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Electrochemical and Spectroscopic Properties of BODIPYThiophene-Triphenylamine Based Dyes for Dye-Sensitized Solar Cells Nelly Kaneza , Jingtuo Zhang, Haiying Liu, Archana Sathyaseelan Panikar, Zhichao Shan, Monica Vasiliu, Seth H. Polansky, David A Dixon, Rebecca E Adams, Russell H. Schmehl, Arunava Gupta, and Shanlin Pan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b01611 • Publication Date (Web): 11 Apr 2016 Downloaded from http://pubs.acs.org on April 17, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Electrochemical and Spectroscopic Properties of BODIPY-ThiopheneTriphenylamine Based Dyes for Dye-Sensitized Solar Cells Nelly Kaneza1, Jingtuo Zhang2, Haiying Liu2, Archana Panikar1, Zhichao Shan1, Monica Vasiliu1, Seth H. Polansky1, David A. Dixon1, Rebecca E. Adams3, Russell H. Schmehl3, Arunava Gupta1, and Shanlin Pan1*

1. Department of Chemistry, The University of Alabama, Tuscaloosa, AL 35487 2. Department of Chemistry, Michigan Technological University, Houghton, Michigan 49931 3. Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States *Corresponding Author: [email protected]; Tel: 1-205-348-6381

Abstract Due to the current increase in consumption of fossil fuels and its negative impact on environment, clean energy technologies such as solar cells are highly desirable to address this global energy challenge. Among these, dye-sensitized solar cells (DSSCs) have emerged as potential substitutes to traditional silicon based solar cells. In this study, a series of boron dipyrromethene (BODIPY)-based dyes (1-5) which contain thiophene and/or triphenylamine (TPA) as redox relays of chromophorebridges are synthesized and characterized using electrochemical and optical spectroscopic methods for their potential applications in DSSCs. Their electrochemical and photophysical properties are investigated and compared with the computational results. DSSCs made of these BODIPY-based dyes exhibit incident photon-tocurrent conversion efficiencies (IPCE) that correspond to their absorption profiles. BODIPY dye 5 bearing TPA provides the highest power efficiency because of its reversible redox activities,

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while the dyes bearing thiophene yield decrease in overall solar cell efficiency because of the irreversible oxidation and electropolymerization of thiophene. Despite their low overall conversion efficiencies, these dyes show interesting structural dependence in their DSSC performance. TiO2 electrodes loaded with these BODIPY dyes are characterized by XRD, XPS, and FTIR to illustrate the surface bonding characteristics of these dyes. 1. Introduction Solar cell technology is a critical component in the search for viable renewable energy resources to satisfy the future global energy consumption needs. In 2000, the energy consumption was reported to be 13 terawatts (TW) and has been projected to double (28TW) in 2050.1,2 DSSCs are an attractive solar energy conversion technology due to their low cost, environment friendliness, and relative ease of fabrication.3,4 First introduced by O’Regan and Grätzel,5 an efficient dye-sensitized solar cell, in contrast to conventional solar cells, fulfills both the optical absorption and charge transfer requirements by using a sensitizer and a nanocrystalline semiconductor with a wide band gap. The sensitizer, an organic and/or inorganic dye, is used as a light absorbing material when attached to the semiconductor, in this case TiO2 anatase. TiO2 has many advantages such as optical transparency and optimal electronic levels for charge transfer,6 which allows efficient charge transfer into the semiconductor upon light absorption followed by charge collection by a conductive electrode (e.g., fluorine-doped-tinoxide (FTO) glass). The sensitizer is then restored to its original state by harvesting an electron from redox mediators (e.g., the iodide/triiodide couple) in the electrolyte solution. The oxidized redox mediator is in turn restored at the counter electrode of the DSSC.7 In short, a DSSC can produce electricity instantly from sunlight, although its power efficiency may suffer from irreversible chemical transformations and the difference in redox potentials of dyes and redox 2 ACS Paragon Plus Environment

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mediators. The efficiency of DSSCs is determined by the structure of the photosensitizer as the latter determines factors such as light harvesting, electron injection, dye regeneration, stability, and charge recombination.8 Previous studies have reported DSSCs with power conversion efficiencies up to 13% under AM 1.5 of irradiation9. Improvements would help to open the door for the use of DSSCs for practical applications in competition with their inorganic counterparts. Many DSSCs based on ruthenium (Ru) organometallic compounds have been reported;10 however, since Ru is expensive and rare, the use of metal-free organic dyes offers greater practical use.11 Stimulated by recent developments in the use of fluorescent dyes in multidisciplinary areas, the class of boron dipyrromethene (BODIPY) dyes has received particular attention due to their versatility.12 Numerous BODIPY-based dyes have been investigated and commercialized because of their strong absorption in the visible spectral region, easily tunable emission, good solubility, high fluorescence quantum yields, small Stokes shift, and photochemical stability.13 BODIPY-based compounds have been proposed as efficient candidates in a wide range of applications, for example, fluorescent labels for bio-imaging, laser dyes, light-emitting devices, photosensitizers, and chemosensors.14,15,16,17Additionally, they exhibit a strong correlation between their photophysical and spectroscopic properties and the nature and location of their substituents.18,19 Possessing all these characteristics, BODIPY dyes have merged as promising photosensitizers in dye-sensitized solar cells (DSSCs). Although BODIPY-based DSSCs show a lower power efficiency of 6.06 %20 as compared to porphyrin-based ones (13%) 9, progress is being made in order to improve their efficiencies. Recent studies have focused on the development of BODIPY dyes with a donorchromophore -acceptor system (Figure 1) to increase the light-harvesting properties.21,22,23 A donor is usually composed of a redox-active molecule with electron-donor ability such as



