Quasi-Solid-State Dye-Sensitized Solar Cells on Plastic Substrates

Jan 22, 2014 - Interest has recently grown around flexible dye-sensitized solar cells (DSCs) for their potential low cost roll-to-roll production proc...
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Quasi-Solid-State Dye-Sensitized Solar Cells on Plastic Substrates Yasmina Dkhissi,† Fuzhi Huang,‡ Yi-Bing Cheng,*,‡ and Rachel A. Caruso*,†,§ †

PFPC, School of Chemistry, The University of Melbourne, Melbourne, Victoria 3010, Australia Department of Materials Engineering, Monash University, Melbourne, Victoria 3800, Australia § CSIRO Materials Science and Engineering, Private Bag 33, Clayton South, Victoria 3169, Australia ‡

S Supporting Information *

ABSTRACT: Interest has recently grown around flexible dye-sensitized solar cells (DSCs) for their potential low cost roll-to-roll production process and their wide range of applications. However, flexible DSCs do not perform as well as their glass substrate equivalents, and standard devices using liquid electrolytes are challenged with long-term stability issues. Consequently, constructing stable flexible solar cells presents a major challenge. This article focuses on flexible quasi-solid-state DSCs (QS-DSCs), constructed with poly(vinylidenefluoride-co-hexafluoropropylene)-based gel electrolytes and submicrometer mesoporous TiO2 beads on plastic substrates. The influence of the gel electrolyte composition was investigated and optimized by varying the polymer content and introducing inorganic fillers. The diffusion behavior of the gel electrolytes was studied by means of voltammetric measurements. Electrochemical impedance spectroscopy gave an understanding of the role of polymer and inorganic fillers with regard to the charge recombination process. Transient photocurrent measurements and scanning electron microscopy coupled with energy X-ray dispersive spectrometry revealed that infiltration of the electrolyte through the photoanode was advantageous for films made with TiO2 beads over TiO2 P25 particles. A record power conversion efficiency of 6.4% for flexible QS-DSCs was obtained with an optimized gel electrolyte, constituting a promising step toward the fabrication of stable flexible DSCs.

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pretreatment of TiCl4 to improve the overall device efficiency,5 and presensitized7 to extend the light absorption capacity, and finally, the strong interparticle connectivity within the beads results in more robust films on plastic substrates. There is a growing need for the construction of more stable devices. The most commonly used I−/I3−-based liquid electrolytes present disadvantages such as solvent leakage, dye degradation, and electrode corrosion of the devices. Much research has been focused on constructing more stable DSCs via for instance the addition of nonvolatile ionic liquids in liquid electrolytes8 or the use of new sensitizers.9 To overcome the drawbacks of liquid electrolytes, much effort has been directed toward the development of solid-state DSCs, replacing the liquid electrolyte by a hole conductor. Recent work using perovskite materials and hole conductors has demonstrated great progress toward highly efficient solid-state DSCs.2,10,11 However, high charge recombination at the mesoporous film/ TCO glass interface requires the use of an effective blocking layer, and high temperatures (450−500 °C) are currently applied for the preparation of the blocking layer. To our understanding, producing a good blocking layer on flexible indium tin oxide coated polyethylene naphthalate substrates (ITO−PEN) remains a challenge. To the best of our

INTRODUCTION Dye-sensitized solar cells (DSCs) constitute a promising and renewable alternative for generating electricity and have attracted great attention since they were reported by Grätzel and O′Regan in 1991.1 In particular, interest in flexible solar cells has recently increased due to their potential low-cost rollto-roll production process and the varied ways in which they can be applied. However, while a certified solar to electric power conversion efficiency of 14.1% has recently been obtained for a perovskite-sensitized solar cell on transparent oxide conductive (TCO) glass,2 the efficiency of flexible DSCs remains relatively low. This is mostly due to the difficulty of making good quality TiO2 films on plastic substrates and their low-temperature tolerance. On glass, efficient electron transport through TiO2 films is obtained through high-temperature treatment (500 °C). On plastic, however, poor interparticle connections for electron transport through TiO2 films remain a challenge. Only a few groups have managed to bypass the latter limitation,3−5 and a record efficiency of 7.6% was reported by Yamaguchi et al.3 Furthermore, the use of high performance materials such as submicrometer mesoporous TiO2 beads has potential to develop more efficient devices. Mesoporous TiO2 beads have shown several advantages in DSC applications, stemming from their tunable pore size and bead diameter, high surface area leading to improved dye loading, and enhanced light harvesting due to their light-scattering properties. In flexible DSC applications, the beads can be precalcined at 500− 650 °C5,6 to increase charge transport properties, subjected to © XXXX American Chemical Society

