Toward Higher Photovoltage: Effect of Blocking Layer on Cobalt

Article pubs.acs.org/JPCC

Toward Higher Photovoltage: Effect of Blocking Layer on Cobalt Bipyridine Pyrazole Complexes as Redox Shuttle for Dye-Sensitized Solar Cells Jun-Ho Yum,*,†,‡ Thomas Moehl,† Junghyun Yoon,†,§ Aravind Kumar Chandiran,† Florian Kessler,†,∥ Paul Gratia,† and Michael Graẗ zel*,† †

Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, School of Basic Sciences, Ecole Polytechnique Fédérale de Lausanne, CH - 1015 Lausanne, Switzerland § R & D Center DSC Team, Dongjin Semichem Co., LTD. 445-935 Hwasung, South Korea S Supporting Information *

ABSTRACT: Fluorine-doped tin-oxide (FTO) transparent conducting substrates, for use in cobalt bis(bipyridine pyrazole) complex based dye-sensitized solar cells (DSCs), were compared using the conventional TiCl4 treatment with conformal TiO2 blocking layers formed by atomic layer deposition (ALD). The conformal blocking layer prepared by ALD promotes a decrease in the dark current, owing to retarded recombination between the FTO and the cobalt electrolyte. The thickness for the conformal blocking layer was optimized to attain the best photovoltaic performance. Optimized photovoltaic devices employing a double layer provided the best results, and a peak power conversion efficiency of 10.6% was achieved under full sun light intensity.

■

INTRODUCTION Dye-sensitized solar cells (DSCs) represents one of the most promising alternatives to compete with the traditional inorganic semiconductor based solar cells because of their ecological and economical features, in which sunlight is absorbed by a dye monolayer located at the junction between an n-type wide band gap semiconductor and an electrolyte containing the redox shuttle.1,2 Upon photoexcitation, the photoexcited dye injects an electron into the n-type material (typically TiO2 anatase nanoparticles) while the redox shuttle (typically triiodide/ iodide redox (I3−/I−) system) regenerates the oxidized dye formed as a result of the injection. The injected electron flows through the TiO2 network to the back contact and then through the external load to the counter electrode.3 Although over 11% of the power conversion efficiency (PCE) has been obtained,4−8 the main drawback of the commonly used I3−/I− system is a mismatch between its oxidation potential (E0(I3−/ I−) = 0.35 V versus standard hydrogen electrode (SHE)) and the dye (mostly >1.0 V vs SHE), limiting the open circuit voltage (Voc) to between 0.7 and 0.8 V.9 The large expenditure needed for efficient dye regeneration is plausibly due to a complex regeneration process involving the formation of intermediates, that is, the I2− radical.10,11 In addition, the I3−/ I− redox couple corrodes a number of metals such as Ag and Cu, imposing restrictions on the use of such materials as current collectors in DSC modules.9,12 For this reason, development of noncorrosive novel redox mediators, with decreased gap between the oxidation potential of the dye and © XXXX American Chemical Society

the redox couple, is of paramount importance to enhance open circuit potential and the efficiency of DSCs. Recently, polypyridine complexes of Co(III)/Co(II) coupled with D-π-A organic sensitizers have attained promising results.13−18 For instance, [Co(bpy)3]3+/2+ (bpy = 2,2′bipyridine) possessing an oxidation potential of 0.56 V versus SHE has yielded DSC devices generating Vocs exceeding 900 mV13,17 and a power conversion efficiencies over 12%.17 A higher Voc, over 1000 mV, has been most recently reported with a novel tridendate cobalt [Co(bpy-pz)2]3+/2+(PF6)3/2 (bpy-pz is 6-(1H-pyrazol-1-yl)-2,2′-bipyridine]) (see the molecular structure in Figure 1) possessing a redox potential of 0.86 V vs SHE.18 The [Co(bpy-pz)2]3+/2+ whose redox potential is offset only by 230 mV from that of the dye, gave a dye regeneration yield of 0.93, which is very interesting since larger energy expenditure is usually required for the effective regeneration of the dye.11,19−21 While this is a remarkable advance, we have observed a higher dark current with these types of Co complexes arising from the reduction of the oxidized mediator by electrons at the fluorine doped tin oxide (FTO) or the TiO2 nanocrystals. A. K. Chandiran et al. have recently reported that a subnanometer thick gallium oxide layer, when deposited by Special Issue: Michael Grätzel Festschrift Received: December 30, 2013 Revised: April 8, 2014

A

dx.doi.org/10.1021/jp412777n | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