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thiophene24 or triphenylamine (TPA)25 to facilitate electron delocalization. Due to its aromaticity and preferred electronic characteristics, thiophene is known to extend conjugation in DSSC dyes. In a similar way, TPA also increases conjugation and has reversible redox activity to facilitate efficient charge transfer.25 The donor is then linked by a BODIPY moiety to the acceptor, an anchoring group such as cyanoacetic acid that binds to TiO2.26,27

Figure 1: Schematic of a donorchromophore- acceptor system for solar energy harvesting and conversion.

This study investigates five BODIPY-based dyes (Figure 2) which contain redox relays of thiophene and/or TPA as efficient photosensitizers for efficient charge transfer and other benefits based on the above consideration. These dyes possess different conjugation configurations and redox properties through incorporating different numbers of thiophene and/or triphenylamine moieties onto the conjugation. Steady-state fluorescence, steady-state and transient absorption, electrochemistry, device characteristics, and the nature of their adsorption onto the surface of the TiO2 electrode are reported. Measured energy levels of these dyes are compared with results calculated from molecular geometries optimized at the density functional theory (DFT) level. 4 ACS Paragon Plus Environment

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2. Experimental

Figure 2: Chemical structures of BODIPY-thiophene-TPA-dyes (1-5) 2.1 Materials Preparation. All chemicals and solvents were purchased and used as received without any further purification. Cyclic voltammetry (CV) measurements were carried out in a glovebox under a N2 atmosphere condition. The dye solutions were prepared in dimethylformamide (DMF) or dichloromethane (DCM) for photophysical or electrochemical characterizations, respectively. 5 ACS Paragon Plus Environment


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2.2 Synthesis of BODIPY-Thiophene-TPA Based Dyes. The detailed synthetic routes and syntheses of BODIPY-thiophene-TPA based dyes (1-5) and their intermediates are presented in the Supporting Information. The general procedure of forming 1-5 via Knoevenagel condensation is described as follows. The precursor aldehyde g, h, i, j or k (see Scheme S1 for compounds numbers) (1 eq.) and cyanoacetic acid (10 eq.) were dissolved in a mixture of benzene, piperidine and acetic acid, and the mixture was stirred for 1.5 hours in 95 oC. Any water formed during the reaction was removed azeotropically by using a Dean-Stark apparatus. After the reaction was quenched by water at room temperature, the mixture was concentrated under reduced pressure and redissolved in dichloromethane. It was then washed in the order by saturated NH4Cl solution, water, and brine solution; dried over Na2SO4; and filtered. The filtrate was concentrated under reduced pressure and the crude product was purified by preparative TLC plates to obtain BODIPY-thiophene-TPA based 1-5. Detailed results of NMR characterization of all BODIPY dyes 1-5 and their intermediates are summarized in Supporting Information as listed in Figure S1-S28. 2.3 DSSC Fabrication. The DSSC consisted of a counter electrode, an organic electrolyte (I-/I3-), a parafilm spacer, and a dye-sensitized semiconductor (TiO2) electrode. Firstly, TiO2 paste was made by grinding 0.5 g of TiO2 powder (P25) with 6 drops of nonionic surfactant Triton-X and 2 drops of tetraethyl orthosilicate in an agate mortar. Secondly, TiO2 films with the geometric area of 0.5 × 0.5 cm2 were prepared by casting the TiO2 paste using the doctor blading technique28 on a FTO glass conductive glass. The TiO2 coated FTO substrates were then annealed at 500 °C for 30 minutes to form TiO2 electrodes. Lastly, after cooling to room temperature, the TiO2 electrodes were immersed in DMF solution containing 0.3 mM dye overnight to form dye 6 ACS Paragon Plus Environment

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sensitized electrodes at 25 °C. They were then taken out and rinsed with ethanol to remove any excess dye and dried in air at room temperature. The counter platinum (Pt) electrode was prepared by sputtering a 100 nm Pt on a cleaned FTO glass. The cell was then filled with a liquid electrolyte made of 0.1 M anhydrous lithium iodide (LiI), 0.12 M iodine solution (I2), 1.0 M 1, 2dimethyl-3-n-propylimidazolium iodide, and 0.5 M tert-butylpyridine in dehydrated acetonitrile. Two devices of each dye were fabricated and tested under 1.5 air mass (AM) irradiation (100 mW/cm2) at 25 °C. 2.4 Characterization of the Dyes. The absorption spectra of the dyes in DCM solution were obtained by using a Perkin Elmer Lambda 35 UV/Vis spectrometer. Emission spectra of the dyes were measured on a Jobin Yvon Fluoromax-4 spectrofluorometer. The electrochemical properties of the dyes were investigated on a CHI 670C potentiostat workstation. A platinum disk working electrode, a graphite counter electrode, and silver wire reference electrode calibrated against a SCE reference electrode were used with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte and DCM as the solvent. The photocurrent-voltage properties and the action spectra of the fabricated DSSCs were measured using a home-built solar cell composed of a Keithley 2400 source meter and a solar simulator with a power intensity of 100 mW/cm2 from a Newport 66902 Xenon Arc lamp modified with an Oriel 1.5 AM spectral filter. The overall power conversion efficiency (ƞ) and the incident photon-to-current efficiency (IPCE) value of each DSSC were calculated using the following equations5:

ƞ% =

× ×

% =

× 100

× ×

(1)

× 100

(2) 7 ACS Paragon Plus Environment


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Where VOC is the open circuit voltage; JSC is the short circuit current density; FF is the fill factor; Pin is the incident simulated solar light; and λ is the excitation wavelength. The nature of the binding between the anchoring group (-COOH) of this class of BODIPY-thiophene-TPA dyes and the TiO2 surface was investigated using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The XRD patterns of the dyes adsorbed onto the TiO2 were obtained using a Bruker XRD system, and a Kratos XIS 165 system was used to determine their XPS spectral peaks. The nature of the bond formed between the COOH group of the dye and the TiO2 surface was investigated using Diffuse Reflectance Infrared Fourier Transform (DRIFT) Spectroscopy. DRIFT measurements were done using The Praying MantisTM setup inside a Bruker-Vertex 70 FT-IR instrument. 2.5 Transient Spectra for Charge Carrier Dynamics and Spectroelectrochemistry. Transient spectra for kinetic analysis of electron transfer were obtained from an Applied Photophysics LKS 60 optical system with an OPOTEK optical parametric oscillator pumped by a Quantel Brilliant Laser equipped with doubling and tripling crystals. Excitation of the dye-sensitized TiO2 films (10 micron thick on ITO) was achieved using 470 nm or 480 nm light (7.5 to 9 mJ/pulse; < 4 ns pulse). The ITO substrate was placed at a 45 degree angle relative to the excitation and analyzing light sources in a 1 cm path cuvette (laser scatter directed away from monochromator). Spectra were collected as 100 µs transient decays using 1 mm slits on the monochromator. Data were collected with linear oversampling on a 600 MHz Agilent Infiniium oscilloscope and averaged over two laser pulses. All kinetic measurements were done under an argon or nitrogen atmosphere. The Kohlrausch-Williams-Watts function (KWW, below), also ∆ = ∆0exp −$#%

(3)


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known as the stretched exponential function, was used to fit decays in which a distribution of rate constants contributed to the overall decay.30 The parameter β is inversely proportional to the width of the distribution at values between 0 to 1; a higher β value represents a more narrow distribution of rate constants, while a lower β value represents a broader distribution of rate constants. Commonly, a mean lifetime value is calculated from the fit using a gamma function -1

distribution of β as shown below. #&'' = #( )* Γ( )*

(4)

Spectroelectrochemical analysis for charged species was carried out using CH Instruments 630 electrochemical workstation for potential control and an Ocean Optics USB 2000 spectrophotometer. For measurements done in a 0.1 M TBAPF6 DMF solution, a Pine Instruments platinum honeycomb spectroelectrochemical electrode card provided both the working and counter electrodes in a 0.2 cm path length cell. For measurements done on a 10 micron thick TiO2 films, ITO/TiO2 served as the working electrode and platinum wire as counter electrode. The supporting electrolyte was either TBAPF6 or LiClO4 in acetonitrile. Potentials are referenced to a Ag/AgCl electrode. 2.6 Computational Approaches. The molecular geometries were optimized at the density functional theory (DFT)31 level with the hybrid B3LYP32,33exchange-correlation functional and the DFT-optimized DZVP2 basis set for all atoms except I where we used the DZVP basis set.34 Vibrational frequencies were calculated to show that the structures were minima. Time dependent-DFT (TD-DFT) calculations35,36 were performed to analyze the UV-Vis spectra at the same computational level. The calculations were performed in the gas phase and in DMF as a solvent, the latter by using a self-consistent reaction field approach37 with the COSMO



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parameters.38,39 All calculations were done with Gaussian 09.40Although the B3LYP TD-DFT values agree reasonably well with the experiment, in order to potentially improve the agreement with the experiment. We predicted the spectra in DMF with different functions following other workers.41424344We used the CAM-B3LYP,45 BMK,46 and PBE47functionals. 3. Results and Discussion 3.1 Optical Spectroscopic Characteristics. UV-Vis spectra of all dyes in solution are shown in Figure 3 A and B. The corresponding fluorescence spectra in Figure 4 show weak emission in the range of 570 ‒ 700 nm because of the charge delocalization from BODIPY core to subunits of TPA and/or thiophene, and the COOH group. Dye 5 showed two unexpected emissions higher in energy probably because of the molecular subunit emission due to its poor conjugation yielding more rotation freedom for TPA unit. All these optical results are summarized in Table 1. Fluorescence quantum yields were calculated using Rhodamine 6G (R6G) as a standard reference with a fluorescence quantum efficiency of 95% in ethanol.48 As expected, the BODIPY-thiophene-TPA dyes showed intense absorption in the UV (π to π* transitions) and visible region (S0 to S1 transitions). The absorption maxima of thiophene substituted BODIPY dyes (1 - 4) are more red-shifted than 5, due to charge delocalization from BODIPY core to form a reduced gap between the HOMO and LUMO levels.24 The molar extinction coefficients at the maximum absorbance are in the order of 5 ˂ 2 ˂ 1 ˂ 4 ˂ 3; with the dyes with two thiophene moieties (3 and 4) exhibiting higher molar extinction values than 2 and 1 which only has one and 5 which has none. The enhanced red shifting and high molar extinction of 3 suggests an increase in the DSSC device performance in the near infrared region since more red photons are expected to be harvested. This might be a result of better electronic communication from the planar structure of thiophene and the redox stability of


