Accepted Article
Received Date : 07-Oct-2014 Accepted Date : 26-Nov-2014 Article type
: Research Article
Improved Efficiency and Stability of Flexible Dye Sensitized Solar Cells on ITO/PEN Substrates Using an Ionic Liquid Electrolyte
Yu Han,a Jennifer M. Pringle,b* Yi-Bing Chenga* a
. Department of Materials Engineering, Monash University, Clayton, Victoria 3800, Australia
b
. Australian Research Council Centre of Excellence for Electromaterials Science, Deakin University, Melbourne Burwood Campus, 221 Burwood Highway, Burwood, VIC 3125, Australia
* Corresponding authors. Email: [email protected]; [email protected]
ABSTRACT
Flexible dye-sensitized solar cells (DSSCs) built on plastic substrates have attracted great interest as they are lightweight and can be roll-to-roll printed to accelerate production and reduce cost. However, plastic substrates such as PEN and PET are permeable to water, This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/php.12399 This article is protected by copyright. All rights reserved.
Accepted Article
oxygen and volatile electrolyte solvents, which is detrimental to the cell stability. Therefore, to address this problem, in this work, an ionic liquid (IL) electrolyte is used to replace the volatile solvent electrolyte. The initial IL-based devices only achieved around 50% of the photovoltaic conversion efficiency of the cells using the solvent electrolyte. Current-voltage and electrochemical impedance spectroscopy (EIS) analysis of the cells in the dark indicated that this lower efficiency mainly originated from (i) a lack of blocking layer to reduce recombination, and (ii) a lower charge collection efficiency. To combat these problems, cells were developed using a 12nm thick blocking layer, produced by atomic layer deposition, and 1 μm thick P25 TiO2 film sensitized with the hydrophobic MK-2 dye. These flexible DSSCs utilizing an IL electrolyte exhibit significantly improved efficiencies and a less than 10% drop in performance after 1000 hours aging at 60ºC under continuous light illumination.
INTRODUCTION Nanostructured dye-sensitized solar cells (DSSCs) can directly convert solar energy into electricity, with state of the art devices currently achieving a photovoltaic conversion efficiency of up to 12.3% [1]. Commercialization of this technology requires not only high efficiency but also long term stability and low manufacturing costs. Conventionally, such devices are produced using coating technologies on transparent conductive oxides (TCO) glass substrates that are heavy, rigid and have high manufacturing costs[2]. Plastic substrates, on the other hand, are lightweight and flexible and suitable for lower cost mass production via industrial roll-to-roll printing. In addition, the use of flexible substrates can widen the range of possible applications, such as mounting DSSC modules on large roof-top areas of factories and warehouses that are typically not designed to withstand the weight
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incorporating photoanodes prepared by cold isostatic pressing, using a solvent based electrolyte and N719 dye: by varying the thickness from 1 µm to 16 µm, the Jsc increased from ca. 3 mA/cm2 to 12 mA/cm2, with a reduction in Voc from 760mV to 700mV [14]. This decrease in Voc was attributed to increased electron recombination as a result of the larger surface area of the working electrode [14].
The relationship between the thickness of the TiO2 photoanode film and the photovoltaic parameters for the flexible devices, using the IL electrolyte, is shown in Figure 4. These cells show a decrease in Voc similar to those cells reported previously using solvent electrolyte, but surprisingly, also a decrease in Jsc with increasing film thickness from 2 to 8 µm.
To investigate the cause of this decrease in Jsc, incident photo to current efficiency (
)
measurements, using both working electrode (front) and counter electrode (back) illumination, were used to assess the charge collection efficiency of the cells. Front illumination produces two peaks, at 420nm and 535 nm, corresponding to the absorption peaks of the C106 dye (Figure 5A). With increasing film thickness, a slight decrease of
is observed, which is consistent with the change of
Jsc.
The
measurements using back illumination shows a larger difference in the spectra with
increasing film thickness (Figure 5B). The absorption peak at 420nm is no longer present, probably due to the absorption of light by the I-/I3- in the IL electrolyte[35]. Also, the absorption peak at 535nm
This article is protected by copyright. All rights reserved. 11
Accepted Article
then show a consistent drop in PCE with aging, similar to previous stability testing on DSSCs with molecular solvents performed by Lee et al.[38] at 60 °C and Miyasaka et al.[30] at 50 °C. This drop of efficiency with aging was mainly due to leaking or permeation of volatile electrolyte solvent out of the cell through the plastic substrates at high temperatures. However, the cells with IL electrolyte retained ca. 90% of their initial PCE after 1000 hours of aging. The drop of efficiency occurred mainly between 100 and 300 hours testing and was probably due to the evaporation of NMBI from the electrolyte and/or ingression of H2O through the plastic substrate which gradually shifted the energy levels of the TiO2/dye/electrolyte interface, reaching equilibrium at around 300 hours of aging. The stable performance observed between 300 to 1000 hours of aging is similar to the previous long-term stability results obtained on cells built on glass substrates using IL electrolytes [8,39,40].
CONCLUSIONS The use of an IL based electrolyte, in combination with a hydrophobic dye, for flexible DSSCs was explored, with the intention of improving cell stability with minimal sacrifice of photovoltaic power conversion efficiency. The results show that over 50% PCE may be lost by simple substitution of a molecular solvent electrolyte with an IL electrolyte for cells built with compressed TiO2 films on flexible PET or PEN substrates with C106 dye. This loss is concluded to firstly be a result of the recombination of electrons from the ITO to electron acceptors in the electrolyte. This is most significant when a higher concentration of triiodide is present, as in the IL electrolyte, which leads to a significant reduction in Voc and FF. To combat this, an ALD deposited blocking layer of ~12nm thickness was applied on the ITO coated substrate. Secondly, the pressing technique used to prepare the TiO2 layer for the flexible DSSCs is known to reduce the pore size of the film compared to those on FTO glass, leading to a slow I-/I3- diffusion rate within the working electrode film, causing a higher
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Accepted Article
consisted of 1-ethyl-3-methylimidazolium tetracyanoborate (EMITCB), 1-ethyl-3-methylimidazolium iodide (EMII), dimethylimidazolium iodide (DMII) (>99.5% purity, received from Solarpur, Merck), GNCS, Iodine (I2) and NMBI in a molar ratio of 16:12:12:0.67:1.67:3.33 [8].
Cell fabrication and measurement: The preparation procedure for the cells was described in our previous work [13]. Briefly, the mesoporous TiO2 layer was produced by doctor blading a TiO2 slurry onto the conductive substrate, followed by cold isostatic pressing to enhance the inter-particle contact and the contact between the particles and the substrate[14]. The WE and CE were sealed together using a heat curable gasket (Bynel) with a uniform thickness of 25 μm. The electrolyte was injected into the gap between the WE and CE, through a small hole pre-drilled on the CE side, by vacuum backfilling and this was followed by sealing using a UV curable epoxy resin (3035B, ThreeBond). The TiOx blocking layer was produced either by dip-coating of the TiOx precursor (20mM titanium butoxide in dry ethanol solution) followed by UV-Ozone decomposition at 150 °C, or by thermal atomic layer deposition (ALD) at 150 °C with an ALD machine (Fuji-F200) using tetrakis(dimethylamido)titanium (TDMAT) as a precursor and a pulse time of 0.06 seconds for water or 0.2 seconds for TDMAT, with a time interval between each pulse of 30 seconds. Ellipsometry (M-2000, J.A. Woollam Co., Inc) was used to characterize the thin blocking layer in the spectral range from 200nm to 1700nm. The data from the ellipsometer were fitted using the Cody-Lorentz dispersion model [15] to get film thickness, roughness, crystallinity and bandgap. Currentvoltage (IV) curves and parameters, such as open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF) and PCEof the DSSCs were obtained using a Keithley 2400 Source Meter under illumination of simulated sunlight provided by an Oriel solar simulator with an AM1.5 filter. Typically, four cells were measured for each batch of cells to obtain the average IV parameters and deviations. The electrochemical impedance spectra of the DSSCs were recorded on a potentiostat (BioLogic VSP) with a frequency range of
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Accepted Article
0.05 Hz – 1 KHz. The magnitude of the alternating signal was 10 mV. The impedance measurements were carried out under different voltage biases ranging from 0.3 to 0.9 V in the dark. The obtained impedance spectra were fitted with the Z-view software (v2.8, Scribner Associates Inc.), using a general transmission line model of DSSC described in the literature [16][17][18], to derive parameters such as the chemical capacitance Cμ, and interfacial charge transfer resistance Rct.
The stability tests were performed on two batches of cells with 11.6nm ALD TiO2 blocking layer, ~1µm thick compressed P25 mesoporous TiO2 layer sensitized with MK-2 dye and commercially available platinized counter electrode and either the molecular solvent or the IL electrolyte. The cells were subjected to one sun illumination at 60 °C for over 1000 hours in a home-made light soaking box equipped with a metal halide lamp. The temperature of the cells was monitored using thermocouples and was controlled by ventilation through a fan installed on the side of the box.
RESULTS AND DISCUSSION The photovoltaic parameters of DSSCs assembled using plastic substrates with either the MPN-based electrolyte or the ionic liquid electrolyte, and the Ru-based C106 dye, are shown in Table 1 and Figure 1. The change from the solvent electrolyte to the IL electrolyte resulted in a decrease in photovoltaic efficiency of ca. 50 %. In comparison, for DSSCs made on glass substrates with the same dye and either a solvent electrolyte or the optimum IL electrolyte composition, typically over 70% photovoltaic conversion efficiency is retained [8,19–21].
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The influence of blocking layer on Voc and FF As shown in Table 1, all three device parameters (Voc, Jsc, Fill factor) for the flexible DSSCs dropped significantly with use of the IL electrolyte, leading to a substantial reduction in photovoltaic conversion efficiency. This drop is largely related to the higher concentration of electron acceptors (I3) in the IL based electrolyte, which is used to overcome mass transport limitations in these more viscous systems [22]. This high concentration of I3- leads to a higher recombination rate of electrons from the mesoporous TiO2 photoanode with the acceptors (I3-) in the electrolyte, causing a reduction of Voc. Moreover, this electron recombination can also occur at the interface of the conducting substrate that is exposed to the electrolyte [23][24][25].
