Zinc Porphyrins Possessing Three P-carboxyphenyl Groups: Effect of the Donor Strength of Push-groups on the Efficiency of Dye Sensitized Solar Cells

Zinc porphyrins decorated with three p-carboxyphenyl anchoring groups and various " push " substituents of varied electron-donating strengths were prepared in good yields by facile and straightforward ways. The effect of electron-donating strength of the donor molecules on the overall power conversion efficiency was evaluated with the help of photophysical, electrochemical, photovoltaic spectroscopy and quantum chemical calculations. It is observed from the photophysical and Infrared (IR) spectroscopic data that multi-anchoring dyes are more stable and bind more strongly to the TiO 2 surface than their one-anchor counterparts. The properties like a three-step synthesis, high overall yields, possible mass production on a gram-scale and strong binding affinities with TiO 2 surfaces make them a suitable choice for commercial applications. Zn 1 NH 3 A, with electron donating and anti-aggregation characteristics, achieved the highest efficiency of 6.50%.


Introduction
The constantly growing global energy demand is stimulating scientists to pursue alternative energy supplies.Among all renewable energy technologies, solar photovoltaics are believed to be the most encouraging ones as solar energy is the most abundant, lavish, and promising energy source on Earth.Since first reported in 1991, dye sensitized solar cells (DSSCs) have attracted great attention due to their low cost and environmental friendly properties [1].Among several crucial components of DSSCs, the dye sensitizing molecule is of key importance [1,2].In the design of an efficient sensitizer, properties like high efficiency, chemical stability, and simple step-economic syntheses play a very important role.The primary role of porphyrins in photosynthetic systems have inspired researchers worldwide to develop highly efficient chromophores based on the porphyrin backbone [3][4][5][6][7].Porphyrins have emerged as the leaders among DSSCs molecules, with exceptional overall conversion efficiencies of 12%-13% under a push-pull framework [8][9][10][11] in competition with ruthenium dyes (11.4%) [12] and organic sensitizers (14.3%) [13][14][15][16].Though efficient, their disadvantages like multistep syntheses, low overall yields and poor stability may hamper their practical applications.Therefore, the search of an efficient porphyrin sensitizer with step-economic synthesis, better overall yield and greater stability is worth pursuing.In DSSCs an anchoring group is necessary to adsorb the dye on a TiO 2 surface, so as to obtain greater stability and fast electron injection [17].The mode of attachment of sensitizer to the semiconductor surface affects the efficiency of the electron-transfer at the dye-semiconductor interface [18][19][20][21].Organic and phthalocyanine dyes with two anchoring groups are found to be more efficient and stable compared to their single anchoring group counterparts [22,23].Porphyrins with different anchoring groups through different Energies 2016, 9, 513 2 of 12 binding modes have been used to study their effect on the photovoltaic performance in DSSCs [24][25][26][27][28][29][30].Following our long-term interest in porphyrin-sensitized solar cells [31][32][33][34][35][36][37][38], we have shown that multi-anchoring porphyrins serve as proficient, stable, and cost-effective sensitizers in DSSCs [39][40][41].Three-step syntheses, comparatively high overall yields, scalable production, and strong binding with TiO 2 surface using two anchoring groups are some of the key features of these porphyrins.An impressive power conversion efficiency >6% is achieved for the devices assembled with these multi-anchoring porphyrins.
Herein we have prepared a series of 1D-π-3A porphyrin sensitizers substituted with a variety of donor molecules with variable electron-donating strengths, as shown in Scheme 1.The nature and strength of the donor groups have significant effects on their photovoltaic performance.These porphyrins were prepared from commercially available starting compounds in just three steps.The overall reaction yields of these porphyrins are in the range of 2%-6% which are comparatively higher than the reported push-pull porphyrins (<1%) [8].
Herein we have prepared a series of 1D-π-3A porphyrin sensitizers substituted with a variety of donor molecules with variable electron-donating strengths, as shown in Scheme 1.The nature and strength of the donor groups have significant effects on their photovoltaic performance.These porphyrins were prepared from commercially available starting compounds in just three steps.The overall reaction yields of these porphyrins are in the range of 2%-6% which are comparatively higher than the reported push-pull porphyrins (<1%) [8].

