Improvement of the optoelectronic and photovoltaic properties of a cyanopyrid-2,6-dione-based donor via molecular engineering
Graphical abstract
Introduction
The growing demand for clean energy to address serious environmental issues means technologies such as organic solar cells, including dye-sensitized solar cells, have attracted considerable attention [1,2]. Organic, otherwise termed bulk hetero-junction (BHJ), solar cells promise unprecedented advantages over their inorganic counterparts, such as light weight, low-cost and device flexibility. As an important part of BHJ solar cell development, immense efforts have been devoted to the small molecule-based organic solar cells (OSCs), primarily due to their well-defined molecular weight, high purity and the reproducibility of synthetic procedures without batch-to-batch variation [[3], [4], [5], [6]]. At present, impressive power conversion efficiencies (PCEs) > 10% have been reported for small molecule-based organic and tandem solar cells [[7], [8], [9]]. Though this achievement is highly encouraging, the performance of these small molecule-based OSCs still trail polymer-based solar cells, and even their inorganic counterparts. Therefore, further development of small molecules is required to enhance the photovoltaic performance within these classes of solar cells, primarily through a greater understanding of the relationship between molecular structure and properties.
It has been established in previous work that in order to develop highly efficient small molecule donors that can compete with polymeric counterparts, there are some design requirements that must be met [10,11]. The design requirements include a low optical band gap, broad absorption profile, high mobility, and multiple reversible redox potentials. Also, appropriate highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels that can complement those of the standard soluble fullerene acceptors [6,6],-phenyl-C61-butyric acid methyl ester (PC61BM) and its C71 analogue (PC71BM), are required. It has been observed that low band gap materials can be generated by incorporating an electron donor–π-bridge–acceptor (D–π–A or D–A) motif within the structure [[12], [13], [14], [15], [16], [17]]. These D and A units together with the π-bridge in the molecular backbone can be used to fine-tune absorption, energy levels, and photovoltaic performance. Such a D–π–A format allows intramolecular charge transfer (ICT) transitions and offers insight into relationships between structure and performance.
Although there are several mentions of D–π–A design in the literature, this configuration has been somewhat overlooked by the wider research community, which is surprising considering the gains that can be achieved with a relatively small amount of effort. A recent review by X. He et al. indicates a number of examples based on D–π–A design where the efficiencies in the excess of 5% are reported [18]. Given the reported literature based on D–π–A formatted small molecule donors [10,11,18], it is evident that the use of target materials based on cyanopyrid-2,6-dione acceptor unit is confined, thus, providing a strong incentive for its exploration. That being said, the D–π–A class of materials has much potential as they (1) can be synthesized using well established chemical reactions, (2) offer an insight into structure-property relationship, (3) provide flexibility of the structural format, and (4) present a vast variety of building blocks to be explored. Moreover, they offer a great potential to apply subtle changes to a given chemical structure so as to allow tuning of optoelectronic and photovoltaic properties.
The target materials based on the D–π–A format use a simple donor moiety, such as triphenylamine (TPA) or carbazole, with an electron withdrawing subunit, such as cyanopyrid-2,6-dione, attached to the other side of the molecule. Research into the structure of such compounds indicates that this D–A or push–pull design causes an increase in both the ICT transition as well as the open circuit voltage [19]. Reports in the literature further suggest that alterations in the properties of the terminal acceptor unit, such as size, type and accepting strength, can enhance absorption, ICT and hole-mobility, and alignment of the HOMO and the LUMO energy levels, increasing the overall power conversion efficiency (PCE) within a BHJ device. With such criteria in mind, and learnings from the work reported by others and us [[20], [21], [22], [23]], we thought to expand our knowledge by evaluating further the D–π–A format. We and others have shown in the literature that small molecule donor and acceptor chromophores based on a variety of formats, such as D–π–A and A–A–A, to name two, comprising commonly named cyanopyridone acceptor unit has a positive effect on the optoelectronic properties and performance of photovoltaic devices [[24], [25], [26], [27], [28], [29], [30], [31]]. Furthermore, an additional advantage of the cyanopyrid-2,6-dione acceptor unit is that a variety of alkyl/aryl groups can be substituted on the nitrogen atom so as to tune solubility of the target materials. With this in mind, we have used 4-hexyloxyaniline to synthesize a cyanopyrid-2,6-dione unit and generate our target donor, (Z)-5-((5'-(4-(diphenylamino)phenyl)-[2,2′-bithiophen]-5-yl)methylene)-1-(4-(hexyloxy)phenyl)-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile (coded as CP2), shown in Fig. 1. We have also synthesized a structural analogue of CP2, namely, (E)-5-((5'-(4-(diphenylamino)phenyl)-[2,2′-bithiophen]-5-yl)methylene)-1-(2-ethylhexyl)-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile (coded as CP1), where we have used 2-ethylhexylamine to synthesize the cyanopyrid-2,6-dione unit. It is important to note that the design concept of CP2 was greatly motivated by our interest in generating a small molecule electron donor with improved optoelectronic properties, tuned energy levels and superior solubility when compared with its structural analogue. Furthermore, it should display thermal stability, and miscibility with an acceptor counterpart such as PC61BM.
