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PVAc based gel polymer electrolyte
Optical Materials 96 (2019) 109349
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Improved long-term stability of dye-sensitized solar cell employing PMA/ PVAc based gel polymer electrolyte
T
Amisha Azmara, R.H.Y. Subbana,b, Tan Winiea,b,∗ a b
Faculty of Applied Sciences, Universiti Teknologi MARA, 40450, Shah Alam, Selangor, Malaysia Institute of Science, Universiti Teknologi MARA, 40450, Shah Alam, Selangor, Malaysia
A R T I C LE I N FO
A B S T R A C T
Keywords: PMA PVAc Gel polymer electrolyte Dye-sensitized solar cells Stability
Gel polymer electrolytes based on blends of poly(methyl acrylate) (PMA) and poly(vinyl acetate) (PVAc) were prepared using tetrapropylammonium iodide (TPAI) salt, 1-butyl-3-methylimidazolium iodide (BMII) ionic liquid and ethylene carbonate (EC) plasticizer. The mass fraction of EC, WEC was varied while the masses of the other components were kept constant in order to study the dependence of dye-sensitized solar cell (DSSC) performance on EC concentration. Incorporation of EC in PMA/PVAc-TPAI-BMII has enhanced the efficiency of the DSSC. The efficiency enhancement is due to the increase in short circuit current density, Jsc, arising from the conductivity enhancement brought about by the EC. A DSSC with TPAI-BMII-EC liquid electrolyte was fabricated and its photovoltaic and stability performance were investigated and compared with DSSC with PMA/PVAcTPAI-BMII-EC gel electrolyte. PMA/PVAc decreases the Jsc from 27.87 to 22.91 mA cm−2 and efficiency, η from 12.28 to 9.67%. Linear sweep voltammetry studies reveal that PMA/PVAc decreases the ion motion in the electrolyte. Although PMA/PVAc deteriorates the performance of DSSC, but it improves the stability performance of DSSC by suppressing the recombination loss as evidenced from the increase in charge transfer resistance at the TiO2 electrode and longer electron recombination lifetime.
1. Introduction Dye-sensitized solar cell (DSSC), first introduced by O'Regan and Grätzel in 1991 [1] is deemed as a candidate for the next generation solar cell, owing to their advantages such as ease of fabrication, design opportunity and flexibility, lightweight, better performance at diffuse light and higher temperatures as well as possibility for indoor application [2]. A DSSC consists of three main components i.e. dye-absorbed photoanode, electrolyte and counter electrode. The photo-electrochemical nature of DSSC allows its efficiency to be optimized by varying the chemical composition of its components [3]. Efficiency of DSSC can be improved by modifying the sensitizer [4–6] and redox complexes [7–9], choosing the photoanode [10–12] and counter electrode [13–15], optimizing the electrolyte component [16–18] as well as introducing additives to fine-tune the semiconductor-electrolyte interface [3,19,20]. Electrolyte in DSSC serves as a medium for transportation of redox ions. Liquid and gel electrolytes are commonly used in DSSCs. Gel electrolyte can be prepared by incorporating a polymer into a liquid electrolyte. The polymeric property of gel electrolyte gives it advantages over liquid electrolytes in terms of leak- and corrosion-proof,
∗
as well as minimization of electrolyte evaporation. In addition, gel polymer electrolyte provides better electrolyte-electrode contact and higher conductivity than solid-state polymer electrolyte. Many types of polymers have been used for gel electrolytes in DSSCs such as poly (ethylene oxide) (PEO) [21–23],poly(methyl methacrylate) (PMMA) [24–26], poly(acrylonitrile) (PAN) [27,28], poly(methyl acrylate) (PMA) [5,29], poly(vinyl acetate) (PVAc) [25,30] and etc. In our earlier works, PMA-tetrapropylammonium iodide (TPAI) and PVAc-TPAI solid polymer electrolytes have been prepared and tested in DSSCs [31]. Then, 1-butyl-3-methylimidazolium iodide (BMII) has been incorporated into PMA/PVAc-TPAI. The effects of BMII on the structural, thermal and conductivity properties of PMA/PVAc-TPAI have been discussed in [32]. In the same work [32], the efficiencies achieved for DSSC with and without BMII were 4.62% and 2.79%, respectively. In another work [33], we reported the dielectric properties and conduction mechanism of PMA/PVAc-TPAI-BMII electrolyte system. The correlated barrier hopping model was used to interpret its conduction mechanism. In the work of [33], the dependence of DSSC performance on BMII concentration was discussed, which was not found in [32]. As a continuation, in this work, we have incorporated ethylene carbonate (EC) into PMA/PVAc-TPAI-BMII to form gel-like polymer electrolyte.
Corresponding author. Faculty of Applied Sciences, Universiti Teknologi MARA, 40450, Shah Alam, Selangor, Malaysia. E-mail address: [email protected] (T. Winie).
https://doi.org/10.1016/j.optmat.2019.109349 Received 26 July 2019; Received in revised form 22 August 2019; Accepted 22 August 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
Optical Materials 96 (2019) 109349
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concentration of iodide, I−. For DSSC assembly, a small amount of the electrolyte was sandwiched between the TiO2 photoanode and the Pt counter electrode. The current-voltage (J-V) characteristics and impedance measurements of the DSSCs were measured under illumination of 100 mWm−2 simulated sunlight (AM1.5), from an Oriel Newport LCS-100 solar simulator, with a Metrohm Autolab potentiostat (PGSTAT128 N). The effective area of the cell was 0.20 cm2. The conversion efficiency, η of the DSSC was calculated from the short-circuit current density, Jsc, the open circuit voltage, Voc, the fill factor, ff and the power of incident light, Pin.
The effects of EC on the Tg and conductivity are reported. FTIR reveals the interactions among components in PMA/PVAc-TPAI-BMII-EC. Many DSSCs with polymer electrolytes are reported in the literature. However, the effect of polymer on the stability of DSSC is seldom highlighted. In this work, the photovoltaic and stability performance of DSSCs assembled with TPAI-BMII-EC liquid electrolyte and PMA/PVAcTPAI-BMII-EC gel electrolyte are investigated and compared. This paper sheds some lights on the long-term stability improvement brought about by the PMA/PVAc. 2. Experimental
η= 2.1. Polymer electrolyte preparation and characterization
Jsc Voc ff Pin
(1)
The impedance measurements of the DSSCs were carried out over a frequency range of 0.1 Hz–1 MHz and the AC modulation signal was 0.5 V. All impedance measurements applied a potential bias equal to the Voc.
PMA in toluene, PVAc, TPAI, BMII and EC were purchased from Sigma-Aldrich. Prior to use, PMA was purified with methanol and the precipitation was dried. PMA/PVAc-TPAI-BMII electrolytes were prepared according to our previous work [32]. EC was added to the highest conducting sample [32]. The mass fraction of EC, WEC was varied from 0.10 to 0.40 while the masses of PMA/PVAc, TPAI and BMII were kept constant. Composition of PMA and PVAc was fixed at 90:10. ATR-FTIR spectroscopic studies were carried out using a Thermo Fisher Scientific Nicolet iS10 spectrophotometer (USA). The FTIR spectra were recorded in the frequency range of 600–4000 cm−1 with a resolution of 2 cm−1. The glass transition temperatures, Tgs were determined by a TA Q200 DSC. Samples weighing around 10 mg were tested in crimped aluminum pans at an underlying heating rate of < q > = 10 °C min−1 over a temperature range between −10 and 80 °C. Conductivities at different temperatures from room temperature to 363 K were determined from the impedance measurements using a HIOKI 3532-50 LCR Hi-tester impedance spectrometer in the frequency range of 50 Hz - 1 MHz. The diffusion coefficient of triiodide ions, DI3was determined by measuring the triiodide limiting current using linear sweep voltammetry (LSV). The electrolyte was sandwiched between two platinum (Pt) electrodes. A spacer of 0.13 mm thickness and 0.20 cm2 hole area was used for the fabrication of symmetrical cell. The cell was polarized from −0.60 to +0.60 V at a rate of 10 mV s−1.
3. Results and discussion 3.1. Electrolyte characterization 3.1.1. FTIR analysis Fig. 1 shows the overlaid FTIR spectra of EC, PMA/PVAc blend and PMA/PVAc-EC at WEC = 0.20. The assignment of peaks of PMA/PVAc blend and EC are based on the literature [32,34]. For PMA/PVAc blend, the O–CH3 stretching, ν(OCH3) peaks are observed at 1164 and 1198 cm−1. Peaks observed at 1240, 1436 and 1733 cm−1 are corresponding to the C–O–C symmetric stretching, νs(COC), CH3 symmetric bending, δs(CH3) and C]O stretching, ν(C]O), respectively. The CH3 symmetric, νs(CH3), CH3 asymmetric stretching, νa(CH3) and C–H asymmetric stretching, νa(CH) are observed at 2849, 2924 cm−1 and 2955 cm−1, respectively. Characteristic peaks exhibited by EC are observed at 1069 cm−1 (C–O stretching, ν(C–O)), 1159 cm−1 (C–O symmetric stretching, νs(C–O)), 1392 (CH2 wagging, ω(CH2)), 1772 and 1798 cm−1 (C]O stretching, ν(C]O)). Mixing EC and PMA/PVAc has resulted in changes of the peaks for both EC and PMA/PVAc. The ν(C–O) of EC gets shifted from 1069 to 1073 cm−1. The ν(C]O) of EC at 1772 and 1798 cm−1 are found to shift to 1775 and 1802 cm−1, respectively, with decrease in their relative intensities. The 1164 cm−1 of PMA/PVAc has superimposed with the 1159 cm−1 of EC to form an intense peak at 1162 cm−1. On the other hand, the ν(C]O) of PMA/PVAc at 1733 cm−1 shows no appreciable deviation from its position, but with decrease in its relative intensity. These observations indicate that interaction has occurred between EC and PMA/PVAc. Figs. 2 and 3 show the evidence of interaction between EC with TPAI and BMII. Table 1 summarizes the EC-TPAI and EC-BMII
2.2. DSSC assembly and characterization Titanium dioxide (TiO2) with average particle size of ~16 nm (P90) and ~21 nm (P25) were acquired from Evonik Industries. Fluorinedoped tin oxide (FTO) conducting glass, Pt solution and cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium (II) dye solution (N3) were obtained from Solaronix. Carbowax, Triton X-100, 4-tert-butylpyridine (TBP) and guanidinium thiocyanate (GuNCS) were from Sigma-Aldrich. Nitric acid and iodine (I2) chips were purchased from Friedemann Schmidt Chemical and Amcochemie-Humburg, respectively. TiO2 photoanodes were prepared on FTO glass coated with two layers of TiO2. The first TiO2 layer was prepared by grinding 0.5 g of P90 TiO2 in 2 mL of nitric acid (pH = 1) for 30 min and followed by spin-coating on FTO glass. The spin-coated FTO glass was then sintered at 450 °C for about 30 min. For the second TiO2 layer, 0.5 g of P25 TiO2, 0.1 g of carbowax and 2 drops of Triton X-100 were ground in 2 mL of nitric acid (pH = 1) for 30 min. The P25 TiO2 slurry obtained was doctor bladed on the P90 TiO2-coated FTO glass followed by sintering at 450 °C for 30 min. After cooling down to room temperature, the TiO2 photoanodes were soaked in a 3 mM N3 dye solution for 24 h. The N3 dye-coated photoanodes were then immersed in 0.5 M TBP and 0.1 M GuNCS in acetonitrile for 15 s. The Pt counter electrodes were prepared by spin-coating a Pt solution on FTO glass and sintered at 450 °C for 30 min. Iodine chips were added to the electrolytes and stirred until homogeneous. The concentration of iodine was one tenth of the
Fig. 1. Overlaid FTIR spectra of EC, PMA/PVAc and PMA/PVAc-EC. 2
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Table 1 FTIR peak assignment for PMA/PVAc, EC, EC-TPAI and EC-BMII. Component
Wavenumber (cm−1)
Peak Assignment
PMA/PVAc (90:10)
1164, 1198 1240 1436 1733 2849 2924 2955
ν(OCH3) νs(COC) δs(CH3) ν(C]O) νs(CH3) νa(CH3) νa(CH)
EC
1069 1159 1392 1481 1772, 1798
ν(C–O) νs(C–O) ω(CH2) δ(CH2) ν(C]O)
EC-TPAI
1077
ν(C–O) (shifted from 1069 cm−1 of EC) νs(C–O) (shifted from 1159 cm−1 of EC) ν(C]O) (shifted from 1772 cm−1 of EC) ν(C]O) (shifted from 1798 cm−1 of EC)
1164
Fig. 2. Overlaid FTIR spectra of EC and EC-TPAI.
