The impact of aromatic π-spacers and internal acceptors in triphenylamine dyes for DSSCs: A DFT approach

Rising energy demand calls for immediate action to provide cheap, clean, and renewable energy sources for humanity's continued survival. Despite the depletion of fossil fuels energy resources, energy-intensive industries must focus their consumption on renewable energy sources. Among these renewable energies, sunlight is a potent, environmentally friendly and abundant source of energy. Due to its environmental friendliness, low cost, and convenience in manufacturing, dye-sensitized solar cells (DSSCs) have grown much devotion from academics over the past 2 decades [[1], [2], [3], [4]]. A DSSCs consists mainly four elements, namely dye sensitizer, photoanode, photocathode, and redox electrolyte. The sensitizer is useful in light harvesting, charge transfer and performance of DSSCs. The sensitizers can be organic based or metal-based dyes. Zn-complex-based dyes have been reported to have power conversion efficiencies (PCE) of 12.3%, whereas Ru-complex-based dyes had PCEs of 11.5% [5,6]. Metal-based dyes primarily have two disadvantages: they are very expensive and unfriendly to the environment. Lingling Zhan et al. designed symmetric-asymmetric molecule ternary organic photovoltaics and reported the efficiencies around 19% [7]. Metal-free organic dyes are becoming more popular because they are convenient to make and have a greater molar absorption coefficient [[8], [9], [10], [11]]. Although, some limitations are also associated with metal-free organic dyes, as they have comparatively with limited light harvesting region, which leads to inferior PCE. In recent years, certain organic dyes' PCE for DSSCs increased up to 14% [12,13]. Further, to improve the overall performance including various internal groups and variation in structural design has been described time to time.

Metal-free dyes typically have a donor and an acceptor connected by a π-spacer, or a D-π-A arrangement, to effectively separate charges during photoirradiation. Triphenylamine, coumarin, indoline and phenothiazine are some commonly used donor groups in D-π-A scaffold for DSSCs [[12], [13], [14], [15], [16], [17], [18], [19]]. Despite the fact that cyanoacrylic acid (CAA) anchoring group being known to dissociate from semiconductor under long-time operation remains the most often employed acceptor group in the synthesis and design of DSSCs [20].

In conjunction with experiment, theoretical chemistry is useful for creating efficient organic dye sensitizers. The boundary between the dye attached to the TiO2 surface and the redox electrolyte is determined using the theoretical approaches for electron transport, charge separation, and dye renewal [[21], [22], [23], [24], [25]]. Various modifications have been described by using internal donors, acceptors, etc. to enhance the efficacy in D-π-A scaffold. Up to the present time, abundant dyes with numerous measures, like, D-(π-A)2, D-A-π-A, D-D-π-A, (D-π-A)3L2, etc. have been described and invented for DSSCs [[26], [27], [28], [29], [30]].

Chiu et al. revealed how an internal acceptor moiety affected the development of an electron-withdrawing group in a D-π-A scaffold [31]. Zhu et al. described D-A-π-A systems with internal acceptors and revealed that the extra internal acceptor not only balanced the electron distribution but also tuned the energy levels [32]. Theoretical studies revealed that adding an auxiliary acceptor is crucial to improve DSSC performance [[33], [34], [35]]. Sun et al. designed carbazole based asymmetric butterfly structure dyes with the different π-spacer and studied their photovoltaic properties. They concluded that among various combinations of π-groups, group having two phenyl rings is showing better results [36]. The addition of a second donor group to the D-π-A arrangement enhances the functionality of the DSSCs, raises the molar extinction coefficient, lowers aggregation, and increases the light absorption area [37,38]. Additionally, Xie et al. explored the effects of auxiliary groups with and without electrons on their photovoltaic characteristics and demonstrated how these effects affected HOMO and LUMO levels [38]. Li et al. demonstrated the effect of electron-rich groups in TPD based dyes for DSSCs [39]. Lu et al., described D-A-π-A system with benzoselenodiazole, demonstrating the usage of benzene as a π-spacer to enhance the JSC and VOC of the DSSCs [40]. Benzoselenadiazole and benzothiadiazole were used as internal acceptor groups in the dyes, while thiophene served as a π-spacer for DSSCs. Velusamy et al. published these dyes and demonstrated that they are superior to several hemicyanine and cyanine dyes [41]. O. Britel et al. described the theoretical design of novel organic dyes based on carbazole for use in DSSCs [42]. Fluorene-based organic dyes were created by Li et al., who also detailed their optical and photovoltaic characteristics [43]. The quantum chemical investigation of Z-shaped heptazethrenes derivatives for photovoltaic applications reported with the power conversion efficiency of 9% [44]. M. Afzal et al. designed dithieno[2,3-D:2ʹ,3ʹ-Dʹ]-benzo[1,2-B:4,5-Bʹ] dithiophene based donor molecules for plausible performance in organic solar cell [45]. The impact of a thiophene-based π-spacers in D-A-π-A scaffold along with the internal acceptor has also demonstrated and shown the effect of internal donor and acceptor groups with various π-spacers for DSSCs in our previous work [46,47]. Motivated from previous work and recent literature, we have designed the present work i.e., D-A-π-A scaffold to see the effect of heteroaromatic based internal acceptors along with aromatic/heteroaromatic π-spacers and investigated the photovoltaic properties for efficient Dye sensitized solar cell (DSSC). Here, five different internal acceptor moieties (quinoline (A1), cinnoline (A2), pyridopyrimidine (A3), naphthyridine (A4), and 2-pyridone (A5)) and four π-spacers, including benzene (π1), furan (π2), thiophene (π3) and benzothiadiazole (π4), are taken into account in this work to examine the effects of the π-spacer and internal acceptor moieties on triphenylamine-based dyes. The donor group for all the dyes under study is triphenylamine (TPA), while the acceptor group is cyanoacrylic acid (CAA). These dyes are referred to by the names B1, B2, B3, B4, B5, F1, F2, F3, F4, F5, T1, T2, T3, T4, T5, BT1, BT2, BT3, BT4 and BT5 (Scheme 1).

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