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Copper Phenanthroline as Fast and High Performance Redox Mediator for Dye Sensitized Solar Cells Marina Freitag, Fabrizio Giordano, Wenxing Yang, Meysam Pazoki, Yan Hao, Burkhard Zietz, Michael Grätzel, Anders Hagfeldt, and Gerrit Boschloo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b01658 • Publication Date (Web): 15 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

Copper Phenanthroline as Fast and High Performance Redox Mediator for Dye Sensitized Solar Cells.

Marina Freitag†§*, Fabrizio Giordano#, Wenxing Yang†, Meysam Pazoki†, Yan Hao†, Burkhard Zietz†, Michael Grätzel#, Anders Hagfeldt†§ and Gerrit Boschloo† †Department of Chemistry- Ångström Laboratory, Uppsala University, Box 523, SE-751 20 Uppsala, Sweden # Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering École Polytechnique Fédérale de Lausanne 1015, Lausanne, Switzerland Corresponding Author; E-mail: [email protected] Phone: +41 21 69 33621 §Current address: Laboratory for Photomolecular Science, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, CH 1015 Lausanne, Switzerland.

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Abstract The most commonly used redox mediators in dye-sensitized solar cells (DSCs), iodide/triiodide and cobalt trisbipyridine ([Co(bpy)3]2+/3+), were successfully replaced by bis (2,9-dimethyl-1,10phenanthroline)copper(I/II) ([Cu(dmp)2]1+/2+). The use of the copper complex based electrolyte led to an exceptionally high photovoltaic performance of 8.3 % for LEG4-sensitized TiO2 solar cells, with a remarkably high open-circuit potential of above 1.0 V at 1000 W m-2 under AM1.5G conditions. The copper complex based redox electrolyte has higher diffusion coefficients and is considerably faster in dye regeneration than comparable cobalt trisbipyridine-based electrolytes. A driving force for dye regeneration of only 0.2 eV is sufficient to obtain unit yield, pointing to new possibilities for improvement in DSC efficiencies. The interaction of the excited dye with components of the electrolyte was monitored using steady state emission measurements and timecorrelated single photon counting (TC-SPC). Our results indicate bimolecular reductive quenching of the excited LEG4 dye by the [Cu(dmp)2]2+ complex through a dynamic mechanism. Excited state dye molecules can readily undergo bimolecular electron transfer with a suitable donor molecule. In DSCs this process can occur when the excited dye is unable to inject electrons into the TiO2. With a high electrolyte concentration the excited dye can be intercepted with an electron from the electrolyte resulting in the reduced state of the dye. Quenching of the reduced dye bye the electrolyte competes with electron injection and results in a lower photocurrent. Quenching of excited LEG4 by complexes of [Cu(dmp)2]1+, [Co(bpy)3]2+ and [Co(bpy)3]3+ followed a static mechanism, due ground state dye - quencher binding. Inhibition of unwanted quenching processes by structural modifications may open ways to further increase the overall efficiency.


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Introduction The worldwide demand for energy is increasing and with it also the interest in renewable energy. A special focus is on solar energy, which shows rapid progress especially in the field of photovoltaics, opening up vast opportunities in research and technology for using renewable energy sources. Among the emerging photovoltaic technologies are the dye-sensitized solar cells (DSCs), developed in 1991 by O’Regan and Grätzel, which are based on dye molecules attached to a nanostructured semiconductor (photoanode).1 They are a potential low-cost alternative for large-scale solar energy conversion2 with top efficiencies of about 13%.3,4 The use of low cost materials, scalable manufacturing technologies with low investments costs, design possibilities with different colors, and high performance in low or diffuse light conditions are attractive features for the DSC technology.5,6,7 The redox couple is a key component in the liquid electrolyte-based DSCs, assuming the tasks of dye regeneration and charge transport between the two electrodes,8 which is crucial for the photovoltaic performance of DSCs.9 Finding an efficient, non-corrosive electron-transfer mediator is an important step towards higher efficiency, longevity, and thus, the practical applications of DSCs.10–13 The development of new redox mediator systems is still far behind the efforts which were made to develop new dyes or any other component of the DSCs. The majority of studies were focused on traditional iodide/triiodide electrolytes, which show major drawbacks including a large potential drop (driving force) for dye regeneration and competitive light absorption.14–16 In 2010 Feldt et al. demonstrated for the first time high efficiency DSCs using cobalt trisbipyridine ([Co(bpy)3]2+/3+) as redox mediator and an organic triphenylamine based dye (D35).



