J. Phys. Chem. C 2007, 111, 1853-1860
1853
Influence of π-Conjugation Units in Organic Dyes for Dye-Sensitized Solar Cells Peng Qin,† Xichuan Yang,*,† Ruikui Chen,† and Licheng Sun*,†,‡ State Key Laboratory of Fine Chemicals, DUT- KTH Joint Education and Research Center on Molecular DeVices, Dalian UniVersity of Technology (DUT), 116012 Dalian, China, and Center of Molecular DeVices, Department of Chemistry, Organic Chemistry, Royal Institute of Technology (KTH), Teknikringen 30, 10044 Stockholm, Sweden
Tannia Marinado, Tomas Edvinsson, Gerrit Boschloo, and Anders Hagfeldt* Center of Molecular DeVices, Department of Chemistry, Physical Chemistry, Royal Institute of Technology, Teknikringen 30, 10044 Stockholm, Sweden ReceiVed: August 27, 2006; In Final Form: October 26, 2006
Two organic dyes with the general structure donor-conjugated chain-acceptor (D-π-A) have been investigated as sensitizers for nanocrystalline TiO2 solar cells. The electron donor and acceptor groups were pyrrolidine and cyano acrylic acid, respectively. The conjugated chain of 2-cyano-3-{5-[2-(4-pyrrolidin-1ylphenyl)vinyl]thiophen-2-yl}acrylic acid contains one phenyl ring and a thiophene unit and is therefore denoted PT, while for 2-cyano-3-{5-[2-(5-pyrrolidin-1-ylthiophen-2-yl)vinyl]thiophen-2-yl}acrylic acid the phenyl ring is replaced by a second thiophene unit (TT). Solar-to-electrical energy conversion efficiencies under simulated AM 1.5 irradiation (1000 W m-2) of 2.3% were obtained for solar cells based on PT but of less than 0.05% for those based on TT. The reasons for the dramatic difference of the efficiencies were analyzed. Photoinduced absorption measurements revealed that the TT dye was not properly regenerated by redox electrolyte after electron injection. This sluggish regeneration is probably due to the 0.3 V less positive HOMO level for TT dye compared to the PT dye, resulting in a lower driving force for regeneration of the oxidized dye by iodide in the electrolyte. In addition, regeneration of the oxidized TT dye and electron injection from the excited TT dye may be poor due to formation of dye aggregates/complexes, as FT-IR measurements show an excess of not properly and/or unidentate bound TT dye molecules instead of bidentate bound PT dye molecules. The results highlight that small structural change of dyes results in significant changes in redox energies and binding features, affecting dramatically the performance of these dyes in dye-sensitized solar cells.
Introduction Dye-sensitized solar cells, DSSC, have attracted a great deal of interest due to the potential of low cost production and the possibilities to design new functional material components to increase the solar-to-electrical energy conversion efficiency.1,2 Dye-sensitized solar cells based on ruthenium complexes have broad absorption spectra extending into the near-IR region and produce solar-to-electrical energy conversion efficiencies of up to 11% under AM 1.5 irradiation.3-11 A broad absorption is necessary to attain good overlap with the solar spectrum to produce a large photocurrent response. In addition, suitable energy levels and location of the HOMO and LUMO orbitals of the photosensitizer are required to match the iodine/iodide redox potential and the conduction band edge level of the TiO2 semiconductor. Natural dyes and organic dyes can also be used in DSSCs. Because they have high molar absorption coefficients and do seldom contain noble metals, sensitizers such as coumarin,12-13 perylene,14 xanthene,15 and porphyrin16 dyes have been extensively investigated. Expansion of the π-conjugated CdC backbone to extend the absorption spectrum to the red is one * To whom correspondence should be addressed. † DUT- KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT). ‡ Organic Chemistry, Royal Institute of Technology (KTH).
