Journal of Organometallic Chemistry 714 (2012) 88e93
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A new thiocyanate-free cyclometallated ruthenium complex for dye-sensitized solar cells: Beneficial effects of substitution on the cyclometallated ligand Claudia Dragonetti a, b, *, Adriana Valore b, c, Alessia Colombo a, b, Dominique Roberto a, b, c, Vanira Trifiletti d, Norberto Manfredi d, Matteo Marco Salamone d, Riccardo Ruffo d, Alessandro Abbotto d, * a
Dip. di Chimica Inorganica, Metallorganica e Analitica “Lamberto Malatesta” dell’Università degli Studi di Milano, Italy UdR di Milano dell’INSTM, Via Venezian 21, I-20133 Milano, Italy Istituto di Scienze e Tecnologie Molecolari del CNR, Via Venezian 21, I-20133 Milano, Italy d Department of Materials Science and Milano-Bicocca Solar Energy Research Center e MIB-Solar, University of Milano-Bicocca, Via Cozzi 53, I-20125 Milano, Italy b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 February 2012 Received in revised form 7 March 2012 Accepted 8 March 2012
A new thiocyanate-free cyclometallated 2-phenylpyridine Ru(II) complex, [Ru(dFppyCF3)(dcbpy)2]þPF 6 (dFppyCF3 ¼ 2-(2,4-difluorophenyl)-5-trifluoromethyilpyridine; dcbpy ¼ 2,20 -bipyridine-4,40 -dicarboxylic acid), containing an electronwithdrawing trifluoromethyl group on the pyridine ring of the cyclometallated ligand, was synthesized and used as photosensitizer in DSSC devices. Its optical and electrochemical properties and stability behaviour towards ligand exchange with the common solar cell additive 4-tert-butylpyridine was compared to that of benchmark DSSC dye N719 and of the reference complex [Ru(dFppy)(dcbpy)2]þPF 6. Substitution of the pyridine ring by the CF3 group afforded enhanced optical properties and a larger overall power conversion efficiency of the corresponding DSSC (3.7%), with a significant improvement compared to the reference cyclometallated complex under the same fabrication conditions. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Ruthenium(II) complexes Cyclometallated phenylpyridine Dye-sensitized solar cells
1. Introduction One of the present technological main challenges is to address the problem of the increasing global energy demand in a clean and renewable manner by exploiting energy from Sun, thus avoiding issues related to fossil fuels such as pollution and finite available amounts [1]. For this reason, there is a great interest of the scientific community towards the development of efficient and cheap thinfilm solar cells. Dye-sensitized solar cells (DSSCs) are considered a realistic solution for harnessing the energy of the Sun and converting it into electrical energy, with power conversion efficiencies now exceeding the value of 11% [2]. One of the most important components of DSSC is the photosensitizer, that is the dye which absorbs light from Sun and transfers an electron from its excited state to the conduction band of TiO2. The most common sensitizers are Ru(II) complexes based on 2,20 -bipyridine (bpy) ligands [3], such as cis-di(thiocyanato) bis(bpy-4,40 -dicarboxylate)ruthenium(II) (N3 [4] and N719) [5].