Special Issue: Michael Grätzel Festschrift Received: September 3, 2013 Revised: January 5, 2014

A

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Table 1. Composition of Liquid Electrolyte L, Gel Electrolytes A, B, C, D, E, F, G, and H, and Respective Viscosities Measured with an AR-G2 Rheometer Using a 2° Steel Conea electrolyte

L

A

B

C

D

E

F

G

H

P(VDF-HFP) (wt %) TiO2 P25 (wt %) SiO2 P7 (wt %) viscosity (Pa·s)

2.5 × 10−3

10 1.1 × 10−1

7 4.0 × 10−2

10 10 2.8 × 10−1

7 10 7.4 × 10−2

7 5 4.5 × 10−2

5 5

2.5 5

5 2

a

The viscosities of the SiO2 P7 composite gel electrolytes could not be measured due to the viscoelastic nature of the gels as the gels adhere to the cone during measurement.

(I2, 99.99% Aldrich), 1-methyl-3-n-propylimidazolium iodide (PMII, Alfa Aesar), N-methyl-benzimidazole (NMBI, Aldrich), and 3-methoxypropionitrile (MPN, 98% Aldrich) were required for the preparation of electrolytes. P(VDF-HFP) (Mw ∼ 400 000 Aldrich), P25 TiO2 nanoparticles (Evonik, approximate particle size 25 nm), and P7 SiO2 nanoparticles (Evonik Aerosil, approximate particle size 7 nm) were used for the composite gel electrolytes. UV-cured TB3035B resin (ThreeBond) was used to seal the devices for stability assessment. Materials Preparation. Mesoporous TiO2 Beads. The amorphous precursor TiO2 beads were prepared via a sol−gel process in the presence of HDA as a structure-directing agent. Submicrometer-sized (830 ± 40 nm) mesoporous TiO2 beads were prepared following the published synthesis process.27 HDA (7.95 g) was dissolved in 800 mL of ethanol, followed by the addition of 3.20 mL of an aqueous KCl solution (0.1 M). TIP (18.10 mL) was added to this solution under vigorous stirring at ambient temperature. The milky white precursorbead suspension was kept static for 18 h and then centrifuged, and the beads were washed with ethanol three times and dried in air at room temperature. For the synthesis of these precursor beads, a HDA:Ti molar ratio of 0.5 and a H2O:Ti molar ratio of 3 were used. To prepare mesoporous TiO2 beads with a highly crystalline framework, a solvothermal treatment of the precursor beads was performed. An amount of 1.6 g of the amorphous precursor beads was dispersed in a 20 mL ethanol and 10 mL Milli-Q water mixture with an ammonia concentration of 0.45 M. This was sealed within a Teflonlined autoclave (50 mL) and heated at 160 °C for 16 h. After filtration and ethanol washing, the air-dried powders were calcined at 650 °C for 2 h in air to remove organic components and produce the mesoporous TiO2 beads for characterization. Electrolytes. Liquid electrolyte L was prepared in the glovebox with the following composition: 0.1 M LiI, 0.1 M I2, 0.6 M PMII, 0.45 M NMBI in MPN. Gel electrolytes were prepared by adding for A 10 wt % and for B 7 wt % P(VDFHFP) (vs liquid electrolyte L) into liquid electrolyte L and heating to 100 °C under stirring for 3 h. TiO2 nanoparticle (P25, Degussa) composite gel electrolytes (electrolytes C to E) were prepared as follows: 10 wt % P(VDF-HFP) and 10 wt % P25 for electrolyte C, 7 wt % P(VDF-HFP) and 10 wt % P25 for electrolyte D, 7 wt % P(VDF-HFP) and 5 wt % P25 for electrolyte E were added to liquid electrolyte L. SiO 2 nanoparticle (P7, Evonik Aerosil) composite gel electrolytes (electrolytes F to H) were prepared as following: 5 wt % P(VDF-HFP) and 5 wt % P7 for electrolyte F, 2.5 wt % P(VDF-HFP) and 5 wt % P7 for electrolyte G, 5 wt % P(VDFHFP) and 2 wt % P7 for electrolyte H were added to liquid electrolyte L. The weight percentages are expressed versus liquid electrolyte L (Table 1). At room temperature, inverting a vial containing gel or composite gel electrolyte did not result in noticeable flow of the electrolyte.