The latter route for recombination has been shown to significantly increase the non-ideality factor (m) of a DSSC, which is normally in the range of ~1.25 to ~1.9. This factor is used to describe the experimentally observed non-ideal charge recombination via the conductive substrate/electrolyte interface and/or via surface states distributed in the band gap of TiO2 [26], measured by the light intensity dependence of Voc, such that:
(1)
where T is the temperature (293K), I0 is the intensity of light, and e is the elementary charge [26,27]. Table 1 shows that with a 10 fold increase in light intensity, Voc increased by 89 ±22 mV for the molecular solvent based cells and 117 ± 31 mV for IL based cells. From equation 1, the non-ideality
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factor of the cells using solvent based electrolyte was estimated to be 1.51, compared to 1.98 for those using IL electrolytes. This indicates that electron recombination at the uncovered ITO surface is more significant with the IL electrolyte than with the solvent electrolyte, causing the observed reduction in FF [26].
To combat this problem, a compact TiOx blocking layer can be deposited onto the ITO film [26,28]. This is typically achieved by TiCl4 treatment, or the spray coating of a TiOx precursor followed by high temperature (~450 °C) decomposition, producing a dense TiOx blocking layer on the FTO glass substrate. However, as the maximum endurable temperature for the plastic substrate is 150 °C for PET and 180 °C for PEN, and the conductive ITO layer is vulnerable to acids and bases, these techniques are not suitable for flexible ITO/PET or ITO/PEN substrates. Low temperature processing methods, such as the reactive sputtering of Ti metal followed by anodic oxidation to produce a 15nm thick dense blocking layer [29,30] has been shown to significantly improve the fill factor of DSSC cells on plastic substrates. The use of an atomic layer deposited blocking layer on ITO/PET was investigated by Miettunen et al. [23] and found to increase the open circuit voltage at low light intensity but reduce the fill factor at high light intensity. For plastic substrates, restricted to low temperature processing, thinner blocking layers were identified as preferable compared to the thicker layers used with glass substrates.
Here we investigate two low temperature processing methods to form the blocking layer: thermal atomic layer deposition at 150 °C using a tetrakis-dimethylamido titanium (TDMAT) precursor, and dip-coating of a TiOx precursor (20mM titanium butoxide in dry ethanol solution) followed by UVOzone decomposition at 150 °C. Analysis of the films by ellipsometry [15] shows that using ALD for
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300 cycles, or dip coating one layer, can produce films with a similar thickness, while the latter method produces a rougher surface (Table 2). The ellipsometry spectra of the ALD deposited thin film shows a sharper absorption peak compared to the dip coated one (Figure 2), indicating that the ALD film has a better long range crystallinity; the absorption peaks are well defined when the materials have a long-range crystal structure, but with a more amorphous structure the absorption peaks broadens and the Urbach tail widens [15,31]. Fitting of the ellipsometry data gives a conduction band position of Eg = 3.54 eV for the ALD TiOx blocking layer, which displays a narrower Urbach tail, while the dip coated TiOx blocking layer has a peak that is too broad to get a reliable fit. The performances of the DSSCs utilizing these different blocking layers are shown in Figure 3 and Table 3.
For the cells with either the dip coated or ALD blocking layers, the dark currents are significantly reduced indicating that both layers can effectively block electron recombination from the ITO to electron acceptors (I3-) in the electrolyte. However, the cells with the dip coated blocking layers do not produce any photocurrent under illumination, which may be due to the fact that dip coating produces a blocking layer that is amorphous in structure (as supported by the ellipsometry studies, Figure 2) and/or it may be too thick and acts as an electron barrier so that electrons generated at the mesoporous working electrode under illumination are also completely blocked. Thus, for the dip coated blocking layer to work properly, a thinner layer of less than 11 nm may need to be used. On the other hand, the cells with an ALD blocking layer have the photocurrent reduced by about 20% compared to cells without a blocking layer. Also, the cells show a notable increase of Voc (125 ± 27 mV at 10% sun, 74 ± 26 mV at 100% sun) and FF (0.14 ± 0.02) that leads to an overall increase in PCE, particularly at low light intensity.
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The effect of the ALD blocking layer was analyzed by a.c. impedance spectroscopy, performed on the cells without illumination. To obtain the charge recombination resistance, Rct, and chemical capacitance, Cct, an applied bias (Vapp) of between 450mV and 750 mV was used on cells with and without the ALD blocking layer. Rct and Cct can be determined by equations 2 and 3 [18]
where R0 and C0 are constants, independent of applied bias voltage, kB is the Boltzmann constant and T0 is a characteristic temperature used to describe the depth of trap states distribution within the bandgap of TiO2[32] . The results (Figure 3C) show that the recombination resistance, Rct, is greater with an ALD blocking layer at any given chemical capacitance. This indicates that the ALD blocking layer can effectively reduce charge recombination from the uncovered ITO to charge acceptors in the electrolyte, thereby offering the desired improvements in Voc and FF.
The influence of photoanode film thickness and dye on photocurrent Typically, photocurrent density increases almost linearly with an increase of mesoporous TiO2 film thickness from ~1 µm to over 14 µm, if the dye used is the same, as more light can be harvested from the portion of the spectrum where the dye absorbs weakly (i.e. giving a higher light harvesting efficiency) [14,33,34]. This relationship has also previously been observed for flexible cells
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incorporating photoanodes prepared by cold isostatic pressing, using a solvent based electrolyte and N719 dye: by varying the thickness from 1 µm to 16 µm, the Jsc increased from ca. 3 mA/cm2 to 12 mA/cm2, with a reduction in Voc from 760mV to 700mV [14]. This decrease in Voc was attributed to increased electron recombination as a result of the larger surface area of the working electrode [14].
The relationship between the thickness of the TiO2 photoanode film and the photovoltaic parameters for the flexible devices, using the IL electrolyte, is shown in Figure 4. These cells show a decrease in Voc similar to those cells reported previously using solvent electrolyte, but surprisingly, also a decrease in Jsc with increasing film thickness from 2 to 8 µm.
To investigate the cause of this decrease in Jsc, incident photo to current efficiency (
)
measurements, using both working electrode (front) and counter electrode (back) illumination, were used to assess the charge collection efficiency of the cells. Front illumination produces two peaks, at 420nm and 535 nm, corresponding to the absorption peaks of the C106 dye (Figure 5A). With increasing film thickness, a slight decrease of
is observed, which is consistent with the change of
Jsc.
The
measurements using back illumination shows a larger difference in the spectra with
increasing film thickness (Figure 5B). The absorption peak at 420nm is no longer present, probably due to the absorption of light by the I-/I3- in the IL electrolyte[35]. Also, the absorption peak at 535nm
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Accepted Article
is shifted to higher wavelengths, and with significantly decreasing
, with increasing film
thickness.
The
of a DSSC consists of three components: the light harvesting efficiency,
injection efficiency,
, and the electron collection efficiency,
wavelength dependent[26]. One method for qualitatively probing
, the electron
. They are all generally
is through assessment of how
depends on the direction of illumination [34], if we assume a constant electron injection
efficiency and light harvesting efficiency for a film with a given thickness. Plotting the /
ratio (Figure 5C) shows that at wavelengths > ca. 600nm the curves approach
similar values, to a ratio of ca. 0.8 – 0.9, indicating that the collected flux of electrons generated by absorbing red shifted photons is less dependent on light illumination direction. This is probably because longer wavelength red light is weakly absorbed by the electrolyte and the dye, so it has a greater penetration depth than other visible light. However, for wavelengths around the absorption peak of the dye (535nm), the curves are noticeably different, with significant troughs in the spectra. Furthermore,
/
is predominantly higher for the thinner TiO2 films, indicating that
although the added film thickness may increase the amount of adsorbed dye molecules and thus more light being harvested, it actually does not produce a higher photocurrent. This is postulated to
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Accepted Article
be due to increased recombination of charges from the thicker TiO2 film with the higher concentration of I-/I3- in the IL, which decreases
.
One possible explanation for the decrease in Voc and Jsc with increasing film thickness (Figure 4) is the effect of the pressing technique used to produce the TiO2 films. The mechanical pressure enhances the particle to particle interconnection, facilitating charge diffusion within the mesoporous TiO2 network [13]. However, it also reduces the porosity and pore size of the TiO2 film compared to sintered mesoporous TiO2 films on glass substrates; the average pore size is 3-4 nm for compressed films[14], and > 25 nm for sintered films[22]. Also, with the increase of film thickness from 2 µm to 16 µm on a compressed TiO2 film, the porosity of the film could reduce from 60% to 45% [34]. These can lead to a slower I-/I3- diffusion rate within the working electrode film, causing a higher electron recombination rate and a lower dye regeneration rate. This is a particular problem with the more viscous IL electrolytes with a high triiodide concentration. Such a phenomenon is also observed for DSSCs using electrolytes with the bigger cobalt couple, where the mesoporous TiO2 film needs to be thinner and have larger pores[36]. Thus, when an IL electrolyte is used with a compressed TiO2 film, a very thin working electrode film (less than 2 µm) is preferred, to reduce recombination. However, such thin films may not have a large enough surface area to allow sufficient light harvesting, which would cause a reduction in the charge harvesting efficiency.
To overcome these problems, and to provide a higher resilience towards attack from water, hydrophobic dyes with higher light extinction coefficients can be used. In this study, MK-2 dye was selected as it has a light extinction coefficient of 35,800 M-1 cm-1,[37] with a water contact angle of
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Accepted Article
137°. It also displays good long-term stability under white-light irradiation, including UV light, or at high temperatures (80 °C) under dark conditions [12].
Cells were prepared using working electrodes with ALD deposited 11.6nm thick blocking layers on the ITO, to reduce the secondary route of charge recombination, and a ~1µm thick compressed P25 mesoporous TiO2 layer sensitized with MK-2 dye. These DSSCs, with the IL electrolyte, produced 4.4% efficiency under 10% sun and 4.0% efficiency under one sun (Table 4, Figure 6). These DSSCs show a much improved
/
with both front and back illumination (Figure 5A &B), and the curve of
(Figure 5C) shows a predominantly higher value than with the C106 sensitized
thicker films, indicating that the charge collection efficiency is much improved.
