First Pseudohalogen Polymer Electrolyte for Dye ... - ACS Publications

Article pubs.acs.org/JPCC

First Pseudohalogen Polymer Electrolyte for Dye-Sensitized Solar Cells Promising for In Situ Photopolymerization Federico Bella,*,†,‡ Adriano Sacco,† Gian Paolo Salvador,† Stefano Bianco,† Elena Tresso,†,‡ Candido Fabrizio Pirri,†,‡ and Roberta Bongiovanni*,‡ †

Center for Space Human Robotics at Polito, Istituto Italiano di Tecnologia, Corso Trento 21, 10129 Torino, Italy Department of Applied Science and Technology, DISAT, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy

‡

S Supporting Information *

ABSTRACT: The incorporation of selenocianated-based redox couple in a polymer electrolyte for dye-sensitized solar cells is reported for the first time. The pseudohalogen redox mediator was integrated in two kinds of acrylic/methacrylic membranes prepared by photocopolymerization of multifunctional monomers. Before activation, the obtained membranes were transparent, self-standing and flexible, and the physicochemical characterizations of the films showed the formation of highly crosslinked architectures. Membranes were activated by swelling in an optimized solution containing the SeCN−/(SeCN)2 redox mediator with 4-tert-butylpyridine in acetonitrile, and the electrochemical behavior of the electrolytes revealed fast charge transfer kinetics. The photovoltaic performances of quasi-solid dye-sensitized solar cells were evaluated and compared with the results of the liquid counterpart, showing promising photoharvesting properties. No diminution in photoconversion efficiencies was evidenced in the comparison between solid and liquid cells, demonstrating an optimal kinetics of the redox species in the polymer cage, associated with a noteworthy increase in device durability, as demonstrated by aging tests. In addition, the in situ photopolymerization in the presence of the redox mediator is presented with outstanding results: this process, hardly feasible for the traditional I−/I3− couple (inhibitor of radical polymerization processes), enables at the same time the creation of an excellent electrode/electrolyte interface and the sealing of the device.

1. INTRODUCTION The growth of energy consumption and the limited fossil fuel availability are bringing out renewable energy as one of the most important issues for global energy policy. In this context, solar energy is the most hopeful and sustainable energy resource among the renewable alternatives.1 For this reason, a large number of research groups investigate the possibility to develop new photovoltaic devices as alternatives to the expensive silicon solar cells. After their invention in 1991,2 dye-sensitized solar cells (DSSCs) have received considerable attention as a cost-effective alternative to conventional solar devices.3 In a standard DSSC, the photoanode consists of a relatively thick (around 12 μm) layer of TiO2 nanoparticles electrically interconnected and sensitized with ruthenium complexes-based dye molecules. The TiO2 layer is deposited on a glass slice, covered with a transparent conducting oxide (TCO) for electrical contact purposes. An I−/I3− redox couple usually acts as hole conductor, with the aim of regenerating the dye molecules. The counter electrode consists of a TCO-covered glass slice with a nanometric layer of Pt, useful to promote the triiodide reduction. The selection and optimization of all these materials is crucial to obtain good and long-lasting performances. In particular, in the last two years the scrupulous research in the field of dyes and redox couples allowed to get efficiencies higher than 12%.4 © 2013 American Chemical Society

Nowadays, as a key component of a DSSC, the electrolyte is thoroughly investigated with the aim to solve two important technological problems: low durability and corrosion due to the redox couple. As regards the first factor, the poor long-term durability of DSSCs, mainly caused by leakage and volatilization of organic solvent-based liquid electrolytes, has hampered the widespread practical use of these devices. To solve this awkward problem, scientists have investigated the use of high-boiling solvents or the development of quasi-solid/solid electrolytes. In particular, ionic liquids,5 nanoparticles,6 gelling agents,7 p-type semiconductors,8 polymer membranes,9 and perovskites10 have been tested, as reported in a recent review.11 The second drawback of electrolytes concerns the redox couple. The most used pair (i.e., I−/I3−) shows easy penetration into the porous TiO2 layer, allows fast dye regeneration and slow recombination with the injected electrons. However, several disadvantages limit its application: (a) I3− and other polyiodides formed in the electrolyte absorb a good portion of the visible light, thus, subtracting photons to dye molecules; (b) iodide-based redox couple corrodes copper/silver lines used to collect the electrons in large scale modules; (c) the redox potential of the I−/I3− couple limits the maximum photovoltage Received: May 31, 2013 Revised: August 29, 2013 Published: September 9, 2013 20421

dx.doi.org/10.1021/jp405363x | J. Phys. Chem. C 2013, 117, 20421−20430

The Journal of Physical Chemistry C

Article

containing a pseudohalogen redox couple and the first attempt of its in situ photopolymerization.