Scheme 1. Schematic Diagram to Explain the Function of the BL Displaying the Kinetic Processes in DSC: A Layer (Depicted by Blue) on the FTO Alleviates the Back Electron Flow to an Electrolyte

Figure 1. Molecular structures of (a) cobalt complex [Co(bpypz)2]3+/2+ and (b) dye (Y123).

atomic layer deposition (ALD) on top of the TiO2 film, is an effective blocking layer for the electron back reaction from the TiO2 to the [Co(bpy-pz)2]3+/2+ redox system.22 Herein, we describe how effectively a blocking layer on FTO can alleviate the dark current through the FTO and achieved an increase of Voc. P. J. Cameron et al. have shown the importance of a thin compact blocking layer to suppress the charge recombination via the FTO substrate, where nonideal light intensity dependence of Voc particularly at low light intensities is associated with the recombination.23,24 B. A. Gregg et al. showed that the recombination could be inhibited via an insulating film of poly(phenylene oxide-co-2-allylphenylene) in DSCs employing two different redox couples, ferrocene/ ferrocenium and TPD+/TPD (TPD is N,N′-di-m-tolyl-N,N′diphenylbenzidine). Nevertheless, the authors pointed to limitations in processability, including nonuniform coating, limited control of coating thickness, and propensity for TiO2 pore blockage.25 Another passivation method was reported by T. W. Hamann et al., ensuring a retardation of the recombination process through conformal insulating oxide blocking layer by ALD on the FTO surface.26 Atomic layer deposition can produce pinhole-free films with atomic scale control over thickness.27 The self-limiting growth process determined by the surface hydroxyl group and the introduction of metal and oxidizing precursors in two different stages avoids any gas phase and leads to the conformal growth on high aspect ratio structures. In this study, we deposited a thin and conformal TiO2 blocking layer (BL) by ALD on fluorine-doped tin-oxide in order to reduce the adverse recombination via FTO with our [Co(bpy-pz)2]3+/2+ redox system (see Scheme 1). DSCs with this blocking layer showed a substantial change in the dark currents leading to a change in their overall performance, especially in Vocs when compared to a bare FTO. The Vocs generated at low light intensities from a DSC with the bare FTO or the typical TiCl4-treated FTO dropped drastically compared to DSCs with ALD BL. An optimized photovoltaic device employing the ALD TiO2 film with a controlled thickness produced a power conversion efficiency over 10% and an open circuit voltage over 1020 mV at AM1.5G full sun.

■

EXPERIMENTAL SECTION Materials Synthesis. The synthetic details of 3-{6-{4[bis(2′,4′-dihexyloxybiphenyl-4-yl)amino-]phenyl}-4,4-dihexylcyclopenta-[2,1-b:3,4-b′]dithiphene-2-yl}-2-cyanoacrylic acid coded Y123 (see the molecular structure in Figure 1) have been described in a previous study.15 The synthetic details of [Co(bpy-pz)2]3+/2+ (bpy-pz = 6-(1H-pyrazol-1-yl)-2,2′-bipyridine) have been described in our previous study.18 Solar Cell Fabrication and Characterization. A TiCl4 treatment was done in a typical way via hydrolysis of TiCl4: fluorine doped thin oxide (FTO, Solar 4 mm thickness, 10 ohm/sq, Nippon Sheet Glass, Japan) conducting glass were immersed in 40 mM TiCl4 aq at 70 °C for 30 min and washed with water and ethanol. Similarly, different thicknesses (5, 10, and 20 nm) of underlayer titanium dioxide are deposited using atomic layer deposition (Model: Savannah 100, Cambridge Nanotech, U.S.A.). The deposition is carried out using a sequence of pulses of metal and oxidizing precursor in the exposure mode. The metal precursor, tetrakis(dimethylamido)titanium (99.99%, Aldrich) is pulsed for 100 ms simultaneously closing the exit stop valve for 5 s, which is followed by a nitrogen gas purge for 10 s. The process is repeated using H2O (18.2 MΩ, Millipore), but the water pulsing time is reduced to 15 ms. The thickness is evaluated using spectroscopic ellipsometry by following the deposition sequence on a silicon wafer (CMI, EPFL). The photon energies ranging between 1.5 and 5.5 eV is used for the ellipsometric study and the obtained data is fitted using modified-Cauchy dispersion law to extract the thickness. The deposition rate is found to be 0.066 nm/ cycle. The TiO2 blocking layer deposited on the FTO glass is characterized using X-ray diffraction technique. The measurement is made using D8-Bruker Discover instrument in θ−2θ mode fitted with a monochromator. The TiO2 transparent electrodes composed of ∼20 nm anatase resulting in ∼32 nm pore on a FTO were controlled to be ∼2.5 μm for general study about blocking layer effect. For the best performance, a ∼4.0 μm thick TiO2 transparent electrode with a ∼4.0 μm thick B