Stability tests To examine whether or not the flexible DSSCs with IL electrolyte and a hydrophobic dye offer better stability to those with solvent electrolyte, two batches of cells with the same configuration as above, with either the molecular solvent or IL electrolyte, were subject to long-term stability testing at 60 °C under one sun illumination for over 1000 hours. The results, presented in Figure 7, show that both batches of cells have an initial increase of photovoltaic conversion efficiency (PCE) as observed previously [10,12,13,38]. This increase may be due to either an improvement in electrolyte infiltration into the mesoporous TiO2 film with time, and/or the system reaching an equilibrium with the adsorption of electrolyte additives (e.g., NMBI and GNCS) on the TiO2 particles surface, which then reduces recombination and/or improves regeneration of the dye. The cells with solvent electrolyte
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Accepted Article
then show a consistent drop in PCE with aging, similar to previous stability testing on DSSCs with molecular solvents performed by Lee et al.[38] at 60 °C and Miyasaka et al.[30] at 50 °C. This drop of efficiency with aging was mainly due to leaking or permeation of volatile electrolyte solvent out of the cell through the plastic substrates at high temperatures. However, the cells with IL electrolyte retained ca. 90% of their initial PCE after 1000 hours of aging. The drop of efficiency occurred mainly between 100 and 300 hours testing and was probably due to the evaporation of NMBI from the electrolyte and/or ingression of H2O through the plastic substrate which gradually shifted the energy levels of the TiO2/dye/electrolyte interface, reaching equilibrium at around 300 hours of aging. The stable performance observed between 300 to 1000 hours of aging is similar to the previous long-term stability results obtained on cells built on glass substrates using IL electrolytes [8,39,40].
CONCLUSIONS The use of an IL based electrolyte, in combination with a hydrophobic dye, for flexible DSSCs was explored, with the intention of improving cell stability with minimal sacrifice of photovoltaic power conversion efficiency. The results show that over 50% PCE may be lost by simple substitution of a molecular solvent electrolyte with an IL electrolyte for cells built with compressed TiO2 films on flexible PET or PEN substrates with C106 dye. This loss is concluded to firstly be a result of the recombination of electrons from the ITO to electron acceptors in the electrolyte. This is most significant when a higher concentration of triiodide is present, as in the IL electrolyte, which leads to a significant reduction in Voc and FF. To combat this, an ALD deposited blocking layer of ~12nm thickness was applied on the ITO coated substrate. Secondly, the pressing technique used to prepare the TiO2 layer for the flexible DSSCs is known to reduce the pore size of the film compared to those on FTO glass, leading to a slow I-/I3- diffusion rate within the working electrode film, causing a higher
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Accepted Article
electron recombination rate and a lower dye regeneration rate. The dependence of
on
illumination directions shows that the recombination is exacerbated with thicker working electrode films. This problem was combatted using a ~1 µm thick TiO2 film and hydrophobic dye (MK-2) with a high light extinction coefficient. Thus, more efficient and stable, flexible DSSCs that take advantage of the negligible vapor pressure of IL electrolytes, with minimal sacrifice of photovoltaic conversion efficiency, were successfully produced. These devices demonstrated 4.0% efficiency under one sun, and less than 10% drop in efficiency after 1000 hours light soaking at 60 °C.
ACKNOWLEDGMENTS: This work has been supported financially by the Victorian Organic Solar Cell Consortium. Research facilities such as ALD, ellipsometer provided by the Melbourne Centre for Nanofabrication are greatly acknowledged.
REFERENCES [1]
Yella, A., H. Lee, H.N. Tsao, C. Yi, A.K. Chandiran, M.K. Nazeeruddin, E.W. Diau, C. Yeh, S.M.
Zakeeruddin, M. Grätzel (2011) Porphyrin-Sensitized Solar Cells with Cobalt (II/III)-Based Redox Electrolyte Exceed 12 Percent Efficiency. Science. 334, 629–634. [2]
Kalowekamo, J., E. Baker (2009) Estimating the manufacturing cost of purely organic solar
cells. Sol. Energy. 83, 1224–1231. [3]
Miettunen, K., J. Halme, P. Lund (2013) Metallic and plastic dye solar cells. WIREs. Energy
Environ. 2, 104–120.
This article is protected by copyright. All rights reserved. 16
Accepted Article
[4]
Yamaguchi, T., N. Tobe, D. Matsumoto, T. Nagai, H. Arakawa (2010) Highly efficient plastic-
substrate dye-sensitized solar cells with validated conversion efficiency of 7.6%. Sol. Energy Mater. Sol. Cells. 94, 812–816. [5]
Kijitori, Y., M. Ikegami, T. Miyasaka (2007) Highly efficient plastic dye-sensitized
photoelectrodes prepared by low-temperature binder-free coating of mesoscopic titania pastes. Chem. Lett. 36, 190–191. [6]
Zardetto, V., T.M. Brown, A. Reale, A. Di Carlo (2011) Substrates for flexible electronics: A
practical investigation on the electrical, film flexibility, optical, temperature, and solvent resistance properties. J. Polym. Sci. Part B Polym. Phys. 49, 638–648. [7]
Lu, H.-L., T.F.R. Shen, S.-T. Huang, Y.-L. Tung, T.C.K. Yang (2011) The degradation of dye
sensitized solar cell in the presence of water isotopes. Sol. Energy Mater. Sol. Cells. 95, 1624–1629. [8]
Bai, Y., Y. Cao, J. Zhang, M. Wang, R. Li, P. Wang, S.M. Zakeeruddin and M. Grätzel (2008)
High-performance dye-sensitized solar cells based on solvent-free electrolytes produced from eutectic melts. Nat. Mater. 7, 626–630. [9]
Kuang, D., P. Wang, S. Ito, S.M. Zakeeruddin, M. Grätzel (2006) Stable Mesoscopic Dye-
Sensitized Solar Cells Based on Tetracyanoborate Ionic Liquid Electrolyte. J. Am. Chem. Soc. 128, 7732–7733. [10]
Harikisun, R., H. Desilvestro (2011) Long-term stability of dye solar cells. Sol. Energy. 85,
1179–1188. [11]
Asghar, M.I., K. Miettunen, J. Halme, P. Vahermaa, M. Toivola, K. Aitola and P. Lund (2010)
Review of stability for advanced dye solar cells. Energy Environ. Sci. 3, 418–426.
This article is protected by copyright. All rights reserved. 17
Accepted Article
[12]
Hara, K., Z.-S. Wang, Y. Cui, A. Furube, N. Koumura (2009) Long-term stability of organic-dye-
sensitized solar cells based on an alkyl-functionalized carbazole dye. Energy Environ. Sci. 2, 1109– 1114. [13]
Han, Y., J.M. Pringle, Y.B. Cheng (2014) Photovoltaic characteristics and stability of flexible
dye-sensitized solar cells on ITO/PEN substrates. RSC Adv. 4, 1393-1400. [14]
Weerasinghe, H.C., P.M. Sirimanne, G.P. Simon, Y.B. Cheng (2011) Cold isostatic pressing
technique for producing highly efficient flexible dye-sensitised solar cells on plastic substrates. Prog. Photovoltaics Res. Appl. 20, 321–332. [15]
Ferlauto, A. S., G.M. Ferreira, J.M. Pearce, C.R. Wronski, R.W. Collins, X. Deng and G. Ganguly
(2002) Analytical model for the optical functions of amorphous semiconductors from the nearinfrared to ultraviolet: Applications in thin film photovoltaics. J. Appl. Phys. 92, 2424-2436. [16]
Bisquert, J. (2002) Theory of the Impedance of Electron Diffusion and Recombination in a Thin
Layer. J. Phys. Chem. B. 106, 325–333. [17]
Fabregat-Santiago, F., J. Bisquert, G. Garcia-Belmonte, G. Boschloo, A. Hagfeldt (2005)
Influence of electrolyte in transport and recombination in dye-sensitized solar cells studied by impedance spectroscopy. Sol. Energy Mater. Sol. Cells. 87, 117–131. [18]
Wang, Q., S. Ito, M. Grätzel, F. Fabregat-Santiago, I. Mora-Seró, J. Bisquert, T. Bessho and H.
Imai (2006) Characteristics of High Efficiency Dye-Sensitized Solar Cells. J. Phys. Chem. B. 110, 25210– 25221. [19]
Zhang, M., J. Zhang, Y. Bai, Y. Wang, M. Su, P. Wang (2011) Anion-correlated conduction band
edge shifts and charge transfer kinetics in dye-sensitized solar cells with ionic liquid electrolytes. Phys. Chem. Chem. Phys. 13, 3788–3794.
This article is protected by copyright. All rights reserved. 18
Accepted Article
[20]
Cao, Y., Y. Bai, Q. Yu, Y. Cheng, S. Liu, D. Shi, F. Gao and P. Wang (2009) Dye-Sensitized Solar
Cells with a High Absorptivity Ruthenium Sensitizer Featuring a 2-(Hexylthio)thiophene Conjugated Bipyridine. J. Phys. Chem. C. 113, 6290–6297. [21]
Yu, Q., Y. Wang, Z. Yi, N. Zu, J. Zhang, M. Zhang and P. Wang (2010) High-Efficiency Dye-
Sensitized Solar Cells: The Influence of Lithium Ions on Exciton Dissociation, Charge Recombination, and Surface States. ACS Nano. 4, 6032–6038. [22]
Zakeeruddin, S.M., M. Grätzel (2009) Solvent-Free Ionic Liquid Electrolytes for Mesoscopic
Dye-Sensitized Solar Cells. Adv. Funct. Mater. 19, 2187–2202. [23]
Miettunen, K., J. Halme, P. Vahermaa, T. Saukkonen, M. Toivola, P. Lund (2009) Dye Solar Cells
on ITO-PET Substrate with TiO2 Recombination Blocking Layers. J. Electrochem. Soc. 156, B876-B883. [24]
Cameron, P.J., L.M. Peter (2005) How Important is the Back Reaction of Electrons via the
Substrate in Dye-Sensitized Nanocrystalline Solar Cells?. J. Phys. Chem. B. 109, 930–936. [25]
Xia, J., N. Masaki, K. Jiang, S. Yanagida (2007) Sputtered Nb2O5 as a Novel Blocking Layer at
Conducting Glass / TiO2 Interfaces in Dye-Sensitized Ionic Liquid Solar Cells. J. Phys. Chem. C. 111, 8092–8097. [26]
Halme, J., P. Vahermaa, K. Miettunen, P. Lund (2010) Device physics of dye solar cells. Adv.
