SONG Jun-Ling LU Rui-Feng LI An

a (School of Chemical & Material Engineering, Jiangnan University, Wuxi 214122, China)

b (Division of Chemistry and Biological Chemistry, School of Physical & Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore)

1 INTRODUCTION

Dye-Sensitized Solar Cells (DSSCs) have attracted considerable attention as conventional solid-state photovoltaic devices for the conversion of solar energy to electric power[1].Organic and inorganic dyes suitable for DSSCs have been under intensive investigation[1-3].The major ongoing research has focused on ruthenium (Ru) polypyridyl complexes,however, traditional ruthenium dyes are expensive and comparatively lower molar extinction coefficient,limiting their wide applications.Thus, it is necessary to find cheaper and safer alternatives for the dyes in DSSCs.Recently, some new types of organic dyes have been investigated as the sensitizers in solar cell.For example, Hara[4,5], and Yanagida[6]and coworkers reported an organic dye containing a donor (D)group (dialkylaminophenyl group) and an acceptor(A) group (cyanoacrylic acid), which exhibit good cell preformance.Another cheaper dyes, porphyrin derivatives, with large π-conjugation show interesting photo physical and electrochemical properties.The characteristic strong absorption intensity at Soret and moderate Q bands make porphyrin and their metal complexes as optimal candidates for DSSCs.In comparison with Ru-based DSSCs, porphyrinbased DSSCs could be fine-tuned through varying substitution patterns and/or complexing with metals.To date, diverse porphyrins with different donating or anchoring group have been designed, synthesized and employed in DSSCs and the maximum 13%conversion efficiencies were achieved[7-11].It has been demonstrated that dyes with more effective intramolecular charge transfer (ICT) molecules will show better cell performance, thus an effective strategy is to introduce strong electron donating(D) and electron-withdrawing (A) groups to get a D-π-A structure within the molecule.It is well-known that dyes with electron-rich triphenylamine-based derivatives as donor, such as diphenyl-benzenamine,and electron-withdrawing groups, such as cyanoacrylic acid, have been widely investigated[7-13].Furthermore, the introduction of strong electron withdrawing groups to a triphenylamine backbone can easily tune the absorption spectra of dye molecules, thus controlling the extent of intramolecular charge transfer.In addition, the positions and kinds of donor-acceptor substituents had significant affects on the efficiency of porphyrins in DSSCs[12,13].The stronger electron-accepting effects can be further obtained by introducing several cyano groups into a conjugated system.However, attaching the nitro group as one of the strong electron-withdrawing groups to a triphenylamine backbone is still rarely studied.Thus, optimization of these porphyrin dyes becomes increasingly important to enhance the overall device efficiency.The design and synthesis of highly efficient sensitizers is still a challenging task.

Recently, we have prepared a series of metal-free porphyrin dyes with different D-A units.This study showed that by introducing appropriate substituent groups at appropriate position on the porphyrin ring,the cell performance could be dramatically improved due to the increase of the intramolecular charge separation[7].This push-pull type of dyes has appropriate structures that allow efficient charge separation.Moreover, a strong conjugation across the donor, bridge, and anchor which allows good electronic coupling between the LUMO of the dye and the TiO2conduction band should exist to ensure high charge transfer rates.

In this article, as part of our efforts to develop more efficient porphyrin dyes, we designed a series of new push-pull type of metalloporphyrin derivatives as shown in Fig.1.Firstly, we performed Density Functional Theory (DFT) calculations to virtually screen candidate analogues and then we synthesized two new D2 and D3 porphyrin dyes containing N,N-diphenyl amine as the electron donor,carboxylic acid as the acceptor, and nitro group acting as the second withdrawing group for D3.The Zn(II)-5,10,15-tris phenyl-20-(4-carboxy methyl phenyl)-porphyrin (D1) was employed as a reference to evaluate the effects of these unsymmetrical metallo porphyrin analogues on the photovoltaic properties (Chart 1).Finally, D1, D2 and D3 are adsorbed onto TiO2electrodes to test cell performances in DSSCs.

