Electronic excitation and injection of Ru-N3 dye anchored to TiO2 surface

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Abstract

A step-by-step theoretical protocol based on the density functional theory (DFT) and time-dependent DFT (TD-DFT) has been performed to study a Ruthenium polypyridyl complex named cis-dithiocyanatobis-(4,4′-dicarboxy-2,2′-bipyridine) ruthenium(II) (N3) sensitized TiO2 solar cell including dye excitations and electronic injection. Three binding structures of N3 anchored to a TiO2 nanoparticle (TiO2)38 have been adopted. The electronic structures and optical properties of N3 dye in gas phase, in solutions (with the explicit and implicit solvent models), and interfaced with TiO2 have been calculated. The hybrid DFT exchange-correlation (XC) functional B3LYP and PBE0, the newly-modified PBE0 functional PBE0-1/3, and the long-range corrected DFT (LRC-DFT) XC functional Cam-B3LYP have been applied. It is found that both the DFT XC functionals and molecular environments play a crucial role on molecular properties, which affect both the spectral lineshapes and peak positions. Coupled with PCM model, TD-PBE0-1/3 produces the best result compared with the experimental spectrum of N3 in CH2Cl2, TD-B3LYP slightly underestimates the excitation energies, and TD-Cam-B3LYP overestimates the excitation energies of 0.4–0.5 eV. As the solvent polarity increases, the electronic absorption spectra of N3 dye blue shift. The absorption manners of N3 anchored to (TiO2)38 affect the electronic excitations, leading to different electronic injection pathways. Two of three anchoring manners favor the interfacial electronic transfer. In the end, three possible electronic injection pathways from the different parts of the excited dye to TiO2 nanoparticle have been suggested, indicating the different timescales of electron injections.

Introduction

To satisfy the currently increased global electricity needs, educe air pollution and cut global warming emissions, it is urgent to develop the practical and affordable renewable energy sources. Silicon solar cells have been commonly used for converting solar radiation to electricity. However, the complex manufacturing processes and high production costs involved limit their commercial application. Dye-sensitized solar cells (DSSCs) or Grätzel cells are an efficient type of thin-film photovoltaic cell. [1] They promise viable solutions to future large-scale solar-energy conversion issues on the basis of high efficiency and low cost, and have thus attracted considerable research activities involved to design and optimize DSSCs. DSSC technology arises from the concept of “artificial photosynthesis”. In DSSCs, chlorophyll is replaced by a light absorbing dye. We know that the main components of a DSSC is a wide band gap semiconductor that is able to accept an electron injected into its conduction band from the excited photosensitizer. Since the semiconductor is transparent to the major part of the solar spectrum, a light absorbing dye is required to allow the absorption of light. A good photosensitizer not only should have broad and strong absorption bands but also should adsorb to the semiconducting antiparticles firmly and infuse electron from a high energy state generated by absorption of the light that can inject the photoexcited electron into the conduction band of semiconductors. Charge injection occurs very rapidly, often requiring less than 1 ps.

To enhance the performance of DSSCs, the improvement of semiconductor, redox mediator and dye-sensitizer has been the focus of previous publications. Among them, the development of more efficient dye-sensitizer is the key aspect for advancing these devices. A variety of dye sensitizers have thus been suggested, such as the metal-organic or fully organic dyes or porphyrin derivatives. Organic dye as an alternative to the dye-sensitizer generally possesses a common character of donor-π-bridge-acceptor (D-π-A) structure. When an organic sensitizer absorbs light, intramolecular charge transfer occurs from the subunit D to A through the bridge. To date, organic dyes exhibit high efficiencies in DSSCs. Porphyrin sensitizers were among the first examined natural types of dyes [2], [3], and this day they continue to be one of the most frequently studied sensitizers [4], [5], [6], [7], [8], [9], [10], [11]. Based on Zn-porphyrin dye and Cobalt(II/III)-based redox electrolyte, the power conversion efficiency of the best DSSC device has reached a record of 12.3% [12]. Among the organometallic complexes, Ruthenium polypyridyl complexes, such as N3 and N719 dyes, have shown the best photovoltaic properties due to their broad light absorption and relatively long excite-state lifetime [13], [14]. The best of Ru complex-based DSSCs has been reported to convert solar energy to electrical energy with an efficiency around 11% [15].

