A tetrahydropyrene-based organic dye for solar cell application

Abstract

A novel tetrahydropyrene-based D–π–A organic dye D2 was designed and synthesized for the first time, featuring 4,5,9,10-tetrahydropyrene as a π conjugation linker to bridge the diphenylamine unit and the thienyl acrylic acid moiety. Its counterpart D1 was prepared for comparison, where biphenyl was used as the linker. Both dyes were characterized by photophysical, electrochemical, and theoretical computational methods. It was found that the introduction of two ethylene groups on the C2, C2′ and C6, C6′ of the biphenyl in D2 can prevent the rotation of the adjacent phenyl rings, and ensure the coplanarity of the bridge. As a result, the maximum absorption peak (λ_max) of D2 was 29 nm red shifted as compared with D1. Nanocrystalline TiO₂-based dye-sensitized solar cells were fabricated using the dyes as light harvesting sensitizers, and exhibited power conversion efficiencies of 6.75% for D2 and 4.73% for D1 under AM 1.5 conditions.

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Introduction

Dye-sensitized solar cells (DSCs) have attracted significant attention as low cost alternatives to conventional solid-state photovoltaic devices.¹ In these devices, metal-free organic dyes have been investigated as sensitizers to replace noble ruthenium based complexes due to their low cost and excellent flexibility of molecular tailoring.² A variety of organic dyes have been developed, and some of them have shown power conversion efficiencies comparable to those of ruthenium dyes.³ Generally, most efficient organic dyes possess an electron donor π-conjugation, exhibiting facile intramolecular charge transfer and efficient light harvesting ability. Increasing the conjugation length of the linker in the dye molecules would help to extend their absorption spectra towards the long wavelength region and thus increase photocurrent density and power conversion efficiencies in the DSCs. Although increasing lengths of methine units between the donor and acceptor are found as efficient approaches to expand optical absorption ranges in polyene based dyes,⁴ aromatic hydrocarbons can be benefit as linkers for stable DSCs. In the previous reports, a series of aromatic hydrocarbons, such as phenyl,^5a fluorene,^5b 9,10-dihydrophenanthrene,^5c and some fused aromatic compounds,⁶ are employed as the bridges in the dyes for DSCs.

In this report, 4,5,9,10-tetrahydropyrene was firstly employed as the linker to bridge diphenylamine unit and thienyl acrylic acid unit to construct a D–π–A organic dye used in DSC. We have also prepared its biphenyl analogue to study the effect of rigid planar structure of tetrahydropyrene on the performance of DSCs. Our results show that 4,5,9,10-tetrahydropyrene is a better π-conjugator when compared to the biphenyl group due to the presence of two ethylene groups which prevents the rotation of the adjacent phenyl rings, ensures the coplanarity of the bridge, extends the spectral absorption range, and improves the device performance.

Results and discussion

The chemical structures of D1 and D2 are shown in Scheme 1. D1 was prepared according to the report.⁷ The synthetic rout of D2 was shown in Scheme 2. The starting compound 4,5,9,10-tetrahydropyrene was prepared according to the previous report.⁸ Firstly, dibromotetrahydropyrene was prepared in the solution of tetrahydropyrene with bromine and NaOH in the mixture of acetic acid and water following the method by Christoph. The dibromo compound was monoaminated by Hartwig's palladium-catalyzed C–N cross-coupling with diphenylamine. The resulting amine was subjected to Suzuki–Miyaura cross-coupling reaction with 5-formylthiophene-boronic acid, 5-bromo-2-thiophenecarboxyaldehyde. The aldehyde obtained was converted to the dyes via Knoevenagel condensation with cyanoacetic acid in the acetic acid in the presence catalytic amount of ammonium acetate. The synthetic details were described in Experimental section.

Scheme 1 Chemical structures of D1 and D2.

Scheme 2 Synthetic route of D2. (i) Br₂/NaOH; (ii) Pd(OAc)₂/dppf/NaOBu^t/toluene; (iii) Pd(PPh₃)₄/K₂CO₃/toluene/H₂O; (iv) cyanoacetic acid/piperidine/THF/reflux.

The electronic absorption spectra of the two dyes were measured in dichloromethane solution, as shown in Fig. 1. Their maximum absorption peaks (λ_maxs) were located at 439 nm (ε = 3.75 × 10⁴ M⁻¹ cm⁻¹) and 468 nm (3.1 × 10⁴ M⁻¹ cm⁻¹) for D1 and D2, respectively. The 29 nm redshift for D2 was observed when compared with D1, which can be explained by the higher degree of the planarity in tetrahydropyrene, resulting in more efficient intramolecular charge transfer (ICT) from the donor to the acceptor in the molecule upon optical excitation. The detailed photophysical parameters are listed in Table 1.