Chart 1. Chemical structures of porphyrin derivatives used in this study

Fig.1. Molecular orbital energy levels (eV) of compounds 1～8 calculated using B3LYP/6-31G* in vacuo for various dyes originated from compared to results for a model of a TiO2 nanoparticle

2 EXPERIMENTAL

2.1 Calculation method

All calculations were performed with the Gaussian 03[14]program package.The molecular structure of each compound in vacuo was fully optimized both at the ground and the first excited states using density functional theory (DFT) with the B3LYP density functional[14]and the 6-31G* basis set, along with the corresponding pseudopotential for the metal atom.All optimized geometries were subjected to vibrational frequency analysis, and characterized as a minimum (no imaginary frequencies).The time-dependent DFT (TD-DFT) at the level of B3LYP/6-31G*was carried out to study the excited state of selected molecules.The 20 lowest spin-allowed singlet transitions were involved to simulate the absorption spectra.The C-PCM[15]frameworks were used to describe the electrostatic solute-solvent interactions.The contribution of the singly excited state configurations to each electronic transition of the three metalloporphyrin analogues was calculated using the Swizard program, version 4.2[16].Graphical molecular orbitals were generated with the Gauss View software program (version 3.09)[14]and the molecular orbital contributions from the groups of atoms were obtained from Aomix program[16].

2.2 Materials and instruments

All chemicals were obtained from commercial sources and used without further purification.Unless otherwise noted, all reactions were carried out in oven dried glassware under an atmosphere of nitrogen and distilled solvents were transferred by syringe.Solvents and reagents were purified according to the standard procedure prior to use.Evaporation of organic solutions was achieved by rotary evaporation with the water bath temperature below 40 ℃.Product purification by flash column chromatography was accomplished using silica gel 60(0.010～0.063 nm).Technical grade solvents were used for chromatography and distilled prior to use.NMR spectra were recorded at room temperature on a 300 MHz Bruker ACF 300, 400 MHz Bruker DPX 400 and 500 MHz Bruker AMX 500 NMR spectrometers, respectively.The residual solvent signals were taken as the reference (7.26 ppm for1H NMR spectroscopy).Chemical shift (δ) is referred in terms of ppm, and coupling constants (J) are given in Hz.Following abbreviations classify the multiplicity: s =singlet, d = doublet, t = triplet, q = quartet, m =multiplet or unresolved, br = broad signal.Infrared spectra were recorded on a Bio-RAD FTS 165 FT-IR Spectrometer and reported in cm-1.Samples were prepared in thin film technique.HRMS (ESI) spectra were recorded on a Finnigan/MAT LCQ quadrupole ion trap mass spectrometer coupled with the TSP4000 HPLC system and the Crystal 310 CE system.Unless otherwise stated, all reactions were carried out in oven dried glassware under a nitrogen atmosphere and distilled solvents were transferred by syringe.Solvents and reagents were purified according to the standard procedure prior to use.Evaporation of organic solutions was achieved by rotary evaporation with the water bath temperature below 40 ℃.Product purification by flash column chromatography was accomplished using silica gel 60.Technical grade solvents were used for chromatography and distilled prior to use.NMR spectra were recorded at room temperature on a 300 MHz Bruker ACF 300, 400 MHz Bruker DPX 400 and 500 MHz Bruker AMX 500 NMR spectrometers, respectively.The residual solvent signals were taken as the reference (7.26 ppm for1H NMR spectroscopy).Infrared spectra were recorded on a Bio-RAD FTS 165 FT-IR Spectrometer and reported in cm-1.Samples were prepared in thin film technique.HRMS (ESI) spectra were recorded on a Finnigan/MAT LCQ quadrupole ion trap mass spectrometer coupled with the TSP4000 HPLC system and the Crystal 310 CE system.Absorption spectra were measured on a JASCO V-670 UV-Vis-Near IR spectrophotometer and photoluminescence analyses were performed on a Cary Eclipses fluorescence spectrometer.

