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Linker Unit Modification of Triphenylamine-based Organic Dyes for Efficient Cobalt Mediated Dye-Sensitized Solar Cells Hanna Ellis, Susanna K Eriksson, Sandra Feldt, Erik Gabrielsson, Peter Lohse, Rebecka Lindblad, Licheng Sun, Håkan Rensmo, Gerrit Boschloo, and Anders Hagfeldt J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp403619c • Publication Date (Web): 18 Sep 2013 Downloaded from http://pubs.acs.org on October 3, 2013

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The Journal of Physical Chemistry

Linker Unit Modification of Triphenylamine-based Organic Dyes for Efficient Cobalt Mediated DyeSensitized Solar Cells Hanna Ellis,a Susanna K. Eriksson,a Sandra M. Feldt,a Erik Gabrielsson,b Peter W. Lohse,a Rebecka Lindblad,c Licheng Sun,b Håkan Rensmo,c Gerrit Boschloo,a,* Anders Hagfeldt a

a

Department of Chemistry – Ångström Laboratory; Physical Chemistry, Uppsala University,

Box 523, 751 20 Uppsala, Sweden. b

Royal Institute of Technology, Organic Chemistry. Center of Molecular Devices, Chemical

Science and Engineering, 100 44 Stockholm, Sweden. c

Department of Physics and Astronomy; Molecular and Condensed Matter Physics, Uppsala

University, Box 516, 751 20 Uppsala, Sweden. * Corresponding author: Box 523, 751 20 Uppsala Sweden, Tel: +46 18 4713303, E-mail: [email protected]

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Abstract Linker unit modification of donor-linker-acceptor-based organic dyes was investigated with respect to the spectral and physicochemical properties of the dyes. The spectral response for a series of triphenylamine (TPA)-based organic dyes, called LEG1-4, was shifted into the red wavelength region and the extinction coefficient of the dyes was increased by introducing different substituted dithiophene units on the π-conjugated linker. The photovoltaic performance of dye-sensitized solar cells (DSCs) incorporating the different dyes in combination with cobaltbased electrolytes was found to be dependent on dye binding. The binding morphology of the dyes on the TiO2 was studied using photoelectron spectroscopy, which demonstrated that the introduction of alkyl chains and different substituents on the dithiophene linker unit resulted in a larger tilt angle of the dyes with respect to the normal of the TiO2-surface, and thereby a lower surface coverage. The good photovoltaic performance for cobalt electrolyte-based DSCs found here and by other groups using TPA-based organic dyes with a cyclopentadithiophene linker unit substituted with alkyl chains was mainly attributed to the extended spectral response of the dye, whereas the larger tilt angle of the dye with respect to the TiO2-surface resulted in less efficient packing of the dye molecules and enhanced recombination between electrons in TiO2 and Co(III) species in the electrolyte. KEYWORDS: Photovoltaics TiO2 Electron transfer kinetics Dye adsorption Dye aggregation


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Introduction

Dye-sensitized solar cells (DSCs) have the potential to convert solar energy into electricity at a low cost. In the DSC, a dye sensitizer attached to a mesoporous semiconductor, normally TiO2, is responsible for capturing the sunlight. After electron injection into the semiconductor the oxidized dye molecules are in turn regenerated by a redox mediator, normally iodide/triiodide, in a surrounding electrolyte. In a recent publication we showed for the first time that high efficiency DSCs can be obtained using a triphenylamine (TPA)-based organic sensitizer with steric substituents, called D35, in combination with a cobalt redox mediator, instead of the iodide/triiodide redox couple.1 Other interesting results have, recently, also been reported for organic dyes in combination with other one-electron transition metal redox mediators.2,3 Organic dyes are beneficial compared to the conventional used ruthenium dyes since they can have simpler synthetic routes and higher extinction coefficients, enabling good light harvesting efficiencies. The world record for DSCs of 12.3 % is currently obtained using co-sensitization of a zinc-porphyrin dye and a TPA-based dye in combination with the cobalt tris(2,2’-bipyridyl) redox mediator.4 The photovoltaic response of donor-π-acceptor organic sensitizers can be enhanced by increasing the spectral response of the dyes into the red wavelength region and by tuning the steric and electronic properties of the dyes in order to get a better blocking of the TiO2 surface to diminish recombination processes. The introduction of alkoxy chains on the dyes has been shown to prevent dye aggregation5 and to suppress recombination, allowing the use of cobalt redox mediators in the DSC.1,6,7 The length and the position of the alkyl chains can, however, affect the surface coverage.7 Common strategies to enhance the spectral response of TPA dyes is to narrow 3 ACS Paragon Plus Environment


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down the molecular energy gap by modifying the π-conjugated extension8 or by suppressing the rotational disorder of the molecules by rigidification of the conjugated linker.9 Here we have extended the spectral response of the D35 dye, by modifying the conjugated linker using different substituted dithiophene units. Figure 1 shows the series of different dyes investigated, called the LEG1-4 series. LEG1 has bithiophene, LEG2 has 5-(thiophen-2-yl)-2,3dihydrothieno[3,4-b][1,4]dioxine, LEG3 has 3-hexyl-2,2'-bithiophene and LEG4 has 4,4-dihexyl4H-cyclopenta[2,1-b:3,4-b´]dithiophene as the π-conjugated linker. The LEG 4 dye is similar to the Y123 dye10 which gives 10.0 % power conversion efficiency (PCE) with cobalt electrolyte, the only difference being the slightly longer alkoxy chains on the donor unit of Y123 compared the LEG4 (C6 and C4, respectively). The photovoltaic performance of the solar cells incorporating the different dyes in combination with cobalt tris(2,2’-bipyridyl) electrolyte, was investigated with respect to the spectral and physicochemical properties of the dyes. The binding morphology of the dyes with respect to the TiO2-surface, studied using photoelectron spectroscopy, was found to impact the recombination kinetics, and thus the photovoltaic performance.


