Exploration of charge transfer and absorption spectra of porphyrin–polyoxometalate hybrids to search for high performance sensitizers

Abstract

Based on a porphyrin derivative (system 1), Lindqvist-, Keggin-, and Anderson-type polyoxometalate (POM) organic–inorganic hybrids (systems 2–4) were designed with the aim of investigating their charge transfer character and screening them as high performance p-type sensitizers. The electronic structures and absorption spectra of systems 1–4 were systematically investigated by means of density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The results indicate that Lindqvist- and Keggin-type POMs affect the lowest unoccupied molecular orbital (LUMO) energy levels, while the Anderson-type POM does not contribute to the frontier molecular orbitals (FMOs). Furthermore, the absorption spectrum of the Lindqvist-type POM porphyrin derivative (system 2) exhibits strong and broad absorption in the visible region and is red shifted about 100 nm in comparison with system 1. Further studies point out that system 2 can balance the photovoltaic parameters, LHE, HJE, CRE and DRE, indicating that it will be a promising high performance dye sensitizer in p-type dye-sensitized solar cells (DSSCs).

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Introduction

The frequent occurrence of fog and haze in Asian countries is attributed to the burning of fossil fuels, and the energy crisis also makes the problem of environmental pollution more and more serious. Therefore, we urgently need to search for clean and safe energy resources and technologies.¹ As promising renewable energy devices to solve the energy crisis, dye-sensitized solar cells (DSSCs) have attracted tremendous attention recently due to their flexibility, cost effectiveness, easy production processes, and environmentally friendly fabrication.^2–6 Since the seminal work on the sensitization of an n-type semiconductor TiO₂ reported by O’Regan and Grätzel in 1991, enormous efforts have been devoted to DSSCs.^7,8 Currently, the highest reported efficiency of an n-DSSC is up to 15%.⁹ In 1999, Lindqvist and co-workers first reported the performance and characterization of a p-type DSSC based on a porous nickel oxide (NiO) electrode sensitized by a free-base porphyrin or by erythrosine B.¹⁰ Although many efforts have been made, the efficiencies of p-DSSCs are so far relatively very low,¹¹ but the potential use of p-DSSCs is leading to the fabrication of tandem DSSCs (pn-DSSCs), whose overall efficiencies could be higher than those of n-DSSCs.¹² Thus, developing efficient p-DSSCs is of great significant for pn-DSSCs to further enhance the overall efficiencies. There are many paths to improve the efficiencies of p-DSSCs, such as expanding the range of the absorption of the dyes in the solar spectrum and maintaining the charge separation, etc. A dye with a donor–π linker–acceptor (D–π–A) structural motif exhibits a high molar extinction coefficient and can facilitate photo-induced charge separation; therefore we adopted this type of dye.^13–16

Polyoxometalates (POMs), as one kind of significant anionic metal-oxide cluster, have wide potential applications in catalysis, photochemistry, biology and medicine because of their structural diversity, and unique physical and chemical properties.^17–19 POMs have become a subject of general interest not only for their excellent electronic properties and strong electron-withdrawing ability, but also for their extensive use as inorganic building blocks for the construction of organic–inorganic hybrid materials. Recently, covalent organic modifications of POMs have drawn much attention due to the synergistic properties between POMs and organic groups.^20–22 Up to now, a great deal of Lindqvist-, Keggin-, Dawson- and Anderson-type POM organic hybrids have been synthesized.^23–26 POM-based hybrid materials have great potential in DSSCs due to their favorable absorption properties. In our previous work, Lindqvist-type POM-based organic–inorganic hybrids were studied using density functional theory (DFT) methods.^{21,22,27–30} It turns out that this kind of POM-based hybrid has high stability and intense absorption in the visible region, and can be a promising photovoltaic material for DSSCs.

Porphyrins, as attractive components in materials, are widely used in material chemistry and biochemistry because of their fascinating chemical properties, such as long-lived excited states and tunability by chemical derivatization.³¹ Herein, POM-based organic–inorganic hybrids, covalently linked with porphyrins, were designed in the search for high efficiency dyes for DSSCs.