of the device; (d) the complex chemistry of this electrolyte (involving two reactions) causes great energy loss.12 Redox couples alternative to the I−/I3− pair include copper complex, pseudohalogen redox couples (SeCN−/(SeCN)2 and SCN−/ (SCN)2), Br/Br3− and Co(II,III) complexes.13 Pseudohalogens are inorganic ion/molecules with the general forms Ps−Ps or Ps−X, where Ps is a pseudohalogen group (i.e., cyanide, cyanate, thiocyanate, selenocyanate) and X is a halogen. The behavior and chemical properties of pseudohalides are quite identical to that of the true halide ions, and the presence of internal double or triple bonds does not affect their chemical behavior. As redox shuttles for DSSCs, pseudohalogens are very promising, because their redox potentials are more positive than that of I−/I3− (i.e., 0.19 V more for the SeCN−/(SeCN)2 couple). As a consequence, a larger energy level difference between the redox potential of the electrolyte and the quasi-Fermi level of TiO2 is observed, thus, leading to significant gain in open-circuit voltage (Voc) values. The application of selenocianate-based redox couple SeCN−/ (SeCN)2 in liquid electrolyte-based DSSC was introduced by Oskam et al. in 2001, by using acetonitrile as solvent and N3 as sensitizer.14 Sunlight conversion efficiency values were not reported, but the incident photon-to-current conversion efficiency (IPCE) was low (20%), and this was attributed to the inefficient dye regeneration by the SeCN− ion. Three years later, Grätzel et al. prepared a selenocianated-based lowviscosity ionic liquid (i.e., 1-ethyl-3-methylimidazolium selenocyanate, EMISeCN), which led to outstanding efficiency (8.3%), demonstrating that pseudohalogens are a valid alternative to the iodine-based couple.15 Bergeron et al. investigated an unconventional SnO2-based electrode for regenerative solar cells containing the SeCN−/(SeCN)2 couple, obtaining IPCEs and Vocs comparable with those of I−/I3−.16 A combined electrochemical and nuclear magnetic resonance (NMR) study of the diffusion of some SeCN−/(SeCN)2/ (SeCN)3−-based room temperature ionic liquids (RTILs) was undertaken by Solangi et al., exploiting the fact that Se, C, and N nuclei are NMR active.17 Selenocyanate-based redox couple was also mixed with the iodine-based one by Song et al., and favorable properties such as reduced visible light absorption and higher ionic conductivity were observed.18 All of these studies were carried out in liquid electrolytes-based DSSCs. In principle, SeCN−/(SeCN)2 redox shuttle possesses good mass transport characteristics, with a valuable mobility and thus could be particularly suitable for an integration in quasi-solid polymer electrolytes. Moreover, it is possible to conceive a onepot chemical formulation containing the reactive oligomers and the redox couple, to be directly deposited and polymerized in situ, impregnating with a close contact the semiconductor nanostructure. In fact, the direct in situ photo-crosslinking of the polymer electrolyte is hindered using conventional I−/I3− couple, because iodine is a polymerization inhibitor, but would become possible with a selenocyanate-based redox shuttle. Our recent studies on DSSCs concern the preparation of polymer electrolyte membranes, able to trap and retain for a long time a liquid electrolyte, thus, ensuring good performance and durability to the devices.9,19,20 Because the potentialities of the SeCN−/(SeCN)2 redox couple are promising, in this work we propose their incorporation in two different polymer networks. Chemico-physical and electrochemical characterizations of the obtained polymer electrolytes and on the assembled quasi-solid DSSCs are presented. To the best of our knowledge, this is the first ever reported polymer electrolyte

2. EXPERIMENTAL SECTION 2.1. Materials. Bisphenol A ethoxylate dimethacrylate (BEMA, average Mn = 1700), polyethylene glycol diacrylate (PEGDA, average Mn = 575), poly(ethylene glycol) methyl ether methacrylate (PEGMA, average Mn = 475), acetonitrile (CH3CN), chloroform (CHCl3), 4-tert-butylpyridine (TBP), potassium selenocyanate (KSeCN) and bromine (Br2) were purchased from Sigma-Aldrich. 2-Hydroxy-2-methyl-1-phenyl1-propanone (Darocur 1173) from Ciba Specialty Chemicals was used as free radical photoinitiator. Conducting glass plates (FTO glass, fluorine-doped tin oxide overlayer, sheet resistance 7 Ω/sq, purchased from Solaronix) were cut into 2 × 2 cm2 sheets and used as substrate for the deposition of a TiO2 porous film from a paste (DSL 18NR-AO, Dyesol) and for the fabrication of platinized counter-electrodes. Sensitizing dye cis-diisothiocyanato-bis(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium(II) bis(tetrabutylammonium) (N719, Ruthenizer 535 bis-TBA) was purchased from Solaronix. 2.2. Preparation of SeCN−/(SeCN)2 Redox Couple. The SeCN−/(SeCN)2 solution was prepared in agreement with the procedure published by Oskam et al.14 A solution of KSeCN (5 mmol) in 50 mL of CH3CN was prepared under stirring, in an ice bath and in the dark. In the meantime, a bromine solution was prepared by dissolving Br2 (2.5 mmol) in 25 mL of CH3CN. After complete dissolution of both solutions, the second one was added to the first one dropwise, under the same conditions. The resulting suspension was filtered to remove precipitated KBr, resulting in a yellow selenocyanogen solution. Then, upon slowly adding 10 mmol KSeCN in 25 mL of CH3CN, the solution darkened in color. An additional filtration step was necessary in order to have a clear solution. Accordingly to the stoichiometry of the quantitative reaction, the electrolyte solution prepared in this way consisted of 25 mM of the oxidized form of the redox couple and 100 mM of the reduced form. The optimization of the concentration and the use of additives will be discussed in section 3.1. 2.3. Preparation of Polymer Electrolytes. Two reactive mixtures were prepared, each of which containing a monofunctional oligomer (PEGMA), a difunctional one (BEMA or PEGDA) and the photoinitiator (Darocur 1173, 3 wt%). BEMA/PEGMA and PEGDA/PEGMA weight ratios were set at 35:65 and 20:80, respectively, according to previous studies.9,19 Each mixture was sandwiched between two UVtransparent glasses, separated by a 100 μm thick tape and UV cured for 3 min under N2 flux, using a medium vapor pressure Hg lamp (Helios Italquartz), with a radiation intensity on the surface of 30 mW cm−2, measured by an Oriel photometer. Membranes were peeled off from the glass plates and activated by swelling into the liquid electrolyte solution described in sections 2.2 and 3.1. The swelling time was set at 5 min, which was the saturation time for the liquid uptake in all the membranes. Chemical structure formulas of the reactive monomers and the photoinitiator are reported in Scheme 1. Moreover, a formulation containing the electrolyte solution and the reactive mixture (BEMA/PEGMA, 35:65 wt%/wt%) in a 50:50 weight ratio was prepared. The solution was deposited by drop casting directly on the photoanode, sandwiched with the counter electrode (see below) and photoreticulated in 1 min under simulated solar light, using a Newport 91195A class A solar simulator with AM 1.5G radiation intensity. 20422



Article

prepare symmetrical cells and using two binder clips to close the structures. The measurements were performed using a CH Instruments 760D electrochemical workstation in the frequency range 10−2−104 Hz, with a 10 mV AC signal. Experimental curves were fitted exploiting an equivalent circuit19 in order to obtain information about the charge transfer mechanisms. The circuit is shown in Scheme 2. Here,

Scheme 1. Structures of the Reactive Monomers and the Photoinitiator Used for the Preparation of Polymer Electrolytes Membranesa

Scheme 2. Equivalent Circuit Exploited for the Fitting of the Impedance Spectra of Polymer Electrolyte Membranes

Rs represents the series resistance, Rct and Qdl are the charge transfer resistance and the double layer constant phase element (CPE) at the interface between the electrode and the membrane, and Zw is the Warburg impedance related to the charge diffusion inside the membrane.21 The electron charge transfer time τct and the electrolyte diffusion coefficient De were evaluated by using the following equations:

a

n represents the number of ethoxy repeating units in the individual species: n is equal to 22 for BEMA, 9 for PEGMA, and 3 for PEGDA.