2.3 Syntheses of metalloporphyrins D1, D2 and D3

The synthesis procedures of D1, D2 and D3 are shown in Schemes 1 and 2.Porphyrin P1 was synthesized by the cross condensation of phenyldiphyrromethane with 4-carbomethoxybenzaldehyde and benzaldehyde followed by hydrolysis under basic condition[12].Then porphyrin P1 was treated with Zn(OAc)2to obtain D1 (Scheme 1).Similarly,syn-thetic routes to D2 and D3 are displayed in Scheme 2.Structures of all new compounds were characterized by spectroschopic analyses including1H NMR, FT-IR and mass spectra.

Scheme 1. Synthetic routes of D1

Scheme 2. Synthetic routes of D2 and D3

2.3.1 General procedure for the synthesis of 4-(10,15,20-triphenyl-21H,23H-porphin-5-yl)-phenyl acid-Zn (D1)

To a solution of 4-(10,15,20-triphenyl-21H,23H-porphin-5-yl)-phenyl acid (0.06 g, 0.07 mmol) in methanol was added zinc acetate dihydrate (0.15 g,0.69 mmol) in methanol.The solution was stirred at 80 ℃ for 2 h.The solvent was removed by using a rotatory evaporator, and the residue dissolved in dichloromathane and passed through the filter paper.The filtrate was washed with water, dried over anhydrous sodium sulfate, and then the solvent was removed in vacuo to give the pure porphyrin product D1 (0.06 g, 90%) as a purple power.1H NMR (500 MHz, CDCl3): δ in ppm = 8.97～8.94 (m, 6H), 8.90(d, J = 4.2 Hz, 2H), 8.58 (bs, 2H), 8.31 (d, J = 8.9 Hz,2H), 8.22 (d, J = 7.2 Hz, 6H), 7.79～7.73 (m, 9H).IR (CHCl3): νmax= 3018, 2399, 1558, 1215, 758 cm-1.H RMS (ESI): m/z: calcd.for C45H29N4O2Zn:721.1582 [M]+; found: 721.1611.The syntheses of 4-(15-N,N-diphenyl-benzenamine-10,20-biphenyl-21H,23H-porphin-5-yl)-phenyl acid methyl ester and 4-(15-N,N-diphenyl-benzenamine-10,20-biphenyl-21H,23H-porphin-5-yl)-3-nitro-benzoic acid methyl ester were carried out by a modified Aldler-Longo method[12].

2.3.2 General procedure for the synthesis of 4-(15-N,N-diphenyl-benzenamine-10,20-biphenyl-21H,23H-porphin-5-yl)-phenyl acid-Zn (D2)

To a solution of 4-(15-N,N-diphenyl-benzenami-ne-10,20-biphenyl-21H,23H-porphin-5-yl)-phenyl acid 7 (0.1 g, 0.12 mmol) in methanol was added zinc acetate dihydrate (0.26 g, 1.2 mmol) in methanol.The solution was stirred at 80 ℃ for 2 h.The solvent was removed by using a rotatory evaporator, and the residue dissolved in dichloromathane and passed through the filter paper.The filtrate was washed with water, dried over anhydrous sodium sulfate, and then the solvent was removed in vacuo to give the pure porphyrin product D2 (0.1 g, 93%) as a purple power.1H NMR (500 MHz, CDCl3): δ in ppm = 9.04～9.03 (m, 1H),8.92～8.86 (m, 4H), 8.84 (d, J = 4.6 Hz, 2H), 8.58(d, J = 7.9 Hz, 2H), 8.40 (d, J = 6.1 Hz, 2H), 8.27～8.25 (m, 5H), 8.11 (d, J = 5.1 Hz, 1H), 7.83～7.79(m, 7H), 7.49 (d, J = 8.2 Hz, 1H), 7.45 (d, J = 4.2 Hz,5H), 7.28 (d, J = 7.5 Hz, 4H), 7.27～7.16 (m, 2H).IR (CHCl3): νmax= 3018, 2399, 1506, 1215, 756 cm-1.H RMS (ESI): m/z: calcd.for C57H38N5O2Zn:888.2317 [M]+; found: 888.2348.

