Zinc-Porphyrin Based Dyes for Dye-Sensitized Solar Cells - American

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Zinc-Porphyrin Based Dyes for Dye-Sensitized Solar Cells S. Karthikeyan and Jin Yong Lee* Department of Chemistry, Sungkyunkwan University, Suwon, 440-746 Korea

J. Phys. Chem. A 2013.117:10973-10979. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/14/18. For personal use only.

S Supporting Information *

ABSTRACT: We have designed seven efficient sensitizers based on the zincporphyrin structure for dye sensitized solar cells (DSSCs). The geometries, electronic properties, light harvesting efficiency (LHE), and electronic absorption spectra of these sensitizers are studied using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. We found that the designed sensitizers have smaller HOMO− LUMO energy with broadened and red-shifted absorption bands (300−1100 nm) having high molar extinction coefficient compared to the so far known best sensitizer (YD2-o-C8). The position of HOMO−LUMO energy level of these sensitizers ensures a positive effect on the process of electron injection and dye regeneration. Our theoretical calculations reveal that the new sensitizer can be used as a potential sensitizer for DSSCs compared to YD2-oC8.

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INTRODUCTION Dye-sensitized solar cells (DSSCs) have attracted great attention in scientific research for practical applications due to the potential advantages of a low cost, easy production, flexibility, and transparency in comparison with conventional crystalline silicon solar cells.1−9 In recent years, many attempts have been made to develop new efficient sensitizers for practical usage. Sensitizers based on ruthenium complexes,10−19 metal free organic dyes,20−23 and porphyrins24−28 were developed and used as efficient sensitizers for DSSCs. Among them, ruthenium dyes and porphyrin dyes have reached power conversion efficiencies of more than 10%. However, ruthenium complex dyes are not suitable due to cost effectiveness and environmental concerns, as ruthenium is a rare and expensive metal which limits the potential for wide applications of these complexes. Thus, porphyrin dyes as an alternative to ruthenium complexes have attracted much attention due to many advantages such as the diversity of their molecular structures, high molar extinction coefficient, simple synthesis route, low cost, and environmental friendliness. Recently, Gratzel reported28 a D-π-A based zinc porphyrin dye (YD2-o-C8) and achieved an efficiency of solar to electric conversion up to 12.3% under standard global AM 1.5 solar conditions with enduring stability. YD2-o-C8 is the best reported porphyrin dye other than the ruthenium based dyes. However, the major problem of YD2-o-C8 and its derivatives is that they show very low molar extinction coefficients in the region of 500−600 nm as well as in the long wavelength region, which greatly decreases the light harvesting abilities. Many groups have put effort trying to overcome this problem but failed to do so completely.29−33 Here, we designed seven new zinc-porphyrin sensitizers (Scheme 1). We have found that these new sensitizers have high molar extinction coefficient and broad absorption region from 300 to 1100 nm. Furthermore, for © 2013 American Chemical Society

better understanding these new zinc-porphyrin sensitizers, we investigated energy level alignment, energy gap, electrons localization, light harvest efficiency, and UV−visible-nearinfrared absorption spectra. We have found that our new sensitizers would have better performance as compared with the so far known the best porphyrin sensitizer, YD2-o-C8.

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COMPUTATIONAL METHOD The geometries of our new sensitizers were optimized by density functional theory (DFT) calculations using the B3LYP exchange-correlation functional with 6-31G(d) basis sets. The vibrational frequency calculations were performed to confirm that the new zinc-porphyrin structures are local minima (no imaginary frequencies) on potential energy surfaces. This level of theory and basis sets were deemed appropriate for calculations on zinc-porphyrin sensitizers and resulted in good agreement with experimental results.34 The location of HOMO, LUMO, and the electronic properties of new porphyrin dyes were investigated by using the B3LYP/631(d) method in THF solvent. The electronic populations of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were calculated to show the position of the localization of electron populations along with the molecular orbital energy. To provide insight into the electronic properties of zinc-porphyrin sensitizers, timedependent density functional theory (TD-DFT) calculations were performed on ground state geometries at the B3LYP/631G(d) level in THF solvent. All calculations were performed using a suite of Gaussian 09 packages.35 Received: August 24, 2013 Revised: September 26, 2013 Published: October 3, 2013 10973

