. The use of such condensations is a common strategy to generate metal-free organic dyes for dye-sensitized solar cells [33]. However, the use of the same strategy to develop materials for BHJ photovoltaic devices has been limited [9], [10]. In this report, we demonstrate the Knoevenagel condensation reaction between the aldehyde group present on a rigidified spacer and an aromatizable acceptor. To the best of our knowledge this is the first time that rigidified thiophenes have been used in conjunction with the aromatizable cyanopyridone acceptor.
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 Speciality Chemicals (Sydney, Australia) and were used as received. 1-(2-ethylhexyl)-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile and 6-(4-(diphenylamino)phenyl)-4-(2-ethylhexyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-carbaldehyde were synthesized as per our previous reports [24], [34].
Spectroscopic measurements
Unless otherwise specified, all 1H and 13C
Synthesis and characterization
The materials 1 and 2 were synthesized by reacting the aldehyde precursors, 5-(4-(diphenylamino)phenyl)thieno[3,2-b]thiophene-2-carbaldehyde and 6-(4-(diphenylamino)phenyl)-4-(2-ethylhexyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-carbaldehyde, at reflux with 1-(2-ethylhexyl)-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile in chloroform, respectively, in the presence of pyridine as a base. Both the materials were purified through column chromatography. These solids were characterized
Conclusions
In conclusion, we have demonstrated the use of D–π–A small molecules containing rigidified π-spacer thiophenes in organic solar cells. The new materials 1 and 2 using thienothiophene and dithienopyrrole respectively as π-spacers were soluble in common organic solvents and were applied as p-type semiconducting components along with the n-type material PC61BM in BHJ photovoltaic devices. By comparison with a reference dye using bithiophene as the π-spacer (R1), compound 1 shows a larger band gap,
Acknowledgements
This research was funded through the Flexible Electronics Theme of the CSIRO Future Manufacturing Flagship and was also supported by the Victorian Organic Solar Cell Consortium (Victorian Department of Primary Industries, Sustainable Energy Research and Development Grant, Victorian Department of Business and Innovation, Victoria's Science Agenda Grant and the Australian Renewable Energy Agency (ARENA)) and the Australian Centre for Advanced Photovoltaics (ACAP).
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