knowledge, high efficiency solid-state DSCs on ITO−PEN have yet to be published. Furthermore, the charge recombination phenomenon is not as critical in I−/I3−-based systems as it is in solid-state systems. Thus, quasi-solid-state DSCs (QS-DSC) are an intermediate option between liquid and solid-state DSCs, designed to mitigate the technological difficulties of the previously mentioned devices on plastic substrates. Polymer gel electrolytes exhibit relatively high ionic conductivity and thermal stability and have been seen as a viable alternative to liquid electrolytes.12,13 The addition of polymers into liquid electrolytes has been shown to decrease the electrolyte volatility and increase its viscosity and stability. Numerous systems have been studied that vary in complexity, molecular weight, ionic conductivity, transport capability, and hence the photovoltaic performance of the resulting QS-DSC. Among them, a photochemically stable fluorine polymer, poly(vinylidenefluoride-co-hexafluoropropylene) (P(VDFHFP)), has attracted much interest.14,15 Various nanofillers have been dispersed into gel electrolytes16 including TiO2 nanoparticles,17,18 TiO2 nanorods,19 Al2O3 nanoparticles,20 SiO2 nanoparticles,21,22 carbon particles,23 or carbon nanotubes.24 The introduction of the nanofillers into the polymer matrix reduced the crystallinity of the polymer and enhanced the mobility of the I−/I3− redox couple, improving the device efficiency.25 While many researchers have studied QS-DSCs on glass substrates, only a few have, to our knowledge, successfully applied gel electrolytes in DSCs on plastic substrates. Consequently, constructing stable highly efficient flexible solar cells presents a major challenge. Recently, Fan et al. reported a promising efficiency of 4.3% for flexible QS-DSCs using sea-urchin-like TiO2 microspheres.26 Herein we report a 6.4% efficient flexible QS-DSC using a P(VDF-HFP)/TiO2based composite gel electrolyte and TiO2 bead photoelectrodes. The charge transport mechanism involved in the fabricated QS-DSCs and the advantages of the beads over standard commercial TiO2 particles are assessed.

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EXPERIMENTAL METHODS Chemicals. Titanium(IV) isopropoxide (TIP, 97%) and hexadecylamine (HDA, 90%) were purchased from SigmaAldrich. Absolute ethanol (99.7%, Merck), potassium chloride (AR, BDH), ammonia solution (25%, BDH), and Milli-Q water (18.2 MΩ cm) were used for the synthesis of beads. The absolute ethanol was also used for the preparation of the slurry for film fabrication. The sensitizer solution contained bistetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II), known as N719 (B2, Dyesol), acetonitrile (GG, Merck), and tert-butanol (Merck). Hexachloroplatinic acid (H2PtCl6) ethanol solution was used to platinize the counter electrodes for the preparation of symmetric cells. Lithium iodide (LiI, 99.9% Aldrich), iodine B

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equipped with an Oriel Cornerstone 260 monochromator. Electrochemical impedance spectroscopy (EIS) measurements were performed by using a computer-controlled AutoLab PSTA30 in a frequency range of 0.005−105 Hz in the dark when a bias corresponding to the reference was applied (800 mV). ZView software was then used to fit the impedance spectra by applying equivalent-circuit models of the DSC device. Voltammetric measurements were carried out on symmetrical platinized glass cells. Current−voltage voltammograms were obtained with a potentiostat running at a scan rate of 5 mV s−1. Scanning electron microscopy (SEM, NOVA Nanosem 450) associated with energy-dispersive X-ray spectrometry (EDS, Brüker Esprit 1.9 software) was used to quantify the abundance of polymer and inorganic fillers along the film cross sections.