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LUMO levels of PorCND1A1-PorCND1A7 sensitizers are between the narrow ranges of −3.04 to −3.45 eV. These values are higher than the lower bound level of the conduction band of TiO2 (−4.0 eV), indicating that the efficiency of charge injection from the excited sensitizer molecule to the TiO2 conduction band is viable. The HOMO energy levels of new zinc-porphyrin sensitizers are in the range of −4.93 to −4.99 eV. These values are lower than the electrolyte oxidation potential (I−/I3: −4.8 eV), ensuring effective sensitizer regeneration process, so that the oxidized sensitizers could be regenerated by the reduced species (I−) in the electrolyte to guarantee the efficient charge separation in the photovoltaic devices. The energy gap decreases as the length of the πconjugated linker and the number of the heteroatoms increase. The energy gaps of the zinc-porphyrin based dyes were calculated to be in the following order: A2 (1.54 eV) < A7 (1.66 eV) < A6 (1.75 eV) < A3 (1.82 eV) < A1 (1.86 eV) < A4 (1.87 eV) < A5 (1.89 eV) < A0 (2.29 eV). The energy gaps of our new sensitizers are much smaller than the so far known best sensitizer YD2-o-C8. It is expected from previous studies38,39 that the sensitizers that have smaller band gap values show higher efficiency in the DSSCs; therefore, our new zincporphyrin sensitizers would have better light harvesting ability than the so far known best sensitizer dye YD2-o-C8. Figure S1 in the Supporting Information clearly shows that the decrease in energy gap is mainly due to the substantial stabilizing effect on the LUMO energy levels of the new zinc-porphyrin sensitizers. This result shows that the energy gap of the dye sensitizer can be tuned by substituting a suitable electron donating or withdrawing group. The HOMO and LUMO energies provide an estimation of the ionization potential and electron affinity, respectively.40 Thus, a group with high HOMO energy could be a strong electron donating group, whereas that with low LUMO energy could be a strong electron withdrawing group. On the basis of this rationale, the order of electron accepting ability was calculated to be in the following order: A2 > A7 > A6 > A3 > A5 > A4 > A1 > A0. In particular, all these acceptor groups are much better than the acceptor of YD2-o-C8. From Table 2, D1 is a stronger electron donating group than D2. Figure 2 shows the frontier molecular orbitals of zincporphyrin based dyes. The HOMOs of the dyes are mainly localized on the ancillary ligands and porphyrin ring moiety, and there are much less electron density around the acceptor, which shows a good electron donating effect of the −N(PhCH3)2 group. The LUMOs are localized on the π-linker and the anchoring group, showing the ability to transfer electrons from donor region to acceptor region when dyes are excited by light. Since the dyes are connected with semiconductor through acceptor, the excited electron in the dyes would be injected into semiconductor. The suitability of a dye molecule as a sensitizer for DSSCs depends on its ability to form charge separated states. A longer charge separated state of a dye increases the chance of positive and negative charges reaching the electrodes by avoiding charge recombination. In order to obtain a longer charge separated state, HOMO must be localized on the donor subunit and LUMO on the acceptor and anchoring groups. Detailed analysis of the molecular orbital contribution41 to the HOMO and LUMO can predict the formation of charge separated states. Figure 2 shows that electron densities of HOMOs of these dyes are mostly localized on the ancillary ligand and porphyrin unit, and the LUMOs are mainly localized