Mater. 22, E210–E234. [27]
Guillén, E., L.M. Peter, J. A. Anta (2011) Electron Transport and Recombination in ZnO-Based
Dye-Sensitized Solar Cells. J. Phys. Chem. C. 115, 22622–22632. [28]
Yu, Z., N. Vlachopoulos, M. Gorlov, L. Kloo (2011) Liquid electrolytes for dye-sensitized solar
cells. Dalton. Trans. 40, 10289-10303.
This article is protected by copyright. All rights reserved. 19
Accepted Article
[29]
Lee, K.M., S.J. Wu, C.Y. Chen, C.G. Wu, M. Ikegami, K. Miyoshi, T. Miyasaka, K.C. Ho (2009)
Efficient and stable plastic dye-sensitized solar cells based on a high light-harvesting ruthenium sensitizer. J. Mater. Chem. 19, 5009–5015. [30]
Miyasaka, T. (2011) Dye-sensitized solar cells built on plastic substrates by low-temperature
preparation of semiconductor films. Key Eng. Mater. 451, 1–19. [31]
Sancho-Parramon, J., M. Modreanu, S. Bosch, M. Stchakovsky (2008) Optical characterization
of HfO2 by spectroscopic ellipsometry: Dispersion models and direct data inversion. Thin Solid Films. 516, 7990–7995. [32]
Bisquert, J., A. Zaban, M. Greenshtein, I. Mora-Seró (2004) Determination of rate constants
for charge transfer and the distribution of semiconductor and electrolyte electronic energy levels in dye-sensitized solar cells by open-circuit photovoltage decay method. J. Am. Chem. Soc. 126, 13550– 13559. [33]
Ito, S., S.M. Zakeeruddin, R. Humphry-Baker, P. Liska, R. Charvet, P. Comte, M.K. Nazeeruddin,
P. Pechy, M. Takata, H. Miura, S. Uchida, M. Grätzel (2006) High-efficiency organic-dye-sensitized solar cells controlled by nanocrystalline-TiO2 electrode thickness. Adv. Mater. 18, 1202–1205. [34]
Halme, J., G. Boschloo, A. Hagfeldt, P. Lund (2008) Spectral characteristics of light harvesting,
electron injection, and steady-state charge collection in pressed TiO2 dye solar cells. J. Phys. Chem. C. 112, 5623–5637. [35]
Awtrey, A.D., R.E. Connick (1951) The Absorption Spectra of I2, I3-, I-, IO3-, S4O6= and S2O3=.
Heat of the Reaction I3- = I2 + I-, J. Am. Chem. Soc. 73, 1842–1843. [36]
Tsao, H.N., P. Comte, C. Yi, M. Grätzel (2012) Avoiding diffusion limitations in cobalt(III/II)-
tris(2,2'-bipyridine)-based dye-sensitized solar cells by tuning the mesoporous TiO2 film properties. ChemPhyChem. 13, 2976–2981.
This article is protected by copyright. All rights reserved. 20
Accepted Article
[37]
Koumura, N., Z.-S. Wang, S. Mori, M. Miyashita, E. Suzuki, K. Hara (2006) Alkyl-Functionalized
Organic Dyes for Efficient Molecular Photovoltaics. J. Am. Chem. Soc. 128, 14256–14257. [38]
Lee, K.M., W.H. Chiu, M.D. Lu, W.F. Hsieh (2011) Improvement on the long-term stability of
flexible plastic dye-sensitized solar cells. J. Power Sources. 196, 8897–8903. [39]
Harikisun, R., H. Desilvestro (2011) Long-term stability of dye solar cells. Sol. Energy. 85,
1179–1188. [40]
Wang, P.,
.
lein,
.
umphry-Baker, . . akeeru
in,
.
r t el (2005) Stable ≥8%
efficient nanocrystalline dye-sensitized solar cell based on an electrolyte of low volatility. Appl. Phys. Lett. 86, 123508.
FIGURE CAPTIONS Figure 1. IV curves of flexible cells using the solvent or the IL based electrolyte (one sun, C106 dye, without blocking layer)
Figure 2. Elipsometry of TiOx blocking layers prepared by ALD or dip coating, showing the different film crystallinities
Figure 3. (A) Photovoltaic performance at 1 sun, and (B) dark current of cells using the different blocking layers, (C) the charge recombination resistance of the cells
Figure 4. Voc and Jsc of cells with varying TiO2 film thickness, using the IL electrolyte and C106 dye without a blocking layer
This article is protected by copyright. All rights reserved. 21
Accepted Article
Figure 5. (A)
and (B)
for cells with different TiO2 film thickness, and (C)
of the cells
Figure 6. Current-Voltage curves of cells using ~12nm ALD blocking layer, ~1 µm TiO2 film with MK-2 dye, eutectic IL electrolyte
Figure 7. Comparison of stability of flexible DSSC devices using molecular solvent or IL based electrolyte (with ~12nm ALD blocking layer, ~1 µm TiO2 film and MK-2 dye) at 60 °C under one sun illumination
TABLE CAPTIONS Table 1. Comparison of photovoltaic performances of DSSCs using the solvent or the IL electrolyte, with C106 dye
Table 2. Thickness and roughness of the TiOx blocking layer deposited by ALD or dip coating Table 3. Photovoltaic performance of flexible DSSCs using IL electrolyte and the different blocking layers
Table 4. Photovoltaic parameters of cells using ~12nm ALD blocking layer, ~1 µm TiO2 film with MK-2 dye, and the eutectic IL electrolyte
This article is protected by copyright. All rights reserved. 22
Accepted Article
Table 1. Comparison of photovoltaic performances of DSSCs using the solvent or the IL electrolyte, with C106 dye
Illumination Voc (mV)
Jsc (mA/cm2)
FF
PCE (%)
intensity Molecular solvent electrolyte 10% sun
570±10
1.07±0.02
0.73±0.02
4.4
100% sun
659±12
10.95±0.06
0.69±0.02
5.0
Ionic liquid electrolyte 10% sun
446±15
0.79±0.04
0.63±0.01
2.2
100% sun
563±16
7.33±0.05
0.61±0.01
2.5
Table 2. Thickness and roughness of the TiOx blocking layer deposited by ALD or dip coating
Deposition method
Thickness (nm)
Roughness (nm)
ALD (thermal, 300 cycles)
11.65
0.34
Dip coating ( 1 layer)
11.81
1.36
This article is protected by copyright. All rights reserved. 23
Accepted Article
Table 3. Photovoltaic performance of flexible DSSCs using IL electrolyte and the different blocking layers
Without blocking layer
With ALD blocking layer
With dip
Illumination Voc (mV)
Jsc (mA/cm2) FF
PCE (%)
intensity 10% sun
446±15
0.79±0.04
0.63±0.01
2.2
100% sun
563±16
7.33±0.05
0.61±0.02
2.5
10% sun
571±12
0.58±0.04
0.78±0.01
2.5
100% sun
637±10
5.49±0.03
0.75±0.01
2.6
10% sun
552
0.01
0.80
0.04
100% sun
615
0.08
0.80
0.04
coated blocking layer
Table 4. Photovoltaic parameters of cells using ~12nm ALD blocking layer, ~1 µm TiO2 film with MK-2 dye, and the eutectic IL electrolyte
Light intensity
Voc (mV)
Jsc (mA/cm2)
FF
PCE (%)
10% Sun
609±6
0.93±0.02
0.77±0.01
4.4
100% Sun
661±4
8.49±0.03
0.72±0.01
4.0
This article is protected by copyright. All rights reserved. 24
Current (mA/cm2)
10
solvent electrolyte ionic liquid electrolyte
8
6
4
2
0 0
200
400
600
Voltage (mV)
18
Imaginary Dielectric Function (e2)
Accepted Article
12
16
Dip coating ALD
14 12 10 8 6 4 2 0 0
200
400
600
800
1000 1200 1400 1600 1800
Wavelength (nm)
This article is protected by copyright. All rights reserved. 25
8
W ithout Blocking Layer (one sun) W ith ALD Blocking Layer (one sun) W ith Dip Coated Blocking Layer (one sun)
Current (mA/cm2)
6
4
2
0
0
200
400
600
Voltage (mV)
2
W ithout Blocking Layer W ith ALD Blocking Layer W ith Dip Coated Blocking Layer
2 Dark Current (mA/cm )
B 0
-2
-4
0
200
400
600
800
Voltage (mV)
Charge Recombination Resistance, Rct (Ohm)
Accepted Article
A
C
With ALD blocking layer Without blocking layer
100
10 2E-4
Chemical Capacitance, Cct (F)
This article is protected by copyright. All rights reserved. 26
4E-4
7
Voc
595
Jsc
585 5 580 575
4
570 565
3 2
3
4
5
6
Film Thickness ( m)
This article is protected by copyright. All rights reserved. 27
7
8
Jsc (mA/cm2)
6
590
Voc (mV)
Accepted Article
600
Incident photon-to-current efficiency, front (%)
50 40 30 20 10 0 360
400
440
480
520
560
600
640
680
720
760
Incident photon-to-current efficiency, back (%)
Wavelength (nm)
2 •m thick, C106 4 •m thick, C106 8 •m thick,C106 1 •m thick, MK-2
B
40 35 30 25 20 15 10 5 0 -5
360
400
440
480
520
560
600
640
680
720
760
Wavelength (nm)
1.0
2 •m thick, C106 4 •m thick, C106 8 •m thick, C106 1 •m thick, MK-2
C
0.9 0.8
•IPCE,back/•IPCE,front
Accepted Article
2 •m thick, C106 4 •m thick, C106 8 •m thick, C106 1 •m thick, MK-2
A
60
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 400
440
480
520
560
600
640
Wavelength (nm)
This article is protected by copyright. All rights reserved. 28
680
720
760
10% Sun 100% Sun
Current (mA/cm2)
8
6
4
2
0 0
100
200
300
400
500
600
Voltage (mV)
140
Normalized Efficiency (%)
Accepted Article
10
120
100
80
Cells with ionic liquid electrolyte Cells with solvent electrolyte
60
40 0
200
400
600
Time (hours)
This article is protected by copyright. All rights reserved. 29
800
1000
700
Received Date : 07-Oct-2014 Accepted Date : 26-Nov-2014 Article type
: Research Article
Improved Efficiency and Stability of Flexible Dye Sensitized Solar Cells on ITO/PEN Substrates Using an Ionic Liquid Electrolyte
Yu Han,a Jennifer M. Pringle,b* Yi-Bing Chenga* a
. Department of Materials Engineering, Monash University, Clayton, Victoria 3800, Australia
b
. Australian Research Council Centre of Excellence for Electromaterials Science, Deakin University, Melbourne Burwood Campus, 221 Burwood Highway, Burwood, VIC 3125, Australia
* Corresponding authors. Email: [email protected]; [email protected]
ABSTRACT
Flexible dye-sensitized solar cells (DSSCs) built on plastic substrates have attracted great interest as they are lightweight and can be roll-to-roll printed to accelerate production and reduce cost. However, plastic substrates such as PEN and PET are permeable to water, This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/php.12399 This article is protected by copyright. All rights reserved.