The mixtures were placed in an ultrasonic bath for 15 min and subsequently heated to 100 °C to melt the polymer under stirring for 3 h. After cooling to room temperature, the gel electrolytes were formed. The gelation process is thermoreversible, and the gels were heated to 80 °C to decrease the viscosity before injection into the devices. Device Fabrication. Flexible Solar Cells. The calcined TiO2 beads were dispersed in ethanol with a 25 wt % solid loading and sonicated for 30 min. The slurry was used to coat three layers (3L) on the indium tin oxide-coated polyethylene naphthalate substrate (ITO−PEN, Peccell Technologies, Inc., Japan, 15 Ω □−1) by doctor blading. The film was then pressed at 100 MPa for 3 min in a cold isostatic press (CIP), which applies a pressure in all directions through a pressurized liquid to the TiO2 film that is sealed in a flexible bag. Preparing films using P25 nanoparticles required the use of the CIP after every doctor-bladed layer (two layers, both pressed at either 100, 150, or 200 MPa). The films were soaked in the 0.3 mM N719 tertbutanol and acetonitrile (1:1, by volume) solution overnight. The dye-sensitized working electrode on the ITO−PEN substrate was sandwiched with a Pt-sputtered ITO−PEN counter electrode (Peccell, Japan, 15 Ω □−1). A 25 μm thick Surlyn gasket (DuPont) was used as a spacer. A drop of liquid electrolyte was sandwiched between the counter electrode and the sensitized photoanode. The jig containing the cell was then closed by tightening the screws, thus avoiding leakage of electrolyte during testing. The testing jig was equipped with a mask of 0.16 cm2. For quasi-solid-state flexible devices, a drop of the heated gel electrolyte (80 °C) was deposited on each flexible sensitized photoanode. The flexible working electrode and counter electrode were sandwiched according to the procedure described above. Symmetric Cell for Voltammetric Measurements. To prepare a counter electrode, one drop of a 10 mM hexachloroplatinic acid (H2PtCl6) isopropanol solution was placed on an FTO-coated glass substrate (Nippon, 10 Ω □−1), smeared by using the tip of a pipet, and then dried. A hot gun (400 °C for 15 min) was applied to platinize the counter electrode. Two platinized counter electrodes were sealed by using a laser-engraved 25 μm Surlyn gasket (DuPont) under heat and pressure. By using a vacuum chamber, the electrolyte was back-injected into the sandwiched cell through a hole drilled in one of the counter electrodes. The hole was sealed by applying a Surlyn-attached aluminum film. Finally, copper wires were attached to the counter electrodes. Characterization. The viscosity of the gel electrolytes was measured using an AR-G2 controlled stress rheometer with a 2° steel cone 988134. The viscosity of the liquid electrolyte was measured at 25 °C, and those of the TiO2 P25 composite gel electrolytes were measured at 80 °C. The thickness of the TiO2 films was determined by using a profilometer (DEKTAK 150, Veeco Instruments Inc.). Current−voltage (I−V) curves of the solar cells were characterized by using a Keithley 2400 source meter under illumination of simulated sunlight, which was provided by an Oriel solar simulator equipped with an AM 1.5 filter. Flexible devices were shielded by a black metal mask with an aperture area of 0.16 cm2. Transient photocurrents were measured by switching on and off the solar simulated light when current steady state was reached, i.e., when saturated current was attained. Incident photon-to-current conversion efficiency (IPCE) plots as a function of excitation wavelength (λ = 330−800 nm) were characterized by using a Keithley 2400 source meter under the irradiation of a 300 W xenon lamp

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RESULTS AND DISCUSSION The gel electrolyte composition was varied (Table 1) to optimize the performance of the corresponding QS-DSCs as well as to understand the charge transport mechanism involved. DSC Photovoltaic Performances. DSCs containing liquid electrolyte L, gel electrolyte A, and composite gel electrolyte C at 1 sun showed efficiencies of 6.1%, 4.1%, and 5.1%, respectively (Table 2). Examples of current−voltage plots and IPCE spectra are shown in Figure 1. Table 2. Photovoltaic Performances of DSCs Constructed with 3L Bead Films and Electrolytes L, A, B, C, D, E, F, G, and Ha electrolyte

Voc (mV)

Jsc (mA cm−2)

FF

η (%)

L A B C D E F G H

728 739 771 748 811 782 751 754 770

13.1 8.2 9.0 10.5 8.8 12.3 9.3 9.2 9.7

0.64 0.68 0.71 0.65 0.73 0.67 0.70 0.67 0.71

6.1 4.1 4.9 5.1 4.7 6.4 4.8 4.6 5.3

a

Photocurrent density−voltage characteristics displayed here were measured for one cell at 100 mW cm−2. Average current−voltage results for three identical cells can be found in the Supporting Information (SI).

The addition of 10 wt % P(VDF-HFP) to the liquid electrolyte L gelled the electrolyte. As a result of this gelation, the short-circuit photocurrent density of the now quasi-solidstate device using the electrolyte A dropped significantly (from 13.1 to 8.2 mA cm−2). By decreasing the polymer content in the gel electrolytes (from A 10 wt % to B 7 wt % of P(VDFHFP) in liquid electrolyte L) the performance of the corresponding QS-DSC was improved giving higher efficiency, Voc, Jsc, and fill factor. Noteworthy, below 7 wt % P(VDF-HFP) in the MPN-based liquid electrolyte, the electrolyte remained in the liquid state: i.e., it flowed when the sample vial was inverted. Introducing 10 wt % TiO2 nanoparticles to electrolyte A enhanced the QS-DSC performance (electrolyte C). Electrolyte E (7 wt % P(VDF-HFP) and 5 wt % P25) gave the best results with outstanding quasi-solid-state flexible device characteristics: η = 6.4%, Voc = 782 mV, Jsc = 12.3 mA cm−2, and FF = 0.67. For a gel with an identical polymer concentration but a higher inorganic filler content (electrolyte D), the device performance C