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Figure 1. The molecular structure of the LEG-series of dyes.

Experimental Section Materials used All chemicals were purchased from Sigma Aldrich if not otherwise mentioned. For working and counter electrodes fluorine-doped tin oxide (FTO) glass from Pilkington TEC15 and predrilled TEC8 were used, respectively. Detergent solution (RBS 25 from Fluka analytical), ethanol (VWR DBH Prolabo purity of 99.9%) and deionized water were used for the cleaning of substrates. The same ethanol was also used for the dye baths. Screen printing was performed with TiO2-paste from Dyesol (DSL 18 NR-T) and a light-scattering paste (JGC C&C Ltd. PST-400C, gratefully received from JGC Catalysts and Chemical Ltd.). Sodium dodecyl sulfate (SDS) with 5 ACS Paragon Plus Environment


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>99% purity and 3,4-ethylenedioxythiophene (EDOT) was purchased from Aldrich and used for preparation of poly(3,4-ethylenedioxythiophene) (PEDOT) counter electrodes. LiClO4 and 4-tertbutylpyridine (TBP) were purchased from Aldrich of 99.9%, respectively 96% purity, TBP was distilled before use. Acetonitrile anhydrous 99.8% was used as received. Dyes were synthesized accordingly to the procedure published by the team of Professor Licheng Sun.11 Dichloromethane (DCM) anhydrous >= 99.8%, Tetrabutylammonium hydroxide (TBAOH) solution 1.0 M in methanol and sulfuric acid 95.0-98.0% were used for the dye desorption measurements. Ferrocene 98% from Aldrich and Tetrabutylammonium hexafluorophosphate (TBAPF6) ≥ 99.0% from Fluka analytical were used for electrochemical analysis of the dye HOMO-levels. Chenodeoxycholic acid (cheno) of ≥ 95% purity was used as additive in the dye baths in order to investigate possible aggregation of the dyes on the TiO2-surface.

Synthesis of cobalt complexes Co(bpy)3(PF6)2 and Co(bpy)3(PF6)3 were synthesized according to a modified procedure published by our group.1 Briefly, 1 equivalent of CoCl2 . 6H2O and 3.3 equivalent of the (2,2’bipyridyl) ligand were dissolved in a minimal amount of methanol (Merck) to give a brownyellow solution. The solution was left to stir at reflux for 2 hours and an excess of TBAPF6 was added to the solution to precipitate the compound. The product was then filtered, washed with methanol, ethanol and diethylether, dried under vacuum and used without further purification. Oxidation of the cobalt (II) complex to cobalt (III) was performed by adding H2O2 (30%) drop wise to an acetonitrile solution of the complex. The solution was left to stir and a large amount of saturated TBAPF6 in water was then added to the solution to precipitate the compound. The 6 ACS Paragon Plus Environment

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final product was washed with water, filtered, dried under vacuum and used without further purification. The NMR-spectrum of Co(bpy)3(PF6)3 is shown in Supporting Information Figure S1. Fabrication of Dye-sensitized Solar Cells Fluorine-doped tinoxide (FTO)-coated glass substrates (TEC15 and TEC8, Pilkington) were cleaned in ultrasonic bath for one hour in the following media: detergent, deionized water and ethanol. The TEC15 glass substrates were pre-treated in a 40 mM aqueous TiCl4 solution at 70°C for 30 minutes and then rinsed with ethanol and water. The TEC15 substrates were then screen printed (active area 0.25 cm2) with diluted Dyesol DSL 18 NR-T paste. The paste was diluted by adding 40 w% polymer mixture containing 90w% terpineol and 10w% etylcellulose to 60w% Dyesol paste. After drying at 125 °C for 10 minutes the substrates were printed with a second layer of a 20 w% terpineol-diluted TiO2 light-scattering paste. The samples were then treated by a heating process: 180 °C (10 minutes), 320 °C (10 minutes), 390 °C (10 minutes) and 500 °C (60 minutes) in an oven (Nabertherm Controller P320) at air atmosphere. After sintering, the samples were once again treated in 40 mM aqueous TiCl4, by the aforementioned procedure. A final heating step (500 °C for 60 minutes) was performed. The final thicknesses of the TiO2 films used for solar cell assembly were 8-9.5 µm. Before immersing the electrodes in the dye bath the electrodes were cooled to 90 °C. The dye bath had a concentration of 0.2 mM D35/LEG1/LEG2/LEG3/LEG4 in ethanol, for the solar cells assembled with cheno in the dye bath a concentration of 10 mM cheno was used. The films were left in the dye baths overnight (16 hours) before being rinsed with ethanol and assembled in a sandwich structure. A 30 µm thick thermoplastic Surlyn frame was used and a cobalt-based electrolyte consisting of 0.22 M Co(bpy)3(PF6)2, 0.05 M Co(bpy)3(PF6)3, 0.2 M TBP, 0.1 M LiClO4 in acetonitrile was introduced 7 ACS Paragon Plus Environment


