Synthesis of two tri-arylamine derivatives as sensitizers in dye-sensitized solar cells: Electron injection studies and photovoltaic characterization

Synthetic Metals 188 (2014) 77–85

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Synthesis of two tri-arylamine derivatives as sensitizers in dye-sensitized solar cells: Electron injection studies and photovoltaic characterization M. Can a , M.Z. Yigit b , K. Seintis c , D. Karageorgopoulos c,e , S. Demic d,∗ , S. Icli b , V. Giannetas c , M. Fakis c,∗ , E. Stathatos e,∗∗ a

Department of Engineering Sciences, Faculty of Engineering, Izmir Katip Celebi University, Cigli, 35620 Izmir, Turkey Ege University, Solar Energy Institute, Bornova, 35040 Izmir, Turkey c Department of Physics, University of Patras, 26500 Patras, Greece d Department of Materials Science and Engineering, Faculty of Engineering and Architecture, Izmir Katip Celebi University, Cigli, 35620 Izmir, Turkey e Electrical Engineering Department, Technological-Educational Institute of Western Greece, 26334 Patras, Greece b

a r t i c l e

i n f o

Article history: Received 21 June 2013 Received in revised form 21 October 2013 Accepted 19 November 2013 Keywords: Donor/␲-bridge/acceptor structure Dye sensitized solar cells Electron injection dynamics

a b s t r a c t Two organic sensitizers with the structure donor-␲-acceptor, having tri-arylamine as electron donor, cyano-acetic acid as electron acceptor and a phenyl or thiophene ring as ␲-bridge, are synthesized. The synthetic procedure, the photophysical and electrochemical properties of both dyes are described in detail. In particular, cyclic voltammetry and spectroscopic measurements were performed while the energy gap between highest occupied and lowest unoccupied molecular orbital for both dyes was calculated. Besides, electron injection dynamics on a series of titanium dioxide nanostructured films sensitized with the two dyes were investigated with time resolved fluorescence spectroscopy using femtosecond temporal resolution, while nanostructured alumina films were used as reference. The photovoltaic performances of both dyes in quasi-solid state dye sensitized solar cells were also investigated. A maximum overall conversion efficiency of 3.8% was monitored for two micrometer thin titanium dioxide films and quasi-solid state electrolyte. The performances of the solar cells are explained in terms of photophysical properties and electron injection dynamics of both dyes. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Almost two decades ago, dye sensitized solar cells (DSSCs) were proposed as low cost alternatives to the conventional amorphous silicon solar cells, owing to the simplicity of their fabrication procedures, practically under ambient conditions with mild chemical processes [1,2]. DSSCs are placed in the category of third generation photovoltaics where new trends in the photovoltaic technology are applied. One of the basic components of the DSSCs is the photosensitizer which is responsible for the visible and near-infra-red utilization of the solar light. At present, ruthenium complexes and metal porphyrins photosensitizers have an overall solar to energy conversion efficiency more than 12% for small area devices [3,4]. However metal free organic dyes could be also considered as an alternative choice for the conversion of solar energy into electricity using DSSCs [5–8]. The reason is that pure organic dyes have

∗ Corresponding authors. ∗∗ Corresponding author. Tel.: +30 2610 369242; fax: +30 2610 369193. E-mail addresses: [email protected] (S. Demic), [email protected] (M. Fakis), [email protected] (E. Stathatos). 0379-6779/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2013.11.015

certain advantages over metal based photosensitizers: They are easily modified with common synthetic procedures while they exhibit high molar extinction coefficients compared to metal complexes. Organic dyes with high molar extinction coefficients could be advantageously used in thin titanium dioxide film based solar cells which are mainly required in solid state devices where the mass transport and pore filling are limited affecting the performance of the cells [9–11]. Organic sensitizers with the structure donor-␲-spacer-acceptor (D-␲-A) have proved to be a promising choice for efficient DSSCs as they exhibit large molar extinction coefficients, tunable absorption depending on ␲ spacer and facile synthesis with low cost [12–16]. The donor moieties could be one of diphenylamine [17], carbazole [18], indoline [19] and the acceptor cyanoacetic acid, or rhodamine-3-acetic acid. The ␲-conjugation bridge can be thiophene [20], methine [21] or a benzene unit [22] while furan, naphthalene, anthracene and fluorene have also been referred [23–26]. Moreover, it is generally accepted that a change in the conjugation pathway that bridges the donor and acceptor units is an effective strategy to tune the electronic properties of the organic dyes. Coplanar ␲-conjugation is beneficial to the red shift of the absorption peak of the dye but sometimes non-coplanar bridges have referred to enhance the charge separation and

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M. Can et al. / Synthetic Metals 188 (2014) 77–85

state electrolyte and the results are discussed in terms of the photophysical properties of the dyes and electron injection dynamics.