Compound CP2 was synthesized in a straightforward manner with commercially available substrates via the Knoevenagel condensation reaction of 5'-(4-(diphenylamino)phenyl)-[2,2′-bithiophene]-5-carbaldehyde with the active methylene group of the cyanopyrid-2,6-dione acceptor unit (Scheme 1), and its chemical structure was confirmed by 1H and 13C NMR spectroscopies, and mass spectrometry [for experimental spectra, see Supplementary Information (SI)]. Of particular note is the fact that the synthesis of CP2 did not need extensive column purification, a convenience that is consistent with our previous study where we reported similar materials comprising cyanopyrid-2,6-dione functionality [24]. This result is of great importance for cyanopyrid-2,6-dione-based donor molecules where scale-up is required for large area applications. The newly engineered CP2 demonstrated a better positioning of energy levels complementing those of the conventional acceptor PC61BM. The photovoltaic performance of the BHJ device based on a blend of CP2 and PC61BM (1: 1, w/w) exhibited an impressive power conversion efficiency (PCE) of 6.28%, which is more than twice the efficiency of the best CP1-based device (2.92%), with a short circuit current density (Jsc) of 12.81 mA/cm2, and a Voc of 0.98 V under the illumination of AM.1.5, 100 mW/cm2. Not only is CP2 the first reported example in the literature where a cyanopyrid-2,6-dione acceptor unit has been engineered, but the device performance is amongst the highest PCE numbers for solution-processable, D–A formatted donors based on cyanopyrid-2,6-dione.
Section snippets
Materials and methods
All the reactions were carried out under inert atmosphere, unless otherwise stated. Solvents used for various reactions were dried using a commercial solvent purification/drying system. Solvents used for extractions and column chromatography, and all other reagents were used as supplied by commercial vendors without further purifications or drying.
Thin layer chromatography (TLC) was performed using 0.25 mm thick plates pre-coated with Merck Kieselgel 60 F254 silica gel, and visualized using UV
Optical and electrochemical properties
The solution UV–Vis spectra of both CP1 and CP2 were measured in chloroform. Dye CP2 had a higher molar extinction coefficient (ε) and absorption maximum (λmax) when compared with CP1 [ε (CP2) = 70,067 L M−1 cm−1; ε (CP1) = 58,950 L M−1 cm−1]. Typically, CP2 gave a strong λmax at 607 nm, whereas the λmax for CP1 was at 592 nm. Of particular relevance, we observed a similar bathochromic absorption shift in the thin film spectrum of CP2 relative to CP1 (see Fig. 2 for film and Fig. S1 (SI) for
Conclusions
In conclusion, we have demonstrated that molecular engineering of cyanopyrid-2,6-dione acceptor unit is of great importance to improve the performance of small molecule donors in organic photovoltaic devices. In a direct comparison we have shown that the use of 4-hexyloxyaniline rather than 2-ethylhexylamine helps to generate a target (1) with improved optoelectronic properties, (2) having greater miscibility with the acceptor counterpart so as to have favourable blend morphology, and (3)
Acknowledgements
The CSIRO Division of Manufacturing, Clayton, Victoria Australia is acknowledged for providing support through a Visiting Scientist position for A.G. A.K.H., A.A., J.Y.C. and A.G. acknowledge the vast variety of facilities at Swinburne, Deakin and RMIT Universities, and CSIRO Manufacturing, Clayton, Victoria Australia. A.G. acknowledges the assistance of Dr J. Subbiah at the University of Melbourne, Parkville, Australia. M.J. and A.G. acknowledge the supercomputer support from Swinburne
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A.K.H. and A.A. contributed equally to this work.