A Rational Design for Dye Sensitizer: Density Functional Theory Study

Article pubs.acs.org/JPCC

A Rational Design for Dye Sensitizer: Density Functional Theory Study on the Electronic Absorption Spectra of OrganoimidoSubstituted Hexamolybdates Jing Wang, Sha Cong, Shizheng Wen, Likai Yan,* and Zhongmin Su* Institute of Functional Material Chemistry, Department of Chemistry, Northeast Normal University, Changchun 130024, P. R. China S Supporting Information *

ABSTRACT: The novel dyes of organoimido-substituted hexamolybdates for positive type dye-sensitized solar cells (p-type DSSCs) have been studied on the basis of timedependent density functional theory (TDDFT) calculations. The electronic absorption spectra, light harvesting efficiency (LHE), charge separation efficiency (CSE), and holes injecting efficiency (HJE) of designed systems have been systematically investigated. The results reveal that the long π-conjugated bridge and auxochrome play crucial roles in red-shifting the absorption bands and reinforcing the intensity of the bands. Based on [(n-C4H9)4N]2[Mo6O18(N-1-C10H6-2-CH3)], the designed systems 6 and 4 are good candidates for p-type DSSC dyes due to the strong absorption in the visible region as well as high LHE, CSE, and HJE. The maximum absorption of the one-electronreduced system obviously red-shifts to the visible region. Therefore, the highly efficient dyes of p-type DSSC can be prepared by reducing POM-based organic−inorganic hybrids which have both long π-conjugated bridge and auxochrome.

1. INTRODUCTION In recent years, the energy resource has become an urgent problem to be solved for the whole world. Solar cells are regarded as one key technology to use solar energy, which is one of the wonderful energy resources.1 Huge efforts have been invested for seeking the better photovoltaic materials to take advantages of solar irradiation (5% UV, 43% visible, 52% IR) efficiently.2 Compared to the conventional silicon-based semiconductor photovoltaic devices, which were first reported in 1991 by O’Regan and Grätzel,3 dye-sensitized solar cells (DSSCs) are becoming more attractive for their numerous potential advantages such as low cost, large-area capability, and easy processing. The efficiencies of p-type DSSCs are crucial to make highly efficient tandem cells, in which both electrodes are photoactive.4−8 DSSCs based on the sensitization have been investigated since 1990 and built a valuable knowledge in this area. The self-operating p-DSSC based on a porous nickel oxide electrode sensitized by a free base porphyrin or erythrosine B was reported by Lindquist and co-workers in 1999.9,10 Since then, a few photosensitizers including coumarin, perylene, and porphyrin derivatives were synthesized.11 More recently, the most efficient sensitizers consisting of oligothiophene units with long-lived charge separated excited states have been reported.12 One of the main directions to improve DSSC efficiency is to enhance the solar light absorption of efficient sensitizers (400−900 nm). The other one is to keep the chargeseparated states. To solve these problems, the donor−(π spacer)−acceptor systems were used for the active layers in organic solar cells.13,14 As electron donor materials, one of the most important properties is strong absorption covering a broad spectral region, while the electron acceptor materials can efficiently reduce the charge recombination reaction. © 2013 American Chemical Society

Polyoxometalates (POMs), a class of metal−oxygen cluster compounds, play an important role in diverse disciplines, such as catalysis, medicine, and materials science with fascinating structural, electrochemical, magnetic, catalytic, and photophysical properties.15−17 In recent years, the functionalization of POM clusters with organic species has become a convenient and effective way to generate novel donor (π spacer)−POM hybrid materials. Thanks to the works of Wei18−22 and Peng,23,24 a number of organoimido derivatives of hexamolybdates have been synthesized, and the related properties have been explored. For organoimido derivatives of POM, the strong d−π interaction between the organic delocalized π-electrons and the cluster d-electrons may result in fascinating synergistic effects. One of the important changes is that the absorption of organoimido derivative of POM is red-shifted in the UV/vis absorption spectra compared to that of parent POM.18−24 Thus, the organic−inorganic hybrid materials based on POMs might be a promising potential for applications in DSSCs. Particularly, the reduced process of POM is usually reversible with marginal structure variation, which is crucial to the stability of dye.25 It was reported that combining the POMs with phthalocyanine films can improve photoelectron chemical performance by facilitating photogenerated electron transfer and retarding electron recombination.26 Peng and co-workers synthesized the novel POM-based photovoltaic materials, in which the hexamolybdate clusters were embedded into the main chain of poly(phenylene acetylene)s and the side chains of conjugated polymers.23,25 These polymers show intense Received: October 27, 2012 Revised: January 10, 2013 Published: January 11, 2013 2245