Accepted Article
oxygen and volatile electrolyte solvents, which is detrimental to the cell stability. Therefore, to address this problem, in this work, an ionic liquid (IL) electrolyte is used to replace the volatile solvent electrolyte. The initial IL-based devices only achieved around 50% of the photovoltaic conversion efficiency of the cells using the solvent electrolyte. Current-voltage and electrochemical impedance spectroscopy (EIS) analysis of the cells in the dark indicated that this lower efficiency mainly originated from (i) a lack of blocking layer to reduce recombination, and (ii) a lower charge collection efficiency. To combat these problems, cells were developed using a 12nm thick blocking layer, produced by atomic layer deposition, and 1 μm thick P25 TiO2 film sensitized with the hydrophobic MK-2 dye. These flexible DSSCs utilizing an IL electrolyte exhibit significantly improved efficiencies and a less than 10% drop in performance after 1000 hours aging at 60ºC under continuous light illumination.
INTRODUCTION Nanostructured dye-sensitized solar cells (DSSCs) can directly convert solar energy into electricity, with state of the art devices currently achieving a photovoltaic conversion efficiency of up to 12.3% [1]. Commercialization of this technology requires not only high efficiency but also long term stability and low manufacturing costs. Conventionally, such devices are produced using coating technologies on transparent conductive oxides (TCO) glass substrates that are heavy, rigid and have high manufacturing costs[2]. Plastic substrates, on the other hand, are lightweight and flexible and suitable for lower cost mass production via industrial roll-to-roll printing. In addition, the use of flexible substrates can widen the range of possible applications, such as mounting DSSC modules on large roof-top areas of factories and warehouses that are typically not designed to withstand the weight
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incorporating photoanodes prepared by cold isostatic pressing, using a solvent based electrolyte and N719 dye: by varying the thickness from 1 µm to 16 µm, the Jsc increased from ca. 3 mA/cm2 to 12 mA/cm2, with a reduction in Voc from 760mV to 700mV [14]. This decrease in Voc was attributed to increased electron recombination as a result of the larger surface area of the working electrode [14].
The relationship between the thickness of the TiO2 photoanode film and the photovoltaic parameters for the flexible devices, using the IL electrolyte, is shown in Figure 4. These cells show a decrease in Voc similar to those cells reported previously using solvent electrolyte, but surprisingly, also a decrease in Jsc with increasing film thickness from 2 to 8 µm.
To investigate the cause of this decrease in Jsc, incident photo to current efficiency (
)
measurements, using both working electrode (front) and counter electrode (back) illumination, were used to assess the charge collection efficiency of the cells. Front illumination produces two peaks, at 420nm and 535 nm, corresponding to the absorption peaks of the C106 dye (Figure 5A). With increasing film thickness, a slight decrease of
is observed, which is consistent with the change of
Jsc.
The
measurements using back illumination shows a larger difference in the spectra with
increasing film thickness (Figure 5B). The absorption peak at 420nm is no longer present, probably due to the absorption of light by the I-/I3- in the IL electrolyte[35]. Also, the absorption peak at 535nm
This article is protected by copyright. All rights reserved. 11
Accepted Article
then show a consistent drop in PCE with aging, similar to previous stability testing on DSSCs with molecular solvents performed by Lee et al.[38] at 60 °C and Miyasaka et al.[30] at 50 °C. This drop of efficiency with aging was mainly due to leaking or permeation of volatile electrolyte solvent out of the cell through the plastic substrates at high temperatures. However, the cells with IL electrolyte retained ca. 90% of their initial PCE after 1000 hours of aging. The drop of efficiency occurred mainly between 100 and 300 hours testing and was probably due to the evaporation of NMBI from the electrolyte and/or ingression of H2O through the plastic substrate which gradually shifted the energy levels of the TiO2/dye/electrolyte interface, reaching equilibrium at around 300 hours of aging. The stable performance observed between 300 to 1000 hours of aging is similar to the previous long-term stability results obtained on cells built on glass substrates using IL electrolytes [8,39,40].
CONCLUSIONS The use of an IL based electrolyte, in combination with a hydrophobic dye, for flexible DSSCs was explored, with the intention of improving cell stability with minimal sacrifice of photovoltaic power conversion efficiency. The results show that over 50% PCE may be lost by simple substitution of a molecular solvent electrolyte with an IL electrolyte for cells built with compressed TiO2 films on flexible PET or PEN substrates with C106 dye. This loss is concluded to firstly be a result of the recombination of electrons from the ITO to electron acceptors in the electrolyte. This is most significant when a higher concentration of triiodide is present, as in the IL electrolyte, which leads to a significant reduction in Voc and FF. To combat this, an ALD deposited blocking layer of ~12nm thickness was applied on the ITO coated substrate. Secondly, the pressing technique used to prepare the TiO2 layer for the flexible DSSCs is known to reduce the pore size of the film compared to those on FTO glass, leading to a slow I-/I3- diffusion rate within the working electrode film, causing a higher
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Accepted Article
consisted of 1-ethyl-3-methylimidazolium tetracyanoborate (EMITCB), 1-ethyl-3-methylimidazolium iodide (EMII), dimethylimidazolium iodide (DMII) (>99.5% purity, received from Solarpur, Merck), GNCS, Iodine (I2) and NMBI in a molar ratio of 16:12:12:0.67:1.67:3.33 [8].
Cell fabrication and measurement: The preparation procedure for the cells was described in our previous work [13]. Briefly, the mesoporous TiO2 layer was produced by doctor blading a TiO2 slurry onto the conductive substrate, followed by cold isostatic pressing to enhance the inter-particle contact and the contact between the particles and the substrate[14]. The WE and CE were sealed together using a heat curable gasket (Bynel) with a uniform thickness of 25 μm. The electrolyte was injected into the gap between the WE and CE, through a small hole pre-drilled on the CE side, by vacuum backfilling and this was followed by sealing using a UV curable epoxy resin (3035B, ThreeBond). The TiOx blocking layer was produced either by dip-coating of the TiOx precursor (20mM titanium butoxide in dry ethanol solution) followed by UV-Ozone decomposition at 150 °C, or by thermal atomic layer deposition (ALD) at 150 °C with an ALD machine (Fuji-F200) using tetrakis(dimethylamido)titanium (TDMAT) as a precursor and a pulse time of 0.06 seconds for water or 0.2 seconds for TDMAT, with a time interval between each pulse of 30 seconds. Ellipsometry (M-2000, J.A. Woollam Co., Inc) was used to characterize the thin blocking layer in the spectral range from 200nm to 1700nm. The data from the ellipsometer were fitted using the Cody-Lorentz dispersion model [15] to get film thickness, roughness, crystallinity and bandgap. Currentvoltage (IV) curves and parameters, such as open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF) and PCEof the DSSCs were obtained using a Keithley 2400 Source Meter under illumination of simulated sunlight provided by an Oriel solar simulator with an AM1.5 filter. Typically, four cells were measured for each batch of cells to obtain the average IV parameters and deviations. The electrochemical impedance spectra of the DSSCs were recorded on a potentiostat (BioLogic VSP) with a frequency range of
This article is protected by copyright. All rights reserved. 5
Accepted Article
0.05 Hz – 1 KHz. The magnitude of the alternating signal was 10 mV. The impedance measurements were carried out under different voltage biases ranging from 0.3 to 0.9 V in the dark. The obtained impedance spectra were fitted with the Z-view software (v2.8, Scribner Associates Inc.), using a general transmission line model of DSSC described in the literature [16][17][18], to derive parameters such as the chemical capacitance Cμ, and interfacial charge transfer resistance Rct.
The stability tests were performed on two batches of cells with 11.6nm ALD TiO2 blocking layer, ~1µm thick compressed P25 mesoporous TiO2 layer sensitized with MK-2 dye and commercially available platinized counter electrode and either the molecular solvent or the IL electrolyte. The cells were subjected to one sun illumination at 60 °C for over 1000 hours in a home-made light soaking box equipped with a metal halide lamp. The temperature of the cells was monitored using thermocouples and was controlled by ventilation through a fan installed on the side of the box.
RESULTS AND DISCUSSION The photovoltaic parameters of DSSCs assembled using plastic substrates with either the MPN-based electrolyte or the ionic liquid electrolyte, and the Ru-based C106 dye, are shown in Table 1 and Figure 1. The change from the solvent electrolyte to the IL electrolyte resulted in a decrease in photovoltaic efficiency of ca. 50 %. In comparison, for DSSCs made on glass substrates with the same dye and either a solvent electrolyte or the optimum IL electrolyte composition, typically over 70% photovoltaic conversion efficiency is retained [8,19–21].
This article is protected by copyright. All rights reserved. 6
Accepted Article
The influence of blocking layer on Voc and FF As shown in Table 1, all three device parameters (Voc, Jsc, Fill factor) for the flexible DSSCs dropped significantly with use of the IL electrolyte, leading to a substantial reduction in photovoltaic conversion efficiency. This drop is largely related to the higher concentration of electron acceptors (I3) in the IL based electrolyte, which is used to overcome mass transport limitations in these more viscous systems [22]. This high concentration of I3- leads to a higher recombination rate of electrons from the mesoporous TiO2 photoanode with the acceptors (I3-) in the electrolyte, causing a reduction of Voc. Moreover, this electron recombination can also occur at the interface of the conducting substrate that is exposed to the electrolyte [23][24][25].