Synthesis
The 1D-π-3A porphyrinic sensitizers were synthesized in three steps, namely mixed condensation, zinc metalation, and base hydrolysis.The detailed synthetic route is presented in Scheme 2. Condensation of pyrrole, methyl-4-formylbenzoate, and the corresponding aldehyde under Lindsey conditions catalysed by boron trifluoride diethyl etherate, followed by subsequent oxidation by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) yielded the triester derivatives in decent yields.In the attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) spectra all the porphyrins show one stretching frequency at around 1720 cm −1 confirming the presence of the ester carbonyl group.In the ultraviolet visible (UV/Vis) spectra the free-base porphyrins show a strong Soret band and four moderate Q bands.The subsequent step of zinc metallation has been readily achieved in high yields by reacting the free-based porphyrin with zinc acetate.The formation of zinc complexes of all the porphyrins was confirmed through the complete disappearance of the nuclear magnetic resonance (NMR) signal of the inner NH protons with slightly upfield shifts for all remaining protons.Hydrolysis of these metal complexes have been achieved straightforwardly by reacting them in a mixture solution of tetrahydrofuran (THF) and methanol with excess aqueous KOH.ATR-FTIR spectra of final acid products show shifting of carbonyl peaks in the range of 1675-1700 cm −1 because of intermolecular hydrogen bonding.All of the porphyrins were fully characterized by optical spectroscopy, ATR-FTIR, nuclear magnetic resonance spectroscopy, and high-resolution mass spectrometry.

Synthesis
The 1D-π-3A porphyrinic sensitizers were synthesized in three steps, namely mixed condensation, zinc metalation, and base hydrolysis.The detailed synthetic route is presented in Scheme 2. Condensation of pyrrole, methyl-4-formylbenzoate, and the corresponding aldehyde under Lindsey conditions catalysed by boron trifluoride diethyl etherate, followed by subsequent oxidation by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) yielded the triester derivatives in decent yields.In the attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) spectra all the porphyrins show one stretching frequency at around 1720 cm ´1 confirming the presence of the ester carbonyl group.In the ultraviolet visible (UV/Vis) spectra the free-base porphyrins show a strong Soret band and four moderate Q bands.The subsequent step of zinc metallation has been readily achieved in high yields by reacting the free-based porphyrin with zinc acetate.The formation of zinc complexes of all the porphyrins was confirmed through the complete disappearance of the nuclear magnetic resonance (NMR) signal of the inner NH protons with slightly upfield shifts for all remaining protons.Hydrolysis of these metal complexes have been achieved straightforwardly by reacting them in a mixture solution of tetrahydrofuran (THF) and methanol with excess aqueous KOH.ATR-FTIR spectra of final acid products show shifting of carbonyl peaks in the range of 1675-1700 cm ´1 because of intermolecular hydrogen bonding.All of the porphyrins were fully

Photophysical Properties
The UV/Vis absorption spectra of the studied zinc(II) porphyrin dyes, as displayed in Figure 1, shows one strong Soret band and two Q bands.The peak positions and their molar absorption coefficients (ε) are summarized in Table 1.The shift in absorption bands of porphyrins are substituent-dependent.Porphyrin Zn1TH3A has the Soret band absorption at 427 nm while Zn1TC3A, Zn1BC3A, Zn1OD3A, Zn1OH3A and Zn1NP3A display Soret band absorptions at 426 nm.In the cases of porphyrins Zn1NH3A, Zn1NN3A, and Zn1ND3A, the Soret bands shift to 425 nm.The overall highest molar ε has been observed for Zn1OH3A.It is interesting to observe that the porphyrins Zn1TH3A, Zn1TC3A, Zn1BC3A, Zn1OD3A, and Zn1OH3A have relatively high molar ε than the N,N'-alkylaniline-substituted porphyrins.Scheme 2. Synthetic route to the studies 1D-π-3A porphyrin sensitizers.

Photophysical Properties
The UV/Vis absorption spectra of the studied zinc(II) porphyrin dyes, as displayed in Figure 1, shows one strong Soret band and two Q bands.

