Elsevier

Dyes and Pigments

Volume 119, August 2015, Pages 122-132
Dyes and Pigments

Small molecules containing rigidified thiophenes and a cyanopyridone acceptor unit for solution-processable bulk-heterojunction solar cells

https://doi.org/10.1016/j.dyepig.2015.03.028Get rights and content

Highlights

  • Use of rigidified thiophenes in conjunction with aromatizable cyanopyridone acceptor.

  • Linkage of rigidified thiophenes to aromatizable acceptor for spectral red-shift.

  • Two fused thiophenes display better film morphology comparing to three fused heterocycles incorporating thiophenes.

Abstract

We designed two solution processable small molecules, 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), consisting of a triphenylamine electron donating group and a cyanopyridone electron accepting group linked by two different rigidified π-spacer thiophenes. Ultraviolet–visible absorption spectra revealed that the use of three fused rings (dithienopyrrole) as the conjugated π-spacer resulted in an enhanced intramolecular charge transfer transition and reduction of band gap, when compared with a non-fused bithiophene and two fused thiophenes (thienothiophene) analogues. Power conversion efficiencies (PCEs) of 2.39% and 2.14% were achieved for simple photovoltaic devices based on 1/PC61BM and 2/PC61BM under simulated AM 1.5 illumination (100 mW cm−2), respectively. It has been observed that the degree of conjugation of the central π-bridge was not a key factor for the enhancement of photovoltaic performance.

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).

References (39)

  • J.E. Anthony

    Small-molecule, nonfullerene acceptors for polymer bulk heterojunction organic photovoltaics

    Chem Mater

    (2011)
  • A. Mishra et al.

    Small molecule organic semiconductors on the move: promises for future solar energy technology

    Angew Chem Int Ed

    (2012)
  • Y. Lin et al.

    Small molecule semiconductors for high-efficiency organic photovoltaics

    Chem Soc Rev

    (2012)
  • B.C. Thompson et al.

    Polymer–fullerene composite solar cells

    Angew Chem Int Ed

    (2008)
  • S. Günes et al.

    Conjugated polymer-based organic solar cells

    Chem Rev

    (2007)
  • G. Dennler et al.

    Polymer-fullerene bulk-heterojunction solar cells

    Adv Mater

    (2009)
  • J. Chen et al.

    Development of novel conjugated donor polymers for high-efficiency bulk-heterojunction photovoltaic devices

    Acc Chem Res

    (2009)
  • Y.-J. Cheng et al.

    Synthesis of conjugated polymers for organic solar cell applications

    Chem Rev

    (2009)
  • Y. Li et al.

    Conjugated polymer photovoltaic materials with broad absorption band and high charge carrier mobility

    Adv Mater

    (2008)
  • Cited by (0)

    1

    Tel.: +61 3 9545 2507.

    View full text