The latter route for recombination has been shown to significantly increase the non-ideality factor (m) of a DSSC, which is normally in the range of ~1.25 to ~1.9. This factor is used to describe the experimentally observed non-ideal charge recombination via the conductive substrate/electrolyte interface and/or via surface states distributed in the band gap of TiO2 [26], measured by the light intensity dependence of Voc, such that:
(1)
where T is the temperature (293K), I0 is the intensity of light, and e is the elementary charge [26,27]. Table 1 shows that with a 10 fold increase in light intensity, Voc increased by 89 ±22 mV for the molecular solvent based cells and 117 ± 31 mV for IL based cells. From equation 1, the non-ideality
This article is protected by copyright. All rights reserved. 7
Accepted Article
factor of the cells using solvent based electrolyte was estimated to be 1.51, compared to 1.98 for those using IL electrolytes. This indicates that electron recombination at the uncovered ITO surface is more significant with the IL electrolyte than with the solvent electrolyte, causing the observed reduction in FF [26].
To combat this problem, a compact TiOx blocking layer can be deposited onto the ITO film [26,28]. This is typically achieved by TiCl4 treatment, or the spray coating of a TiOx precursor followed by high temperature (~450 °C) decomposition, producing a dense TiOx blocking layer on the FTO glass substrate. However, as the maximum endurable temperature for the plastic substrate is 150 °C for PET and 180 °C for PEN, and the conductive ITO layer is vulnerable to acids and bases, these techniques are not suitable for flexible ITO/PET or ITO/PEN substrates. Low temperature processing methods, such as the reactive sputtering of Ti metal followed by anodic oxidation to produce a 15nm thick dense blocking layer [29,30] has been shown to significantly improve the fill factor of DSSC cells on plastic substrates. The use of an atomic layer deposited blocking layer on ITO/PET was investigated by Miettunen et al. [23] and found to increase the open circuit voltage at low light intensity but reduce the fill factor at high light intensity. For plastic substrates, restricted to low temperature processing, thinner blocking layers were identified as preferable compared to the thicker layers used with glass substrates.
Here we investigate two low temperature processing methods to form the blocking layer: thermal atomic layer deposition at 150 °C using a tetrakis-dimethylamido titanium (TDMAT) precursor, and dip-coating of a TiOx precursor (20mM titanium butoxide in dry ethanol solution) followed by UVOzone decomposition at 150 °C. Analysis of the films by ellipsometry [15] shows that using ALD for
This article is protected by copyright. All rights reserved. 8
Accepted Article
300 cycles, or dip coating one layer, can produce films with a similar thickness, while the latter method produces a rougher surface (Table 2). The ellipsometry spectra of the ALD deposited thin film shows a sharper absorption peak compared to the dip coated one (Figure 2), indicating that the ALD film has a better long range crystallinity; the absorption peaks are well defined when the materials have a long-range crystal structure, but with a more amorphous structure the absorption peaks broadens and the Urbach tail widens [15,31]. Fitting of the ellipsometry data gives a conduction band position of Eg = 3.54 eV for the ALD TiOx blocking layer, which displays a narrower Urbach tail, while the dip coated TiOx blocking layer has a peak that is too broad to get a reliable fit. The performances of the DSSCs utilizing these different blocking layers are shown in Figure 3 and Table 3.
For the cells with either the dip coated or ALD blocking layers, the dark currents are significantly reduced indicating that both layers can effectively block electron recombination from the ITO to electron acceptors (I3-) in the electrolyte. However, the cells with the dip coated blocking layers do not produce any photocurrent under illumination, which may be due to the fact that dip coating produces a blocking layer that is amorphous in structure (as supported by the ellipsometry studies, Figure 2) and/or it may be too thick and acts as an electron barrier so that electrons generated at the mesoporous working electrode under illumination are also completely blocked. Thus, for the dip coated blocking layer to work properly, a thinner layer of less than 11 nm may need to be used. On the other hand, the cells with an ALD blocking layer have the photocurrent reduced by about 20% compared to cells without a blocking layer. Also, the cells show a notable increase of Voc (125 ± 27 mV at 10% sun, 74 ± 26 mV at 100% sun) and FF (0.14 ± 0.02) that leads to an overall increase in PCE, particularly at low light intensity.
This article is protected by copyright. All rights reserved. 9
Accepted Article
The effect of the ALD blocking layer was analyzed by a.c. impedance spectroscopy, performed on the cells without illumination. To obtain the charge recombination resistance, Rct, and chemical capacitance, Cct, an applied bias (Vapp) of between 450mV and 750 mV was used on cells with and without the ALD blocking layer. Rct and Cct can be determined by equations 2 and 3 [18]
where R0 and C0 are constants, independent of applied bias voltage, kB is the Boltzmann constant and T0 is a characteristic temperature used to describe the depth of trap states distribution within the bandgap of TiO2[32] . The results (Figure 3C) show that the recombination resistance, Rct, is greater with an ALD blocking layer at any given chemical capacitance. This indicates that the ALD blocking layer can effectively reduce charge recombination from the uncovered ITO to charge acceptors in the electrolyte, thereby offering the desired improvements in Voc and FF.
The influence of photoanode film thickness and dye on photocurrent Typically, photocurrent density increases almost linearly with an increase of mesoporous TiO2 film thickness from ~1 µm to over 14 µm, if the dye used is the same, as more light can be harvested from the portion of the spectrum where the dye absorbs weakly (i.e. giving a higher light harvesting efficiency) [14,33,34]. This relationship has also previously been observed for flexible cells
This article is protected by copyright. All rights reserved. 10
Accepted Article
incorporating photoanodes prepared by cold isostatic pressing, using a solvent based electrolyte and N719 dye: by varying the thickness from 1 µm to 16 µm, the Jsc increased from ca. 3 mA/cm2 to 12 mA/cm2, with a reduction in Voc from 760mV to 700mV [14]. This decrease in Voc was attributed to increased electron recombination as a result of the larger surface area of the working electrode [14].
The relationship between the thickness of the TiO2 photoanode film and the photovoltaic parameters for the flexible devices, using the IL electrolyte, is shown in Figure 4. These cells show a decrease in Voc similar to those cells reported previously using solvent electrolyte, but surprisingly, also a decrease in Jsc with increasing film thickness from 2 to 8 µm.
To investigate the cause of this decrease in Jsc, incident photo to current efficiency (
)
measurements, using both working electrode (front) and counter electrode (back) illumination, were used to assess the charge collection efficiency of the cells. Front illumination produces two peaks, at 420nm and 535 nm, corresponding to the absorption peaks of the C106 dye (Figure 5A). With increasing film thickness, a slight decrease of
is observed, which is consistent with the change of
Jsc.
The
measurements using back illumination shows a larger difference in the spectra with
increasing film thickness (Figure 5B). The absorption peak at 420nm is no longer present, probably due to the absorption of light by the I-/I3- in the IL electrolyte[35]. Also, the absorption peak at 535nm
This article is protected by copyright. All rights reserved. 11
Accepted Article
is shifted to higher wavelengths, and with significantly decreasing
, with increasing film
thickness.
The
of a DSSC consists of three components: the light harvesting efficiency,
injection efficiency,
, and the electron collection efficiency,
wavelength dependent[26]. One method for qualitatively probing
, the electron
. They are all generally
is through assessment of how
depends on the direction of illumination [34], if we assume a constant electron injection
efficiency and light harvesting efficiency for a film with a given thickness. Plotting the /
ratio (Figure 5C) shows that at wavelengths > ca. 600nm the curves approach
similar values, to a ratio of ca. 0.8 – 0.9, indicating that the collected flux of electrons generated by absorbing red shifted photons is less dependent on light illumination direction. This is probably because longer wavelength red light is weakly absorbed by the electrolyte and the dye, so it has a greater penetration depth than other visible light. However, for wavelengths around the absorption peak of the dye (535nm), the curves are noticeably different, with significant troughs in the spectra. Furthermore,
/
is predominantly higher for the thinner TiO2 films, indicating that
although the added film thickness may increase the amount of adsorbed dye molecules and thus more light being harvested, it actually does not produce a higher photocurrent. This is postulated to
This article is protected by copyright. All rights reserved. 12
Accepted Article
be due to increased recombination of charges from the thicker TiO2 film with the higher concentration of I-/I3- in the IL, which decreases
.
One possible explanation for the decrease in Voc and Jsc with increasing film thickness (Figure 4) is the effect of the pressing technique used to produce the TiO2 films. The mechanical pressure enhances the particle to particle interconnection, facilitating charge diffusion within the mesoporous TiO2 network [13]. However, it also reduces the porosity and pore size of the TiO2 film compared to sintered mesoporous TiO2 films on glass substrates; the average pore size is 3-4 nm for compressed films[14], and > 25 nm for sintered films[22]. Also, with the increase of film thickness from 2 µm to 16 µm on a compressed TiO2 film, the porosity of the film could reduce from 60% to 45% [34]. These can lead to a slower I-/I3- diffusion rate within the working electrode film, causing a higher electron recombination rate and a lower dye regeneration rate. This is a particular problem with the more viscous IL electrolytes with a high triiodide concentration. Such a phenomenon is also observed for DSSCs using electrolytes with the bigger cobalt couple, where the mesoporous TiO2 film needs to be thinner and have larger pores[36]. Thus, when an IL electrolyte is used with a compressed TiO2 film, a very thin working electrode film (less than 2 µm) is preferred, to reduce recombination. However, such thin films may not have a large enough surface area to allow sufficient light harvesting, which would cause a reduction in the charge harvesting efficiency.
To overcome these problems, and to provide a higher resilience towards attack from water, hydrophobic dyes with higher light extinction coefficients can be used. In this study, MK-2 dye was selected as it has a light extinction coefficient of 35,800 M-1 cm-1,[37] with a water contact angle of
This article is protected by copyright. All rights reserved. 13
Accepted Article
137°. It also displays good long-term stability under white-light irradiation, including UV light, or at high temperatures (80 °C) under dark conditions [12].
Cells were prepared using working electrodes with ALD deposited 11.6nm thick blocking layers on the ITO, to reduce the secondary route of charge recombination, and a ~1µm thick compressed P25 mesoporous TiO2 layer sensitized with MK-2 dye. These DSSCs, with the IL electrolyte, produced 4.4% efficiency under 10% sun and 4.0% efficiency under one sun (Table 4, Figure 6). These DSSCs show a much improved
/
with both front and back illumination (Figure 5A &B), and the curve of
(Figure 5C) shows a predominantly higher value than with the C106 sensitized
thicker films, indicating that the charge collection efficiency is much improved.
