Theoretical Studies on MetalloporphyrinâPolyoxometalates Hybrid

Article pubs.acs.org/JPCC

Theoretical Studies on Metalloporphyrin−Polyoxometalates Hybrid Complexes for Dye-Sensitized Solar Cells Ting Zhang,† Wei Guan,† Shizheng Wen,‡ Tengying Ma,† Likai Yan,*,† and Zhongmin Su*,† †

Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024, Jilin, China School of Physics and Electronic Electrical Engineering, Huaiyin Normal University, Huaian 223001, China

‡

S Supporting Information *

ABSTRACT: We use time-dependent density functional theory methods to discuss the absorption spectra, electronic transition properties, and photovoltaic performance of metalloporphyrin−polyoxometalates (POM) complexes for p-type dye-sensitized solar cells (DSSCs). The results show that the energy levels of the frontier molecular orbitals for dyes 2−6 match the requirements of p-type DSSCs. The absorption spectra of dyes 2−6 exhibit larger and broader absorptions compared to that of dye 1 by the introduction of POM. In addition, the photovoltaic performances of dyes 2−6 are suitable for high-efficiency DSSCs. This paper is expected to advance the design of metalloporphyrin−POM hybrid dyes with excellent performance in DSSCs.

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stability.17 They are an extremely interesting class of organic− inorganic hybrids, which are constituted by covalent attachments of organic groups to POMs via linkages. The organic− inorganic hybrids combine the merits of organic and inorganic materials and result in “value-added properties”.18−20 Therefore, there is no doubt that the combination of porphyrins and POMs within a hybrid material will keep the individual functionality and also introduce novel properties, such as catalysis, adsorption, and nonlinear optical, especially optoelectronic, properties. In the past decade, we have systematically accomplished the computational studies of electronic properties on Keggin-, Lindquist-, Anderson-, and Dawson-type POMs and their related derivatives.21−29 The density functional theory (DFT) method has been confirmed to be appropriate for explaining their bonding character, stability, redox, and spectra properties as well as charge transfer. In the field of DSSCs, joint theoretical−experimental analysis is a particularly useful method for investigating the complexity of the processes involved in these devices. The time-dependent DFT (TDDFT), which provides an accurate result and maintains reasonable computational time, has emerged as the most applied theory in the field. In this work, we designed six dyes based on the POM conjugated zinc(II)-centered-porphyrin to investigate the performance on DSSCs by using the TDDFT method. As far as we know, this work is the first to explore this kind of new complex combined porphyrin with Lindquist-type POM as dye sensitizers.

INTRODUCTION Dye-sensitized solar cells (DSSCs) have attracted significant attention as alternatives to traditional solar cells owing to their low-cost fabrication combined with high solar energy to electricity power conversion efficiency (PCE).1−3 In 1991, the mesoscopic DSSC using ruthenium complex dye and nanocrystalline titania (TiO2) mesoporous film with PCE ∼7% efficiency was developed by O’Regan and Grätzel.4 This discovery opened new frontiers in the development of solar energy technologies. The development of photocathode DSSCs (p-DSSCs) can provide a method for designing series connection solar cells (pn-DSSCs). However, the relative efficiencies of p-DSSCs are far less than that of their n-type counterparts.5−9 The efficiencies of p-type DSSCs are critical for obtaining efficient series connection pn-DSSCs. In 1999, Lindquist’s group reported the self-operating p-DSSC based on a porous nickel oxide electrode sensitized by a free base erythrosine B or porphyrin.10,11 After that, some photosensitizers comprising perylene, carbazole, and porphyrin derivants were synthesized.12 One of the main paths of promoting DSSCs efficiency is by maintaining the charge separation. The other way is to expand the range of absorption spectra of efficient sensitizers in the sunlight. To solve these problems, donor−π−acceptor dyes were used for DSSCs.13 Polyoxometalates (POMs), which are made up of high oxidation state transition metals linked by oxygen bridges, show great promise in a diverse range of fields such as catalysis, electronics, and optics.14−16 Moreover, porphyrins are fascinating parts in materials that are often used in organic−inorganic hybrid complexes because of their appealing photochemical properties, such as intense visible absorption bands, excellent chemical properties, and high © XXXX American Chemical Society

Received: September 26, 2014 Revised: December 2, 2014

A

dx.doi.org/10.1021/jp509765a | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Article

METHOD AND COMPUTATIONAL DETAILS Theoretical Background. There are five main steps in ptype DSSCs, which are shown in the following:30 Dye|NiO + hν → Dye*|NiO

(excitation)

