Fabrication and analysis of dye-sensitized solar cells (DSSCs) using porphyrin dyes with catechol anchoring groups

Abstract

Herein we report the preparation and application of 4 different zinc(II) tetrakis(dihydroxyphenyl) porphyrins (ZnTDHPP) as the sensitizing dyes in dye-sensitized solar cells (DSSCs). The experimental results include solution and solid state UV-Vis data, steady state current–voltage characteristics, and our theoretical analysis comprised of density functional theory (DFT) and Langmuir isotherm adsorption formalism. The results show that the Zn tetrakis(2,3-dihydroxyphenyl) porphyrin (Zn2,3TDHPP) and Zn tetrakis(3,4-dihydroxyphenyl) porphyrin (Zn3,4TDHPP) with adjacent hydroxyl groups attaches to a TiO₂ surface much more strongly than carboxylate. The catechol anchoring group showed high stability of the dye on the TiO₂ surface. The cells prepared from these porphyrins showed no significant desorption of dye from the TiO₂ surface after several days. The DSSCs based on Zn2,3TDHPP showed the best photovoltaic performance under AM 1.5 irradiation comparable to the conventional Zn tetrakis(p-carboxyphenyl) porphyrin (ZnTCPP), despite lower dye loading on the TiO₂ surface. However, non-cooperative OH bonding to TiO₂ for Zn2,4TDHPP and Zn2,5TDHPP shows weak attachment to the TiO₂ surface and lower efficiencies. DFT calculations showed that the Zn2,3TDHPP structure is more non-planar than the others, which may suppress dye aggregation. The adopted adsorption modeling is fitted to the experimental data to study the kinetics of dye loading. The study can herald development of a new class of porphyrin sensitizer for DSSC applications.

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1 Introduction

With population increase, resource depletion and the rising cost of fossil energy sources, researchers have tried to develop clean and renewable energy such as solar energy. Solar energy is an abundant, renewable and clean energy source that could supply energy for societies. Dye-sensitized solar cells (DSSCs) are the third generation of solar cells, first developed by O'Regan and Gratzel in 1991.¹ DSSC generally comprises a photoanode, which is made of a semiconductor, a sensitizer (dye molecules) on a transparent conducting oxide (TCO) substrate and a hole conducting material as electrolyte. Photoexcitation of dye molecules leads to electron injection into the conduction band of the semiconductor metal oxide (generally titanium dioxide), which is in contact with an electrolyte solution, generating the oxidized dye. The process is followed by electron transfer from the electrolyte to the sensitizer and dye regeneration.^2–4

Inspired by the important role of porphyrins in photosynthesis, many researchers have used porphyrin derivatives as sensitizers in DSSCs.^5–14 To date, the highest DSSC efficiency has been reported for a porphyrin dye.¹² Porphyrins are macrocyclic compounds with highly π-conjugated systems exhibiting intense absorptions in the visible region. Some functional groups can extend the porphyrin and increase their ability to absorb light and therefore making them good choices for use in solar energy conversion. Catechol is such a functional group, which could extend the porphyrin π-system.¹⁵ Catechol porphyrins have potential application in solar cells.^16,17 ortho-Dihydroxy phenyl functional groups in catechol porphyrins are directed at the meso positions. ortho Dihydroxy phenyl can easily be oxidized to ortho-quinone by electrochemical methods.¹⁸ Porphyrins and catechol are both active in electrochemistry and coupling these two compounds could make new compounds with interesting properties.^15–17,19

Anchoring groups play an important role in DSSCs. To have a fast electron injection into the TiO₂ semiconductor, the existence of anchoring groups is crucial to attach the dye onto the TiO₂ surface. Carboxylate is an anchoring group, which has been widely studied.^20–25 The major binding modes for the carboxylate anchoring group are bidentate chelating and bidentate bridging of the carboxylate on the TiO₂ surface but an monodentate mode is also possible (Scheme 1).²⁶

Scheme 1 (a) Monodentate, (b) bidentate chelating and (c) bidentate bridging modes of carboxylate binding on a TiO₂ surface.

Similar to the carboxylate anchoring group, catechol can also bind to the TiO₂ surface in monodentate or bidentate modes. Li et al.¹⁹ have shown that catechol can form two full coverage H-bonded structures, containing monodentate only, or mixed monodentate and bidentate molecules on the TiO₂ surface (Scheme 2). Monodentate and bidentate structures can simply interconvert via proton exchange between catechol and the TiO₂ surface. In the bidentate mode, the formation of both bidentate chelating (Scheme 2b) and bidentate bridging (Scheme 2c) complexes have been reported.^27–29

Scheme 2 (a) Monodentate, (b) bidentate chelating and (c) bidentate bridging modes of catechol binding on a TiO₂ surface.

In this study, we used zinc(II) tetrakis(dihydroxyphenyl) porphyrin (Zn-TDHPP) as the sensitizer in dye-sensitized solar cells and compared the results with tetrakis(p-carboxyphenyl) zinc(II) porphyrin (ZnTCPP), which has been previously studied.^22,30 To the best of our knowledge, this is the first report of a porphyrin dye with a catechol anchoring group thus far. Photosensitizers are characterized using NMR, FT-IR and UV-Vis spectroscopy. To investigate application of these sensitizers in DSSCs, the solution and solid state UV-Vis spectroscopy are used, and the dye loading amount is derived from these experiments. The current–voltage characteristics are also provided for the DSSCs with the synthesized photosensitizers. To obtain further evidence, density functional theory (DFT) calculations were carried out. Dye adsorption modeling was also used to investigate the adsorption kinetics and estimate electron injection efficiencies from dye to the TiO₂ surface. General structures of metalloporphyrins used in this study are shown in Scheme 3.