way to improve the solar cell performance.17 It would, however complicate the synthetic procedure and decrease the stability of the dye molecules. The introduction of π-conjugated ring moieties, such as benzene, thiophene, or pyrrole, into the methine backbone is, therefore, used as an economical way to simultaneously expand the π-conjugation system and sustain the stability of the dye molecule. In this paper, we present the synthesis and solar cell performance of two D-π-A chromophores that have a pyrrolidine moiety as electron donor (D) and a cyano acrylic acid moiety as electron acceptor (A). We will refer to the dyes according to their different π-backbones, PT for phenyl, thiophene and TT for thiophene, thiophene; see Figure 1. Although their structures are almost identical, PT and TT dyes gave dramatically different solar cell efficiencies. It was found that this originates from the diverse positions of the HOMO energy levels, leading to different driving forces for regeneration. The results also indicate dissimilar anchoring modes for the two different dyes, affecting the binding to the TiO2. Experimental Section 2.1. Materials. Screen-printed nanostructured TiO2 photoelectrodes were prepared by IVF Industrial Research and Development Corp. using a TiO2 paste from ECN, The Netherlands.18 The fluorine-doped SnO2 conducting glass TEC8
10.1021/jp065550j CCC: $37.00 © 2007 American Chemical Society Published on Web 01/06/2007
1854 J. Phys. Chem. C, Vol. 111, No. 4, 2007
Figure 1. Molecular structures of the conjugated dyes: (top) PT; (bottom) TT.
was obtained from Pilkington and was washed in ethanol in an ultrasonic bath. n- Butyllithium (2.93M in hexane), potassium tert-butoxide, and cyanoacetic acid were purchased from Aldrich. THF, DMF, and piperidine were purified before use. All other solvents and chemicals used in this work were of reagent grade and used without further purification. 2.1.1. 5-[2-(4-Pyrrolidin-1-yl-phenyl)Vinyl]thiophene-2-carbaldehyde (PPTC). To a solution of t-BuOK (190.4 mg, 1.70 mmol) in 20 mL of anhydrous THF was added diethyl 2-thienylmethylphosphonate (397.8 mg, 1.70 mmol) under nitrogen atmosphere. The mixture was vigorously stirred in an ice bath for 20 min. Then 4-pyrrolidin-1-ylphenylcarbaldehyde (200.0 mg, 1.14 mmol) in 10 mL of anhydrous THF was added dropwise during ca. 1 h and the solution was stirred continuously for another 2.5 h at 0 °C. After removal of the solvent by rotary evaporation, the residue was purified by column chromatography (silica gel, 2:1 v/v hexane/dichloromethane) and 1-[4-(2thiophen-2-ylvinyl)phenyl]pyrrolidine (PPT) was obtained as a dark yellow solid with a yield of 61%. To a stirring anhydrous THF solution (30 mL) of PPT (100.0 mg, 0.39 mmol) was slowly added 0.33 mL of n-BuLi (2.93 M in hexane) at -40 °C under nitrogen atmosphere, and the solution was stirred for 1 h. Then the mixture was heated to room temperature and DMF (71.5 mg, 0.98 mmol) was added dropwise. After 2.5 h, the solution was poured into a saturated aqueous solution of ammonium chloride and extracted with dichloromethane. The organic layer was dried over anhydrous magnesium sulfate and evaporated. After evaporation of the solvent, the residue was purified by column chromatography (Al2O3 gel, 2:1 v/v hexane/dichloromethane) and a dark orange solid (PPTC) was obtained with a yield of 50%. 1H NMR (CDCl3, 400 MHz, ppm): δ 1.96 (t, J ) 6.4 Hz, 4H), 3.28 (t, J ) 6.4 Hz, 4H), 6.56 (d, J ) 8.0 Hz, 2H), 7.15 (m, J ) 15.6 Hz, 1H), 7.22 (d, J ) 16.0 Hz, 1H), 7.27 (d, J ) 3.6 Hz, 1H), 7.46 (d, J ) 8.4 Hz, 2H), 7.91 (d, J ) 3.6 Hz, 1H), 9.81 (s, 1H). MS (APCI) (m/z): [M + H]+, 284.0. 2.1.2. 