Dye*|NiO → Dye−|NiO + h+(NiO)

reduction potential of the redox mediator. Accordingly, the lower ΔGinj and ΔGreg and higher ΔGCR may result in high efficiency of dye in DSSCs. Computational Details. Herein, the metalloporphyrin− POM hybrid complexes were chosen to study, and all calculations were performed by Gaussian 09W software package.32 The geometries of all dyes were optimized by B3LYP33,34 functional, and the ground states of all dyes studied in this work are closed-shell singlet states. The solvent effect of CH2Cl2 was employed by polarizable continuum model (PCM)35 in optimization calculations. Additionally, the basis set Lanl2DZ was employed for Zn, Br, and Mo atoms, and other atoms are described with 6-31g*. On the basis of the optimized geometries, the essence of excited states of studied complexes were explored using TDDFT calculations at the PBE0/6-31g* level, and Lanl2DZ basis set for Zn, Br, and Mo atoms in CH2Cl2; the PCM was also employed in TDDFT calculations.

(1)

(hole injection) (2)

−

+

Dye |NiO + h (NiO) → Dye|NiO (geminate recombination) (3) −

Dye |NiO +

1/2I3−

−

→ Dye|NiO + 3/2I (dye regeneration) (4)

−

3/2I →

1/2I3−

−

+ e (Pt)

(I3−

regeneration)

(5)

It can be seen in Figure 1 that the first step is the dye excitation by light absorption, followed by the hole injection

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RESULTS AND DISCUSSION Molecular Structure. Herein, six dyes based on the POM conjugated zinc(II)-centered-porphyrin were chosen for investigating the properties of DSSCs by using TDDFT method. As shown in Figure 2, dye 1 is the zinc(II)-centered

Figure 1. Schematic diagram illustrating the main processes in a p-type DSSC.

from the excited dye into the valence band of the semiconductor. In the third step, the reduced dye may recombine with the hole if it cannot react with the electrolyte within the charge-separated lifetime. Next, the dye is regenerated because of electron transfer from the reduced dye to the oxidized species (I3−). Finally, the holes that are accumulated in the semiconductor move to the back collector of the electrode and the reduced species (I−) diffuses to the Pt electrode. On the basis of the working theory, the effective way to improve the efficiency of DSSCs is by adjusting the parameter of light-harvesting efficiency (LHE), hole-injecting efficiency (HJE), dye regeneration efficiency (DRE), and reducing charge recombination efficiency (CRE). The LHE is relative to the oscillator strength, f, of the dye, which corresponds to maximum absorption wavelengths (λmax), as shown in following:31

Figure 2. Structure of dyes 1−6.

5,10,15-triphenyl-porphyrin and benzoic acid linked by the ethynyl. Dye 2 is designed by adding the arylimido hexamolybdate to dye 1. For the sake of understanding the conjugated bridge influence on the DSSCs properties for studied dyes, the substituting groups R are introduced into the zinc(II)-centered-porphyrin. Based on dye 2, atoms F, Cl, and Br are introduced into position (5, 15) of zinc(II)-centeredporphyrin and the related derivatives are denoted as dyes 3−5. The five Mo atoms of dye 2 were substituted by W atoms, which is named dye 6. Electronic Structure. In n-type DSSC principle, the oxidation potential of dyes that are analogous to the highest occupied molecular orbital (HOMO) energy level of the dyes should be more negative than −4.80 eV, which is the redox couple of I−/I3−. This can ensure an efficient and fast regeneration of the dye while avoiding the charge recombination between photoinjected electrons in TiO2 and oxidized dye sensitizers. The lowest unoccupied molecular orbital (LUMO)

(6) LHE = 1 − 10−f The free energies (in electronvolts ) of HJE, DRE, and CRE are ΔGinj, ΔGreg, and ΔGCR, respectively. They can be expressed as

ΔGinj = E VB − (E00(dye*) + E(dye/dye−))

ΔGreg =

E(I 2 /I3−)

−

− E(dye/dye )

ΔGCR = E(dye/dye−) − E VB

(7) (8) (9)

where EVB is the valence band potential of the semiconductor, E00(dye*) the energy of excited-state sensitizer, E(dye/dye−) the reduction potential of sensitizer, and E(I2/I3−) the B



Article

overlap with the valence band orbital of NiO. In addition, the LUMO distributions of dyes should locate far from the NiO to reduce the hole combination. The distributions on HOMO and LUMO of dyes 2−6 are shown in Figure 5. The electrons in

energies of dyes should be higher than the conduction band of TiO2 (−4.00 eV versus vacuum) to provide the thermodynamic driving force for charge generation. Figure 3 displays the