Scheme 3 General structures of sensitizers used in this study.

2 Experimental

2.1 Materials and reagents

Tetrakis(dihydroxyphenyl) porphyrins were obtained from tetrakis(dimethoxyphenyl) porphyrins (ZnTDMPP). Tetrakis(dimethoxyphenyl) porphyrins and tetrakis(p-carboxyphenyl) porphyrin were prepared by the condensation of the appropriate dimethoxybenzaldehyde with pyrrole in refluxing propionic acid.²⁷ The tetrakis(dihydroxyphenyl) porphyrins were prepared by boron tribromide demethylation of the corresponding methyl ethers, according to the method presented by Milgrom.³¹ Solvents were dried over appropriate drying agents. All other chemicals were reagent grade and used without further purification.

2.2 Analytical instruments and measurements

Solution absorption spectra were obtained on an Analytik Jena s600 spectrometer using a 1 cm cuvette. Solutions were made at concentrations of 5 × 10⁻⁵ M in ethanol. Solid state absorption spectra were obtained on a Shimadzu UV-2100 spectrometer.

¹H NMR and ¹³C NMR spectra were obtained in DMSO-d₆ using a 300 MHz Bruker instrument. Photocurrent–voltage (I–V) measurements were performed using a Sharif solar (I–V) source measure unit and Prova 8300 sun simulator.

2.3 Synthesis of dyes

2.3.1 Tetrakis(dimethoxyphenyl) porphyrins. Propionic acid (150 ml) was brought to the boil and then the dimethoxybenzaldehyde (20 mmol) was added, followed by pyrrole (20 mmol) added dropwise. After refluxing for 1.5 h, the solution was cooled to room temperature and then crystallized over 3 days. The product was filtered and washed with hot water and methanol. In the case of tetrakis(3,4-dimethoxyphenyl) porphyrin and tetrakis(2,5-dimethoxyphenyl) porphyrin, no precipitate was achieved; therefore, the propionic acid was removed from the crude mixture. The solid black residue was purified using column chromatography with dichloromethane and methanol (50 : 1) as eluents.

Tetrakis(3,4-dimethoxyphenyl) porphyrin: yield: 350 mg 4%, ¹H NMR (CDCl₃, 300 MHz), δ, (ppm): 8.9 (s, 8H), 7.8 (s, 4H), 7.7 (d, 4H), 7.2 (d, 4H), 4.1 (s, 12H); 3.9 (s, 12H), −2.70 (s, 2H). ¹³C NMR (CDCl₃, 100 MHz), δ, (ppm): 148.8, 147.0, 134.7, 131.1, 130.0, 127.34, 126.7, 119.8, 118.2, 110.2, 109.4, 108.7, 56.0, 55.9; UV-Vis, (CHCl₃ λ_max): 421 (Soret); 518; 556; 592; 649.

Tetrakis(2,3-dimethoxyphenyl) porphyrin: yield: 680 mg 8%, ¹H NMR (300 MHz, CDCl₃, 25 °C): δ, (ppm), 8.7 (s, 8H), 7.2 (m, 4H), 7.1 (d, 4H), 7.0 (m, 4H), 3.9 (s, 12H), 1.8 (s, 12H), −2.8 (s, 2H).

Tetrakis(2,4-dimethoxyphenyl) porphyrin: yield: 682 mg 8%, ¹H NMR (CDCl₃, 300 MHz), δ, (ppm), 8.7 (s, 8H), 7.7 (s, 4H), 7.0 (s, 8H), 4.1 (s, 12H), 1.8 (s, 12H), −2.7 (s, 2H).

Tetrakis(2,5-dimethoxyphenyl) porphyrin: yield: 500 mg 6%, ¹H NMR (CDCl₃, 300 MHz), δ, (ppm), 9 (s, 8H), 7.4 (d, 4H), 7.1 (s, 8H), −2.8 (s, 2H), 3.9 (s, 12H), 1.9 (s, 12H).

2.3.2 Tetrakis(p-carboxyphenyl) porphyrin (TCPP). Propionic acid (150 ml) was brought to the boil and then the p-phthalaldehydic acid (20 mmol) was added followed by pyrrole (20 mmol) added dropwise. After refluxing for 1.5 h, the solution was cooled to room temperature and then crystallized overnight. The product was filtered and washed with hot water and a small amount of methanol.

Yield: 10%, ¹H NMR (300 MHz, DMSO-d₆, 25 °C) δ (ppm): 8.7 (s, 8H), 8.3 (d, 8H), 8.1 (d, 8H), −2.9 (s, 2H).

2.3.3 Zinc tetrakis(p-carboxyphenyl) porphyrin. A mixture of TCPP and 1.2 equivalents of Zn(OAc)₂·4H₂O in methanol was stirred at room temperature. The metalation process was followed by UV-Vis spectroscopy. After the completion of the reaction, the solvent was evaporated with a rotor evaporator at low temperature. The crude product was washed several times with water to remove the excess zinc salt and then dried under vacuum.

ZnTCPP: yield: 94%, ¹H NMR (300 MHz, DMSO-d₆, 25 °C) δ (ppm): 8.9 (s, 8H), 8.4 (d, 8H), 8.3 (d, 8H). UV-Vis (EtOH, λ_max, nm): 425 (Soret), 558, 601.