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Figure 1. (a) Current−voltage and (b) IPCE plots for DSCs constructed with electrolytes L (liquid), B (7 wt % P(VDF-HFP)), E (7 wt % P(VDFHFP) + 5 wt % P25 TiO2 particles), and H (5 wt % P(VDF-HFP) + 2 wt % P7 SiO2 particles).

was significantly lower than for electrolyte E and also lower than the equivalent gel without the fillers (electrolyte B). The viscosities of the electrolytes were studied to see if there was any correlation between electrolyte viscosity and device performance. However, the viscoelastic nature of the SiO2 gels caused them to adhere to the cone during measurement, and hence the viscosities could not be measured. Noteworthy, the SiO2 gels solidified upon addition of P7 SiO2 particles without addition of P(VDF-HFP). Furthermore, although the polymer and inorganic filler weight percentages utilized for the SiO2 gels were lower than that of the TiO2 gels, the SiO2 gels F, G, and H appeared more viscous. Looking at the viscosities of the other electrolytes (Table 1), it can be observed that as expected addition of polymer increased the viscosity of the electrolyte and that the gel viscosity decreased as the polymer content was decreased (L: 2.5 × 10−3 Pa·s, A: 1.1 × 10−1 Pa·s, and B: 4.0 × 10−2 Pa·s). Furthermore, although the addition of P25 TiO2 particles in the electrolyte resulted in higher viscosities of the electrolytes (C: 2.8 × 10−1 Pa·s and E: 4.5 × 10−2 Pa·s), the efficiency of the resulting devices was increased: η = 5.1% for electrolyte C and η = 6.4% for electrolyte E. Extraordinarily, the device fabricated with the viscous electrolyte E surpassed the efficiency of that constructed with the nonviscous liquid electrolyte L. From these observations, the particle to polymer ratio in the electrolyte was a determining factor with regard to the electrolyte performance. Similar conclusions can be drawn from the devices containing SiO2 composite gel electrolytes. The best power conversion efficiency obtained for the silicacontaining QS-DSCs was 5.3% for electrolyte H. This implied that the type of inorganic fillers (nature and/or size) had a significant impact on the electrolyte performance. To gain an understanding of the effect of the difference in filler type, the properties of the electrolyte and charge recombination at the electrolyte/TiO2 interface were probed. Ion Transport in Electrolytes. Voltammetric measurements on symmetric electrochemical cells were performed to give an understanding of the ion transport performance in the synthesized electrolytes (Figure 2). Due to the large excess of I− in the electrolyte,28 the current was mainly limited by the diffusion of I3− species. The apparent diffusion coefficients were calculated for every electrolyte according to eq 128 and listed in Table 3. D(I3−) =

l J 2nFC(I3−) lim

Figure 2. Voltammograms obtained for the electrolytes L, A, B, C, D, E, F, G, and H in a Pt symmetric cell.

where n = 2 is the number of electrons involved in the electrochemical reduction of triiodide at the electrode, F Faraday’s constant, C(I3−) the I3− concentration per volume unit, assumed to be uniform along the whole cell, l the distance between the platinized electrodes, and Jlim the saturated current density (eq 1). The saturated current, Jlim, was read from the voltammograms when the plot became flat, i.e., indicating a saturated and constant current (Figure 2). When P(VDF-HFP) was added to liquid electrolyte L, D(I3−) decreased with increasing viscosity of the electrolytes, B (7 wt % P(VDF-HFP)) being less viscous than A (10 wt % P(VDF-HFP)). In terms of the inorganic fillers, the overall trend was that D(I3−) decreased with increasing filler quantity for both TiO2 and SiO2 fillers. The highest coefficient was obtained for the composite electrolyte E, exceeding the diffusion coefficient of the liquid electrolyte L. This indicated that the 3D network of the composite gel electrolyte E formed an effective pathway for the transport of I3−/I− redox species. This could be due to an increased charge exchange mechanism, i.e., Grotthuss mechanism,29−31 favored by polyiodide bond exchange and adsorbed imidazolium cations on the surface of TiO2 nanoparticles. The latter has been described in the literature to facilitate the electron transport through the aligned anionic species by creating preferential electrostatic pathways.17,19 Nevertheless, QS-DSC efficiencies and apparent diffusion coefficients did not follow a linear correlation. For instance, composite gel electrolytes D and G exhibited high D(I3−) (4.0 × 10−6 and 4.2 × 10−6 cm2 s−1, respectively),

(1) D

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Table 3. Apparent Diffusion Coefficients D(I3−) of Triiodide in the Electrolytes Obtained from Jlim and Equation 1 Jlim (mA) 106D(I3−) (cm2 s−1)