2.3.3 4-(15-N,N-diphenyl-benzenamine-10,20-biphenyl-21H,23H-porphin-5-yl)-3-nitro-benzoic acid methyl ester

4-(Diphenylamino) 2-nitrobenzaldehyde (0.61 g,2.25 mmol) and methyl 4-formyl 3-nitrobenzoate(0.47 g, 2.25 mmol) was added to a dry roundbottomed flask and placed under high vacuum.Acetic acid (80 mL) was then added to the flask and mix the solid uniformly under nitrogen atmosphere.This reaction mixture was heated to reflux and 5-phenyldiphrromethane (1.0 g, 4.50 mmol) in acetic acid (20 mL) was added slowly over 20 min.The mixture was refluxed for 2 h, and then the crude product was evaporated to dryness under vacuum.The resulting crude product was purified by column chromatography on silica gel to give compound as a purple solid (0.4 g, 10%).1H NMR (500 MHz,CDCl3): δ in ppm = 9.08 (s, 1H), 9.01 (d, J = 4.5 Hz,2H), 8.87～8.85 (m, 4H), 8.61～8.59 (m, 3H), 8.38(d, J = 7.9 Hz, 1H), 8.34～8.23 (m, 4H), 8.07 (bs,2H), 7.81～7.75 (m, 6H), 7.47～7.35 (m, 10H),7.17～7.14 (m, 2H), 4.16 (s, 3H), –2.72 (s, 2H).IR(CHCl3): νmax= 3018, 2399, 1732, 1506, 1215, 756 cm-1.HRMS (ESI): m/z: calcd.for C58H41N6O4:885.3189 [M]+; found: 885.3207.

2.3.4 4-(15-N,N-diphenyl-benzenamine-10,20-biphenyl-21H,23H-porphin-5-yl)-3-nitro-benzoic acid

4-(15-N,N-diphenyl-benzenamine-10,20-biphenyl-21H,23H-porphin-5-yl)-3-nitro-benzoic acid methyl ester (0.2 g) was dissolved in 75 mL of isopropanol and 10 mL of 2 M aq KOH was added slowly to the reaction mixture.This reaction mixture was refluxed for 6 hours, which was then cooled to room temperature.Once cooled, an equivalent amount of 1 M HCl was slowly added and the resulting solution (pH = 3)was extracted several times with ethyl acetate.The crude product was purified by column chromatography on silica gel to give the desired porphyrin product as a purple solid (0.18 g, 91%).1H NMR(500 MHz, CDCl3): δ in ppm = 9.02 (d, J = 5.5 Hz,2H), 9.00～8.87 (m, 5H), 8.61 (bs, 2H), 8.51 (bs,1H), 8.35 (d, J = 7.4 Hz, 1H), 8.28 (bs, 2H), 8.20 (d,J = 7.5 Hz, 2H), 8.09 (d, J = 8.2 Hz, 2H), 7.80 (bs,6H), 7.49～7.43 (m, 10H), 7.17 (t, J = 5.2 Hz, 2H), –2.72 (s, 2H).IR (CHCl3): νmax= 3018, 2399, 1506,1419, 1215, 756 cm-1.HRMS (ESI): m/z: calcd.for C57H39N6O4: 871.3033 [M]+; found: 871.3029.

2.3.5 4-(15-N,N-diphenyl-benzenamine-10,20-biphenyl-21H,23H-porphin-5-yl)-3-nitro-benzoic acid-Zn (D3)

