Synthesis, optical, electrochemical and photovoltaic properties of organic dyes containing trifluorenylamine donors

Dyes and Pigments 113 (2015) 78e86

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Synthesis, optical, electrochemical and photovoltaic properties of organic dyes containing trifluorenylamine donors Abhishek Baheti a, Satyanarayana Reddy Gajjela b, Palani Balaya b, c, K.R. Justin Thomas a, * a

Organic Materials Laboratory, Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee, India Department of Mechanical Engineering, National University of Singapore, 117576 Singapore, Singapore c Engineering Science Program, National University of Singapore, 117576 Singapore, Singapore b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 May 2014 Received in revised form 25 July 2014 Accepted 28 July 2014 Available online 6 August 2014

Two new organic dyes based on trifluorenylamine donor and cyanoacrylic acid acceptor have been synthesized and characterized by optical and electrochemical measurements and density functional theory calculations. It is found that the trifluorenylamine donor is beneficial to red-shift the absorption and to lower the oxidation potential when compared to the triphenylamine donor. The variations in the photovoltaic performance of the dyes are corroborated by the dye loading data, incident photon to current conversion efficiency and the interfacial kinetic parameters estimated from the intensity modulated photovoltage/photocurrent spectral measurements. A dye with fluorene and bithiophene segments in the p-linker exhibited device efficiency up to 5.8%. The enhanced power conversion efficiency exhibited by this dye when compared to its analogue containing diphenylaminofluorne donor is attributed to its superior anti-aggregation ability and the comparatively prolonged electron lifetime. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Organic dyes Dye-sensitized solar cells Optical spectra TDDFT calculations Fluorene Donor-acceptor compounds

1. Introduction €tzel [1], dyeEver since the pioneering work of O'Regan and Gra sensitized solar cell (DSSC) [2] fabricated using organic and organometallic dyes has received immense attention due to their potential advantages such as low cost, facile chemical modifications to fine-tune the functional properties and environmental friendliness. Typically, a DSSC is constituted by four major components namely mesoporous nanocrystalline semiconductor oxide such as titanium dioxide or zinc oxide based photoanode, an organic or organometallic sensitizer, redox shuttle in an electrolyte solution and a counter electrode. The sensitizer acts as light harvester and electron generator. The absorption of sun light by the sensitizer leads to an intramolecular charge transfer (CT) electronic excitation followed by electron injection into the conduction band (CB) of semiconductor oxide. Finally, the oxidized sensitizer is regenerated by the redox mediator. Ground and excited state potentials of the sensitizer predominantly affect the efficiency of the DSSC as they are directly related to the photocurrent generation and dye regeneration kinetics. Wealth of knowledge on the structural features affecting the ground and excited state properties are available

* Corresponding author. Tel.: þ91 1332 285376; fax: þ91 1332 273560. E-mail addresses: [email protected], [email protected] (K.R. Justin Thomas). http://dx.doi.org/10.1016/j.dyepig.2014.07.036 0143-7208/© 2014 Elsevier Ltd. All rights reserved.

in recent literature dealing with the structure-property relationship investigations on different classes of sensitizers including ruthenium polypyridyl complexes [3], porphyrins [4] and metalfree organic dyes [5]. Among these sensitizers, metal free organic dyes are most attractive due to their easy synthesis, higher molar extinction coefficient for the charge transfer absorption, lower cost and environment compatibility. An organic dye is typically composed of three functionally unique structural elements: donor (D), acceptor (A) and p-linker. A large number of organic dyes with D-p-A configuration and possessing amine donors such as triphenylamine [6], carbazole [7], indoline [8], phenothiazine [9], phenoxazine [10], etc. have been explored as sensitizers in DSSC. Polycyclic fused heteroaromatics have also been demonstrated as decent donors [11]. Heterocyclic rings possessing reduced aromaticity have been found to be beneficial for CT from the donor to acceptor [1d,12]. Also, the presence of electron accepting units such as benzothiadiazole [13], benzotriazole [14], thienopyrazine [15], quinoxaline [16], diketeopyrrolopyrrole [6a,17], triazine [18], cyanovinyl [19], etc., has been found to alter the excited state energetics of the dyes and consequently the light harvesting properties. Despite the wealth of design rules evolved for organic dyes, most of the dyes known till now suffer from one or more of the detrimental processes such as dye aggregation at the surface of TiO2, enhanced charge recombination at the TiO2/electrolyte interface and the inherently narrow absorption below 600 nm. It has been found that