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into the solar cells through a predrilled hole in the counter electrode. The devices were sealed with thermoplastic Surlyn covers and glass coverslips. PEDOT counter electrodes were used and manufactured using an aqueous micelle solution as published by our group12. In short, PEDOT was electropolymerized onto an FTO substrate in a solution containing a micellar aqueous solution of 0.1 M sodium dodecyl sulfate (SDS) and 0.01 M 3,4-ethylenedioxythiophene (EDOT), where EDOT and SDS had been dissolved in deionized water by ultrasonicating for one hour. A two electrode system where another FTO plate of the same dimensions was used as counter electrode connected to an IviumStat.XR (Ivium Technologies) electrochemical instrument in the galvanostatic mode. A current of 13 mA was applied for 175 seconds, resulting in a poly(3,4-ethylenedioxythiophene) (PEDOT) layer of 270±25 nm. Solar Cell Characterization Current-Voltage (IV) characteristics of the solar cells were investigated by a Keithley 2400 source/meter and a Newport solar simulator (model 91160). The light intensity was calibrated using a certified reference solar cell (Fraunhofer ISE), to an intensity of 1000 W m-2. When performing the IV-measurements a black mask of 0.5×0.5 cm2 was used in order to avoid significant additional contribution from light impinging on the device outside the active area (0.5×0.5 cm2). The apparatus for Incident Photon to Current Conversion Efficiency (IPCE) consisted of a computer-controlled setup with a xenon light source (Spectral Products ASB-XE175), a monochromator (Spectral Products CM 110), and a Keithley 2700 multimeter. The same certified reference solar cell was used for calibration as previously mentioned. Electron lifetime and extracted charge measurements were performed using a white LED (Luxeon Star 1W) as light source. The voltage and current signals were recorded with a 16-bit


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resolution digital acquisition board (National Instruments) in combination with a current amplifier (Stanford Research Systems SR570) and a custom-made system using electromagnetic switches. Lifetimes were determined by monitoring photovoltage transients at different light intensities upon applying a small square wave light modulation to the base light intensity. The photovoltaic responses were fitted using first-order kinetics to obtain time constants13. Extracted charge measurements were performed by illuminating the cell under open-circuit conditions and then turning the lamp off to let the voltage decay to a voltage V. The solar cell was then shortcircuited and the photocurrent was measured under a certain time and integrated to obtain the extracted charge at open circuit conditions, QOC (C). Nanosecond Transient Absorption Spectroscopy (TAS) Dye regeneration by cobalt-based redox mediator and electron recombination to the oxidized dye were monitored using a Nd:YAG laser (Continuum Surelight II, repetition rate: 10 Hz, pulse length: 10 ns) in combination with an Optical Parametrical Oscillator (Continuum Surelite OPO Plus). Pump pulses of 580 nm for D35, 605 nm for LEG1 and LEG3, 620 nm for LEG4 and 635 nm for LEG2 were used. The intensity of the laser output was attenuated to 0.2 mJ cm-2. Probe light was provided by a near-infrared LED (Osram SHF 484, λmax 880 nm, FWHM 80 nm) or a laser diode (Laser Components ADL-78051TL, 780 nm, 5 mW), and kinetic traces were measured with an amplified Si-photodiode (Thorlabs PDA10A-EC). In order to avoid stray light from the laser a cutoff filter (RG 715) was used in front of the detector. Photo-induced absorption (PIA) spectra were measured for all dyes in the LEG-series adsorbed on TiO2 to determine the spectrum of the oxidized dyes (see Figure S9 in Supporting Information). Based on these spectra 780 nm was chosen as probe light in ns-TAS for measuring LEG1 and LEG3 while 880 nm was chosen for measuring D35, LEG2 and LEG4. The electrolyte consisted of 0.22 M 9 ACS Paragon Plus Environment


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Co(bpy)3(PF6)2, 0.05 M Co(bpy)3(PF6)3, 0.1 M LiClO4 and 0.2 M TBP in acetonitrile. Recombination dynamics were measured with a solution of 0.2 M TBP and 0.1 M LiClO4 in acetonitrile. Measurements were performed on screen printed TiO2 films of 3 µm thickness which were sensitized according to already mentioned procedure. A drop of the electrolyte was applied on the samples for regeneration measurements while for the recombination measurements the solution without redox couple was applied. A glass coverslips was used for sealing the sample. The kinetic traces represent an average of 320 laser shots. Photoelectron spectroscopy measurements The X-ray photoelectron spectroscopy (PES) measurements were conducted at the Swedish national synchrotron radiation facility MAX-lab in Lund at beamline I411.14 The kinetic energy of the photoelectrons was detected with a Scienta R4000 WAL analyzer. The take-off angle for the electrons was 60° and the take-off direction was collinear with the e-vector of the incoming radiation. The pressure in the analyzing chamber was in the order of 10-8 mbar. All PES core level spectra were energy calibrated by fixing the binding energy for the Ti 2p2/3 level originating from the TiO2-substrate to 458.56 eV15. The samples were dye-sensitized thin films of mesoporous TiO2 (2-3 µm) on conductive glass. The same paste and sintering procedure was used as for the solar cell electrodes. The films were sensitized for approximately 16 hours in dye baths with a concentration of 0.2 mM for LEG1/LEG2/LEG3/LEG4 and 0.3 mM for D35. This small difference in concentration will not affect the dye coverage on the outermost surface. After sensitization, the films were rinsed with ethanol and immediately transferred into the vacuum chamber. The core level spectra were fitted using a voigt shaped line profile with a Lorentzian contribution set to 0.3 eV. All S 2p core level spectra were intensity calibrated by dividing with the intensity of the corresponding Ti 2p substrate signal. D35 has only one sulphur atom whereas 10 ACS Paragon Plus Environment

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the LEG1-4 dyes have two and to compensate for this, the S 2p signal from D35 was in addition multiplied by 2. Dye desorption measurements Dye loads were estimated from dye desorption measurements. TiO2-films with one layer of Dyesol DSL 18 NR-T paste, (diluted by 40 w% polymer mixture, containing 90w% terpineol and 10w% ethylcellulose) with a thickness of about 5 µm were immersed in dye baths with a concentration of 0.2 mM D35/LEG1/LEG2/LEG3/LEG4 in ethanol and after sensitization put in alkaline solution consisting of 0.05 M TBAOH in DCM to desorb the dyes. Sulfuric acid was added into the solution in order to re-protonate the dye. UV-Vis measurements of the solutions in a 1 cm cuvette were done with an Ocean Optics HR2000 spectrometer and a Micropack DH2000-BAL light-source. Dye coverage was estimated using Lambert-Beers law.