* Corresponding authors. Dipartimento di Chimica Inorganica, Metallorganica e Analitica “Lamberto Malatesta”, Università degli Studi di Milano, Via Venezian 21, I20133 Milano, Italy. Fax: þ39 02 50314405. E-mail addresses:
[email protected] (C. Dragonetti),
[email protected] (A. Abbotto). 0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2012.03.011
However, a limitation of such Ru(II) complexes is the presence of thiocyanate, NCS, ancillary ligands. Thiocyanate is an ambidentate ligand, which can coordinate either at the sulfur or at the nitrogen atom [2c,6], and a monodentate ligand, which can be easily replaced by other competing ligands yielding less efficient species. In particular, often, with the goal to optimize the efficiency of the DSSCs, 4-tert-butylpyridine is used as an additive in the electrolyte solution in order to up-shift the conduction band of the semiconductor oxide and thus increase the cell photovoltage [4,7]. It has been demonstrated [8] that N719 or other NCS-based Ru dyes (for example Z-907 [9]) may free a thiocyanate ligand at elevated temperatures (80e100 C) by ligand exchange with 4-tert-butylpyridine. The efficiency of N719-4-tert-butylpyridine cells compared to N719 in the otherwise similar DSSC is approximately 50% lower, due to ca. 30 nm blue shift in its optical absorption spectrum [10]. In order to circumvent the issues related to the use of thiocyanatebased complexes, an alternative organometallic compound with a cyclometallated 2-phenylpyridine ligand,[Ru(dFppy)(dcbpy)2]þPF 6 (dppy ¼(2,4-difluorophenyl))pyridine; dcbpy ¼ 2,20 -bipyridine-4,40 dicarboxylic acid), has been recently reported, with power conversion efficiencies comparable to that of N719 under the same cell fabrication conditions [11]. Cyclometallating ligands are routinely incorporated in neutral Ir(III) complexes for organic light-emitting diodes (OLEDs) [12], in cationic complexes for organic light-emitting electrochemical cells (OLECs) [13], and in nonlinear optical materials [14].
C. Dragonetti et al. / Journal of Organometallic Chemistry 714 (2012) 88e93
It is known that in cyclometallated phenylpyridine based Ir(III) and Pt(II) systems, the presence of substituents on the phenyl or pyridine ring allows to modulate the HOMO and LUMO energy levels and consequently the absorption energy of the complex [15,16]. For example, in [Pt(C^N-ppy-4-styryl-R)(acetylacetonate)] systems the introduction of an electronwithdrawing R group leads to a red-shift in the absorption spectrum [16]. Therefore, of particular interest is the design and development of functionalized cyclometallated Ru(II) complexes for light-harvesting applications [17]. In particular, recently, Berlinguette has described an interesting series of Ru(II) complexes related to [Ru(ppy)(dcbpy)2]þPF 6 (ppy ¼ cyclometallated 2-phenylpyridine) with various substituents on the phenyl ring [17a,b]. However, to our knowledge, the introduction of substituents on the pyridine ring of phenylpyridines has not been investigated. Here, we present the synthesis, UVevis spectroscopy, electrochemistry and photovoltaic performances in a DSSC cell of a new thiocyanate-free cyclometallated 2-phenylpyridine Ru(II) complex substituted both on the phenyl and the pyridine rings, [Ru(dFppyCF3)(dcbpy)2]þPF 6 (dFppyCF3 ¼ 2-(2,4-difluorophenyl)5-trifluoromethyilpyridine). Its power conversion efficiency and its stability towards ligand exchange with the solar cell additive 4-tertbutylpyridine was compared to that of N719 and of the related complex [Ru(dFppy)(dcbpy)2]þPF 6 in order to understand the effect of an electronwithdrawing group such as CF3 on the pyridine ring of the cyclometallated phenylpyridine.