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TPA.24,25 The absorbance of dyes adsorbed onto TiO2 (Figure S31) leads to a blue shift and a broadening of the spectra, most likely due to the interaction between the dyes and the TiO2 surface. 4 1

5

(A) Normalized Absorbance

3

2

1.0 0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

800

Wavelength (nm)

1 5

(B)

2

3 4

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Normalized Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8

0.6

0.4

0.2

0.0 300

400

500

600

700

800

Wavelength(nm)

Figure 3: (A and B) Normalized absorbance spectra of (1-10 µM) BODIPY-thiophene-TPA dyes (1-5) in dichloromethane (DCM) (A) and dimethylformamide (DMF) (B).



1 5

Normalized Fluorescence Intensity

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4

2

3

1.0 0.8 0.6 0.4 0.2 0.0 600

650

700

750

800

Wavelength (nm) Figure 4: Normalized fluorescence spectra of (1-10 µM) BODIPY-thiophene-TPA dyes (1-5) in dichloromethane. Table 2 summarizes the calculated and experimental energies for the HOMO and LUMO. The experimental LUMO is estimated from the CV reduction potential measurements and the experimental HOMO is estimated from the UV-Vis spectrum or from the CV oxidation potential measurements. The calculated HOMOs (BMK, see the B3LYP values in the Supporting Information) become slightly more bound in the solvent as do the LUMOs. As a consequence, the calculated HOMO-LUMO GAPs do not have a strong solvent dependence. The calculated LUMOs in the solvent follow basically the same trends as the estimated experimental values with the LUMO for 5 being the least stable. The calculated HOMOs do not follow the same trends as the estimated experimental values and the calculated range for the HOMOs is more than double that from experiment. Figure 5 shows the two highest energy occupied MOs and the


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two lowest energy unoccupied MOs of 5. The calculated molecular orbitals for 1-4 can be found in Figure S29.

Figure 5: Calculated two highest occupied and two lowest unoccupied molecular orbitals for Bodipy dye 5.

We initially calculated the absorption spectra with the B3LYP functional using TD-DFT and the maximum deviation from experiment for the most intense bands was 0.4 eV. In addition a long wavelength weak transition to the red of the first large peak was sometimes found. A more detailed discussion of the B3LYP results is given in the Supporting Information. The best overall agreement with experiment for the first peak was with the BMK functional and the differences between the calculated values and experiment are now0.15, 0.23, 0.12, 0.14, and 0.1eV for 1, 2, 13 ACS Paragon Plus Environment


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3, 4, and 5 respectively. All of the calculated values are consistently blue-shifted from experiment. We note that there are now no additional peaks to the red of the main peak. At the BMK level, the largest error in the predicted transition is 0.23 eV, which we consider to be good agreement with experiment. For 1, 3, and 5, the first transition is a combination of the HOMO → LUMO and HOMO-1 → LUMO and for 2 and 4, the first transition is the HOMO → LUMO. The calculated values for the most intense transitions are in the order of 5 ˂ 1 ˂ 2 ˂ 3 ˂ 4 with an inversion in the order of 3 and 4 and 1 and 2 as compared to experiment. The higher energy peaks starting at about 460 nm are composed of a significant number of transitions. The HOMOs in 2 and 4 are localized on the carbons of the central BODIPY chromophore as are the LUMOs consistent with the HOMO-LUMO being the most intense transition. For 1, 3, and 5 the HOMO is localized on the acetylene-phenyl group and the LUMO is localized on the BODIPY moiety as in 2 and 4. The HOMO-1 in 5 is localized on the BODIPY with some acetylene character so it is similar to the HOMOs of 2 and 4. The HOMO-1 in 1 has some component on the BODIPY core but also has contributions from the C=C connecting to the 5-member thiophene ring and contributions on the thiophene ring. The HOMO-1 in 3 with two thiophene rings resembles the HOMO-1 in 1 except that there are contributions from each thiophene. The dominant components of the intense first transition are localized in the BODIPY chromophore.