To further investigate the properties of POM-based hybrids, quantum chemical calculations as reliable and suitable avenues are crucial to explore the relationship between their structures and properties. In the past several years, the stability, redox, optical and nonlinear optical properties of Lindqvist-, Keggin- and Anderson-type POM hybrids were systematically investigated.^21,32–37 Obviously, DFT and time-dependent DFT (TDDFT) can shed light on the electronic and optical properties of POM-based complexes.³⁸ Herein, a series of POM-based organic–inorganic hybrids were constructed in which the porphyrin derivative was covalently linked with three kinds of POM, Lindqvist-, Keggin- and Anderson-type POMs. DFT and TDDFT calculations on their structural and electronic properties, absorption spectra and transition nature were performed in order to screen out the high efficiency dyes for DSSCs. Furthermore, the results are expected to provide theoretical guidance and prediction for the experimental synthesis of POM-based organic–inorganic hybrids.

Methods

Theoretical background

Based on the working principle of p-type DSSCs, the energy conversion efficiency is closely connected to the light harvesting efficiency (LHE), hole injection efficiency (HJE), dye regeneration efficiency (DRE) and charge recombination efficiency (CRE).

The LHE is an important parameter to assess the performance of DSSCs, which is under the influence of the oscillator strength (f) of the dye corresponding to the maximum absorption λ_max. The LHE is expressed as follows:³⁹

LHE = 1 − 10^−f (1)

The HJE is affected by the free enthalpy ΔG_inj, which is related to the hole injection from the excited states of the dye into the valance band (VB) of NiO. It can be determined using:^40,41

ΔG_inj = −[E₀₀(S*) − E(S/S⁻)] − E_VB (2)

where E_VB is the VB edge of NiO (−4.96 eV),²⁹ E₀₀(S*) is the energy of the excited state sensitizer, and E(S/S⁻) is the reduction potential of the dye.

The DRE can be measured using ΔG_reg, which is determined with the following equation:

ΔG_reg = E(I₂/I₃⁻) − E(S/S⁻) (3)

where E(I₂/I₃⁻) is the reduction potential of the redox mediator (−4.80 eV).²⁹

The free energy of the CRE is ΔG_CR, which can be expressed as follows:

ΔG_CR = E(S/S⁻) − E_VB (4)

The four equations indicate that reducing ΔG_inj and ΔG_reg and increasing ΔG_CR as well as the LHE might be effective ways to achieve higher efficiency dyes.

Computational details

All calculations were carried out using the Gaussian 09 D01 program package.⁴² The geometrical structures were optimized using the B3LYP functional with 6-31G* for non-metal atoms and the LanL2DZ basis set for Mo and Sn atoms.⁴³ As a range-separated hybrid functional, the CAM-B3LYP functional can wonderfully describe the charge transfer situation of excited states. Therefore, the absorption spectra and transition characteristics were analyzed using TDDFT calculations at the CAM-B3LYP/6-31G* level, and the LanL2DZ basis set for Mo and Sn atoms. The number of excited states we calculated via TDDFT method is 20, 50, 50 and 80 for systems 1–4, respectively. The solvent effect was taken into account during the geometry optimization and TDDFT calculations in the solvent N,N-dimethyl formamide (DMF) by means of the polarizable continuum model (PCM).⁴⁴

Results and discussion

Molecular structures

In this work, 10-ethynyl-5,15-biphenyl-porphyrin that benzoic acid is linked to by the ethynyl group is named as system 1. The carboxyl is added as an anchoring group to bond to the semiconductor surface. In 2007, Wei et al. synthesized the hexamolybdate derivative, [Mo₆O₁₈(N-1-C₁₀H₆-2-CH₃)]²⁻ in which the naphthylamine was linked with the hexamolybdate through a Mo≡N bond.⁴⁵ It is found that the introduction of hexamolybdate (Lindqvist-type POM) into naphthylamine can red shift the UV-vis spectrum. Theoretical study indicates that the hexamolybdate acts as an electron acceptor and the organic group as an electron donor.²⁷ Therefore, this phenomenon brings up an interesting question: whether the other types of POM, such as Keggin- and Anderson-type POMs in POM–porphyrin hybrids have the same charge transfer character and red shift the absorption spectra. With the purpose of searching for high-performance dyes, systems 2, 3 and 4 were designed by introducing system 1 to [Mo₆O₁₈N]²⁻, [PSnMo₁₁O₃₉]³⁻ (Keggin-type POMs) and [MnMo₆O₂₄[C(CH₂)₃]CH₃]³⁻ (Anderson-type POM), respectively. The molecular structures of systems 1–4 are shown in Fig. 1.