1777 1807
EC-BMII
ν(C–O) (shifted from 1069 cm−1 of EC) νs(C–O) (shifted from 1159 cm−1 of EC) ν(C]O) (shifted from 1772 cm−1 of EC) ν(C]O) (shifted from 1798 cm−1 of EC)
1075 1163 1776 1803
Fig. 3. Overlaid FTIR spectra of EC and EC-BMII.
interactions. Upon incorporation of TPAI or BMII, the characteristic peaks of EC are observed to experience appreciable changes in their vibrational frequencies and intensities. 3.1.2. Tg analysis The interaction between EC and PMA/PVAc-TPAI-BMII is supported from the Tg studies. The Tg of PMA/PVAc-TPAI-BMII was recorded as 22.6 °C [32]. Fig. 4 illustrates that Tg decreases with increasing WEC. Incorporation of EC leads to the formation of TPAI-EC and BMII-EC complexes, which reduces the fraction of PMA/PVAc-TPAI and PMA/ PVAc-BMII complexes. The reduced fraction of PMA/PVAc-TPAI and PMA/PVAc-BMII complexes increases the flexibility of polymer chains. The increase in chain flexibility is reflected in a decrease in Tg. EC molecules that remain between the adjacent polymer chains, reduce the polymer-polymer chain interaction. Hence, the easy movement of polymer chain increases the free volume, which facilitates the movement of ions.
Fig. 4. Tg values of PMA/PVAc-TPAI-BMII with respect to WEC.
connected in series with another CPE [35]. The CPE is a “leaky capacitor”. The use of a CPE instead of a capacitor is to compensate for the inhomogeneity of electrolyte, roughness of the electrolyte-electrode interface and etc [36]. The real, Zr and imaginary, Zi parts of the impedance associated to the equivalent circuit can be expressed as [37]:
Zr =
3.1.3. Impedance analysis The Nyquist plots for PMA/PVAc-TPAI-BMII containing WEC = 0.20 at various temperatures are shown in Fig. 5. The Nyquist plots for temperatures below 363 K consist of a semicircular spur at high frequency followed by a tilted spike at low frequency. Thus, the Nyquist plots of Fig. 5 (a)–(c) can be represented by a parallel combination of bulk resistance, Rb and constant phase element (CPE) that are
1 Rb 1 ⎡R ⎣ b
+ Q1
ωα1
+ Q1 ωα1 cos
cos
2
( ) ⎤⎦ α1 π 2
( ) α1 π 2
+ ⎡Q1 ⎣
ωα1
sin
2
( ) ⎤⎦ α1 π 2
+
1 απ cos ⎛ 2 ⎞ Q2 ωα2 ⎝ 2 ⎠ (2)
and
3
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Fig. 6. Frequency dependence of dielectric constant of PMA/PVAc-TPAI-BMII with WEC = 0.20 at (a) 303 K, (b) 313 K, (c) 323 K, (d) 333 K, (e) 343 K, (f) 353 K and (g) 363 K. Fig. 5. Nyquist plots and their corresponding fitted lines for PMA/PVAc-TPAIBMII containing WEC = 0.20 at (a) 303 K, (b) 313 K, (c) 333 K and (d) 363 K.
Q1 ωα1 sin Zi =
1
⎡ R + Q1 ωα1 cos ⎣ b
2
( ) ⎤⎦ α1 π 2
( ) α1 π 2
+ ⎡Q1 ωα1 sin ⎣
( ) α1 π 2
σ=
1 απ cos ⎛ 2 ⎞ Q2 ωα2 ⎝ 2 ⎠
Q=
(4)
1 απ sin ⎛ 2 ⎞ Q2 ωα2 ⎝ 2 ⎠
Table 2 Circuit parameters Rb, Q and α for PMA/PVAc-TPAI-BMII containing WEC = 0.20 at various temperatures.
303 313 323 333 343 353 363
Rb (Ω) 5210 1030 440 302 110 55 16
Q1 (F)
Q2 (F) −11
8.5 × 10 7.0 × 10−9 1.0 × 10−8 3.1 × 10−8 7.6 × 10−8 8.4 × 10−8 –
−6
7.3 × 10 2.0 × 10−5 2.8 × 10−5 3.3 × 10−5 5.2 × 10−5 1.1 × 10−4 1.9 × 10−4
α1 (rad)
α2 (rad)
0.91 0.64 0.71 0.63 0.62 0.63 –
0.84 0.84 0.84 0.83 0.83 0.81 0.70
(7)
3.1.4. Conductivity analysis Fig. 7 depicts the temperature dependence of conductivity for PMA/ PVAc-TPAI-BMII containing various WEC. It is observed that conductivities of all samples increase with increasing temperature and at comparable temperature, conductivity increases with WEC. However, no further addition of WEC beyond 0.40 because the electrolyte has become liquid-like for WEC > 0.20. Enhancement of conductivity by temperature can be attributed to the increment of the number of free ions by temperature [cf. section 3.1.3]. The presence of high dielectric constant of EC helps in dissociation of TPAI and BMII. This increases the number of free ions. TPAI contains Pr4N+ cations and I− anions; BMII contains BMI+ cations and I− anions. Pr4N+ (4.59 Å) and BMI+ (3.85 Å) [28] are bulky cations. Movement of bulky ions in the polymer matrix is easily hindered by the entanglement of polymer chains. Hence, contribution to conductivity from Pr4N+ and BMI+ cations is minimal. The contribution to conductivity is mostly from I− anions. Conductivity enhancement can also be attributed to the plasticizing effect of EC as Tg is observed to decrease upon incorporation of EC (cf. Fig. 4). Increased chain flexibility facilitates the movement of ions. In addition, EC maintains a gel-like state within the polymer matrix. This allows the diffusive transport
(5)
Equations (2) and (3) were used to fit the Nyquist plots of Fig. 5 (a)–(c) whereas equations (4) and (5) were used to fit the Nyquist plots of Fig. 5 (d). Table 2 lists the values of Rb, Q1, Q2, α1 and α2. It is observed that the value of Rb decreases with increasing temperature. This indicates an increment in conductivity, σ with temperature because σ is inversely proportional to Rb according to equation (6).
Temperature (K)
εo εr A t
where εo is the permittivity of free space, A and t have their usual meanings. Many studies reported that the value of εr decreases with increasing frequency [39–41]. The lower value of εr at high frequency results in lower value of Q at high frequency. This explains the lower value of Q1 as compared with Q2. The value of Q is found to increase with temperature. This is because the value of εr increases with temperature as shown in Fig. 6. The εr represents the number of free ions stored in a sample [42]. The increase of εr with temperature indicates that the number of free ions increases with temperature.
and
Zi =
(6)
where t is the electrolyte thickness and A is the electrolyte-electrode contact area. The variation of conductivity with respect to temperature will be discussed further in section 3.1.4. Again from Table 2, the values of Q2 are found higher than Q1. Capacitance, Q is directly proportional to the dielectric constant, εr of a material as shown in equation (7).
1 απ sin ⎛ 2 ⎞ + 2 Q2 ωα2 2 ⎠ ⎝ ⎤ ⎦ (3)
where Q1 is the capacitance at high frequency and Q2 is the capacitance at low frequency. α1 is the deviation of the diameter of the semicircle from the Zi axis and α2 is the deviation of the spike from the Zr axis. ω is the angular frequency. In contrast, the Nyquist plot at 363 K consist only a steeply rising spike, the semicircular spur completely disappeared (cf. Fig. 5 (d)). The equivalent circuit representation is a Rb in series with a CPE [38]. Hence, the Zr and Zi associated to the equivalent circuit are [38]:
Zr = Rb +
t Rb A
4
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Fig. 8. J-V characteristics of (a) cell A, (b) cell B, (c) cell C, (d) cell D and (e) cell E.
cations do not absorb easily to the TiO2 surface. Thus, bigger value of Voc in the range of 0.70–0.76 V can be observed for the current electrolyte system. The adsorption ability of cations on the TiO2 surface can be reflected in the value of Voc. From Table 3, the Voc is seen to increase slowly with WEC. This means that with increasing WEC, the adsorption ability of cations on the TiO2 surface reduces. This is because, on top of the absorption difficulty of big cations on the TiO2 surface, EC molecules block the cations to the TiO2. Table 4 compares the performance of the DSSC in this work with other polymer electrolyte-based DSSCs. The efficiency obtained in this work is better or on par with the works reported by other researchers. It is worth to highlight here that the efficiency obtained using blend of PMA and PVAc is higher than other PMA- and PVAc-based single polymer electrolytes.
Fig. 7. Temperature dependence of conductivity for PMA/PVAc-TPAI-BMII containing (a) 0.00, (b) 0.10, (c) 0.15, (d) 0.20, (e) 0.25, (f) 0.30, (g) 0.35 and (h) 0.40 of WEC.
properties of liquids.
3.2. DSSC characterization 3.2.1. Dependence of DSSC performance on EC concentration DSSCs with the configuration FTO/TiO2/N3 dye/electrolyte/Pt/ FTO were assembled and Table 3 lists the composition of the electrolyte. Fig. 8 shows the J-V characteristics of the cells and their performance parameters are summarized in Table 3. The efficiency of the cell increases from 4.62% (cell A) to 7.39% (cell B) upon incorporation of WEC = 0.10. The efficiency of the DSSC continues to increase to 11.22% (cell E) as WEC increases to 0.40. It is to note here that no-flow gel-like electrolyte samples are obtained for WEC ≤ 0.20. Electrolyte conductivity is one important factor governing the performance of DSSCs particularly the Jsc and η [18,21,23]. The higher efficiency of DSSCs with EC is due to the higher Jsc value. The Jsc of the DSSCs is found to gradually increase from 10.04 mA cm−2 (cell A) to 25.88 mA cm−2 (cell E). The increase in Jsc is caused by the conductivity enhancement brought about by the EC. The roles of EC as the conductivity enhancer are 1) assists the dissociation of TPAI and BMII, which allows greater numbers of ions for conduction and 2) decreases the Tg of the polymer, which increases the segmental flexibility of polymer chain and hence facilitates the ions movement. Adsorption of cations on the TiO2 surface shifts the Fermi level, EF of TiO2 towards the redox potential of I−/I3− and hence results in smaller value of Voc [28,43]. For electrolyte with big cations such as Pr4N+ and BMI+, shift of EF of TiO2 towards the redox potential is less as the big
Table 4 Performance of various polymer electrolyte-based DSSCs.