Photophysical Properties
The UV/Vis absorption spectra of the studied zinc(II) porphyrin dyes, as displayed in Figure 1, shows one strong Soret band and two Q bands.The peak positions and their molar absorption coefficients (ε) are summarized in Table 1.The shift in absorption bands of porphyrins are substituent-dependent.Porphyrin Zn1TH3A has the Soret band absorption at 427 nm while Zn1TC3A, Zn1BC3A, Zn1OD3A, Zn1OH3A and Zn1NP3A display Soret band absorptions at 426 nm.In the cases of porphyrins Zn1NH3A, Zn1NN3A, and Zn1ND3A, the Soret bands shift to 425 nm.The overall highest molar ε has been observed for Zn1OH3A.It is interesting to observe that the porphyrins Zn1TH3A, Zn1TC3A, Zn1BC3A, Zn1OD3A, and Zn1OH3A have relatively high molar ε than the N,N'-alkylaniline-substituted porphyrins.The peak positions and their molar absorption coefficients (ε) are summarized in Table 1.The shift in absorption bands of porphyrins are substituent-dependent.Porphyrin Zn 1 TH 3 A has the Soret band absorption at 427 nm while Zn 1 TC 3 A, Zn 1 BC 3 A, Zn 1 OD 3 A, Zn 1 OH 3 A and Zn 1 NP 3 A display Soret band absorptions at 426 nm.In the cases of porphyrins Zn 1 NH 3 A, Zn 1 NN 3 A, and Zn 1 ND 3 A, the Soret bands shift to 425 nm.The overall highest molar ε has been observed for Zn 1 OH 3 A. It is interesting to observe that the porphyrins Zn 1 TH 3 A, Zn 1 TC 3 A, Zn 1 BC 3 A, Zn 1 OD 3 A, and Zn 1 OH 3 A have relatively high molar ε than the N,N'-alkylaniline-substituted porphyrins. -- 1 Absorption maximum (ε/103 M ´1¨cm ´1) of porphyrin in THF;2 Emission maximum were measured in THF by exciting at Soret band; 3 First oxidation potentials were determined by cyclic voltammetry in THF;4 E 0-0 values were estimated from the intersection of the absorption and emission spectra;5 E ox * approximated from E ox and E 0-0 ; 6 From reference [40].
Although the molar ε are not high for Zn 1 NP 3 A, Zn 1 NH 3 A, Zn 1 NN 3 A, and Zn 1 ND 3 A, the low absorbance might be compensated by the broadened absorption and better electron injection to result in higher overall conversion efficiencies (vide infra).To better understand the adsorption behavior of porphyrins on TiO 2 , the thin film absorption spectra of these porphyrins were studied.The UV/Vis spectra of studied porphyrins on TiO 2 displayed in Figure 2 shows slightly red-shifted absorption wavelengths with broadening in shape.Although the molar ε are not high for Zn1NP3A, Zn1NH3A, Zn1NN3A, and Zn1ND3A, the low absorbance might be compensated by the broadened absorption and better electron injection to result in higher overall conversion efficiencies (vide infra).To better understand the adsorption behavior of porphyrins on TiO2, the thin film absorption spectra of these porphyrins were studied.The UV/Vis spectra of studied porphyrins on TiO2 displayed in Figure 2 shows slightly red-shifted absorption wavelengths with broadening in shape.The absorption wavelengths of porphyrins Zn 1 TH 3 A, Zn 1 TC 3 A and Zn 1 BC 3 A adsorbed on TiO 2 are red-shifted by 5 nm with slight broadening in shape, forming J-aggregates on TiO 2 by end-to-end stacking, whereas in TiO 2 films absorbing the rest of the porphyrins only peak broadening without any shift in the absorption wavelength was observed.The emission spectra of Zn 1 TH 3 A, Zn 1 TC 3 A, Zn 1 BC 3 A, Zn 1 OD 3 A, and Zn 1 OH 3 A show two peaks while the other N,N'-alkylaniline substituted porphyrins Zn 1 NP 3 A, Zn 1 NH 3 A, Zn 1 NN 3 A, and Zn 1 ND 3 A show only one peak with a red-shifted emission.

Electrochemical Properties
To determine the first oxidation potential (E ox ) of porphyrins, cyclic voltammetry measurements of the studied porphyrins were performed in degassed THF solution containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF 6 ) as electrolyte.Supplementary Material (Figure S1) shows cyclic voltammograms of the porphyrin sensitizers.All of the porphyrins give well resolved and reversible waves for the first oxidation couple which is stable over multiple scans.The values of oxidation potential as listed in Table 1 indicate that the peripheral substituents on the meso positions moderately alter the oxidation potentials.A lowest first oxidation potential of 1.06 V has been observed for Zn 1 OD 3 A. Porphyrins Zn 1 TH 3 A, Zn 1 TC 3 A, Zn 1 BC 3 A, and Zn 1 OD 3 A gave one reversible oxidation couple while for the remaining porphyrins two reversible oxidation couples were observed.The oxidation potentials of all studied porphyrins are greater than that of I ´/I 3 ´couple assuring the regeneration of oxidized dyes.The excited-state oxidation potentials (E ox *) are obtained from the equation E ox * = E ox1 ´E0-0 where E ox1 is the first oxidation potential of a porphyrin dye and E 0-0 is the zero-zero excitation energy obtained from the intersection of the absorption and emission spectra.The calculated E ox * energy levels of these porphyrins depicted in Table 1 are more negative than the conduction band of TiO 2 (´0.50 V versus NHE), exhibiting the sufficient driving force for electron injection from the excited state of the dye to the conduction band (CB) of TiO 2 .The systematic energy level diagrams of studied porphyrins are shown in Figure 3 demonstrating feasibility of dye regeneration with electrolyte and electron injection to conduction band.
Energies 2016, 9, 513 5 of 12 The absorption wavelengths of porphyrins Zn1TH3A, Zn1TC3A and Zn1BC3A adsorbed on TiO2 are red-shifted by 5 nm with slight broadening in shape, forming J-aggregates on TiO2 by end-to-end stacking, whereas in TiO2 films absorbing the rest of the porphyrins only peak broadening without any shift in the absorption wavelength was observed.The emission spectra of Zn1TH3A, Zn1TC3A, Zn1BC3A, Zn1OD3A, and Zn1OH3A show two peaks while the other N,N'-alkylaniline substituted porphyrins Zn1NP3A, Zn1NH3A, Zn1NN3A, and Zn1ND3A show only one peak with a red-shifted emission.