2.4. Characterization of Polymer Electrolyte Membranes. The kinetics of the photopolymerization was determined by real-time Fourier transform infrared spectroscopy (RT-FTIR), employing a Thermo-Nicolet 5700 instrument. To this purpose, a thin coating with the reactive mixture was deposited on a silicon wafer and exposed simultaneously to the UV beam (which induced the polymerization) and to the IR beam (which analyzed in situ the extent of the reaction). The conversion of acrylate/methacrylate double bonds as a function of time was calculated monitoring the peak area under the 1634 cm−1 band (acrylate CC stretching), normalized by a constant signal in the spectra (CO stretching at 1726 cm−1), according to the following equation: conversion =

[A CC]0 − [A CC]t [A CC]0

τct = (R ctQ dl)1/ βdl

(2)

De = ωede2/4

(3)

where βdl is the CPE index, ωe is the electrolyte characteristic frequency, and de is the membrane thickness.22 2.5. Fabrication and Testing of DSSCs. As regards the preparation of photoanodes, FTO covered glasses were rinsed with acetone and ethanol in an ultrasonic bath for 10 min. A TiO2 paste layer with a circular shape was deposited on FTO by doctor-blade technique and dried at 100 °C for 10 min on a hot plate. Then, a sintering process at 520 °C for 30 min allowed the formation of a nanoporous TiO2 film with a mean thickness of 8 μm, measured by profilometry (P.10 KLA-Tencor Profiler). Finally, photoelectrodes were soaked into a 0.4 mM N719 dye solution in ethanol for 12 h at room temperature and then rinsed in ethanol to remove the unadsorbed dye. Counter-electrode FTO glasses were cleaned with the same rinsing method described above and a 5 nm Pt thin film was deposited by thermal evaporation. A quasi-solid state DSSC was assembled by positioning the polymer electrolyte membrane onto the sensitized photoanode. Then, the two electrodes were clipped together and a cyanoacrylate glue was used as a sealant to prevent leakages of the electrolyte solution. This sandwich-type cell was pressed with two binder clips for few seconds, which were then removed when the glue took hold. The cell prepared with the in situ polymerization of the electrolyte was assembled by drop casting the uncured electrolyte on the photoanode and positioning the counter electrode directly above it, locking with two binder clips. An inert 70 μm thick spacer was used to separate the electrodes. The cell was placed in the solar simulator, where the complete photopolymerization was obtained in only 1 min. As regards liquid cells, useful to optimize the concentration of redox mediator and additives, a microfluidic housing system developed in our laboratory was used.23 The active area of the cell was 0.78 cm2 and the photovoltaic measurements were performed with a 0.22 cm2 rigid black

(1)

where [ACC]0 and [ACC]t represent the peak areas under the 1634 cm−1 band at the beginning of the reaction and at time t, respectively.19 The gel content was determined on the cured membranes by measuring the weight loss after 24 h extraction in chloroform at room temperature, according to the standard test method ASTM D2765−84. The final thickness of the membranes was measured with a Mitutoyo Series 547 Thickness Gauge, equipped with an Absolute Digimatic Indicator (Model IDC112XBS), with a resolution of ±1 μm and a maximum measuring force of 1.5 N. The thermal stability of the UV-cured polymer membranes was tested by thermogravimetric analysis (TGA), with a TGA/ SDTA-851 instrument from Mettler, over a temperature range of 25−600 °C under N2 flux at a heating rate of 10 °C min−1. Glass transition temperature (Tg), storage (E′) and loss (E″) modules of the membranes were measured by dynamic mechanical thermal analysis (DMTA), performed with a TTDMA instrument (Triton Technology Ltd.), at a frequency of 1 Hz and a ramp rate of 2 °C min−1 in tensile configuration. Electrochemical performances of the membranes were evaluated through electrochemical impedance spectroscopy (EIS) measurements. Polymer electrolyte membranes were sandwiched between two platinized electrodes (whose fabrication procedure is described in section 2.5) in order to 20423



Article

prepared solution, placing it on a hot plate (70 °C) and waiting for the solvent (CH3CN) evaporation. In this way, we obtained higher concentrations of (SeCN)2 and SeCN−: 37.5:150 mM and 75:300 mM. I−V characteristics under AM1.5G illumination were evaluated, and they are reported in Table 1 as a function of the concentration of the redox couple.

mask. I−V electrical characterizations under AM1.5G illumination (100 mW cm−2) were carried out using a class A solar simulator (91195A, Newport) and a Keithley 2440 source measure unit. The experimental results reported in this manuscript are the average of three replicates. IPCE measurements were performed in DC mode using a 100 W QTH lamp (Newport) as light source and a 150 mm Czerny Turner monochromator (Omni-λ 150, Lot-Oriel). EIS measurements were carried out on complete cells in order to get information about transport and recombination of charges inside the devices. The same electrochemical workstation presented above was employed for this characterization, which was performed in dark condition by varying the applied bias voltage, with a superimposed small signal of 10 mV amplitude and a frequency variable in the range 10−1−104 Hz. Measured spectra were fitted through an equivalent circuit24 presented in Scheme 3: RTCO models the TCO series

Table 1. Photovoltaic Parameters of DSSCs Assembled with SeCN−/(SeCN)2-Based Liquid Electrolytes, with Different Concentrations, and in Presence/Absence of TBP 0.5 M as Additivea

Scheme 3. Equivalent Circuit Exploited for the Fitting of the Impedance Spectra of Polymer Electrolyte Membrane-Based DSSCs

(5)

Indicating the TiO2 thickness as d, the fitting parameters RR, RT, Qμ, and βμ can be used to calculate the electron diffusion length Ln and the electron diffusion coefficient Dn, exploiting the following formulas: (6)

Dn = Ln2 /τn

(7)

FF

η (%)