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Table 1. Spectroscopic and electrochemical properties of BODIPY-thiophene-TPA dyes 1-5 Dyes

1 2 3 4 5 a

E1/2/ V vs. SCE Reduction

Oxidation 1

Oxidation 2a Oxidation 3

λabs/nm in DCM

-1.04 -1.04 -0.99 -0.97 -1.19

0.55 0.55 0.62

Irr.(Epa:0.90) Irr.(Epa:0.92) Irr.(Epa:0.93) -

613, 641 584, 624 699 672 555

1.23 1.24 1.19

λabs/nm in DMF 645 615 695 700 560

λem/nm in DCM

εmax/10⁴ M¯¹cm¯¹

ɸf (%)

653 657 762 703 572, 617

5.72 5.02 7.76 7.42 3.55

< 0.1 2.1 < 0.01 1.0 < 0.1

Irr: oxidation peak is irreversible with missing reduction peak; Epa: peak position of oxidation reaction;

Table 2. Experimental and calculated HOMO and LUMO energies (gas/DMF) in eV Calc Calc Expt Expt Calc Expt Calc LUMO HOMOa HOMO HOMO-1 LUMOb LUMO+1 GAP -5.71 -5.60/-5.67 -6.03/-6.18 -3.48 -2.58/-2.75 -1.42/-1.74 2.23 1 -5.50 -6.06/-6.16 -7.19/-7.18 -3.48 -2.74/-2.79 -1.43/-1.74 2.02 2 -5.72 -5.52/-5.64 -5.88/-6.03 -3.54 -2.65/-2.88 -1.39/-1.76 2.18 3 -5.40 -5.87/-5.99 -6.57/-6.65 -3.55 -2.80/-2.91 -1.48/-1.76 1.85 4 -5.67 -5.60/-5.68 -6.50/-6.61 -3.33 -2.55/-2.59 -1.49/-1.70 2.34 5 GAP = LUMO-HOMO GAP1 = LUMO-(HOMO-1) a,b Values were calculated using the absolute potential of the standard hydrogen electrode E= 4.28 V49 Dye


Calc GAP 3.02/2.92 3.32/3.37 2.86/2.76 3.07/3.07 3.05/3.09

Calc GAP1 3.45/3.43 3.23/3.15 3.95/4.02


0.015

(A)

Current Intensity (mA)


0.010

1

0.005 0.000 -0.005 -0.010 -0.015

(B)

0.010

0.005

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0.000

-0.005

-0.010

-0.020

-0.015 -0.025 1.5

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Potential (V vs. SCE)



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0.00 -0.01 -0.02 -0.03 -0.04

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(D) 0.02

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5

0.005

0.000

-0.005

-0.010

-0.015

-0.020 1.5

1.0

0.5

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-0.5

-1.0

-1.5


Figure 6: Cyclic Voltammograms of 4.4 mM BODIPY-thiophene-TPA dyes (1-5) measured in dichloromethane with 0.1 M TBAPF₆ at a scan rate of 100 mV/s.


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3.2 Electrochemical Properties. CV was used to determine the redox potentials of all the dyes and their energy levels. The quasi-reversible oxidation waves (Eox) were used to estimate the 0.010 0.006

(A)

(B)

0.004



0.005 0.002

50 mV/s

0.000 -0.002 -0.004 -0.006 -0.008

100 mV/s 0.000

-0.005

-0.010

-0.010 1.0

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-0.025 -0.020 1.0

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0.5

0.4

0.3

Potential (V s. SCE)


0.02


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(E)

0.01

1 V/s

0.00

-0.01

-0.02

-0.03 1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3


Figure 7: Experimental (solid) and simulated (dashed) Cyclic Voltammograms of 0.0044 mM of BODIPY dye 5 first oxidation at scan rates of 50 mV/s (A), 100 mV/s (B), 250 mV/s (C), 500 mV/s (D) and 1V/s (E). Experimental data measured in dichloromethane as the solvent and 0.1 M of TBAPF6 as the supporting electrolyte. Simulated data is summarized in Table S1. Working electrode: Platinum disk (Area: 0.0314 cm2/s), reference: SCE electrode, and a graphite counter electrode.



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HOMO energies whereas the quasi-reversible reduction waves (Ered) were used to estimate LUMO energies. Although during CV measurements, the dyes are in solution contrary to when they are in solar devices (when they adsorbed onto the TiO2 surface), the energy levels estimated from the CV are important to understand the electron transfer in these dyes. As shown in Figure 6, these BODIPY-thiophene -TPA dyes showed two or three oxidations (depending on whether TPA and/or thiophene are present) and one-electron reversible reduction. The first reversible oxidation around 0.5 V vs. SCE shows the oxidation of TPA (1, 3 and 5), the irreversible second oxidation around 0.9 V vs. SCE results from the presence of thiophene (1, 2 and 4). Thiophene is known to go through electrochemical polymerization24,50 which explains its irreversible oxidation wave. The third oxidation around 1.2 V vs. SCE is the oxidation of the BODIPY core. In addition to this, all the BODIPY-thiophene-TPA dyes showed reduction of the BODIPY core around -1.0 V vs. SCE. As shown in Figure 7, digital simulation results of the first oxidation peak of 5 and the first reduction peak (Figure S32-S35) for 1-4, redox potentials and diffusion constants (D) can be obtained with a simple one electron transfer reversible redox reaction mechanism as summarized in Table S1. The diffusion constants of 1-5 are in the range of 1.3-5.1 × 10-6 cm2/s at a planar electrode. Digital simulation of the entire CVs at various scan rates as shown in Figure S35-39 were attempted but failed for 1-4 because of irreversible reaction of thiophene. For example, no well-defined redox peaks can be identified in the CVs of 4 (Figure S39a) because of the surface electropolymerization of thiophene as shown by the progressive current increase feature with continuous potential scanning (Figure S39b). The CV of 5 exhibits reversible oxidation and reduction in the entire scanned potential region as shown by the mass-transfer controlled revisable features in Figure 6E and Figure S40.