Scheme 1. Molecular Structures of New Porphyrin Dyes

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RESULTS AND DISCUSSION The formation of π-stacked aggregation of sensitizer is a major problem responsible for the low conversion efficiency of DSSC. The aggregation of sensitizer molecules may lead to intermolecular quenching or cause the molecules not to attach to the semiconductor surface.36 Therefore, to obtain good photovoltaic performance, aggregation of sensitizers needs to be avoided through appropriate chemical modification In this paper, we made a systematic search for sensitizer based on zinc-porphyrin to obtain improved light absorption properties at UV−visible-near IR region. For this, −N(PhCH3)2 ancillary ligands and different π-bridge moieties were substituted with a basic zinc-porphyrin structure and thereby we designed different structure of zinc-porphyrin based sensitizers and studied their electronic properties using density functional theory and time-dependent density functional theory calculations. Although a variety of anchoring groups like carboxylic acid, cynoacrylic acid, phosphonic acid, sulfonate, and silane derivatives are employed to ensure efficient electron injection into the TiO2 conducting band,37 the present study employs cynoacrylic acid as the anchoring group. The chemical structures of different functional group substituted porphyrins considered in this study are shown in Scheme 1. In the basic porphyrin structure, the central two H+ ions were replaced by Zn2+ for all of the structures used. The optimized structure and frontier molecular orbital are shown in Figures 1 and 2, respectively. The optimized molecular structures of all new zinc-porphyrin sensitizers are nonplanar. Calculated HOMO, LUMO, and energy gap of new sensitizers are given in Table 1. From Table 1, both the HOMO and LUMO levels of these sensitizers match well with the energy requirement for an efficient photosensitizer. The 10974

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Figure 1. Optimized geometries of new porphyrin sensitizer and YD2-o-C8 computed at the B3LYP/6-31(d) level.

The electronic absorption spectra of the new zinc-porphyrin sensitizers were simulated using the TD-DFT method at the TD-B3LYP/6-31G* level of theory. The PCM model was employed to incorporate the effect of solvent. All TD-B3LYP calculations were carried out using THF as the solvent. Figure 3 shows the simulated electronic absorption spectra of the new zinc-porphyrin sensitizers and the reference sensitizer (YD2-oC8). In applications, the ideal absorption spectra of sensitizers should cover the visible spectral region (from 400 to 800 nm) showing Q-bands and even extend to the near-infrared region (∼1000 nm). However, the so far known best sensitizers YD2 and YD2-o-C8 do not have good absorption in the region of 500−600 nm and longer than 700 nm. Figure 3 shows that all of the new sensitizers have excellent absorption in the visible to near-infrared region with high molar extinction coefficients. It is evident that the simulated absorption spectra at the B and Q

on acceptor units (π linker and anchoring unit), which shows a good charge separated state. The frontier molecular orbital shapes of the HOMOs are similar for all new dyes, but shapes of the LUMOs are different. This difference in LUMO is due to the different withdrawing strength of the electron withdrawing groups that are substituted in the basic zinc-porphyrin structure. Molecular orbital composition of the HOMO and LUMO of the new zinc-porphyrin dyes and YD2-o-C8 are given in Table 1. The contribution of anchoring groups of our new dyes to LUMO is 18−28%, whereas the so far known best dye YD2-o-C8 contributed 2%. Therefore, we may anticipate that new zinc-porphyrin dyes have more electronic coupling with the TiO2 surface and would be more favorable for electron injection into the TiO2 surface than the so far known best YD2o-C8 dye. 10975

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Figure 2. Frontier molecular orbitals of PorCNDA1-PorCNDA7 sensitizers at the B3LYP/6-31G(d) level.

Table 1. Percent (%) Molecular Orbital Contribution of the HOMO and LUMO of the Zinc-Porphyrin Based Sensitizer at the B3LYP/6-31G(d) Level acceptor (%) YD2-o-C8 PorCND1A1 PorCND1A2 PorCND1A3 PorCND1A4 PorCND1A5 PorCND1A6 PorCND1A7

HOMO LUMO HOMO LUMO HOMO LUMO HOMO LUMO HOMO LUMO HOMO LUMO HOMO LUMO HOMO LUMO

E (eV)

band gap

porphyrin linker (%)

ancillary ligand (%)

−4.99 −2.70 −4.96 −3.10 −4.99 −3.45 −4.97 −3.15 −4.98 −3.12 −4.93 −3.04 −4.99 −3.24 −4.96 −3.29

2.29

46 71 38 41 37 19 37 38 37 45 39 35 36 34 38 17

48 10 57 7 57 0 56 8 56 9 53 7 58 6 55 4

1.86 1.54 1.82 1.87 1.89 1.75 1.66

π linker 6 17 5 24 5 58 5 27 6 20 8 33 6 32 7 61

anchoring group 0 2 0 28 1 23 2 27 1 26 0 25 0 28 0 18

λmax

LHE

437, 655 405, 461, 532, 802 411, 482, 585, 961 414, 467, 538, 820 414, 461, 535, 808 437, 482, 540, 788 411,467, 564, 852 420, 488, 582,890