Stability tests To examine whether or not the flexible DSSCs with IL electrolyte and a hydrophobic dye offer better stability to those with solvent electrolyte, two batches of cells with the same configuration as above, with either the molecular solvent or IL electrolyte, were subject to long-term stability testing at 60 °C under one sun illumination for over 1000 hours. The results, presented in Figure 7, show that both batches of cells have an initial increase of photovoltaic conversion efficiency (PCE) as observed previously [10,12,13,38]. This increase may be due to either an improvement in electrolyte infiltration into the mesoporous TiO2 film with time, and/or the system reaching an equilibrium with the adsorption of electrolyte additives (e.g., NMBI and GNCS) on the TiO2 particles surface, which then reduces recombination and/or improves regeneration of the dye. The cells with solvent electrolyte
This article is protected by copyright. All rights reserved. 14
Accepted Article
then show a consistent drop in PCE with aging, similar to previous stability testing on DSSCs with molecular solvents performed by Lee et al.[38] at 60 °C and Miyasaka et al.[30] at 50 °C. This drop of efficiency with aging was mainly due to leaking or permeation of volatile electrolyte solvent out of the cell through the plastic substrates at high temperatures. However, the cells with IL electrolyte retained ca. 90% of their initial PCE after 1000 hours of aging. The drop of efficiency occurred mainly between 100 and 300 hours testing and was probably due to the evaporation of NMBI from the electrolyte and/or ingression of H2O through the plastic substrate which gradually shifted the energy levels of the TiO2/dye/electrolyte interface, reaching equilibrium at around 300 hours of aging. The stable performance observed between 300 to 1000 hours of aging is similar to the previous long-term stability results obtained on cells built on glass substrates using IL electrolytes [8,39,40].
CONCLUSIONS The use of an IL based electrolyte, in combination with a hydrophobic dye, for flexible DSSCs was explored, with the intention of improving cell stability with minimal sacrifice of photovoltaic power conversion efficiency. The results show that over 50% PCE may be lost by simple substitution of a molecular solvent electrolyte with an IL electrolyte for cells built with compressed TiO2 films on flexible PET or PEN substrates with C106 dye. This loss is concluded to firstly be a result of the recombination of electrons from the ITO to electron acceptors in the electrolyte. This is most significant when a higher concentration of triiodide is present, as in the IL electrolyte, which leads to a significant reduction in Voc and FF. To combat this, an ALD deposited blocking layer of ~12nm thickness was applied on the ITO coated substrate. Secondly, the pressing technique used to prepare the TiO2 layer for the flexible DSSCs is known to reduce the pore size of the film compared to those on FTO glass, leading to a slow I-/I3- diffusion rate within the working electrode film, causing a higher
This article is protected by copyright. All rights reserved. 15
Accepted Article
electron recombination rate and a lower dye regeneration rate. The dependence of
on
illumination directions shows that the recombination is exacerbated with thicker working electrode films. This problem was combatted using a ~1 µm thick TiO2 film and hydrophobic dye (MK-2) with a high light extinction coefficient. Thus, more efficient and stable, flexible DSSCs that take advantage of the negligible vapor pressure of IL electrolytes, with minimal sacrifice of photovoltaic conversion efficiency, were successfully produced. These devices demonstrated 4.0% efficiency under one sun, and less than 10% drop in efficiency after 1000 hours light soaking at 60 °C.
ACKNOWLEDGMENTS: This work has been supported financially by the Victorian Organic Solar Cell Consortium. Research facilities such as ALD, ellipsometer provided by the Melbourne Centre for Nanofabrication are greatly acknowledged.
REFERENCES [1]
Yella, A., H. Lee, H.N. Tsao, C. Yi, A.K. Chandiran, M.K. Nazeeruddin, E.W. Diau, C. Yeh, S.M.
Zakeeruddin, M. Grätzel (2011) Porphyrin-Sensitized Solar Cells with Cobalt (II/III)-Based Redox Electrolyte Exceed 12 Percent Efficiency. Science. 334, 629–634. [2]
Kalowekamo, J., E. Baker (2009) Estimating the manufacturing cost of purely organic solar
cells. Sol. Energy. 83, 1224–1231. [3]
Miettunen, K., J. Halme, P. Lund (2013) Metallic and plastic dye solar cells. WIREs. Energy
Environ. 2, 104–120.
This article is protected by copyright. All rights reserved. 16
Accepted Article
[4]
Yamaguchi, T., N. Tobe, D. Matsumoto, T. Nagai, H. Arakawa (2010) Highly efficient plastic-
substrate dye-sensitized solar cells with validated conversion efficiency of 7.6%. Sol. Energy Mater. Sol. Cells. 94, 812–816. [5]
Kijitori, Y., M. Ikegami, T. Miyasaka (2007) Highly efficient plastic dye-sensitized
photoelectrodes prepared by low-temperature binder-free coating of mesoscopic titania pastes. Chem. Lett. 36, 190–191. [6]
Zardetto, V., T.M. Brown, A. Reale, A. Di Carlo (2011) Substrates for flexible electronics: A
practical investigation on the electrical, film flexibility, optical, temperature, and solvent resistance properties. J. Polym. Sci. Part B Polym. Phys. 49, 638–648. [7]
Lu, H.-L., T.F.R. Shen, S.-T. Huang, Y.-L. Tung, T.C.K. Yang (2011) The degradation of dye
sensitized solar cell in the presence of water isotopes. Sol. Energy Mater. Sol. Cells. 95, 1624–1629. [8]
Bai, Y., Y. Cao, J. Zhang, M. Wang, R. Li, P. Wang, S.M. Zakeeruddin and M. Grätzel (2008)
High-performance dye-sensitized solar cells based on solvent-free electrolytes produced from eutectic melts. Nat. Mater. 7, 626–630. [9]
Kuang, D., P. Wang, S. Ito, S.M. Zakeeruddin, M. Grätzel (2006) Stable Mesoscopic Dye-
Sensitized Solar Cells Based on Tetracyanoborate Ionic Liquid Electrolyte. J. Am. Chem. Soc. 128, 7732–7733. [10]
Harikisun, R., H. Desilvestro (2011) Long-term stability of dye solar cells. Sol. Energy. 85,
1179–1188. [11]
Asghar, M.I., K. Miettunen, J. Halme, P. Vahermaa, M. Toivola, K. Aitola and P. Lund (2010)
Review of stability for advanced dye solar cells. Energy Environ. Sci. 3, 418–426.
This article is protected by copyright. All rights reserved. 17
Accepted Article
[12]
Hara, K., Z.-S. Wang, Y. Cui, A. Furube, N. Koumura (2009) Long-term stability of organic-dye-
sensitized solar cells based on an alkyl-functionalized carbazole dye. Energy Environ. Sci. 2, 1109– 1114. [13]
Han, Y., J.M. Pringle, Y.B. Cheng (2014) Photovoltaic characteristics and stability of flexible
dye-sensitized solar cells on ITO/PEN substrates. RSC Adv. 4, 1393-1400. [14]
Weerasinghe, H.C., P.M. Sirimanne, G.P. Simon, Y.B. Cheng (2011) Cold isostatic pressing
technique for producing highly efficient flexible dye-sensitised solar cells on plastic substrates. Prog. Photovoltaics Res. Appl. 20, 321–332. [15]
Ferlauto, A. S., G.M. Ferreira, J.M. Pearce, C.R. Wronski, R.W. Collins, X. Deng and G. Ganguly
(2002) Analytical model for the optical functions of amorphous semiconductors from the nearinfrared to ultraviolet: Applications in thin film photovoltaics. J. Appl. Phys. 92, 2424-2436. [16]
Bisquert, J. (2002) Theory of the Impedance of Electron Diffusion and Recombination in a Thin
Layer. J. Phys. Chem. B. 106, 325–333. [17]
Fabregat-Santiago, F., J. Bisquert, G. Garcia-Belmonte, G. Boschloo, A. Hagfeldt (2005)
Influence of electrolyte in transport and recombination in dye-sensitized solar cells studied by impedance spectroscopy. Sol. Energy Mater. Sol. Cells. 87, 117–131. [18]
Wang, Q., S. Ito, M. Grätzel, F. Fabregat-Santiago, I. Mora-Seró, J. Bisquert, T. Bessho and H.
Imai (2006) Characteristics of High Efficiency Dye-Sensitized Solar Cells. J. Phys. Chem. B. 110, 25210– 25221. [19]
Zhang, M., J. Zhang, Y. Bai, Y. Wang, M. Su, P. Wang (2011) Anion-correlated conduction band
edge shifts and charge transfer kinetics in dye-sensitized solar cells with ionic liquid electrolytes. Phys. Chem. Chem. Phys. 13, 3788–3794.
This article is protected by copyright. All rights reserved. 18
Accepted Article
[20]
Cao, Y., Y. Bai, Q. Yu, Y. Cheng, S. Liu, D. Shi, F. Gao and P. Wang (2009) Dye-Sensitized Solar
Cells with a High Absorptivity Ruthenium Sensitizer Featuring a 2-(Hexylthio)thiophene Conjugated Bipyridine. J. Phys. Chem. C. 113, 6290–6297. [21]
Yu, Q., Y. Wang, Z. Yi, N. Zu, J. Zhang, M. Zhang and P. Wang (2010) High-Efficiency Dye-
Sensitized Solar Cells: The Influence of Lithium Ions on Exciton Dissociation, Charge Recombination, and Surface States. ACS Nano. 4, 6032–6038. [22]
Zakeeruddin, S.M., M. Grätzel (2009) Solvent-Free Ionic Liquid Electrolytes for Mesoscopic
Dye-Sensitized Solar Cells. Adv. Funct. Mater. 19, 2187–2202. [23]
Miettunen, K., J. Halme, P. Vahermaa, T. Saukkonen, M. Toivola, P. Lund (2009) Dye Solar Cells
on ITO-PET Substrate with TiO2 Recombination Blocking Layers. J. Electrochem. Soc. 156, B876-B883. [24]
Cameron, P.J., L.M. Peter (2005) How Important is the Back Reaction of Electrons via the
Substrate in Dye-Sensitized Nanocrystalline Solar Cells?. J. Phys. Chem. B. 109, 930–936. [25]
Xia, J., N. Masaki, K. Jiang, S. Yanagida (2007) Sputtered Nb2O5 as a Novel Blocking Layer at
Conducting Glass / TiO2 Interfaces in Dye-Sensitized Ionic Liquid Solar Cells. J. Phys. Chem. C. 111, 8092–8097. [26]
Halme, J., P. Vahermaa, K. Miettunen, P. Lund (2010) Device physics of dye solar cells. Adv.