Results and Discussion Photovoltaic properties The LEG-series was synthesized in order to improve the photovoltaic properties of the D35 sensitizer. D35 has been shown in earlier publications to be efficient with cobalt electrolytes.1 The D35 sensitizer is a so called Donor-π-Acceptor (D-π-A) sensitizer, where the π-unit is used to control the HOMO-LUMO gap which governs the absorption edge. The D35 sensitizer only harvest light below 620 nm, and therefore the π-unit was changed in the LEG-series in order to extend the absorption spectrum and also other properties of the sensitizers. Information of synthetic procedures and further spectral- and electrochemical data for the LEG-series is



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published elsewhere11, and a summary of these properties is found in Table S1 in the Supporting Information. Solar cells based on the LEG-series of sensitizers were assembled and the results are shown in Table 1. Despite the wider absorption spectra obtained for all dyes in the LEG-series compared to the standard used D35 dye11, only LEG4 and LEG3 show an improved power conversion efficiency (PCE) in the cobalt-based electrolyte. Table 1. Photovoltaic parameters of solar cells sensitized with the D35 and the LEG-series of dyes. Four solar cells of each dye were assembled and average with error is given.

Dye

Intensity / W m-2

D35

1000

LEG 1

LEG 2

LEG 3

LEG 4

VOC / V 0.900 ± 0.01

JSC / m Acm-2

FF

PCE / %

7.9 ± 0.4

0.66 ± 0.02

4.7 ± 0.3

320

0.866 ± 0.01

2.7 ± 0.1

0.70 ± 0.05

5.5 ± 0.5

1000

0.878 ± 0.01

7.6 ± 0.3

0.62 ± 0.01

4.2 ± 0.1

320

0.854 ± 0.01

2.7 ± 0.2

0.65 ± 0.01

5.0 ± 0.2

1000

0.801 ± 0.01

7.9 ± 0.6

0.65 ± 0.01

4.1 ± 0.6

320

0.766 ± 0.01

2.9 ± 0.2

0.70 ± 0.04

5.1 ± 0.5

1000

0.915 ± 0.00

8.9 ± 0.1

0.68 ± 0.03

5.5 ± 0.5

320

0.898 ± 0.01

3.2 ± 0.1

0.70 ± 0.05

6.7 ± 0.3

1000

0.847 ±0.01

10.8 ± 0.2

0.71 ± 0.02

6.5 ± 0.3

320

0.815 ± 0.01

3.7 ± 0.1

0.77 ± 0.02

7.7 ± 0.1

LEG4 shows the highest photocurrent and the highest PCE, which can be explained by the high extinction coefficient of the dye and the red shift in the absorption spectrum. However, LEG4 together with LEG2 show relatively lower open circuit voltages (VOC) compared to the other sensitizers. The reason for the lower VOC of the LEG4 and LEG2 was investigated by assembling solar cells of all the dyes with chenodeoxycholic acid (cheno) added in the dye baths. Cheno has 12 ACS Paragon Plus Environment

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been used in other studies to avoid dye aggregation.16 If dye aggregation is the reason for the low VOC of LEG4 and LEG2 an increase in VOC can be expected by the addition of cheno. As seen in Table 2 and Figure S2 in Supporting Information the VOC increases for LEG2, but decreases for LEG4 upon the addition of cheno. The increase in PCE, JSC and VOC for LEG2 indicates that dye aggregation can be a problem for this dye, which will be discussed below. Table 2. Current-voltage characteristics of D35 and the LEG-series of dyes at 1 sun illumination. 10 mM chenodeoxycholic acid was added in the dye baths, the ratio of dye:cheno was 1:50. Two solar cells of every dye were prepared; average and error was calculated. The Current-voltage plot is shown in Figure S2 in Supporting Information.

Dye

Intensity / W m-2

VOC / V

JSC / m Acm-2

FF

PCE / %

D35

1000

0.795 ± 0.005

8.53 ± 0.1

0.67 ± 0.01

4.5 ± 0.1

320

0.758 ± 0.008

3.09 ± 0.1

0.75 ± 0.01

5.5 ± 0.1

1000

0.815 ± 0.035

8.80 ± 0.4

0.60 ± 0.02

4.3 ± 0.1

320

0.778 ± 0.048

3.09 ± 0.1

0.70 ± 0.04

5.2 ± 0.2

1000

0.830 ± 0.005

11.2 ± 0.1

0.51 ± 0.01

4.7 ± 0.1

320

0.795 ± 0.005

3.96 ± 0.1

0.69 ± 0.01

6.8 ± 0.1

1000

0.885 ± 0.005

9.74 ± 0.2

0.57 ± 0.01

4.9 ± 0.1

320

0.863 ± 0.005

3.47 ± 0.1

0.67 ± 0.01

6.2 ± 0.1

1000

0.820 ± 0.005

10.18 ± 0.9

0.52 ± 0.01

4.3 ± 0.4

320

0.783 ± 0.005

3.98 ± 0.1

0.72 ± 0.01

7.0 ± 0.1

LEG1

LEG2

LEG3

LEG4

In Figure 2a the Incident Photon Conversion Efficiency (IPCE) is shown for the LEG-series of dyes. The IPCE is in agreement with the spectral properties of the dyes shifted into the red wavelength region for the LEG-series11. The IPCE is high for all the dyes, but LEG1 and LEG2 show slightly lower IPCE values compared to the other sensitizers. LEG1 shows in agreement with the IPCE results the lowest short circuit current (JSC). According to Equation 1 the IPCE is



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generally determined by the light harvesting efficiency (LHE), the quantum yield of electron injection from the excited sensitizer into the TiO2 conduction band ( φinj ), the efficiency of dye regeneration ( ηreg ), and the collection efficiency of the photo-generated charge carriers (ηcoll ).