Fig. 1 Molecular structures of systems 1–4.

Electronic structures

It is known that the energy levels and distributions of the frontier molecular orbitals (FMOs) affect the electronic transition character of dyes. The distributions of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) for systems 1–4 are displayed in Fig. 2. It should be noted that system 4 has an unpaired electron, and its multiplicity and charge are 5 and −3, respectively, so the FMOs are discussed using α and β spin states. In system 1, the HOMO is delocalized over the whole molecule, and the LUMO is mainly localized on the porphyrin ring and extended to the anchoring group. As can be seen, the FMO distributions of systems 2 and 3 are very similar; the HOMO is mainly delocalized over the organic segment and also has some distribution over the Mo≡N part of system 2, while the LUMO is localized on the POM clusters, indicating that the Lindqvist- and Keggin-type POMs play a vital role in the LUMO distributions. There is an excellent synergistic effect between the organic segment and POM cluster in systems 2 and 3, and the intramolecular charge transfer is predicted to be better than that of system 1. As for system 4, the distributions of electron density in the FMOs are almost the same as that of system 1. It is clearly seen that the Lindqvist- and Keggin-type POMs contribute to the FMOs of the studied POM-based porphyrin hybrids, while the Anderson-type POM almost has no effect on the FMO distributions. With the purpose of accelerating hole injection from the excited state of the dye into the VB of NiO, the HOMO distribution of the dye should significantly extend to the anchoring group and overlap with the VB of NiO, which is beneficial for strong electron coupling with the semiconductor. Furthermore, the LUMO of the dye should predominantly be located far away from the surface of NiO, which can reduce the combination of electron and hole, and promote dye regeneration. From the distributions of the FMOs, it can be reasonably inferred that the electron transitions of the Lindqvist- and Keggin-type POM porphyrin derivatives are mainly from the organic groups to the POM clusters, which favors efficient hole injection and facilitates the regeneration of reduced dyes.

Fig. 2 Frontier molecular orbital profiles of systems 1–4.

The energy levels of the FMOs and the HOMO–LUMO gaps were computed and are shown in Table 1. Comparing with system 1, the HOMO levels of systems 2–4 are slightly up-shifted, while the LUMOs of systems 2 and 3 are down-shifted, and the LUMO energy level of system 4 is slightly changed. In order to explain this phenomenon, the compositions of the LUMOs for systems 1–4 were further analyzed and the results are listed in Table S1.† The LUMOs of systems 2 and 3 mainly consist of d orbitals from the Mo atoms (63% for system 2, 69% for system 3) and have some contribution from the p orbitals of the O atoms (33% for system 2, 19% for system 3), while the LUMOs are both composed of 86% p orbitals from the C atoms for systems 1 and 4. For the studied clusters in this work, the LUMOs are localized on the Lindqvist- and Keggin-type POM clusters for systems 2 and 3, respectively, while the LUMO of system 4 is located on the porphyrin ring, and the Anderson-type POM does not contribute to the LUMO. Thus, the LUMO energy levels of systems 2 (−3.45 eV) and 3 (−3.58 eV) are determined using the LUMO energies of Lindqvist- (−3.49 eV) and Keggin-type (−3.29 eV) POMs, which were computed at the B3LYP/6-31G* (LanL2DZ) level (Fig. S1†), while the LUMO energy of system 4 (−2.87 eV) is determined using that of porphyrin (−2.97 eV). Therefore, it can be concluded that Lindqvist- and Keggin-type POMs affect the FMO energies of POM-based porphyrin hybrids, while the Anderson-type POM has a slight influence, which is consistent with the distributions of the FMOs. The HOMO–LUMO energy gaps of the Lindqvist- and Keggin-type POM porphyrin derivatives decrease, predicting that their absorption spectra may red shift in comparison with that of system 1. It is worth mentioning that the HOMO energies of the Lindqvist- and Keggin-type POM porphyrin derivatives are lower than the VB of NiO and the LUMO energy levels are higher than the I⁻/I₃⁻ redox level, which is beneficial to a high hole injection quantum yield and dye regeneration.