Article

In general, several pathways of recombination have been considered in DSCs. Under illumination, the injected electrons into the conduction band from optically excited dyes, can traverse the TiO2 network and can be collected at the transparent conducting glass or can react either with an oxidized dye molecule or with the oxidized redox couple via the surface states of the TiO2. In this study, the recombination path via the surface states could be excluded because we used identical TiO2 mesoporous films. Therefore, the higher dark current, particularly at low bias potential, is plausibly attributed to the faster recombination of electrons at the underlayer or the FTO with the Co3+ species in the vicinity of the substrate.31 Figure 3 shows the modulus of the dark currents of complete

scattering layer (400 nm, CCIC, HPW-400) was used. The TiO2 electrodes were immersed into a 0.1 mM solution of Y123 with 5 mM 3α,7α-dihydroxy-5β-cholic acid (chenodeoxycholic acid) in 4-tert-butanol/acetonitrile mixture (1:1 v/v) and kept for 15 h at room temperature. The applied electrolyte, JH70 consists of 0.22 M Co(II), 0.05 M Co(III), 0.1 M LiClO4, and 0.2 M 4-tert-butylpyridine in acetonitrile. The dye-adsorbed TiO2 electrode and thermally platinized counter electrode on FTO (TEC 15 ohms/sq, Pilkington) were assembled into a sealed sandwich type cell with a gap of a hot-melt ionomer film, Surlyn (25 μm, Du-Pont) for general study about blocking layer effect. It is noted that a carbonaceous catalyst, graphene nanoplatelets (Cheap Tubes, Inc. (U.S.A.)) was used for the highly performing cells. The carbonaceous typed catalyst has been in general known to perform better than Pt owing to the low charge transfer resistance particularly for the cobalt redox system.28−30 For photovoltaic measurements of the DSCs, a 450 W xenon light source (Oriel, U.S.A.) was used to characterize the solar cells. The spectral output of the lamp was matched in the region of 350−750 nm with the aid of a Schott K113 Tempax sunlight filter (Präzisions Glas & Optik GmbH, Germany) so as to reduce the mismatch between the simulated and true solar spectra to less than 4%. The current− voltage characteristics of the cell under these conditions were obtained by applying external potential bias to the cell and measuring the generated photocurrent with a Keithley model 2400 digital source meter (Keithley, U.S.A.). The devices were masked to attain an illuminated active area of 0.2 cm2.

■

RESULTS AND DISCUSSION The titanium dioxide deposited by ALD technique is sintered at 500 °C for 30 min and its crystallographic phase is investigated using X-ray diffraction and the diffractogram is given in Figure 2. It contains the mixed reflections from both the conducting F-

Figure 3. Dark currents measured with the DSCs employing the [Co(bpy-pz)2]3+/2+ based electrolyte (JH70) and Y123 adsorbed ∼2.5 μm thick TiO2 films with various blocking layer: no BL (black dotted line), TiCl4 (black dashed line), double TiCl4 (green dot-dashed line), 5 nm thick ALD (blue dot-dashed line), 10 nm thick ALD (red solid line), and 20 nm thick ALD (orange double-dot-dashed line). Scan rate is 125 mV/s.

devices composed of various types of blocking layers. As can be observed, bare FTO in the absence of a blocking layer or TiCl4 treatment shows higher dark current over the whole potential range compared other conditions, that is, double TiCl4 treated FTO or ALD TiO2 coated FTO. It has been shown that a TiCl4 treatment forms a layer composed of small nanoparticles leading to increased surface roughness,32,33 which was reconfirmed in our study (see SEM images in Supporting Information, Figures S1 and S2). The higher dark current is plausibly associated with the higher surface area of the TiO2 made by the TiCl4 treatment and a less conformal coverage by TiCl4 treatment compared to layers made by ALD. In fact, the conformal TiO2 layers deposited by ALD on the FTO (see SEM images in Figure S2) show noticeable improvement in reduction of the dark currents by ∼2 orders of magnitude near 0 V. It is noteworthy that a FTO treated twice with TiCl4 treatment (denoted hereafter double TiCl4) shows a comparable low dark current to FTO protected by ALD layers. This implies that coverage of the FTO with TiO2 was improved compared to the typical one step TiCl4 treatment, which was observed also by SEM (see SEM images in Figure S2). We

Figure 2. X-ray diffraction pattern of 20 nm ALD TiO2 deposited on the fluorine doped tin oxide transparent glass. The letter A (in red color with red arrow) refers to the anatase crystal phase of the titanium oxide and the * (in black) refers to the F-SnO2 Bragg reflections.