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the presence of electron rich donor groups produce red-shift in the absorption spectrum [8a,20] and introduction of longer alkyl/ alkoxy chains on the donor [21] or p-linker [22] can in principle retard the electron recombination and dye aggregation. Alternatively, externally added co-adsorbants [23] have been used to inhibit pep stacking interactions of the organic dyes at the expense of the dye loading. Ko and co-workers [24] have designed several organic dyes containing fluorene units in the donor part and found to efficiently function as sensitizer in DSSC. Difluorenylaniline moiety was found to display enhanced photo- and thermal-stability when compared to the corresponding triphenylamine-based dyes. Herein, we report two new organic dyes (Fig. 1) possessing trifluorenylamine donor and cyanoacrylic acid acceptor. We believe that the presence of fluorene moiety will be advantageous for several reasons: (a) Due to electron-richness and elongated conjugation fluorene will modulate the donor acceptor interactions and red-shift the corresponding CT transition. (b) The rigidity of the fluorene segment will suppress the vibrational relaxation pathways in the excited state [25]. (c) Furthermore, the alkyl chains on the fluorene nucleus may help to inhibit the dye aggregation and interfacial electron recombination processes. (d) Finally, the dye cations formed after electron injection is expected to be stabilized by fluorene unit due to delocalization of the charge and prolong the lifetime of the charge separated state. This will be beneficial for the electron injection from the dye to the TiO2 CB [26]. We have used thiophene units in the conjugation pathway to increase the CT transition probability and hence the molar extinction coefficients for the longer wavelength absorption. The light harvesting properties of the new dyes (JA1 and JA2) are also compared with the known dyes (D1 and D2) featuring diphenylaminofluorene donor [27]. 2. Experimental details 2.1. Materials and methods All the chemicals were commercially available and they were used without further purification. All the solvents were dried using standard methods prior to use. 1H NMR and 13C NMR were recorded on a spectrometer operating at 500.13 and 125.77 MHz, respectively. Deuterated chloroform (CDCl3) and dimethyl sulfoxide (DMSO-d6) were used as solvent. UVevis spectra were recorded at room temperature in quartz cuvettes using a spectrophotometer for dichloromethane (DCM). The cyclic voltammetry (CV) recorded on an electrochemical workstation in DCM by using 0.1 M tetrabutylammonium perchlorate as supporting electrolyte. The experiments were performed at room temperature with a threeelectrode cell consisting of a platinum wire as auxiliary electrode,

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a non-aqueous Ag/AgNO3 reference electrode and a glassy carbon working electrode. Mass spectra were recorded in positive-ion mode on an ESI TOF high-resolution mass spectrometer. 2.2. Synthesis 2.2.1. Synthesis of 7-bromo-N,N-bis(9,9-diethyl-9H-fluoren-2-yl)9,9-diethyl-9H-fluoren-2-amine (1) In a pressure tube bis(9,9-diethyl-9H-fluoren-2-yl)amine (1.1 g, 2.4 mmol), 2,7-dibromo-9,9-diethyl-9H-fluorene (1.82 g, 4.8 mmol) was mixed with sodium t-butoxide (0.35 g, 3.61 mmol), Pd(dba)2 (27.6 mg), 1,2-bis(diphenylphosphino)ferrocene, (26.6 mg) in toluene (10 mL) under nitrogen atmosphere. This was heated at 80  C for 36 h. After the completion of the reaction the volatiles were removed by evaporation. The residue was triturated with water and extracted with dichloromethane. The combined organic layer was dried over anhydrous sodium sulfate and evaporated in vacuum to produce a crude product. It was adsorbed on silica gel and purified by column chromatography by using hexane/dichloromethane mixture as eluant. White solid. Yield: 0.90 g (50%). mp 203e205  C. 1 H NMR (500 MHz, CDCl3) d 0.35e0.38 (m, 18H), 1.82e2.01 (m, 12H), 7.09 (d, J ¼ 7.5 Hz, 3H), 7.15e7.17 (m, 3H), 7.25e7.34 (m, 6H), 7.40e7.44 (m, 2H), 7.48 (d, J ¼ 8.0 Hz, 1H), 7.53e7.55 (m, 1H), 7.59 (d, J ¼ 8.5 Hz, 2H), 7.64 (d, J ¼ 7.5 Hz, 2H); 13C NMR (125.77 MHz, CDCl3) d 152.0,151.3,150.9, 149.7,148.1,147.4,141.4,140.5,136.6,135.1,130.0, 126.9, 126.4, 126.1, 123.2, 122.9, 122.8, 120.4, 120.3, 120.1, 119.1, 118.8, 118.3, 56.4, 56.1, 32.8, 32.7, 8.7. HRMS calcd. for C51H50BrN [M þ Naþ] m/z 788.3204 found 788.3200. 2.2.2. Synthesis of 5-(7-(bis(9,9-diethyl-9H-fluoren-2-yl)amino)9,9-diethyl-9H-fluoren-2-yl)thiophene-2-carbaldehyde (2a) A mixture of (5-(1,3-dioxolan-2-yl)thiophen-2-yl)tributylstannane (0.58 mmol) and 7-bromo-N,N-bis(9,9-diethyl-9H-fluoren-2-yl)-9,9-diethyl-9H-fluoren-2-amine, 1 (0.400 g, 0.52 mmol) were taken in dry DMF (4 mL) and degassed with nitrogen followed by the addition of Pd(PPh3)2Cl2 (4 mg). The reaction mixture was heated at 80  C for 24 h under nitrogen. After that, it was poured into water and extracted with dichloromethane. The organic layer was washed with brine solution followed by water and dried over anhydrous Na2SO4. The solvent was evaporated and the solid residue was dissolved in glacial acetic acid (5 mL). The acetic acid solution was stirred for 30 min at 60  C then water 10 mL was added. Heating was continued for 6 h. Then after cooling it up to room temperature water was added and extracted with dichloromethane. The organic layer was washed liberally with water and dried over anhydrous Na2SO4. After removal of solvent, the residue obtained was purified by column chromatography on silica gel using hexane/dichloromethane as eluant. Orange solid. Yield:

Fig. 1. Structures of the dyes.

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0.22 g (54%). mp 190e191  C. IR (KBr, cm1) 1656 (nC]O). 1H NMR (500 MHz, CDCl3) d 0.35e0.42 (m, 18H), 1.86e2.03 (m, 12H), 7.10e7.12 (m, 3H), 7.18e7.19 (m, 3H), 7.25e7.34 (m, 6H), 7.46 (d, J ¼ 3.5 Hz, 1H), 7.57e7.61 (m, 4H), 7.64e7.68 (m, 4H) 7.75 (d, J ¼ 3.5 Hz, 1H), 9.89 (s, 1H); 13C NMR (125.77 MHz, CDCl3) d 182.7, 151.8, 151.3, 150.8, 149.7, 148.4, 147.3, 143.2, 141.7, 137.6, 136.8, 135.0, 130.8, 126.9, 126.4, 125.7, 123.6, 123.2, 122.8, 122.7, 120.8, 120.5, 120.4, 119.5, 119.1, 119.0, 118.0, 56.3, 56.1, 32.8, 8.7, 8.6. HRMS calcd. for C56H53NOS [Mþ] m/z 787.3848 found 787.3828. 2.2.3. Synthesis of 50 -(7-(bis(9,9-diethyl-9H-fluoren-2-yl)amino)9,9-diethyl-9H-fluoren-2-yl)-[2,20 -bithiophene]-5-carbaldehyde (2b) It was obtained from (50 -(1,3-dioxolan-2-yl)-2,20 -bithiophen-5yl)tributylstannane (0.85 mmol) and 1 (0.50 g, 0.78 mmol) by following a procedure described above for 2a. Orange solid. Yield: 0.25 g (55%). mp 210e211  C. IR (KBr, cm1) 1660 (nC]O). 1H NMR (500 MHz, CDCl3) d 0.36e0.43 (m, 18H), 1.87e2.02 (m, 12H), 7.08e7.12 (m, 4H), 7.18 (d, J ¼ 4.0 Hz, 2H), 7.25e7.34 (m, 7H), 7.36 (d, J ¼ 4.5 Hz, 2H), 7.49e7.55 (m, 2H), 7.57e7.61 (m, 3H), 7.63e7.65 (m, 2H), 7.28 (dd, J ¼ 8.5 Hz, 4.0 Hz, 2H), 9.86 (s, 1H); 13C NMR (125.77 MHz, CDCl3) d 182.8, 155.4, 151.8, 151.3, 150.7, 149.7, 148.4, 147.3, 143.2, 141.7, 141.4, 137.6, 136.7, 135.0, 130.8, 126.9, 126.4, 125.7, 123.6, 122.8, 120.8, 120.5, 120.4, 119.5, 119.1, 118.9, 118.0, 56.3, 56.1, 32.8, 8.7, 8.6. HRMS calcd. for C60H55NOS2 [Mþ] m/z 869.3725 found 869.3718. 2.2.4. (E)-3-(5-(7-(bis(9,9-diethyl-9H-fluoren-2-yl)amino)-9,9diethyl-9H-fluoren-2-yl)thiophen-2-yl)-2-cyanoacrylic acid (JA1) 5-(7-(bis(9,9-diethyl-9H-fluoren-2-yl)amino)-9,9-diethyl-9Hfluoren-2-yl)thiophene-2-carbaldehyde, 2a (0.16 g, 0.20 mmol), cyanoacetic acid (0.020 g, 0.24 mmol), acetic acid (5 mL) and ammonium acetate (4 mg) were mixed together and kept on reflux at 120  C for 12 h. The resulting red solution was poured into icecold water to produce an orange precipitate. This was filtered and washed thoroughly with water and dried. The solid was further recrystallized with chloroform. Redebrown solid. Yield: 0.154 g (90%). mp 180e182  C. IR (KBr, cm1) 2213 (nC^N). 1H NMR (500 MHz, CDCl3) d 0.36e0.41 (m, 18H), 1.84e2.02 (m, 12H), 7.10e7.12 (m, 3H), 7.17e7.19 (m, 3H), 7.28e7.34 (m, 6H), 7.49 (d, J ¼ 4.0 Hz, 1H), 7.59e7.61 (m, 4H), 7.64e7.66 (m, 3H), 7.70e7.71 (m, 1H), 7.78 (d, J ¼ 3.5 Hz, 2H), 8.36 (s, 1H); 13C NMR (125.77 MHz, CDCl3) d 151.9, 151.3, 150.9, 149.7, 147.2, 141.3, 136.9, 134.9, 130.4, 126.9, 125.9, 123.4, 122.8, 122.6, 121.2, 120.9, 120.6, 120.4, 119.1, 119.0, 117.9, 56.4, 56.1, 32.8, 8.7, 8.6. HRMS calcd. for C59H54N2O2S [M þ H] m/z 855.3979 found 855.3970. 2.2.5. (E)-3-(50 -(7-(bis(9,9-diethyl-9H-fluoren-2-yl)amino)-9,9diethyl-9H-fluoren-2-yl)-[2,20 -bithiophen]-5-yl)-2-cyanoacrylic acid (JA2) It was prepared from 2b by following a procedure similar to that described above for JA1. Redebrown solid. Yield: 0.118 g (91%). mp 210e212  C. IR (KBr, cm1) 2217 (nC^N). 1H NMR (500 MHz, CDCl3) d 0.36e0.43 (m, 18H), 1.86e2.01 (m, 12H), 7.10 (d, J ¼ 8.0 Hz, 3H), 7.18 (s, 3H), 7.28e7.36 (m, 8H), 7.41e7.44 (m, 2H), 7.52e7.55 (m, 1H), 7.58e7.65 (m, 6H), 7.69e7.72 (m, 1H), 8.32 (s, 1H); 13C NMR (125.77 MHz, CDCl3) d 151.29, 151.26, 149.7, 148.0, 147.3, 141.4, 141.3, 138.9, 136.7, 134.42, 134.36, 132.5, 126.9, 126.4, 124.9, 123.2, 122.8, 120.7, 120.6, 120.3, 119.1, 118.9, 56.2, 32.8, 8.72, 8.65. HRMS calcd. for C63H56N2O2S2 [Mþ] m/z 936.3783 found 936.3755.