Figure 3. HOMO and LUMO distributions and energies of dye 1.

distributions and energy levels of HOMO and LUMO for dye 1. Obviously, the HOMO is mainly from the contributions of C atoms in porphyrin and the energy level is more negative than −4.80 eV; the LUMO is made up by C atoms in prophyrin and carboxyl, and the energy level of the LUMO is higher than the conduction band of TiO2. Dye 1 is proposed to match the requirements of n-type DSSCs. However, in p-type DSSCs, the HOMO energy of dye should be situated under the valence band edge of the semiconductor so as to ensure a high hole injection quantum yield. Moreover, the regeneration reaction of the sensitizer with I−/I3− also should be thermodynamically permitted, indicating that the LUMO energy level for the sensitizer is above that of the mediator. The plots of molecular orbital energies diagram of dyes 2−6 are shown in Figure 4. The results displayed in

Figure 5. HOMO and LUMO distributions of dyes 2−6.

Figure 4. Computed energy levels of HOMO and LUMO (versus vacuum) for dyes 2−6, the experimental NiO conduction valence band edge, EVB, and I2/I3− redox level, E(I2/I3−).

dyes 2−6 on HOMOs are promotions of electrons delocalized predominantly on the organic groups and MoN between the organic group and hexamolybdate cluster, while the LUMOs delocalize over POM clusters. Therefore, the electrons may mainly transfer from organic groups to POM clusters. Absorption Spectrum. It is known that the dyes with large photocurrent response should have a broad absorption, which overlaps with the solar spectra. TDDFT was used to investigate the excited states of studied dyes to gain insight into the spectral properties. The simulated maximum absorption wavelength of dye 1 is 445 nm, which agrees well with the experimental data (425 nm),36 confirming the PBE0 functional is exact for simulating the absorption spectra of these dyes. The ultraviolet−visible (UV−vis) spectra of dyes 1 and 2 are depicted in Figure 6 to further evaluate the effect of POM on absorption spectra. The blue and pink lines represent the absorption spectra of dyes 1 and 2, respectively. It is obvious that a strong absorption band appearing at 400−475 nm is found from the absorption spectra of dye 1. When compared with that of dye 1, the absorption of dye 2 in the visible region

Figure 4 reveal that the HOMO energy levels for the studied dyes are below the valence band of NiO, which benefits the hole injection from the excited dye to the valence band of NiO. LUMOs of all dyes are suitably located above −4.8 eV, which is the redox couple of I−/I3− electrolyte, ensuring the dye regeneration process in the DSSC dye is thermodynamically allowed. For dyes 3−5, the HOMO levels decrease in the order dye 3 (−5.13 eV) > dye 4 (−5.26 eV) > dye 5 (−5.31 eV), and LUMO levels also decrease in the order dye 3 (−2.94 eV) > dye 4 (−2.96 eV) > dye 5 (−2.97 eV). These results indicate that the introductions of different units have no apparent effect on the frontier molecular orbital levels. The LUMO energy of dye 6 decreases compared with that of dye 2, while the HOMO energy level has no significant change. In conclusion, the energy levels of frontier molecular orbitals for dyes 2−6 match the requirements of p-type DSSCs. To obtain a fast hole tunneling from the sensitizer excited state into the valence band of NiO, the distribution of HOMO for dyes should evidently extend on the anchoring groups and

Figure 6. Calculated absorption spectra of dyes 1 and 2. C



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compared to that of dye 1, the λmax of dyes 2−6 are all redshifted and possess broad and strong absorptions between 400 and 800 nm due to the introduction of POM. Therefore, introducing POM cluster to prophyrin for enhancing the absorption in the solar spectra is a feasible method. For describing the electronic transition nature simply and intuitively, the electronic difference density maps (EDDMs) of dyes 2−6 are shown in Figure 8, and the charge-transfer

of the solar spectra is broad, and it can be divided into two regions. From Figure 6, it is obvious that the POM plays an important role in the red-shift and broadening of the absorption spectra. The absorption spectra of dye 2 were also calculated by using different basis sets with PBE0 functional, such as 6-31g*, 631+g*, and 6-311+g* to check the effect of basis set. The computed λmax and oscillator strengths (f) at different basis sets are collected in Table S1 of the Supporting Information. It can be found that the basis set has slight effect on λmax. This result demonstrates that the absorption spectra are almost converged with the basis set size used in this work. In the following, we will discuss the absorption spectra of dyes with PBE0/6-31g*. The calculated absorption spectra of dyes 2−6 are shown in Figure 7. The transition energies, λmax, oscillator strengths, and

Figure 8. Electronic difference density maps of dyes 2−6.