WEC
Voc (V)
Jsc (mA cm−2)
ff (%)
η (%)
A B C D E
0.00 0.10 0.20 0.30 0.40
0.70 0.72 0.73 0.74 0.76
10.04 16.71 22.91 23.87 25.88
66 61 58 58 59
4.62 7.39 9.67 10.18 11.22
Sensitizer
η (%)
References
PVAc-TBAI-PMII-EC/PV/AN-I2 PVAc-LiI-TBP-AN-I2 PMA-LiI-TBP-I2 PMA-LiI-TBP-AN-I2 PMA/PEG-LiI-EC/PC-I2 PMA/PVDF-LiI-TBP-I2 PMA/PVDF/PEG- TBP-I2 PMMA-EC/PC/DMC-NaI-I2
N3 N3 N3 N3 Organic dye TC4 N719 N719 cis-[(dcbH2)2 Ru (SCN)2] N3 N719 N3 N719 N719
5.74 5.62 1.40 7.17 3.77 6.60 7.00 4.80
[25] [30] [44] [30] [5] [29] [29] [45]
5.51 8.48 4.90 5.50 3.45
[25] [4] [21] [24] [26]
N719 N3 N719 N3
3.6 5.75 5.8 7.5
[46] [25] [47] [6]
N3 N719 N719 N3 N3 N3
6.26 5.47 5.18 3.7 1.4 9.67
[18] [23] [16] [28] [27] This work
PMMA-TBAI-PMII-EC/PV/AN-I2 PMMA/PEO-LiI-DMII-TBP-I2 PMMA/PEO-LiI-TBP-EC/PCI2 PMMA/PVA-LiI-MPII-PEG-I2 PMMA/PVDF-EC-TBAI-I2 PEO-EC/PC/DMC-NaI-I2 PEO-TBAI-PMII-EC/PV/AN-I2 PEO-LiI-KI- I2 PEO/PVA-DMSO-EC-TBAI- BMIII2 PEO/PVA-TBAI-LiL-EC-DMSO-I2 PEO/PEG-KI-TiO2-I2 PAN-MgI2-TPAI-EC-PC-I2 PAN–NH4I–I2 PAN-LiBOB-BMII-I2 PMA/PVAc-TPAI-BMII-EC
Table 3 Performance parameters for DSSCs assembled with PMA/PVAc-TPAI-BMII-EC electrolytes. Cell
Gel polymer electrolyte
Notes: N3(Cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)rutheN719 (Di-tetranium(II) [Ru(4,4′-dicarboxy-2,2′-bipyridine)2(NCS)2]), butylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium(II)). 5
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Fig. 9. J-V characteristics of (a) cell C and (b) cell F. Table 5 Performance parameters for cells C and F. Cell
Voc (V)
Jsc (mA cm−2)
ff (%)
η (%)
DI3- (cm2 s−1)
C F
0.73 0.77
22.91 27.87
58 60
9.67 12.28
6.7 × 10−7 2.4 × 10−6
Notes: Electrolyte for cell C: PMA/PVAc-TPAI-BMII-EC at WEC = 0.20 gel polymer electrolyte; electrolyte for cell F: TPAI-BMII-EC liquid electrolyte.
Fig. 11. Stability studies of DSSCs (a) as-fabricated, (b) after 96 h, (c) after 120 h and (d) after 288 h. Table 6 Performance parameters for the as-fabricated and aged DSSCs. Cell
Time duration
Voc (V)
Jsc (mA cm−2)
ff (%)
η (%)
Rct (Ω cm2)
τe (ms)
C
As-fabricated After 96 h After 120 h After 288 h
0.73 0.75 0.73 0.71
22.91 18.11 15.15 12.14
58 61 71 69
9.67 8.24 7.69 6.05
653 597 531 498
23.27 18.45 9.34 5.97
F
As-fabricated After 96 h After 120 h After 288 h
0.77 0.73 0.70 0.67
27.87 22.91 14.20 11.75
60 58 60 58
12.28 9.67 6.13 4.92
608 558 512 435
22.1 15.9 8.24 4.27
Fig. 10. Linear sweep voltammograms of (a) TPAI-BMII-EC liquid electrolyte and (b) PMA/PVAc-TPAI-BMII-EC gel polymer electrolyte (WEC = 0.20).
3.2.2. Performance of DSSC with and without PMA/PVAc The J-V characteristics of cells C and F are presented in Fig. 9. Cell F contains TPAI-BMII-EC-I2 liquid electrolyte, without PMA/PVAc. The amounts of TPAI, BMII, EC and I2 in cell F are the same as in cell C. Table 5 presents the effect of PMA/PVAc on the photovoltaic performance of DSSCs. Cell F without PMA/PVAc shows a higher η of 12.28% and Jsc of 27.87 mA cm−2 as compared to cell C with PMA/PVAc. During regeneration of dye sensitizer, S, the oxidized sensitizer molecules, S+ are reduced by iodide, I− in the electrolyte by
2S+ + 3I − → 2S + I3−
determining the performance of DSSC. The LSV technique can be used to study the redox reaction of I−/I3− couple [6,48–51]. When voltage is applied, current will be produced as a result of the diffusion of I− and I3− ions in the electrolyte. The current increases with voltage until a steady-state current is achieved. Fig. 10 shows the LSV curves for cells C and F. For both cells, the currents saturate above ~0.3 V for both polarities. Both anodic and cathodic limiting current plateaus are quite similar. This indicates the equilibrium steady-state condition. For our electrolytes, the iodide concentration, I− is in excess over the concentration of iodine (10:1). Hence, the I3− ions are the limiting current
(8)
At the Pt counter electrode, the triiodide ion, I3− recombines with the electron and reduces to I−:
I3− + 2e− → 3I − −
(9) −
Thus, I and I3 ions in the electrolyte are playing important role in 6
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Fig. 12. Nyquist plots of DSSCs (a) as-fabricated, (b) after 96 h, (c) after 120 h and (d) after 288 h.
ions. The diffusion of triiodide ions, DI3- can be calculated from the limiting current, Jlim using the following equation [48–50]:
D I3− =
Jlim d 2nFCI3−
(10)
where d is the thickness of the cell and n = 2 is the number of electron required for the reduction of I3− to I− (cf. Equation (9)). The symbol F is the Faraday constant and CI3− is the concentration of I3− per unit volume. The calculated values of DI3- for cells C and F are listed in Table 5. The values of DI3- obtained for the liquid and gel electrolytes in this work are comparable with that reported in the literature [6,48–51]. Cell C shows lower value of DI3- than cell F. This is because PMA/PVAc in cell C decreases the motion of I−/I3− ions in the electrolyte. As a result, presence of PMA/PVAc decreases the Jsc from 27.87 to 22.91 mA cm−2, which leads to the decrease in η from 12.28 to 9.67%.
Fig. 13. Bode plots of DSSCs (a) as-fabricated, (b) after 96 h, (c) after 120 h and (d) after 288 h.
cell C can be due to 1) minimal evaporation of liquid electrolyte due to entrapment of liquid electrolyte by the PMA/PVAc, and 2) reduction of charge recombination by blocking the redox ions to the TiO2 electrode by the PMA/PVAc. The kinetics of charge recombination can be elucidated by the impedance studies of DSSC. Fig. 12 shows the Nyquist plots for the as-fabricated and aged DSSCs. There are two semi-circles in the plots. The resistance of the second semicircle is the charge transfer resistance at the TiO2 electrode, Rct [15,31]. As seen from Table 6, the values of Rct for cells C and F after 288 h are 498 and 435 Ω, respectively. Cell C exhibits higher value of Rct indicates higher charge transfer resistance at the TiO2 electrode, which implies lower charge recombination. Presence of PMA/PVAc suppresses the recombination loss and hence improves the performance of the cell with respect to time. Electron recombination lifetime, τe can be obtained from the Bode plots of Fig. 13. The τe can be calculated from
3.2.3. Improvement of DSSC stability with PMA/PVAc Liquid electrolyte DSSCs are known to have higher efficiencies than polymer electrolyte-based DSSCs. The highest conversion efficiency of DSSCs with liquid electrolyte has been recorded at ~14% [52]. Liquid electrolytes can offer higher efficiency because they give good conductivity and make good contact with the photoanode. However, they are likely to leak, evaporate and corrode the electrodes, which disrupts the long-term stability of DSSCs. In this section, the stability performance of cell C with PMA/PVAc are investigated and compared with cell F without PMA/PVAc. The J-V characteristics of cells C and F as a function of time are presented in Fig. 11 and the effect of PMA/PVAc on the stability performance of DSSC is summarized in Table 6. For cell F, Jsc at 288 h is 42% of Jsc as-fabricated and η at 288 h is 40% of η as-fabricated. On the other hand, for cell C, Jsc at 288 h is 53% of Jsc as fabricated and η at 288 h is 63% of η as-fabricated. The improved stability performance of
τe =
1 2πfmax
(11)
where fmax is the maximum frequency of the Bode plot. The calculated values of τe are shown in Table 6. At comparable aging duration, the τe are longer with PMA/PVAc. For instance, at 288 h, the values of τe increases from 4.27 to 5.97 ms with PMA/PVAc. The longer τe means 7
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that PMA/PVAc makes the redox ions more difficult to recombine with the excited electron. The τe analysis is in agreement with the Rct analysis.
based dye-sensitized solar cells, J. Appl. Electrochem. 47 (11) (2017) 1239–1249. [13] M. Wu, T. Ma, Platinum-free catalysts as counter electrodes in dye-sensitized solar cells, ChemSusChem 5 (8) (2012) 1343–1357. [14] M.W. Khan, J. Yao, K. Zhang, X. Zuo, Q. Yang, H. Tang, K.M. Ur Rehman, G. Li, M. Wu, K. Zhu, H. Zhang, Facile fabrication of hollow mesosphere of crystalline SnO2 nanoparticles and synthesis of SnO2@SWCNTs@Reduced Graphene Oxide nanocomposite as efficient Pt-free counter electrode for dye-sensitized solar cells, Opt. Mater. 80 (2018) 271–278. [15] M.A. Mingsukang, M.H. Buraidah, M.A. Careem, I. Albinsson, B.E. Mellander, A.K. Arof, Investigation of counter electrode materials for gel polymer electrolyte based quantum dot sensitized solar cells, Electrochim. Acta 241 (2017) 487–496. [16] M.A.K.L. Dissanayake, C.A. Thotawatthage, G.K.R. Senadeera, T.M.W.J. Bandara, W.J.M.J.S.R. Jayasundara, B.E. Mellander, Efficiency enhancement in dye sensitized solar cells based on PAN gel electrolyte with Pr4NI + MgI2 binary iodide salt mixture, J. Appl. Electrochem. 43 (9) (2013) 891–901. [17] R.A. Senthil, J. Theerthagiri, J. Madhavan, K. Murugan, P. Arunachalam, A.K. Arof, Enhanced performance of dye-sensitized solar cells based on organic dopant incorporated PVDF-HFP/PEO polymer blend electrolyte with g-C3N4/TiO2 photoanode, J. Solid State Chem. 242 (2016) 199–206. [18] L.P. Teo, T.S. Tiong, M.H. Buraidah, A.K. Arof, Effect of lithium iodide on the performance of dye sensitized solar cells (DSSC) using poly(ethylene oxide) (PEO)/ poly(vinyl alcohol) (PVA) based gel polymer electrolytes, Opt. Mater. 85 (2018) 531–537. [19] X. Yin, W. Tan, J. Zhang, Y. Lin, X. Xiao, X. Zhou, X. Li, B.R. Sankapal, Synthesis of pyridine derivatives and their influence as additives on the photocurrent of dyesensitized solar cells, J. Appl. Electrochem. 39 (1) (2009) 147–154. [20] C. Zhang, Y. Huang, Z. Huo, S. Chen, S. Dai, Photoelectrochemical effects of guanidinium thiocyanate on dye-sensitized solar cell performance and stability, J. Phys. Chem. C 113 (52) (2009) 21779–21783. [21] E. Aram, M. Ehsani, H.A. Khonakdar, Improvement of ionic conductivity and performance of quasi-solid-state dye sensitized solar cell using PEO/PMMA gel electrolyte, Thermochim. Acta 615 (2015) 61–67. [22] M.H. Buraidah, L.P. Teo, C.M. Au Yong, S. Shah, A.K. Arof, Performance of polymer electrolyte based on chitosan blended with poly(ethylene oxide) for plasmonic dyesensitized solar cell, Opt. Mater. 