IPCE = LHE × φinj ×ηreg ×ηcoll

(1)

Figure 2. IPCE for the LEG-series of dyes; (a) without and (b) with the addition of chenodeoxycholic acid in the dye bath.

The IPCE can thus be used to provide useful information of the factors that limits the device performance of the DSC. The high extinction coefficients for the dyes is expected to give LHE close to 100 % at the maximum absorption point and LHE should therefore not be the limiting factor of the IPCE for the LEG1 and LEG2 dyes. The quantum yield of electron injection from the excited sensitizer into the TiO2 conduction band ( φinj ), is neither expected to be the reason for the lower IPCE obtained for LEG1 and LEG2, since the calculated LUMO-levels of the dyes (see Table S1 in Supporting Information) are similar and thus also the driving force for injection. For the triphenylamine-based organic dyes the LUMO is situated on the cyanoacetic acid unit binding to the TiO2-surface,17 and since all the dyes have the same anchoring groups, differences in the 14 ACS Paragon Plus Environment

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electronic coupling are minor. Moreover, comparable reorganization energies for the dyes are expected, as a result of their similar molecular structures, in agreement with previous calculation on reorganization energies for triphenylamine-based organic dyes.18 The lower IPCE found for the LEG2 and LEG1 dye can, however, be due to dye aggregation since the addition of cheno increases the IPCE and JSC significantly, see Figure 2b and Table 2. Normally monomers inject electrons to TiO2 more efficiently than aggregates.19 Electron lifetime measurements The reason for the lower VOC obtained for the LEG4 and LEG2 dyes can be correlated with the differences in the electron lifetime for the dyes, as seen in Figure 3a. The VOC is determined by the difference between the Fermi-level of the TiO2 and the redox level of the redox couple in the electrolyte. Since all solar cells were assembled with the same electrolyte the electron quasiFermi level in the TiO2 determines the VOC. The position of the electron quasi-Fermi level is, in turn, limited by the conduction band edge. Insignificant shifts of the conduction band edge were, however, observed (see Figure S3 in Supporting Information). Thus, the different VOC values obtained for the sensitizers are more likely determined by differences in the electron lifetimes. LEG3 has the best electron lifetime of the whole LEG-series, which is consistent with its high VOC.



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Figure 3. Electron lifetime for the D35 and the LEG-series of dyes; (a) without and (b) with the addition of chenodeoxycholic acid in the dye baths.

When cheno was added in the dye baths the lifetimes changed for the D35 and the LEG-series dyes, see Figure 3b. Interestingly, the electron lifetime improved for the entire LEG-series of dyes with respect to the D35 dye, for which it became notably lower. The photovoltaic performance of the D35 dye has previously been found to be independent of the addition of cheno, due to the inherent structural nature of the D35 molecule.5 The lifetime for LEG2 increased in agreement with the VOC upon the addition of cheno. Regeneration and recombination to the oxidized dye molecules Nanosecond transient absorption spectroscopy measurements were performed to determine the recombination and regeneration rate of the dye molecules by the cobalt redox electrolyte (see Figures S4-S8 in Supporting Information). The transient optical signal recorded at 780 nm or 880 nm, depending on dye, after laser pulse excitation for the different dyes is caused by absorption of the oxidized dye molecules, and to a lesser extent the absorption of electrons in the TiO2. Photo-induced absorption (PIA) spectra were measured for all dyes on TiO2, which reflect the absorption of the oxidized dye molecules (see Figure S9 in Supporting Information). From these


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spectra a probe wavelength of 780 nm was chosen for measuring LEG1 and LEG3 while 880 nm was chosen for measuring D35, LEG2 and LEG4. The decrease in absorbance signal shows the recombination of conduction band electrons with the oxidized dye molecules. When the cobalt electrolyte is added, the decay of the signal accelerates, indicating that the cobalt redox couples regenerate the dye molecules. The obtained recombination and regeneration half times (t1/2) are listed in Table 3 (transients and fits are shown in supporting information). Table 3. Recombination and regeneration halftimes for dye-sensitized TiO2 substrates. The efficiency of the regeneration process is calculated with the half times for the recombination and regeneration processes.

Dye

D35

LEG1

LEG2

LEG3

LEG4

τ1/2,rec / µs

126

41

111

44.8

74

τ1/2,reg / µs

5.5

8.4

26.9

7.0

6.2

ηreg / %

96

80

76

84

92

13.8

17.7

17.7

17.6

17.7

r/Å

The driving force for recombination of photoinjected electrons to the oxidized dye molecules is determined by the potential difference between the conduction band of the TiO2 and the ground state oxidation potential of the dye. The ground state oxidation potential of the dyes measured on sensitized TiO2 films are listed in Table S2 Supporting Information. Since the oxidation potentials of the dyes are similar, the driving force for recombination should be similar and the differences in recombination rate obtained would be due to structural differences between the dyes. By using the optimized molecular structures of the dyes in gas phase calculated at the B3LYP/6-31G(d) level using Gaussian 0920 the distances between the attachments to the TiO2surface and the positive charge, taken as situated on the triphenylamine nitrogen of the dyes21,22, given as r in Table 3, were determined. A distance-dependence of the electron transfer reaction



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between the electron at the TiO2-surface and the hole on the dye may be expected,23 but the results show no clear trends. The regeneration efficiency can be estimated from the regeneration half times using Equation 2,

η reg =

k reg k rec + k reg

= 1−

t1 / 2 redox t1 / 2 inert

(2)

where kreg is the rate constant for regeneration of the sensitizer by the redox couple, and krec is the rate constant for back electron transfer from electrons in the TiO2 conduction band to the oxidized dye molecules. The lowest regeneration efficiencies are found for LEG1 and LEG2, as a result of relatively fast recombination between electron and oxidized dye (LEG1) or slow regeneration half time (LEG2). This agrees well with the low IPCE values found for these dyes. Dye aggregation may also, as mentioned above, affect the injection efficiency for the LEG2 and LEG1 dye, lowering the obtained IPCE values.