doped SnO2 and the titanium oxide. The peaks at 25.3° and 55.1° denoted as A (in red) corresponds to (101) and (211) reflection of the anatase phase of the TiO2 and is nearly identical to the mesoporous photo anode film. All other peaks denoted with * are due to the underlying FTO substrate. The broad peak over a range of 20−30° is due to the amorphous glass. C



Article

Table 1. Averaged Photovoltaic Characteristics of DSC with Different BL Layers, no BL, ALD_10 nm, and ALD_20 nm; Electrolytes JH70 and Y123 Adsorbed on ∼2.5 μm thick TiO2 Films are Employed BL no

TiCl4

double TiCl4

ALD5

ALD10

ALD20

I0 (mW cm−1) 100 51 9.5 100 51 9.5 100 51 9.5 100 51 9.5 100 51 9.5 100 51 9.5

Jsc (mA cm−2)

Voc (mV)

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1025 ± 16 1000 ± 24 882 ± 80 1036 ± 13 1016 ± 16 944 ± 27 1053 ± 8 1034 ± 9 963 ± 8 1047 ± 17 1029 ± 21 963 ± 20 1045 ± 9 1022 ± 11 956 ± 11 1026 ± 2 1008 ± 2 949 ± 2

7.83 4.21 0.78 8.27 4.34 0.81 8.14 4.31 0.80 8.51 4.49 0.85 8.30 4.36 0.87 7.42 3.85 0.70

0.53 0.23 0.05 0.12 0.09 0.03 0.06 0.04 0.02 0.18 0.07 0.01 0.11 0.06 0.01 0.11 0.08 0.04

FF (%) 67.7 67.5 59.6 67.6 70.4 70.0 69.3 71.3 71.2 68.5 71.4 70.7 71.1 73.2 78.8 67.0 72.3 78.8

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.2 5.9 12 1.6 2.1 4.5 1.4 1.8 2.8 1.1 1.6 0.7 1.0 2.1 0.1 1.0 0.7 0.5

η (%) 5.45 5.60 4.46 5.80 6.09 5.93 5.94 6.23 5.78 6.11 6.46 6.08 6.00 6.67 6.44 5.10 5.50 5.48

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.49 0.81 1.36 0.29 0.40 0.68 0.28 0.35 0.42 0.12 0.11 0.16 0.05 0.11 0.11 0.14 0.15 0.32

Figure 4. Dark currents measured with the DSCs employing the [Co(bpy-pz)2]3+/2+ photovoltaic characteristics of DSCs with different BL layers.

envisaged that the low dark current could lead to a higher Voc from solar cells, which will be discussed below. Table 1 and Figure 4 summarize photocurrent−voltage characteristics of DSCs with different BL under simulated sunlight with various intensities: 9.5, 51, and 100 mW·cm−2. The power conversion efficiency (η) was derived from eq 1: η = Jsc ·Voc·FF/I0

open circuit voltages (Voc) are strongly dependent upon the type of BL. For instance, a Voc of 1025 mV was obtained by bare FTO photoanode, whereas 10−30 mV higher Vocs were generated by TiCl4 treated or ALD 5 and 10 nm thick TiO2 coated FTO. It is noteworthy that a big variation in PV performance of bare FTO (10 to 30% from high to low light intensity) is observed (see Table 1 and Figure 4). It is not clear how to explain the poor reproducibility at this stage of the study; however, the low performances are in agreement with previous studies.22,25,26 Cells with ALD TiO2 BL and double TiCl4 treated FTO showed particularly higher efficiency due to their higher Vocs than bare or single TiCl4 treated substrates. At low light intensities, 51 and 9.5 mW cm−2, Vocs exceeding 1000

(1)

where Jsc is a short circuit current density, Voc is an open circuit voltage, FF is a fill factor, and I0 is the photon flux illuminated on the solar cells. The short circuit current densities of (Jsc) ranging from 8.1−8.5 mA cm−2 were almost independent of BL type. On the other hand, it is evident that D