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The advantages of cobalt complexes is that they require less driving force for dye regeneration, since they are one-electron outer-sphere transition metal complexes, that are non-volatile, noncorrosive, light-colored, and have a tunable potential (E0’ = 0.3−1.0 V vs. NHE) through modification of the ligands. However, owing to the large size of the cobalt complex, limited solubility, and relatively fast electron transfer kinetics, Co-complex redox electrolytes are subject to mass-transport limitations and recombination losses.17–19 Apart from cobalt-based redox couples, other metal complexes and clusters, based on Ni(III)/Ni(IV)20,21, Cu(I)/Cu(II)13,22, and Fe(II)/Fe(III)9,23, have also been investigated. Fukuzumi and coworkers were the first to use Cu(I)/Cu(II)-based electrolytes as redox mediators in dye-sensitized solar cells (DSC),22 and attained a power conversion efficiency (PCE) of 2.2% using bis(2,9-dimethyl-1,10phenanthroline)copper(I/II) ([Cu(dmp)2]1+/2+, Fig. 1) under reduced solar light irradiation (20 mW cm-2). More recently, Peng Wang and coworkers reached 7.0% efficiency using the same complex under full sunlight, utilizing an organic dye.13 The use of copper complexes with a distorted tetragonal geometry, in which the structural change between the [Cu(dmp)2]1+and [Cu(dmp)2]2+ complexes is minimized, clearly provides a promising strategy to electron mediators

for DSCs, because they have a fast electron transfer and a redox potential suitable for dye regeneration.24 In the present work, we have studied the [Cu(dmp)2]1+/2+, redox shuttle in combination with a high-absorption-coefficient organic photosensitizer LEG425–27 displaying an impressive solar to electricity conversion efficiency of 8.3 % and open-circuit potentials above 1.0 V under 100 mW cm-2 AM1.5G illumination conditions. Further, the interaction between the excited state of the dye and the components of the electrolyte was investigated by steady state emission measurements and time-correlated single photon counting (TC-SPC).28–32 Redox mediators can 4 ACS Paragon Plus Environment

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potentially lead to affect reductive or oxidative quenching of the excited states prior to electron injection into the semiconductor nanoparticle.33,34 Here we provide experimental evidence for efficient quenching of the excited state of LEG4, following different quenching mechanisms depending exclusively on the metal complex used. Comparative studies were performed with the sensitizer anchored on ZrO2 nanostructured films, where a static quenching mechanism was found to be present for [Cu(dmp)2]2+, complexes.

Figure 1. Molecular structure of the copper complex redox mediator [Cu(dmp)2]1+/2+, (left) and the dye LEG4 (right) studied in this work. The counter ions for the copper complexes are trifluoromethanesulfonimide (TFSI) for [Cu(dmp)2]1+ and chloride (Cl) and TFSI for [Cu(dmp)2]2+



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Experimental Section Materials. All chemicals were purchased from Sigma Aldrich unless otherwise noted. Bis(2,9dimethyl-1,10-phenanthroline)copper(I/II)

bis(trifluoromethane)sulfonimide

/

chloride

(Cu(dmp)2 TFSI and Cu(dmp)2 TFSI Cl) were received from Merck. LEG4 was purchased from Dyenamo AB (Stockholm, Sweden). Solar Cell Preparation. Fluorine-doped tin oxide (FTO) glass substrates (Pilkington, TEC15) were cleaned in an ultrasonic bath for 1h in the following order of solvents; water, ethanol, and acetone. The conducting glass substrates were pretreated by immersion in a 40 mM aqueous TiCl4 solution at 70 °C for 90 min and then washed with water. To prepare 0.25 cm2 mesoporous TiO2 films, colloidal TiO2 paste (Dyesol DSL 30 NRD-T) was screen printed and dried at 120°C for 6 min between each deposition. Subsequently, a light-scattering TiO2 layer was deposited (Dyesol WER2-0) on top of the mesoporous TiO2 film. A profilometer (Veeco Dektak 3) measured the thickness of the active TiO2 films at 5 µm and the thickness of the scattering layer at 2 µm. An oven (Nabertherm Controller P320) gradually heated the electrodes in an air atmosphere, applying a four level program: 180°C (10 min), 320°C (10 min), 390°C (10 min), and 450°C (30 min). After sintering, the electrodes were treated in aqueous TiCl4 at 70°C for 30 min and afterwards washed with water. A final heating step at 500°C (60 min) was performed followed by overnight immersion of the electrodes in the dye bath solution. The dye bath contains 0.2 mM LEG4 in tert-butanol and acetonitrile (1:1). The mesoporous ZrO2 films were prepared on glass substrates by a doctorblading procedure described previously35 and sensitized by immersing them into 0.1 mM LEG4 in tert-butanol and acetonitrile (1:1). After the sensitization procedure all films were then rinsed in acetonitrile to remove excess dye. Solar cells 6 ACS Paragon Plus Environment