Electrochemical Properties
To determine the first oxidation potential (Eox) of porphyrins, cyclic voltammetry measurements of the studied porphyrins were performed in degassed THF solution containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as electrolyte.Supplementary Material (Figure S1) shows cyclic voltammograms of the porphyrin sensitizers.All of the porphyrins give well resolved and reversible waves for the first oxidation couple which is stable over multiple scans.The values of oxidation potential as listed in Table 1 indicate that the peripheral substituents on the meso positions moderately alter the oxidation potentials.A lowest first oxidation potential of 1.06 V has been observed for Zn1OD3A.Porphyrins Zn1TH3A, Zn1TC3A, Zn1BC3A, and Zn1OD3A gave one reversible oxidation couple while for the remaining porphyrins two reversible oxidation couples were observed.The oxidation potentials of all studied porphyrins are greater than that of I − /I3 − couple assuring the regeneration of oxidized dyes.The excited-state oxidation potentials (Eox*) are obtained from the equation Eox* = Eox1 − E0-0 where Eox1 is the first oxidation potential of a porphyrin dye and E0-0 is the zero-zero excitation energy obtained from the intersection of the absorption and emission spectra.The calculated Eox* energy levels of these porphyrins depicted in Table 1 are more negative than the conduction band of TiO2 (−0.50 V versus NHE), exhibiting the sufficient driving force for electron injection from the excited state of the dye to the conduction band (CB) of TiO2.The systematic energy level diagrams of studied porphyrins are shown in Figure 3 demonstrating feasibility of dye regeneration with electrolyte and electron injection to conduction band.

Quantum Chemical Results
To gain insights into the electronic structures for the frontier orbitals of the porphyrins, density functional theory calculations were carried out for studied porphyrins at the B3LYP/6-31G(d) level using Gaussian 09 Software [42,43].Figure 4 illustrates the electron density distributions of studied porphyrins in their respective HOMO, LUMO, LUMO + 1, and LUMO + 2 molecular orbitals.
Analyzing the orbitals in detail, for HOMO orbitals, the electron density distributions for porphyrin dyes varies greatly with the electron-donating tendency of the push group.The electronic cloud density over the HOMO and LUMO estimate the hole and electron carrying ability, respectively [44].In the studied porphyrins the electron density is mainly localized on the porphyrin core in the HOMO orbitals for Zn1TH3A, Zn1OD3A, and Zn1OH3A, with a small part of it extended

Quantum Chemical Results
To gain insights into the electronic structures for the frontier orbitals of the porphyrins, density functional theory calculations were carried out for studied porphyrins at the B3LYP/6-31G(d) level using Gaussian 09 Software [42,43].Figure 4 illustrates the electron density distributions of studied porphyrins in their respective HOMO, LUMO, LUMO + 1, and LUMO + 2 molecular orbitals.
Analyzing the orbitals in detail, for HOMO orbitals, the electron density distributions for porphyrin dyes varies greatly with the electron-donating tendency of the push group.The electronic cloud density over the HOMO and LUMO estimate the hole and electron carrying ability, respectively [44].In the studied porphyrins the electron density is mainly localized on the porphyrin core in the HOMO orbitals for Zn 1 TH 3 A, Zn 1 OD 3 A, and Zn 1 OH 3 A, with a small part of it extended Energies 2016, 9, 513 6 of 12 over the weak electron-donating substituent, while the majority of the electron density is localized on the strong electron donating groups has seen for Zn 1 TC 1 A, Zn 1 BC 3 A, Zn 1 NP 3 A, Zn 1 NH 3 A, Zn 1 NN 3 A and Zn 1 ND 3 A.Although the porphyrin core is still the main electron population site, in both LUMO and LUMO+1 orbitals, a significant amount of electron density is distributed over the meso carboxyphenyl anchoring group.Interestingly, Zn 1 TH 3 A, and Zn 1 TC 3 A have electron density localized at the carboxylphenyl group opposite to the electron donating substituent for LUMO orbitals and at the 5-and 15-carboxyphenyl, the meso substituents neighboring the donating group, for LUMO+1 orbitals but a reverse distribution pattern is observed for the rest of the zinc porphyrin dyes.Given that our ATR-IR analyses demonstrate a cis two arm anchoring with one dangling carboxyphenyl group for these tricarboxyphenyl-containing porphyrin dyes [39,40], it is likely that the LUMO and LUMO + 1 synchronize to pump electron density into the conduction band of TiO 2 .
Energies 2016, 9, 513 6 of 12 over the weak electron-donating substituent, while the majority of the electron density is localized on the strong electron donating groups has seen for Zn1TC1A, Zn1BC3A, Zn1NP3A, Zn1NH3A, Zn1NN3A and Zn1ND3A.Although the porphyrin core is still the main electron population site, in both LUMO and LUMO+1 orbitals, a significant amount of electron density is distributed over the meso carboxyphenyl anchoring group.Interestingly, Zn1TH3A, and Zn1TC3A have electron density localized at the carboxylphenyl group opposite to the electron donating substituent for LUMO orbitals and at the 5-and 15-carboxyphenyl, the meso substituents neighboring the donating group, for LUMO+1 orbitals but a reverse distribution pattern is observed for the rest of the zinc porphyrin dyes.Given that our ATR-IR analyses demonstrate a cis two arm anchoring with one dangling carboxyphenyl group for these tricarboxyphenyl-containing porphyrin dyes [39,40], it is likely that the LUMO and LUMO + 1 synchronize to pump electron density into the conduction band of TiO2.Also for the LUMO + 2 orbital all the electron density is exclusively localized on the carboxyphenyl anchoring group, indicating that the electron injection from higher excited states involving the LUMO + 2 orbital is also possible.Nevertheless, it is clearly evident from the electron density distribution that the porphyrin dyes containing a strong electron-donating meso substituent provides better charge separation which slows down the charge recombination and results in a more efficient electron injection.