The binding morphology of the dyes with respect to the TiO2-surface PES is a powerful tool for studying surfaces and is well suited for the dye-sensitized surfaces investigated here. The binding morphology of the dyes to the TiO2-surface was measured by PES in order to correlate the trends in photovoltaic performance and the dye structures. The technique is highly surface sensitive, element specific and can with suitable data analysis give information on electronic structure, binding configurations and coverage on surfaces. In this study PES has been used to examine the binding configuration using the fact that the dye molecules contain two chemically different nitrogen atoms. The inelastic mean free path (IMFP) for electrons is strongly dependent on the photon energy, hence, by measuring N 1s with different photon energies it was possible to compare the photoemission intensities from the different nitrogens and draw


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conclusions about the binding configuration.24,25 The N 1s core levels of the different dyes were measured with the photon energies 758 eV and 540 eV, with the latter being significantly more surface sensitive (see Figure S10 in Supporting Information). The intensity of the N 1s core level for the two different nitrogen atoms, here named NA (acceptor) and ND (donor), were measured and the relative intensities of the different contributions were compared (see Table 4). As seen in Table 4, the percentage of the NA is smaller for LEG3 and LEG4 compared to LEG1 and LEG2 when measured with a photon energy of 758 eV. This could be due to the fact that LEG3 and LEG4 have alkyl groups on the linker units which can screen the signal of the anchoring groups and the NA. When the photon energy is increased an increase in intensity of the NA part is observed for all dyes. It can thus be concluded that the dyes are placed as expected with the anchoring group close to the surface and the donor unit sticking out from the surface. Table 4. The N 1s intensity ratios (NA/Ntot) for two different photon energies where Ntot is the total intensity from N 1s. The spectra can be seen in Figure S10 in Supporting Information. The N-N distance seen from the surface was estimated using equation 3.

LEG 1

LEG2

LEG3

LEG4

758 eV

0.127

0.126

0.108

0.104

540 eV

0.064

0.075

0.064

0.076

N-N distance

16 Å

12 Å

12 Å

7Å

A more detailed picture of the molecular surface structure can be obtained by assuming a flat surface and analyzing relative peak intensities of the N 1s core levels measured with different photon energies. When the photon energy is varied, the surface sensitivity is changed as described by the inelastic mean free path for electrons (IMFP). The IMFP for the N 1s signal is assumed to be 6 Å and 8 Å25,26 for photon energies of 540 and 758 eV, respectively. Assuming



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normal emission, the change in relative peak intensities can to a first approximation be estimated as −d

I =eλ I0

(3)

where d is the vertical distance with respect to the TiO2-surface between NA and ND and λ is the IMFP for the molecule. Using Equation 3, the vertical distances between the two different nitrogens were calculated see Table 4. From this variation one can estimate the binding angles to the substrate surface by knowing the approximate size of the dyes molecules. From such reasoning the LEG1 dye is standing rather vertical on the TiO2-surface, whereas the tilt angle of the D-π-A backbone of the dyes with respect to the TiO2-surface increases when different substituents are introduced on the dithiophene linker unit. The tilt angle with respect to the normal of the TiO2-surface for the LEG4 dye is significantly larger compared to the other dyes. This is in contrast to the result found by Cao et al.,6 where TPA-based organic dyes with elongated alkyl side chains were ascribed to have a smaller tilt angle with respect to the normal of the TiO2-surface.

Surface coverage The dye coverage on the TiO2-surface for the dyes can be obtained from PES measurements by comparing the intensities of the S 2p core level with the Ti 2p level for each dye. The sulphur atoms are in all dyes positioned close to the TiO2-surface with small differences of the distance to the surface, which makes it a suitable choice for comparing dye coverage. Figure 4 shows the S 2p core level spectra measured with a photon energy of 758 eV. The two peaks seen in the figure


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corresponds to the spin orbit split doublet of the S 2p signal with the S 2p3/2 at a binding energy of 163.7 eV and S 2p1/2 at a binding energy of 164.9 eV. As seen in Figure 4 the intensity of the S 2p level, which relates to the dye coverage, varies significantly between the different dyes. If the S 2p intensity is set to 1 for LEG4 the following coverage series is calculated (LEG1-LEG4): 1.72, 1.42, 1.35 and 1 concluding that there are over 70% more dye molecules on the surface for LEG1 compared to LEG4 and 42% and 35% more dye for LEG2 and LEG3, respectively.

Figure 4. The S 2p spectra normalized versus the corresponding Ti 2p signal. This gives a relative measure of the dye coverage.

Since PES is a very surface sensitive technique, the estimation of the dye coverage is true for the outermost surface of the TiO2 film. Additional knowledge about the dye packing on the TiO2 surface can be obtained by measuring the dye coverage from dye desorption measurements, since this gives information about the surface coverage within the entire TiO2 film. The surface coverage of the dyes obtained from measurements of dye desorption and the relative dye coverage measured by PES is given in Table 5. Table 5. Dye coverage of the TiO2-surface, measured both by dye desorption measurements and by PES.