Article

hand gives a slope exceeding 250 mV/decade over the entire intensity range, except at high light intensity over 25 mW cm−1. The nonideal slope implies that high recombination via the FTO substrate occurs most likely over the entire intensity range.24 A fall off of Voc from single TiCl4 treated substrates, particularly at low intensity below 1 mW cm−1, was observed, although high Voc was obtained at high intensity. The observation of voltage change versus light intensity provides strong dependence of the voltage behavior on BL type. As a consequence, DSCs containing substrates with a 5−10 nm TiO2 BL by ALD outperformed others with no BL or single TiCl4 treated substrates. It is obvious that the optimized thickness of ALD BL is much lower than the desired 25−50 nm thickness formed via the TiCl4 treatment.32 Considering the comparable performance of double TiCl4 treated substrates, this facile process would however be interesting and advantageous from an industrial application point of view. It is well-known that superior DSC performance has been obtained by optimizing the comprising elements. For example, an optical element, a scattering layer composed of large particles,37−40 has been used to increase incoming light path leading to a higher Jsc. DSCs with a BL, for example, double TiCl4, ALD 5 nm, or ALD 10 nm all generated a Jsc of 13.0− 14.0 mA cm−2, a fill factor over 0.74, and a η over 10.0% at full sun (see Table 2). At low intensities, 51 and 9.5 mW cm−2, the

and 950 mV were generated from the ALD TiO2 BL or double TiCl4 treated FTO. The overall efficiencies are slightly higher than those at 1 sun due to higher fill factor though. Bare FTO on the other hand produced lower Voc than those of ALD or double TiCl4 treatment, which mainly led to a big loss in power conversion efficiency at 9.5 mW cm−2. For instance, if the bare FTO device is compared to 5 nm ALD it shows a maximum difference in Voc at low light intensity of about 80 mV and a drop of performance by 28%. Overall, the presence of the blocking layer resulted in more pronounced enhancement under low light intensities, which is consistent with previous studies.23,24 It is also interesting to see the effect of a thicker ALD film, that is, 20 nm. It turns out that the thicker ALD layer resulted in a lower efficiency mainly due to a drop in Jsc by 13% when compared to that of thinner ALD. This result implies that though the thicker ALD BL reduce shunting of the device, the low conductivity of the ALD BL reduces the FF at high light intensities due to the higher series resistance in these devices. B. Yoo et al., have reported achievement of a maximum Jsc with 15 nm thick TiO2 dense layer and then a drop of the current with 20 nm thicker film in solar cells employing I3−/I− redox couple and ruthenium complex as sensitizer.34 Or a thicker film could sacrifice its transmittance, owing to increased reflectance. D. H. Kim et al. have presented a thickness dependence of ALD deposited TiO2 film on transmittance, where films thicker than 20 nm lost their transmittance, particularly over absorption spectrum of N719 dye, leading to a loss in a photocurrent.35 Figure 5 shows the Voc change versus illumination light intensity generated with an array of light emitting diodes. The

Table 2. Photovoltaic Characteristics of the Best DSC with Double TiCl4, ALD 5 nm, and ALD10 nm; A TiO2 Film Consisting of ∼4 μm Thick TiO2 Transparent Layer with ∼4 μm Thick TiO2 Scattering Layer is Employed BL double TiCl4

ALD5

ALD10

I0 (mW cm−1)

Jsc (mA cm−1)

Voc (mV)

FF (%)

η (%)

100 51 9.5 100 51 9.5 100 51 9.5

13.00 6.76 1.28 13.91 7.32 1.36 13.22 6.93 1.28

1020 1001 941 1024 1006 948 1026 1006 945

77.6 80.3 79.8 74.3 77.4 78.7 75.9 78.9 82.3

10.24 10.66 10.09 10.59 11.18 10.66 10.30 10.79 10.45

device still exhibited excellent Vocs of 1000 and 940 mV, respectively, leading to a η of 10.0−11.2% (see Table 2). It should be noted that the higher fill factor is achieved by employing a carbonaceous film as cathode instead of Pt. The higher fill factor than general is plausibly ascribed to lower charge transfer resistance at the counter electrode, owing to the use of carbonaceous film, which was investigated in our previous study.30 As a concluding PV result, we confirmed that high performance is achievable by controlling charge loss through the FTO of the photoanode. An increase in the Voc, which showed further improvement compared to our previous study lead to the efficiency of 10.6% at full sun with a 5 nm ALD BL as the highest performance after optimization (see Figure 6).