Photovoltaic Properties
The 1D-π-3A porphyrins were fabricated into solar cell devices in the presence of chenodeoxycholic acid (CDCA) as a co-adsorbent according to the procedures detailed in the Experimental Section and evaluated for the photovoltaic performance under standard AM 1.5 G simulated solar conditions.The best photovoltaic performance for each porphyrin is summarized in Table 2.The optimized I-V curves of all the studied three p-carboxyphenyl substituted porphyrins are shown in Figure 5.As observed from the photovoltaic data and I-V curves, the strong electrondonating N,N'-alkylaniline-substituted porphyrins outperform the devices using the rest of the porphyrins as the sensitizer.The Zn1TH3A porphyrin having a hexylthiophene group as electron donor at the meso-position exhibits an efficiency of 5.36% based on a short-circuit current density (Jsc) of 12.41 mA cm −2 , an open-circuit voltage (Voc) of 0.65 V and a fill factor (FF) of 67%, whereas the Also for the LUMO + 2 orbital all the electron density is exclusively localized on the carboxyphenyl anchoring group, indicating that the electron injection from higher excited states involving the LUMO + 2 orbital is also possible.Nevertheless, it is clearly evident from the electron density distribution that the porphyrin dyes containing a strong electron-donating meso substituent provides better charge separation which slows down the charge recombination and results in a more efficient electron injection.