L

A

B

C

D

E

F

G

H

18.0 3.9

10.7 2.3

15.7 3.4

13.0 2.8

18.6 4.0

21.4 4.6

15.9 3.4

19.3 4.2

17.0 3.7

Figure 3. (a) Fitted impedance Nyquist plots for DSCs constructed with electrolytes L (liquid), B (7 wt % P(VDF-HFP)), E (7 wt % P(VDF-HFP) + 5 wt % P25 TiO2 particles), and H (5 wt % P(VDF-HFP) + 2 wt % P7 SiO2 particles), tested in the dark under −0.8 V bias. (b) Equivalent-circuit model of the DSC device applied to fit the impedance spectra. (c) Enlargement of (a) for Z′ values between 20 and 30 Ω.

bead film, allowing efficient I3−/I− charge transport along the surface of the P25 TiO2 nanoparticles and efficient dye regeneration with reduced charge recombination at the sensitized TiO2/electrolyte interface. The addition of SiO2 nanoparticles, however, did not increase the recombination resistance. While the SiO2 gels had lower polymer content than the TiO2 gels, the EIS (Figure S3, SI) showed that the polymer content had little influence on the recombination process despite increasing the gel viscosity. In addition, it was observed that the SiO2 gels solidified upon addition of P7 SiO2 particles without addition of P(VDF-HFP), suggesting higher viscosities of the SiO2 gels when compared to TiO2 gels. This solidification process could be interpreted as a result of the agglomeration of SiO2 particles in the liquid electrolyte, increasing its overall viscosity. This would suggest that SiO2 nanoparticles did not efficiently improve the charge transport of the electrolyte species through the photoanode. The photovoltaic performances obtained with electrolyte H (2 wt % of SiO2 nanoparticles, Table 2) suggested that lower concentrations of SiO2 nanoparticles may be favorable. Overall, this implied that the type of nanoparticles in the composite electrolyte played an important role in the retardation of the charge recombination between injected electron and electron acceptor I3− in the electrolyte (see section 2, Electrochemical Impedance Spectroscopy in SI). Finally, the charge transport process through the TiO2 film was facilitated when using liquid electrolyte L or electrolyte E (low Rt values ∼ 3 Ω), slightly hindered when using electrolyte H with a higher transport resistance (5.4 Ω), and even more difficult with gel electrolyte B (10.1 Ω). Hence, by improving the charge transport properties of the I3−/I− redox couple in the gel electrolytes, the charge combination at the TiO2 photoanode/electrolyte interface was reduced, and the best device performances were obtained for electrolyte composition E, providing the most efficient pathway for the electrolyte species. Comparison with P25 Films. To compare the performance of the beads to that of P25 TiO2 particles, the influence of the CIP pressure on P25 films was first investigated (see section

whereas the efficiencies of their corresponding QS-DSCs remained below 5% (Table 2). This suggested that additional parameters should be taken into account in the interpretation of the electrolyte transport properties. Electrochemical Impedance Spectroscopy. Electrochemical impedance spectroscopy (EIS) measurements were performed in the dark with a −0.8 V applied bias on DSCs assembled with liquid electrolyte L, gel electrolyte B, TiO2 composite gel electrolyte E, and SiO2 composite gel electrolyte H. The corresponding fitted Nyquist plots and resistances are represented in Figure 3 and Table 4, respectively. Table 4. Resistances Obtained by Fitting the Nyquist Plots of the Cells Constructed with Electrolytes L, B, E, and H electrolyte

Rt (Ω)

Rct (Ω)

RPt (Ω)

L B E H

3.7 10.1 3.2 5.4

22.2 33.5 41.2 32.3

3.4 1.7 2.8 2.3

The first semicircle (high frequency) corresponds to the charge transfer resistance between the platinized electrode and the electrolyte (RPt). The Pt/electrolyte interface showed comparable resistances for all electrolytes (RPt varies between 1.7 and 3.4 Ω). The second semicircle (midfrequency) is associated with the charge-transfer resistance of the charge recombination process between electrons in the mesoscopic TiO2 film and I3− in the electrolyte; the wider the circle, the larger the recombination resistance at the TiO2/electrolyte interface (Rct). First, the Rct value for the gel electrolyte B based device was higher than the liquid-based one (33.5 vs 22.2 Ω). This could be explained by the higher viscosity of the gel electrolyte B, decreasing the I3−/TiO2 bead contact, hence reducing the recombination process. Second, the addition of TiO2 nanoparticles in the gel electrolyte further increased the charge recombination resistance. This could be interpreted as a result of the infiltration of P25 TiO2 nanoparticles through the E