Figure 5. Light intensity dependence of the Voc for the DSC with various BL layer no BL (empty circles), TiCl4 (blue dots), double TiCl4 (empty squares), and ALD (red squares).

light intensity was adjusted finely by controlling power for the white light emitting diodes. It is evident that slopes of Voc/ decade are varied drastically with FTO treatment, particularly at low light intensity. Substrates with BL or double TiCl4 treatment show around 130 and 73 mV/decade at low intensity below 10 mW cm−1 and high intensity, respectively. The slope at high light intensity is close to the ideal value (59 mV/ decade)23,24,36 and increases at low light intensity, which implies that recombination via the substrate is still not negligible despite the presence of BL. Bare FTO on the other

■

CONCLUSIONS A conformal TiO2 blocking layer deposited on fluorine-doped tin-oxide substrates by atomic deposition layer technique was employed in dye-sensitized solar cells (DSC) based on cobalt bipyridine pyrazole tridentate complex as redox shuttle. This E



Article

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Mr. Pascal Comte for the TiO2 paste preparation. The authors also thank Mr. Terumasa Shimoyama from NEC, Nippon Electric Co., Ltd, for providing Y123 dye. M.G. thanks the Swiss National Science Foundation and the ECR advanced Grant Agreement No. 247404 under the CEMesolight project funded by the European community’s seventh FWP for financial support. J.H.Y. thanks Dr. Aswani Yella and Dr. Yongjoo Kim for fruitful discussion and acknowledge the joint development project funded by Dongjin Semichem Co., Ltd. (S. Korea). A.K.C. acknowledges the financial contribution from EU FP7 Project “ORION”, Grant Agreement No. NMP-229036, and the Balzan Foundation as part of the 2009 Balzan Prize award to Michael Grätzel.

■

Figure 6. Photovoltaic characteristics of DSC employing ALD 10 nm BL, ∼4 + 4 μm double layered TiO2 film, [Co(bpy-pz)2]3+/2+ redox couples, and carbonaceous film on FTO as counter electrode: Scheme 1.

ALD blocking layer was compared to the bare FTO photoanode and the other commonly applied FTO BL from TiCl4. The ALD blocking layer enabled a reduction of the recombination process via fluorine-doped tin-oxide and as a result decreases dark current effectively. It was obvious that the blocking effect of the charge recombination was more pronounced at low light intensities than at high light intensities. Substrates with a 5−10 nm TiO2 BL by ALD outperformed others with no BL or single TiCl4 treated substrates. Double TiCl4 treated substrates showed interestingly comparable performance to ALD BLs. Optimized DSCs with a scattering layer achieved the best power conversion efficiency, 10.6% at full sun and exceeding 11% at low light intensity. These promising results confirm that our device engineering even with relative simple methods like in the case of the double TiCl4 treated photoanode assures improved performance of DSCs.

■

ASSOCIATED CONTENT

S Supporting Information *

Figure S1: SEM images of (a) FTO, (b) TiCl4 treated FTO, and (c) double TiCl4 treated FTO. Figure S2: Comparison of SEM images of (a) ALD 10 nm on FTO and (b) double TiCl4 treated FTO. This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

(1) O’Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar-Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737−740. (2) Grätzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338− 344. (3) Grätzel, M. Recent Advances in Sensitized Mesoscopic Solar Cells. Acc. Chem. Res. 2009, 42, 1788−1798. (4) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Y. Dye-Sensitized Solar Cells with Conversion Efficiency of 11.1%. Jpn. J. Appl. Phys. 2 2006, 45, L638−L640. (5) Chen, C. Y.; Wang, M. K.; Li, J. Y.; Pootrakulchote, N.; Alibabaei, L.; Ngoc-le, C. H.; Decoppet, J. D.; Tsai, J. H.; Grätzel, C.; Wu, C. G.; et al. Highly Efficient Light-Harvesting Ruthenium Sensitizer for ThinFilm Dye-Sensitized Solar Cells. ACS Nano 2009, 3, 3103−3109. (6) Yu, Q. J.; Wang, Y. H.; Yi, Z. H.; Zu, N. N.; Zhang, J.; Zhang, M.; Wang, P. High-Efficiency Dye-Sensitized Solar Cells: The Influence of Lithium Ions on Exciton Dissociation, Charge Recombination, and Surface States. ACS Nano 2010, 4, 6032−6038. (7) Han, L. Y.; Islam, A.; Chen, H.; Malapaka, C.; Chiranjeevi, B.; Zhang, S. F.; Yang, X. D.; Yanagida, M. High-Efficiency Dye-Sensitized Solar Cell with a Novel Co-Adsorbent. Energy Environ. Sci. 2012, 5, 6057−6060. (8) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar Cell Efficiency Tables (Version 39). Prog. Photovoltaics 2012, 20, 12−20. (9) Boschloo, G.; Hagfeldt, A. Characteristics of the Iodide/Triiodide Redox Mediator in Dye-Sensitized Solar Cells. Acc. Chem. Res. 2009, 42, 1819−1826. (10) Pelet, S.; Moser, J. E.; Grätzel, M. Cooperative Effect of Adsorbed Cations and Iodide on the Interception of Back Electron Transfer in the Dye Sensitization of Nanocrystalline TiO2. J. Phys. Chem. B 2000, 104, 1791−1795. (11) Clifford, J. N.; Palomares, E.; Nazeeruddin, M. K.; Grätzel, M.; Durrant, J. R. Dye Dependent Regeneration Dynamics in Dye Sensitized Nanocrystalline Solar Cells: Evidence for the Formation of a Ruthenium Bipyridyl Cation/Iodide Intermediate. J. Phys. Chem. C 2007, 111, 6561−6567. (12) Olsen, E.; Hagen, G.; Lindquist, S. E. Dissolution of Platinum in Methoxy Propionitrile Containing LiI/I2. Sol. Energy Mater. Sol. Cells 2000, 63, 267−273. (13) 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 Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2010, 132, 16714−16724. (14) 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, 11442−11445.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: junho.yum@epfl.ch. *E-mail: michael.graetzel@epfl.ch. Present Addresses ‡