Photovoltaic Properties
The 1D-π-3A porphyrins were fabricated into solar cell devices in the presence of chenodeoxycholic acid (CDCA) as a co-adsorbent according to the procedures detailed in the Experimental Section and evaluated for the photovoltaic performance under standard AM 1.5 G simulated solar conditions.The best photovoltaic performance for each porphyrin is summarized in Table 2.The optimized I-V curves of all the studied three p-carboxyphenyl substituted porphyrins are shown in Figure 5.As observed from the photovoltaic data and I-V curves, the strong electron-donating N,N'-alkylanilinesubstituted porphyrins outperform the devices using the rest of the porphyrins as the sensitizer.The Zn1TH3A porphyrin having a hexylthiophene group as electron donor at the meso-position exhibits an efficiency of 5.36% based on a short-circuit current density (J sc ) of 12.41 mA cm ´2, an open-circuit voltage (V oc ) of 0.65 V and a fill factor (FF) of 67%, whereas the porphyrin dyes Zn1TC3A and Zn1BC3A with carbazole donor groups obtained overall power conversion efficiencies of 4.49% and 4.40%, respectively.The Zn 1 OD 3 A porphyrin with an ortho-didodecylphenyl substituent on the meso position achieved a conversion efficiency of 4.78%, while the hexyloxyphenyl-substituted porphyrin Zn 1 OH 3 A attained an overall performance of 4.81%.
Zn1OH3A is most likely because of its stronger electron-donating character, inhibition of charge recombination, and effective charge separation.The N,N'-alkylaniline-substituted porphyrin sensitizers Zn1NP3A, Zn1NH3A, Zn1NN3A and Zn1ND3A gave efficiencies higher than 5.6%.The intrinsic anti-aggregation properties resulting from the presence of long alkyl chains and electrondonating character have both been instrumental in their superior performance.For comparison we varied the alkyl chain length from propyl, hexyl, nonyl, to dodecyl.The propyl-substituted porphyrin Zn1NP3A gave a Jsc of 13.86 mA cm −2 , Voc of 0.64 V and FF of 66% corresponding to an overall performance of 5.86%.The porphyrin Zn1NH3A with hexyl substituent achieved a power conversion efficiency of 6.5%.The nonyl derivative Zn1NN3A obtained a power conversion efficiency of 5.65%, while the N-dodecyl-decorated porphyrin Zn1ND3A gives a Jsc of 13.61 mA•cm −2 and a moderate Voc of 0.65 V contributed to its efficiency of 6%.The incident photon-to-current efficiency (IPCE) characteristics of studied porphyrins show similar trends with the observed Jsc and are shown in Figure 6.The IPCE values demonstrated at the Soret band region are consistent with the performance of the porphyrin dyes.The highest IPCE value of 81% is observed for Zn1NH3A having the highest efficiency of 6.50%, whereas the IPCE value of 40% is observed for Zn1BC3A having an efficiency of 4.40%.It is also evident from the IPCE spectra  It is noteworthy that the dye loading for the porphyrin Zn 1 OD 3 A is 90 nmol¨cm ´2 which is the least among the studied porphyrin dyes due to the presence of two ortho-dodecyl side chains, which might hinder close packing of the molecules in TiO 2 surface.
The higher efficiency of the Zn 1 TH 3 A porphyrin than Zn 1 TC 3 A, Zn 1 BC 3 A, Zn 1 OD 3 A and Zn 1 OH 3 A is most likely because of its stronger electron-donating character, inhibition of charge recombination, and effective charge separation.The N,N'-alkylaniline-substituted porphyrin sensitizers Zn 1 NP 3 A, Zn 1 NH 3 A, Zn 1 NN 3 A and Zn 1 ND 3 A gave efficiencies higher than 5.6%.The intrinsic anti-aggregation properties resulting from the presence of long alkyl chains and electron-donating character have both been instrumental in their superior performance.For comparison we varied the alkyl chain length from propyl, hexyl, nonyl, to dodecyl.The propyl-substituted porphyrin Zn 1 NP 3 A gave a J sc of 13.86 mA¨cm ´2, V oc of 0.64 V and FF of 66% corresponding to an overall performance of 5.86%.The porphyrin Zn 1 NH 3 A with hexyl substituent achieved a power conversion efficiency of 6.5%.The nonyl derivative Zn 1 NN 3 A obtained a power conversion efficiency of 5.65%, while the N-dodecyl-decorated porphyrin Zn 1 ND 3 A gives a J sc of 13.61 mA¨cm ´2 and a moderate V oc of 0.65 V contributed to its efficiency of 6%.
The incident photon-to-current efficiency (IPCE) characteristics of studied porphyrins show similar trends with the observed J sc and are shown in Figure 6.The IPCE values demonstrated at the Soret band region are consistent with the performance of the porphyrin dyes.The highest IPCE value of 81% is observed for Zn 1 NH 3 A having the highest efficiency of 6.50%, whereas the IPCE value of 40% is observed for Zn 1 BC 3 A having an efficiency of 4.40%.It is also evident from the IPCE spectra that the N,N'-alkylaniline-substituted porphyrins show intense, broad, and red-shifted IPCE absorption features compared to other porphyrins.The dip around 500-550 nm is also slightly higher for them than the latter.The dye loading of the porphyrins on TiO 2 films was examined and summarized in Table 2.The dye densities and efficiencies of porphyrins are roughly consistent; Zn 1 NH 3 A has the highest dye loading density giving the highest efficiency whereas, despite a low dye density, Zn 1 OD 3 A is able to give good performance (η = 4.78%).It is worth mentioning that a regular desorption solvent, 0.1 M aqueous KOH in THF (2:8, v/v), failed to completely desorb these three p-carboxyphenyl substituted sensitizing dyes, even after exposure for 24 h.It requires a solution of 0.2 M KOH(aq) in THF (1:1, v/v) to completely desorb the dye attached onto the TiO 2 surface.The use of a high concentration of base to desorb the dye is an additional support to prove the higher stabilities of three meso p-carboxyphenyl groups-possessing porphyrins.
Energies 2016, 9, 513 8 of 12 that the N,N'-alkylaniline-substituted porphyrins show intense, broad, and red-shifted IPCE absorption features compared to other porphyrins.The dip around 500-550 nm is also slightly higher for them than the latter.The dye loading of the porphyrins on TiO2 films was examined and summarized in Table 2.The dye densities and efficiencies of porphyrins are roughly consistent; Zn1NH3A has the highest dye loading density giving the highest efficiency whereas, despite a low dye density, Zn1OD3A is able to give good performance (η = 4.78%).It is worth mentioning that a regular desorption solvent, 0.1 M aqueous KOH in THF (2:8, v/v), failed to completely desorb these three p-carboxyphenyl substituted sensitizing dyes, even after exposure for 24 h.It requires a solution of 0.2 M KOH(aq) in THF (1:1, v/v) to completely desorb the dye attached onto the TiO2 surface.The use of a high concentration of base to desorb the dye is an additional support to prove the higher stabilities of three meso p-carboxyphenyl groups-possessing porphyrins.