CH3

2. Experimental 2.1. Materials and methods

N S O

MZ-173

N HO

CH3 N O N

OH

MZ-175 Scheme 1. Molecular structure of the dyes MZ-173 and MZ-175.

injection [27]. However, the D-␲-A structure for efficient dye for DSSCs cannot avoid possible aggregation of the dyes on TiO2 surface which could lead to the drop of cell efficiency [28]. Aggregation of the dyes can be efficiently hindered introducing co-adsorbents like cholic acid, hexadecyl malonic acid or 1-decyl phosphonic acid [29,30]. These co-adsorbers decrease the amount of aggregates penetrating among the dye molecules but on the same time they cause a decrease of the total amount of the adsorbed dye molecules. The efficiency of a DSSC is highly determined by the energetics of the materials involved and the exact dynamics of the various charge transfer processes [31–41]. Electron injection from the dye’s excited state to the conduction band of the semiconductor is a most important mechanism. It takes place within hundreds of femtoseconds and it has recently gained a reinforced interest due to several reports referring to its retardation in the presence of electrolyte [37,42–45]. Based on above considerations we present the synthesis of two new organic dyes with D-␲-A structure where the donor is a tri-arylamine derivative, substituted with a methyl group, while the acceptor is cyanoacetic acid moiety (Scheme 1). The effect of the methyl group on the electron donating part, regarding electrochemical properties and solar cell performance, is discussed. The linker between donor and acceptor differentiate the two dyes which is a phenyl group in the first and a thiophene at the second case, respectively. The dyes are designed for thin film based solid or quasi solid state DSSCs applications exploring the advantage of such chromophores with short length and high extinction coefficients. We describe the synthesis of both dyes and their photophysical characterization while the electron injection in TiO2 films by means of femtosecond time resolved fluorescence spectroscopy using nanocrystalline Al2 O3 films as reference is also presented. The results of electron injection efficiency are correlated to the structure and excited state potential of the sensitizers. Finally, the electrical characterization of DSSCs employing the two dyes was examined for thin nanocrystalline films (∼2 ␮m) and quasi-solid

All solvents and reagents, unless otherwise stated, were of puriss quality and used as received. Copper(I) iodide was purchased from Fluka. Dichloromethane, toluene, 1,10-phenantroline, 1,2dimethoxyethane (DME), [1,1 -bis(diphenylphospino)ferrocene] dichloropalladium(II), 3-methyldiphenylamine were obtained from Sigma–Aldrich. Potassium carbonate and potassium hydroxide were purchased from Riedel de Haen. For the solar cells, TiO2 and Al2 O3 films: lithium iodide, iodine, 1-methyl-3-propylimidazolium iodide, hydrogen hexachloroplatinate(IV) hydrate (H2 PtCl6 ), poly(propylene glycol) bis(2-aminopropyl) ether, 3-isocyanatopropyltriethoxysilane and all solvents were purchased from Sigma–Aldrich. SnO2 :F transparent conductive electrodes (FTO, TEC15) 15 Ohm/square were purchased from Hartford Glass Co., USA. Commercial ultra pure titanium isopropoxide (TTIP, 97%, Aldrich), aluminum tributoxide (Al(BuO)3 , 95%, Aldrich), Triton X-100 (polyoxyethylene-10isooctylphenyl ether) surfactant (99.8%, Aldrich), and glacial acetic acid (AcOH, Aldrich) were used to make the precursors for TiO2 and Al2 O3 sol–gels. Preparation of TiO2 and Al2 O3 films are described elsewhere [46,47]. Electrochemical investigations were carried out with CH 660B Instruments potentiometer. The steady state absorption and fluorescence spectra were taken using a Hitachi U-2900 UV–vis spectrophotometer and a Hitachi F-2500 fluorescence spectrophotometer, respectively. The accuracy of the fluorescence measurements was checked by detecting the emission band of europium Eu3+ (5 D0 → 7 F2 transition) at 613 nm as a reference. The fluorescence spectra of solid samples were collected using a specific holder assuring the exact geometry in all measurements (inclination angle: 10◦ ). Unsensitized TiO2 and Al2 O3 films were used as reference samples for obtaining the spectra of sensitized films. The thickness of the oxide films was ∼2 ␮m. The fluorescence dynamics were studied by using a femtosecond time resolved upconversion system that has been described in details elsewhere [31,32,44,45]. The excitation beam was the second harmonic of a Ti:Sapphire femtosecond laser, at 400 nm, and the average excitation power was less than 3 mW. The samples were dilute solutions of the two dyes in tetrahydrofuran (10−5 M concentration), or TiO2 and Al2 O3 films sensitized with the dyes. All dynamics were detected under magic angle conditions. 2.2. Synthesis of the dyes 2.2.1. (4-Bromophenyl)(3-methylphenyl)phenylamine CuI (57 mg; 0.3 mmol) and 1,10-phenanthroline (54 mg; 0.3 mmol) were added to a two-necked flask and dried under vacuum at 120 ◦ C for 1 h. Then 3-methyldiphenyl amine (550 mg, 3 mmol), 1-bromo-4-iodobenzene (1.02 g, 3.6 mmol) and dry pxylene (50 mL) were added to the flask and the mixture was heated to 150 ◦ C. After that, KOH powder (730 mg, 13 mmol) was added to the flask and it was refluxed for 20 h. After the completion of the reaction, it was cooled to room temperature and glacial acetic acid (3 mL) was added to the reaction medium. The mixture was then extracted with toluene (3 × 15 mL). Combine organic phase was washed with NaHCO3 and brine solutions, and the solvent was evaporated. Crude product in brown liquid was purified by column chromatography (SiO2 , CH2 Cl2 /n-hexane 2:1) to give white powder, which was then precipitated in methanol (600 mg, 60%). 1 H