2.3.4 Tetrakis(dihydroxyphenyl) porphyrins (TDHPP). The appropriate tetrakis(dimethoxyphenyl) porphyrin (0.5 mmol) was dissolved in the minimum amount of dry dichloromethane. The solution was cooled in a methanol–liquid nitrogen bath (to −98 °C). Boron tribromide (6 mmol) was slowly added to this solution. The mixture was stirred for 1 h at −98 °C, and then allowed to come to room temperature, and was then stirred overnight at room temperature. Water (20 ml) was added to quench the ice-cold reaction mixture. Neutralization with triethylamine produced a brown-purple precipitate, which was collected by filtration and washed with water and dichloromethane. Characterization for 3,4TDHPP: yield: 75%, ¹H NMR (300 MHz, DMSO-d₆, 25 °C): δ, (ppm) 9.4 (s, 8H), 8.9 (s, 8H), 7.7 (s, 4H), 7.4 (s, 4H), 7.1 (s, 4H), −2.9 (s, 2H). ¹³C NMR (75 MHz, DMSO-d₆, 25 °C): δ, (ppm) 145.0, 144.1, 132.8, 131.6, 126.7, 122.9, 120.4, 115.9, 114.7. IR (KBr): ν, cm⁻¹ 3386, 1653, 1597, 1508, 1433, 1346, 1260, 1201, 1108, 928, 871, 801, 711. UV-Vis (EtOH, λ_max, nm): 428 (Soret), 520, 559, 592, 653.

2,3TDHPP: yield: 80%, ¹H NMR (300 MHz, DMSO-d₆, 25 °C): δ, (ppm) 9.7 (s, 8H), 8.7 (s, 8H), 7.2 (m, 4H), 7.1 (d, 4H), 7.0 (m, 4H), −2.8 (s, 2H). ¹³C NMR (75 MHz, DMSO-d₆, 25 °C): δ, (ppm), 146.5, 145.5, 129.1, 128.6, 127.8, 126.8, 118.1, 116.8, 116.1. IR (KBr): ν, cm⁻¹, 3423, 2923, 2853, 1615, 1586, 1466, 1346, 1204, 1145, 927, 844, 804, 768, 730. UV-Vis (EtOH, λ_max, nm): 420 (Soret), 516, 550, 591, 647.

2,4TDHPP: yield: 78%, ¹H NMR (300 MHz, DMSO-d₆, 25 °C): δ, (ppm) 9.1 (s, 8H), 8.8 (s, 8H), 7.7 (s, 4H), 7.0 (s, 8H), −2.7 (s, 2H). ¹³C NMR (75 MHz, DMSO-d₆, 25 °C): δ, (ppm), 146.5, 145.5, 129.1, 128.6, 127.8, 126.8, 118.1, 116.8, 116.1.

2.3.5 Zn tetrakis(dihydroxyphenyl) porphyrins. A mixture of TDHPP and 1.2 equivalents of Zn(OAc)₂·4H₂O in methanol was stirred at room temperature. The metalation process was followed by UV-Vis spectroscopy. After the completion of the reaction, the solvent was evaporated with a rotor evaporator at low temperature. The crude product was washed several times with water to remove the excess zinc salt and then dried under vacuum.

Zn2,3TDHPP: yield: 95%, ¹H NMR (300 MHz, DMSO-d₆, 25 °C): δ, (ppm) 9.7 (s, 8H), 8.8 (s, 8H), 7.4 (m, 4H), 7.2 (d, 4H), 7.0 (m, 4H). ¹³C NMR (75 MHz, DMSO-d₆, 25 °C): δ, (ppm), 146.6, 145.5, 129.2, 128.8, 128.0, 126.8, 118.3, 116.9, 115.9. IR (KBr): ν, cm⁻¹, 3426, 2926, 2851, 1620, 1586, 1466, 1346, 1210, 1145, 927, 844, 809, 768, 735, 547, 469. UV-Vis (EtOH, λ_max, nm): 425 (Soret), 556, 592. Elemental analysis calc. for C₄₄H₂₈N₄O₈ (%) Zn: C, 65.56; H, 3.50; N, 6.95; found (%) C, 65.58; H, 3.45; N, 6.98.

Zn3,4TDHPP: yield: 95%, ¹H NMR (300 MHz, DMSO-d₆, 25 °C): δ, (ppm) 9.4 (s, 8H), 8.8 (s, 8H), 7.7 (s, 4H), 7.4 (s, 4H), 7.1 (s, 4H). ¹³C NMR (75 MHz, DMSO-d₆, 25 °C): δ, (ppm) 145.0, 144.1, 132.8, 131.6, 126.7, 122.9, 120.4, 115.9, 114.7. UV-Vis (EtOH, λ_max, nm): 428 (Soret), 559, 602.

Zn2,4TDHPP: yield: 96%, ¹H NMR (300 MHz, DMSO-d₆, 25 °C): δ, (ppm) 9.4 (s, 8H), 8.8 (s, 8H), 7.7 (s, 4H), 6.9 (s, 8H). UV-Vis (EtOH, λ_max, nm): 425 (Soret), 556, 597.

Zn2,5TDHPP: yield: 95%, ¹H NMR (300 MHz, DMSO-d₆, 25 °C): δ, (ppm) 9.7 (s, 8H), 9 (s, 8H), 7.4 (d, 4H), 7.1 (s, 8H). UV-Vis (EtOH, λ_max, nm): 424 (Soret), 556, 591.