To a solution of 4-(15-N,N-diphenyl-benzenamine-10,20-biphenyl-21H,23H-porphin-5-yl)-3-nitro benzoic acid (0.07 g, 0.07 mmol) in methanol was added zinc acetate dihydrate (0.15 g, 0.7 mmol) in methanol.The solution was stirred at 80 ℃ for 2 h.The solvent was removed by using a rotatory evaporator, and the residue dissolved in dichloromathane and passed through the filter paper.The filtrate was washed with water, dried over anhydrous sodium sulfate, and then the solvent was removed in vacuo to give the pure porphyrin product D3 (0.065 g, 86%)as a purple power.1H NMR (500 MHz, CDCl3): δ in ppm = 9.02 (d, J = 5.5 Hz, 2H), 9.00～8.87 (m, 5H),8.61 (bs, 2H), 8.51 (bs, 1H), 8.35 (d, J = 7.4 Hz, 1H),8.28 (bs, 2H), 8.20 (d, J = 7.5 Hz, 2H), 8.09 (d, J =8.2 Hz, 2H), 7.80 (bs, 6H), 7.49～7.43 (m, 10H),7.17 (t, J = 5.2 Hz, 2H).IR (CHCl3): νmax= 3018,2399, 1506, 1215, 756 cm-1.HRMS (ESI): m/z: calcd.for C57H37N6O4Zn: 933.2168 [M]+; found: 933.2153.

2.4 Preparation of the metalloporphyrinmodified TiO2 electrode

The 5.0 ± 0.2 μm thick films of nanocrystalline TiO2(Ti-nanoxide T/SP, Solaronix) were deposited on transparent conducting glass (SnO2:F, FTO, 15 Ω/square, Solaronix) with the doctor blade squeegee method.After heating to 450 ℃ for 2 h, TiO2substrates were cooled down and were prepared for dye loading.Then the TiO2plates (electrodes) were dipped into a 4 × 10-4M dye solution in THF and left for 2 h, and taken out and rinsed with methanol.A sandwich cell was prepared using the dye-sensitized electrode as the working electrode and a platinumcoated conducting glass electrode as the counter electrode.The counter electrode used was a 400 ? Pt fabricated by e-beam evaporation on a commercial indium tin oxide glass.The two electrodes separated by 50 μm thermal-plastic Suryln spacers were bonding together, and the electrolyte composed of 0.1 M I2, 0.1 M LiI, 0.5 M terbutylpyridine, and 0.6 M 1-hexyl-3-methylimidazolium iodide in methoxyacetonitrile was introduced between the electrodes by capillary action.

2.5 Photovoltaic characterization

A Sun 2000 solar simulator light source (Abettechnologies, U.S.A.) was used to give an irradiance of 100 mW·cm-2(the equivalent of one sun at air mass (AM) 1.5) at the surface of solar cells.The spectral output of the lamp was matched in the region of 350～750 nm with the aid of a sunlight filter (bettechnologies, U.S.A.) so as to reduce the mismatch between the simulated and the true solar spectra.Various incident light intensities were regu-lated with wavelength neutral wire mesh attenuators.The current-voltage characteristics of the cell under these conditions were obtained by applying external potential bias to the cell and measuring the generated photocurrent with a Keithley model 2440 digital source meter (Keithley, U.S.A.).This process was fully automated using lab tracer software.A similar data acquisition system was used to control the incident photon-to-collected electron conversion efficiency measurement with Oriel QE/IPCE Kit equipment.Under full computer control, light from a 300W xenon lamp (Oriel, U.S.A.) was focused through a monochromator (Oriel, U.S.A) onto the photovoltaic cell under test.The monochromator was incremented through the visible spectrum to generate the IPCE (λ) as defined by IPCE (λ) = 12400 Jsc/λφ,where λ is the wavelength, Jscis short-circuit photocurrent density (mA·cm-2), and φ is the incident radiative flux (mW·cm-2).

3 RESULTS AND DISCUSSION

3.1 DFT calculations

Before any attempt of syntheses, we designed different electron donating and withdrawing groups,and filtered the appropriate candidates according to the electron density of the ground-state of metalloporphyrin derivatives.Fig.1 shows the relative mole-cular orbitals of metalloporphyrin analogs.A good candidate for dye molecules should have more positive HOMO level than the reduction potential of Iˉ/I3ˉ, at the same time; its LUMO level should be more negative than the conduction band of TiO2and an intense absorption in the visible region small band-gap and possess photo-induced intramolecular charge transfer properties.From Fig.1, compounds 4(D2) and 5 (D3) are the best candidates as dye sensitizer that satisfy these requirements.The ground state of compound 4 has LUMO at –2.18 eV right above the conduction band of TiO2, and has HOMO at –5.02 eV right below the Iˉ/I3ˉ reduction potential,while the ground state of compound 5’s LUMO is–2.73 eV and HOMO locates at –5.00 eV.Thus we selected compound 4(D2), 5(D3) to further study in the following experiments.Compound 2 (D1) has been chosen as reference to investigate the electron donating and withdrawing effects.