57 (2016) 202–211. [23] K. Prabakaran, S. Mohanty, S.K. Nayak, Improved electrochemical and photovoltaic performance of dye sensitized solar cells based on PEO/PVDF–HFP/silane modified TiO2 electrolytes and MWCNT/Nafion® counter electrode, RSC Adv. 5 (51) (2015) 40491–40504. [24] D.H. Kim, M.S. Park, D.J. Kim, H.H. Cho, J.H. Kim, Solid polymer electrolyte dyesensitized solar cells with organized mesoporous TiO2 interfacial layer templated by poly(vinyl alcohol)–poly(methyl methacrylate) comb copolymer, Solid State Ion. 300 (2017) 195–204. [25] Y.K. Kim, S.H. Park, W.P. Hwang, M.H. Seo, H.W. Park, Y.W. Jang, M.R. Kim, J.K. Lee, Impedance spectroscopy on dye-sensitized solar cells with a poly(ethylenedioxythiophene):poly(styrenesulfonate) counter electrolyte, J. Korean Phys. Soc. 60 (12) (2012) 2049–2053. [26] J. Theerthagiri, R.A. Senthil, M.H. Buraidah, J. Madhavan, A.K. Arof, Effect of tetrabutylammonium iodide content on PVDF-PMMA polymer blend electrolytes for dye-sensitized solar cells, Ionics 21 (10) (2015) 2889–2896. [27] A.K. Arof, H.K. Jun, L.N. Sim, M.Z. Kufian, B. Sahraoui, Gel polymer electrolyte based on LiBOB and PAN for the application in dye-sensitized solar cells, Opt. Mater. 36 (1) (2013) 135–139. [28] T.M.W.J. Bandara, W.J.M.J.S.R. Jayasundara, M.A.K.L. Dissanayake, M. Furlani, I. Albinsson, B.E. Mellander, Effect of cation size on the performance of dye sensitized nanocrystalline TiO2 solar cells based on quasi-solid state PAN electrolytes containing quaternary ammonium iodides, Electrochim. Acta 109 (2013) 609–616. [29] M. Fathy, J.E. Nady, M. Muhammed, S. Ebrahim, M.B. Soliman, A.E.B. Kashyout, Quasi-solid-state electrolyte for dye sensitized solar cells based on nanofiber PMAPVDF and PMA-PVDF/PEG membranes, Int. J. Electrochem. Sci. 11 (7) (2016) 6064–6077. [30] C.W. Tu, K.Y. Liu, A.T. Chien, C.H. Lee, K.C. Ho, K.F. Lin, Performance of gelledtype dye-sensitized solar cells associated with glass transition temperature of the gelatinizing polymers, Eur. Polym. J. 44 (3) (2008) 608–614. [31] A. Azmar, M.D. Rozana, T. Winie, Characterization of PMA-TPAI and PVAc-TPAI solid polymer electrolytes and application in dye-sensitized solar cell, J. Appl. Polym. Sci. 135 (47) (2018) 46835. [32] T. Winie, A. Azmar, M.D. Rozana, Ionic liquid effect for efficiency improvement in poly(methyl acrylate)/poly(vinyl acetate)-based dye-sensitized solar cells, High Perform. Polym. 30 (8) (2018) 937–948. [33] A. Azmar, T. Winie, Development of PMA/PVAc-TPAI-BMII solid polymer electrolytes for application in dye-sensistized solar cell, E3S Web of Conferences, vol. 67, 201803032. [34] H.J. Woo, A.K. Arof, Vibrational studies of flexible solid polymer electrolyte based on PCL–EC incorporated with proton conducting NH4SCN, Spectrochim. Acta A Mol. Biomol. Spectrosc. 161 (2016) 44–51. [35] T. Winie, A.K. Arof, Impedance spectroscopy: basic concepts and application for electrical evaluation of polymer electrolytes, in: C.H. Chan, C.H. Chia, S. Thomas (Eds.), Physical Chemistry of Macromolecules, Apple Academic Press Inc., USA, 2014, pp. 333–363. [36] X. Qian, N. Gu, Z. Cheng, X. Yang, E. Wang, S. Dong, Impedance study of (PEO)10LiClO4-Al2O3 composite polymer electrolyte with blocking electrodes, Electrochim. Acta 46 (12) (2001) 1829–1836. [37] F.H. Muhammad, R.H.Y. Subban, T. Winie, Solid solutions of hexanoyl chitosan/ poly(vinyl chloride) blends and NaI for all-solid-state dye-sensitized solar cells, Ionics 25 (7) (2019) 3373–3386.
4. Conclusions Both FTIR and DSC studies suggest interactions exist between the components in the PMA/PVAc-TPAI-BMII-EC. Incorporation of EC increases the conductivity of PMA/PVAc-TPAI-BMII. EC increases the conductivity by 1) assists the dissociation of TPAI and BMII, which allows greater numbers of ions for conduction and 2) decreases the Tg of the polymer, which increases the segmental flexibility of polymer chain and hence facilitates the ions movement. Presence of EC has enhanced the efficiency of the DSSC. The highest efficiency of 9.67% is obtained with no flow gel-like electrolyte containing WEC = 0.20. The efficiency enhancement is due to the increase in short circuit current density, arising from the conductivity enhancement brought about by the EC. The effect of PMA/PVAc on the performance of DSSC is also discussed. DSSC without PMA/PVAc shows higher efficiency than DSSC with PMA/PVAc. This is because PMA/PVAc decreases the ion motion in the electrolyte. Although DSSC with PMA/PVAc exhibits lower efficiency, but it maintains 63% of its efficiency after 288 h as compared to 40% of efficiency retention for DSSC without PMA/PVAc. The PMA/PVAc improves the stability of DSSC by suppressing the recombination loss as evidenced from the impedance studies. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The authors wish to thank the Ministry of Education Malaysia for supporting this work through FRGS/1/2018/STG07/UITM/02/12. References [1] B. O'Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films, Nature 353 (1991) 737–740. [2] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Peteerson, Dye-sensitized solar cells, Chem. Rev. 110 (11) (2010) 6595–6663. [3] T. Stergiopoulos, E. Rozi, C.S. Karagianni, P. Falaras, Influence of electrolyte coadditives on the performance of dye-sensitized solar cells, Nanoscale Res. Lett. 6 (2011) 307. [4] S. Venkatesan, I.P. Liu, J.C. Lin, M.H. Tsai, H. Teng, Y.L. Lee, Highly efficient quasisolid-state dye-sensitized solar cells using polyethylene oxide (PEO) and poly(methyl methacrylate) (PMMA)-based printable electrolytes, J. Mater. Chem. A 6 (21) (2018) 10085–10094. [5] J. Shi, S. Peng, J. Pei, Y. Liang, F. Cheng, J. Chen, Quasi-solid-state dye-sensitized solar cells with polymer gel electrolyte and triphenylamine-based organic dyes, ACS Appl. Mater. Interfaces 1 (4) (2009) 944–950. [6] M.H. Buraidah, S. Shah, L.P. Teo, F.I. Chowdhury, M.A. Careem, I. Albinsson, B.E. Mellander, A.K. Arof, High efficient dye sensitized solar cells using phthaloylchitosan based gel polymer electrolytes, Electrochim. Acta 245 (2017) 846–853. [7] S.A. Sapp, C.M. Elliott, C. Contado, S. Caramori, C.A. Bignozzi, Substituted polypyridine complexes of cobalt(II/III) as efficient electron-transfer mediators in dyesensitized solar cells, J. Am. Chem. Soc. 124 (37) (2002) 11215–11222. [8] Z.S. Wang, K. Sayama, H. Sugihara, Efficient eosin Y dye-sensitized solar cell containing Br-/Br3- electrolyte, J. Phys. Chem. B 109 (47) (2005) 22449–22455. [9] G. Oskam, B.V. Bergeron, G.J. Meyer, P.C. Searson, Pseudohalogens for dye-sensitized TiO2 photoelectrochemical cells, J. Phys. Chem. B 105 (29) (2001) 6867–6873. [10] G.K.R. Senadeera, A.M.J.S. Weerasinghe, M.A.K.L. Dissanayake, C.A. Thotawatthage, A five-fold efficiency enhancement in dye sensitized solar cells fabricated with AlCl3 treated, SnO2 nanoparticle/nanofibre/nanoparticle triple layered photoanode, J. Appl. Electrochem. 48 (11) (2018) 1255–1264. [11] M.A. Waghmare, M. Naushad, H.M. Pathan, A.U. Ubale, Rose bengal-sensitized ZrO2 photoanode for dye-sensitized solar cell, J. Solid State Electrochem. 21 (9) (2017) 2719–2723. [12] M.A.K.L. Dissanayake, H.N.M. Sarangika, G.K.R. Senadeera, H.K.D.W.M.N.R. Divarathna, E.M.P.C. Ekanayake, Application of a nanostructured, tri-layer TiO2 photoanode for efficiency enhancement in quasi-solid electrolyte-
8
Optical Materials 96 (2019) 109349
A. Azmar, et al.
[38] M.F. Shukur, R. Ithnin, M.F.Z. Kadir, Electrical characterization of corn starchLiOAc electrolytes and application in electrochemical double layer capacitor, Electrochim. Acta 136 (2014) 204–216. [39] T. Winie, C.H. Chan, R.H.Y. Subban, Ac conductivity and dielectric properties of hexanoyl chitosan-LiClO4-TiO2 composite polymer electrolytes, Adv. Mater. Res. 335–336 (2011) 873–880. [40] S.K. Tripathi, A. Gupta, M. Kumari, Studies on electrical conductivity and dielectric behaviour of PVdF–HFP–PMMA–NaI polymer blend electrolyte, Bull. Mater. Sci. 35 (6) (2012) 969–975. [41] C.Y. Tan, N.K. Farhana, N.M. Saidi, S. Ramesh, K. Ramesh, Conductivity, dielectric studies and structural properties of P(VA-co-PE) and its application in dye sensitized solar cell, Org. Electron. 56 (2018) 116–124. [42] M.F. Shukur, R. Ithnin, M.F.Z. Kadir, Ionic conductivity and dielectric properties of potato starch-magnesium acetate biopolymer electrolytes: the effect of glycerol and 1-butyl-3-methylimidazolium chloride, Ionics 22 (7) (2016) 1113–1123. [43] H. Wang, L.M. Peter, Influence of electrolyte cations on electron transport and electron transfer in dye-sensitized solar cells, J. Phys. Chem. C 116 (19) (2012) 10468–10475. [44] M. Fathy, A.B. Kashyout, J. El Nady, Sh Ebrahim, M.B. Soliman, Electrospun polymethylacrylate nanofibers membranes for quasi-solid-state dye sensitized solar cells, Alexandria Eng. J. 55 (2) (2016) 1737–1743. [45] H. Yang, M. Huang, J. Wu, Z. Lan, S. Hao, J. Lin, The polymer gel electrolyte based on poly(methyl methacrylate) and its application in quasi-solid-state dye-sensitized solar cells, Mater. Chem. Phys. 110 (1) (2008) 38–42. [46] Y. Ren, Z. Zhang, S. Fang, M. Yang, S. Cai, Application of PEO based gel network
[47]
[48]
[49]
[50]
[51]
[52]
9
polymer electrolytes in dye-sensitized photoelectrochemical cells, Sol. Energy Mater. Sol. Cells 71 (2) (2002) 253–259. S. Agarwala, L.N.S.A. Thummalakunta, C.A. Cook, C.K.N. Peh, A.S.W. Wong, L. Ke, G.W. Ho, Co-existence of LiI and KI in filler-free, quasi-solid-state electrolyte for efficient and stable dye-sensitized solar cell, J. Power Sources 196 (3) (2011) 1651–1656. Y. Cui, X. Zhang, J. Feng, J. Zhang, Y. Zhu, Enhanced photovoltaic performance of quasi-solid-state dye-sensitized solar cells by incorporating a quaternized ammonium salt into poly(ethylene oxide)/poly(vinylidene fluoride-hexafluoropropylene) composite polymer electrolyte, Electrochim. Acta 108 (2013) 757–762. W.M.N.M.B. Wanninayake, K. Premaratne, G.R.A. Kumara, R.M.G. Rajapakse, Use of lithium iodide and tetrapropylammonium iodide in gel electrolytes for improved performance of quasi-solid-state dye-sensitized solar cells: recording an efficiency of 6.40%, Electrochim. Acta 191 (2016) 1037–1043. E. Chatzivasiloglou, T. Stergiopoulos, A.G. Kontos, N. Alexis, M. Prodromidis, P. Falaras, The influence of the metal cation and the filler on the performance of dye-sensitized solar cells using polymer-gel redox electrolytes, J. Photochem. Photobiol. A Chem. 192 (1) (2007) 49–55. H.T. Chou, H.C. Hsu, C.H. Lien, S.T. Chen, The effect of various concentrations of PVDF-HFP polymer gel electrolyte for dye-sensitized solar cell, Microelectron. Reliab. 55 (11) (2015) 2174–2177. K. Kakiage, Y. Aoyama, T. Yano, K. Oya, J.I. Fujisawa, M. Hanaya, Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes, Chem. Commun. 51 (88) (2015) 15894–15897.