The high energy conversion efficiency of the widely used N3 sensitizer and other related Ru(II) polypyridyl complexes comes from the efficient exciton generation and exciton separation processes. Ru polypyridyl complexes are typical organometallic complexes and they absorb to the semiconducting surface via polypyridyl ligands. When Ru polypyridyl complexes are excited, the photo-induced intramolecular electron transfer from Ru metal center to the ligands takes place. The small spatial separation between the dye’s polypyridyl ligands and the semiconductors (such as TiO2) surface facilitates the interfacial electron injection and inhibits the charge recombination. The injection time of the electron from excited dye to semiconductor is dominated by the alignment of the excited electron energy level of the dye and the conduction band minimum (CBM) of the semiconductor. Because this alignment is affected by the anchoring manner, the adsorption structure of the dye is still an essential property, especially for the initial electron injection to the semiconductor surface. Microscopic examinations of the adsorption manner in DSSC are therefore very important for a better understanding of the working principle of the solar cell, as well as for the design of new DSSC device with improved high performances. The improvement based on the atomistic understanding from both the time-resolved experiments and theory as well as modeling is crucial for further breakthroughs. In the recent years, many works have been devoted to atomistic simulation of DSSCs [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33].

In this work, we focus on one representative prototype organic dye molecule N3. We use the density functional theory (DFT) and time-dependent DFT (TD-DFT) approaches to investigate the effects of the solvent and the absorption manners of N3 anchored to TiO2 surfaces on the molecular properties as well as the electronic injection pathways from excited dye to TiO2. Through the studies on the geometric and electronic structures of isolated dye and the dye-TiO2 complexes, we give a deeper insight into the working principle of N3-sensitized DSSC device. In detail, we show the adsorption geometries of N3 anchored on (TiO2)38 nanoparticle, the UV–visible absorption spectroscopies of isolated dye and dye-TiO2 complexes, and the energy alignment of molecular levels with respect to the substrate band edges. The interactions between the N3 dye and TiO2 will be revealed. Especially, the mechanisms of interface electron separation of N3-sensitized TiO2 system will be revealed. The overall picture extracted from these works indicates an important role of the dye adsorption energy and geometry on the electrochemical properties and the DSSCs efficiency.

The paper is organized as follow. Section 2 briefly introduces the theoretical methods and the computational details. Section 3 presents the calculated results and discussions. Finally, a summary is provided in section 4.

Section snippets

Computational details

All the calculations were carried out within the Gaussian 09 program package [34]. The DFT and TD-DFT approaches with different DFT exchange-correlation (XC) functional have been applied. The 6-31G basis set was used for all the atoms except Ru and Ti. For 3d transition metals Ti and 4d transition metal Ru, the effective core potential LANL2DZ [35] was adopted. Solvent effects were introduced by means of the explicit solvents or the implicit conductor-like polarizable continuum model (PCM) [36].

Ru-N3 dye

The Ru-complex with a cis arrangement of the thiocyanate ligands as shown in Fig. 1 is employed. The geometry was optimized both in vacuo and water solution without any constraint. Table 1 shows the calculated and experimental-measured [48] main geometric parameters of N3. The computed results agree well with the experimental measurement and the geometry of Ru-N3 dye is slightly affected by the solvent effect.

The computed absorption spectra of N3 molecule in gas phase and solutions are shown in

Concluding remarks

We have presented a detail theoretical study on the electronic excitation and injection mechanism of Ru-N3 dye anchored to TiO2 surface and provided a microscopic atomistic simulation of N3-sensitized solar cell system. Through the studies on the geometric and electronic structures of isolated dye and dye-TiO2 complexes, we have given a deeper insight into the solvent effect on N3’s properties and the working principle of N3-sensitized solar cell device. At first, we instigated the optical

Acknowledgments

The work is supported by National Science Foundation of China (Grant Nos. 21290193, 21373163 and 21573177).

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