89
Sephadex LH-20 was purchased from GE Helthcare. 1H NMR spectra were obtained on a Bruker Avance DRX 400 MHz instrument. Microwave synthesis were carried out by CEM Discover S-Class system. The Ruthenium dimer, [(p-C6H6)RuCl2]2, was prepared with the synthetic procedure reported in the literature [18], and the fluorinated ligand, dFppyCF3 ¼ 2-(2,4-difluorophenyl)-5-trifluoromethylpyridine, was prepared by a Suzuki coupling [19]. 2.2. Synthesis of [Ru(CH3CN)4(dFppyCF3)]þPF 6 (2 step of Scheme 1)
A suspension of [(pC6H6)RuCl2]2 (0.75 mmol, 424.8 mg) [18], 2-(2,4-difluorophenyl)-5-trifluoromethyilpyridine (1.5 mmol, 390 mg), NaOH (1.5 mmol, 60.2 mg) and KPF6 (3.01 mmol, 554 mg) in CH3CN (1.9 mL) was stirred at 45 C for 15 h under nitrogen. The obtained yellow-brown suspension was filtered on Al2O3, using a mixture of CH2Cl2/CH3CN (3%) as eluent. The yellow fraction was dried under vacuum affording a solid residue which was dissolved in the minimum quantity of CH2Cl2 and precipitated by addition of diethylether and hexane. 1 H NMR (400 MHz; CD3CN) d ppm: 9.18 (s, 1H) 8.30 (dd, J1 ¼ 1.47 Hz, J2 ¼ 8.86 Hz, 1H) 8.06 (dd, J1 ¼ 1.02 Hz, J2 ¼ 9.23 Hz, 1H) 7.60 (dd, J1 ¼ 2.34 Hz, J2 ¼ 8.68 Hz, 1H) 6,57 (ddd, J1 ¼ 2.37 Hz, J2 ¼ 9.55 Hz, J3 ¼ 12.93 Hz, 1H) 2.50 (s, 3H, CH3CN) 2.15 (s, 6H, CH3CN) 2.05 (s, 3H, CH3CN). FABþ ¼ 523. Anal. Calcd for RuC20H17N5PF11: C, 35.94; H, 2.56; N,10.48. Found: C, 35.60; H,2.44; N, 10.40. 2.3. Synthesis of [Ru(dFppyCF3)(dcbpy)2]þPF 6 (3 step of Scheme 1)
2. Experimental section 2.1. General information All reagents and solvents were purchased from SigmaeAldrich and Fluka. All reactions were carried out under nitrogen atmosphere.
In a vial containing [Ru(CH3CN)4(dFppyCF3)]þPF 6 (0.030 mmol, 20.0 mg) in methanol (10 ml), were added 2,20 -bipyridine-4,40 dicarboxylic acid (dcbpy; (0.06 mmol, 14.6 mg)) and NaOH (0.12 mmol, 4.8 mg). The solution was put in a microwave reactor,
Step 1 EtOH aq 90%
RuCl33H2O+
Cl
Ru
Cl Cl
Ru
Cl
Cl Cl
Ru
N
Cl
+ X
+
Y
Y
Step 2
Ru
Cl
45°C, 3h
KPF6, NaOH CH3CN, 45°C, 15h N2
PF6-
CH3CN N
X
CH3CN
Ru
CH3CN
CH3CN X
X
+ Step 3 +
Y CH3CN N X
Ru
CH3CN
HOOC PF61)
CH3CN
CH3CN X
PF6-
COOH
2)
COOH
N N NaOH, MeOH 100°C, 30min, MW
Y
N N
X
N
Ru N
HNO3 0.2 M
COOH
N
X COOH For [Ru(dFppyCF3)(dcbpy)2]+PF6- X= F, Y= CF3; for [Ru(dFppy)(dcbpy)2]+PF6- X= F, Y= H þ Scheme 1. Synthesis of [Ru(dFppyCF3)(dcbpy)2]þPF 6 and of[Ru(dFppy)(dcbpy)2] PF6 .