M. Can et al. / Synthetic Metals 188 (2014) 77–85

NMR (CHCl3 ): 8.56 (s, 1H), 7.31 (d, 2H), 7.22 (t, 2H), 7.12 (t, 1H), 7.05 (d, 2H), 6.99 (t, 1H), 6.93 (dd, 2H), 6.87 (m, 2H), 2.26 (s, 3H). 2.2.2. 4 -[(3-Methylphenyl)(phenyl)amino]biphenyl-4-carbaldehyde (4-Bromophenyl)(3-methylphenyl)phenylamine (405.88 mg, 1.2 mmol), (4-formylphenyl)boronic acid (119.95 mg, 0.8 mmol) and Pd(dppf)Cl2 (81.4 mg, 0.1 mmol) were added to a Schlenk flask. Then aqueous solution of K2 CO3 (8 mL, 6 mmol) and 30 mL of DME were added under argon atmosphere and the reaction mixture was heated and stirred overnight at 90 ◦ C. After the reaction completed, CHCl3 was added and the reaction mixture was washed with water (2 × 15 mL). Then the organic solvent was evaporated. The crude product was purified by column chromatography (SiO2 , CH2 Cl2 /n-hexane 1:2) to afford the product as a yellow powder (232.60 mg, 80%). 1 H NMR (CHCl3 ): 10.02 (s, 1H), 7.92 (d, 2H), 7.74 (d, 2H), 7.52 (d, 2H), 7.29 (dd, 2H), 7.12 (m, 5H), 7.07 (t, 1H), 6.95 (m, 3H), 2.28 (s, 3H). 2.2.3. 2-Isocyano-3-{4 -[(3methylphenyl)(phenyl)amino]biphenyl-4-yl}acrylic acid (MZ-175) 4 -[(3-Methylphenyl)(phenyl)amino]biphenyl-4-carbaldehyde (232.60 mg, 0.6 mmol) cyanoacrilic acid (153.11 mg, 1.8 mmol) and piperidine (0.65 mL, 6.6 mmol) were added into a flask. Then freshly distilled CHCl3 (20 mL) was added to the flask under argon atmosphere. After refluxing for 30 h, to the reaction medium, another 20 mL of CHCl3 was added. The mixture was then washed with water (3× 15 mL). The organic solvent was finally evaporated under vacuum. The crude product was purified by column chromatography (SiO2 , CH2 Cl2 /MeOH, 9:1) to give the product as a yellow powder (232.46 mg, 90%). 1 H NMR (DMSO): 8.02 (s, 1H), 7.92 (d, 2H), 7.73 (d, 2H), 7.62 (d, 2H), 7.31 (t, 2H), 7.21 (t, 1H), 7.06 (m, 5H), 6.89 (t, 2H), 6.83 (d, 1H), 3.49 (s, 3H); FT-IR (KBr, cm−1 ): OH, 3423; (C N, nitrile), 2215, C O, 1590; (C H, aliphatic), 2935; C C H, 3030; C C , 1523; (C C, Ph) 1495. 1 H NMR spectrum for MZ-175 is presented as supplementary material in Fig. 1S. 2.2.4. 5-{4-[(3-Methylphenyl)(phenyl)amino]phenyl}thiophene2-carbaldehyde (4-Bromo-phenyl)(3-methylphenyl)phenylamine (405.88 mg, 1.2 mmol), (5-formyl-2-thienyl) boronic acid (124.77 mg, 0.8 mmol) and Pd(dppf)Cl2 (81.4 mg, 0.1 mmol) were added to a Schlenk flask. Then aqueous solution of K2 CO3 (8 mL, 6 mmol) and 30 mL of DME were added under argon atmosphere and the reaction mixture was heated and stirred overnight at 90 ◦ C. After that, CHCl3 was added (30 mL) and the mixture was washed with distilled water (2× 15 mL). After the evaporation of organic solvent, the crude product was purified by column chromatography (SiO2 , CH2 Cl2 /n-hexane, 1:2) to afford the product as an orange powder (236.46 mg, 80%). 1 H NMR (CHCl3 ): 9.85 (s, 1H), 7.70 (d, 1H), 7.52 (dd, 2H), 7.31 (m, 3H), 7.13 (t, 1H), 7.14 (dd, 2H), 7.10 (dd, 1H), 7.06 (dd, 2H), 6.96 (dd, 3H), 2.28 (s, 3H). 2.2.5. 2-Isocyano-3-(5-{4-[(3methylphenyl)(phenyl)amino]phenyl}-2-thienyl)acrylic acid (MZ-173) 5-{4-[(3-Methylphenyl)(phenyl)amino]phenyl}thiophene-2carbaldehyde (236.46 mg, 0.6 mmol) cyanoacrilic acid (153.11 mg, 1.8 mmol) and piperidine (0.65 mL, 6.6 mmol) were added into a flask. Then 20 mL of freshly distilled CHCl3 was added under argon atmosphere. The reaction mixture refluxed 30 h. Then, another 15 mL of CHCl3 was added into the mixture and the reaction mixture was washed with distilled water (2 × 15 mL). The organic solvent was evaporated and the crude product was purified by column chromatography (SiO2 , CH2 Cl2 /MeOH, 9:1) to afford the