2.4 Fabrication of dye-sensitized solar cells and photovoltaic measurements

2.4.1 Preparation of nanoporous TiO₂ paste. Two types of TiO₂ paste containing nanocrystalline TiO₂ (20 nm, paste A) and submicro particle TiO₂ (350 nm, paste B) were prepared by a previously reported procedure.^32,33 TiO₂ nanopowder (3 g) was added to mortar and ground with gradual addition of 0.5 ml of acetic acid, 2.5 ml of deionized water and 15 ml of ethanol. The TiO₂ dispersion was transferred with 50 ml of ethanol to a round bottom flask and homogenized in an ultrasonic bath for 30 min. 10 g of terpineol was added to the dispersion and sonication was resumed for another 30 min. Alternating stirring and sonication were used three consecutive times after adding 1.5 g of ethyl cellulose (10% solution in ethanol) to the TiO₂ dispersion. Finally, ethanol was removed by a rotary evaporator. The resulting screen-printing paste corresponds to 21 wt% TiO₂, 10 wt% ethyl cellulose and 69 wt% terpineol (paste A).

Paste B, which was used in the light-scattering layers, was prepared with the same method using 350 nm TiO₂ nanoparticles.

2.4.2 Fabrication of porous TiO₂ electrodes. The FTO glass was first cleaned with detergent solution, distilled water and methanol using an ultrasonic bath (10 min for each solution), and then rinsed with methanol. After treatment in 40 mM aqueous TiCl₄ solution at 70 °C for 30 min, the FTO glass was again washed with water and methanol. Paste A was screen printed (manual screen printer, 90T, Estal Mono, Schweiz. Seidengazefabrik, AG, Thal) on the FTO glass and placed in a clean box for 10 min to let the paste relax and reduce the surface irregularity. Subsequently, FTO glasses were dried for 10 min at 125 °C. This screen-printing procedure with paste A (coating, storing and drying) was repeated to get an appropriate thickness of 10–12 μm for the working electrode. After drying the (paste A) films at 125 °C, one layer of paste B was deposited by the screen-printing method, resulting in a light-scattering TiO₂ film containing a 350 nm layer of anatase TiO₂ particles. The electrodes coated with the TiO₂ pastes were gradually heated under airflow at 325 °C for 5 min, at 375 °C for 5 min, and at 450 °C for 15 min, and finally, at 500 °C for 15 min.

The TiO₂ films were once again treated with 40 mM TiCl₄ solution, as previously described, then rinsed with water and methanol and heated at 450 °C for 15 min and 500 °C for 15 min. After cooling to 80 °C, the TiO₂ electrodes were immersed into 0.1 mM dye solution with 35 mM 4-tert-butylpyridine as additive in a mixture of ethanol and THF (volume ratio, 4 : 1) and maintained at room temperature for 2 h to take up the dye.

2.4.3 Preparation of counter Pt electrodes and DSSC assemblage. FTO glasses were washed with the same method as used for the working electrode. 0.05 M H₂PtCl₆ solution was spin-coated on the FTO glass substrate at 2000 rpm for 15 s. The formed film was annealed at 450 °C for 1 hour in the atmosphere.

Clamps assembled the dye-adsorbed TiO₂ electrode and counter electrode into a sandwich-type cell. Finally, a drop of electrolyte solution (0.10 M lithium iodide, 0.60 M butylmethylimidazolium iodide, 0.03 M I₂, and 0.05 M 4-tert-butylpyridine in acetonitrile/valeronitrile 75 : 25) was introduced into the clamped electrodes.

2.5 Desorption measurement

The loading of each dye was determined by dye desorption in a basic solution. The dye-loaded anodes were soaked in a 10% solution of tetrabutylammonium hydroxide in water/EtOH. The solution absorbances were measured with time until the dyes completely desorbed from electrode.

3 Results and discussion

We have studied four types of porphyrin dyes, which are Zn2,3TDHPP, Zn2,4TDHPP, Zn2,5TDHPP, and Zn3,4TDHPP, and compared them with previously studied dye ZnTCPP (Scheme 3). Each of these dyes was chosen because they have two hydroxyl groups on each porphyrin meso-phenyl as anchoring groups. In the case of Zn2,3TDHPP and Zn3,4TDHPP these two hydroxyl groups were adjacent and cooperatively bond to the TiO₂ surface, but for Zn2,4TDHPP and Zn2,5TDHPP they were non-adjacent.

3.1 Photophysical properties of the dyes in solution and on TiO₂ film

3.1.1 Photophysical properties of the dyes in solution. UV-Vis spectra of metalloporphyrins are shown in Fig. 1. Absorption spectra were obtained between 400 and 750 nm in ethanolic solution. All studied metalloporphyrins have strong Soret bands at around 425–428 nm and two Q bands at around 500–700 nm, which are UV-Vis spectra characteristics of metalloporphyrins and makes them potential candidates for light harvesting in DSSCs.

Fig. 1 UV-Vis spectra of metalloporphyrins at 0.05 mM concentration.

Zinc is a closed shell metal and has very little effect on the porphyrin π to π* energy gap in porphyrin electronic spectra. The Soret bands for ZnTDHPPs were 5–8 nm red-shifted relative to zinc tetraphenylporphyrin, which indicates the conjugation of the lone pair electrons of the eight oxygen atoms that are directly attached to the phenyl rings, with π electrons of the macrocycle rings.^18,34 Extinction coefficients for ZnTDHPPs were lower than ZnTPP due to both electronic and steric effects.^35,36 ZnTDHPPs with adjacent hydroxyl groups have relatively higher extinction coefficients than the porphyrins with non-adjacent hydroxyl groups. The lower extinction coefficient in such porphyrins, i.e. Zn2,5TDHPP, could be due to the resonance forms shown in Scheme 4.³⁷ The π system extension, results in a red-shift in the porphyrin UV-Vis spectra and the electronegativity of oxygen attracts electron density from the porphyrin ring and lowers the extinction coefficient.³⁶

Scheme 4 The resonance forms of 2,5TDHPP.