COOH
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C. Dragonetti et al. / Journal of Organometallic Chemistry 714 (2012) 88e93
modality dynamic, for 25 min at 100 C (Maximum pressure: 2 bar; Maximum power: 280 W). The obtained dark red product was dissolved in distilled water and acidified with HNO3 0.2 M, until a precipitate was formed. The dark red precipitate was isolated by filtration under vacuum using fritted glass G4, washed first with some drops of water and then with diethylether. (Yield: 65%);1H NMR (400 MHz; CD3OH) d ppm: 9.13 (s, 1H) 9.06 (s, 1H) 9.00 (s, 2H) 8.51 (d, J ¼ 8.12, 1H) 8.17 (d, J ¼ 6.08 Hz, 1H) 8.08 (d, J ¼ 9.21 Hz, 1H) 8.04 (d, J ¼ 5.45 Hz, 1H) 8.00 (dd, J1 ¼ 1.05 Hz, J2 ¼ 5.45 Hz, 1H) 7.95 (d, J ¼ 5.73 Hz, 1H) 7.83 (m, 5H) 6.51 (m, 1H) 5.96 (dd, J1 ¼ 2.46 Hz, J2 ¼ 8.12 Hz, 1H). FABþ ¼ 848. Anal. Calcd for RuC36H21N5O8PF11: C, 43.56; H, 2.13; N,7.06. Found: C, 43.61; H,2.14; N, 7.10. 2.4. Synthesis of [Ru(CH3CN)4(dFppy)]þPF 6 (2 step of Scheme 1)
A suspension of [(pC6H6)RuCl2]2 (0.360 mmol, 205.6 mg) [18], 2-(2,4-difluorophenyl)pyridine (0.73 mmol, 139.3 mg), NaOH (0.73 mmol, 29.2 mg), KPF6 (1.46 mmol, 268.2 mg) in CH3CN (5 mL) was stirred at 45 C for 15 h under nitrogen. The obtained yellowbrown suspension was filtered on Al2O3, using a mixture of CH2Cl2/ CH3CN (3%) as eluent. The yellow fraction was dried under vacuum affording a solid residue which was dissolved in the minimum quantity of CH2Cl2 and precipitated by addition of diethylether and hexane. 1 H NMR (400 MHz; CD3CN) d ppm: 8.99 (dd, J1 ¼ 0.82 Hz, J2 ¼ 5.66, 1H) 8,17 (d, J ¼ 8.02 Hz, 1H) 7.81 (m, 1H) 7.54 (dd, J1 ¼ 2.37 Hz, J2 ¼ 8.74 Hz, 1H) 7.22 (m, 1H) 6.51 (m, 1H) 2.51 (s, 3H, CH3CN) 2.15 (s, 6H, CH3CN) 2.05 (s, 3H, CH3CN). FABþ ¼ 455. Anal. Calcd for RuC19H18N5PF8: C, 38.01; H, 3.02; N,11.66. Found: C, 38.11; H,3.00; N, 11.71. 2.5. Synthesis of [Ru(dFppy)(dcbpy)2]þPF 6 (3 step of Scheme 1)
In a vial containing [Ru(CH3CN)4(dFppy)]þPF 6 (0.022 mmol, 13.4 mg) in methanol (10 ml), were added 2,20 -bipyridine-4,40 dicarboxylic acid (dcbpy; 0.022 mmol, 5.5 mg) and NaOH (0.044 mmol, 1.8 mg). The solution was put in a microwave reactor, modality dynamic, for 25 min at 100 C (Maximum pressure: 2 bar; Maximum power: 280 W). The obtained dark red product was dissolved in distilled water and acidified with HNO3 0.2 M, until a precipitate was formed. The dark red precipitate was isolated by filtration under vacuum using fritted glass G4, washed first with some drops of water and then with diethylether. (Yield: 62%);1H NMR (400 MHz; CD3OD) d ppm: 9.10 (s, 1H) 9.03 (s, 1H) 8.99 (s, 2H) 8.35 (d, J ¼ 8.15, 1H) 8.23 (d, J ¼ 5.78 Hz, 1H) 7.95 (m, 4H) 7.81 (m, 4H) 7.61 (d, J ¼ 5.48 Hz, 1H) 7.04 (t, J1,2 ¼ 6.56 Hz, 1H) 6.45 (m, 1H) 5.87 (dd, J1 ¼ 2.08 Hz, J2 ¼ 8.04 Hz, 1H). FABþ ¼ 780. Anal. Calcd for RuC35H22N5O8PF8: C, 45.47; H, 2.40; N,7.57. Found: C, 45.40; H, 2.42; N, 7.58. 2.6. Preparation and characterization of DSSCs DSSCs have been prepared adapting a procedure reported in the literature [20]. In order to exclude metal contamination, all of containers were in glass or teflon and were treated with EtOH and 10% HCl prior to use. Plastic spatulas and tweezers have been used throughout the procedure. FTO glass plates (Solaronix TCO 22-7, 2.2 mm thickness, 7 U/square) were cleaned in a detergent solution for 30 min using an ultrasonic bath, rinsed with pure water and absolute EtOH. FTO plates were treated with a freshly prepared 40 mM aqueous solution of TiCl4 for 30 min at 70 C and then rinsed with water and EtOH. A first transparent layer was screen-printed using 20-nm transparent TiO2 paste (Solaronix Ti-