dx.doi.org/10.1021/jp3106452 | J. Phys. Chem. C 2013, 117, 2245−2251

The Journal of Physical Chemistry C

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absorption in the visible range and possess great potential for applications in photovoltaic cells. To further improve the device performances, the theoretical calculations play an important role, which provide a deep understanding for the relationship between chemical structures and the optical properties of the D (π spacer)−A system.27−29 It is well-known that density functional theory (DFT) and time-dependent DFT (TDDFT) can successfully reveal the electronic and optical properties of compounds. The useful information from quantum calculations is essential to design the multifunctional molecular materials.30,31 In order to explore the novel dye-sensitizers, which have broad and intense absorption in the visible (vis) region, we design a series of POM-based organic−inorganic systems as p-type DSSCs dyes, and their electronic properties, absorption spectra, and transition nature were systematically studied.

dye−|NiO + h+(NiO) → dye|NiO (geminate recombination)

dye−|NiO +

(dye regeneration) 3 − 1 I → I3− + e−(Pt) 2 2

According to the working theory, in order to enhance the efficiency of the p-type DSSCs, the dyes should have high light harvesting efficiency (LHE) in the visible region and then result in full charge separation and generate enough holes. In addition, the holes should efficiently inject to the valence band of the semiconductor and reduce the geminate recombination. Therefore, for the same p-type DSSCs with only different dyes, it is an effective way to enhance the energy conversion efficiency of DSSCs by improving LHE, charge separation efficiency (CSE), and holes injecting efficiency (HJE). The LHE is closely to the oscillator strength f of the dye corresponding to λmax, which is expressed as30 LHE = 1 − 10−f

3. RESULTS AND DISCUSSION 3.1. Theory of p-Type DSSC. For p-type DSSCs, there are five dominating steps when they work.4 The first step is dye excitation: the dye is excited by visible light absorption. The second is hole injection: the hole of the excited dye is injected to the valence band of the semiconductor (or, in other words, electron transfer from the valence band of the semiconductor to the dye). However, if the reduced dye cannot react with the electrolyte within the charge-separated lifetime, it may recombine with the hole in the semiconductor (the third step). The fourth step is dye regeneration: the dye is regenerated by electron transfer from the reduced dye to the oxidized species (I3−) in the electrolyte. The fifth step is I3− regeneration: the holes in the semiconductor move to the back collector of the working electrode and the reduced species (I−) in the electrolyte diffuses to the Pt electrode (Scheme 1).

(1)

In the hole generation, the electron and hole must overcome their mutual Coulomb attraction.36

V=

e2 4πεr ε0r

(2)

where e is the charge of an electron, εr is the dielectric constant of the surrounding medium, ε0 is the permittivity of vacuum, and r is the separation distance between electron and hole. From the equation, the larger r can lead to the lower V, which is helpful to enhance the CSE. The HJE is determined by the driving force ΔERP. In the hole injection process, the injection driving force ΔERP37 has been adopted and expressed within Koopmans’ approximation as38 dye dye dye dye ΔΕ RP = [E LUMO + 2E HOMO ] − [E LUMO + E HOMO NiO dye NiO + E VB ] = E HOMO − E VB

Edye HOMO

(excitation)

dye*|NiO → dye−|NiO + h+(NiO)

(I3− regeneration)

Scheme 1. Key Processes in a p-Type DSSC under Illumination

2. COMPUTATIONAL METHODS In this work, all calculations were performed by using the Amsterdam Density Functional (ADF) 2009.01 program.32−34 All the systems were optimized at the BP86 level within the framework of the generalized gradient approximation. The zero-order regular approximation (ZORA) was adopted in all the calculations. Solvent effects were considered by conductorlike screening model (COSMO) continuum method with the parameters of ethyl acetate solvent (epsilon = 6.02, radius = 3.39). The ionic radii for the atoms which actually define the size of the solvent cavity were chosen to be 2.09, 1.40, 1.70, 1.41, 1.08, and 1.98 Å for Mo, O, C, N, H, and S, respectively. Geometrical optimizations of all systems under Cs symmetry constraints were carried out. To describe the electrons, we used Slater-type basis sets. Triple-ξ plus polarization basis sets were used to describe the valence electrons of all atoms, whereas for transition metal molybdenum atom, a frozen core composed of 1s to 3spd shells was described by means of single Slater functions. Moreover, the value of the numerical integration parameter used to determine the precision of numerical integrals was 6.0. TDDFT calculations were performed using statistical average of orbital potentials (SAOP) by Gritsenko and Baerends et al.35

dye|NiO + hν → dye*|NiO

1 − 3 I3 → dye|NiO + I− 2 2

(3)

Edye LUMO

where and are the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energies of dye, respectively. ENiO VB is the valence band edge of NiO. The increase of −ΔERP will result in higher HJE.