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Dye coverage / mol cm-2

Relative dye coverage

Relative dye coverage

(dye desorption)

(dye desorption)

(PES)

D35

1.09×10-7

1.09

0.84

LEG1

1.35×10-7

1.35

1.72

LEG2

1.40×10-7

1.40

1.42

LEG3

1.73×10-7

1.73

1.35

LEG4

9.99×10-8

1

1

Dye

The results show in agreement with both dye desorption measurements and PES measurements that there is less LEG4 on the TiO2-surface, compared to the other LEG dyes. The larger tilt angle found for the LEG4 dye with respect to the TiO2-surface normal compared to the other dyes is likely to result in less efficient packing of the dye molecules and thereby a poor blocking effect of the TiO2- surface, leading to enhanced recombination losses. The better conversion efficiency obtained for the LEG4 dye is therefore mainly attributed to the more red-shifted absorption edge of the dye and the higher extinction coefficient. Thus, future research needs to be focused on controlling the binding morphology of the dye in order to improve the efficiency of the dye further. Despite that the estimated tilt angle with respect to the TiO2 normal for the LEG3 dye is larger than for the LEG1 dye, the packing of the dye molecules is still effective and the introduction of a hexyl chain on the linker unit efficiently decreases recombination losses further and therefore increases the VOC. The introduction of alkyl chains on the dyes has previously been found to be an efficient concept in preventing recombination losses6, but the position of the alkyl chains was here also found to affect the binding morphology of the dyes and thus the surface coverage.


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Conclusions The spectral response of the D35 dye was efficiently enhanced into the red wavelength region by modifying the π- conjugated linker unit using dithiophene units with different substituents. The LEG4 dye had the best photovoltaic performance of the different modified dyes, but even though, it showed lower dye coverage compared to the other dyes. The lower dye packing on the TiO2surface of the LEG4 leads to poor blocking effect of the TiO2-surface, leading to enhanced recombination losses and shorter electron life time in the TiO2. The LEG4 dye was showed to have lower dye packing by dye desorption measurements and by using PES as technique for calculating the morphology of the dyes attached to the TiO2-surface. This illustrates that despite the better photovoltaic performance found for the LEG4 dye, having a cyclopentadithiophene linker unit substituted with hexyl chains, more research needs to be focused on controlling the binding morphology of the dyes with respect to the TiO2-surface, in order to diminish recombination losses and to improve the efficiency further. The introduction of alkyl chains on the dye has been proven a useful concept to prevent recombination losses6,7, but the position of the alkyl chains and other substituents can affect the binding morphology of the dyes and thus the surface coverage.

Acknowledgment: We acknowledge the Swedish Energy Agency, Knut and Alice Wallenberg Foundation, the Swedish Research Council, EU Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 246124 of the SANS project, and STandUP for Energy program for financial support.

Supporting Information Available: NMR-spectrum of Co(bpy)3(PF6)3, spectral- and electrochemical data for the dyes, IV curve of solar cells sensitized with D35 and the LEG-series of 23 ACS Paragon Plus Environment


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dyes with cheno added in the dye baths, extracted charge measurements for solar cells sensitized with D35 and the LEG-series, table with HOMO-levels of the dyes measured on dye-sensitized

TiO2 films, transient absorption traces showing recombination and regeneration to the oxidized dye molecules, photo induced absorption spectra of D35 and the LEG-series. Also the N1s spectra for the LEG-series of dyes and full author lists for references 14 and 20. This information is available free of charge via the Internet at http://pubs.acs.org.


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References (1) Feldt, S. M.; Gibson, E. A.; Gabrielsson, E.; Sun, L.; Boschloo, G.; Hagfeldt, A. Design of Organic Dyes and Cobalt Polypyridine Redox Mediators for High-Efficiency DyeSensitized Solar Cells. J. Am. Chem. Soc. 2010, 132, 16714-16724. (2) Daeneke, T.; Kwon, T.-H.; Holmes, A. B.; Duffy, N. W.; Bach, U.; Spiccia, L. High-efficiency Dye-Sensitized Solar Cells with Ferrocene-Based Electrolytes. Nat Chem. 2011, 3, 211-215. (3) Bai, Y.; Zhang, J.; Zhou, D.; Wang, Y.; Zhang, M.; Wang, P. Engineering Organic Sensitizers for Iodine-Free Dye-Sensitized Solar Cells: Red-Shifted Current Response Concomitant with Attenuated Charge Recombination. J. Am. Chem. Soc. 2011, 133, 1144211445. (4) Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.; Grätzel, M. Porphyrin-Sensitized Solar Cells with Cobalt (II/III) Based Redox Electrolyte Exceed 12 Percent Efficiency. Science. 2011, 334, 629-634. (5) Jiang, X.; Marinado, T.; Gabrielsson, E.; Hagberg, D. P.; Sun, L.; Hagfeldt, A. Structural Modification of Organic Dyes for Efficient Coadsorbent-Free Dye-Sensitized Solar Cells. J. Phys. Chem. C. 2010, 114, 2799-2805. (6) Cao, Y.; Cai, N.; Wang, Y.; Li, R.; Yuan, Y.; Wang, P. Modulating the Assembly of Organic Dye Molecules on Titania Nanocrystals via Alkyl Chain Elongation for Efficient Mesoscopic Cobalt Solar Cells. Phys. Chem. Chem. Phys. 2012, 14, 8282-8286. (7) Murakami, T. N.; Koumura, N.; Uchiyama, T.; Uemura, Y.; Obuchi, K.; Masaki, N.; Kimura, M.; Mori, S. Recombination Inhibitive Structure of Organic Dyes for Cobalt Complex Redox Electrolytes in Dye-Sensitised Solar Cells. J. Mater. Chem. A. 2013, 1, 792-798. (8) Hagberg, D. P.; Marinado, T.; Karlsson, K. M.; Nonomura, K.; Qin, P.; Boschloo, G.; Brinck, T.; Hagfeldt, A.; Sun, L. Tuning the HOMO and LUMO Energy Levels of Organic Chromophores for Dye Sensitized Solar Cells. J. Org. Chem. 2007, 72, 9550-9556 . (9) Xu, M.; Zhang, M.; Pastore, M.; Li, R.; De Angelis, F.; Wang, P. Joint Electrical, Photophysical and Computational Studies on D-π-A Dye Sensitized Solar Cells: The Impacts of Dithiophene Rigidification. Chem. Sci. 2012, 3, 976-983. (10) Tsao, H. N.; Comte, P.; Yi, C.; Grätzel, M. Avoiding Diffusion Limitations in Cobalt(III/II)-Tris(2,2′-Bipyridine)-Based Dye-Sensitized Solar Cells by Tuning the Mesoporous TiO2 Film Properties. ChemPhysChem. 2012, 13, 2976-2981. (11) Gabrielsson, E.; Ellis, H.; Feldt, S.; Tian, H.; Boschloo, G.; Hagfeldt, A.; Sun, L. Convergent/Divergent Synthesis of a Linker-Varied Series of Dyes for Dye-Sensitized Solar Cells Based on the D35 Donor. Adv. Energy Mater. 2013. DOI: 10.1002/aenm.201300367. (12) Ellis, H.; Vlachopoulos, N.; Häggman, L.; Perruchot, C.; Jouini, M.; Boschloo, G.; Hagfeldt, A. PEDOT Counter Electrodes for Dye-Sensitized Solar cells Prepared by Aqueous Micellar Electrodeposition. Electrochim Acta. 2013, 107, 45-51. (13) Boschloo, G.; Häggman, L.; Hagfeldt, A. Quantification of the Effect of 4-tertButylpyridine Addition to I-/I3- Redox Electrolytes in Dye-Sensitized Nanostructured TiO2 Solar Cells. J. Phys. Chem. B. 2006, 110, 13144-13150.