Fig. 1 Absorption spectra of D1 and D2 measured in dichloromethane solution.

Table 1 Photoelectrochemical properties of D1 and D2, and their solar cell performance

Dye	λmax^a [nm]	ε [L mol^−1 cm^−1]	E0–0^b [eV]	Es+/0^c [V]	Es+/*^d [V]	Jsc [mA cm^−2]	Voc [mV]	FF	η [%]
a Absorption in CH2Cl2 solutions (1 × 10^−5 M) at rt.b E0–0 values were estimated from the X-intercepts from the tangent of their absorption edges.c The oxidation potentials of the dyes were measured in CH2Cl2 solutions with tetrabutylammoniumhexafluorophosphate (TBAPF6 0.1 M) as electrolyte, Pt wires as working and counter electrode, Ag/Ag^+ as reference electrode; calibrated with ferrocene/ferrocenium (Fc/Fc^+) as an internal reference and converted to NHE by addition of 630 mV.d The estimation was determined by subtracting E0–0 from Es+/0.
D1	439	37500	2.38	0.95	−1.43	9.30	717	0.71	4.73
D2	468	31000	2.17	0.91	−1.26	13.21	730	0.70	6.75

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Cyclic voltammetry measurements were performed to investigate the molecular energy levels of the three dyes. The first oxidation potentials (E_s+/0 vs. NHE) were observed to be 0.95 V for D1 and 0.91 V for D2. The relatively lower value for D2 means the easier oxidation due to the presence of two ethylene groups with donor ability. In both cases, the potentials are more positive compared to the redox potential of the iodide/tri-iodide couple (0.4 V vs. NHE),⁹ indicating that the ground state dye regeneration is energetically favourable for DSC application. The optical transition energies (E_0–0) were 2.38 eV for D1 and 2.17 eV for D2, estimated from the X-intercepts from the tangent of their absorption edges. Their excited-state redox potentials, E_s+/*, determined by subtracting E_0–0 from E_s+/0, were −1.43 eV for D1, and −1.26 eV for D2. All the values are negative enough to allow their excited state electron transfer into the TiO₂ conduction band (−0.5 V vs. NHE).¹⁰

In order to gain insight into the geometrical configuration and electron distribution of the frontier orbitals of the two dyes, density functional theory (DFT) calculations were made on a B3LYP/6-31G level. Fig. 2 shows the optimized ground state molecular structures and dihedral angles for D1 and D2. The dihedral angles of the adjacent biphenyl are 32.4°in D1 and 16.9°in D2, which indicates more planar configuration of the D2 dye than D1, allowing better electronic communication between the arylamine and thienyl acrylic acid. The calculated HOMO and LUMO energies were −5.21 and −2.84 eV for D1 and −5.04 eV and −2.78 eV for D2, corresponding the energy gaps of 2.37 eV and 2.26, respectively. The frontier molecular orbitals of the two dyes are shown in Fig. 3. At the HOMO state, the electron density is mainly distributed on the donor and part of the biphenyl or the tetrahydropyrene for both the dyes, while at the LUMO level, electron density is located on the thienyl acrylic acid and part of the biphenyl or the tetrahydropyrene. As compared to D1, the electron density on the linker is higher in D2 at the HOMO state, implying easier electron delocalization in the latter due to its coplanarity property. Generally, electronic distributions of HOMO and LUMO relates to the charge transfer from electron donor to acceptor in a molecule. Thus, the heterogeneous electron distribution in the HOMO of D2 will favour photo-excited charge transfer and create high device performance.

Fig. 2 The geometry optimized ground state molecular structures of D1 and D2 with corresponding dihedral angles between each plain.

Fig. 3 Frontier molecular orbitals of the HOMO and LUMO for D1 and D2, calculated with DFT on a B3LYP/6-31+G(d) Level (isodensity = 0.03 au).

For preparation of the DSCs, a double TiO₂ film containing an 11 μm transparent layer and a 6 μm scattering layer was employed. The final device was prepared in a sandwich-type, where the dyed TiO₂ electrode and Pt-counter electrode was sealed with a hot-melt film, and filled with the electrolyte containing an I⁻/I₃⁻ redox couple-based electrolyte. Detailed procedures for the fabrication of the DSCs were described in Experimental. Fig. 4 shows action spectra of the DSCs in the form of monochromatic incident photon-to-current conversion efficiencies (IPCEs). Both the DSCs exhibited the peak values around 80% with different trends in the range of 350–650 nm. The IPCE spectrum of the D2-based device is red shifted about 50 nm compared with that of the D1-based device, which is in accordance with the corresponding absorption spectra. The photocurrent density–voltage (I–V) curves of both the devices were recorded under AM 1.5 irradiation (100 mW cm⁻²), and shown in Fig. 5. D2 based-device gave a short circuit photocurrent density (J_sc) of 13.21 mA cm⁻², an open circuit voltage (V_oc) of 730 mV, and a fill factor (FF) of 0.70, corresponding to an overall conversion efficiency (η), derived from the equation η = J_scV_ocFF/light intensity, of 6.75%, while D1 based device gave J_sc of 9.30 mA cm⁻², V_oc of 717 mV, FF of 0.71, and η of 4.73% under the same conditions. The large difference in the efficiencies between D1 and D2 comes mainly from the difference in the J_scs between them. The higher J_sc for D2 is consistent with its broader IPCE spectrum as shown in Fig. 4.