To gain insight into the electronic structure of excited states, TD-B3LYP calculations were performed in tetrahydrofuran (THF) solution (C-PCM model) with geometries obtained in vacuo.Since molecular orbital (MO) contribution is important in determining the charge-separated states of metalloporphyrin analogues, we examine the molecular orbital contributions for D1, D2 and D3.The chemical structures of these three dyes are shown in Chart 1.The excited states molecular orbitals of the three compounds are plotted in Fig.2.There are similar electronic structures for the cores of D1, D2 and D3.The changes of the withdrawing and donating moieties cause the difference in electron densities on the anchor group (carboxyl group) in the LUMOs.The HOMO is localized on the donor subunit and the LUMO on the acceptor subunit,which suggests the formation of a charge-separated state between the donor and acceptor during photoinduced electron transfer.Table 1 shows the major portions of molecular orbital.The HOMO-LUMO gaps are calculated at 2.80, 2.64 and 2.37 eV for the excited states of D1, D2 and D3, respectively.When porphyrin ring substituents increase the splitting between the key filled orbitals or lift the degeneracy presenting in the unoccupied orbitals, the Q-transition increases in intensity[17-24].The HOMO of D1 is mainly localized on the porphyrin ring with few contributions from the benzoic acid moieties, the almost degenerate HOMO and HOMO-1 couples.The LUMO is mainly distributed on the porphyrin ring with small contributions from the benzoic acid moieties.The LUMO of D2 is similar to that of D1,whereas, its HOMO contains the porphyrin ring and substituted diphenylamino moieties.Obviously, there is spatial charge separation between HOMO and LUMO of D2.

When anchoring group attached withdrawing group (nitro), the HOMO and LUMO of D3 are more separated than that of D2.The LUMO is close to the acceptor moieties, although nitro-withdrawing group has a few contributions, it can further stabilize the whole porphyrin molecule by the strong electronwithdrawing group.The HOMO is mainly localized on the porphyrin ring and substituted diphenylamino moieties, which is similar to D2.Obviously, the structure with a strong electron-donating group on one side and a strong electron-withdrawing group on the other side will facilitate the spatial charge separation.Importantly, the HOMOs of D2 and D3 were also partially distributed on the porphyrin ring,suggesting considerable interactions between the donor and acceptor entities.The locations of HOMO and LUMO of D2 and D3 also suggest the formation of a diphenyl-benzenamine-benzoic-acid/nitro-benzoic-acid (donor?+-acceptor?ˉ) charge-separated state during photoinduced charge transfer.The excitation energies of D1, D2 and D3 are shown in Table 2.Introducing Ph2N group in the donor (D) moiety and nitro-group in the acceptor (A) moiety can obviously increase the oscillator strength of the Q-band and also give rise to the solar spectrum overlap especially at the visible range.

Fig.2. Molecular orbital diagrams of D1, D2 and D3

Table 1. Molecular Orbital Contribution (MOC) of the Five Highest Occupied and Five Lowest Unoccupied Molecular Orbitals of D1, D2 and D3 Calculated at the B3LYP/6-31G* Level in THF Solution

Table 2. Computed Excited Energies (eV), Oscillator Strengths (f) and Two Highest Electronic Transition Configurations for the Optical Transitions below 3.2 eV for D1, D2 and D3 in THF Calculated Using TD-B3LYP/6-31G*//B3LYP/6-31G* Using the C-PCM Framework