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O M U L

-3.0 -3.48 eV

-3.33 eV -3.54 eV

-3.55 eV

-4.0 -4.26 eV

2.02 eV

-4.5

2.18 eV

1.85 eV

-4.63 eV 2.34 eV

2.23 eV

-I3 / -I

Energy, eV vs. Vacuum

-3.5

-3.48 eV

-5.0

-5.03 eV

TPA -5.60 eV

-5.5 -6.0

3.2 eV

-5.71eV

1

-5.40 eV

-5.50 eV

2

-5.72eV

3

4

-5.67 eV

Thiophene

5

O M O H

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-6.5 -7.0 -7.5

-7.46 eV

TiO 2

Figure 8: HOMO and LUMO energy values for the BODIPY-thiophene-TPA dyes (1-5) estimated from the CV (Figure 6) and absorption (Figure 3a) measurements. For 2 and 4, the HOMO energy levels were estimated from the absorption measurements because of the lack of well-defined oxidation peaks in their CVs as shown by Figure 6B and D. From the oxidation and reduction potentials of all dyes as shown in Figure 6, the HOMO and LUMO energies were estimated and summarized in Figure 8. There is no significant change in LUMO energies between the BODIPY dyes that contain TPA versus those that have thiophene. The HOMO energies, on the other hand, shows a trend similar to the one observed in the UV-Vis absorption measurements for dye 3, 1 and 5. The discrepancy in the HOMO energy level trend for dye 2 and 4 from Figure 3 is caused by their HOMO energy levels estimated from the absorption measurements indirectly. There are no well-defined oxidation peaks detected by CV for these two dyes as shown by Figure 6 B and D. Based on the HOMO-LUMO gaps shown in Figure 9, thiophene modified dyes are predicted to have the most efficient light absorption 19 ACS Paragon Plus Environment


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toward near IR region because of their highest absorptivity in this region. For efficient DSSC fabrication, efficient regeneration of the dye after electron injection is required. The oxidation potential of the dye is expected to be more positive than that of redox mediators I-/I3-.51 The calculated HOMO energy values for the dyes are above the iodine redox couple energy level (4.63 eV), suggesting that all the oxidized dyes will be regenerated. In a similar manner, the location of the LUMO energies has to be above the TiO2 conduction band energy level for the electrons to be injected. The LUMO energies of 1-5 (-3.55 eV to -3.33 eV) are above the conduction band of TiO2 (-4.26 eV). Considering these findings 1-5 can be suggested as promising potential sensitizers in DSSCs. Table 3: DSSC Photovoltaic values of BODIPY-thiophene-TPA dyes 1-5 Dye 1 2 3 4 5

Jsc (mAcm-2) 0.39 0.11 0.17 0.07 0.53

Voc (V) 0.48 0.46 0.46 0.35 0.56

FF 0.66 0.30 0.42 0.22 0.65

ƞ (%) 0.12 0.05 0.03 0.01 0.22

3.3 Photovoltaic Properties of the DSCCs. DSSCs containing these five BODIPY dyes are fabricated and their solar energy harvesting and conversion characteristics are fully characterized. The photocurrent-voltage (I-V) parameters exhibited by each DSSC under AM 1.5 illumination are summarized in Table 3. Detailed characteristics of dark and photocurrent responses of all devices are shown in Figure S41. Figure 9 shows the IPCE versus the incident wavelength for these DSSCs. 1 and 2 have much more near IR photocurrent response than all other three dyes; 3 and 4 exhibit broad photocurrent responses in the near IR region resembling their absorption spectra. The weak photocurrent observed for dyes bearing thiophene (1-4) can be explained by the irreversible redox reaction of thiophene that may form a charge 20 ACS Paragon Plus Environment

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recombination region or trap that might inhibit efficient charge separation and collection toward a good device. 1 and 5 are the ones with higher JSC values, 0.39 and 0.53 mA/cm2 and therefore exhibit higher IPCE maxima compared to 2, 3, and 4 (Figure 9). The DSSC based on 5 shows a better power efficiency (ƞ = 0.22 %) under AM 1.5 irradiation which is not consistent with the absorption and energy levels measurements of the dyes in solution. Its IPCE spectrum also shows a strong absorption in the visible region that reaches a value of 17.2 % at λ= 535 nm. This is consistent with the absorption spectrum of 5 adsorbed on the TiO2 film. The efficiency of 5, which only has a TPA substituent as the donor, shows better electron transfer since it exhibits complete reversible oxidation; therefore, does not form any bottleneck in the system. Our results strongly suggest that the presence of thiophene can help shift the solar energy absorption to the near IR region although the device would suffer from an irreversible redox reaction of thiophene in a DSSC.