3.1.2 Photophysical properties of the dyes adsorbed on the TiO₂ film. The solid state UV-Vis spectra of zinc tetrakis(dihydroxyphenyl) porphyrin and zinc tetrakis(p-carboxyphenyl) porphyrin dyes adsorbed on the TiO₂ film have been shown in Fig. 2. The broadened absorption bands of the dyes on TiO₂ might be a sign of intermolecular interactions of the molecules aggregated on the TiO₂ surfaces.³⁸ All zinc tetrakis(dihydroxyphenyl) porphyrin dyes have sharper Soret bands than that of ZnTCPP indicating lower aggregation in the former compounds. Dihydroxophenyl porphyrins also show red-shifts upon binding to the TiO₂ film, which could be ascribed to the stabilization of the LUMO level in TiO₂–porphyrin complexes.³⁹ In the case of ZnTCPP, the existence of blue shoulders for the Soret bands may have been derived from the deprotonation of the carboxylic acid upon binding to the TiO₂ surface⁴⁰ and/or H-type aggregation of the dye molecules.^41,42

Fig. 2 The solid state UV-Vis spectra of ZnTDHPP dyes adsorbed on the TiO₂ film.

3.1.3 Desorption of dyes from TiO₂ film and the adsorption model. To measure the dye loading amount and also the time for complete desorption of dye from the TiO₂ surface, we carried an experiment to desorb dye from the TiO₂ surface. Fig. 3 shows desorption of dyes from the TiO₂ surface versus time in TBAOH solution.

Fig. 3 Desorption of dyes from TiO₂ surface during the 30 h (○-experimental ●-model).

ZnTCPP and porphyrins with non-adjacent hydroxyl groups desorbed from TiO₂ surface very quickly; in comparison, Zn2,3TDHPP and Zn3,4TDHPP desorbed more slowly from the TiO₂ surface. Zn2,4TDHPP and Zn2,5TDHPP almost completely desorbed from the TiO₂ surface after 30 minutes and ZnTCPP after two hours yet this process took six hours for Zn2,3TDHPP and Zn3,4TDHPP. There was no significant desorption in acidic and neutral media after days. Therefore, Zn2,3TDHPP and Zn3,4TDHPP showed almost three times more stability on the TiO₂ surface. This significant increase in stability of the catechol anchoring group can arise from cooperative bonding of dihydroxy phenyl via a stable five-membered ring (form b in Scheme 2).

Table 1 shows data concerning spectral properties and dye loading amount of all the synthesized porphyrin derivatives on the TiO₂ surface calculated from desorption and calibration data.

Table 1 Absorption wavelength, extinction coefficient and loading amount of dyes on TiO₂

Dye	λmax (nm) ethanol	ε × 10^−5 (M^−1 cm^−1)	λmax (nm) on TiO2 surface	Dye loading (nmol cm^−2)	Injection efficiency% (φinj)	Adsorption coefficient (ka) nmol cm^−1	Desorption coefficient (kd) nmol cm^−1
ZnTCPP	425	3.466	425	33.5	2.63	100 × 10^−3.0	2.85 × 10^−3.9
Zn2,3TDHPP	425	4.538	430	25.3	2.6	100 × 10^−3.1	3.5 × 10^−3.9
Zn3,4TDHPP	428	3.098	435	36.7	2.5	100 × 10^−3.01	2.85 × 10^−3.9
Zn2,5TDHPP	424	2.84	429	15.1	0.9	57 × 10^−3.01	3.8 × 10^−3.75
Zn2,4TDHPP	425	2.24	432	10.4	0.5	55 × 10^−3.01	2.5 × 10^−3.75

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For non-adjacent hydroxyl group dyes, the loading amount and also desorption time were very low, which indicates that in such porphyrins, there was no effective attachment of dye on the TiO₂ surface. Although all the electrodes were soaked in dye solutions of the same concentration for the same period of time, the amount of adsorbed dye for Zn2,3TDHPP was lower than that for Zn3,4TDHPP. This observation suggests that the flat orientation of adsorbed porphyrin with respect to the TiO₂ surface of Zn2,3TDHPP (25.3 nmol cm⁻²) results in less dye uptake compared to Zn3,4TDHPP (36.7 nmol cm⁻²) and ZnTCPP (33.5 nmol cm⁻²)^20,43 (Fig. 4).

Fig. 4 Orientation of adsorbed porphyrin with respect to the TiO₂ surface.

Herein, we implement a model, developed in our previous study,⁴⁴ for investigation of time evolution of dye loading to extract the dye adsorption. The adsorption from dye solution by solid nanoparticles is studied assuming a kinetic form of the time-dependent Langmuir formalism framework. In the present microscopic model, it is assumed that the bulk density of dye molecules are adsorbed only by the nanoparticles network by a constant adsorption and desorption rates, also the interconnection of nanoparticles is influenced by particle necking, which leads to partial aggregation of nanoparticles.

Using k_a and k_d as the rate constants for adsorption and desorption, respectively, we can write the net rate of adsorption for solution phase species i as the difference between the rate of adsorption and the rate of desorption as follows:

R_i,ads,net = k_aiC_isol(C_i-site,total − C_{i-sites,occupied by i}) − k_d,sC_{i-sites,occupied by i} (1)

which means that the rate of adsorption should be proportional to the concentration of dye molecules in the solution phase (C_isol) and to the number of sites available on the surface of the particles. If the dye molecules in the solution around the adsorbent solid particles occupy some fraction ε of a volume V, and if this volume contains an adsorbing dye molecule i, then the rate of adsorption of the dye molecules onto the adsorbent and the rate of depletion of that species from the solution phase are coupled batch processes. The component mass balances for the solution and solid phases are as follows:

Solution phase

Solid phase

where A_s is effective surface area of nanoparticle matrix per unit of mass and ρ_s is the solid nanoparticle density. Further derivation of equations can be found in the literature.⁴⁴ The obtained adsorption amount on the semiconductor matrix can be assumed to calculate the injection efficiency by relation (4).