PV-Center, Centre Suisse d’Electronique et de Microtechnique SA, Jaquet-Droz 1, CH − 2002 Neuchâtel, Switzerland (J.-H.Y.). ∥ Siemens AG, Corporate Technology, CT RTC MAT IEC-DE, Günther-Scharowsky Strasse 1, D-91058 Erlangen, Germany (F.K.). Funding

The authors declare no competing financial interest. F



Article

(15) Tsao, H. N.; Yi, C.; Moehl, T.; Yum, J.-H.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Grätzel, M. Cyclopentadithiophene Bridged Donor-Acceptor Dyes Achieve High Power Conversion Efficiencies in Dye-Sensitized Solar Cells Based on the Tris-Cobalt Bipyridine Redox Couple. ChemSusChem 2011, 4, 591−594. (16) Zhou, D. F.; Yu, Q. J.; Cai, N.; Bai, Y.; Wang, Y. H.; Wang, P. Efficient Organic Dye-Sensitized Thin-Film Solar Cells Based on the Tris(1,10-Phenanthroline)Cobalt(II/III) Redox Shuttle. Energy Environ. Sci. 2011, 4, 2030−2034. (17) Yella, A.; Lee, H. W.; Tsao, H. N.; Yi, C. Y.; 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% Efficiency. Science 2011, 334, 629−634. (18) Yum, J.-H.; Baranoff, E.; Kessler, F.; Moehl, T.; Ahmad, S.; Bessho, T.; Marchioro, A.; Ghadiri, E.; Moser, J.-E.; Yi, C.; et al. A Cobalt Complex Redox Shuttle for Dye-Sensitized Solar Cells with High Open-Circuit Potentials. Nat. Commun. 2012, 3, 631. (19) Kuciauskas, D.; Freund, M. S.; Gray, H. B.; Winkler, J. R.; Lewis, N. S. Electron Transfer Dynamics in Nanocrystalline Titanium Dioxide Solar Cells Sensitized with Ruthenium or Osmium Polypyridyl Complexes. J. Phys. Chem. B 2001, 105, 392−403. (20) Hamann, T. W.; Ondersma, J. W. Impedance Investigation of Dye-Sensitized Solar Cells Employing Outer-Sphere Redox Shuttles. J. Phys. Chem. C 2010, 114, 638−645. (21) Feldt, S. M.; Wang, G.; Boschloo, G.; Hagfeldt, A. Effects of Driving Forces for Recombination and Regeneration on the Photovoltaic Performance of Dye-Sensitized Solar Cells Using Cobalt Polypyridine Redox Couples. J. Phys. Chem. C 2011, 115, 21500− 21507. (22) Chandiran, A. K.; Tetreault, N.; Humphry-Baker, R.; Kessler, F.; Baranoff, E.; Yi, C.; Nazeeruddin, M. K.; Grätzel, M. Subnanometer Ga2O3 Tunnelling Layer by Atomic Layer Deposition to Achieve 1.1 V Open-Circuit Potential in Dye-Sensitized Solar Cells. Nano Lett. 2012, 12, 3941−3947. (23) Cameron, P. J.; Peter, L. M. How Does Back-Reaction at the Conducting Glass Substrate Influence the Dynamic Photovoltage Response of Nanocrystalline Dye-Sensitized Solar Cells? J. Phys. Chem. B 2005, 109, 7392−7398. (24) Cameron, P. J.; Peter, L. M.; Hore, S. How Important Is the Back Reaction of Electrons via the Substrate in Dye-Sensitized Nanocrystalline Solar Cells? J. Phys. Chem. B 2005, 109, 930−936. (25) Gregg, B. A.; Pichot, F.; Ferrere, S.; Fields, C. L. Interfacial Recombination Processes in Dye-Sensitized Solar Cells and Methods to Passivate the Interfaces. J. Phys. Chem. B 2001, 105, 1422−1429. (26) Hamann, T. W.; Martinson, A. B. F.; Elam, J. W.; Pellin, M. J.; Hupp, J. T. Atomic Layer Deposition of TiO2 on Aerogel Templates: New Photoanodes for Dye-Sensitized Solar Cells. J. Phys. Chem. C 2008, 112, 10303−10307. (27) Ritala, M.; Leskela, M. Handbook of Thin Film Materials; Academic Press: San Diego, CA, 2001; Vol. 1. (28) Sapp, S. A.; Elliott, C. M.; Contado, C.; Caramori, S.; Bignozzi, C. A. Substituted Polypyridine Complexes of Cobalt(II/III) as Efficient Electron-Transfer Mediators in Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2002, 124, 11215−11222. (29) Kavan, L.; Yum, J.-H.; Grätzel, M. Graphene Nanoplatelets Outperforming Platinum as the Electrocatalyst in Co-BipyridineMediated Dye-Sensitized Solar Cells. Nano Lett. 2011, 11, 5501−5506. (30) Kavan, L.; Yum, J.-H.; Nazeeruddin, M. K.; Grätzel, M. Graphene Nanoplatelet Cathode for Co(III)/(II) Mediated DyeSensitized Solar Cells. ACS Nano 2011, 5, 9171−9178. (31) Cameron, P. J.; Peter, L. M.; Zakeeruddin, S. M.; Grätzel, M. Electrochemical Studies of the Co(III)/Co(II)(dbbip)2 Redox Couple as a Mediator for Dye-Sensitized Nanocrystalline Solar Cells. Coord. Chem. Rev. 2004, 248, 1447−1453. (32) Choi, H.; Nahm, C.; Kim, J.; Moon, J.; Nam, S.; Jung, D. R.; Park, B. The Effect of TiCl4-Treated TiO2 Compact Layer on the Performance of Dye-Sensitized Solar Cell. Curr. Appl. Phys. 2012, 12, 737−741.