direction is from the purple portion to the light blue. Because the electronic transition properties of dyes 2−6 are similar, the charge transfer for dye 2 is selected as a representative for analysis. It can be seen that two types of electron transitions are mixed in the absorption, the π → π* transition within the porphyrin fragment and the charge transfer from the porphyrin fragment to POM cluster. This indicates that the POM cluster plays a significant role in charge transfer. Photovoltaic Performance. In p-type DSSCs principle, the high-efficiency dye must have high HJE, LHE, and DRE but low CRE. Therefore, the LHE, ΔGinj, ΔGreg, and ΔGCR of dyes 2−6 are calculated and displayed in Table 2. The LHEs of dyes

Figure 7. Calculated absorption spectra of dyes 2−6.

Table 2. Computed LHE, E00(dye*) (eV), E(dye/dye−) (eV), ΔGinj (eV), ΔGreg (eV), and ΔGCR (eV) of Dyes 2−6

Table 1. Computed Transition Energy (E, eV), Maximum Absorption Wavelengths (λmax, nm), Oscillator Strengths ( f), and Transition Nature of Dyes 2−6 dye

E (eV)

λmax (nm)

f

2 3 4 5 6

1.90 1.79 1.85 1.87 1.92

652 692 668 661 647

1.66 2.01 1.74 1.73 1.84

major assignment HOMO→LUMO HOMO→LUMO HOMO→LUMO HOMO→LUMO HOMO→LUMO

(94%) (95%) (95%) (95%) (93%)

dye

LHE

Eexc (eV)

E(S/S−) (eV)

ΔGinj (eV)

ΔGreg (eV)

ΔGCR (eV)

2 3 4 5 6

0.98 0.99 0.98 0.98 0.99

1.90 1.79 1.86 1.87 1.92

−2.82 −2.93 −2.96 −2.98 −3.05

−4.04 −3.82 −3.86 −3.85 −3.83

−1.98 −1.87 −1.84 −1.82 −1.75

2.14 2.03 2.00 1.98 1.91

2−6 are similar and the range of 0.98−0.99 and higher than those of other dyes, which were reported in the literature.28,29 The ΔGinj, ΔGreg, and ΔGCR of all dyes are negative, which benefits the hole injection and dye regeneration. Compared with the those of dye 2, the absolute values of ΔGinj, ΔGreg, and ΔGCR of dyes 3−6 are similar, indicating that the effects of halogen elements on the bridge of porphyrin and the substitution from Mo atoms to W atoms on photovoltaic performance of dyes are slight.

transition nature of dyes 2−6 are shown in Table 1. As shown in Figure 7, the absorption spectra of all dyes are divided into two regions; the first intense peak is in the range of 400−500 nm, and the second one is located in the 600−800 nm region. The values of λmax for dyes 2−6 are in the order 6 (647 nm) < 2 (652 nm) < 5 (661 nm) < 4 (668 nm) < 3 (692 nm). Compared with that of dye 2, the maximum absorption wavelengths of dyes 3, 4, and 5 red shift about 40, 20, and 10 nm, respectively. This result indicates that the λmax of dyes are further red-shifted by the introduction of halogen elements on the bridge of porphyrin. In addition, dye 6 exhibits strong and broad absorptions in the range of 400−800 nm within the visible region as dye 2. Therefore, the spectral feature is slightly changed by the substitution from Mo atoms to W atoms. As

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CONCLUSIONS The photovoltaic performance for DSSCs on the basis of metalloporphyrin and polyoxometalates complexes were first investigated by using DFT and TDDFT calculations on the electronic properties, absorption spectra, and charge-transfer character of excited states. The results show that dyes 2−6 D



Article

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match the requirements of p-type DSSCs as their HOMO energy levels are lower than the valence band of NiO and their LUMO energies are higher than I−/I3−. In comparison with dye 1, the absorption spectra of dyes 2−6 exhibit stronger and broader absorptions in the solar spectra region, which are attributed to the charge transfer from porphyrin to POM cluster. The LHEs of dyes 2−6 are similar and the range of 0.98−0.99, and are higher than those of other reported dyes. This work is inferred to be beneficial for designing metalloporphyrin and polyoxometalates dyes with excellent properties to enhance the efficiency of dye-sensitized solar cells.

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ASSOCIATED CONTENT

S Supporting Information *

Transition energy, maximum absorption wavelengths, and oscillator strength of dye 2 by using different basis sets. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*L.-K.Y.: phone, +86-431-85099108; fax, +86-431-85684009; email, [email protected]. *Z.-M.S.: phone, +86-431-85099108; fax, +86-431-85684009; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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

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