6.96 7.70 8.06 6.87 6.90 6.92

0.56 0.57 0.58 0.62 0.61 0.61

0.45 0.46 0.55 0.49 0.59 0.62

1.75 2.03 2.56 2.08 2.46 2.60

As a further improvement, we added TBP to the first three solutions listed in Table 1. The resulting effect was an improvement in open-circuit voltage (Voc) and fill factor (FF), with a slight decrease of short-circuit current density (Jsc): overall, the efficiencies of the DSSCs increased, reaching 2.60%. All photovoltaic parameters of the cells tested with different liquid electrolytes are given in Table 1 and Figure S1. These results, together with the tuning of the concentrations and the addition of TBP, are the first results that have ever been published on the redox couple SeCN−/(SeCN)2 prepared according to the procedure of Oskam et al. and tested in DSSC under condition of standard irradiation. In the following paragraphs, we will consider only the best electrolyte among those reported in Table 1. 3.2. Preparation and Characterization of UV-Cured Membranes. Polymer membranes were prepared by photocopolymerizing two acrylic/methacrylic reactive mixtures: BEMA/PEGMA and PEGDA/PEGMA. Under UV light, the free-radical polymerization took place and the photosensitive initiator (PI) generated radicals (R•), which reacted with the oligomer (M) according to the following reaction:

(4)

Ln = d(RR /RT)1/2

Voc (V)

25 mM/100 mM 37.5 mM/150 mM 75 mM/300 mM 25 mM/100 mM + TBP 37.5 mM/150 mM + TBP 75 mM/300 mM + TBP

The addition of pure TBP does not significantly vary the concentration of the redox couple.

where RR is the recombination resistance, RT is the transport resistance, βμ is the index of the CPE Qμ, and τn is the effective electron lifetime:

τn = (R CTQ μ)1/ βμ

Jsc (mA cm−2)

a

resistance, RCE and CCE are the counter electrode charge transfer resistance and capacitance, respectively, and ZTL represent the transmission line model of the photoanode/ electrolyte interface, given by the expression:25 1/2 βμ /2 ⎧ ⎫ ⎡ ⎪⎛ R ⎞ ⎪ RR R T ⎤ ⎥ Z TL = ⎢ coth⎨⎜ T ⎟ [1 + (jωτn)βμ ]1/2 ⎬ β ⎪ ⎪ ⎢⎣ 1 + (jωτn) μ ⎥⎦ R ⎝ ⎠ R ⎩ ⎭

liquid electrolyte

hν

M

(8) PI → R• → RM• In this equation, M represents the difunctional oligomer (BEMA, PEGDA), and the propagation reaction triggered by RM• leads to a three-dimensional cross-linked network.19 When the monomethacrylate PEGMA is added, it takes part in the propagation (changing the polymerization kinetics) and is incorporated into the network. A monofunctional monomer like PEGMA reduces the crosslinking density of the polymeric network and diminishes the Tg of the polymer, thus making the membrane more flexible and increasing the mobility of the redox mediator in the polymer matrix.9 As can be seen in Figure 1, the membranes prepared were transparent, selfstanding, and flexible. The kinetics of photopolymerization was followed by RTFTIR measurements. The conversion curves of the two reactive mixtures are shown in Figure 2. The typical profile of the polymerization processes of multifunctional systems, leading to the formation of crosslinked architectures, was observed. In this process, the reaction rate decreases while the polymer is

3. RESULTS AND DISCUSSION 3.1. Optimization of SeCN−/(SeCN)2 Solution. The SeCN−/(SeCN)2 solution was prepared in agreement with the procedure published by Oskam et al.,14 as reported in section 2.2. The prepared solution contains 25 mM of the oxidized form of the redox couple, (SeCN)2, and 100 mM of the reduced form, SeCN−. As also observed by Oskam et al., it is not possible to increase the amount of the reactants in the preparation phase, as this would lead to unstable solutions, resulting in precipitation of (SeCN)x. As an alternative, we found that it was possible to concentrate a posteriori the 20424



Article

good agreement with the high gel content values (>98%, Table 2), thus indicating the formation of a fully crosslinked network. The thermal stability of the two polymer membranes was assessed by TGA, and the resulting thermograms, reported in Figure 3, show a one-step weight loss degradation process

Figure 1. UV-cured acrylic/methacrylic transparent and flexible membrane.

Figure 3. TGA measurements, under flowing nitrogen, in the temperature range between 25 and 800 °C, of the UV-cured PEGDA/PEGMA and BEMA/PEGMA polymer membranes.

above 300 °C. The temperature corresponding to a weight loss of 5% is reported in Table 2. These values are very high, especially if one considers that, generally, the operating temperature of DSSCs does not exceed 70 °C.28 The viscoelastic behavior of the polymer membranes was investigated by means of DMTA experiments, which consist in the storage (E′) and loss (E″) modules measurements and in the evaluation of the damping factor tan δ as E″/E′ ratio. The curves of E′ and tan δ as a function of temperature are shown in Figure 4. As one can note, a strong decrease of E′ in the Tg

Figure 2. Acrylic/methacrylic double bonds conversion of PEGDA/ PEGMA and BEMA/PEGMA reactive mixtures, checked by RT-FTIR measurements as a function of the irradiation time.

formed, the concentration of the reactive species decreases and the viscosity increases. At the end, when the gel point is reached, the reactive species still present have poor mobility: their diffusion is hindered and the reaction rate drops to zero.26 The plateau value of these curves gives the final acrylic/ methacrylic double bonds conversion (reported in Table 2), Table 2. Conversion Values after 100 s of Irradiation (convt=100s), Thickness and Gel Content Values, and Temperatures of 5% Weight Loss (T5) of the UV-Cured PEGDA/PEGMA and BEMA/PEGMA Polymer Membranes membrane

convt=100s (%)

thickness (μm)

gel content (%)

T5 (°C)

BEMA/PEGMA PEGDA/PEGMA

93 90

103 102

99 98

219 178

Figure 4. DMTA E′ and tan δ curves of the UV-cured PEGDA/ PEGMA and BEMA/PEGMA polymer membranes.