Mater. 22, E210–E234. [27]
Guillén, E., L.M. Peter, J. A. Anta (2011) Electron Transport and Recombination in ZnO-Based
Dye-Sensitized Solar Cells. J. Phys. Chem. C. 115, 22622–22632. [28]
Yu, Z., N. Vlachopoulos, M. Gorlov, L. Kloo (2011) Liquid electrolytes for dye-sensitized solar
cells. Dalton. Trans. 40, 10289-10303.
This article is protected by copyright. All rights reserved. 19
Accepted Article
[29]
Lee, K.M., S.J. Wu, C.Y. Chen, C.G. Wu, M. Ikegami, K. Miyoshi, T. Miyasaka, K.C. Ho (2009)
Efficient and stable plastic dye-sensitized solar cells based on a high light-harvesting ruthenium sensitizer. J. Mater. Chem. 19, 5009–5015. [30]
Miyasaka, T. (2011) Dye-sensitized solar cells built on plastic substrates by low-temperature
preparation of semiconductor films. Key Eng. Mater. 451, 1–19. [31]
Sancho-Parramon, J., M. Modreanu, S. Bosch, M. Stchakovsky (2008) Optical characterization
of HfO2 by spectroscopic ellipsometry: Dispersion models and direct data inversion. Thin Solid Films. 516, 7990–7995. [32]
Bisquert, J., A. Zaban, M. Greenshtein, I. Mora-Seró (2004) Determination of rate constants
for charge transfer and the distribution of semiconductor and electrolyte electronic energy levels in dye-sensitized solar cells by open-circuit photovoltage decay method. J. Am. Chem. Soc. 126, 13550– 13559. [33]
Ito, S., S.M. Zakeeruddin, R. Humphry-Baker, P. Liska, R. Charvet, P. Comte, M.K. Nazeeruddin,
P. Pechy, M. Takata, H. Miura, S. Uchida, M. Grätzel (2006) High-efficiency organic-dye-sensitized solar cells controlled by nanocrystalline-TiO2 electrode thickness. Adv. Mater. 18, 1202–1205. [34]
Halme, J., G. Boschloo, A. Hagfeldt, P. Lund (2008) Spectral characteristics of light harvesting,
electron injection, and steady-state charge collection in pressed TiO2 dye solar cells. J. Phys. Chem. C. 112, 5623–5637. [35]
Awtrey, A.D., R.E. Connick (1951) The Absorption Spectra of I2, I3-, I-, IO3-, S4O6= and S2O3=.
Heat of the Reaction I3- = I2 + I-, J. Am. Chem. Soc. 73, 1842–1843. [36]
Tsao, H.N., P. Comte, C. Yi, M. Grätzel (2012) Avoiding diffusion limitations in cobalt(III/II)-
tris(2,2'-bipyridine)-based dye-sensitized solar cells by tuning the mesoporous TiO2 film properties. ChemPhyChem. 13, 2976–2981.
This article is protected by copyright. All rights reserved. 20
Accepted Article
[37]
Koumura, N., Z.-S. Wang, S. Mori, M. Miyashita, E. Suzuki, K. Hara (2006) Alkyl-Functionalized
Organic Dyes for Efficient Molecular Photovoltaics. J. Am. Chem. Soc. 128, 14256–14257. [38]
Lee, K.M., W.H. Chiu, M.D. Lu, W.F. Hsieh (2011) Improvement on the long-term stability of
flexible plastic dye-sensitized solar cells. J. Power Sources. 196, 8897–8903. [39]
Harikisun, R., H. Desilvestro (2011) Long-term stability of dye solar cells. Sol. Energy. 85,
1179–1188. [40]
Wang, P.,
.
lein,
.
umphry-Baker, . . akeeru
in,
.
r t el (2005) Stable ≥8%
efficient nanocrystalline dye-sensitized solar cell based on an electrolyte of low volatility. Appl. Phys. Lett. 86, 123508.
FIGURE CAPTIONS Figure 1. IV curves of flexible cells using the solvent or the IL based electrolyte (one sun, C106 dye, without blocking layer)
Figure 2. Elipsometry of TiOx blocking layers prepared by ALD or dip coating, showing the different film crystallinities
Figure 3. (A) Photovoltaic performance at 1 sun, and (B) dark current of cells using the different blocking layers, (C) the charge recombination resistance of the cells
Figure 4. Voc and Jsc of cells with varying TiO2 film thickness, using the IL electrolyte and C106 dye without a blocking layer
This article is protected by copyright. All rights reserved. 21
Accepted Article
Figure 5. (A)
and (B)
for cells with different TiO2 film thickness, and (C)
of the cells
Figure 6. Current-Voltage curves of cells using ~12nm ALD blocking layer, ~1 µm TiO2 film with MK-2 dye, eutectic IL electrolyte
Figure 7. Comparison of stability of flexible DSSC devices using molecular solvent or IL based electrolyte (with ~12nm ALD blocking layer, ~1 µm TiO2 film and MK-2 dye) at 60 °C under one sun illumination
TABLE CAPTIONS Table 1. Comparison of photovoltaic performances of DSSCs using the solvent or the IL electrolyte, with C106 dye
Table 2. Thickness and roughness of the TiOx blocking layer deposited by ALD or dip coating Table 3. Photovoltaic performance of flexible DSSCs using IL electrolyte and the different blocking layers
Table 4. Photovoltaic parameters of cells using ~12nm ALD blocking layer, ~1 µm TiO2 film with MK-2 dye, and the eutectic IL electrolyte
This article is protected by copyright. All rights reserved. 22
Accepted Article
Table 1. Comparison of photovoltaic performances of DSSCs using the solvent or the IL electrolyte, with C106 dye
Illumination Voc (mV)
Jsc (mA/cm2)
FF
PCE (%)
intensity Molecular solvent electrolyte 10% sun
570±10
1.07±0.02
0.73±0.02
4.4
100% sun
659±12
10.95±0.06
0.69±0.02
5.0
Ionic liquid electrolyte 10% sun
446±15
0.79±0.04
0.63±0.01
2.2
100% sun
563±16
7.33±0.05
0.61±0.01
2.5
Table 2. Thickness and roughness of the TiOx blocking layer deposited by ALD or dip coating
Deposition method
Thickness (nm)
Roughness (nm)
ALD (thermal, 300 cycles)
11.65
0.34
Dip coating ( 1 layer)
11.81
1.36
This article is protected by copyright. All rights reserved. 23
Accepted Article
Table 3. Photovoltaic performance of flexible DSSCs using IL electrolyte and the different blocking layers
Without blocking layer
With ALD blocking layer
With dip
Illumination Voc (mV)
Jsc (mA/cm2) FF
PCE (%)
intensity 10% sun
446±15
0.79±0.04
0.63±0.01
2.2
100% sun
563±16
7.33±0.05
0.61±0.02
2.5
10% sun
571±12
0.58±0.04
0.78±0.01
2.5
100% sun
637±10
5.49±0.03
0.75±0.01
2.6
10% sun
552
0.01
0.80
0.04
100% sun
615
0.08
0.80
0.04
coated blocking layer
Table 4. Photovoltaic parameters of cells using ~12nm ALD blocking layer, ~1 µm TiO2 film with MK-2 dye, and the eutectic IL electrolyte
Light intensity
Voc (mV)
Jsc (mA/cm2)
FF
PCE (%)
10% Sun
609±6
0.93±0.02
0.77±0.01
4.4
100% Sun
661±4
8.49±0.03
0.72±0.01
4.0
This article is protected by copyright. All rights reserved. 24
Current (mA/cm2)
10
solvent electrolyte ionic liquid electrolyte
8
6
4
2
0 0
200
400
600
Voltage (mV)
18
Imaginary Dielectric Function (e2)
Accepted Article
12
16
Dip coating ALD
14 12 10 8 6 4 2 0 0
200
400
600
800
1000 1200 1400 1600 1800
Wavelength (nm)
This article is protected by copyright. All rights reserved. 25
8
W ithout Blocking Layer (one sun) W ith ALD Blocking Layer (one sun) W ith Dip Coated Blocking Layer (one sun)
Current (mA/cm2)
6
4
2
0
0
200
400
600
Voltage (mV)
2
W ithout Blocking Layer W ith ALD Blocking Layer W ith Dip Coated Blocking Layer
2 Dark Current (mA/cm )
B 0
-2
-4
0
200
400
600
800
Voltage (mV)
Charge Recombination Resistance, Rct (Ohm)
Accepted Article
A
C
With ALD blocking layer Without blocking layer
100
10 2E-4
Chemical Capacitance, Cct (F)
This article is protected by copyright. All rights reserved. 26
4E-4
7
Voc
595
Jsc
585 5 580 575
4
570 565
3 2
3
4
5
6
Film Thickness ( m)
This article is protected by copyright. All rights reserved. 27
7
8
Jsc (mA/cm2)
6
590
Voc (mV)
Accepted Article
600
Incident photon-to-current efficiency, front (%)
50 40 30 20 10 0 360
400
440
480
520
560
600
640
680
720
760
Incident photon-to-current efficiency, back (%)
Wavelength (nm)
2 •m thick, C106 4 •m thick, C106 8 •m thick,C106 1 •m thick, MK-2
B
40 35 30 25 20 15 10 5 0 -5
360
400
440
480
520
560
600
640
680
720
760
Wavelength (nm)
1.0
2 •m thick, C106 4 •m thick, C106 8 •m thick, C106 1 •m thick, MK-2
C
0.9 0.8
•IPCE,back/•IPCE,front
Accepted Article
2 •m thick, C106 4 •m thick, C106 8 •m thick, C106 1 •m thick, MK-2
A
60
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 400
440
480
520
560
600
640
Wavelength (nm)
This article is protected by copyright. All rights reserved. 28
680
720
760
10% Sun 100% Sun
Current (mA/cm2)
8
6
4
2
0 0
100
200
300
400
500
600
Voltage (mV)
140
Normalized Efficiency (%)
Accepted Article
10
120
100
80
Cells with ionic liquid electrolyte Cells with solvent electrolyte
60
40 0
200
400
600
Time (hours)
This article is protected by copyright. All rights reserved. 29
800
1000
700