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evenly distributed over the working electrode, and accessible dye molecules were efficiently regenerated. However, under prolonged illumination, when the diffusion of the electrolyte species was hindered through the sensitized photoanode, the dye regeneration decreased, thus resulting in lower photocurrent.32 Figure 4(a) and (b) exhibits the transient photocurrent measurements for P25 and bead films (1L and 3L) for electrolyte L, B, and E. From Figure 4(a), the 5 μm thick P25 films experienced diffusion issues with the liquid electrolyte L, which was more viscous than standard electrolytes due to the presence of the immidazolium additive (PMII) and MPN. The photocurrent decay was more significant when using electrolytes B and E, demonstrating infiltration was hindered more with increased electrolyte viscosity. In addition, the voids between P25 particles in the working electrode film would be too small for P25 particles in the composite gel electrolyte E to enter. This suggested the so-called Grotthuss mechanism probably only took place on top of and not through the TiO2 film. As mentioned above, the TiO2 working electrodes on plastic substrates underwent a cold isostatic press treatment to obtain a good necking between TiO2 particles as high temperature sintering was not permitted by the substrate temperature limitation. Consequently, the resulting pressed films were compact, rendering the diffusion of viscous electrolyte through the dense TiO2 film difficult. The large voids between 830 nm TiO2 beads were advantageous here as they permitted electrolyte infiltration. The limited electrolyte diffusion into the P25 film was confirmed by SEM coupled with EDS (Figure 5). As observed in the SEM image of a P25 film and associated EDS line scans, the relative homogeneous content of iodine (I) over the film thickness suggested liquid electrolyte infiltrated through the film. However, the polymer gel electrolyte E containing P(VDF-HFP) and P25 particles did not infiltrate well through the dense P25 film (low carbon (C) and fluorine (F) content in the P25 film, Figure 5(a)). The titanium (Ti) and oxygen (O) contents were related to the dense P25 TiO2 film and not to the TiO2 particles present in the electrolyte E. For a thick bead film (15 μm, Figure 4(b)), the transient photocurrent measurements showed that liquid electrolyte L can move freely. For more viscous gel electrolytes, the percolation capability of the electrolyte decreased with bead film thickness, particularly in the case of gel electrolyte B. The diffusion issue remained moderate for composite gel electrolyte E. This proved that while the thin P25 film suffered from significant diffusion issues, these issues only appeared for much thicker

1.2 in SI). Due to the close packing of pressed P25 particles, infiltration of the viscous electrolyte through the dense P25 film was expected to be a major limiting effect; hence, efforts were made to reduce its impact on the cell performance. The best photovoltaic performance was obtained for P25 films CIP pressed at 150 MPa, thus films pressed under this pressure were used for the comparison with the bead films (Table 5). The Table 5. Photovoltaic Performances of DSCs Constructed with One Layer of Beads (1LB), Three Layers of Beads (3LB), and Two Layers of P25 (P25 2L) with Electrolyte E and Pt Mirror Counter Electrodea 1LB − 5 μm 3LB − 15 μm P25 2L − 5 μm

Voc (mV)

Jsc (mA cm−2)

FF

η (%)

766 751 793

5.9 10.9 8.0

0.65 0.64 0.72

2.9 5.2 4.6

a

Photocurrent intensity−voltage characteristics displayed here were measured for one cell at 100 mW cm−2. Pt mirror counter electrodes exhibited lower catalytic performances than Pt-sputtered electrodes.

standard bead film thickness was around 15 μm (applied in three layers, 3L), and for direct comparison with the P25 films (applied in two layers, 2L) based on film thickness, 5 μm thick bead films (one layer, 1L) were also prepared. For the same film thickness, devices constructed with P25 films showed better photovoltaic performance than bead films, as a low Jsc was obtained for thin bead films (5.9 mA cm−2). The beads are monodisperse in diameter, thus the closest packing of these beads would still leave about 25% space as voids in the film. As a result, a thin layer (e.g., 5 μm) of the beads would not allow the same amount of dye loading as a compressed P25 particle film of the same thickness, thus showing lower light-harvesting capacity and photocurrent. This effect was corrected by doctor blading more bead layers on the thin bead films. Furthermore, the robustness of bead films on the flexible substrate allowed formation of fairly thick films, whereas the binder-free P25 slurry suffered film cracking during preparation of thick films (>2L). Electrolyte Diffusion through the TiO2 Film. Transient photocurrent measurements were carried out to assess the infiltration of various electrolytes (L, B, and E) through the TiO2 films. The time-varying photocurrent response of the different DSCs was achieved by switching the illumination on and off after a steady state had been reached. When the light was turned on, the mass transport issue through the TiO2 film was revealed by the observed current decay before reaching saturation. At the beginning of illumination, I3− species were

Figure 4. Transient photocurrent measurements under 1000 W cm−2 illumination for DSCs constructed with (a) P25 2L films CIP pressed at 150 MPa and (b) 1LB and 3LB films CIP pressed at 100 MPa tested with electrolyte L, B, and E. F

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Figure 5. Cross-sectional SEM images of (a) 2L P25 film and (b) 2L bead film with electrolyte E and corresponding EDS line scans of the same devices indicating elemental content.