(hole injection) 2246



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3.2. Designed Systems. In this work, we chose [Mo6O18(N-1-C10H6-2-CH3)]2− (system 1) which was synthesized by Wei as the parent system.22 In system 1, the organic group is an electron donor, while the POM cluster is the electron acceptor. First, we focused on designing highly effective dyes for DSSCs, which contain the anchoring group and have good absorption in the UV−vis region. The system 2 was designed by adding a carboxyl as the anchoring group on system 1. In order to red-shift the maximum absorption of UV−vis spectra, three ways were suggested: increasing the naphthalene rings, enhancing the π conjugated links, and adding the auxochrome. The system 3 was designed by replacing the naphthalene in system 2 with perylene. To further red-shift the maximum absorption, we adopted the second way to design system 4, which the double triple bonds are linked with naphthalene rings. The triple bond not only increases the π conjugated link but also provides more planar conformation.39 Considering the auxochrome effect, the systems 5 and 6 were designed by adding double thiophenol groups to systems 3 and 4, respectively. The molecular structures of systems 1−6 are shown in Scheme 2.

extend across the dye, especially the anchoring group, which can achieve strong coupling with the semiconductor.27 In order to clearly know the electronic properties, the HOMO and LUMO energy levels as well as frontier molecular orbitals of systems were studied, and the results are shown in Figure 1. All the HOMOs of studied systems delocalize over the organic groups, and extend to the anchoring group, which is helpful to the efficient holes injection. The LUMOs localize on the POM cluster, which is far from the anchoring site, and can effectively prevent the charge recombination. Both HOMO and LUMO are important to make high effective dye of DSSCs. From the molecular structure, the r for systems 2−6 increases in the following order: system 2 < systems 3 and 5 < systems 4 and 6. For Coulomb attraction, the opposite order of V is obtained: systems 4 and 6 < systems 3 and 5 < system 2. Therefore, systems 4 and 6 would have the higher CSE than others. The HOMO energy of system 2 is lower than system 1 by 0.19 eV due to the introduction of carboxyl. Comparing with system 2, the HOMO energy of system 3 is increased by replacing the naphthalene with perylene, while the HOMO energy of system 4 is slightly lower than system 3 by 0.02 eV. The results indicate that the long π-conjugated link slightly decreases the HOMO energy, and the thiophenol groups can enhance the HOMO energy. The HOMO energies of systems 1−6 are lower than 0.1 eV (the VB of NiO). Therefore, all systems have possibility to be dyes of p-type DSSCs with NiO as the SC and I−/I3− as the redox system. 3.4. Absorption Spectra. We have performed TDDFT calculations to obtain an electronic absorption spectrum. It should be stressed that the TDDFT sometimes underestimates the excitation energy, but it is the most accurate method that can be used on large molecules at an affordable computational cost. The calculated absorption spectra of system 1 by SAOP are in good agreement with the experimental result of α-type crystal, and the difference of maximum absorption band between them is only 13 nm (Supporting Information figure).22 Therefore, the credible electronic absorption spectra of systems can be obtained by SAOP. The calculated maximum absorption, oscillator strength, LHE, and dominant molecular orbital transitions are summarized in Table 1. The maximum absorption bands of systems 2−6 is in the order system 2 < system 3 < system 5 < system 4 < system 6. Systems 3−6 have strong absorption between 600 and 800 nm in the visible region. Compared with system 2, system 3 is red-shifted by 276 nm as the naphthalene ring is replaced with perylene. The double−triple bonds in system 4 provide a more planar

Scheme 2. Molecular Structures of Systems 1−6

3.3. Electronic Structure. Generally, as p-type DSSC, the HOMO must be below the energy level of the top of the semiconductor (SC) valence band (VB) (measured at 0.1 eV).40 Thus, the electron on the reduced dye should be far from the hole on the semiconductor. It requires that the LUMO of the dye should be far from the anchoring site. In order to ensure that the hole is injected from the excited state of dye into the semiconductor, the HOMO of the dye should