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Improved long-term stability of dye-sensitized solar cell employing PMA/ PVAc based gel polymer electrolyte
T
Amisha Azmara, R.H.Y. Subbana,b, Tan Winiea,b,∗ a b
Faculty of Applied Sciences, Universiti Teknologi MARA, 40450, Shah Alam, Selangor, Malaysia Institute of Science, Universiti Teknologi MARA, 40450, Shah Alam, Selangor, Malaysia
A R T I C LE I N FO
A B S T R A C T
Keywords: PMA PVAc Gel polymer electrolyte Dye-sensitized solar cells Stability
Gel polymer electrolytes based on blends of poly(methyl acrylate) (PMA) and poly(vinyl acetate) (PVAc) were prepared using tetrapropylammonium iodide (TPAI) salt, 1-butyl-3-methylimidazolium iodide (BMII) ionic liquid and ethylene carbonate (EC) plasticizer. The mass fraction of EC, WEC was varied while the masses of the other components were kept constant in order to study the dependence of dye-sensitized solar cell (DSSC) performance on EC concentration. Incorporation of EC in PMA/PVAc-TPAI-BMII has enhanced the efficiency of the DSSC. The efficiency enhancement is due to the increase in short circuit current density, Jsc, arising from the conductivity enhancement brought about by the EC. A DSSC with TPAI-BMII-EC liquid electrolyte was fabricated and its photovoltaic and stability performance were investigated and compared with DSSC with PMA/PVAcTPAI-BMII-EC gel electrolyte. PMA/PVAc decreases the Jsc from 27.87 to 22.91 mA cm−2 and efficiency, η from 12.28 to 9.67%. Linear sweep voltammetry studies reveal that PMA/PVAc decreases the ion motion in the electrolyte. Although PMA/PVAc deteriorates the performance of DSSC, but it improves the stability performance of DSSC by suppressing the recombination loss as evidenced from the increase in charge transfer resistance at the TiO2 electrode and longer electron recombination lifetime.
1. Introduction Dye-sensitized solar cell (DSSC), first introduced by O'Regan and Grätzel in 1991 [1] is deemed as a candidate for the next generation solar cell, owing to their advantages such as ease of fabrication, design opportunity and flexibility, lightweight, better performance at diffuse light and higher temperatures as well as possibility for indoor application [2]. A DSSC consists of three main components i.e. dye-absorbed photoanode, electrolyte and counter electrode. The photo-electrochemical nature of DSSC allows its efficiency to be optimized by varying the chemical composition of its components [3]. Efficiency of DSSC can be improved by modifying the sensitizer [4–6] and redox complexes [7–9], choosing the photoanode [10–12] and counter electrode [13–15], optimizing the electrolyte component [16–18] as well as introducing additives to fine-tune the semiconductor-electrolyte interface [3,19,20]. Electrolyte in DSSC serves as a medium for transportation of redox ions. Liquid and gel electrolytes are commonly used in DSSCs. Gel electrolyte can be prepared by incorporating a polymer into a liquid electrolyte. The polymeric property of gel electrolyte gives it advantages over liquid electrolytes in terms of leak- and corrosion-proof,
∗
as well as minimization of electrolyte evaporation. In addition, gel polymer electrolyte provides better electrolyte-electrode contact and higher conductivity than solid-state polymer electrolyte. Many types of polymers have been used for gel electrolytes in DSSCs such as poly (ethylene oxide) (PEO) [21–23],poly(methyl methacrylate) (PMMA) [24–26], poly(acrylonitrile) (PAN) [27,28], poly(methyl acrylate) (PMA) [5,29], poly(vinyl acetate) (PVAc) [25,30] and etc. In our earlier works, PMA-tetrapropylammonium iodide (TPAI) and PVAc-TPAI solid polymer electrolytes have been prepared and tested in DSSCs [31]. Then, 1-butyl-3-methylimidazolium iodide (BMII) has been incorporated into PMA/PVAc-TPAI. The effects of BMII on the structural, thermal and conductivity properties of PMA/PVAc-TPAI have been discussed in [32]. In the same work [32], the efficiencies achieved for DSSC with and without BMII were 4.62% and 2.79%, respectively. In another work [33], we reported the dielectric properties and conduction mechanism of PMA/PVAc-TPAI-BMII electrolyte system. The correlated barrier hopping model was used to interpret its conduction mechanism. In the work of [33], the dependence of DSSC performance on BMII concentration was discussed, which was not found in [32]. As a continuation, in this work, we have incorporated ethylene carbonate (EC) into PMA/PVAc-TPAI-BMII to form gel-like polymer electrolyte.
Corresponding author. Faculty of Applied Sciences, Universiti Teknologi MARA, 40450, Shah Alam, Selangor, Malaysia. E-mail address: [email protected] (T. Winie).
https://doi.org/10.1016/j.optmat.2019.109349 Received 26 July 2019; Received in revised form 22 August 2019; Accepted 22 August 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
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concentration of iodide, I−. For DSSC assembly, a small amount of the electrolyte was sandwiched between the TiO2 photoanode and the Pt counter electrode. The current-voltage (J-V) characteristics and impedance measurements of the DSSCs were measured under illumination of 100 mWm−2 simulated sunlight (AM1.5), from an Oriel Newport LCS-100 solar simulator, with a Metrohm Autolab potentiostat (PGSTAT128 N). The effective area of the cell was 0.20 cm2. The conversion efficiency, η of the DSSC was calculated from the short-circuit current density, Jsc, the open circuit voltage, Voc, the fill factor, ff and the power of incident light, Pin.
The effects of EC on the Tg and conductivity are reported. FTIR reveals the interactions among components in PMA/PVAc-TPAI-BMII-EC. Many DSSCs with polymer electrolytes are reported in the literature. However, the effect of polymer on the stability of DSSC is seldom highlighted. In this work, the photovoltaic and stability performance of DSSCs assembled with TPAI-BMII-EC liquid electrolyte and PMA/PVAcTPAI-BMII-EC gel electrolyte are investigated and compared. This paper sheds some lights on the long-term stability improvement brought about by the PMA/PVAc. 2. Experimental
η= 2.1. Polymer electrolyte preparation and characterization
Jsc Voc ff Pin
(1)
The impedance measurements of the DSSCs were carried out over a frequency range of 0.1 Hz–1 MHz and the AC modulation signal was 0.5 V. All impedance measurements applied a potential bias equal to the Voc.
PMA in toluene, PVAc, TPAI, BMII and EC were purchased from Sigma-Aldrich. Prior to use, PMA was purified with methanol and the precipitation was dried. PMA/PVAc-TPAI-BMII electrolytes were prepared according to our previous work [32]. EC was added to the highest conducting sample [32]. The mass fraction of EC, WEC was varied from 0.10 to 0.40 while the masses of PMA/PVAc, TPAI and BMII were kept constant. Composition of PMA and PVAc was fixed at 90:10. ATR-FTIR spectroscopic studies were carried out using a Thermo Fisher Scientific Nicolet iS10 spectrophotometer (USA). The FTIR spectra were recorded in the frequency range of 600–4000 cm−1 with a resolution of 2 cm−1. The glass transition temperatures, Tgs were determined by a TA Q200 DSC. Samples weighing around 10 mg were tested in crimped aluminum pans at an underlying heating rate of < q > = 10 °C min−1 over a temperature range between −10 and 80 °C. Conductivities at different temperatures from room temperature to 363 K were determined from the impedance measurements using a HIOKI 3532-50 LCR Hi-tester impedance spectrometer in the frequency range of 50 Hz - 1 MHz. The diffusion coefficient of triiodide ions, DI3was determined by measuring the triiodide limiting current using linear sweep voltammetry (LSV). The electrolyte was sandwiched between two platinum (Pt) electrodes. A spacer of 0.13 mm thickness and 0.20 cm2 hole area was used for the fabrication of symmetrical cell. The cell was polarized from −0.60 to +0.60 V at a rate of 10 mV s−1.
3. Results and discussion 3.1. Electrolyte characterization 3.1.1. FTIR analysis Fig. 1 shows the overlaid FTIR spectra of EC, PMA/PVAc blend and PMA/PVAc-EC at WEC = 0.20. The assignment of peaks of PMA/PVAc blend and EC are based on the literature [32,34]. For PMA/PVAc blend, the O–CH3 stretching, ν(OCH3) peaks are observed at 1164 and 1198 cm−1. Peaks observed at 1240, 1436 and 1733 cm−1 are corresponding to the C–O–C symmetric stretching, νs(COC), CH3 symmetric bending, δs(CH3) and C]O stretching, ν(C]O), respectively. The CH3 symmetric, νs(CH3), CH3 asymmetric stretching, νa(CH3) and C–H asymmetric stretching, νa(CH) are observed at 2849, 2924 cm−1 and 2955 cm−1, respectively. Characteristic peaks exhibited by EC are observed at 1069 cm−1 (C–O stretching, ν(C–O)), 1159 cm−1 (C–O symmetric stretching, νs(C–O)), 1392 (CH2 wagging, ω(CH2)), 1772 and 1798 cm−1 (C]O stretching, ν(C]O)). Mixing EC and PMA/PVAc has resulted in changes of the peaks for both EC and PMA/PVAc. The ν(C–O) of EC gets shifted from 1069 to 1073 cm−1. The ν(C]O) of EC at 1772 and 1798 cm−1 are found to shift to 1775 and 1802 cm−1, respectively, with decrease in their relative intensities. The 1164 cm−1 of PMA/PVAc has superimposed with the 1159 cm−1 of EC to form an intense peak at 1162 cm−1. On the other hand, the ν(C]O) of PMA/PVAc at 1733 cm−1 shows no appreciable deviation from its position, but with decrease in its relative intensity. These observations indicate that interaction has occurred between EC and PMA/PVAc. Figs. 2 and 3 show the evidence of interaction between EC with TPAI and BMII. Table 1 summarizes the EC-TPAI and EC-BMII
2.2. DSSC assembly and characterization Titanium dioxide (TiO2) with average particle size of ~16 nm (P90) and ~21 nm (P25) were acquired from Evonik Industries. Fluorinedoped tin oxide (FTO) conducting glass, Pt solution and cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium (II) dye solution (N3) were obtained from Solaronix. Carbowax, Triton X-100, 4-tert-butylpyridine (TBP) and guanidinium thiocyanate (GuNCS) were from Sigma-Aldrich. Nitric acid and iodine (I2) chips were purchased from Friedemann Schmidt Chemical and Amcochemie-Humburg, respectively. TiO2 photoanodes were prepared on FTO glass coated with two layers of TiO2. The first TiO2 layer was prepared by grinding 0.5 g of P90 TiO2 in 2 mL of nitric acid (pH = 1) for 30 min and followed by spin-coating on FTO glass. The spin-coated FTO glass was then sintered at 450 °C for about 30 min. For the second TiO2 layer, 0.5 g of P25 TiO2, 0.1 g of carbowax and 2 drops of Triton X-100 were ground in 2 mL of nitric acid (pH = 1) for 30 min. The P25 TiO2 slurry obtained was doctor bladed on the P90 TiO2-coated FTO glass followed by sintering at 450 °C for 30 min. After cooling down to room temperature, the TiO2 photoanodes were soaked in a 3 mM N3 dye solution for 24 h. The N3 dye-coated photoanodes were then immersed in 0.5 M TBP and 0.1 M GuNCS in acetonitrile for 15 s. The Pt counter electrodes were prepared by spin-coating a Pt solution on FTO glass and sintered at 450 °C for 30 min. Iodine chips were added to the electrolytes and stirred until homogeneous. The concentration of iodine was one tenth of the
Fig. 1. Overlaid FTIR spectra of EC, PMA/PVAc and PMA/PVAc-EC. 2
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Table 1 FTIR peak assignment for PMA/PVAc, EC, EC-TPAI and EC-BMII. Component
Wavenumber (cm−1)
Peak Assignment
PMA/PVAc (90:10)
1164, 1198 1240 1436 1733 2849 2924 2955
ν(OCH3) νs(COC) δs(CH3) ν(C]O) νs(CH3) νa(CH3) νa(CH)
EC
1069 1159 1392 1481 1772, 1798
ν(C–O) νs(C–O) ω(CH2) δ(CH2) ν(C]O)
EC-TPAI
1077
ν(C–O) (shifted from 1069 cm−1 of EC) νs(C–O) (shifted from 1159 cm−1 of EC) ν(C]O) (shifted from 1772 cm−1 of EC) ν(C]O) (shifted from 1798 cm−1 of EC)
1164
Fig. 2. Overlaid FTIR spectra of EC and EC-TPAI.