Stability Test of Porphyrin Sensitizers
For the commercialization of DSSCs, along with a high efficiency, the long term stability is an important issue to be considered.We have used a simple method to examine the stability of the studied dyes.To obtain the stability information, the absorption spectra of three p-carboxyphenyl group-possessing porphyrins Zn1ND3A were compared with that of a single anchoring group possessing porphyrin Zn3TPA1A (Zn3TPA1A possesses three triphenyl amine and one p-carboxyphenyl group on its meso positions) upon irradiation.The TiO2 film adsorbed with Zn1ND3A, and Zn3TPA1A were irradiated under standard one-sun illumination and the absorption spectra were monitored after irradiation for 0, 5, and 30 min.The observed results depicted in Figure 7 suggest that the absorbance of dyes decreases at various rates without any shifting of the wavelength of maximum absorbance.The decrease in the absorbance is significantly larger for Zn3TPA1A in comparison with Zn1ND3A which suggests that the porphyrins possessing three p-carboxyphenyl groups are more stable compared to porphyrins possessing a single anchoring group.

Stability Test of Porphyrin Sensitizers
For the commercialization of DSSCs, along with a high efficiency, the long term stability is an important issue to be considered.We have used a simple method to examine the stability of the studied dyes.To obtain the stability information, the absorption spectra of three p-carboxyphenyl group-possessing porphyrins Zn 1 ND 3 A were compared with that of a single anchoring group possessing porphyrin Zn 3 TPA 1 A (Zn 3 TPA 1 A possesses three triphenyl amine and one p-carboxy-phenyl group on its meso positions) upon irradiation.The TiO 2 film adsorbed with Zn 1 ND 3 A, and Zn 3 TPA 1 A were irradiated under standard one-sun illumination and the absorption spectra were monitored after irradiation for 0, 5, and 30 min.The observed results depicted in Figure 7 suggest that the absorbance of dyes decreases at various rates without any shifting of the wavelength of maximum absorbance.The decrease in the absorbance is significantly larger for Zn 3 TPA 1 A in comparison with Zn 1 ND 3 A which suggests that the porphyrins possessing three p-carboxyphenyl groups are more stable compared to porphyrins possessing a single anchoring group.
Energies 2016, 9, 513 8 of 12 that the N,N'-alkylaniline-substituted porphyrins show intense, broad, and red-shifted IPCE absorption features compared to other porphyrins.The dip around 500-550 nm is also slightly higher for them than the latter.The dye loading of the porphyrins on TiO2 films was examined and summarized in Table 2.The dye densities and efficiencies of porphyrins are roughly consistent; Zn1NH3A has the highest dye loading density giving the highest efficiency whereas, despite a low dye density, Zn1OD3A is able to give good performance (η = 4.78%).It is worth mentioning that a regular desorption solvent, 0.1 M aqueous KOH in THF (2:8, v/v), failed to completely desorb these three p-carboxyphenyl substituted sensitizing dyes, even after exposure for 24 h.It requires a solution of 0.2 M KOH(aq) in THF (1:1, v/v) to completely desorb the dye attached onto the TiO2 surface.The use of a high concentration of base to desorb the dye is an additional support to prove the higher stabilities of three meso p-carboxyphenyl groups-possessing porphyrins.

Stability Test of Porphyrin Sensitizers
For the commercialization of DSSCs, along with a high efficiency, the long term stability is an important issue to be considered.We have used a simple method to examine the stability of the studied dyes.To obtain the stability information, the absorption spectra of three p-carboxyphenyl group-possessing porphyrins Zn1ND3A were compared with that of a single anchoring group possessing porphyrin Zn3TPA1A (Zn3TPA1A possesses three triphenyl amine and one p-carboxyphenyl group on its meso positions) upon irradiation.The TiO2 film adsorbed with Zn1ND3A, and Zn3TPA1A were irradiated under standard one-sun illumination and the absorption spectra were monitored after irradiation for 0, 5, and 30 min.The observed results depicted in Figure 7 suggest that the absorbance of dyes decreases at various rates without any shifting of the wavelength of maximum absorbance.The decrease in the absorbance is significantly larger for Zn3TPA1A in comparison with Zn1ND3A which suggests that the porphyrins possessing three p-carboxyphenyl groups are more stable compared to porphyrins possessing a single anchoring group.

Optical Spectroscopy
UV/Vis absorption spectra of the porphyrins in THF and adsorbed onto TiO 2 electrodes were recorded on a V-670 UV/Vis/NIR spectrophotometer (JASCO, Easton, MD, USA).For the absorption spectra of the thin films on TiO 2 , TiO 2 films (area: 1 ˆ1 cm 2 ) were prepared with thicknesses of about 1 µm to obtain comparable shapes and peak positions.The films were immersed in 2 ˆ10 ´4 M solutions of the porphyrins in THF for 12 h and the films were rinsed with THF, dried, and the absorbance was measured.Steady-state fluorescence spectra were acquired on a Cary Eclipse fluorescence spectrophotometer (Varian, Palo Alto, CA, USA).