2-Cyano-3-{5-[2-(4-pyrrolidin-1-yl-phenyl)Vinyl]thiophen2-yl}acrylic Acid (PT). To a solution of PPTC (100.0 mg, 0.35 mmol) and cyanoacetic acid (29.8 mg, 0.35 mmol) in 15 mL of acetonitrile was added 12 drops of piperidine. The reaction mixture was then heated to reflux for 12 h. After removal of the solvent, the residue was purified by column chromatography (silica gel, 7:1 v/v dichloromethane/methanol) to give a dark red solid (yield 50%). 1H NMR (DMSO, 400 MHz, ppm): δ 1.96 (t, J ) 6.4 Hz, 4H), 3.28 (t, J ) 6.4 Hz, 4H), 6.55 (d, J ) 8.8 Hz, 2H), 7.06 (d, J ) 16.0 Hz, 1H), 7.22 (d, J ) 16.0 Hz, 1H), 7.23 (d, J ) 4.0 Hz, 1H), 7.47 (d, J ) 8.4 Hz, 2H), 7.70 (d, J ) 4.0 Hz, 1H), 8.17 (s, 1H). 13C NMR (DMSO, 500 MHz, ppm): δ 24.92, 47.21, 98.46, 111.81, 115.34, 117.38, 122.83, 125.43, 128.66, 132.69, 133.59, 140.64, 145.13, 148.08, 153.25, 164.09. TOF MS-EI+ (m/z) ([M]+): calcd for C20H18N2O3S, 350.1089; found, 350.1084. 2.1.3. 2-Cyano-3-{5-[2-(5-pyrrolidin-1-ylthiophen-2-yl)Vinyl]thiophen-2-yl}acrylic Acid (TT). The synthesis of TT was
Qin et al. performed in a procedure similar to that for PT, using pyrrolidine thiophene carbaldehyde instead. It was obtained with a yield of 65% as a purple solid. 1H NMR (DMSO, 400 MHz, ppm): δ 1.92 (t, J ) 6.4 Hz, 4H), 3.29 (t, J ) 6.4 Hz, 4H), 5.76 (d, J ) 4.0 Hz, 1H), 6.64 (d, J ) 16.0 Hz, 1H), 7.07 (d, J ) 4.0 Hz, 1H), 7.15 (d, J ) 4.0 Hz, 1H), 7.24 (d, J ) 16.0 Hz, 1H), 7.70 (d, J ) 3.8 Hz, 1H), 8.20 (s, 1H). 13C NMR (DMSO, 500 MHz, ppm): δ 25.33, 50.41, 69.75, 101.55, 112.23, 117.51, 123.77, 124.86, 127.84, 131.85, 132.93, 141.03, 144.94, 153.67, 156.77, 164.27. HRMS-EI- (m/z) ([M - H]-): calcd for C18H16N2O3S2, 355.0575; found, 355.0658. 2.2. Preparation of Dye-Sensitized TiO2 Electrodes. The screenprinted TiO2 photoelectrodes were sintered at 450 °C for 45 min and cooled to 80-100 °C before immersing into a 200 µM dye solution in acetonitrile. The TiO2 films were sensitized for 12-14 h at room temperature. After completion of the dye adsorption, the electrode was washed with (99.9%) acetonitrile and dried under a stream of air. It was noted that the TT dye bath was not stable as precipitation and color change was observed after 2 weeks. Another observation was that the TT dye partly detached from the TiO2 electrodes in the presence of acetonitrile-based electrolyte. 2.3. Photovoltaic Measurements of the Solar Cell. The twoelectrode electrochemical cell used for photovoltaic measurements consisted of a dye-sensitized, 9-11 µm thick TiO2 electrode, a counter electrode, a 50 µm Surlyn frame, and an organic electrolyte. The FTO, fluorine-doped tin oxide, counter electrode was thermally platinized by depositing 15 µL of 5 mM H2PtCl6 in 2-propanol and heating to 380 °C for 15 min. The surface area of the TiO2 film electrode was 0.48 cm2. The electrolyte was a 0.6 M tetrabutylammonium iodide (TBA)I, 0.1 M LiI, 0.05 M I2 solution in acetonitrile with 0.5 M 4-tertbutylpyridine (4-TBP), unless indicated otherwise. A xenon lamp with a Schott Tempax 113 filter was used to simulate sunlight (AM 1.5) and was calibrated to 1000 W m-2 with a reference silicon diode. The DSSCs were masked to expose only the active area under all measurements. 