Figure 1. Frontier molecular orbital energy level diagram for systems 1−6. 2247



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Table 1. Excitation Energy E (eV), Injection Driving Force −ΔERP (eV), Oscillator Strength (f), Light Harvesting Efficiency (LHE), Maximum Absorption λmax (nm), and the Corresponding Dominant MO Transitions for Systems 1−6 system

E

−ΔERPa

f

LHEb

λmax

main configurations

1

2.79

4.80

0.23

0.41

437

2

3.22

4.99

0.17

0.32

382

3

1.98

4.54

0.38

0.58

658

4

1.66

4.56

0.42

0.62

694

5

1.60

4.42

0.29

0.49

685

6

1.49

4.43

0.44

0.64

794

H → L + 8 (62%) H → L + 6 (12%) H → L + 15 (11%) H-2 → L + 5 (40%) H → L + 15 (15%) H-1 → L + 8 (15%) H → L + 4 (55%) H → L + 8 (35%) H → L + 4 (67%) H → L + 8 (27%) H → L + 5 (64%) H → L + 8 (32%) H → L + 4 (76%) H → L + 8 (18%)

a The −ΔERP is calculated according to eq 3. bThe LHE is calculated according to eq 1. This factor has to be as high as possible to maximize the photocurrent response of the cell.38

conformation; thus, the maximum absorption band of system 4 red-shifts by 36 nm compared with system 3. After adding the thiophenol groups as auxochrome, the maximum absorption is further red-shifted. For example, the maximum absorption of systems 5 and 6 are larger than those of systems 3 and 4 by 27 and 100 nm, respectively. The high LHE is important to enhance the Jsc and finally results in increasing η of DSSCs. The LHE of systems 2−6 is in the order system 2 < system 5 < system 3 < system 4 < system 6. Both the replacement of naphthyl by perylene and the inserted double−triple bonds increase the LHE. Therefore, the LHE of system 3 is larger than system 2 by 0.26 and system 4 is larger than system 3 by 0.04. However, the thiophenol group has different influence on the LHE of systems. The LHE is decreased from 0.58 (system 3) to 0.49 (system 5), while it is increased from 0.62 (system 4) to 0.64 (system 6). The reason may be result from the two triple bonds, which increase the π-conjugated links. In addition, systems 4 and 6 have higher HJE than corresponding systems 3 and 5 due to the larger −ΔERP. In order to further investigate the electronic absorption spectra, the UV−vis spectra of systems 2−6 were simulated and shown in Figures 2 and 3. The black, red, purple, blue, and green lines represent the absorption spectra of systems 2 to 6, respectively. It can be clearly seen that the simulated electronic absorption spectra of all studied systems can be divided into two regions: band 1 and band 2. Systems 3−6 exhibit superior absorption properties than that of system 2. As is shown in Figure 2, both two bands of system 3 are red-shifted. Compared

Figure 3. Simulated absorption spectra of systems 3, 4, and 6.

with system 3, the band 1 of system 5 is only red-shifted by 6 nm. Figure 3 shows that the absorption bands of systems 4 and 6 are more intense and broad in the visible region than that of system 3. The two bands of system 4 are red-shifted and strengthened due to the two triple bands. It is worthy to stress that the band 2 of system 6 is red-shifted 100 nm in comparison with system 4. The result proves that the triple bonds linking plays a crucial role to red-shift and strengthen the absorption spectrum, while the thiophenol group does not obviously affect the position of the band 1 but can reinforce its intensity. That is to say, the absorption in the visible region of the solar spectrum can be enhanced and broadened by both inserting the triple bonds and adding the auxochrome. Thus, system 6 has the most strong and broad absorption in the visible region of the solar spectrum. It proposes that this kind of POM-based organic−inorganic hybrid is a promising candidate for DSSCs. In order to consider the solvent effect, the absorption spectra of system 6 are also calculated with acetonitrile or water media. The result is shown in Figure 4, and the red, black, and green lines represent the absorption spectra in ethyl acetate, acetonitrile, and water, respectively. It can be seen that the solvent effect on absorption band of system 6 is slight. To clarify the origin of spectra for designed systems, the absorption spectra of system 6 were analyzed as a representative in terms of the relevant absorption wavelength, the main transitions, and the oscillator strength (f) (Table 1). The molecular orbitals (MOs) involved in the dominant electron transitions of system 6 are shown in Figure 5. The

Figure 2. Simulated absorption spectra of systems 2, 3, and 5. 2248



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theoretically assessed by analyzing the corresponding oneelectron-reduced systems.41 Therefore, the hole injection is investigated in the same way. Here, the spin density of the reduced system 6/(NiO)4 cluster (Figure 6) was analyzed using the BP86 method. The result indicates that the spin density of POM cluster is 0.81, and it is a benefit to the hole injection into the (NiO)4 cluster.