79

product as a yellow powder (235.72 mg, 90%). 1 H NMR (DMSO): 8.19 (s, 1H), 7.73 (d, 1H), 7.63 (d, 2H), 7.49 (d, 1H), 7.33 (t, 2H), 7.22 (t, 1H), 7.09 (m, 3H), 6.95 (t, 3H), 6.84 (t, 2H), 3.49 (s, 3H); FT-IR (KBr, cm−1 ): OH, 3455; (C N, nitrile), 2214; C O, 1583; (C H, aliphatic), 2927; C C H, 3040; C C , 1490; (C C, Ph) 1440 and 1380. 1 H NMR spectrum for MZ-173 is presented in Fig. 2S as supplementary material. Finally, all steps given above for the synthesis of both chromophores are presented in Scheme 2. 2.3. Preparation of solar cells In the construction of the solar cells a quasi-solid state electrolyte was used. This was chosen as it combines the high ionic conductivity of liquids while it reduces the risk of leaks and minimizes sealing problems in the cells. For the gel electrolyte applied to the DSSCs, we used a hybrid organic–inorganic material ICSPPG230 which was prepared according to a procedure described in previous publications [46,47]. The gel electrolyte was synthesized by the following procedure: 0.7 g of the functionalized alkoxide precursor ICS-PPG230 were dissolved in 2.4 g of sulfolane under vigorous stirring. Then, 0.6 mL AcOH were added followed by 0.3 M 1-methyl-3-propylimidazolium iodide, 0.1 M LiI and 0.05 M I2 in a final molar ratio of AcOH:LiI:MPImI:I2 = 2.5:0.1:0.3:0.05. Finally 0.5 M of tert-butyl pyridine was added to the above mixture. After few hours of stirring, one drop of the obtained sol was placed on the top of the titania electrode with adsorbed dye molecules and a slightly platinized FTO counter electrode was pushed by hand on the top. The platinized FTO glass was made by exposing it to a H2 PtCl4 solution (5 mg/1 mL of i-PrOH) followed by heating at 450 ◦ C for 10 min. The two electrodes tightly stuck together by Si O Si bonds developed by the presence of ICS-PPG230. The cell active area for the electrical measurements was 0.28 cm2 determined by a black mask. Three quite similar cells were tested under the same conditions in order to avoid any misleading estimation of their efficiency. For the J–V curves, the samples were illuminated with Xe light using a Solar Light Co. solar simulator (model XPS 400) equipped with AM 0 and AM 1.5 direct Air Mass filters. The light intensity was kept constant at 1000 W/m2 measured with a CMP 3 Kipp&Zonen pyranometer. Finally, the J–V curves were recorded by connecting the cells to a Keithley Source Meter (model 2601) which was controlled by Keithley computer software (LabTracer). Cell performance parameters, including short-circuit current density (JSC ), open circuit voltage (VOC ), maximum power (Pmax ), fill factor (ff) and overall cell conversion efficiency, were measured and calculated from each J–V characteristic curve for the three cells and their mean values are presented. 3. Results and discussion 3.1. Electrochemical properties The electrochemical properties of two dyes were examined by cyclic voltammetry in a 0.1 M solution of Bu4 NPF6 in acetonitrile with a scan rate of 200 mV s−1 . A glassy carbon electrode was used as the working electrode while platinum wire and Ag/AgCl were used as the counter and the reference electrodes, respectively. The ferrocene/ferricenium (Fc/Fc+ ) redox couple was used as an internal reference. Before the analyses, electrolyte was subjected to argon gas for about 10 min. The electrochemical calculations were carried out by following the steps reported in Ref. [48] ELUMO = −e(E1/2(red.) + 4.4) EHOMO = −e(E1/2(ox.) + 4.4)

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M. Can et al. / Synthetic Metals 188 (2014) 77–85