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were assembled, using a 25 µm thick thermoplastic Surlyn frame, with a PEDOT-coated counter electrode (TEC8). The PEDOT electrodes were prepared by electropolymerization of EDOT from a micellar aqueous solution of 0.1 M SDS and 0.01 M EDOT.36 In the next step the electrolyte solution was introduced through a hole, predrilled in the counter electrode, under vacuum. The cells were sealed with thermoplastic Surlyn covers and a glass coverslip. Unless otherwise noted, the electrolyte consisted of 0.20 M Cu(dmp)2TFSI, 0.04 M Cu(dmp)2TFSI Cl, 0.1 M LiTFSI, and 0.5 M 4-tert-butylpyridine (TBP) in acetonitrile. For comparison, DSCs were prepared using 0.22 M Co(bpy)3(PF6)2, 0.05 M Co(bpy)3(PF6)3, 0.1 M LiTFSI, and 0.2 M TBP in acetonitrile. Solar Cell Characterization. Current-voltage (I-V) characteristics were measured with a Keithley 2400 source and a Newport solar simulator (model 91160). The simulator gave light with an AM 1.5 G spectral distribution and was calibrated using a certified reference solar cell (Fraunhofer ISE). The intensity was calibrated to 1000 W m-2 and using a neutral density filter to 100 W m-2. A black metal mask with a 0.5x0.5 cm2 aperture was used to define the active area. Incident Photon to Current Conversion Efficiency (IPCE). IPCE spectra were recorded using a computer-controlled setup consisting of a Xenon light source (Spectral Products ASB-XE175), a monochromator (Spectral Products CM110) and a multimeter (Keithley 2700), calibrated using a certified reference solar cell (Fraunhofer ISE). Electron lifetime measurements were performed using a white LED (Luxeon Star 1W) as a light source. Voltage traces were recorded with a 16-bit resolution digital acquisition board (National Instruments) and lifetimes were determined by monitoring photovoltage transients at different light intensities upon applying a small square wave modulation to the base light 7 ACS Paragon Plus Environment


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intensity. The photovoltage responses were fitted using first-order kinetics to obtain time constants. Photoinduced Absorption Spectroscopy (PIA). PIA measurements were performed using the setup as previously described.32 A square wave modulated blue LED (Luxeon Star 1 W, Royal Blue, 460 nm) used for excitation was superimposed on a white probe light provided by a 20 W tungsten-halogen lamp. The transmitted probe light was focused onto a monochromator (Acton Research Corp. SP-150) and detected using a UV-enhanced Si photodiode, connected to a lockin amplifier (Stanford Research Systems model SR830) via a current amplifier (Stanford Research Systems model SR570). The intensity of the probe light was approximately 100 Wm-2, and the intensity of the excitation LED was approximately 80 W m-2. The modulation frequency of the LED was 9.33 Hz. Transient Absorption Spectra (TAS). TAS and electron transfer kinetics were recorded by a frequency tripled Q-switched Nd:YAG laser coupled with a MOPO (Spectra-Physics S12 Quanta-Ray system) to obtain the desired wavelength for the pump light of 520 nm (~13 ns pulse at 10 Hz). The energy of the pump pulse was kept around 1 mJ. The cross-section area was estimated at 0.35 cm2, from a pump beam diameter of ca 0.5 cm2 and 45° positioning of the film vs. the incoming pump. A detection system (Edinburgh Instrument, L920) was used, equipped with a Xe arc lamp (continuous wave), a monochromator and an R928-type PMT connected to a 500MHz oscilloscope (Tektronix TDS 3052B) for kinetic traces. Electrochemical Measurements. Cyclic voltammetry was completed on a CH Instruments 660 potentiostat with a three-electrode setup. Ferrocene was used to calibrate the potential before and after each measurement. 8 ACS Paragon Plus Environment