Table 1 Frontier molecular orbital energies and energy gaps of systems 1–4 (eV)^a

System	1	2	3	4
a H = HOMO, L = LUMO.
LUMO	−2.97	−3.45	−3.58	−2.87
HOMO	−5.26	−5.21	−5.12	−5.13
H–Lgap	2.29	1.76	1.54	2.26

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Absorption spectra

As we all know, dyes with an excellent performance in p-type DSSCs should have wide absorption which overlaps well with the solar spectrum. To investigate the effect of different types of POM moieties on the optical properties, we performed TD-DFT calculations on systems 1–4 to simulate the ultraviolet-visible (UV-vis) absorption spectra in DMF solution. The excitation energies (E_v), maximum absorption wavelengths (λ_max), oscillator strengths (f) and dominant transition configurations for the studied systems are listed in Table 2, and the UV-vis spectra of systems 1–4 are presented in Fig. 3. It is clearly seen that the simulated spectra of all the systems are divided into two distinct absorption ranges: the first intense peak is located in the 300–500 nm region and the other one is in the range of 500–800 nm. The spectrum of system 1 consists of two peaks at 399 and 633 nm with corresponding oscillator strengths of 2.01 and 0.23, respectively. It is worth mentioning that band 2 in system 2 not only red shifts from 633 nm (system 1) to 730 nm (system 2) but also becomes broader. The result certifies that the Lindqvist-type POM is a strong electron acceptor, which plays a vital role to red-shift and reinforce the UV-vis spectrum. The maximum absorption wavelengths for systems 3 and 4 are similar to those of system 1, while the corresponding oscillator strengths (f) slightly increase. It can be found that the introduction of Keggin- and Anderson-type POMs has negligible influence on λ_max. As a consequence, different kinds of POMs produce different effects on the spectra. For the studied systems, the spectrum of the Lindqvist-type POM porphyrin derivative overlaps well with the solar spectrum. Therefore, based on the absorption spectra, we infer that the Lindqvist-type POM porphyrin derivative may be a promising candidate for DSSCs.

Table 2 Excitation energies (E_v), maximum absorption wavelengths (λ_max), dominant transition configurations, oscillator strengths (f), and the electron density difference maps (EDDMs) of systems 1–4^a

System	Ev (eV)	λmax (nm)	Major assignment	f	EDDMs
a The charge transfer is from the purple area to the blue area.
1	1.96	633	H → L (51%)	0.23
			H → L+1 (17%)
			H−1 → L (16%)
			H−1 → L+1 (14%)
	3.11	399	H−1 → L+1 (45%)	2.01
			H−1 → L (20%)
			H → L+1 (17%)
			H → L (14%)
2	1.70	730	H → L (82%)	1.40
	3.02	411	H−1 → L+6 (28%)	2.47
			H−1 → L+7 (28%)
			H → L (10%)
3	1.94	640	H → L+2 (39%)	0.38
			H → L+3 (16%)
			H → L+8 (15%)
			H−1 → L+8 (14%)
	3.09	401	H−1 → L+8 (43%)	2.06
			H → L+8 (17%)
			H → L+3 (14%)
			H−1 → L+2 (14%)
4	1.94	637	H(α) → L(α) (28%)	0.33
			H(β) → L(β) (27%)
	3.10	401	H−1(α) → L+1(α) (22%)	2.12
			H−1(β) → L+1(β) (22%)
			H−1(α) → L(α) (11%)
			H−1(β) → L(β) (11%)

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Fig. 3 Simulated absorption spectra of systems 1–4 in DMF.

To give an intuitional impression of the origin of the absorption spectra, the electron density difference maps (EDDMs) in Table 2 characterize the dominant electron transitions for systems 1–4. For system 1, both band 1 and band 2 are mainly assigned to the electron promotion from the HOMO to the LUMO/LUMO+1, and mix with some transition from the HOMO−1 to the LUMO/LUMO+1. The charge transfer of system 1 is within the porphyrin ring, which is assigned to a π → π* transition. With regard to systems 2 and 3, it is clear that the two systems have similar charge transfer characters, that is, the major transition originates from the porphyrin segment to the POM cluster through the C≡C moiety and the minor one is a π → π* transition within the porphyrin ring, which is in good agreement with the electronic structure analysis discussed above. For system 4, the charge transfer character is almost the same as that of system 1, indicating that the Anderson-type POM does not participate in the charge transfer process. Considering the analysis of the electronic structure and absorption spectra, the introduction of Lindqvist- and Keggin-type POMs would be favorable for hole injection to the VB of NiO, while the Anderson-type POM fails to promote the above process.