I

CH3

CH3

+

i N

Br

NH

Br

i: CuI/1,10-phenanthroline/p-xylene/KOH/glacial acetic acid/150 Co/20 h

CH3

O CH3

N

HO B

COOH NC

O

MZ-175 iii

S

HO

ii N

Br CH3

ii HO

O O

COOH

S

B

NC

N

HO

MZ-173 iii

ii: Pd(dppf)Cl2/K2CO3/DME/90 Co iii: Piperidine/CHCl3/reflux Scheme 2. Synthetic protocols of the sensitizers.

where E1/2(red.) and E1/2(ox.) are the first reduction and oxidation potential values found from the voltammograms respectively. In cyclic voltammograms, MZ-173 and MZ-175 gave one reversible oxidation peak attributed to the tri-arylamine moieties of these compounds and one irreversible reduction peak resulted from the presence of cyanoacrylic acid part. The oxidative potential values are 0.93 V for MZ-173 and 1.02 V for MZ-175 and the reductive potentials are about −1.24 V for MZ-173 and −1.34 V for MZ-175 (Fig. 1). Tri-arylamine and cyanoacrylic acid groups are known as strong electron donating and accepting groups, respectively. As expected, the presence of an electron donating group on tri-arylamine moiety reduces its oxidation potential, and the presence of an electron withdrawing group performs the opposite effect [49]. In MZ-173 and MZ-175, tri-arylamine group has an electron donating methyl substituent. Due to the fact that methyl groups have electron releasing properties toward the tri-arylamine moiety, then, these two dyes exhibit oxidation behavior at a lower potential values than corresponding tri-arylamines without alkyl groups [50]. Triaryl-amines usually give one oxidation peak at 1–1.2 V and the effect of methyl substituent is seen as a small decrease in the oxidation value [50]. HOMO and LUMO levels of these dyes were calculated from their oxidation and reduction potential values, respectively. HOMO levels are calculated as −5.33 eV for MZ-173 and −5.42 eV for MZ-175 while their LUMO levels are −3.16 and −3.06 eV, respectively. Both of the dyes exhibit lower HOMO energy level than the potential of I− /I− 3 (−4.90 eV), indicating that there is enough driving force for the dye’s regeneration. Besides the LUMO energy levels of the dyes are higher than the TiO2 conduction band (−3.9 eV) indicating that the electron injection process is energetically favorable compared with the conduction band edge energy level of the TiO2 electrode.

Fig. 2 demonstrates the energy levels of the materials used in DSSC device. 3.2. Photophysical properties 3.2.1. Steady state spectroscopy Fig. 3 shows the absorption and fluorescence spectra of MZ-173 and MZ-175 in dilute THF solutions. The corresponding photophysical results are summarized in Table 1. MZ-175 exhibits absorption and fluorescence peaks at 387 and 505 nm respectively. Upon replacing the phenylene ring with the thiophene one, both peaks are red-shifted to 412 and 514 nm respectively. This could be due to the more polarizable nature of thiophene compared to the phenyl ring. The molar extinction coefficients for both dyes have been measured in THF and found to be 22,078 and 11,333 M−1 cm−1 for MZ-173 and MZ-175, respectively. Due to the thiophene ring being more polarizable than the phenyl, a better coupling between the donor and acceptor groups is expected in MZ-173. This leads to a stronger overlap between the HOMO and LUMO orbitals which is then seen through the higher molar extinction coefficient of the thiophene containing dye MZ-173. It is also known that thiophene Table 1 Photophysical results of MZ-173 and MZ-175 dyes. MZ-173

THF TiO2 film Al2 O3 film

MZ-175

abs (nm)

fluor (nm)

abs (nm)

fluor (nm)

412 439 433

514 567 575

387 395 391

505 549 558

M. Can et al. / Synthetic Metals 188 (2014) 77–85

81

1.0

Absorption MZ-173 MZ-175

Intensity

0.8

Fluorescence MZ-173 MZ-175

0.6

0.4

0.2

0.0 350

400

450

500

550

600

650

700

Wavelength (nm) Fig. 3. Absorption and fluorescence spectra of dyes MZ-173 and MZ-175 in THF solutions.

is due to dye molecules that are energetically relaxed because of interaction with the nanoparticles of the substrate or with other dye molecules. Finally, the fluorescence spectra of both dyes on Al2 O3 are red-shifted compared to TiO2 while the opposite happens in the case of the absorption spectra. This means that on Al2 O3 the dye molecules obtain a more relaxed conformation in the excited state compared to TiO2 .

has a lower energy of transfer transition because of its smaller stabilization energy than benzene (thiophene 19 kcal/mol; benzene 36 kcal/mol) [51–53]. The absorption and fluorescence spectra of the two dyes on TiO2 and Al2 O3 substrates are shown in Fig. 4a and b. All spectra on TiO2 and Al2 O3 are red-shifted compared to the corresponding spectra in THF. Specifically, in the absorption spectra the red-shift ranges from 4 to 27 nm while in the fluorescence ones it ranges from 44 to 61 nm. It is clear that the red-shift is larger in the case of fluorescence spectra than in the absorption ones and in the case of MZ-173 than in MZ-175. The former can be an indication that the emission

1.0

MZ-173 Absorption TiO2

0.8

Al2O3

0.6

Intensity

Fig. 1. Cyclic voltammograms of the MZ-173 (a) and MZ-175 (b) dyes.