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For diffusion coefficients measurements a 20μm diameter platinum microelectrode was used as the working electrode, glassy carbon as the counter electrode, and Ag/AgNO3 (10 mM AgNO3, 0.1 M TBAPF6 in acetonitrile) as a reference electrode. The electrolyte solution contained 5 mM [Cu(dmp)2]+, [Cu(dmp)2]2+, [Co(bpy)3]2+, or [Co(bpy)3]3+ and 0.1 M Li TFSI, 0.5 M TBP in acetonitrile. The scan rate was 10 mV s-1. Electrochemical impedance spectroscopy (EIS) on complete cells was acquired at various potentials (24 steps linearly spaced between 0 and 1.2 V in forward bias). The measurements were performed in dark using a SP-300 bi-potentiostat (Biologic Science Instrument). The voltage perturbation of 10 mV amplitude was superimposed onto the DC bias and modulated at frequencies ranging from 7MHz to 0.1Hz. The impedance data were analyzed using EC-Lab software. Steady-State Spectroscopy. Emission spectra were measured on a Horiba Jobin Yvon Fluorolog and automatically corrected for wavelength dependent instrument sensitivity. Solution based experiments were performed at a right angle in a 1 cm quartz cuvette and mesoporous ZrO2 films were measured at front face at ca. 45° angle. Time-Correlated Single Photon Counting (TC-SPC). A detailed description of the experimental setup has been published previously.31,32 Briefly, the sample was excited with a picosecond diode laser (Edinburgh Instruments, EPL 470) at 470 nm (87 ps pulses). The laser pulse energy was ca. 15 pJ and was attenuated (often more than 1 order of magnitude) to the desired count rate of 1% or less of the excitation frequency. The LEG4 sensitized ZrO2 (0.7 cm2) were immersed in 3.0 ml of acetonitrile and titrated with 5-20 μl aliquots of 0.1 mM solutions of each quencher ([Cu(dmp)2]+, [Cu(dmp)2]2+, [Co(bpy)3]2+, or [Co(bpy)3]3+) in acetonitrile. Decay curves obtained by single photon counting were analyzed by iterative reconvolution using an 9 ACS Paragon Plus Environment


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exponential decay model with one, two, or three components in the Spectra Solve program. The instrument response function (IRF) was free to move relative to the decay during analysis.

Results and Discussion 1. Photovoltaic Performance of Dye-sensitized Solar Cells with Copper Phenanthroline Redox Mediators J-V characteristics were measured for LEG4-sensitized solar cells containing the [Cu(dmp)2]1+/2+ redox shuttle, see Figure 2.

Figure 2. Current density J vs. applied potential V curves of LEG4-sensitized DSCs with Cu(dmp)2 or Co(bpy)3 based electrolyte under AM1.5G illumination (100 mW cm-2) .

In terms of efficiency, the photovoltaic performance of [Cu(dmp)2]1+/2+ was comparable to that of the more frequently used [Co(bpy)3]3+/2+ redox shuttle for thin-film DSCs sensitized with triphenylamine-based organic sensitizers, see Table 1. The open-circuit voltage (VOC) of devices with [Cu(dmp)2]1+/2+ redox mediator was more than 1 V, and exceeded that of [Co(bpy)3]3+/2+ by


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more than 100 mV. This high value is ascribed to the much more positive redox potential of [Cu(dmp)2]2+/1+ (E0’ = 0.94 V vs. NHE, see Supporting Information and Table 2) compared to that of [Co(bpy)3]3+/2+ (E0’ = 0.56 V vs. NHE). The short circuit current density (JSC) was, however, about 10% lower for [Cu(dmp)2]1+/2+. In this study, a top efficiency of 8.1 % at 1000 W m−2 illumination for the DSCs sensitized was obtained with the [Cu(dmp)2]1+/2+ based electrolyte. The DSCs with copper based electrolyte exhibited a lower fill factor in comparison to the devices with cobalt based electrolyte. The reason seems related to higher series resistance. These results are unexpected, since the measured diffusion coefficients are higher for copper complexes. The solar cells were further characterized by Impedance spectroscopy. The Nyquist plot of the copper based device shows a large semicircle related to the diffusion resistance of the redox mediator, which is responsible for the low fill factor in the solar cells. The precise origin of the large diffusion resistance will be investigated in future work. The charge transfer resistance at the counter electrode is well defined and lower than 1 Ohm (Figure S 3.).

Table 1. Current-voltage characteristics of DSCs sensitized with LEG4 with [Cu(dmp)2]1+/2+ or [Co(bpy)3]2+/3+ based electrolytes Dye / electrolyte

Voc [mV]

Jsc [mAcm-2]

FF

η [%]

LEG4 / [Cu(dmp)2] 1+/2+

1045

11.0

0.60

7.0

LEG4 / [Cu(dmp)2] 1+/2+*

1020

12.6

0.62

8.3

LEG4 / [Co(bpy)3] 2+/3+

875

12.7

0.66

7.3

* Best performing device, solar cell preparation and characterization is given in detail in Supporting Information.