while the slope of each curve gives an indication of the initial polymerization rate. Figure 2 shows that the polymerization rate of the PEGDA-based system is higher than that of the BEMA one: this is the proof of the fact that the acrylic groups are more reactive than methacrylic ones, as commonly reported in the literature.27 Overall, final conversions around 90% were obtained in less than 2 min, thus demonstrating the rapidity of the UV-induced polymerization reactions if compared with the traditional processes. Moreover, RT-FTIR experiments are in

region occurs, while tan δ shows a maximum: the temperature corresponding to this maximum is usually assumed as the Tg of the sample (Table 3). As regards BEMA/PEGMA and PEGDA/PEGMA membranes, Tg were −45 and −39 °C, respectively. Both these values are well below 0 °C, ensuring that the membranes are in the desired rubbery state at room temperature, and undesirable phase transitions during the 20425



Article

for the BEMA-PEGMA membrane; according to literature,31 the higher series resistance could lead to a decrease of the FF in the quasi-solid DSSCs, with a consequent drop of efficiency. The results of the fitting process are presented in Figure 5 superimposed to the experimental curves, and the relative parameters are reported in the last two columns of Table 3. As already observed, the BEMA/PEGMA membrane is characterized by a higher charge transfer time; for what concerns the electrolyte diffusion coefficient, instead, no difference can be evidenced between the two samples, proving that the polymeric matrix has no influence on the redox couple diffusion process.19 3.3. Photovoltaic Performance and Electrochemical Characterization of Polymer Electrolyte MembraneBased DSSCs. The two polymer electrolyte membranes were tested as quasi-solid electrolytes for DSSCs. The photocurrent versus photovoltage characteristics of the cells are shown in Figure 6, and the values of photovoltaic parameters are listed in Table 4.

Table 3. Glass Transition Temperatures (Tg), Cross-Linking Density Values (υe), Charge Transfer Times (τct), and Electrolyte Diffusion Coefficients (De) of the UV-Cured PEGDA/PEGMA and BEMA/PEGMA Polymer Membranes membrane

Tg (°C)

υe (mol m−3)

τct (μs)

De (10−6 cm2 s−1)


−45 −39

454 654

35 29

2.9 2.8

functioning of the device can be excluded. Besides guaranteeing flexibility, resilience, and limited fragility, the rubbery state of these UV-cured material also ensures high ionic mobility and conductivity, as demonstrated by a recent investigation carried out by means of pulsed-field-gradient nuclear magnetic resonance (PFG-NMR) on polymer electrolyte for Li-ion batteries.29 DMTA measurements were also useful to calculate the crosslinking density (υe) values of the two polymer networks. In fact, by measuring the elastic modulus at the rubbery plateau, υe is obtained from the classical rubber theory equation: υe =

E′ 3RT

(9)

where T is temperature (measured in K, well above the Tg of the polymer) at which the storage modulus is evaluated, and R is the gas constant.30 Here, T was 300 K, being the glass transition temperature of the samples in the range 228−233 K. υe values are reported in Table 3, together with a brief summary of other data concerning membranes characterization. It can be noted that the PEGDA/PEGMA membrane has a higher crosslinking density, in agreement with the shorter ethylene oxide unit of PEGDA (n = 3), if compared to that of BEMA (n = 22), as illustrated in Scheme 1. The two polymer membranes were swollen in the liquid solution containing the SeCN−/(SeCN)2 redox mediator at a concentration of 75/300 mM + TBP 0.5 M (see Table 1). The swollen polymers were still non tacky, transparent, freestanding, extremely flexible, and easy to be handled. The results of the EIS measurements performed on polymer electrolyte membranes sandwiched between two Pt-covered electrodes are reported in Figure 5. As can be clearly seen, the BEMA/PEGMA membrane exhibits a slightly taller highfrequency arc (the one related to the charge transfer mechanism) and a larger low-frequency arc (associated with the diffusion impedance) with respect to the PEGDA/PEGMA sample. Both these results imply a larger total series resistance

Figure 6. Photocurrent−photovoltage curves for quasi-solid DSSCs fabricated with the UV-cured PEGDA/PEGMA and BEMA/PEGMA polymer electrolyte membranes.

Table 4. Photovoltaic Parameters of the Quasi-Solid DSSCs Assembled with the UV-Cured PEGDA/PEGMA and BEMA/PEGMA Polymer Electrolyte Membranes membrane

Jsc (mA cm−2)

Voc (V)

FF

η (%)


6.68 7.10

0.55 0.57

0.53 0.59

1.95 2.38

Globally, the polymer membranes proved to be valid quasisolid electrolytes for DSSC and were able to produce an efficiency equivalent to the 91% of the corresponding liquid cell (η = 2.60%). The sunlight conversion efficiency of the PEGDA/PEGMA-based DSSC (2.38%) is greater than that of BEMA/PEGMA one (1.95%), having the highest Jsc, Voc and FF values. As regards Jsc values, in our previous works we showed that the Jsc of I−/I3−-based polymer electrolyte membranes was closely related to the amount of liquid electrolyte trapped in the crosslinked network, as well as to the effectiveness with which it was retained over time.9,19 In this work, the liquid uptake of the PEGDA/PEGMA membrane was found to be slightly greater than that of BEMA/PEGMA one (56 and 47% by weight, respectively), and this explains the higher Jsc measured for the PEGDA-PEGMA system. Furthermore, we must also consider that the υe of the PEGDA/PEGMA polymer network is slightly

Figure 5. EIS curves of the UV-cured PEGDA/PEGMA and BEMA/ PEGMA polymer electrolyte membranes. The points are experimental data, while the continuous lines are fitting curves. 20426



