Interfacial properties of high-order aggregation of organic dyes: A combination of static and dynamic properties
Introduction
Since the seminal report by O'Regan and Grätzel in 1991 [1], dye sensitized solar cells (DSSCs) utilizing nanoporous TiO2 electrode has aroused considerable attentions from both academic and engineering community. There are many prominent advantages for DSSCs including low cost, facile fabrication, and acceptable photovoltaic conversion efficiency [[2], [3], [4]]. Sensitizes are responsible for the light harvesting and charge separation, which is one of the most vital components to determine the overall performance of DSSCs [5]. Although the DSSCs based on metal sensitizers have achieved remarkable power conversion efficiency (PCE) so far, their applications have been limited due to the rare materials, tedious synthetic process, and low yields [6]. In contrast, organic sensitizers are promising alternative because of adjustable molecular design, abundant raw materials, and low environmental pollution [7,8].
The relative low PCE is one of the largest obstacles for the large scale application of DSSCs based on organic dyes. Variation of the configuration and/or incorporation of new groups are the powerful pathways to improve the performance of organic dyes, which has been widely investigated in previous literature [9,10]. Alternatively, reduction of the negative effects is also a pathway to enhance the overall performance. Aggregation is inevitable for organic dyes due to the strong attractive force between organic dyes resulting in a reduced electron injection efficiency and lowered conversion efficiency [11,12]. Meanwhile, the aggregation would increase the dye loading on the TiO2 surface leading to an improved amount of light harvesting. Consequently, the recombination of the injected electrons would be effectively inhibited owing to the blocking layer formed by aggregated dyes [13]. In general, dye aggregation dictates structural and optoelectronic properties of photoelectrodes in DSSCs, thereby playing an essential role in their photovoltaic performance. Therefore, it is important to deeply understand dye aggregation with the aim to suitably control the performance of DSSCs. However, it is difficult or almost impossible to study it by experimental method. The theoretical method has been testified to be a powerful tool to understand the aggregation behavior. If the aggregation effect is clearly elucidated, it is not only beneficial to reduce the negative effects but also helpful to enlarge the positive influence by the variation of dye configuration.
Pastore and De Angelis firstly reported the aggregation effect of indoline D102 and D149 dyes on TiO2 surface providing the possibility to calculate the various aggregation modes for screening and predicting the corresponding optical response [14]. Later, Zhang et al. have studied the adsorption properties of high-order aggregates (from monomeric to pentameric aggregates) on TiO2 anatase (101) surface via density functional theory (DFT) [15]. In previous studies, the organic dyes are perpendicular to the TiO2 surface, which is far from the real situation in DSSCs. The bent of organic dyes would result in the different electron coupling between two dyes, diverse possibility of electron recombination, and other properties. It is necessary to investigate the dynamic behaviors of organic dyes rather than only the static properties. Li and co-workers reported the dynamic effect on the dimeric aggregates by joint of first-principle calculations and molecular dynamics (MD) simulations [16]. However, the high-order aggregation of organic dyes has never been studied by MD method to our best knowledge.
In this work, the monomeric, dimeric, trimeric, tetrameric, pentameric, and hexameric aggregates of THI-BTZ-2T-C (See Scheme 1), are investigated by both first principles calculations and MD simulations. THI-BTZ-2T-C is D-A-π-A configuration dye, which has been designed in our previous work [17]. As compared with IK-2 (See Scheme 1) reported by Irgashev et al. [18], it presents more potential applications in DSSCs with stronger light harvesting due to the involvement of electron-withdrawing group in π group. However, the aggregation effect is not considered in our previous work. Here, the variation of band gap, coverage value of dyes, electron injection time, and others are compared for different order of aggregation. Moreover, the variation of aggregation is inspected as a function of time. The deep understanding of the dye aggregation effects would not only throw light on the complex factors related with the DSSCs performance but also pave a way to rationally design high-efficiency sensitizers.
Section snippets
Computational details
In order to analyze the high-order dye aggregation, a 4 × 8 × 6 TiO2 anatase (101) supercell was applied as the adsorption surface since most of the available TiO2 anatase crystals are mainly based on thermodynamically stable (101) surface [19]. In Fig. 1, there are six dye-TiO2 adsorption sites on the five-coordinated unsaturated titanium atoms of the (101) surface of anatase unit cells, named a, b, c, d, e, and f, respectively. Based on these six adsorption sites, six configurations
Structures of dye adsorbed on TiO2 (101) surface
The bridged bidentate mode is more stable than monodentate mode [20], thus, only the former mode is adopted in the following investigations, in which carboxylic group is linked to the five-coordinated titanium atoms of TiO2 (101) surface. The optimized structural geometries of different dye aggregates are plotted in Fig. 2. The distances between Ti and O atoms are in a range of 1.9–2.1 Å, which is almost not changed from monomer to hexamer. It indicates that the aggregation has negligible
Conclusion
The effect of high order aggregation for dye THI-BTZ-2T-C is investigated by combination of first principle and MD. The torsion between various parts in dye is not greatly affected by the aggregation order in static properties indicating that the influence of aggregation on electron injection from dye to TiO2 is not large. Moreover, absorption wavelength presents red-shift from monomer to dimer, then, it shifts towards blue region from dimer to trimer. In general, the absorption spectrum
Acknowledgements
We thank the National Supercomputing Center in Shenzhen (Shenzhen Cloud Computing Center) for providing computational resources and software. This work was supported by the National Natural Science Foundation of China (21476061, 21503069, 21676071, 21703053), Program for He'nan Innovative Research Team in University (15IRTSTHN005).
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