1777 1807
EC-BMII
ν(C–O) (shifted from 1069 cm−1 of EC) νs(C–O) (shifted from 1159 cm−1 of EC) ν(C]O) (shifted from 1772 cm−1 of EC) ν(C]O) (shifted from 1798 cm−1 of EC)
1075 1163 1776 1803
Fig. 3. Overlaid FTIR spectra of EC and EC-BMII.
interactions. Upon incorporation of TPAI or BMII, the characteristic peaks of EC are observed to experience appreciable changes in their vibrational frequencies and intensities. 3.1.2. Tg analysis The interaction between EC and PMA/PVAc-TPAI-BMII is supported from the Tg studies. The Tg of PMA/PVAc-TPAI-BMII was recorded as 22.6 °C [32]. Fig. 4 illustrates that Tg decreases with increasing WEC. Incorporation of EC leads to the formation of TPAI-EC and BMII-EC complexes, which reduces the fraction of PMA/PVAc-TPAI and PMA/ PVAc-BMII complexes. The reduced fraction of PMA/PVAc-TPAI and PMA/PVAc-BMII complexes increases the flexibility of polymer chains. The increase in chain flexibility is reflected in a decrease in Tg. EC molecules that remain between the adjacent polymer chains, reduce the polymer-polymer chain interaction. Hence, the easy movement of polymer chain increases the free volume, which facilitates the movement of ions.
Fig. 4. Tg values of PMA/PVAc-TPAI-BMII with respect to WEC.
connected in series with another CPE [35]. The CPE is a “leaky capacitor”. The use of a CPE instead of a capacitor is to compensate for the inhomogeneity of electrolyte, roughness of the electrolyte-electrode interface and etc [36]. The real, Zr and imaginary, Zi parts of the impedance associated to the equivalent circuit can be expressed as [37]:
Zr =
3.1.3. Impedance analysis The Nyquist plots for PMA/PVAc-TPAI-BMII containing WEC = 0.20 at various temperatures are shown in Fig. 5. The Nyquist plots for temperatures below 363 K consist of a semicircular spur at high frequency followed by a tilted spike at low frequency. Thus, the Nyquist plots of Fig. 5 (a)–(c) can be represented by a parallel combination of bulk resistance, Rb and constant phase element (CPE) that are
1 Rb 1 ⎡R ⎣ b
+ Q1
ωα1
+ Q1 ωα1 cos
cos
2
( ) ⎤⎦ α1 π 2
( ) α1 π 2
+ ⎡Q1 ⎣
ωα1
sin
2
( ) ⎤⎦ α1 π 2
+
1 απ cos ⎛ 2 ⎞ Q2 ωα2 ⎝ 2 ⎠ (2)
and
3
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Fig. 6. Frequency dependence of dielectric constant of PMA/PVAc-TPAI-BMII with WEC = 0.20 at (a) 303 K, (b) 313 K, (c) 323 K, (d) 333 K, (e) 343 K, (f) 353 K and (g) 363 K. Fig. 5. Nyquist plots and their corresponding fitted lines for PMA/PVAc-TPAIBMII containing WEC = 0.20 at (a) 303 K, (b) 313 K, (c) 333 K and (d) 363 K.
Q1 ωα1 sin Zi =
1
⎡ R + Q1 ωα1 cos ⎣ b
2
( ) ⎤⎦ α1 π 2
( ) α1 π 2
+ ⎡Q1 ωα1 sin ⎣
( ) α1 π 2
σ=
1 απ cos ⎛ 2 ⎞ Q2 ωα2 ⎝ 2 ⎠
Q=
(4)
1 απ sin ⎛ 2 ⎞ Q2 ωα2 ⎝ 2 ⎠
Table 2 Circuit parameters Rb, Q and α for PMA/PVAc-TPAI-BMII containing WEC = 0.20 at various temperatures.
303 313 323 333 343 353 363
Rb (Ω) 5210 1030 440 302 110 55 16
Q1 (F)
Q2 (F) −11
8.5 × 10 7.0 × 10−9 1.0 × 10−8 3.1 × 10−8 7.6 × 10−8 8.4 × 10−8 –
−6
7.3 × 10 2.0 × 10−5 2.8 × 10−5 3.3 × 10−5 5.2 × 10−5 1.1 × 10−4 1.9 × 10−4
α1 (rad)
α2 (rad)
0.91 0.64 0.71 0.63 0.62 0.63 –
0.84 0.84 0.84 0.83 0.83 0.81 0.70
(7)
3.1.4. Conductivity analysis Fig. 7 depicts the temperature dependence of conductivity for PMA/ PVAc-TPAI-BMII containing various WEC. It is observed that conductivities of all samples increase with increasing temperature and at comparable temperature, conductivity increases with WEC. However, no further addition of WEC beyond 0.40 because the electrolyte has become liquid-like for WEC > 0.20. Enhancement of conductivity by temperature can be attributed to the increment of the number of free ions by temperature [cf. section 3.1.3]. The presence of high dielectric constant of EC helps in dissociation of TPAI and BMII. This increases the number of free ions. TPAI contains Pr4N+ cations and I− anions; BMII contains BMI+ cations and I− anions. Pr4N+ (4.59 Å) and BMI+ (3.85 Å) [28] are bulky cations. Movement of bulky ions in the polymer matrix is easily hindered by the entanglement of polymer chains. Hence, contribution to conductivity from Pr4N+ and BMI+ cations is minimal. The contribution to conductivity is mostly from I− anions. Conductivity enhancement can also be attributed to the plasticizing effect of EC as Tg is observed to decrease upon incorporation of EC (cf. Fig. 4). Increased chain flexibility facilitates the movement of ions. In addition, EC maintains a gel-like state within the polymer matrix. This allows the diffusive transport
(5)
Equations (2) and (3) were used to fit the Nyquist plots of Fig. 5 (a)–(c) whereas equations (4) and (5) were used to fit the Nyquist plots of Fig. 5 (d). Table 2 lists the values of Rb, Q1, Q2, α1 and α2. It is observed that the value of Rb decreases with increasing temperature. This indicates an increment in conductivity, σ with temperature because σ is inversely proportional to Rb according to equation (6).
Temperature (K)
εo εr A t
where εo is the permittivity of free space, A and t have their usual meanings. Many studies reported that the value of εr decreases with increasing frequency [39–41]. The lower value of εr at high frequency results in lower value of Q at high frequency. This explains the lower value of Q1 as compared with Q2. The value of Q is found to increase with temperature. This is because the value of εr increases with temperature as shown in Fig. 6. The εr represents the number of free ions stored in a sample [42]. The increase of εr with temperature indicates that the number of free ions increases with temperature.
and
Zi =
(6)
where t is the electrolyte thickness and A is the electrolyte-electrode contact area. The variation of conductivity with respect to temperature will be discussed further in section 3.1.4. Again from Table 2, the values of Q2 are found higher than Q1. Capacitance, Q is directly proportional to the dielectric constant, εr of a material as shown in equation (7).
1 απ sin ⎛ 2 ⎞ + 2 Q2 ωα2 2 ⎠ ⎝ ⎤ ⎦ (3)
where Q1 is the capacitance at high frequency and Q2 is the capacitance at low frequency. α1 is the deviation of the diameter of the semicircle from the Zi axis and α2 is the deviation of the spike from the Zr axis. ω is the angular frequency. In contrast, the Nyquist plot at 363 K consist only a steeply rising spike, the semicircular spur completely disappeared (cf. Fig. 5 (d)). The equivalent circuit representation is a Rb in series with a CPE [38]. Hence, the Zr and Zi associated to the equivalent circuit are [38]:
Zr = Rb +
t Rb A
4
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Fig. 8. J-V characteristics of (a) cell A, (b) cell B, (c) cell C, (d) cell D and (e) cell E.
cations do not absorb easily to the TiO2 surface. Thus, bigger value of Voc in the range of 0.70–0.76 V can be observed for the current electrolyte system. The adsorption ability of cations on the TiO2 surface can be reflected in the value of Voc. From Table 3, the Voc is seen to increase slowly with WEC. This means that with increasing WEC, the adsorption ability of cations on the TiO2 surface reduces. This is because, on top of the absorption difficulty of big cations on the TiO2 surface, EC molecules block the cations to the TiO2. Table 4 compares the performance of the DSSC in this work with other polymer electrolyte-based DSSCs. The efficiency obtained in this work is better or on par with the works reported by other researchers. It is worth to highlight here that the efficiency obtained using blend of PMA and PVAc is higher than other PMA- and PVAc-based single polymer electrolytes.
Fig. 7. Temperature dependence of conductivity for PMA/PVAc-TPAI-BMII containing (a) 0.00, (b) 0.10, (c) 0.15, (d) 0.20, (e) 0.25, (f) 0.30, (g) 0.35 and (h) 0.40 of WEC.
properties of liquids.
3.2. DSSC characterization 3.2.1. Dependence of DSSC performance on EC concentration DSSCs with the configuration FTO/TiO2/N3 dye/electrolyte/Pt/ FTO were assembled and Table 3 lists the composition of the electrolyte. Fig. 8 shows the J-V characteristics of the cells and their performance parameters are summarized in Table 3. The efficiency of the cell increases from 4.62% (cell A) to 7.39% (cell B) upon incorporation of WEC = 0.10. The efficiency of the DSSC continues to increase to 11.22% (cell E) as WEC increases to 0.40. It is to note here that no-flow gel-like electrolyte samples are obtained for WEC ≤ 0.20. Electrolyte conductivity is one important factor governing the performance of DSSCs particularly the Jsc and η [18,21,23]. The higher efficiency of DSSCs with EC is due to the higher Jsc value. The Jsc of the DSSCs is found to gradually increase from 10.04 mA cm−2 (cell A) to 25.88 mA cm−2 (cell E). The increase in Jsc is caused by the conductivity enhancement brought about by the EC. The roles of EC as the conductivity enhancer are 1) assists the dissociation of TPAI and BMII, which allows greater numbers of ions for conduction and 2) decreases the Tg of the polymer, which increases the segmental flexibility of polymer chain and hence facilitates the ions movement. Adsorption of cations on the TiO2 surface shifts the Fermi level, EF of TiO2 towards the redox potential of I−/I3− and hence results in smaller value of Voc [28,43]. For electrolyte with big cations such as Pr4N+ and BMI+, shift of EF of TiO2 towards the redox potential is less as the big
Table 4 Performance of various polymer electrolyte-based DSSCs.