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IPCE spectra of DSCs using cobalt and copper complexes based electrolytes are shown in (Figure 3.) The IPCE for the LEG4 based DSCs with the copper phenanthroline complexes reached a maximum IPCE of about 75% at 500 nm in comparison to the ∼80% maximum IPCE achieved with [Co(bpy)3]2+/3+ under similar conditions.37 The cobalt based electrolyte shows higher IPCE value in the region of 400 nm – 475 nm and again 525 nm – 600 nm. The IPCE spectra with copper based electrolyte and LEG4 correspond well with the absorption spectra of the dye itself.37 The drop in the IPCE efficiency of 10-15% is caused by the present absorptivity of the [Cu(dmp)2]1+/2+ based electrolyte in the wavelength range of 400 nm to 525 nm. The difference in IPCE spectra can also be the consequence of the higher recombination between [Cu(dmp)2]1+/2+ and the dye and/or the TiO2/FTO layer in comparison to the [Co(bpy)3]2+/3+ electrolyte. This was further investigated by time-resolved fluorescence measurements that indicate an efficient reductive quenching of LEG4 (Figure 9.). These combined aspects result in the lower light harvesting efficiency throughout the wavelength range. The lower current, which was measured for [Cu(dmp)2]1+/2+ based electrolyte can be explained by the IPCE spectrum. Overall, the relatively high IPCE values suggest good electron injection efficiency from LEG4 into TiO2, as well as good regeneration of the dye and good charge collection efficiencies. In absence of an antireflecting coating, the maximum attainable IPCE is estimated to be about 90%. Nevertheless, about 15% of the absorbed photons are estimated to be lost in the photoconversion process, indicating room for investigation and improvements.


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Figure 3. IPCE spectrum for LEG4-sensitized DSC with [Cu(dmp)2]1+/2+ and [Co(bpy)3]2+/3+ electrolyte 2. Dye Regeneration by Copper Redox Mediator and Recombination The regeneration of the dyes by the copper complexes was investigated with photoinduced absorption spectroscopy, and performed under conditions similar to the operational conditions of DSCs (illumination intensity ca. 100 W m−2). The PIA spectra of LEG4 photoanode in the presence and absence of the copper phenanthroline are shown in Figure 4.

Figure 4 PIA spectra of DSCs employing LEG4 with inert electrolyte (black), and with [Cu(dmp)2]1+/2+ based electrolyte (blue). 13 ACS Paragon Plus Environment


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In the absence of a redox mediator, a bleach is found at 560 nm, and absorption peaks of oxidized dye at 650 nm and 800 nm are found for LEG4 adsorbed on mesoporous TiO2.25 The bleach is caused by a ground-state bleach of the dye upon oxidation, together with a Stark shift caused by the photoinjected electrons that give a local change in the electrical field across the dye molecules.38–41 When the [Cu(dmp)2]1+/2+ based electrolyte is added, the bleach of groundstate dye and the absorption peaks of oxidized dye disappear because the oxidized dye is rapidly regenerated by the redox mediator. A bleach at about 600 nm persists, caused by the Stark effect. A weak featureless absorption is found at wavelengths higher than 700 nm, which is ascribed to electrons in mesoporous TiO2. The kinetics of dye regeneration using the copper phenanthroline redox mediator in the electrolyte was further investigated by nanosecond transient absorption spectroscopy (TAS), Figure 5. The decay of the absorbance signal in the presence of inert electrolyte (0.1 M LiTFSI, 0.5M TBP in acetonitrile) shows the recombination of electrons in TiO2 with oxidized dye molecules with a half-time (t1/2) of 86 µs. In presence of cobalt or copper complex electrolyte the decay of the signal accelerates, indicating that the redox couples regenerate the dye molecules. The regeneration half time (t1/2) for the [Cu(dmp)2]1+ complex was much faster (t1/2 = 1.3 µs), than that of the [Co(bpy)3]2+ redox mediator (4.8 µs) under similar conditions, see Table 2. The four times faster regeneration kinetics for [Cu(dmp)2]1+ are surprising considering the small driving force (–ΔG) for the process, about 0.2 eV for [Cu(dmp)2]1+, while it is 0.5 eV for [Co(bpy)3]2+. Copper phenanthroline has a distorted tetragonal geometry, in which the structural change in conformation between the copper(I) and copper(II) complex has a high self-exchange rate for electron transfer and therefore lower reorganization energy.22 The dye regeneration rate constant, kreg and regeneration efficiency, Φreg were calculated using Equation 1. The calculated


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Φreg is very close to 100 % for the [Cu(dmp)2]1+/2+ based electrolyte, while it is 4% lower for the cobalt electrolyte under the conditions used. 𝜙!"# = !

!!"# !"# !!!"#

≈1−

!!/!,!"#

%$(Eq. 1)

!!/!,!"#

%$Figure 5. Nanosecond laser transient absorption spectroscopy measurements of LEG4 on mesoporous TiO2 with inert electrolyte (black), [Co(bpy)3]2+/3+ (green) and with [Cu(dmp)2]1+/2+ (blue) based electrolytes. Excitation wavelength 520 nm, probe wavelength 750 nm. Table 2. Transient absorption decay half-times and calculated regeneration efficiencies for LEG4 on TiO2 with inert electrolyte, and with [Co(bpy)3]2+/3+ and [Cu(dmp)2]1+/2+ based electrolyte.