geometries were fully optimized without any symmetry constraints at the DFT level with Becke's three parameters hybrid functional and Lee, Yang and Parr's correlational functional (B3LYP) using 631G (D, P) basis set on all atoms. The default parameters for the convergence criteria were used. Vibrational analyses on the optimized structures were performed to confirm the structure. The excitation energies and oscillator strengths for the lowest 10 singlet transitions at the optimized geometry in the ground state were obtained by TD-DFT calculations using the same basis set as for the geometry minimization. 2.4. DSSC fabrication and characterization Screen printable paste was prepared by using mesoporous TiO2 synthesized by surfactant assisted method [28]. Certain quantity of TiO2 was mixed with two different types of ethyl cellulose, EC1 (5e15 mPa s) and EC2 (30e50 mPa s) in the proportion of 28 wt% and 46 wt% of titania respectively. Appropriate quantity of terpineol anhydrous was added carefully to obtain a viscous paste. A few drops of acetyl acetone were added to prevent re-aggregation of TiO2. Prior to screen printing the FTO substrates are pre-treated in 40 mM TiCl4 aq. Solution. The printed films were sintered at 500  C and then treated in 40 mM TiCl4 aqueous solution at 70  C for 30 min, followed by calcinations at 500  C for 30 min. After cooling down to 80  C, the film was soaked in dye solutions for at least 15 h and then washed with respective solvents and dried. The working and Pt-counter electrodes were assembled into a sealed sandwich solar cell with a 60 mm thick surlyn frame. Redox electrolyte (0.6 M 1, 2-dimethyl-3-n-propylimidazolium iodide, 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine, 0.5 guanidinium thiocyanate in acetonitrile:valeronitrile (85:15 mixture) was introduced through the pre-drilled holes on the back of counter electrode. Finally, the two holes were sealed using additional piece of melt Surlyn film covered with a thin glass slide. The dye adsorption was examined using the Shimadzu 1800 UVevis spectrophotometer by measuring the absorption spectra of the desorbed-dye solution in a 0.1 M NaOH aqueous solution. The JeV curves are recorded with a Keithley 2400 source meter under the illumination of Air Mass 1.5G simulated solar light coming from a solar simulator (Newport Instruments, USA equipped with a 1000 W Xe lamp and an AM1.5 filter). Incident monochromatic IPCE spectra were obtained with an IVT solar system (IVT solar Pvt. Ltd., Singapore). The incident light intensity was calibrated using a reference Si solar cell (Oriel-91150). The electron transport times at short circuit were measured by intensity modulated photocurrent spectroscopy (IMPS). The electron recombination times at open circuit were measured by intensity modulated photovoltage spectroscopy (IMVS). The measurements were carried out using a potentiostat equipped with a frequency response analyzer (Auto lab, PGSTAT302N). The DSSC were illuminated using a light-emitting diode (LED, 456 nm) as the light source. The dc component light intensity was superimposed with a small ac component (10% or less of the dc component), the frequency range was 1000 Hz 0.1 mHz and the dc light intensity was varied from 0.2 to 10 mW cm2. The light intensity was measured using a calibrated Si-photodiode. 3. Results and discussion 3.1. Synthesis and characterization