φ_inj = φ(1 − 10^−C_a) (4)

The highest rate of injection efficiency belongs to ZnTCPP. The estimated injection quantum yield together with 100% collection efficiency in DSSCs strongly suggest that the limiting performance factor for DSSCs can be the inefficient injection efficiency of the cells (Table 1).

3.2 Photovoltaic properties

To consider the potential of ZnTDHPP as photosensitizers for DSSCs, their DSSC performances were tested. Fig. 5 shows J–V curves for the studied dyes. Detailed photovoltaic parameters, such as open circuit voltage (V_OC), short circuit current (J_SC), fill factor (ff) and power conversion efficiency (η) for different dyes under AM 1.5 solar illumination, have been listed in Table 2. Power conversion efficiency is derived from the equation: η = J_SC × V_OC × ff, where J_SC is the short circuit current, V_OC is the open circuit potential, and ff is the fill factor.

Fig. 5 Current–voltage curves for DSSCs based on Zn2,3TDHPP, Zn3,4TDHPP, Zn2,4TDHPP and Zn2,5TDHPP under illumination of AM 1.5 simulated sunlight.

Table 2 Photovoltaic parameters of porphyrin-based dye-sensitized solar cells under AM 1.5 illumination

Dye	VOC (mV)	JSC (mA cm^−2)	Fill factor (%)	Efficiency (%)
ZnTCPP	449	2.7	67	0.8
Zn2,3TDHPP	449	2.1	62	0.6
Zn3,4TDHPP	410	1.11	51	0.23
Zn2,4TDHPP	312	0.28	34	0.03
Zn2,5TDHPP	390	0.12	38	0.02

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The V_OC values of all the dye cells ranked: ZnTCPP = Zn2,3TDHPP > Zn3,4TDHPP > Zn2,5TDHPP > Zn2,4TDHPP. Despite the lower dye loading amount for Zn2,3TDHPP compared to ZnTCPP and Zn3,4TDHPP, the V_OC value for this porphyrin is similar to ZnTCPP. This could be because of the flat orientation of Zn2,3TDHPP on the TiO₂ surface, meaning it could cover more of the surface than the other dyes and prevent direct contact of electrolyte with TiO₂ as well as decrease the recombination.

Similar to the power conversion efficiencies, the J_SC values of all the dye cells ranked: ZnTCPP > Zn2,3TDHPP > Zn3,4TDHPP > Zn2,4TDHPP > Zn2,5TDHPP. The order is the same as extinction coefficient for ZnTDHPPs, indicating that short circuit current increases with increase in light absorption ability. However, in the case of Zn2,4TDHPP and Zn2,5TDHPP photovoltaic results were poor because of weak binding on the TiO₂ surface and in some extent due to lower extinction coefficients of the corresponding dye.

3.3 Density functional theory (DFT) calculations

To obtain further evidence for the abovementioned results, DFT calculations of two porphyrins with adjacent hydroxyl group and ZnTCPP dyes were carried out, using the B3LYP/3-21G level. The calculated structures do not show negative frequencies, inferring that the optimized geometries are in the global energy minima. Fig. 6 illustrates the electron density distributions of dyes in their respective HOMO and LUMO levels. The energy levels of frontier orbitals and the HOMO–LUMO energy gaps for these three dyes are listed in Table 3.

Fig. 6 The electron density distributions and energy levels of ZnTCPP, Zn2,3TDHPP and Zn3,4TDHPP.

Table 3 The energy levels of frontier orbitals and the HOMO–LUMO energy gaps for ZnTCPP, Zn2,3TDHPP and Zn3,4TDHPP

Dye	HOMO (eV)	LUMO (eV)	Eg (eV)
ZnTCPP	−5.34	−2.40	2.94
Zn2,3TDHPP	−4.95	−1.91	3.03
Zn3,4TDHPP	−4.85	−1.95	2.90

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Electron distributions of all three dyes, in the frontier orbitals, are mostly over the porphyrin ring, which is likely due to the fact that the phenyl rings are perpendicular to the porphyrin ring and do not contribute to electron distributions. This could block electron transfer to TiO₂ and reduce photovoltaic performance for studied dyes to those that have anchoring group participation in electron conjugation. It is well known that the electron density distribution of the LUMO around an anchoring group affects the electronic coupling between the excited adsorbed dye and 3d orbital of TiO₂.⁴⁵ Consequently, we can expect the low φ_inj values of dyes (Table 1).

For ZnTDHPPs, variations in energies for different positions of the hydroxyl groups for the HOMO level (−4.95 eV for Zn2,3TDHPP and −4.85 eV for Zn3,4TDHPP) are larger than those for the LUMO level (−1.91 eV for Zn2,3TDHPP and −1.95 eV for Zn3,4TDHPP) indicating that variation in the hydroxyl group position mainly affect the occupied orbitals of ZnTDHPP dyes (Table 3). In addition, the dihedral angle between the phenyl unit and the porphyrin macrocycle for Zn2,3TDHPP is 84.1° but the corresponding angles for Zn3,4TDHPP and ZnTCPP are 69.1° and 69.5°, respectively, indicating that Zn2,3TDHPP is more non-planar than the others.