WEC
Voc (V)
Jsc (mA cm−2)
ff (%)
η (%)
A B C D E
0.00 0.10 0.20 0.30 0.40
0.70 0.72 0.73 0.74 0.76
10.04 16.71 22.91 23.87 25.88
66 61 58 58 59
4.62 7.39 9.67 10.18 11.22
Sensitizer
η (%)
References
PVAc-TBAI-PMII-EC/PV/AN-I2 PVAc-LiI-TBP-AN-I2 PMA-LiI-TBP-I2 PMA-LiI-TBP-AN-I2 PMA/PEG-LiI-EC/PC-I2 PMA/PVDF-LiI-TBP-I2 PMA/PVDF/PEG- TBP-I2 PMMA-EC/PC/DMC-NaI-I2
N3 N3 N3 N3 Organic dye TC4 N719 N719 cis-[(dcbH2)2 Ru (SCN)2] N3 N719 N3 N719 N719
5.74 5.62 1.40 7.17 3.77 6.60 7.00 4.80
[25] [30] [44] [30] [5] [29] [29] [45]
5.51 8.48 4.90 5.50 3.45
[25] [4] [21] [24] [26]
N719 N3 N719 N3
3.6 5.75 5.8 7.5
[46] [25] [47] [6]
N3 N719 N719 N3 N3 N3
6.26 5.47 5.18 3.7 1.4 9.67
[18] [23] [16] [28] [27] This work
PMMA-TBAI-PMII-EC/PV/AN-I2 PMMA/PEO-LiI-DMII-TBP-I2 PMMA/PEO-LiI-TBP-EC/PCI2 PMMA/PVA-LiI-MPII-PEG-I2 PMMA/PVDF-EC-TBAI-I2 PEO-EC/PC/DMC-NaI-I2 PEO-TBAI-PMII-EC/PV/AN-I2 PEO-LiI-KI- I2 PEO/PVA-DMSO-EC-TBAI- BMIII2 PEO/PVA-TBAI-LiL-EC-DMSO-I2 PEO/PEG-KI-TiO2-I2 PAN-MgI2-TPAI-EC-PC-I2 PAN–NH4I–I2 PAN-LiBOB-BMII-I2 PMA/PVAc-TPAI-BMII-EC
Table 3 Performance parameters for DSSCs assembled with PMA/PVAc-TPAI-BMII-EC electrolytes. Cell
Gel polymer electrolyte
Notes: N3(Cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)rutheN719 (Di-tetranium(II) [Ru(4,4′-dicarboxy-2,2′-bipyridine)2(NCS)2]), butylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium(II)). 5
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Fig. 9. J-V characteristics of (a) cell C and (b) cell F. Table 5 Performance parameters for cells C and F. Cell
Voc (V)
Jsc (mA cm−2)
ff (%)
η (%)
DI3- (cm2 s−1)
C F
0.73 0.77
22.91 27.87
58 60
9.67 12.28
6.7 × 10−7 2.4 × 10−6
Notes: Electrolyte for cell C: PMA/PVAc-TPAI-BMII-EC at WEC = 0.20 gel polymer electrolyte; electrolyte for cell F: TPAI-BMII-EC liquid electrolyte.
Fig. 11. Stability studies of DSSCs (a) as-fabricated, (b) after 96 h, (c) after 120 h and (d) after 288 h. Table 6 Performance parameters for the as-fabricated and aged DSSCs. Cell
Time duration
Voc (V)
Jsc (mA cm−2)
ff (%)
η (%)
Rct (Ω cm2)
τe (ms)
C
As-fabricated After 96 h After 120 h After 288 h
0.73 0.75 0.73 0.71
22.91 18.11 15.15 12.14
58 61 71 69
9.67 8.24 7.69 6.05
653 597 531 498
23.27 18.45 9.34 5.97
F
As-fabricated After 96 h After 120 h After 288 h
0.77 0.73 0.70 0.67
27.87 22.91 14.20 11.75
60 58 60 58
12.28 9.67 6.13 4.92
608 558 512 435
22.1 15.9 8.24 4.27
Fig. 10. Linear sweep voltammograms of (a) TPAI-BMII-EC liquid electrolyte and (b) PMA/PVAc-TPAI-BMII-EC gel polymer electrolyte (WEC = 0.20).
3.2.2. Performance of DSSC with and without PMA/PVAc The J-V characteristics of cells C and F are presented in Fig. 9. Cell F contains TPAI-BMII-EC-I2 liquid electrolyte, without PMA/PVAc. The amounts of TPAI, BMII, EC and I2 in cell F are the same as in cell C. Table 5 presents the effect of PMA/PVAc on the photovoltaic performance of DSSCs. Cell F without PMA/PVAc shows a higher η of 12.28% and Jsc of 27.87 mA cm−2 as compared to cell C with PMA/PVAc. During regeneration of dye sensitizer, S, the oxidized sensitizer molecules, S+ are reduced by iodide, I− in the electrolyte by
2S+ + 3I − → 2S + I3−
determining the performance of DSSC. The LSV technique can be used to study the redox reaction of I−/I3− couple [6,48–51]. When voltage is applied, current will be produced as a result of the diffusion of I− and I3− ions in the electrolyte. The current increases with voltage until a steady-state current is achieved. Fig. 10 shows the LSV curves for cells C and F. For both cells, the currents saturate above ~0.3 V for both polarities. Both anodic and cathodic limiting current plateaus are quite similar. This indicates the equilibrium steady-state condition. For our electrolytes, the iodide concentration, I− is in excess over the concentration of iodine (10:1). Hence, the I3− ions are the limiting current
(8)
At the Pt counter electrode, the triiodide ion, I3− recombines with the electron and reduces to I−:
I3− + 2e− → 3I − −
(9) −
Thus, I and I3 ions in the electrolyte are playing important role in 6
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Fig. 12. Nyquist plots of DSSCs (a) as-fabricated, (b) after 96 h, (c) after 120 h and (d) after 288 h.
ions. The diffusion of triiodide ions, DI3- can be calculated from the limiting current, Jlim using the following equation [48–50]:
D I3− =
Jlim d 2nFCI3−
(10)
where d is the thickness of the cell and n = 2 is the number of electron required for the reduction of I3− to I− (cf. Equation (9)). The symbol F is the Faraday constant and CI3− is the concentration of I3− per unit volume. The calculated values of DI3- for cells C and F are listed in Table 5. The values of DI3- obtained for the liquid and gel electrolytes in this work are comparable with that reported in the literature [6,48–51]. Cell C shows lower value of DI3- than cell F. This is because PMA/PVAc in cell C decreases the motion of I−/I3− ions in the electrolyte. As a result, presence of PMA/PVAc decreases the Jsc from 27.87 to 22.91 mA cm−2, which leads to the decrease in η from 12.28 to 9.67%.
Fig. 13. Bode plots of DSSCs (a) as-fabricated, (b) after 96 h, (c) after 120 h and (d) after 288 h.
cell C can be due to 1) minimal evaporation of liquid electrolyte due to entrapment of liquid electrolyte by the PMA/PVAc, and 2) reduction of charge recombination by blocking the redox ions to the TiO2 electrode by the PMA/PVAc. The kinetics of charge recombination can be elucidated by the impedance studies of DSSC. Fig. 12 shows the Nyquist plots for the as-fabricated and aged DSSCs. There are two semi-circles in the plots. The resistance of the second semicircle is the charge transfer resistance at the TiO2 electrode, Rct [15,31]. As seen from Table 6, the values of Rct for cells C and F after 288 h are 498 and 435 Ω, respectively. Cell C exhibits higher value of Rct indicates higher charge transfer resistance at the TiO2 electrode, which implies lower charge recombination. Presence of PMA/PVAc suppresses the recombination loss and hence improves the performance of the cell with respect to time. Electron recombination lifetime, τe can be obtained from the Bode plots of Fig. 13. The τe can be calculated from
3.2.3. Improvement of DSSC stability with PMA/PVAc Liquid electrolyte DSSCs are known to have higher efficiencies than polymer electrolyte-based DSSCs. The highest conversion efficiency of DSSCs with liquid electrolyte has been recorded at ~14% [52]. Liquid electrolytes can offer higher efficiency because they give good conductivity and make good contact with the photoanode. However, they are likely to leak, evaporate and corrode the electrodes, which disrupts the long-term stability of DSSCs. In this section, the stability performance of cell C with PMA/PVAc are investigated and compared with cell F without PMA/PVAc. The J-V characteristics of cells C and F as a function of time are presented in Fig. 11 and the effect of PMA/PVAc on the stability performance of DSSC is summarized in Table 6. For cell F, Jsc at 288 h is 42% of Jsc as-fabricated and η at 288 h is 40% of η as-fabricated. On the other hand, for cell C, Jsc at 288 h is 53% of Jsc as fabricated and η at 288 h is 63% of η as-fabricated. The improved stability performance of
τe =
1 2πfmax
(11)
where fmax is the maximum frequency of the Bode plot. The calculated values of τe are shown in Table 6. At comparable aging duration, the τe are longer with PMA/PVAc. For instance, at 288 h, the values of τe increases from 4.27 to 5.97 ms with PMA/PVAc. The longer τe means 7
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that PMA/PVAc makes the redox ions more difficult to recombine with the excited electron. The τe analysis is in agreement with the Rct analysis.