Fig. 4 IPCE of the DSCs sensitized by D1 and D2.

Fig. 5 I–V curves of the DSCs sensitized by D1 and D2.

Conclusions

In conclusion, tetrahydropyrene was firstly employed as π conjugator to bridge diphenylamine unit and thienyl acrylic acid moiety to construct a novel D–π–A organic dye D2. Its analogue biphenyl dye D1 was prepared for comparison. DFT calculations showed that the introduction of two ethylene groups on the C2, C2′ and C6, C6′of the biphenyl in D2 can prevent the rotation of the adjacent phenyl rings, and ensures the coplanarity of the bridge. As a result, the maximum absorption peak (λ_max) of D2 was 29 nm redshift as compared with D1. Dye-sensitized solar cells were fabricated using these dyes as light harvesting sensitizers, and exhibited the power conversion efficiencies of 6.75% for D2 and 4.73% for D1 under AM 1.5 conditions. The results indicates that tetrahydropyrene is a useful π conjugator to construct D–π–A organic dyes.

Experimental

Materials

All reagents and solvents were obtained from commercial sources and used without further purification unless otherwise noted. THF and toluene were dried over sodium/benzophenone and freshly distilled before use. Reactions were performed under nitrogen atmosphere. All reagents and solvents were obtained from commercial sources and used without further purification unless otherwise noted. THF and toluene were dried over sodium/benzophenone and freshly distilled before use. Reactions were performed under nitrogen atmosphere.

Synthesis of 7-bromo-N,N-diphenyl-4,5,9,10-tetrahydropyren-2-amine (2)

An oven-dried 100 mL vacuum reaction tube was charged with dibromotetrahydropyrene (0.722 g, 1 mmol), diphenylamine (0.17 g, 0.5 mmol), sodium hydride (0.2 g of 60% NaH in white oil, 5 mmol), Ni(PPh₃)₂(1-naphthyl)Cl (0.038 g, 5% mol relative to diarylamine, 0.05 mol), and PPh₃ (0.026 g, 10% mmol relative to diarylamine, 0.1 mmol). The tube was evacuated and backfilled with nitrogen with the operation being repeated three times. Dried toluene (15 mL) was added via syringe. The reaction mixture was heated in an oil bath at 120 °C for 12 h, then was allowed to cool to room temperature, filtered through a pad of silica gel. The filtrate was evaporated under reduced pressure and the residue was purified by column chromatography on silica gel with petroleum ether as eluent to obtain 2 (0.5 g, 55%). ¹H-NMR (400 MHz, CDCl₃): δ 7.34 (s, 1H), 7.28 (s, 1H), 7.24 (s, 2H), 7.19 (s, 2H), 7.12 (d, J = 3.6 Hz, 4H), 7.02 (t, 2H), 6.77 (s, 2H), 2.81 (t, 4H), 2.73 (t, 4H).

Synthesis of 5-(7-(diphenylamino)-4,5,9,10-tetrahydropyren-2-yl)thiophene-2- carbaldehyde (3)

Compound 2 (0.37 g, 0.82 mmol), Pd(PPh₃)₄ (0.015 g, 0.013 mmol), and Na₂CO₃ (1 g, 0.01 mol) in 10 mL THF and 5 mL H₂O were heated to 45 °C under a nitrogen atmosphere for 30 min. A solution of 5-formylthiophen-2-yl-boronic acid (0.2 g, 1.34 mmol) in THF (5 mL) was added slowly, and the mixture was refluxed at 70 °C for a further 12 h. After cooling to room temperature, the mixture was extracted with CH₂Cl₂ (3 × 20 mL). The organic phase was dried over anhydrous magnesium sulfate. The solvent was removed with a rotary evaporator and the residue was purified on a silica gel column (petroleum ether–CH₂Cl₂ = 10 : 1, v/v) to give an orange solid 3 (0.19 g, 49%). ¹H-NMR (400 MHz, CDCl₃) δ: 9.87 (s, 1H), 7.73 (d, J = 4.0 Hz, 1H), 7.40 (d, J = 4.0 Hz, 1H), 7.37 (s, 2H), 7.27 (t, 4H), 7.13 (d, J = 8.8 Hz, 4H), 7.04 (t, 2H), 6.79 (s, 2H), 2.90 (t, 4H), 2.78 (t, 4H).