Dye/Electrolyte LEG4/ inert electrolyte LEG4/ [Cu(dmp)2]1+/2+ LEG4/ [Co(bpy)3]2+/3+

E0´ [V vs. NHE] 1.04a 1.04/ 0.94 1.04/ 0.56

t½ [µs]

Φreg [%]

85.7 1.3* 4.8*

98.4 94.4

*the signal offset due absorption of long-live electrons is taken into consideration 15 ACS Paragon Plus Environment


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a

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On mesoporous TiO225

Figure 6. Electron lifetime as a function of quasi Fermi level (EF) of TiO2 under open circuit conditions of DSCs employing the dye LEG4 with [Cu(dmp)2]1+/2+ and, for comparison, [Co(bpy)3]2+/3+ based electrolyte. Complete solar cells were used to investigate the recombination reaction between electrons in TiO2 and the two metal complex redox mediators. In Figure 6, the measured electron lifetimes of DSCs sensitized with LEG4 employing [Cu(dmp)2]1+/2+ and [Co(bpy)3] 2+/3+ based electrolyte are compared. Figure 6 shows a semi-logarithmic plot of the electron lifetime as a function of the open circuit voltage of the TiO2 measured under different light intensities. The electron lifetimes for [Cu(dmp)2]1+/2+ are significantly higher than for [Co(bpy)3]2+/3+ at the quasi Fermi level (EF) of TiO2 under open circuit conditions. Considering that the redox potential of the electrolyte is about 0.38 V more positive for [Cu(dmp)2]1+/2+, it must be concluded that the electron recombination is faster by one order of magnitude for this redox electrolyte under conditions of equal Fermi level in TiO2. The change of the slope of the curve for copper based electrolyte can be attributed to electron recombination to the copper complexes at the FTO substrate, which is 16 ACS Paragon Plus Environment

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covered by a thin TiO2 blocking layer prepared by a chemical bath deposition method using TiCl4. This recombination process becomes apparent at low light intensities, suggesting that there are pinholes in the blocking layer. This indicates that a better blocking layer on the FTO substrate would further improve the solar cell performance, specifically under low-light conditions. 3. Mass Transport Limitations of Copper Dimethyl-Phenanthroline Complexes as Redox Mediators The transport of the electroactive ions is expected to have a great influence on the overall performance of DSCs. The copper phenanthroline complexes are considerably less bulky in comparison to the cobalt trisbipyridine complexes and are expected to diffuse more efficiently in electrolyte solvent and the dye-coated mesoporous TiO2 layer. Nevertheless, the [Cu(dmp)2]1+/2+ complexes will experience mass transport limitations due to their phenanthroline ligands and their positive charge, which may lead to electrostatic attraction towards the TiO2 surface. The diffusion coefficients for [Cu(dmp)2]1+/2+ complexes, determined by cyclic voltammetry using a microelectrode, were found to be more than two times higher than those for the cobalt trisbipyridine complexes, see Table 3. This higher value is attributed to their smaller size. Table 3. Diffusion coefficients at 298 K of [Cu(dmp)2]1+/2+ and [Co(bpy)3]2+/3+ redox mediator in acetonitrile-based electrolytes.a D [10-6 cm2s-1] [Cu(dmp)2]1+

15.0

[Cu(dmp)2]2+

25.0

2+

6.6

[Co(bpy)3]3+

9.4

[Co(bpy)3]

a) Conditions: 5 mM metal complex, 0.1M LiTFSI and 0.5 M TBP in acetonitrile



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The effect of mass transport limitations was further investigated in actual DSC devices by recording the photocurrent transients using a large modulation of the incident light. Figure 7 shows photocurrent transients for a LEG4-sensitized DSC with [Cu(dmp)2]1+/2+ electrolytes under various light intensities. A maximum photocurrent is observed when the light is switched on and followed by a slight decrease until reaching a plateau in the photocurrent. At lower light intensities no peak current was observed. The photocurrent maximum at high light intensities is the result of mass transport of [Cu(dmp)2]2+ to the counter electrode, giving a decrease in the photocurrent at current densities higher than about 10 mA cm-2. Such effects can be minimized by increasing the [Cu(dmp)2]2+ concentration in the electrolytes, but this is not practically possible due to its limited solubility, or by minimizing the thickness of the electrolyte layer. Despite the slight photocurrent limitations at the highest light intensities, the relationship between light intensity and photocurrent is nearly ideally linear, see Fig. 7 (A) and (B).