Figure 6. (a) Fitted impedance Nyquist plots for cells prepared with 1LB, 3LB, and P25 2L films and tested with electrolyte E in the dark under −0.8 V bias. (b) Equivalent-circuit model of the DSC device applied to fit the impedance spectra. (c) Enlargement of (a) for Z′ values between 20 and 35 Ω.

bead films and in the case of highly viscous electrolytes. The SEM/EDS analyses demonstrated the good infiltration of the polymer gel electrolyte E through a 10 μm thick bead film (Figure 5(b), C and F profiles). Nevertheless, although C and F contents through the bead film confirmed the polymer infiltration, it could not be certified that the P25 TiO2 nanoparticles present in electrolyte E infiltrated through the TiO2 bead film. To assess the stability of the devices constructed with a 3L bead film and electrolyte E, UV-cured TB3035B resin was utilized to seal the devices. The closed devices were placed in an oven at 65 °C. Preliminary stability testing showed the QSDSC fabricated with electrolyte E maintained 90% of its initial efficiency for over 300 h at 65 °C. EIS was carried out to compare the charge transport capabilities of the working electrodes fabricated with beads and P25 particles (Figure 6). The charge transfer capabilities at the TiO2/electrolyte interface were comparable for P25 and bead films of a similar film thickness (5 μm) and reduced for a thicker bead film (15 μm) (Table 6). In particular, the chargetransfer resistance values of the charge recombination process between electrons in the TiO2 film and I3− in the electrolyte (Rct) were similar for the 5 μm thick films (76.1 and 68.6 Ω for

Table 6. Resistances Obtained by Fitting the Nyquist Plots of the Cells Prepared with 1LB, 3LB, and P25 2L Films and Tested with Electrolyte E in the Dark under −0.8 V Bias film

Rt (Ω)

Rct (Ω)

RPt (Ω)

1LB 3LB P25 2L

10.7 10.5 21.1

76.1 44.3 68.6

3.1 4.1 6.5

1LB and P25 2L, respectively) and lower for the thicker film (44.3 Ω).

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CONCLUSION The influence of P(VDF-HFP)-based gel electrolyte composition on the photovoltaic performances of flexible quasi-solidstate DSCs was investigated. An optimized composite gel electrolyte (7 wt % P(VDF-HFP), 5 wt % P25 TiO 2 nanoparticles vs liquid electrolyte) provided the most effective pathway for the redox species to move in the electrolyte, thus exhibiting a photon-to-current conversion efficiency of 6.4% in the QS-DSC. Flexible working electrodes prepared with mesoporous TiO2 beads and P25 nanoparticles were compared. Devices fabricated with bead films achieved better photovoltaic performance. A range of techniques was used to understand the G

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charge transport mechanisms involved. Transient photocurrent measurements and EDX spectrometry revealed that electrolyte infiltration was facilitated through bead films that presented larger interparticle voids than P25 particle films. Electrochemical impedance spectroscopy demonstrated the major role of nanoparticulate TiO2 fillers in the electrolyte is in the charge recombination process at the sensitized TiO2/electrolyte interface and comparable charge transport properties in P25 and bead films at similar film thickness. This work is a promising step toward the fabrication of more stable flexible DSCs. The stability of the devices is currently under assessment with preliminary testing showing that the QS-DSC fabricated with electrolyte E maintained 90% of its initial efficiency for over 300 h.

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ASSOCIATED CONTENT

S Supporting Information *

Current-voltage curves and associated data, IPCE, EIS, and transient photocurrent measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +61 3 8344 7146. Fax: +61 3 9347 5180 *E-mail: [email protected]. Tel.: +61 3 9905 4930. Fax: +61 3 9905 4940. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported through the Australian Research Council Discovery Project Scheme, DP110101346. Mr. Yang Chen is acknowledged for DSC construction training. Ms. Lu Cao is acknowledged for help with the bead synthesis. We thank Dr. Alex Wu from Professor Robert Lamb’s Surface Science and Technology Group from The University of Melbourne for the supply of SiO2 particles. The Monash Centre for Electron Microscopy at Monash University is acknowledged for electron microscopy access. Support from the Victorian Organic Solar Cell Consortium is acknowledged. R.A.C. is the recipient of an Australian Research Council Future Fellowship FT0990583.

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REFERENCES

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