2.3. Computational methods All the computations were performed with the Gaussian 09 program package in an HP computer workstation. The ground-state

The new organic sensitizers containing trifluorenylamine donors were synthesized by following the procedure illustrated in Scheme 1. In the first step, bis(9,9-diethyl-9H-fluoren-2-yl)amine

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[29] was reacted with excess 2,7-dibromo-9,9-diethyl-9H-fluorene under the Pd-catalysed CeN coupling conditions [30] to obtain 7bromo-N,N-bis(9,9-diethyl-9H-fluoren-2-yl)-9,9-diethyl-9H-fluoren-2-amine (1) in reasonable yield. Then it was converted to the aldehydes 2a, and 2b by Stille coupling [31] reaction with the protected tin reagents of thiophene or bithiophene aldehydes followed by acid hydrolysis. Finally, Knoevenagel condensation of these aldehydes with cyanoacetic acid produced the desired dyes (JA1 and JA2). 3.2. Photophysical properties The absorption spectra of the dyes were recorded in dichloromethane and displayed in Fig. 2. The pertinent data are compiled in Table 1. Both the dyes display broader absorption spectra covering a wide wavelength range in the visible region. Both the dyes exhibited three prominent absorption peaks while the dye possessing elongated conjugation in the p-bridge, JA2 showed splitting for the middle peak. The absorption bands at the shorter wavelength region (<400) originate from the pep* electronic excitations localized within the trifluorenylamine and p-bridge segments. The band at the longer wavelength corresponds to the intramolecular charge transfer (ICT) transition from trifluorenylamine donor to the cyanoacrylic acid acceptor. The splitting of the middle peak in the region 370e450 nm for the dye JA2 is attributed to the development of new red-shifted absorption from the pbridge which possesses extended conjugation due to the bithiophene unit [1d,25]. This assignment is further supported by the fact that compound 1 showed absorption peak at 377 nm. Dye JA1 also displayed a peak in the same region but with slightly larger molar extinction coefficient. It is probable that the pep* transitions originating from the trifluorenylamine and p-bridge segments in JA1 are overlapping. However, on extension of conjugation for the

Fig. 2. Absorption spectra of the dyes (JA1 & JA2) recorded in dichloromethane solutions.

p-bridge in JA2 the pep* transition corresponding to the bridge is red-shifted and appeared as a separate peak at the longer wavelength side. Further, the origin of the CT transition is also evident on comparing the absorption spectra of the dyes JA1 and JA2 with their precursor bromo (1) and aldehyde (2) derivatives (Fig. S1). It is observed that a new longer wavelength absorption peak appears on introduction of aldehyde or cyanoacrylic acid unit on the trifluorenylamine core while retaining the pep* transition. Also, it is interesting to compare the wavelengths and molar extinction coefficients of the pep* and CT transitions with those of

Scheme 1. Synthetic pathway of the dyes JA1 and JA2.

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Table 1 Optical and electrochemical data of the dyes recorded in dichloromethane solutions. Dye

labs, nm (ε, M1cm1  103)

lTiO2, nm

Eox, V (DEp)a

HOMO, eVb

LUMO, eVc

E0e0, eVd

E0e0*,Ve

JA1 JA2 D1f D2f

492 513 469 487

451 492 471 505

0.27 0.25 0.41 0.31

5.07 5.05 5.21 5.11

3.02 2.99 3.05 3.03

2.05 2.06 2.16 2.08

1.01 1.04 0.98 1.00

a b c d e f

(24.1), 375 (45.5), 275 (23.5) (sh), 421 (53.1), 390 (54.3), 274 (30.1) (34.9), 353 (19.7), 310 (20.8) (40.5), 371 (26.3), 304 (20.1),

(54) (40), 0.74 (70) (43), 0.75 (104)