It is well known that for non-planar and distorted dyes, dye aggregation and charge recombination can be efficiently suppressed, which is favorable for enhancing cell efficiency.⁴² Similarly, the lower V_OC for Zn3,4TDHPP could be a consequence of the lower dihedral angle between the phenyl unit and the porphyrin macrocycle and the more planar structure that favors aggregation. Therefore, Zn2,3TDHPP compared to Zn3,4TDHPP could be considered a better dye due to its different mode of attachment (Fig. 4) and its relatively non-planar structure.^8,42,46

To have fast electron transfer, the HOMO must be sufficiently more positive than the redox potential of I⁻/I₃⁻ to accept electrons effectively; the difference between these two levels is given by ΔE.⁴⁷ HOMO energy levels are −5.34, −4,95 and −4.85 for ZnTCPP, Zn2,3TDHPP and Zn3,4TDHPP, respectively; considering that I⁻/I₃⁻ electrolyte potential is −4.85 eV,⁴⁸ the differences between electrolyte and HOMO energy level (ΔE) are 0.5, 0.1 and 0 for ZnTCPP, Zn2,3TDHPP and Zn3,4TDHPP, respectively. For Zn3,4TDHPP, there is no driving force for electron injection from electrolyte to dye oxidized state, which could be the reason for poor photocurrent in this dye (Fig. 7).⁴⁷

Fig. 7 Illustration of energy levels and electron injection efficiencies of ZnTCPP, Zn2,3TDHPP and Zn3,4TDHPP.

4 Conclusion

ZnTDHPP dyes were successfully synthesized and used as sensitizers in DSSCs and compared with ZnTCPP as the reference dye. This is the first report of a porphyrin dye with a catechol anchoring group to date. The J_SC values of all the dye cells, as well as the power conversion efficiencies, were ranked: ZnTCPP > Zn2,3TDHPP > Zn3,4TDHPP > Zn2,4TDHPP > Zn2,5TDHPP; the order was the same as extinction coefficients for ZnTDHPPs. Despite lower dye loading on the TiO₂ surface for Zn2,3TDHPP, it has comparable photovoltaic performance (0.6 vs. 0.9) and more stability than ZnTCPP. The stability of the dye on TiO₂ for Zn2,3TDHPP and Zn3,4TDHPP might arise from bidentate coordination on the TiO₂ surface via chelation and formation of five-membered rings. The higher efficiency observed for Zn2,3TDHPP relative to Zn3,4TDHPP may be ascribed to the mode of coordination to the TiO₂ surface which results in more efficient electron transfer, suppressed dye aggregation and high extinction coefficient for the former compound. Injection efficiencies of the cells were also calculated; the results indicate that the limiting performance factor for DSSCs can be the inefficient injection efficiency of the cells. Our future strategy is to take advantage of the high stability of the catechol anchoring group to extend the π system contribution and also to synthesise asymmetric porphyrins to make dipole moments by modification of the porphyrin core to enhance the efficiencies.

Acknowledgements

This study was supported by the Iranian National Science Foundation (INSF) and Shahid Beheshti University Research Affaires.