Article

3.4. Aging Test. The liquid and quasi-solid DSSCs assembled with the two different polymer electrolyte membranes were kept at 60 °C for 250 h and checked every 30 h, according to a common procedure useful to evaluate the device aging.19 The stability performance of the solar cells is shown in Figure 9. After 250 h, the cell based on the liquid electrolyte lost 46% of its initial efficiency, while the devices fabricated using the polymer electrolyte membranes showed a reduction in their photoconversion efficiency of only ∼10%. No particular differences between the aging curves of the devices assembled with the two different polymer electrolyte membranes were noticed. Overall, the present findings confirm that crosslinked BEMA/PEGMA and PEGDA/PEGMA membranes effectively trapped the SeCN−/(SeCN)2 redox pair in their polymer matrix and were able to retain it upon prolonged storage at high temperature. 3.5. In Situ Preparation of the Polymer Electrolyte Membrane: Obtaining of an Efficient Electrolyte Able to Seal the Cell. In view of an industrial development of DSSCs, the preparation of the membranes by photopolymerization as described in this paper and in other publications should be optimized. In particular, the curing of the monomers in the presence of the redox couple, thus avoiding the swelling step, was unsuccessful till now: in fact, the traditional I−/I3− mediator is a strong inhibitor of photopolymerization processes proceeding with a radical mechanism.34 Trying to bypass this constraint, a few attempts to make an in situ process in the presence of particular photoinitiating systems have been proposed in literature. However, they required high irradiation times (90−120 min), with a sure damage of the sensitizer due to the prolonged UV exposure; moreover, suitable monomers conversions were not achieved.35 Having obtained promising results with the pseudoalogen couple swelled in the light-cured membrane, we attempted to prepare the polymer electrolyte with an in situ process. A formulation containing the electrolyte solution and the reactive mixture (BEMA/PEGMA, 35:65 wt%/wt%) in a 50:50 weight ratio was drop-casted on the photoelectrode and reticulated in situ, impregnating the pores of the mesoporous TiO2 layer with a close interfacial contact. It is noteworthy that the crosslinking was photoinitiated with visible light (and not ultraviolet one), which ensures that no degradation of the sensitizer occurred. This particular visible light-photoinitiated mechanism in the presence of SeCN−/(SeCN)2 redox pair is completely new, and it is currently investigated in our photochemistry laboratories. The obtained preliminary result showed a sunlight conversion efficiency of 1.61% (Jsc = 5.58 mA cm−2, Voc = 0.58 V, FF = 0.50), and its photocurrent−photovoltage curve is reported in Figure S2. This groundwork result is very positive, not only because it confirms the proper functioning of the cell assembled with the in situ photopolymerized pseudohalogen polymer electrolyte, but also because the measured efficiency was only slightly lower than that obtained for the DSSC assembled with the BEMA/PEGMA membrane (1.95%, Table 4): further improvements will surely be achieved by adjusting the weight ratio between the oligomeric reactive mixture and the electrolyte solution. A striking feature was that the in situ photopolymerization also allowed the sealing of the cell: once removed the binder clips after irradiation, the electrodes maintained a perfect contact between them, also under mechanical stress. This was due to the excellent adhesive properties typical of methacrylates

greater than that of the BEMA/PEGMA (see Table 3), which means that such a membrane is capable of retaining the liquid electrolyte more effectively, without losing it during assembly and operation of the device. This result is also in agreement with IPCE curves of the two cells, shown in Figure 7. The

Figure 7. IPCE curves for quasi-solid DSSCs fabricated with the UVcured PEGDA/PEGMA and BEMA/PEGMA polymer electrolyte membranes.

broad curves, covering almost the entire visible spectrum from 400 to 800 nm, exhibit maximum values of 32 and 27% for the PEGDA/PEGMA- and BEMA/PEGMA-based DSSCs, respectively. It is surprising and relevant to note that both of these IPCEs are higher than those obtained by Oskam et al. for a liquid cell (20%):14 this fact further underlines that the quasisolid device presented in this work is promising. The higher FF value in the PEGDA/PEGMA-based device has to be expected because this membrane was characterized by a reduced charge transfer time, as evidenced by EIS analysis performed on the symmetrical cells (see Table 3).32 As stated above, the faster electronic transfer at the electrolyte/Ptelectrode interface leads to the reduction of the total cell series resistance, which is responsible for the enhancement of the FF. To clarify the dependence of the open circuit voltage on the polymeric matrix (see Table 4), EIS measurements were performed at room temperature also on complete DSSCs. From the analysis of the Nyquist plots reported in Figure 8A, it can be clearly seen that the principal difference between the two kinds of devices lies in the width of the main arc, which is related to the recombination resistance:24 the wider is the arc, the lower will be the recombination rate. The transport and recombination properties were evaluated through eqs 5−7 and the results are presented in Figure 8B. Here it can be observed that no difference is present in the diffusion coefficient behavior, and this result was expected since this parameter depends on the photoanode properties, and identical semiconductor layers were used for the different devices. Concerning the electron lifetime, instead, the use of the PEGDA/PEGMA membrane led to a reduction of the recombination at the interface between the TiO2 layer and the polymer electrolyte. This phenomenon is responsible for the higher photovoltage exhibited by the PEGDA/PEGMAbased DSSC. 33 The combined effect of reduction of recombination and unchanged transport properties brings to a slightly higher diffusion length values for the PEGDA/ PEGMA-based device with respect to the BEMA/PEGMA one. 20427



Article

Figure 8. (A) EIS curves for quasi-solid DSSCs fabricated with the UV-cured PEGDA/PEGMA and BEMA/PEGMA polymer electrolyte membranes measured at open circuit voltage. The points are experimental data, while the continuous lines are fitting curves. (B) Electron lifetime (τn), diffusion coefficient (Dn), and diffusion length (Ln) dependence on applied bias voltage for quasi-solid DSSCs fabricated with the UV-cured PEGDA/PEGMA and BEMA/PEGMA polymer electrolyte membranes.

(especially on glassy substrates as those of the two electrodes),36 undisturbed by the introduction of the redox mediator in the polymeric network. Overall, these preliminary results have shown the possibility of in situ producing the pseudohalogen polymer electrolyte, obtaining not only good performance, but also a good sealing of the device, thus avoiding the use of commercial adhesive tapes, whose application requires the pressing of the cell at high temperature (and these steps also would require machinery, time and money in an industrial plant).

4. CONCLUSIONS In this paper we proposed the first polymer electrolyte for DSSCs based on the pseudohalogen redox couple SeCN−/ (SeCN)2. To optimize the preparation conditions, different liquid electrolyte-based DSSCs have been fabricated varying the concentration of redox mediator and TBP. The optimal conversion efficiency of 2.60% was obtained employing 75 mM (SeCN)2, 300 mM SeCN− and 0.5 M TBP in acetonitrile solution.

Figure 9. Normalized light-to-electricity conversion efficiencies of DSSCs assembled with UV-cured PEGDA/PEGMA and BEMA/ PEGMA polymer electrolyte membranes and with the reference liquid electrolyte vs conservation time under 60 °C.