Redox potentials are reported with reference to the ferrocene internal standard. Deduced from the oxidation potential using the formula HOMO ¼ 4.8 þ Eox. Obtained from the optical band gap and the electrochemically deduced HOMO value. Calculated from optical edge. Excited-state oxidation potential versus NHE. Data from Ref. [27].

the dyes D1 and D2 [27]. The CT transitions in JA1 and JA2 are bathochromically shifted by 23 nm and 26 nm, respectively while the pep* transition shows hike in intensity and broadening when compared to the dyes D1 and D2. Generally, the CT transition in donor-acceptor compounds is influenced by the donor and acceptor strengths and nature of the conjugation bridge. Since the acceptor and p-spacer are similar in these dyes (D1, D2, JA1 and JA2), the red-shift in the CT transition probably originates from the variation in the donor strength attributable to the electron richness contributed by the fluorenyl unit [32]. The effect of solvent polarity on the CT transition is studied by measuring the absorption spectra in the solvents of different polarity (Fig. S2 and S3, Table S1). The dyes exhibited blue-shifted absorption in polar solvents indicative of a polarized structure in the ground state which is preferentially solvated by the polar solvents. A drastic blue-shift was observed in DMF. This may be reasoned to the basic nature of DMF that can cause deprotonation of carboxylic acid and thus weakens the donor-acceptor interactions [32g]. This assumption was further confirmed by the addition of triethylamine (TEA) or trifluoroacetic acid (TFA) to the dye solution in THF and DCM (Fig 3 & Fig. S4). The addition of TFA did not alter the CT absorption peak but on addition of TEA a substantial hypsochromic shift was observed. This indicates that the dye molecules are predominantly in the protonated state (DH) in these solvents. On the addition of TEA deprotonation occurs and the equilibrium shifts towards the deprotonated form (De) [32g]. The absorption spectra of the dyes anchored on TiO2 were also recorded and displayed in Fig. 4. Absorption edges observed for the

Fig. 3. Absorption spectra of the dye JA1 recorded in DCM/THF solutions after addition of TFA or TEA.

dyes in the solid state are very similar to that observed in dichloromethane solution. However, a slight blue-shift of the absorption onset observed for JA2 may probably originate from the deprotonation of the cyanoacrylic acid [33] on interaction with TiO2. Since JA2 possess comparatively elongated electron-rich conjugation in the p-spacer when compared to JA1, the observed blue-shift could be ascribed to the increased acidity of JA2 which may cause stronger interaction with TiO2 [34]. It is interesting to note that the dyes D1 and D2 [27] exhibited slight red-shift in contrast to the behaviour of JA1 and JA2. Probably, the voluminous nature of difluorenylamine unit in JA1 and JA2, when compared to the diphenylamine in D1 and D2 inhibits the close approach of the dyes on the surface of TiO2 [35].

3.3. Electrochemical properties Cyclic voltammetry was performed to ascertain the feasibility of electronic injection from the excited dye molecules into the CB of TiO2 and dye regeneration by the redox couple in the electrolyte. The cyclic voltammograms recorded for the dyes in dichloromethane solutions are displayed in Fig. 5 and data are listed in Table 1. Both the dyes displayed quasi-reversible one electron oxidation couple at the potential higher than the internal ferrocene standard. This originates due to the removal of electron from the trifluorenylamine unit. The oxidation potential for the dye JA2 is lower than the dye JA1. This is attributable to the change in the spacer unit from thiophene to bithiophene. The insertion of bithiophene unit increases the electron richness of the molecule and consequently facilitates the ease of molecular oxidation

Fig. 4. Absorption spectra of the dyes recorded on TiO2 thin films.

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3.4. Theoretical calculations

Fig. 5. Cyclic voltammograms of the dyes (JA1 & JA2) recorded in dichloromethane solutions.

[1d,25,32a,32e]. The oxidation potential of the dyes fall in the range of 1.02e1.04 V versus NHE which is favourable for dye regeneration from the electrolyte redox couple Ie/Ie 3 (ca.~0.4 V versus NHE) [36]. * are more negative (1.01 The excited state oxidation potentials Eox to 1.04 V versus NHE) than the CB edge level of TiO2 (0.5 V versus NHE) [37] and suggest a thermodynamically favourable electron injection from the photo-excited dye molecules into the CB of TiO2. It is noticeable that alternation of D1 and D2 by substitution of phenyl groups by fluorene units (JA1 and JA2) increases the electron richness of the donor unit and cathodically shifts the oxidation potentials marginally by 0.134 and 0.064 V respectively. This results in the negative shift in the ground and excited state oxidation potentials which are beneficial for the efficient injection of electrons into the CB of TiO2 and regeneration of the oxidized dye by the redox shuttle [38].

Fig. 6. Frontier molecular orbitals of the dyes computed using TDDFT/B3LYP/631G(D, P).