Figure 7. (A) Photocurrent transients at increasing (0.02 – 1.4 sun) illumination intensities and (B) dependence of photocurrent to the light intensity for DSCs employing LEG4 as the sensitizer and [Cu(dmp)2]1+/2+ as the redox mediator.


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4. Excited State Quenching of LEG4 on ZrO2 with Copper and Cobalt Complex Redox Mediators Quenching effects of the redox mediator in dye-sensitized solar cells is potentially an important effect, but it has been largely neglected in DSC research. Only a few investigations consider quenching in iodide / triiodide redox electrolytes.28,42 Steady state emission measurements and time-correlated single photon counting (TC-SPC) were performed in order to investigate the interaction of the sensitizer LEG4 with the individual copper and cobalt complexes. LEG4 was anchored to ZrO2 mesoporous films as inert substrate and immersed in a cuvette with acetonitrile. In order to access the nature of the quenching mechanism, fluorescence lifetimes were recorded at various concentrations of the quenchers, [Cu(dmp)2]+, [Cu(dmp)2]2+, [Co(bpy)3]2+, or [Co(bpy)3]3+. Quenching of fluorescence can occur due to different interactions between the dye and the metal complex. Generally, quenching processes can be divided in dynamic quenching, where collisional encounters lead to fluorescent quenching, and static quenching, where a nonfluorescent complex is formed between the fluorophore (in this case the dye LEG4) and the quencher (the metal complex). Both possible mechanisms require contact between the dye and the quencher, and therefore quenching studies can also give information about the accessibility of the fluorophore to the quencher. The emission of LEG4 adsorbed on ZrO2 decreased upon addition of all investigated metal complexes to the solution, see Figure 8A for an example. Stern-Volmer analysis of the steadystate emission spectra showed a linear relationship for all metal complexes (Figure 8B). The slope was strongly depending on the nature of the metal complex: strongest quenching was found for [Cu(dmp)2]2+, followed by [Co(bpy)3]2+, [Co(bpy)3]3+ and finally [Cu(dmp)2]+.



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Considering the strong quenching found for micromolar concentrations of the quencher, effects of quenching in DSCs with millimolar to molar concentration are likely to be important for the functioning of the solar cell. The electron donating or accepting properties of [Cu(dmp)2]1+ and [Cu(dmp)2]2+, respectively, can lead to the quenching of the sensitizer due to electron transfer, as indicated in reactions 1 and 2: LEG4* + [Cu(dmp)2]1+ → LEG4- + [Cu(dmp)2]2+

(r1)

LEG4* + [Cu(dmp)2]2+ → LEG4+ + [Cu(dmp)2]1+

(r2)

LEG4* + [Cu(dmp)2]2+ → LEG4 + [Cu(dmp)2]2+

(r3)

Photoreduction of LEG4 (r1) is not necessarily a negative effect, as the reduced dye can still inject an electron into TiO2. Photoxidation of LEG4 by [Cu(dmp)2]2+ (r2) is, on the other hand, a direct loss mechanism: the formed oxidized dye is subsequently regenerated by [Cu(dmp)2]1+. The [Cu(dmp)2]2+ complex has an unpaired d-electron, which may also lead to direct paramagnetic quenching (r3).43,44 To analyze the quenching processes in more detail timeresolved emission studies were performed.

Figure 8. (A) Steady state emission spectral changes of LEG4 on ZrO2 with increasing concentration of [Cu(dmp)2]2+ in acetonitrile (λex= 510 nm, λem=650nm), (B) fitted Stern-Volmer


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plots for F0/F for LEG4 on ZrO2 with increasing concentration of [Cu(dmp)2]1+, [Cu(dmp)2]2+, [Co(bpy)3]2+ or [Co(bpy)3]3+in acetonitrile (λex= 470 nm, λem=650nm). The results for the time-resolved fluorescence measured using the TC-SPC method are given in Figure 9. The emission lifetime of LEG4 adsorbed on ZrO2 and in contact with acetonitrile was determined to be 3.8 ns. The emission lifetime did not change as function of the quencher concentration for the trisbipyridine cobalt complexes, nor for the copper(I) complex, see Supporting Information. Only for the [Cu(dmp)2]2+ complex as quencher, the emission lifetime of LEG4 was found to decrease with increasing quencher concentration. This implies a dynamic quenching mechanism, resulting from purely collisional encounters between the excited dye and the [Cu(dmp)2]2+ complex. The fluorescence quenching data (Figure 8b) are analyzed by the Stern-Volmer equation: !! !