20428



Article

Electrolyte For High-Efficiency Iodine-Free Dye-Sensitized Solar Cells. J. Power Sources 2013, 221, 328−333. (6) Lee, H. F.; Wu, J. L.; Hsu, P. Y.; Tung, Y. L.; Ouyang, F. Y.; Kai, J. J. Enhanced Photovoltaic Performance and Long-Term Stability of Dye-Sensitized Solar Cells by Incorporating SiO2 Nanoparticles in Binary Ionic Liquid Electrolytes. Thin Solid Films 2013, 529, 2−6. (7) Chen, C. L.; Chang, T. W.; Teng, H.; Wu, C. G.; Chen, C. Y.; Yang, Y. M.; Lee, Y. L. Highly Efficient Gel-State Dye-Sensitized Solar Cells Prepared Using Poly(acrylonitrile-co-vinyl acetate) Based Polymer Electrolytes. Phys. Chem. Chem. Phys. 2013, 15, 3640−3645. (8) Powar, S.; Daeneke, T.; Ma, M. T.; Fu, D.; Duffy, N. W.; Götz, G.; Weidelener, M.; Mishra, A.; Bäuerle, P.; Spiccia, L.; Bach, U. Highly Efficient p-Type Dye-Sensitized Solar Cells Based on Tris(1,2Diaminoethane)Cobalt(II)/(III) Electrolytes. Angew. Chem., Int. Ed. 2013, 52, 602−605. (9) Bella, F.; Pugliese, D.; Nair, J. R.; Sacco, A.; Bianco, S.; Gerbaldi, C.; Barolo, C.; Bongiovanni, R. A UV-Crosslinked Polymer Electrolyte Membrane for Quasi-Solid Dye-Sensitized Solar Cells with Excellent Efficiency and Durability. Phys. Chem. Chem. Phys. 2013, 15, 3706− 3711. (10) Chung, I.; Lee, B.; He, J.; Chang, R. P. H.; Kanatzidis, M. G. AllSolid-State Dye-Sensitized Solar Cells with High Efficiency. Nature 2012, 485, 486−489. (11) Bella, F.; Bongiovanni, R. Photoinduced Polymerization: An Innovative, Powerful and Environmentally Friendly Technique for the Preparation of Polymer Electrolytes for Dye-Sensitized Solar Cells. J. Photochem. Photobiol., C 2013, 16, 1−21. (12) Cong, J.; Yang, X.; Kloo, L.; Sun, L. Iodine/Iodide-Free Redox Shuttles for Liquid Electrolyte-Based Dye-Sensitized Solar Cells. Energy Environ. Sci. 2012, 5, 9180−9194. (13) Lee, J. K.; Yang, M. Progress in Light Harvesting and Charge Injection of Dye-Sensitized Solar Cells. Mater. Sci. Eng., B 2011, 176, 1142−1160. (14) Oskam, G.; Bergeron, B. V.; Meyer, G. J.; Searson, P. C. Pseudohalogens for Dye-Sensitized TiO2 Photoelectrochemical Cells. J. Phys. Chem. B 2001, 105, 6867−6873. (15) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Humphry-Baker, R.; Grätzel, M. A Solvent-Free, SeCN−/(SeCN)3− Based Ionic Liquid Electrolyte for High-Efficiency Dye-Sensitized Nanocrystalline Solar Cells. J. Am. Chem. Soc. 2004, 126, 7164−7165. (16) Bergeron, B. V.; Marton, A.; Oskam, G.; Meyer, G. J. DyeSensitized SnO2 Electrodes with Iodide and Pseudohalide Redox Mediators. J. Phys. Chem. B 2005, 109, 937−943. (17) Solangi, A.; Bond, A. M.; Burgar, I.; Hollenkamp, A. F.; Horne, M. D.; Rüther, T.; Zhao, C. Comparison of Diffusivity Data Derived from Electrochemical and NMR Investigations of the SeCN/ (SeCN)2/(SeCN)3 System in Ionic Liquids. J. Phys. Chem. B 2011, 115, 6843−6852. (18) Song, D.; Kang, M. S.; Lee, Y. G.; Cho, W.; Lee, J. H.; Son, T.; Lee, K. J.; Nagarajan, S.; Sudhagar, P.; Yum, J. H.; et al. Successful Demonstration of an Efficient I−/(SeCN)2 Redox Mediator for DyeSensitized Solar Cells. Phys. Chem. Chem. Phys. 2012, 14, 469−472. (19) Bella, F.; Ozzello, E. D.; Sacco, A.; Bianco, S.; Bongiovanni, R. Polymer Electrolytes for Dye-Sensitized Solar Cells Prepared by Photopolymerization of PEG-Based Oligomers. Int. J. Hydrogen Energy 2013, DOI: 10.1016/j.ijhydene.2013.06.110. (20) Bella, F.; Ozzello, E. D.; Bianco, S.; Bongiovanni, R. Photopolymerization of Acrylic/Methacrylic Gel-Polymer Electrolyte Membranes for Dye-Sensitized Solar Cells. Chem. Eng. J. 2013, 225, 273−279. (21) Hauch, A.; Georg, A. Diffusion in the Electrolyte and ChargeTransfer Reaction at the Platinum Electrode in Dye-Sensitized Solar Cells. Electrochim. Acta 2001, 46, 3457−3466. (22) Macdonald, J. R. Impedance Spectroscopy. Ann. Biomed. Eng. 1992, 20, 289−305. (23) Sacco, A.; Lamberti, A.; Pugliese, D.; Chiodoni, A.; Shahzad, N.; Bianco, S.; Quaglio, M.; Gazia, R.; Tresso, E.; Pirri, C. F. Microfluidic Housing System: A Useful Tool for the Analysis of Dye-Sensitized