To understand the electronic structure and the resultant optical and electrochemical properties density functional theoretical (DFT) [39] calculations were performed on the dyes. The computations were performed using the correlation functional, B3LYP [40] with 6-31G (D, P) basis set. The electronic distributions in the frontier molecular orbitals of the selected dyes are displayed in Fig. 6 and the computed energies of the vertical transitions are collected in Table S2. In the dyes, the highest occupied molecular orbital (HOMO) is generally contributed by the trifluorenylamine unit and the lowest unoccupied molecular orbital (LUMO) by the acceptor unit (cyanoacrylic acid) and spread up to the thiophene/bithiophene linkage toward the amine segment. The presence of such well separated HOMO and LUMO at the two terminal ends of the molecule is characteristic of a donor-acceptor compound exhibiting charge migration from donor to acceptor on electronic excitation. Thus, the HOMOeLUMO excitation induced by light irradiation could move the electron distribution from the triarylamine part to the cyanoacrylic acid segment. Thus, the absorption of light by the dyes in the wavelength range 400e600 nm is expected to lead to charge migration from the trifluorenylamine donor part to the cyanoacrylic acceptor end and subsequent electron injection into the CB of TiO2. TDDFT computations using B3LYP gave significantly red-shifted absorption peaks. The overestimation of CT transitions in dipolar compounds at the B3LYP level is quite expected [41]. However, the qualitative trends in the absorption peak positions and intensities matched well with the calculated vertical transitions. As mentioned in the earlier paragraph, the longer wavelength vertical transition in the dyes originates from HOMO to LUMO electronic excitations with reasonable oscillator strength. Therefore the longer wavelength absorption for the dyes may be assigned to CT transition. Since the shorter wavelength absorptions have contribution from HOMO-1 to LUMO and HOMO-2 to LUMO or HOMO to LUMOþ2, they can be attributed to pep* transition localized within the pspacer and trifluorenylamine, respectively. 3.5. Photovoltaic performance of the dyes The photovoltaic characteristics of the sensitizers have been investigated by fabricating DSSC by adsorbing the dyes on TiO2 from the dichloromethane bath solutions and using 0.6 M 1, 2dimethyl-3-n-propylimidazolium iodide, 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine, 0.5 M guanidinium thiocyanate in acetonitrile:valeronitrile (85:15) mixture as redox electrolyte. The incident photon conversion efficiency (IPCE) spectra as a function of excitation wavelength are presented in Fig. 7a. In the whole action spectrum, the IPCE curve of JA2 lies higher than that of JA1 indicating superior light harvesting efficiency (LHE) of JA2 dye in DSSC. Compared with JA1, the IPCE spectra of DSSC based on JA2 dye is remarkably broader in the visible region which could be due to the presence of bithiophene unit in p-conjugation that enhances the molar extinction coefficients and spectral response in the visible region [42]. Despite, the broader absorption spectrum of JA1 dye adsorbed on TiO2 and higher dye concentration, the observed lower IPCE values are probably due to dye aggregation on TiO2 surface which leads to self-quenching of excited states and hence inefficient electron injection. Therefore, it is prospective that a DSSC based on JA2 dye shows the highest photovoltaic (PV) performance in terms of light harvesting and thus the higher photocurrent. The PV parameters under AM1.5G illumination are summarized in Table 2. Currentevoltage curves recorded under light and in dark are shown in Fig. 7b where a correlation between the structure of

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It is interesting to point out here that the dye JA2 exhibited superior photovoltaic performance than that of the previously reported dye D2, while the power conversion efficiency of the dye JA1 is nearly same as that of the dye D1. The main reason for the enhancement of efficiency for the DSSCs fabricated using JA dyes is their improved VOC, JSC and FF. It is believed that the introduction of fluorene in the donor suppresses the aggregation tendency and increases the donor strength when compared to the diphenylamine donor derivatives. Due to these factors the electron recombination is hindered and the donor-acceptor interactions are pronounced and hence results in high VOC and JSC [44]. Additionally, the voluminous nature of difluorenylamine unit may help to improve FF. Further, to account for these observations, CB edges and electron life time are measured by IMPS/IMVS analyses of JA1 and JA2 based DSSC. Fig. 8a shows the relation between extracted charge density, Q and VOC. The VOC of a DSSC is primarily related to CB position of TiO2 and the charge recombination rate [1d], [42]. At a certain Q, the VOC of JA2 based DSSC is higher by 40 mV compared to JA1 based DSSC, indicating a negative shift of CB [44]. In other words, the downward movement of CB can be observed for JA1 based DSSC which could have been caused by the presence of more protons on TiO2 surface due to its much higher dye loading [45] (JA1 ¼ 53.7 mmol cm3, JA2 ¼ 44.1 mmol cm3). On the other hand, as we have seen in Fig. 8b the electron lifetime for JA1 based DSSC is lower by 4 times. Fig. 8b shows the lifetime of the electrons investigated at open-circuit conditions using IMVS as a function of VOC for DSSCs with JA1 and JA2 dyes. At the same VOC points, te for DSSC based on JA2 is nearly four times