Synthesis of 2-cyano-3-(5-(7-(diphenylamino)-4,5,9,10-tetrahydropyren-2-yl) thiophen-2-yl) acrylic acid (D2)

Compound 3 (0.10 g, 0.21 mmol), 2-cyanoacetic acid (0.068 g, 0.8 mmol) and piperidine (0.3 mL) in 10 mL of THF were heated to refluxed under a nitrogen atmosphere for 8 h. After cooling to room temperature, 20 mL water was added. The solution was acidified with 10% aqueous HCl and extracted with CH₂Cl₂. The organic phase was dried over anhydrous magnesium sulfate. The solvent was removed with a rotary evaporator and the residue was purified on a silica gel column with dichloromethane as eluent to obtain D2 (0.093 g, 80%). ¹H-NMR (400 MHz, DMSO-d₆) δ: 7.72 (d, J = 4.0 Hz, 1H), 7.53 (s, 1H), 7.40 (d, J = 4.0 Hz, 1H), 7.39 (s, 2H), 7.28 (t, 4H), 7.14 (d, J = 8.8 Hz, 4H), 7.06 (t, 2H), 6.80 (s, 2H)2.89 (t, 4H), 2.76 (t, 4H), MS (MALDI-TOF): m/z calcd for (C₃₆H₂₆N₂O₂S): 550.2, found: 550.1.

DSC fabrication

The nanocrystalline TiO₂ pastes (particle size, 20 nm) were prepared using a previously reported procedure.¹¹ Fluorine doped thin oxide (FTO, 4 mm thickness, 10 ohms sq⁻¹, Nippon Sheet Glass, Japan) conducting electrodes were washed with soap and water, followed by sonication for 10 min in acetone and isopropanol, respectively. Following a drying period, the electrodes were then submerged in a 50 mM aqueous solution of TiCl₄ for 30 min at 75 °C, and then washed by water and ethanol. On the electrodes, a 12 μm thick nanocrystalline TiO₂ layer and 5 μm thick TiO₂ light scattering layer (particle size, 400 nm, PST-400C) were prepared by screen-printing method. The TiO₂ electrodes were heated at 500 °C for 30 min, followed by treating with a 50 mM aqueous solution of TiCl₄ for 30 min at 75 °C and subsequent sintering at 500 °C for 30 min. The thickness of TiO₂ films was measured by a profiler, Sloan, Dektak3. The electrodes were immersed in a dye bath containing 0.2 mM D1 or D2 and 2 mM 3α,7α-dihydroxy-5β-cholic acid (chenodeoxycholic acid) in 4-tert-butanol/acetonitrile mixture/tetrahydrofuran (1 : 1 : 0.2, v/v) and kept for 24 h at room temperature. The dyed electrodes were then rinsed with the mixed solvent to remove excess dye. A platinum-coated counter electrode was prepared according to the reporter,¹² and two holes were drilled on its opposite sides. The two electrodes were sealed together with a 25 μm thick thermoplastic Surlyn frame. An electrolyte solution was then introduced through one of the two holes in the counter electrode, and the holes were sealed with the thermoplastic Surlyn. The electrolyte contains 0.68 M dimethyl imidiazolium iodide, 0.05 M iodine, 0.10 M LiI, 0.05 M guanidinium thiocyanate, and 0.40 M tert-butylpyridine in the mixture of acetonitrile and valeronitrile (85 : 15, v/v). All the devices were prepared with a photoactive area of about 0.3 cm², and a metal mask of 0.165 cm² was covered on the device for photovoltaic property measurements.

Characterization of DSCs

The photocurrent–voltage (I–V) characteristics were recorded at room temperature using a computer-controlled Keithley 2400 source meter under air mass (AM) 1.5 simulated illumination (100 mW cm⁻², Oriel, 67005). The action spectra of monochromatic incident photo-to-current conversion efficiency (IPCE) for solar cells were performed using a commercial setup (PV-25 DYE, JASCO). A 300 W Xenon lamp was employed as light source for generation of a monochromatic beam. Calibrations were performed with a standard silicon photodiode.

Acknowledgements

This work is supported by the National Nature Science Foundation (Grant nos 21102150, 21174149, 51173190, 21073203, and 21121001), the National 863 Program (no. 2011AA050521), and the 973 Program (2009CB930404, 2011CB932303, and 2011CB808400).

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