3.2.2. Time resolved fluorescence in solutions The fluorescence dynamics of the two dyes were studied in dilute THF solutions (10−5 M concentration) at various detection wavelengths. The decays at three characteristic wavelengths are shown in Fig. 5 for MZ-173 and MZ-175, respectively. At short wavelengths (high energy side of the fluorescence spectrum), a fast decay is observed which becomes less important as the detection wavelength is shifted to longer wavelengths (low energy side of the fluorescence spectrum) until it becomes a rise. This behavior is an indication of a transient relaxation of the

Fluorescence TiO2

0.4

Al2O3 0.2

(a)

0.0

-3.06 eV

1.0

MZ-175

-3.16 eV

TiO2

MZ173

MZ175

I-/I-3

-4.90 eV -5.33 eV

Intensity

Energy

-3.90 eV

0.8

Absorption TiO2

0.6

Al2O3 Fluorescence TiO2

0.4

Al2O3

0.2

(b)

0.0

-5.42 eV

350

400

450

500

550

600

650

700

750

Wavelength (nm) Fig. 2. Energy levels of the materials used in DSSC device.

Fig. 4. Absorption and fluorescence spectra of (a) MZ-173 and (b) MZ-175 on TiO2 and Al2 O3 films.

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M. Can et al. / Synthetic Metals 188 (2014) 77–85

MZ-173

1.2

1.0 1.0

Intensity

0.8

Intensity

0.6

(a)

0.4 0.2 475nm

0.0

515nm

565nm

λdet=530nm

0.8

TiO2 Al2O3

0.6 0.4

(a)

0.2 0.0

MZ-175 1.0

Intensity

0.8

1.0

0.6

Intensity

0.2 465nm

0.0 0

505nm

2

4

565nm

6

8

MZ-173

αi (λ)

0.5

0.0

MZ-175

0.5

αi (λ)

λdet=610nm

0.8 0.6

(c)

0.4 0.2 0.0 0

5

10

15

20

Time (ps) Fig. 7. Fluorescence dynamics of MZ-173 on TiO2 and Al2 O3 at (a) 530, (b) 570 and (c) 610 nm. The solid lines represent fitting curves obtained by a stretched exponential decay function.

mechanisms, respectively. In both dyes, ˛1 (), associated with  1 , is positive at short wavelengths and becomes negative at long ones. It is noted that in MZ-175, ˛1 () becomes negative at 485 nm (i.e. at a wavelength shorter than the wavelength of the maximum of the fluorescence spectrum) while in MZ-173 it becomes negative at longer wavelengths (i.e. at 545 nm). The values of ˛2 (), associated with  2 , are always positive in MZ-173 corresponding to a decay mechanism. However, the behavior is different in the case of MZ175, where ˛2 () is positive for wavelengths shorter than 545 nm but it becomes negative at longer ones. Finally, ˛3 () is positive at all detection wavelengths for both dyes and therefore  3 can be considered as the lifetime of the S1 state for each dye.

(a)

1.0

0.0

460

1.0

Intensity

fluorescence spectrum toward low energy (i.e. a dynamic red-shift). In order to analyze the fluorescence decays, a global analysis was employed by using a three exponential decay function convoluted to the Instrument’s Response Function. A global fitting analysis provides an improved resolution of correlated parameters. For MZ-173, time constants of  1 = (1.68 ± 0.05) ps,  2 = (39 ± 2) ps and  3 = (1400 ± 200) ps are found. Similarly, for MZ-175 the time constants found by the global fitting analysis are  1 = (1.00 ± 0.07) ps,  2 = (2.4 ± 0.1) ps and  3 = (2000 ± 200) ps. The longer decay times ( 3 ) are underestimated since they are similar or longer than the higher temporal limit of our instrument. A clear view of the different nature of the decay mechanisms is shown in Fig. 6 where the pre-exponential coefficients of the three lifetimes are depicted as a function of wavelength. Positive and negative values of the pre-exponential coefficients are associated to decay and rise

-0.5

(b)

0.4

0.0

Fig. 5. Fluorescence dynamics of (a) MZ-173 and (b) MZ-175 in THF solutions at three characteristic wavelengths. The solid lines represent the fitting curves obtained by a global fitting procedure with a three-exponential decay function.

-0.5

0.6

0.2

10

Time (ps)

1.0

λdet=570nm

0.8

(b)

0.4

(b) 480

500

520

540

560

580

600

620

Wavelength (nm) Fig. 6. Pre-exponential coefficients for (a) MZ-173 and (b) MZ-175 in THF as a function of emission wavelength.