3.2 UV-Vis absorption in solution and on the TiO2 film and fluorescence emission spectra

The UV-Vis spectra of porphyrins in THF solvent are shown in Fig.3a.All the porphyrins compounds exhibit two distinct regions in their UV-Vis absorption spectra.The most intense absorption band, commonly referred to the Soret band, exists in the 400～430 nm region.The Soret band is clearly the most intense peak observed in Fig.3 for all the porphyrin sensitizers investigated in this study.The three compounds possess two Q bands in the 500～650 nm region.The series of bands between 400 and 650 nm attribute to π-π* absorptions of the conjugated macrocycle[21,25].Mainly, the Soret bands are corresponding to the S0→S2transition while the Q-bands are corresponding to the higher vibrational mode Q(1,0) and the lowest energy vibrational mode Q(0,0) of the S0→S1transition according to the calculation results of the excited state of the metalloporphyrins based on time-dependent density functional theory (TD-DFT)[25].The molar extinction coefficients increase in the order of D1 ＜ D2 ＜ D3 and the spectra have a slight red-shift in the same order.It is worth comparing our calculated electronic structure to the available spectroscopic data obtained in the context of the present investigation.The spectroscopic data are consistent with the trends in the HOMOs and LUMOs energies calculated for the excited states of D1, D2 and D3.As shown in Fig.3b,the absorption bands (ca.400～460 nm) of dyes attached to TiO2film are broaden and slightly redshifted for all three compounds compared to those observed in THF solution, implying that the aggregation of dye occurred on the TiO2surface which in turn leads to a red-shift (so-called J-aggregates[25]).Similar observation has been reported on carboxyphenyl zinc porphyrin binds on nanotube TiO2surfaces[21,24]and the J-aggregates of dye molecules were believed to be in a head-to-tail arrangement(end-to-end stacking) to form a J-dimer.

The emission spectra of porphyrins D1, D2 and D3, shown in Fig.3c, were obtained at room temperature in THF solution.The emission maxima are independent of the excitation wavelength between 400 and 600 nm, and the spectra show characteristic double-shoulder peaks are at around 615 and 655 nm for D1, D2 and D3, similar to those reported in other zinc porphyrins[21,24,25].The emission maxima for D3 are slightly red-shifted compared to those for D2 and D1, which is consistent with the results on the UV-visible absorption spectra.The excitation spectra,monitored at 615 nm for zinc-porphyrin compounds,exhibit intense Soret and Q-bands that correspond with the grounds-state absorption spectra, indicating the presence of a single emitting species in each case.

3.3 Photo electrochemical measurements in DSSCs

IPCE measurements were performed to compare the efficiency of each sensitizer over the sunlight spectral region.The overall IPCE results for zinc porphyrin compounds are shown in Fig.4a.The system IPCE values were as high as 23% in the range of 400 to 450 nm.It was evident that upon comparison to the UV-Vis spectrum of the dye, electron injection in this region originated in the excitation of the Soret band.Q-band photoexcitation also contributed to the overall photocurrent at lower photon energies, with the IPCE values about 4%.A significant difference in the IPCE values between Soret and Q-band regions was acceptable considering the large differences in their respective extinction coefficients.The spectra of the metalloporphyrins adsorbed onto the 5-μm-thick TiO2electrodes are similar to those of the corresponding solution spectra but exhibit a small red shift due to the interaction of the anchoring groups with the surface (Fig.3b).The diphenyl-benzenamine-substituted metalloporphyrin dye sensitizers, D2, revealed far larger IPCE values than D1 systems.Maximum efficiencies were 23%and 16%, respectively, both of which occurred in the Soret band region.While photocurrent associated with Q-band absorption was small for D2 and D1 systems.The D3 system has the smallest IPCE values.As stated in the former section, dyes in this study aggregate on the TiO2surface and the absorption of D3 on TiO2was more broadened than the other two zinc dyes.From Fig.3b, there isn’t any aggregate absorption band in the photocurrent action spectrum of the zinc porphyrin compounds here,which suggests that aggregates do not contribute to any current generation.Consequently, we could conclude the photons absorbed by the monomers are most effective to produce charge-separated states as these are probably located at the interface between the stacks and the semiconductor as reported[7,26,27].