Nanoxide T20/SP). The coated films were dried at 125 C for 6 min and then another layer was screen-printed by using a paste of 100e400 nm light scattering TiO2 particles (Solaronix TiNanoxide R/SP). The coated plates were kept in a cabinet for 5 min and then thermally treated under an air flow at 125 C for 6 min, 325 C for 10 min, 450 C for 15 min, and 500 C for 15 min. The heating ramp rate was 5e10 C/min. The sintered layer was treated again with 40 mM aqueous TiCl4 (70 C for 30 min), rinsed with EtOH and heated at 500 C for 30 min. After cooling down to 80 C the TiO2 coated plate was immersed into a 0.1 mM solution of the dye in EtOH for 20 h at room temperature in the dark. The thickness of the layers was measured by means of a VEECO Dektak 8 Stylus Profiler. Counter electrodes were prepared according to the following procedure. A 1-mm hole was made in an FTO plate using diamond drill bits. The electrodes were then cleaned with a detergent solution for 5 min using an ultrasonic bath, 10% HCl, and finally acetone for 10 min using an ultrasonic bath. After heating at 400 C for 15 min a drop of a 5 x 103 M solution of H2PtCl6 in EtOH was added and the thermal treatment at 400 C for 15 min repeated. The dye adsorbed TiO2 electrode and Pt-counter electrode were assembled into a sealed sandwich-type cell by heating with a hotmelt ionomer-class resin (Surlyn 25 mm thickness) as a spacer between the electrodes. A drop of the electrolyte solution was added to the hole and introduced inside the cell by vacuum backfilling. Finally, the hole was sealed with a sheet of Surlyn. An aluminium foil at the back side of the counter electrode was taped to reflect unabsorbed light back to the photoanode. Photovoltaic measurements of DSSCs were carried out using a 500 W Xenon light source (ABET Technologies Sun 2000 Solar Simulator). The power of the simulated light was calibrated to AM 1.5G (100 mW cm2) using a reference Si cell photodiode equipped with an IR-cutoff filter (KG-5, Schott) to reduce the mismatch in the region of 350e750 nm between the simulated light and AM 1.5. Values were recorded after 3 and 24 h, 3 and 7 days of ageing in the dark. IeV curves were obtained by applying an external bias to the cell and measuring the generated photocurrent with a Keithley model 2400 digital source meter. Incident photon-to-current conversion efficiencies (IPCE) were recorded as a function of excitation wavelength by using the incident light 400 W Xenon lamp, which was focused through a monochromator (HORIBA Jobin Yvon). 2.7. Electrochemical measurements The dyes were dissolved (concentration about 104 M) in a 0.1 M solution of tetrabutylammonium hexafluorophosphate (Fluka, electrochemical grade, >99.0%) in DMF (Aldrich, 99.8%). Differential pulsed voltammetry (DPV) and cyclic voltammetry (CV) were carried out at scan rate of 20 and 50 mV/s, respectively, with a PARSTA2273 potentiostat in a single chamber three-electrode electrochemical cell in a glove box filled with argon ([O2]