Figure 4. Simulated absorption spectra of system 6 in different solvents.

Figure 6. Structure of system 6/(NiO)4 cluster. The red, green, gray, yellow, white, deep blue, and light blue balls are O, Mo, C, S, H, N, and Ni, respectively.

3.6. Reduced System 2. Herein, we chose system 2 as the representative to study the reduced system. The system 2(e) was formed by adding one electron to system 2. The result indicates that α-HOMO and LUMO localize on the POM cluster. The maximum absorption of system 2(e) is red-shifted by 114 nm, and the LHE is increased by 0.18 in comparison with system 2. The simulated UV−vis spectrum of system 2(e) in Figure 7 reveals that there is only one absorption band in the spectrum.

Figure 7. Simulated UV−vis spectra. The black and blue lines represent the absorption spectrum of system 2 and system 2(e), respectively.

Figure 5. Contour of frontier molecular orbitals of system 6 for the main configuration.

maximum absorption (band 1) mainly originates from the electron transitions of HOMO-2 → LUMO + 4 and HOMO → LUMO + 9/LUMO + 17. The main electron transitions can be assigned to the charge transfer within organic segment and a little charge transfer from organic segment to POM cluster. The band 2 of system 6 is mainly from the HOMO → LUMO + 4/ LUMO + 8 electron transitions. These transitions are mainly assigned to the charge transfer from organic segment to POM cluster. The inserted triple bonds provide the planar conformation and increase the length of π-conjugated bridge, which results in long wave absorption. As the electron donor, the S atom in thiophenol group increases the conjugated degree of system. The electron transition nature shows that increasing the π-conjugated bridge length and the delocalization is an effective way to improve the absorption ability of system in the visible region. 3.5. Spin Densities of Reduced System 6/(NiO)4 Cluster. The electron injection of the systems can be

The maximum absorption is significantly red-shifted. To clarify the origin of spectra for system 2(e), the molecular orbitals involved in the dominant electron transitions in systems 2 and 2(e) are shown in Figure 8. The absorption band of system 2(e) mainly originates from the electron transitions α-HOMO → α-LUMO + 2 and β-HOMO → β-LUMO + 2. The charge transfer transition of system 2(e) is similar to the band 1 in system 2. The result indicates that the maximum absorption of one-electron-reduced systems obviously red-shifts to the visible region. The highly efficient dye of p-type DSSC can be prepared using reduced organic−POM hybrid materials.

4. CONCLUSIONS In this work, we designed a series of POM-based organic− inorganic hybrids by modifying the organic groups. The electronic structures, absorption spectra, and electronic transition characteristics of designed systems were systemati2249



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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-431-5684009; e-mail: [email protected] (L.Y.), [email protected] (Z.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by NSFC (20971020, 21073030, and 21131001), Program for New Century Excellent Talents in University (NCET-10-318), Doctoral Fund of Ministry of Education of China (20100043120007), and the Science and Technology Development Planning of Jilin Province (20100104).

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Figure 8. Contour of frontier molecular orbitals of systems 2 and 2(e) for the main configuration.

cally studied on the basis of TD-DFT calculations. The results show that all systems are possible to be p-type DSSC dyes as their HOMO energies are lower than the VB of NiO. Molecular orbitals analysis reveal that the systems may have high charge separation efficiency because the HOMOs delocalize over the organic groups and extend to the anchoring groups, while the LUMOs localize on the POM cluster, which is far from the anchoring site. Because of the different r of systems 2−6 in Coulomb attraction, their CSE increases in the order of system 2 < systems 3 and 5 < systems 4 and 6. The absorption spectra show that the systems 1−6 have two absorption bands, and the excitation energies decrease along with the increasing the πconjugated bridge length and delocalization. The spin density of the reduced system 6 /(NiO)4 cluster localize on the POM cluster, indicating that the hole injection occurs from dye to (NiO)4 cluster. Systems 4 and 6 are potential candidates for the dyes of DSSCs due to the broad and strong absorption in the visible region together with the higher CSE and HJE. System 2(e) is superior to system 2 due to its lower excitation energy but higher LHE. In summary, the highly efficient dye of p-type DSSC can be designed by preparing reduced organic−POM hybrid materials with long π-conjugated bridge and high delocalization.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

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A Rational Design for Dye Sensitizer: Density Functional Theory Study

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