Photovoltaic performance

As discussed above, a high efficiency dye requires a high LHE. In the meantime, we should also take the HJE, DRE and CRE into consideration to fully evaluate the performance of the dye. In order to give an intuitional impression whether these systems can serve as dyes, we calculated the LHE, ΔG_inj, ΔG_reg and ΔG_CR, which are listed in Table 3. It is found that the LHEs of the Keggin- and Anderson-type POM porphyrin derivatives are relatively very low. Furthermore, ΔG_inj of the Anderson-type POM porphyrin derivative is positive, which hinders hole injection. The LHE of the Lindqvist-type POM porphyrin derivative is 0.96 and higher than those of other dyes reported in the literature,^21,22 indicating that the covalently linked porphyrin with the Lindqvist-type POM improves the light harvesting ability. Because of the negative ΔG_inj and ΔG_reg, the Lindqvist-type POM porphyrin derivative (system 2) has sufficient driving force to ensure efficient electron injection and dye regeneration. The ΔG_CR is positive, which means that the charge recombination process is not spontaneous. As a consequence of the above results, system 2 meets the requirements of a dye. Therefore, we expect the Lindqvist-type POM porphyrin derivative to be a promising candidate for p-type DSSCs.

Table 3 Excitation energy E_v, E(S/S⁻), ΔG_inj, ΔG_reg, ΔG_CR (eV) and LHE for systems 2–4

System	Ev	E(S/S^−)	ΔGinj	ΔGreg	ΔGCR	LHE
2	1.70	−3.68	−0.42	−1.12	1.28	0.96
3	1.94	−3.73	−0.71	−1.07	1.23	0.58
4	1.94	−3.01	0.01	−1.79	1.95	0.53

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Conclusions

In order to shed light on the influence of different types of POM on the charge transfer of POM-based organic–inorganic hybrids as well as searching for high efficiency p-type dyes, Lindqvist-, Keggin-, and Anderson-type POM porphyrin hybrids were designed. The geometries, electronic structures and absorption spectra of systems 1–4 were systematically investigated by means of DFT and TD-DFT methods. Comparing with the porphyrin derivative (system 1), the LUMO energy levels for the Lindqvist- and Keggin-type POM porphyrin derivatives decrease due to the introduction of the Lindqvist- and Keggin-type POMs. Meanwhile, the LUMO distributions for the Lindqvist- and Keggin-type POM porphyrin derivatives are mainly located on the POM clusters, while the Anderson-type POM does not contribute to the FMOs. The absorption spectrum of system 2 is red shifted about 100 nm in comparison with system 1. Therefore, the Lindqvist-type POM porphyrin derivative exhibits strong and broad absorption in the visible region, which overlaps well with the solar spectrum. As for systems 3 and 4, the absorption spectra are not improved by the introduction of Keggin- and Anderson-type POMs. The analysis of the charge transfer shows that the Lindqvist-type POM is a strong electron acceptor. The performance of system 2 as a dye was further verified, showing that the Lindqvist-type POM porphyrin derivative possesses a high LHE, large HJE and DRE and retarded charge recombination, and matches the requirements of p-type DSSCs. Thus, introducing a Lindqvist-type POM is an effective way to improve the properties of a dye. As a consequence, the Lindqvist-type POM porphyrin derivative is a potential candidate among these designed POM-based complexes. It is expected that our studies can be helpful for the future design and screening of new efficient porphyrin–POM dyes to enhance the performance of p-type DSSCs.

Acknowledgements

The authors gratefully acknowledge financial support by NSFC (21131001), Doctoral Fund of Ministry of Education of China (20100043120007), and the Science and Technology Development Planning of Jilin Province (20100104, 20100320).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Cartesian coordinates of the optimized structures in this work. See DOI: 10.1039/c5ra17343d

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