3.2.3. Time resolved fluorescence on TiO2 and Al2 O3 In Fig. 7, the fluorescence dynamics of dye MZ-173 are shown on TiO2 and Al2 O3 substrates at three characteristic wavelengths. Similar results have been obtained from dye MZ-175. The dynamics become slower as the detection is shifted to longer wavelengths but no rise component is observed. It is obvious that MZ-173 decays much faster on TiO2 than on Al2 O3 indicating an efficient electron injection mechanism. On TiO2 the excited state population of the dye molecules decay by electron injection toward the conduction band of TiO2 , by non-radiative mechanism, including aggregation induced quenching, as well as by radiative decay to the ground state. On the other hand, in the case of Al2 O3 electron injection is negligible because the excited state potential of the dyes is lower than the potential of the conduction band of Al2 O3 and for this reason Al2 O3 is considered as a reference substrate. The other decay mechanisms apart from electron injection are considered similar on

M. Can et al. / Synthetic Metals 188 (2014) 77–85 Table 2 Decay parameters of MZ-173 and MZ-175 on TiO2 and Al2 O3 films after fitting with stretched exponential function. The average decay times and electron injection efficiencies are also given. Wavelength (nm)

Substrate

 (ps)

ˇ

 (ps)

˚inj

MZ173

530

TiO2 Al2 O3 TiO2 Al2 O3 TiO2 Al2 O3

0.23 0.39 0.10 2.60 0.20 8.60

0.58 0.33 0.40 0.37 0.39 0.44

0.36 2.44 0.33 10.8 0.71 22.00

0.85

TiO2 Al2 O3 TiO2 Al2 O3 TiO2 Al2 O3

0.10 0.20 0.10 1.20 0.15 4.60

0.45 0.25 0.37 0.32 0.40 0.40

0.25 4.6 0.42 8.20 0.46 15.00

0.94

570 610 MZ175

510 550 590

0.97 0.97

10 9

Current Density (mA/cm2 )

Dye

83

0.95

8

(a)

7 6 5 4 3 2

MZ-173 Voc=0,75 volts 2 Jsc=9,2 mA/cm ff=0,55 n%=3,8

1

0.97

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Voltage (volts) Al2 O3 and TiO2 . The dynamics on both nanostructured substrates have been fitted with a stretched exponential decay function of the form [31,37,43] (1)

where  is the decay time and ˇ is a dispersion parameter ranging from 0 to 1. Lower values of ˇ mean a higher inhomogeneity of the samples. The average decay time is then determined through the equation stretched-exp  = (/ˇ) (1/ˇ)

(2)

where  is the gamma function. The efficiency of the electron injection from the excited state of the dyes to TiO2 is afterwards calculated through the relationship: ˚inj = 1 − [TiO2 /Al2 O3 ]

(3)

All fitting parameters and electron injection values are summarized in Table 2. It is noted that multi-exponential fittings have also been made (results not given here) showing injection components of ∼100 fs, ∼1 ps and >3 ps which is typical for D-␲-A dye sensitizers [32,54,55]. In addition, the electron injection efficiency values have been found almost identical for multi-exponential and stretched exponential fittings. The values of injection efficiency given in Table 2 are very high ranging from 0.86 to 0.97. Such high electron injection values are one of the conditions that should be satisfied toward efficient solar cell devices. In addition, the electron injection values do not exhibit any significant differences between the two dyes. This is indicative of the fact that adding thiophene ring instead of phenyl one is not a dramatic change regarding the injection efficiency. On the other hand, the molecular size and the distance of electron transfer seem to be determinant factors. In order to investigate the effect of electron transfer distance on injection efficiency, the values of ˚inj reported here for MZ-173 should be compared to the ones found in a previous paper of ours for sensitizer D2 in Ref. [28]. In the dye D2, a fluorene group is inserted between the tri-arylamine and the thiophene ring, thus, increasing the electron transfer distance. A reduced electron injection efficiency of 0.85 was found in D2 compared to 0.97 in MZ-173 (both measured at the peak of the fluorescence spectrum and with similar experimental and analysis conditions). However, the efficiency of a DSSC device with D2 was found 4.6%, i.e. higher than that of MZ-173 (vide infra). This reduction in MZ-173 could not be attributed to a different coverage of the solar spectrum because both MZ-173 and D2 have their absorption maxima on TiO2 at similar wavelength (439 nm for MZ-173 and 430 nm for D2). The difference is attributed to the values for molar extinction coefficients where in the case of D2 is higher than that of MZ-173 (42,600 vs. 22,078 M−1 cm−1 ). On the other hand, it may

2

ˇ

9

Current Density (mA/cm )

I(t) = I0 exp (−t/)

10

MZ-175 Voc=0,69 volts 2 Jsc=6,7 mA/cm ff=0,48 n%=2,2

(b)