10

(A)

1 2 3 4 5

0.6

Current Density, mA/cm2

50

IPCE %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 0.5 500 525 550 575 600 625 650 675 700 Wavelength (nm)

(B)

0.4

0.2

1 2 3 4 5

0.0

-0.2 0.0

0.1

0.2

0.3

0.4

0.5

0.6

Voltage (V)

Figure 9: (A) Action spectra obtained for DSSC of dyes (1-5) under AM 1.5 simulated light; (B) I-V curves of DSSCs based on BODIPY dyes 1-5. 3.4 Transient Absorption Spectroscopic Study for Charge Carrier Dynamics. As shown in Figure 10A, transient absorption spectroscopy on the nanosecond to microsecond time scales exhibited a large positive signal centered around 540 nm for 5. This feature comes from the oxidized dye



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formed after charge injection as shown by literature reports and also our spectroelectrochemistry study in Figure S42.52 Concomitant bleaching of the ground state absorbance was seen around 590 nm. The cation radical decay time was measured in 0.1 M lithium perchlorate and 0 M,

(A)

(B)

Figure 10. (A) Typical time evolution of transient absorption for dye 5 modified TiO2, in CH3CN solution containing 0.05 M LiI; (B) cation radical decays of dye 5 at 540 nm on TiO2 in deaerated solution in the presence and absence of iodine redox mediator. Spectra were taken at excitation of 480 nm, 7.5 mJ/pulse. 0.005 M, and 0.05 M lithium iodide as shown in Figure 10B. Reduction of the cation radical was modeled using a stretched exponential decay function (see experimental section) where τ is the lifetime and β is the relative width of the exponentials making up the decay; the values of lifetime and β were used to obtain a weighted average lifetime for the observed decays. In the absence of iodide the weighted lifetime obtained is 3.9 µs, reflecting a characteristic time for recombination of the cation with the injected electron in the absence of additional electron donors. The addition of a redox mediator such as iodide increases the decay rate of the oxidized dye and, with 0.05 M iodide, the dye cation radical has a weighted lifetime of 290 ns, as shown in Figure 10B. Thus, over 90% of the oxidized dye reacts with the iodide under competitive kinetic conditions with 0.05 M iodide. Consistent with the electrochemistry characterization and 22 ACS Paragon Plus Environment

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calculations, the redox potential difference between 5 (S+/S) and the iodide/triiodide redox couple indicates reaction of S+ with I- will be thermodynamically favorable. Fast dye regeneration is critical for attaining high device efficiencies. The triphenylamine moiety of 5 has an oxidation potential of 0.62 V vs SCE. After conversion to a common reference electrode (E1/2 (S+/S) = 0.86 V vs NHE), the difference between S+/S and I3-/I- potentials is 0.51 V. Detailed data fitting and time evolution of transient absorption features of S+ are shown in Figure S43. The kinetic results clearly illustrate that, at the I- concentrations used in the cells, reduction of the dye cation radical is rapid relative to recombination of the injected electrons and dye cation radical.

120 (101) 100

(Ti Foil)

*

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(200)

(004) (112)

(103)

5

(105) (211)

(220)

80

4

60

3

40

2 1

20

TiO2 0 20

25

30

35

40

45

50

55

60

65

70

75

80

2 Theta degree

Figure 11: X-ray diffraction patterns of bare TiO2 calcined at 500°C and BODIPY dyes (1-5)TiO2 films. 3.5 Surface Bonding Characteristics of Bodipy-Thiophene-TPA Dyes with TiO2 Electrodes. To fully understand how these dye sensitizers adsorb onto the surface of the TiO2 electrode, XPS, XRD and DRIFT-IR measurements were made. Previous studies show that effective coupling is



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achieved when an anchoring group (phosphonate or carboxylate) binds tightly to the TiO2 by electrostatic attraction, hydrogen bonding, or chemical adsorption.25,26,27 The nature of the bond between the anchoring group, carboxylic acid in this case, and the TiO2 provides information that will help towards the understanding of the surface modification of the TiO2 electrode and its effect on the DSSC overall performance.25,26,27 As shown in Figure 11, XRD of TiO2 electrode remains in its anatase phase after the adsorption of the dyes. No additional scattering features are present from dye molecules, indicating the formation of monolayer of dyes without causing major chemical and physical transformation to the content of TiO2 electrode. On the other hand, XRD spectrum is not sensitive enough to resolve the surface chemical bonding formation between dye and on TiO2.


(A)

529.64 eV

5

529.59 eV

4

530.11 eV

3

529.55 eV

2

529.84 eV

1

529.25 eV

TiO2 534

533

532

531

530

529

528

527

526

Binding Energy (eV)

(2p5/2) (2p3/2)

(B) 5

458.362 eV

464.192 eV 458.139 eV

4 Intensity (counts/sec)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity (counts/sec)

Page 25 of 36

463.99 eV 458.788 eV

3 464.527 eV

458.083 eV

2 1

463.878 eV 457.983 eV

463.748 eV 457.914 eV 463.653 eV

TiO2 466

464

462

460

458

456

Binding Energy (eV)

Figure 12: XPS spectra of the O 1s region (A) and Ti 2p region (B) of BODIPY-thiophene-TPA dyes (1-5) XPS and DRIFT-FTIR were also used to further distinguish any binding interactions between the TiO2 surface and the anchoring group (-COOH) of the BODIPY dyes. All the BODIPY dyes (1-5) adsorbed to TiO2 were compared to the bare TiO2. Compared to bare TiO2, the peak attributed to TiO2 is shifted to higher binding energy by 0.3 – 0.86 eV in the O (1s) region, 0.021 – 0.874 eV (2p3/2) and 0.095 ‒ 1.225 eV for the Ti (2p5/2) region upon adsorption of all of the BODIPY dyes (Figure 12). These findings suggest that there are some surface interactions between the TiO2 and the BODIPY dyes.