= 1 + 𝐾!" 𝑄 = 1 + 𝑘! 𝜏! 𝑄 ,

(Eq. 2)

where F0 and F are the emission intensities in absence and presence of the quencher. KSV is the Stern-Volmer constant, [Q] is the concentration of the quencher, kq the bimolecular quenching rate and τ0 the excited state lifetime of the fluorophore (dye) in absence of the quencher. As indicated in Figure 8 and Figure 9 the quenching constant was highest in case of the [Cu(dmp)2]2+ complex with kq = 7.9×1010 M-1s-1 (Table 4). In case of [Cu(dmp)2]1+ the quenching rate was considerably slower with 1.0×1010 M-1s-1. For the dye-sensitized solar cell, a time constant for quenching of LEG4* by [Cu(dmp)2]2+ (0.05 M) of 250 ps is calculated, and 500 ps for quenching by [Cu(dmp)2]1+ (0.20 M). As the former is a recombination reaction, it may result in photocurrent losses in the solar cell. Previous studies indicate very fast electron injection from the dye to TiO2 on a tens of femtosecond time scale.28,33,45



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Figure 9. Fluorescence lifetime decay curves for LEG4 on ZrO2 with increasing concentration of [Cu(dmp)2]2+ (instrument response function, IRF, indicated by round symbols), (insert) fitted time-resolved emission lifetime for LEG4 on ZrO2 with increasing concetration of [Cu(dmp)2]2+ in acetonitrile. These studies were, however, often performed in the absence of the electrolyte and under high energy conditions. Under operational conditions, additional much slower components seem to play a significant role.46 For instance, for the organic dye D149 on TiO2, biphasic electron injection was found to occur with time constants of about 300 fs (50%) and 30 ps (50 %). It is not unreasonable to assume that similar biphasic injection is occurring in the case of LEG4 on TiO2, and that significant competitive quenching by the [Cu(dmp)2]2+ complex may take place. It should be noted that under operational conditions the concentration of [Cu(dmp)2]2+ in the mesoporous TiO2 electrode will be higher than its initial value due to its continuous generation in the mesoporous TiO2 electrode (and simultaneous consumption at the counter electrode).47


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Table 4. Quenching Constants of LEG4 on mesorporous ZrO2 with various quenchers in acetonitrile . Quencher [Cu(dmp)2]1+ [Cu(dmp)2]2+ [Co(bpy)3]2+ [Co(bpy)3]3+

KSV (×105 M-1) kq (×1010 M-1s-1) τ0(ns) 4.5 30.1 12.3 6.1

1.1 7.9 3.2 1.6

3.8

Conclusions We have demonstrated that copper complexes can be implemented as efficient redox mediators in dye-sensitized solar cells with very low driving force for dye regeneration. In this study, LEG4-sensitized DSCs employing a [Cu(dmp)2]1+/2+ redox electrolyte showed high photovoltaic performance of 8.3 % and open-circuit potentials exceeding 1.0 V under 100 mW cm-2 AM1.5G conditions. Considering the mass-transport limitations of previously used mediators, copper based systems are shown to have a higher diffusion coefficients compared the cobalt-based alternatives. Also, they are 4 times faster in the regeneration of the LEG4 dye in comparison with the cobalt-complex based electrolytes and show little mass transport limitations in photocurrent transients measurements. A driving force of only 0.2 eV is sufficient for dye regeneration with unit yield, pointing to new possibilities for improvement in DSC efficiencies. These promising results suggest a bright future for this environmentally friendly redox couple. The results of steady state emission measurements and time-correlated single photon counting (TC-SPC) indicate a higher rate of quenching with a dynamic mechanism only in case of the [Cu(dmp)2]2+ complex, that can compete with the electron injection of the dye leading to lower photocurrent and reduction in performance. In general, quenching of the excited state of the dye in the DSC by



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the components of the electrolyte is a potential loss mechanism that needs to be taken into consideration. The presented results suggest that the quenching of the excited sensitizer by [Cu(dmp)2]2+ species competes with electron injection and lowers the overall DSC performance. The overall characterization of the [Cu(dmp)2]1+/2+ with LEG4 suggest higher recombination in comparison to [Co(bpy)3]2+/3+ based electrolyte. The results indicate multiple recombination pathways with [Cu(dmp)2]1+/2+, including the reductive quenching of the excited dye and with the TiO2/FTO layer. Implementation of synthetic modifications to the phenanthroline ligands of the redox mediator or the steric properties of the dye with the aim of reducing specifically the static quenching and other recombination processes will likely further increase overall solar cell efficiencies.

Supporting Information Device fabrication and photovoltaic characterization of the best performance cells; Cyclic voltammogram of [Cu(dmp)2]1+/2+ redox couple, molar extinction coefficients, impedance spectroscopy. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements The authors acknowledge financial support by Merck KGaA, Darmstadt, Germany, the Swedish Energy Agency, the Swedish Research Council, and the STandUP for Energy program. We also thank Leif Häggman (Uppsala Universitet) for technical support.


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