Cyclic Voltammetry
The cyclic voltammetry (CV) measurements of all of the porphyrins were performed on a CHI 600D electrochemical analyzer (CH Instruments, Austin, TX, USA) in degassed THF with 0.1 M TBAPF 6 as a supporting electrolyte.The cell assembly consisted of a glassy carbon working electrode, Ag wire as a reference electrode, and platinum wire as the auxiliary electrode.A ferrocene/ferrocenium redox couple was used as an internal reference.

Density Functional Theory Calculations
Geometry optimizations and the electronic structures of the porphyrins were performed by using DFT at the B3LYP level of theory with the 6-31G(d) basis set in the Gaussian09 Program Package (Gaussian Inc., Wallingford, CT, USA).

Photovoltaic Measurements
TiO 2 photoanode films were purchased from Yingkou Opvtech New Energy Co., Ltd.(Yingkou Liaoning, China).The films, which were prepared using the screen-printing method, were composed of a transparent layer (thickness « 12 µm), a scattering layer (thickness « 4 µm), and a working area of 0.4 ˆ0.4 cm 2 .The films were pretreated according to the following activation procedures before use: heating at 100 ˝C for 22 min, at 110 ˝C for 60 min, at 450 ˝C for 68 min, at 500 ˝C for 60 min, at 250 ˝C for 60 min, cooling at 80 ˝C and keeping at 80 ˝C before immersion.The TiO 2 films were immersed in a 2 ˆ10 ´4 M solution of the porphyrin and a 6 ˆ10 ´4 M solution of CDCA in THF at 50 ˝C.The dye-sensitized TiO 2 films were washed with THF, dried in hot air, and used as the working electrode.The counter electrode was prepared on an indium-doped tin-oxide glass substrate (typical size: 1.0 ˆ1.5 cm 2 ) by spin-coating a H 2 PtCl 6 /isopropanol solution through thermal decomposition at 380 ˝C for 0.5 h.To fabricate the DSSC device, the two electrodes were tightly clipped together into a sandwich-type cell that was spaced by a 40 µm film spacer.A thin layer of electrolyte, which contained 0.05 M I 2 , 0.1 M lithium iodide (LiI), 0.6 M dimethyl-propyl-benzimidiazole iodide (DMPII), and 0.6 M 4-tert-butylpyridine (TBP) in dry CH 3 CN, was introduced into the space between the two electrodes.The photo-electrochemical characterizations on the solar cells were performed on an Oriel Class A solar simulator (Oriel 91195A, Newport Corp., Andover, MA, USA).Photocurrent-voltage characteristics of the DSSCs were recorded on a potentiostat/galvanostat (CHI650B, CH Instruments, Inc.) at a light intensity of 100 mW¨cm ´2 and calibrated to an Oriel reference solar cell (Oriel 91150).The monochromatic quantum efficiency was recorded on a monochromator (Oriel 74100) under short-circuit conditions.The intensity of each wavelength was within the range 1-3 mW¨cm ´2.

Stability Test of Porphyrin Sensitizers
For the stability study, TiO 2 thin films with areas of 1 ˆ1 cm and thicknesses of 3-4 µm were used.The films were immersed in a 2 ˆ10 ´4 M solution of the porphyrin in THF for 0.5 h, dried, and the absorbance was measured.Then, the same films were irradiated under standard one sun illumination for 5 min and 30 min and the absorbance was measured again.

Conclusions
Porphyrins Zn 1 TH 3 A, Zn 1 TC 3 A, Zn 1 BC 3 A, Zn 1 OD 3 A, Zn 1 OH 3 A, Zn 1 NP 3 A, Zn 1 NHA, Zn 1 NN 3 A, and Zn 1 ND 3 A with meso-substituted electron-donating substitutions and three carboxyphenyl groups have been synthesized from readily available starting materials in high yields.The syntheses of the studied sensitizers are facile, straightforward, and scalable.The redox potentials of the studied porphyrins have a significant effect on the photovoltaic properties.ATR-FTIR spectra of the studied three-carboxyphenyl-group-possessing porphyrins on TiO 2 reveal that two p-carboxy-phenyls are attached on TiO 2 with a double arm binding mode.Higher than 4% efficiency has been achieved by all of the studied porphyrins.Zn 1 NH 3 A gives best performance owing to its electron donating and anti-aggregation characteristics.The stability study performed demonstrates the studied multi-anchored porphyrins are more stable compared to its mono-anchored analogs.

Figure 7 .
Figure 7. Stability study of Zn 1 ND 3 A and Zn 3 TPA 1 A.

Table 1 .
Electrochemical Properties of studied porphyrins.

Table 1 .
Electrochemical Properties of studied porphyrins.