Tailoring dual-channel anchorable organic sensitizers with indolo[2,3-b]quinoxaline moieties: Correlation between structure and DSSC performance
Graphical abstract
Introduction
To deal with the global energy crisis, the most abundant renewable resource is provided by the utilization of solar energy (Lewis and Nocera, 2006). In 1991, the successful breakthrough of dye-sensitized solar cells (DSSCs was published by O’Regan and Grätzel, 1991), indicating that photovoltaic technologies have encountered the boosting of their promotion with decent photon-to-electric conversion efficiencies and their intrinsic features have been systematically studied (Hagfeldt et al., 2010, Gonçalves et al., 2008, Yen et al., 2012, Yun et al., 2018). As the alternative of conventional silicon-based solar cells, one of the highlighted advantages of DSSCs is its superior performance under diffused light conditions (Yen et al., 2012). The sensitizer is the most strategic component for the improvement of DSSC performance highly due to its function of harvesting photons and its representation of the first device interface to solar radiation (Ragoussi and Torres, 2015). Metal-based sensitizers, particularly ruthenium complexes, are still taking a large portion of market share in the area of commercialized photovoltaic devices, including N3, N719 and Z907 with optimum efficiencies exceeding 10% (Nazeeruddin et al., 1993, Nazeeruddin et al., 1999, Wang et al., 2003). Our research group has also thoroughly studied Ru(II) sensitizers for many years with the optimization of the ancillary ligands (El-Shafei et al., 2012, Hussain et al., 2013, Cheema et al., 2014, Su et al., 2017, Ashraf et al., 2018, Su et al., 2019, Ashraf et al., 2019). However, their development ran into a bottleneck with several issues, such as scarcity of noble metal and lengthy purification steps (Pashaei et al., 2016). Alternatively, metal-free organic sensitizers exhibit a number of tactical advantages over their organometallic counterparts including low cost, facile synthesis, tunable absorption spectral response and excellent light-harvesting prowess (Yum et al., 2011, Kim et al., 2013, Wu and Zhu, 2013, Freitag et al., 2017). It is reported that DSSCs employing metal-free dyes have reached the efficiency of 13.0% (Yao et al., 2015). In 2017, the DSSC performance has even leaped to 28.9% under ambient lighting with two judiciously designed metal-free dyes (Freitag et al., 2017).
Since the structures of organic sensitizers provide many design probabilities for modulating their photophysical properties, one feasible constructing approach is to employ fused aromatic heteroarenes as the principal chromophore and then continuously connect supplementary functional segments onto the scaffold via carbon–carbon coupling (Wu and Zhu, 2013). Among published heterocycles, the nitrogen-containing heterocyclic quinoxaline has been widely utilized in creating and synthesizing organic sensitizers (Pei et al., 2012, Pei et al., 2013, Yang et al., 2014, Li et al., 2015, Lyu et al., 2019). Owing to its strong electron withdrawing effect attained from two symmetric unsaturated nitrogen atoms (Cheng et al., 2011), quinoxaline can also be treated as an auxiliary acceptor possessing both electron-transporting and electron-accepting characteristics. Some representative quinoxaline-based organic sensitizers from previously publications were presented in Table 1. In 2018, by exploiting a unique solidification method, the efficiency of solid-state DSSCs based on quinoxaline-derived sensitizers gained the efficiency of 11.7%, which is the current highest efficiency achieved for solid-state DSSCs (Zhang et al., 2018). To further rigidify the conjugating system and enhance the photostability, quinoxaline has been developed into coplanar polycyclic aromatic hydrocarbon (PAH) core such as dithieno[3,2-f:2′,3′-h]quinoxaline (Ni et al., 2015) and pyrido[3,4-b]pyrazine (Zhang et al., 2015). Indolo[2,3-b]quinoxaline (IQ) is another analog derived from quinoxaline by fusing an indole part inside. Risko and Welch synthesized an unsymmetrical non-fullerene acceptor where the use of IQ block as the terminus offered unprecedented ‘red-shifts’ in the absorption (Payne et al., 2017); Hou and co-workers employed IQ as the terminal donor and combined it with phenothiazine as QX07 to achieve the efficiency of 8.28% in DSSCs (Qian et al., 2016); They also produced a series of T-shaped IQ-based sensitizers in which QX23 incorporating a furan as π-bridge and a cyanoacrylic acid as anchoring group gave the efficiency of 7.09% (Qian et al., 2017). The applications of IQ-based dyes in DSSCs including solvent effects (Barati-darband et al., 2019) and anchoring group and π‑spacer effects (Barati-darband et al., 2018) were theoretically investigated by Izadyar’s research group. The triune functions of IQ, i.e., electron donating, electron withdrawing and electron transporting effect, still need to be further studied.