8 7 6 5 4 3 2 1 0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Voltage (volts) Fig. 8. J–V curves of quasi-solid state solar cells with (a) MZ-173 and (b) MZ-175 dyes.

be attributed to an increased back electron transfer in MZ-173/TiO2 because of the short distance between the injected electrons and the positively charged tri-arylamine group. 3.3. Solar cells performance The current–voltage (J–V) characteristic curves of quasi-solid state dye sensitized solar cells using MZ-173 and MZ-175, newly synthesized dyes as sensitizers, are presented in Fig. 8. For each sensitizer, we prepared three cells which were tested under the same conditions and the data presented as insets in Fig. 8 concern the average values obtained from the cells. In all cases, the active area of the cells was 0.28 cm2 . The quasi-solid state DSSCs constructed with MZ-175 showed an overall efficiency of 2.2% attributed to the absorption spectrum of the dye which is located at the left edge of the visible light. The open circuit voltage was measured as 0.69 V and the short circuit current was 6.7 mA/cm2 . As the short circuit current value is rather low we believe that it caused to two factors: The low extinction coefficient value and the ineffective visible light utilization. Under same conditions a higher efficiency, 3.8%, was obtained for cells with MZ-173 as sensitizer. It is rather expected since this dye exhibits higher values for molar extinction coefficient compared to MZ-175 and better spectral properties. The increase of VOC in MZ-173 vs. MZ-175 can be explained by the relative position of conduction band after dye adsorption on TiO2 and charge recombination rate [14]. The devices showed good stability when subjected to 1 sun light irradiance from a Xe lamp as solar

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M. Can et al. / Synthetic Metals 188 (2014) 77–85

simulated light source after 400 h of continuous visible light soaking. The cells were all unsealed and were used without any further treatment after the electrolyte was gelled. Although, a direct comparison with cells prepared with closely related dyes containing unsubstituted triphenylamines cannot be done because of several differences applied during preparation steps of the cells (TiO2 film thickness and different particle size, quasi-solid electrolyte etc.), it is noted that a compound similar to MZ-173 but without the methyl substituent has shown a reduced efficiency and short circuit current of 2.75% and 5.42 mA/cm2 [compound L1 in Ref. [9]]. That dye has also shown a similar molar extinction coefficient as MZ-173 (25,000 vs. 22,078 M−1 cm−1 ) and a slightly blue shifted absorption spectrum (absorption peak at 404 nm vs. 412 nm, both in THF). It is also noted that the quasi solid-state electrolyte used here, generally gives lower solar cell efficiencies because of the lower ionic conductivity values than liquid ones as used in Ref. [9]. We believe the enhanced solar cell behavior observed in MZ-173 is due to better charge transfer and electron injection properties. 4. Conclusions In summary, two new tri-arylamine based organic dyes containing phenyl and thiophene moieties as ␲-bridges are synthesized and examined as sensitizers in thin TiO2 film based DSSCs. The spectral response of the dye with thiophene as a ␲-linker proved to be more efficient in solar cell performance while a maximum overall performance of 3.8% was obtained for a quasi-solid state DSSC. However, electron injection dynamics studies performed for both dyes on TiO2 films showed similar values of their quantum efficiencies. Therefore, we concluded that the difference observed to the values of solar cell efficiencies are mainly attributed to the different spectral response and extinction coefficients values measured for the two dyes. Acknowledgements Prof. E. Stathatos would like to acknowledge that his research has been co-financed by the European Union (European Social Fund – ESF) and Greek National funds through the Operational Program “Education and Lifelong Learning” of the National Strategic Reference Framework (NSRF) – Research Funding Program: Thales MIS: 377756. M. Can, S. Demic and S. Icli acknowledge the supports from the State Planning Organization of Turkey (DPT) for financial support through the project 11-DPT-001. Z.M. Yigit also thanks for Erasmus Student Exchange program for the visit to the TechnologicalEducational Institute of Western Greece. Finally, the authors would like also to thank Dr. Peter Hrobarik for useful discussions. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.synthmet. 2013.11.015. References [1] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chemical Reviews 110 (2010) 6595–6663. [2] J. Gong, J. Liang, K. Sumathy, Renewable and Sustainable Energy Reviews 16 (2012) 5848–5860. [3] A. Yella, H.W. Lee, H.N. Tsao, C. Yi, A.K. Chandiran, M.K. Nazeeruddin, E.W. Diau, C.Y. Yeh, S.M. Zakeeruddin, M. Grätzel, Science 334 (2011) 629–634. [4] F. Gao, Y. Wang, J. Zhang, D. Shi, M. Wang, R. Humphry-Baker, P. Wang, S. Zakeeruddin, M. Grätzel, Chemical Communications (2008) 2635–2637. [5] K. Guo, K. Yan, X. Lu, Y. Qiu, Z. Liu, J. Sun, F. Yan, W. Guo, S. Yang, Organic Letters 14 (2012) 2214–2217.

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