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(14) Bässler, M.; Forsell, J. O.; Björneholm, O.; Feifel, R.; Jurvansuu, M.; Aksela, S.; Sundin, S.; Sorensen, S. L.; Nyholm, R.; Ausmees, A.; et al. Soft X-ray Undulator Beam Line I411 at MAX-II for Gases, Liquids and Solid Samples. J. Electron. Spectr. Rel. Phen 1999, 103, 953-957. (15) Johansson, E. M. J.; Hedlund, M.; Siegbahn, H.; Rensmo, H. Electronic and Molecular Surface Structure of Ru(tcterpy)(NCS)3 and Ru(dcbpy)2(NCS)2 Adsorbed from Solution onto Nanostructured TiO2: A Photoelectron Spectroscopy Study. J. Phys. Chem. B. 2005, 109, 22256-22263. (16) Wang, Z.-S.; Cui, Y.; Dan-oh, Y.; Kasada, C.; Shinpo, A.; Hara, K. ThiopheneFunctionalized Coumarin Dye for Efficient Dye-Sensitized Solar Cells: Electron Lifetime Improved by Coadsorption of Deoxycholic Acid. J. Phys. Chem. C. 2007, 111, 7224-7230. (17) Hagberg, D. P.; Jiang, X.; Gabrielsson, E.; Linder, M.; Marinado, T.; Brinck, T.; Hagfeldt, A.; Sun, L. Symmetric and Unsymmetric Donor Functionalization. Comparing Structural and Spectral Benefits of Chromophores for Dye-Sensitized Solar Cells. J. Mater. Chem. 2009, 19, 7232-7238. (18) Marinado, T.; Hagberg, D. P.; Hedlund, M.; Edvinsson, T.; Johansson, E. M. J.; Boschloo, G.; Rensmo, H.; Brinck, T.; Sun, L.; Hagfeldt, A. Rhodanine Dyes for Dye-Sensitized Solar Cells : Spectroscopy, Energy Levels and Photovoltaic Performance. Phys. Chem. Chem. Phys. 2009, 11, 133-141. (19) Khazraji, A. C.; Hotchandani, S.; Das, S.; Kamat, P. V. Controlling Dye (Merocyanine-540) Aggregation on Nanostructured TiO2 Films. An Organized Assembly Approach for Enhancing the Efficiency of Photosensitization. J. Phys. Chem. B. 1999, 103, 46934700. (20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision A.02; Gaussian, Inc., : Wallingford CT, 2009. (21) Westermark, K.; Tingry, S.; Persson, P.; Rensmo, H. k.; Lunell, S.; Hagfeldt, A.; Siegbahn, H. Triarylamine on Nanocrystalline TiO2 Studied in Its Reduced and Oxidized State by Photoelectron Spectroscopy. J. Phys. Chem. B. 2001, 105, 7182-7187. (22) Nyhlen, J.; Boschloo, G.; Hagfeldt, A.; Kloo, L.; Privalov, T. Regeneration of Oxidized Organic Photo-Sensitizers in Grätzel Solar Cells: Quantum-Chemical Portrait of a General Mechanism. ChemPhysChem. 2010, 11, 1858-1862. (23) Closs, G. L.; Miller, J. R. Intramolecular Long-Distance Electron Transfer in Organic Molecules. Science. 1988, 240, 440-447. (24) Johansson, E. M. J.; Edvinsson, T.; Odelius, M.; Hagberg, D. P.; Sun, L.; Hagfeldt, A.; Siegbahn, H.; Rensmo, H. Electronic and Molecular Surface Structure of a Polyene−Diphenylaniline Dye Adsorbed from Solution onto Nanoporous TiO2. J. Phys. Chem. C 2007, 111, 8580-8586. (25) Hahlin, M.; Johansson, E. M. J.; Plogmaker, S.; Odelius, M.; Hagberg, D. P.; Sun, L.; Siegbahn, H.; Rensmo, H. Electronic and Molecular Structures of Organic Dye/TiO2 Interfaces for Solar Cell Applications: A Core Level Photoelectron Spectroscopy Study. Phys. Chem. Chem. Phys. 2010, 12, 1507-1517. (26) Cumpson, P. J. Estimation of Inelastic Mean Free Paths for Polymers and Other Organic Materials: Use of Quantitative Structure–Property Relationships. Surf. Interface Anal. 2001, 31, 23-34.


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