Another feasible design strategy is to manipulate the anchoring groups, including the replacement of different types of anchors, such as cyanoacetic acid, 1-phenyl-pyrazol-5-one-3-carboxylic acid (Abdellah et al., 2019), and rhodanine-3-acetic acid (Elmorsy et al., 2018), and also the design of multi-branched multi-anchoring dyes (Manfredi et al., 2014). The anchor, which is considered as transfer station by guaranteeing efficient charge injection, has a substantial influence on the affinity of the dye on TiO2 via covalent bonding (Zhang and Cole, 2015). The multi-anchoring sensitizers possess superior properties in terms of light harvesting ability, dye loading amount, suppression of intermolecular interaction and charge recombination, and long-term stability, relative to the corresponding mono-anchoring dyes (Chen and Lin, 2017). Previously a series of IQ-based organic dyes were designed and synthesized with a novel conceptual D-D|A-π-A configuration and their corresponding properties have been studied. With aforesaid benefits, the above motif is converted to a di-branched di-anchoring framework which is expressed as D-D|A-(π-A)2. Two novel IQ-derived sensitizers, FS13 and FS14, were tailored, synthesized and characterized. The edged donors were chosen from triphenylamine and carbazole. Phenylene and cyanoacetic acid were employed as π-spacer and anchoring group correspondingly. Hydrophobic carbon chains were further attached to the N-terminal of the building units to fulfill anti-aggregation. Besides, the previously reported mono-anchoring sensitizer, FS08 (Su et al., 2019), was introduced as the benchmark to investigate the effect of the number of anchors on the photophysical, electrochemical and photovoltaic properties and to compare the structural-property relationship. To further probe the binding energy given by anchors, the co-sensitization method, which is effective in assisting the photon-harvesting abilities of sensitizers over the whole visible region and consequently improving the DSSC performance by integrating the merits of both metal-free organic dyes and Ru(II) complex into the one fabricated device (Babu et al., 2017, Naik et al., 2017, Naik et al., 2018a, Naik et al., 2018b, Babu et al., 2016, Elmorsy et al., 2018, Naik et al., 2018c), was employed by combining FS13, FS14 and FS15 with a previously published polypyridyl Ru(II) sensitizer HD-2 (Fig. 1) (Cheema et al., 2014). The co-sensitization effect on photovoltaic properties is taken into consideration. The molecular structures of FS13; FS14 and FS08 are drawn in Fig. 1.
Section snippets
Molecular design and synthesis
With the purpose of studying the relationship between the number of anchoring group and the corresponding photophysical, electrochemical and photovoltaic properties, two dual-channel anchorable sensitizers were designed by converting the D-D|A-π-A configuration (Su et al., 2019) into a new D-D|A-(π-A)2 framework. Comparing FS08 belonging to the former case (Su et al., 2019), the construction of FS13 and FS14 still utilized indolo[2,3-b]quinoxaline (IQ) “D|A” part due to its high coplanarity
Conclusions
Two new dual-channel di-anchoring organic dyes, FS13 and FS14, have been successfully tailored and synthesized with D-D|A-(π-A)2 building configuration. In the new scheme, indolo[2,3-b]quinoxaline (IQ) was employed as the main building unit with merged donor–acceptor conjugation system, triphenylamine and carbazole were selected as additional groups, respectively, and cyanoacetic acids take the responsibility as anchoring units. The newly produced dyes were systematically characterized via
Declaration of Competing Interest
The authors declared that there is no conflict of interest.
Acknowledgements
The authors are thankful to the Department of Textile Engineering, Chemistry, and Science Department at NC State for providing experimental facilities.
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