2.4. Characterization of the Dyes. 1H NMR spectra were measured on a Varian INOVA 400 NMR spectrometer. Mass spectra were made on a HP1100 MSD instrument. The IPCE was recorded using a computerized setup consisting of a xenon arc lamp (300 W Cermax, ILC Technology), a 1/8 m monochromator (CVI Digikro¨m CM 110), a Keithley 2400 source/meter, and a Newport 1830-C power meter with 818UV detector head. All UV-visible spectra of the dyes in solution and adsorbed on TiO2 films were recorded on an HR-2000 Ocean Optics fiber optics spectrophotometer. A Bio-Rad FTS375C IR spectrometer with near-IR accessory was used to obtain IR spectra of the sensitizer in KBr (1:100) solid pellets at a resolution of 2 cm-1. The ATR data were taken with spectrometer Perkin-Elmer Spectrum FTIR 2000, with a Specac Golden gate accessory crystal using 16 scans at a resolution of 2 cm-1. Cyclic voltammetry was performed with a CH Instruments 660 potentiostat using 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in dry acetonitrile (Fluka >99.9%) as supporting electrolyte. A platinum working electrode, platinum counter electrode, and a Ag/Ag+ reference electrode were used. The system was internally calibrated with ferrocene/ferrocenium (Fc/Fc+). Photoinduced absorption (PIA) experiments were performed using white probe light provided by a tungsten-halogen lamp
π-Conjugation Units in Organic Dyes
J. Phys. Chem. C, Vol. 111, No. 4, 2007 1855
SCHEME 1: Synthetic Route for Organic Dye PT
and superimposed on/off modulated blue light (450 nm) from an LED (Luxeon 1-Watt Star, Royal Blue) for excitation. The transmitted probe light was focused onto a monochromator (Acton Research Corp. SP-150) and detected by an UVenhanced silicon photodiode connected to a current amplifier and lock-in amplifier (Stanford Research Systems models SR570 and SR830, respectively). All PIA measurements were performed slightly above room temperature (25-28 °C). The samples used were dye-sensitized TiO2 films on a microscope glass with a drop of electrolyte and a cover glass. The geometrical and electronic properties of the PT and the TT dye were studied with density functional theory (DFT) calculations using the Gaussian03 program package.19 Optimized geometries were obtained with calculations at a B3LYP/6-31G(d) level where B3LYP20 is a hybrid functional which is a modification of the three-parameter exchangecorrelation functional of Becke,21 while the gradient-corrected exchange and correlation functionals are calculated according to Becke22 and Lee et al.23 The primary S0 f S1 excitation and five additional excitation transitions were calculated using time-dependent DFT calculations with B3LYP/6-31+G(d). 3. Results and Discussion 3.1. Synthesis. PT was synthesized via the synthetic route presented in Scheme 1; TT was prepared in a similar way. The compound 4-pyrrolidin-1-yl-phenylcarbaldehyde was synthesized according to literature procedure.24 The intermediate PPT was obtained by two-step synthesis starting from para-bromobenzaldehyde in reasonable yields. Formylation reaction of PPT was performed with n-butyllithium in DMF affording the aldehyde functional group on the thiophene ring (PPTC). The final target dye molecule PT was synthesized in 65% yield by reaction of PPTC with cyanoacetic acid in acetonitrile. The structures of all synthesized compounds were characterized by NMR and MS. 3.2. Photovoltaic Performance of DSSCs Based on PT and TT. The device performances of solar cells based on PT and TT under AM 1.5 illumination are summarized in Table 1. Intriguingly, the short-circuit currents, Jsc, open-circuit voltage, Voc, and efficiencies, η, for solar cells based on TT are dramatically lower compared to those based on PT. Efficiencies are 2.3% for PT and less than 0.5% for TT based solar cells. It has to be noted that for these comparative studies no complete optimization was performed; i.e., no scattering layer was added nor titanium tetrachloride treatment was done on the TiO2 electrodes.
TABLE 1: Photovoltaic Performance of DSSCs Based on PT and TTa dye
Jsc/mA cm-2
Voc/V
fill factor (FF)
η/%
PT TT
7.57 0.39
0.58 0.26
0.59 0.45
2.3