Fig.3. UV-visible absorption spectra of the three zinc-porphyrins dyes in THF solution (a), adsorbed on TiO2 (b) and emission spectra in THF solution (c)

Fig.4. (a) Photocurrent action spectra obtained with D1, D2, and D3 anchored on nanocrystalline TiO2 films.(b) J-V characteristics measured under the illumination of AM 1.5G full sunlight (100 mW/cm-2) for metalloporphyrins and N719

3.4 Photovoltaic measurements

The dye solution was prepared in THF solution (4× 10-4M).The hot TiO2electrodes (80 ℃) were immersed into the dye solution for 2 h.The colored films exhibited striking performance when tested in a photovoltaic cell in conjunction with a redox electrolyte.The results obtained for current voltage (I-V)curves for all systems when illuminated are shown in Fig.4b.The short-circuit current densities, Jsc, opencircuit voltages, Voc, fill factors, FF and power conversion efficiency of the cell are summarized in Table 3 under standard global AM 1.5 solar conditions.As expected, comparison of the metalloporphyrin-sensitized DSSC results, DSSC based on D2 shows the best performance (Jscof 2.83 mA·cm-2, Vocof 617 mV and FF of 0.459), which reaches 19%with respect to an N719-based device fabricated under the same conditions.Although the metalloporphyrin dye D3 shows the biggest intramolecular charge separation according to calculation results of the excited state of the metalloporphyrin, the poorest performance was obtained.Besides intramolecular charge separation, there are several other critical factors influencing the power conversation efficiency,such as band gap of porphyrin-sensitizers and aggregation degree of metalloporphyrin on TiO2.In D3 case, introduction of the nitro-group may facilitate the self-aggregation of metalloporphyrin by π-packing[13]and the aggregation on the TiO2surface[12,27,28], resulting in a decrease of the conversion efficiency.Comparison of these zinc-porphyrins and N719 shows that N719 performs better cell performance.The reason should be that the planar structure of zinc-porphyrins enhance the recombination of photo-generated electrons, leading to the smaller Voc and FF in zinc-porphyrin-sensitized solar cells[28-30].Meanwhile, the smaller Jscvalues for zinc-porphyrin dyes might be due to the dye aggregates harmful to electron injection, and some pushpull porphyrins likewise suffer from dye aggregation.Some research groups have also demonstrated that the moderate performance of the porphyrin sensitized solar cell ascribe to the porphyrin molecular aggregate onto the TiO2nanoparticle surface[28,30]and we should minimize the aggregation of porphyrin sensitizer when designing and synthesizing the dyes molecules.

Table 3. Detailed Photovoltaic Parameters of Metalloporphyrin as Sensitizera

4 CONCLUSION

In summary, unsymmetrical push-pull type zincporphyrins, as dye photosensitizer applied in DSSCs as demonstrated, can facilitate the intramolecular charge separation.A set of metalloporphyrin derivatives have been synthesized to investigate the effect of substituted acceptor and donor groups on the optical and photovoltaic properties of metalloporphyrin.Together with DFT and TD-DFT calculations, we demonstrated that the TiO2solar cell with unsymmetrical donor-bridge-acceptor structures showed an improved cell performance compared to the D1 sensitized reference cell.Not only can intramolecular charge separation facilitate the improvement of power conversion efficiency of solar cell,but also the aggregations of metalloporphyrin on TiO2affect the cell performance.Because of the interplay of various factors, the best performance achieved of DSSC was D2 sensitized TiO2solar cell and yielded the maximal IPCE value of 23% and power conversion efficiency η of 0.80% under standard AM 1.5 conditions.Our results suggest that push-pull type zinc-porphyrins sensitizers with donor-bridge-acceptor (D-B-A) larger π-conjugating system could increase the photo to current efficiency and the posi-tion of withdrawing group is important for designing and synthesizing the effective metalloporphyrin-sensitized solar cell.We should minimize the aggre-gation of porphyrin sensitizer at the same time when designing and synthesizing the dye molecules.

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