References

1. B. O'Regan and M. Gratzel, Nature, 1991, 353, 737–740.
2. A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson, Chem. Rev., 2010, 110, 6595–6663.
3. M. K. Nazeeruddin, E. Baranoff and M. Grätzel, Solar Energy, 2011, 85, 1172–1178.
4. M. Grätzel, J. Photochem. Photobiol., C, 2003, 4, 145–153.
5. N. Xiang, X. Huang, X. Feng, Y. Liu, B. Zhao, L. Deng, P. Shen, J. Fei and S. Tan, Dyes Pigm., 2011, 88, 75–83.
6. A. Yella, H. W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W. G. Diau, C. Y. Yeh, S. M. Zakeeruddin and M. Grätzel, Science, 2011, 334, 629–634.
7. S. Eu, S. Hayashi, T. Umeyama, Y. Matano, Y. Araki and H. Imahori, J. Phys. Chem. C, 2008, 112, 4396–4405.
8. H.-P. Wu, Z.-W. Ou, T.-Y. Pan, C.-M. Lan, W.-K. Huang, H.-W. Lee, N. M. Reddy, C.-T. Chen, W.-S. Chao, C.-Y. Yeh and E. W.-G. Diau, Energy Environ. Sci., 2012, 5, 9843–9848.
9. D. Arteaga, R. Cotta, A. Ortiz, B. Insuasty, N. Martin and L. Echegoyen, Dyes Pigm., 2015, 112, 127–137.
10. T. Hasobe, S. Hattori, P. V. Kamat, Y. Urano, N. Umezawa, T. Nagano and S. Fukuzumi, Chem. Phys., 2005, 319, 243–252.
11. A. Yella, C.-L. Mai, S. M. Zakeeruddin, S.-N. Chang, C.-H. Hsieh, C.-Y. Yeh and M. Grätzel, Angew. Chem., 2014, 126, 3017–3021.
12. S. Mathew, A. Yella, P. Gao, R. Humphry-Baker, B. F. Curchod, N. Ashari-Astani, I. Tavernelli, U. Rothlisberger, M. K. Nazeeruddin and M. Grätzel, Nat. Chem., 2014, 6, 242–247.
13. D. Daphnomili, G. D. Sharma, S. Biswas, K. R. Justin Thomas and A. G. Coutsolelos, J. Photochem. Photobiol., A, 2013, 253, 88–96.
14. H. Deng, Z. Lu, Y. Shen, H. Mao and H. Xu, Chem. Phys., 1998, 231, 95–103.
15. R. Mosurkal, J.-A. He, K. Yang, L. A. Samuelson and J. Kumar, J. Photochem. Photobiol., A, 2004, 168, 191–196.
16. S. Osati, S. S. H. Davarani, N. Safari and M. H. Banitaba, Electrochim. Acta, 2011, 56, 9426–9432.
17. S. Osati, N. Safari, M. K. Bojdi and S. S. H. Davarani, J. Electroanal. Chem., 2011, 655, 120–127.
18. S. Shahrokhian and A. Hamzehloei, Electrochem. Commun., 2003, 5, 706–710.
19. S.-C. Li, J.-G. Wang, P. Jacobson, X. Q. Gong, A. Selloni and U. Diebold, J. Am. Chem. Soc., 2009, 131, 980–984.
20. W. M. Campbell, A. K. Burrell, D. L. Officer and K. W. Jolley, Coord. Chem. Rev., 2004, 248, 1363–1379.
21. J. Rochford, D. Chu, A. Hagfeldt and E. Galoppini, J. Am. Chem. Soc., 2007, 129, 4655–4665.
22. S. Cherian and C. C. Wamser, J. Phys. Chem. B, 2000, 104, 3624–3629.
23. A. Kay and M. Graetzel, J. Phys. Chem., 1993, 97, 6272–6277.
24. K. Kalyanasundaram, N. Vlachopoulos, V. Krishnan, A. Monnier and M. Graetzel, J. Phys. Chem., 1987, 91, 2342–2347.
25. H. Imahori, S. Hayashi, H. Hayashi, A. Oguro, S. Eu, T. Umeyama and Y. Matano, J. Phys. Chem. C, 2009, 113, 18406–18413.
26. M. K. Nazeeruddin, R. Humphry-Baker, P. Liska and M. Grätzel, J. Phys. Chem. B, 2003, 107, 8981–8987.
27. I. A. Janković, Z. V. Šaponjić, M. I. Čomor and J. M. Nedeljković, J. Phys. Chem. C, 2009, 113, 12645–12652.
28. P. Z. Araujo, P. J. Morando and M. A. Blesa, Langmuir, 2005, 21, 3470–3474.
29. P. Z. Araujo, C. B. Mendive, L. A. G. Rodenas, P. J. Morando, A. E. Regazzoni, M. A. Blesa and D. Bahnemann, Colloids Surf., A, 2005, 265, 73–80.
30. J. Jasieniak, M. Johnston and E. R. Waclawik, J. Phys. Chem. B, 2004, 108, 12962–12971.
31. L. R. Milgrom, J. Chem. Soc., Perkin Trans. 2, 1983, 2535–2539,  10.1039/p19830002535.
32. S. Ito, T. N. Murakami, P. Comte, P. Liska, C. Grätzel, M. K. Nazeeruddin and M. Grätzel, Thin Solid Films, 2008, 516, 4613–4619.
33. M. Späth, P. M. Sommeling, J. A. M. van Roosmalen, H. J. P. Smit, N. P. G. van der Burg, D. R. Mahieu, N. J. Bakker and J. M. Kroon, Prog. Photovoltaics, 2003, 11, 207–220.
34. A. Stone and E. B. Fleischer, J. Am. Chem. Soc., 1968, 90, 2735–2748.
35. M. Rojkiewicz, P. Kuś, P. Kozub and M. Kempa, Dyes Pigm., 2013, 99, 627–635.
36. J. P. Lewtak, D. Gryko, D. Bao, E. Sebai, O. Vakuliuk, M. Scigaj and D. T. Gryko, Org. Biomol. Chem., 2011, 9, 8178–8181.
37. D. A. James, D. P. Arnold and P. G. Parsons, Photochem. Photobiol., 1994, 59, 441–447.
38. C.-L. Wang, Y.-C. Chang, C.-M. Lan, C.-F. Lo, E. Wei-Guang Diau and C.-Y. Lin, Energy Environ. Sci., 2011, 4, 1788–1795.
39. Y. Ooyama, T. Yamada, T. Fujita, Y. Harima and J. Ohshita, J. Mater. Chem. A, 2014, 2, 8500–8511.
40. P. Péchy, T. Renouard, S. M. Zakeeruddin, R. Humphry-Baker, P. Comte, P. Liska, L. Cevey, E. Costa, V. Shklover, L. Spiccia, G. B. Deacon, C. A. Bignozzi and M. Grätzel, J. Am. Chem. Soc., 2001, 123, 1613–1624.
41. F. Nüesch and M. Grätzel, Chem. Phys., 1995, 193, 1–17.
42. Y. Wang, X. Li, B. Liu, W. Wu, W. Zhu and Y. Xie, RSC Adv., 2013, 3, 14780–14790.
43. A. S. Hart, C. B. Kc, H. B. Gobeze, L. R. Sequeira and F. D'Souza, ACS Appl. Mater. Interfaces, 2013, 5, 5314–5323.
44. M. Ameri and E. Mohajerani, RSC Adv., 2015, 5, 92690–92706.
45. H. Imahori, Y. Matsubara, H. Iijima, T. Umeyama, Y. Matano, S. Ito, M. Niemi, N. V. Tkachenko and H. Lemmetyinen, J. Phys. Chem. C, 2010, 114, 10656–10665.
46. J. H. Delcamp, Y. Shi, J.-H. Yum, T. Sajoto, E. Dell'Orto, S. Barlow, M. K. Nazeeruddin, S. R. Marder and M. Grätzel, Chem.–Eur. J., 2013, 19, 1819–1827.
47. Handbook of Photovoltaic Science and Engineering, 2nd edn, 2011.
48. G. Boschloo and A. Hagfeldt, Acc. Chem. Res., 2009, 42, 1819–1826.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra23584g

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