The polymer electrolyte was prepared as membranes of micrometric thickness, and the polymer network was crosslinked with a quick, inexpensive and environmentally friendly process of photoinitiated polymerization. The polymerization kinetics and the physicochemical properties of the membranes (BEMA/PEGMA and PEGDA/PEGMA, respectively) were investigated in detail. After activation with the pseudohalogen redox couple, the membranes were tested as polymer electrolytes for quasi-solid DSSCs. As an appreciable result, the quasi-solid cells showed a conversion efficiency of the solar light equal to 91% of the value of the corresponding liquid electrolyte-based cell. Furthermore, the stability upon time of these polymer electrolytes was investigated, and proved to be very high after 250 h of thermal stress. Moreover, a first attempt of in situ photopolymerization in the presence of the redox mediator has been presented with relevant results: the creation of an intimate electrode/ electrolyte interface and the sealing of the device were obtained at the same time by means of visible light. The UV-cured polymeric networks object of this work proved to be very promising in enabling the effective trapping and diffusion of the SeCN−/(SeCN)2 redox couple, and the photovoltaic performance could be further increased by using selenocianated-based low-viscosity ionic liquids as those reported by Grätzel et al.15

■

ASSOCIATED CONTENT

■

AUTHOR INFORMATION

S Supporting Information *

Photovoltaic parameters of DSSCs assembled with SeCN−/ (SeCN)2-based liquid electrolytes, with different concentrations and in the presence/absence of TBP 0.5 M as additive, and photocurrent−photovoltage curves for quasi-solid DSSCs fabricated by means of in situ photopolymerization of a BEMA/PEGMA/SeCN−/(SeCN)2 reactive mixture are reported. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author

*Phone: +39 0110903448 (F.B.); +39 0110904619 (R.B.). Email: [email protected] (F.B.); roberta.bongiovanni@ polito.it (R.B.). Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Devabhaktuni, V.; Alam, M.; Shekara Sreenadh Reddy Depuru, S.; Green, R. C.; Nims, D.; Near, C. Solar Energy: Trends and Enabling Technologies. Renewable Sustainable Energy Rev. 2013, 19, 555−564. (2) O’Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737−740. (3) Gong, J.; Liang, J.; Sumathy, K. Review on Dye-Sensitized Solar Cells (DSSCs): Fundamental Concepts and Novel Materials. Renewable Sustainable Energy Rev. 2012, 16, 5848−5860. (4) Yella, A.; Lee, H. W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; Wei-Guang Diau, E.; Yeh, C. Y.; Zakeeruddin, S. M.; Grätzel, M. Porphyrin-Sensitized Solar Cells with Cobalt(II/III)Based Redox Electrolyte Exceed 12% Efficiency. Science 2011, 334, 629−634. (5) Wu, H.; Lv, Z.; Hou, S.; Cai, X.; Wang, D.; Kafafy, H.; Fu, Y.; Zhang, C.; Chu, Z.; Zou, D. A New Ionic Liquid Organic Redox 20429



Article

Solar Cell Components. Appl. Phys. A: Mater. Sci. Process. 2012, 109, 377−383. (24) Sacco, A.; Lamberti, A.; Gazia, R.; Bianco, S.; Manfredi, D.; Shahzad, N.; Cappelluti, F.; Ma, S.; Tresso, E. High Efficiency DyeSensitized Solar Cells Exploiting Sponge-Like ZnO Nanostructures. Phys. Chem. Chem. Phys. 2012, 14, 16203−16208. (25) Fabregat-Santiago, F.; Garcia-Belmonte, G.; Bisquert, J.; Zaban, A.; Salvador, P. Decoupling of Transport, Charge Storage, and Interfacial Charge Transfer in the Nanocrystalline TiO2/Electrolyte System by Impedance Methods. J. Phys. Chem. B 2001, 106, 334−339. (26) Priola, A.; Bongiovanni, R.; Gozzelino, G.; Ferrero, F.; Malucelli, G. Properties of Networks Based on Poly(Alkylene Glycol) Diacrylates. Macromol. Symp. 1995, 93, 195−203. (27) Andrzejewska, E. Photopolymerization Kinetics of Multifunctional Monomers. Prog. Polym. Sci. 2001, 26, 605−665. (28) Raga, S. R.; Fabregat-Santiago, F. Temperature Effects in DyeSensitized Solar Cells. Phys. Chem. Chem. Phys. 2013, 15, 2328−2336. (29) Chiappone, A.; Jeremias, S.; Bongiovanni, R.; Schönhoff, M. NMR Study of Photo-Cross-Linked Solid Polymer Electrolytes: the Influence of Monofunctional Oligoethers. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1571−1580. (30) Hill, L. W. Calculation of Crosslink Density in Short Chain Networks. Prog. Org. Coat. 1997, 31, 235−243. (31) Wang, Q.; Ito, S.; Grätzel, M.; Fabregat-Santiago, F.; Mora-Seró, I.; Bisquert, J.; Bessho, T.; Imai, H. Characteristics of High Efficiency Dye-Sensitized Solar Cells. J. Phys. Chem. B 2006, 110, 25210−25221. (32) Sacco, A.; Lamberti, A.; Gerosa, M.; Bisio, C.; Gatti, G.; Carniato, F.; Shahzad, N.; Chiodoni, A.; Tresso, E.; Marchese, L. Towards Quasi-Solid State Dye-Sensitized Solar Cells: Effect of γAl2O3 Nanoparticle Dispersion into Liquid Electrolyte. 2013, under review. (33) Cicero, G.; Musso, G.; Lamberti, A.; Camino, B.; Bianco, S.; Pugliese, D.; Risplendi, F.; Sacco, A.; Shahzad, N.; Ferrari, A. M.; et al. Combined Experimental and Theoretical Investigation of the HemiSquaraine/TiO2 Interface for Dye Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2013, 15, 7198−7203. (34) Bartlett, P. D.; Kwart, H. Dilatometric Studies of the Behavior of Some Inhibitors and Retarders in the Polymerization of Liquid Vinyl Acetate. I. J. Am. Chem. Soc. 1950, 72, 1051−1059. (35) Winther-Jensen, O.; Armel, V.; Forsyth, M.; Macfarlane, D. R. In Situ Photopolymerization of a Gel Ionic Liquid Electrolyte in the Presence of Iodine and Its Use in Dye Sensitized Solar Cells. Macromol. Rapid Commun. 2010, 31, 479−483. (36) Moad, G.; Rizzardo, E.; Thang, S. H. Toward Living Radical Polymerization. Acc. Chem. Res. 2008, 41, 1133−1142.

20430


First Pseudohalogen Polymer Electrolyte for Dye ... - ACS Publications

Recommend Documents