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2854 2927 2963

1606 1636

1503

5-T iO 2

4-T iO 2

Reflectance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1183 1244 1285 1383


3-T iO 2 2-T iO 2 1-T iO 2

T iO 2

1000

1500

2000

2500

3000

3500

-1

W avenum ber (cm )

Figure 13: DRIFT Spectra of Bodipy-thiophene-TPA dyes (1-5) adsorbed onto TiO2 surface in comparison with a blank TiO2 surface. To estimate the amount of dye adsorbed on the TiO2 surface, an attempt to desorb the BODIPY dyes from the TiO2 film was made by dipping them into an aqueous solution of 0.1 M potassium hydroxide (KOH) overnight. Normally dyes can be removed from the TiO2 surface by aqueous acid or base treatment.53 However, this attempt was unsuccessful, suggesting that there might be more than one binding site present that makes desorption of these dyes almost impossible. DRIFT spectra of all BODIPY-thiophene-TPA dyes adsorbed onto the TiO2 surface and bare TiO2 are shown in Figure 13. The bands in the 1600-1400 cm-1 region are attributed to the stretching modes of carboxylate anions (COO-) that are bound to the Ti surface according to previous studies on interaction between COOH group and TiO2.

54,55,56

Two major bands are

observed, one symmetric CO2- stretching at 1383 cm-1, and an asymmetric CO2- stretching band at 1503 cm-1, indicating a chemical bond formation between the CO2- and the TiO2 surface. The difference in these two major stretching modes can be used to determine if the CO2- group binds 26 ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TiO2 surface by monodentate binding, bidentate bridging, or bidentate chelating.42 The ~120 cm1

peak separation of the symmetric and the asymmetric stretching modes strongly indicates a

bidentate bridging bond formation between CO2- and TiO2. These dyes also exhibit weak bands at 1603 and 1636 cm-1 attributed to the C=C stretching vibration from the phenyl ring and 1183, 1244 and 1285 cm-1 attributed to the C-H bend modes. Calculated IR spectra for Dyes 1-5 are shown in Figure S44. The calculated FTIR spectrum of dye 5 qualitatively confirms its structure. The calculated spectrum has a 100-200 cm-1 shift to higher frequency than that from the experiment because the experimental data are anharmonic and are collected on a bulk sample involving ensemble averaging effect; whereas, the calculated values are harmonic and for the isolated, gas phase ion. 4. Conclusions A series of BODIPY-based dyes that contain thiophene and/or TPA were studied and characterized. Their optical, spectroscopic, electrochemical, and photovoltaic properties were investigated due to their promising potential as efficient photosensitizers in DSSCs. The highest power efficiency was achieved by 5 (ƞ= 0.22 %) which has stronger light absorption in the visible region. The weak photocurrent observed for dyes bearing thiophene (1-4) can be explained by the irreversible redox reaction of thiophene that may form a charge recombination region or trap that might inhibit efficient charge separation and collection toward a good device. Dye 5 was found to be more efficient because of its resistance to dye aggregation and redox reversibility. Upon adsorption to the TiO2 surface through a bidentate bridge, these BODIPY dyes (1-5) highlighted the importance of a strong donor/acceptor and improved conjugation towards improving the light harvesting. The presence of thiophene and/or TPA extends the conjugation to enhance light harvesting efficiency by extending the light absorption of a solar cell to near IR region. However, these substituents may not have significantly improved the 27 ACS Paragon Plus Environment


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charge injection due to the limited overall efficiency of the DSCCs that contain these dyes. This issue should be further addressed by arranging the substitution configuration of the BODIPY molecule and selecting other redox mediator for efficient charge injection to thiophene and/or other device configuration (e.g., solid state heterojunction organic solar cells). In addition, coadsorption of these modified BODIPY dyes with small spacer molecules like chenodeoxycholic acid onto TiO2 may help prevent electropolymerization and improve their redox reversibility in a device. This coadsorption concept has been demonstrated experimentally57 and validated theoretically58 previously in the literature. These findings provide a foundation for the design of more efficient DSSCs on the basis of the observed correlations between the structure and the properties for new potential structural directions. ACKNOWLEDGMENT. This work is partially supported by NSF under award OIA-1539035 to SP and RS. AP and ZC are partially supported NSF under award 1153120 to SP and AG. The computational work was supported by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, U.S. Department of Energy (DOE) (catalysis center program) to DAD. DAD also thanks the Robert Ramsay Chair Fund of The University of Alabama for support. We thank the Central Analytical Facility (CAF) of the University of Alabama for its major surface characterization facility support. Supporting Information. Synthesis and structural chatacterization of Bodipy-thiophene-TPA dyes 1-5 and their intermediates; Calculated vs. Experimental UV-Vis absorbance properties; Supplementary figures and tables for electrochemical results; time evolution of transient absorption spectra and lifetime fitting results of Dye 5; HOMO and LUMO (B3LYP) energies; List of first 20 calculated excited states for both the B3LYP and BMK functionals in DMF; and DSSCs device characterization. 28 ACS Paragon Plus Environment

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