(33) Kim, J. K.; Shin, K.; Lee, K. S.; Park, H. Influence of a TiCl4 Treatment Condition on Dye-Sensitized Solar Cells. J. Electrochem. Sci. Technol. 2010, 1, 81−84. (34) Yoo, B.; Kim, K.; Lee, D. K.; Ko, M. J.; Lee, H.; Kim, Y. H.; Kim, W. M.; Park, N. G. Enhanced Charge Collection Efficiency by ThinTiO2-Film Deposition on FTO-Coated ITO Conductive Oxide in Dye-Sensitized Solar Cells. J. Mater. Chem. 2010, 20, 4392−4398. (35) Kim, D. H.; Woodroof, M.; Lee, A.; Parsons, G. N. Atomic Layer Deposition of High Performance Ultrathin TiO2 Blocking Layers for Dye-Sensitized Solar Cells. ChemSusChem 2013, 6, 1014− 1020. (36) Salvador, P.; Hidalgo, M. G.; Zaban, A.; Bisquert, J. Illumination Intensity Dependence of the Photovoltage in Nanostructured TiO2 Dye-Sensitized Solar Cells. J. Phys. Chem. B 2005, 109, 15915−15926. (37) Wang, Z. S.; Kawauchi, H.; Kashima, T.; Arakawa, H. Significant Influence of TiO2 Photoelectrode Morphology on the Energy Conversion Efficiency of N719 Dye-Sensitized Solar Cell. Coord. Chem. Rev. 2004, 248, 1381−1389. (38) Hore, S.; Vetter, C.; Kern, R.; Smit, H.; Hinsch, A. Influence of Scattering Layers on Efficiency of Dye-Sensitized Solar Cells. Sol. Energy Mater. Sol. Cells 2006, 90, 1176−1188. (39) Ito, S.; Zakeeruddin, S. M.; Humphry-Baker, R.; Liska, P.; Charvet, R.; Comte, P.; Nazeeruddin, M. K.; Péchy, P.; Takata, M.; Miura, H.; et al. High-Efficiency Organic-Dye-Sensitized Solar Cells Controlled by Nanocrystalline-TiO2 Electrode Thickness. Adv. Mater. 2006, 18, 1202−1205. (40) Koo, H. J.; Park, J.; Yoo, B.; Yoo, K.; Kim, K.; Park, N. G. SizeDependent Scattering Efficiency in Dye-Sensitized Solar Cell. Inorg. Chim. Acta 2008, 361, 677−683.

G


Toward Higher Photovoltage: Effect of Blocking Layer on Cobalt

Recommend Documents