Fig. 7. (a) IPCE action spectra of the DSSCs employing JA1 & JA2 and (b) corresponding currentevoltage curves measured in light and dark. The thickness of TiO2 films was 14 mm.

two dyes and their DSSC performance is clearly depicted. The JA1 based DSSC showed a conversion efficiency (h) of 4.65% corresponding to a short-circuit current density, JSC of 9.69 mA cm2, an open circuit voltage, VOC of 664 mV, and a fill factor, FF of 0.70. Under similar conditions, JA2 based DSSC exhibited a higher h of 5.80% corresponding to JSC of 11.71 mA cm2, a VOC of 709 mV, and an FF of 0.71. The higher JSC for JA2 is inconsistent with the higher IPCE values shown in Fig. 7a. In a comparison, the VOC of JA1 based DSSC is 45 mV lower than that of the JA2 based DSSC. This can be ascribed to a higher degree of electron recombination with I3 ion in electrolyte as indicated by large dark currents measured for JA1 based DSSC [43]. Therefore, the poor performance of JA1 based DSSC would have been caused by the electron recombination owing to aggregation of its molecules on TiO2 surface. Table 2 Photovoltaic performance parameters for DSSC using JA1 & JA2 under simulated AM1.5G full sun illumination (100 mW cm2)a. Dye JSC, mA cm2 VOC, mV FF, % JA1 JA2 a

9.69 ± 0.07 11.71 ± 0.07

h, %

Amount of dye adsorbed, mmol cm3

664 ± 1 69.60 ± 0.45 4.65 ± 0.03 53.7 709 ± 2 70.86 ± 0.14 5.80 ± 0.04 44.1

Average values are based on data obtained for three similar DSSC devices.

Fig. 8. (a) Dependence of VOC on photo-induced charge density (Q) and (b) electron lifetime (te) as a function of VOC.

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larger than that of DSSC based on JA1, indicating retarded recombination for JA2. Since both the dyes have almost similar structure (trifluorenylamine donor and cyanoacrylic acid acceptor) except the composition of the p-linker (thiophene or bithiophene), the different electron lifetimes indicates that the linker length of the sensitizers plays an important role on the packing of the dyes on TiO2 surface. The long conjugation pathway present in JA2, places the donor further away from TiO2, thus providing a better antiaggregation capability [46]. Therefore, slower is the recombination kinetics in the device and longer the electron life time. The direct result of suppressed recombination is an upward lift of CB edge due to accumulation of more charge and VOC gets larger. [44] Besides the contribution to VOC gain, the retarded recombination of photo-injected electrons could enhance the collection efficiency by reducing electron losses during transport. As a consequence, higher IPCE or JSC can be observed. Thus, the relatively better performance of JA2 based DSSC can be explained on the basis of effective donor-acceptor interactions, stronger electronic coupling with the nanostructured TiO2 and retardation of electron recombination between the oxidized dye and electrons in CB of TiO2. 4. Conclusions We have synthesized metal free organic dyes containing trifluorenylamine donor, cyanoacrylic acid acceptor and fluoreneoligothiophene linker as sensitizers for DSSC. The optical and electrochemical studies of these sensitizers indicate that they possess red-shifted absorption and cathodically shifted oxidation when compared to the reference dyes D1 and D2 featuring similar segments in the conjugation but diphenylamine donor end in place of difluorenylamine. Secondly, the use of oligothiophene unit provided a method to fine tune the electronic properties and recombination kinetics in these dyes. It is found that the conjugation bridge composed of fluorene and bithiophene served as efficient plinker and showed stronger donor-acceptor interactions which favoured the upward shift of CB of TiO2 and rendered hindrance to the distance dependant electron recombination process. As a result of the combination of above factors, the DSSC based on JA2 exhibited high power conversion efficiency attributed to high VOC and high JSC. Our studies reveal that the use of fluorene moiety in the donor as well as linker provides an opportunity to achieve higher extinction coefficient with red-shifted absorption, suitable electronic structure and reasonable steric properties beneficial for photocurrent generation in DSSC.

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Acknowledgments KRJT is thankful to Department of Science and Technology, New Delhi, India for financial support (Ref. No. DST/TSG/PT/2013/ 09). AB acknowledges a senior research fellowship from Council of Scientific and Industrial Research (9/143(0810)/12-EMRI), New Delhi. SRG acknowledges NUS, Singapore for research scholarship and financial support from the MoE, Singapore (WBS R-265000-274-112).

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