97 97 82 95 95 90 94 79

shifted, which make them have the smallest energy gaps. Figure 3 shows that the Q bands of all of the new zinc-porphyrin sensitizers originate from π → π* transition with a strong HOMO → LUMO transitions. Most importantly, the molar extinction coefficients of the Q bands are significantly enhanced compared to that of the reference sensitizer YD2-o-C8. This is due to the introduction of heteroatom in the acceptor part. The

bands for all the new sensitizer are broadened and red-shifted significantly compared to the reference sensitizer YD2-o-C8. The simulated UV−visible spectrum of reference sensitizer YD2-o-C8 shows a Q-band at 656 nm, whereas new sensitizer PorCND1A1-PorCND1A7 at 802, 969, 821, 804, 789, 852, and 894 nm, respectively. Among the new sensitizers, the Q bands of sensitizers PorCND1A2 and PorCND1A7 are the most red10976

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CONCLUSION In this work, we have designed seven zinc-porphyrin based sensitizers with D-π-A structure. We have found that a new sensitizer can absorb light in the UV−visible to the nearinfrared region (300−1100 nm) with a higher molar extinction coefficient compared to the reference sensitizer YD2-o-C8. The energy gaps of the designed sensitizers lie within 1.54−1.89 eV which are lower than that of the reference sensitizer YD2-o-C8 (2.29 eV). The contribution of anchoring groups of our new sensitizer to LUMO is 18−28%, whereas the so far known best sensitizer YD2-o-C8 contributed 2%. Therefore, we may anticipate that new zinc-porphyrin sensitizers have more electronic coupling with the TiO2 surface and would be more favorable for electron injection into the TiO2 surface than the so far known best YD2-o-C8 sensitizer. The LHE values of new dyes are in the range of 79−97%.

Table 2. HOMOs and LUMOs of the Donor (D) and Acceptor (A) in Different Dyesa A1 A2 A3 A4 A5 A6 A7 A0 D1 D2 a

HOMO

LUMO

−6.65 −6.61 −6.46 −6.39 −5.87 −6.81 −6.07 −6.65 −4.48 −5.40

−2.83 −3.46 −2.97 −2.85 −2.86 −3.19 −3.31 −1.78 −0.02 0.48

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All values in eV and calculated using the B3LYP/6-311G(d) method.

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ASSOCIATED CONTENT

S Supporting Information *

Calculated HOMO and LUMO energy levels, excitation energies (nm), and oscillator strengths (f) of new porphyrin sensitizers and reference sensitizer (YD2-o-C8). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +82-31-299-4560. Fax: +8231-290-7075. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NRF Grants (Nos. 2007-0056343 and 2011-0015767) funded by MEST, Republic of Korea (MEST). The authors would like to acknowledge the support from KISTI supercomputing center through the strategic support program for the supercomputing application research [No. KSC-2012-C2-43].

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Figure 3. Calculated absorption spectra of new dyes and reference dye YD2-o-C8.

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broadening and red shift of the absorption bands together with an increase in the molar extinction coefficient of the Q bands compared to that of the Soret band are promising strategies to resolve the inadequate light harvesting properties of the reference sensitizer YD2-o-C8. TDDFT excitation energies (nm) and oscillator strength (f) of PorCND1A0-A7 in the THF solvent are given in the Supporting Information (Tables S1−S8). All sensitizer can effectively absorb light in the UV−visible and near-infrared regions with high molar extinction coefficients (more than 75 000 M−1cm−1). Noticeably, the molar extinction coefficients of all sensitizers (PorCND1A1PorCND1A7) are even higher than N3 dye (16 000 M−1cm−1).42 Light harvest efficiency (LHE) of all dyes can be obtained using a formula, LHE = 1−10−f,43,44 where f is the oscillator strength of dyes. The oscillator strength of a dye is directly obtained from TD-DFT calculations. We calculated LHE (obtained based on the main absorption peaks), the oscillator strength and LHE of all dyes, and the results are listed in Table 1. The LHE values of the new dyes are in the range of 79−97%. 10977

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dx.doi.org/10.1021/jp408473k | J. Phys. Chem. A 2013, 117, 10973−10979