based dye-sensitized solar cells, J. Appl. Electrochem. 47 (11) (2017) 1239–1249. [13] M. Wu, T. Ma, Platinum-free catalysts as counter electrodes in dye-sensitized solar cells, ChemSusChem 5 (8) (2012) 1343–1357. [14] M.W. Khan, J. Yao, K. Zhang, X. Zuo, Q. Yang, H. Tang, K.M. Ur Rehman, G. Li, M. Wu, K. Zhu, H. Zhang, Facile fabrication of hollow mesosphere of crystalline SnO2 nanoparticles and synthesis of SnO2@SWCNTs@Reduced Graphene Oxide nanocomposite as efficient Pt-free counter electrode for dye-sensitized solar cells, Opt. Mater. 80 (2018) 271–278. [15] M.A. Mingsukang, M.H. Buraidah, M.A. Careem, I. Albinsson, B.E. Mellander, A.K. Arof, Investigation of counter electrode materials for gel polymer electrolyte based quantum dot sensitized solar cells, Electrochim. Acta 241 (2017) 487–496. [16] M.A.K.L. Dissanayake, C.A. Thotawatthage, G.K.R. Senadeera, T.M.W.J. Bandara, W.J.M.J.S.R. Jayasundara, B.E. Mellander, Efficiency enhancement in dye sensitized solar cells based on PAN gel electrolyte with Pr4NI + MgI2 binary iodide salt mixture, J. Appl. Electrochem. 43 (9) (2013) 891–901. [17] R.A. Senthil, J. Theerthagiri, J. Madhavan, K. Murugan, P. Arunachalam, A.K. Arof, Enhanced performance of dye-sensitized solar cells based on organic dopant incorporated PVDF-HFP/PEO polymer blend electrolyte with g-C3N4/TiO2 photoanode, J. Solid State Chem. 242 (2016) 199–206. [18] L.P. Teo, T.S. Tiong, M.H. Buraidah, A.K. Arof, Effect of lithium iodide on the performance of dye sensitized solar cells (DSSC) using poly(ethylene oxide) (PEO)/ poly(vinyl alcohol) (PVA) based gel polymer electrolytes, Opt. Mater. 85 (2018) 531–537. [19] X. Yin, W. Tan, J. Zhang, Y. Lin, X. Xiao, X. Zhou, X. Li, B.R. Sankapal, Synthesis of pyridine derivatives and their influence as additives on the photocurrent of dyesensitized solar cells, J. Appl. Electrochem. 39 (1) (2009) 147–154. [20] C. Zhang, Y. Huang, Z. Huo, S. Chen, S. Dai, Photoelectrochemical effects of guanidinium thiocyanate on dye-sensitized solar cell performance and stability, J. Phys. Chem. C 113 (52) (2009) 21779–21783. [21] E. Aram, M. Ehsani, H.A. Khonakdar, Improvement of ionic conductivity and performance of quasi-solid-state dye sensitized solar cell using PEO/PMMA gel electrolyte, Thermochim. Acta 615 (2015) 61–67. [22] M.H. Buraidah, L.P. Teo, C.M. Au Yong, S. Shah, A.K. Arof, Performance of polymer electrolyte based on chitosan blended with poly(ethylene oxide) for plasmonic dyesensitized solar cell, Opt. Mater. 57 (2016) 202–211. [23] K. Prabakaran, S. Mohanty, S.K. Nayak, Improved electrochemical and photovoltaic performance of dye sensitized solar cells based on PEO/PVDF–HFP/silane modified TiO2 electrolytes and MWCNT/Nafion® counter electrode, RSC Adv. 5 (51) (2015) 40491–40504. [24] D.H. Kim, M.S. Park, D.J. Kim, H.H. Cho, J.H. Kim, Solid polymer electrolyte dyesensitized solar cells with organized mesoporous TiO2 interfacial layer templated by poly(vinyl alcohol)–poly(methyl methacrylate) comb copolymer, Solid State Ion. 300 (2017) 195–204. [25] Y.K. Kim, S.H. Park, W.P. Hwang, M.H. Seo, H.W. Park, Y.W. Jang, M.R. Kim, J.K. Lee, Impedance spectroscopy on dye-sensitized solar cells with a poly(ethylenedioxythiophene):poly(styrenesulfonate) counter electrolyte, J. Korean Phys. Soc. 60 (12) (2012) 2049–2053. [26] J. Theerthagiri, R.A. Senthil, M.H. Buraidah, J. Madhavan, A.K. Arof, Effect of tetrabutylammonium iodide content on PVDF-PMMA polymer blend electrolytes for dye-sensitized solar cells, Ionics 21 (10) (2015) 2889–2896. [27] A.K. Arof, H.K. Jun, L.N. Sim, M.Z. Kufian, B. Sahraoui, Gel polymer electrolyte based on LiBOB and PAN for the application in dye-sensitized solar cells, Opt. Mater. 36 (1) (2013) 135–139. [28] T.M.W.J. Bandara, W.J.M.J.S.R. Jayasundara, M.A.K.L. Dissanayake, M. Furlani, I. Albinsson, B.E. Mellander, Effect of cation size on the performance of dye sensitized nanocrystalline TiO2 solar cells based on quasi-solid state PAN electrolytes containing quaternary ammonium iodides, Electrochim. Acta 109 (2013) 609–616. [29] M. Fathy, J.E. Nady, M. Muhammed, S. Ebrahim, M.B. Soliman, A.E.B. Kashyout, Quasi-solid-state electrolyte for dye sensitized solar cells based on nanofiber PMAPVDF and PMA-PVDF/PEG membranes, Int. J. Electrochem. Sci. 11 (7) (2016) 6064–6077. [30] C.W. Tu, K.Y. Liu, A.T. Chien, C.H. Lee, K.C. Ho, K.F. Lin, Performance of gelledtype dye-sensitized solar cells associated with glass transition temperature of the gelatinizing polymers, Eur. Polym. J. 44 (3) (2008) 608–614. [31] A. Azmar, M.D. Rozana, T. Winie, Characterization of PMA-TPAI and PVAc-TPAI solid polymer electrolytes and application in dye-sensitized solar cell, J. Appl. Polym. Sci. 135 (47) (2018) 46835. [32] T. Winie, A. Azmar, M.D. Rozana, Ionic liquid effect for efficiency improvement in poly(methyl acrylate)/poly(vinyl acetate)-based dye-sensitized solar cells, High Perform. Polym. 30 (8) (2018) 937–948. [33] A. Azmar, T. Winie, Development of PMA/PVAc-TPAI-BMII solid polymer electrolytes for application in dye-sensistized solar cell, E3S Web of Conferences, vol. 67, 201803032. [34] H.J. Woo, A.K. Arof, Vibrational studies of flexible solid polymer electrolyte based on PCL–EC incorporated with proton conducting NH4SCN, Spectrochim. Acta A Mol. Biomol. Spectrosc. 161 (2016) 44–51. [35] T. Winie, A.K. Arof, Impedance spectroscopy: basic concepts and application for electrical evaluation of polymer electrolytes, in: C.H. Chan, C.H. Chia, S. Thomas (Eds.), Physical Chemistry of Macromolecules, Apple Academic Press Inc., USA, 2014, pp. 333–363. [36] X. Qian, N. Gu, Z. Cheng, X. Yang, E. Wang, S. Dong, Impedance study of (PEO)10LiClO4-Al2O3 composite polymer electrolyte with blocking electrodes, Electrochim. Acta 46 (12) (2001) 1829–1836. [37] F.H. Muhammad, R.H.Y. Subban, T. Winie, Solid solutions of hexanoyl chitosan/ poly(vinyl chloride) blends and NaI for all-solid-state dye-sensitized solar cells, Ionics 25 (7) (2019) 3373–3386.
4. Conclusions Both FTIR and DSC studies suggest interactions exist between the components in the PMA/PVAc-TPAI-BMII-EC. Incorporation of EC increases the conductivity of PMA/PVAc-TPAI-BMII. EC increases the conductivity by 1) assists the dissociation of TPAI and BMII, which allows greater numbers of ions for conduction and 2) decreases the Tg of the polymer, which increases the segmental flexibility of polymer chain and hence facilitates the ions movement. Presence of EC has enhanced the efficiency of the DSSC. The highest efficiency of 9.67% is obtained with no flow gel-like electrolyte containing WEC = 0.20. The efficiency enhancement is due to the increase in short circuit current density, arising from the conductivity enhancement brought about by the EC. The effect of PMA/PVAc on the performance of DSSC is also discussed. DSSC without PMA/PVAc shows higher efficiency than DSSC with PMA/PVAc. This is because PMA/PVAc decreases the ion motion in the electrolyte. Although DSSC with PMA/PVAc exhibits lower efficiency, but it maintains 63% of its efficiency after 288 h as compared to 40% of efficiency retention for DSSC without PMA/PVAc. The PMA/PVAc improves the stability of DSSC by suppressing the recombination loss as evidenced from the impedance studies. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The authors wish to thank the Ministry of Education Malaysia for supporting this work through FRGS/1/2018/STG07/UITM/02/12. References [1] B. O'Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films, Nature 353 (1991) 737–740. [2] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Peteerson, Dye-sensitized solar cells, Chem. Rev. 110 (11) (2010) 6595–6663. [3] T. Stergiopoulos, E. Rozi, C.S. Karagianni, P. Falaras, Influence of electrolyte coadditives on the performance of dye-sensitized solar cells, Nanoscale Res. Lett. 6 (2011) 307. [4] S. Venkatesan, I.P. Liu, J.C. Lin, M.H. Tsai, H. Teng, Y.L. Lee, Highly efficient quasisolid-state dye-sensitized solar cells using polyethylene oxide (PEO) and poly(methyl methacrylate) (PMMA)-based printable electrolytes, J. Mater. Chem. A 6 (21) (2018) 10085–10094. [5] J. Shi, S. Peng, J. Pei, Y. Liang, F. Cheng, J. Chen, Quasi-solid-state dye-sensitized solar cells with polymer gel electrolyte and triphenylamine-based organic dyes, ACS Appl. Mater. Interfaces 1 (4) (2009) 944–950. [6] M.H. Buraidah, S. Shah, L.P. Teo, F.I. Chowdhury, M.A. Careem, I. Albinsson, B.E. Mellander, A.K. Arof, High efficient dye sensitized solar cells using phthaloylchitosan based gel polymer electrolytes, Electrochim. Acta 245 (2017) 846–853. [7] S.A. Sapp, C.M. Elliott, C. Contado, S. Caramori, C.A. Bignozzi, Substituted polypyridine complexes of cobalt(II/III) as efficient electron-transfer mediators in dyesensitized solar cells, J. Am. Chem. Soc. 124 (37) (2002) 11215–11222. [8] Z.S. Wang, K. Sayama, H. Sugihara, Efficient eosin Y dye-sensitized solar cell containing Br-/Br3- electrolyte, J. Phys. Chem. B 109 (47) (2005) 22449–22455. [9] G. Oskam, B.V. Bergeron, G.J. Meyer, P.C. Searson, Pseudohalogens for dye-sensitized TiO2 photoelectrochemical cells, J. Phys. Chem. B 105 (29) (2001) 6867–6873. [10] G.K.R. Senadeera, A.M.J.S. Weerasinghe, M.A.K.L. Dissanayake, C.A. Thotawatthage, A five-fold efficiency enhancement in dye sensitized solar cells fabricated with AlCl3 treated, SnO2 nanoparticle/nanofibre/nanoparticle triple layered photoanode, J. Appl. Electrochem. 48 (11) (2018) 1255–1264. [11] M.A. Waghmare, M. Naushad, H.M. Pathan, A.U. Ubale, Rose bengal-sensitized ZrO2 photoanode for dye-sensitized solar cell, J. Solid State Electrochem. 21 (9) (2017) 2719–2723. [12] M.A.K.L. Dissanayake, H.N.M. Sarangika, G.K.R. Senadeera, H.K.D.W.M.N.R. Divarathna, E.M.P.C. Ekanayake, Application of a nanostructured, tri-layer TiO2 photoanode for efficiency enhancement in quasi-solid electrolyte-
8
Optical Materials 96 (2019) 109349
A. Azmar, et al.
[38] M.F. Shukur, R. Ithnin, M.F.Z. Kadir, Electrical characterization of corn starchLiOAc electrolytes and application in electrochemical double layer capacitor, Electrochim. Acta 136 (2014) 204–216. [39] T. Winie, C.H. Chan, R.H.Y. Subban, Ac conductivity and dielectric properties of hexanoyl chitosan-LiClO4-TiO2 composite polymer electrolytes, Adv. Mater. Res. 335–336 (2011) 873–880. [40] S.K. Tripathi, A. Gupta, M. Kumari, Studies on electrical conductivity and dielectric behaviour of PVdF–HFP–PMMA–NaI polymer blend electrolyte, Bull. Mater. Sci. 35 (6) (2012) 969–975. [41] C.Y. Tan, N.K. Farhana, N.M. Saidi, S. Ramesh, K. Ramesh, Conductivity, dielectric studies and structural properties of P(VA-co-PE) and its application in dye sensitized solar cell, Org. Electron. 56 (2018) 116–124. [42] M.F. Shukur, R. Ithnin, M.F.Z. Kadir, Ionic conductivity and dielectric properties of potato starch-magnesium acetate biopolymer electrolytes: the effect of glycerol and 1-butyl-3-methylimidazolium chloride, Ionics 22 (7) (2016) 1113–1123. [43] H. Wang, L.M. Peter, Influence of electrolyte cations on electron transport and electron transfer in dye-sensitized solar cells, J. Phys. Chem. C 116 (19) (2012) 10468–10475. [44] M. Fathy, A.B. Kashyout, J. El Nady, Sh Ebrahim, M.B. Soliman, Electrospun polymethylacrylate nanofibers membranes for quasi-solid-state dye sensitized solar cells, Alexandria Eng. J. 55 (2) (2016) 1737–1743. [45] H. Yang, M. Huang, J. Wu, Z. Lan, S. Hao, J. Lin, The polymer gel electrolyte based on poly(methyl methacrylate) and its application in quasi-solid-state dye-sensitized solar cells, Mater. Chem. Phys. 110 (1) (2008) 38–42. [46] Y. Ren, Z. Zhang, S. Fang, M. Yang, S. Cai, Application of PEO based gel network
[47]
[48]
[49]
[50]
[51]
[52]
9
polymer electrolytes in dye-sensitized photoelectrochemical cells, Sol. Energy Mater. Sol. Cells 71 (2) (2002) 253–259. S. Agarwala, L.N.S.A. Thummalakunta, C.A. Cook, C.K.N. Peh, A.S.W. Wong, L. Ke, G.W. Ho, Co-existence of LiI and KI in filler-free, quasi-solid-state electrolyte for efficient and stable dye-sensitized solar cell, J. Power Sources 196 (3) (2011) 1651–1656. Y. Cui, X. Zhang, J. Feng, J. Zhang, Y. Zhu, Enhanced photovoltaic performance of quasi-solid-state dye-sensitized solar cells by incorporating a quaternized ammonium salt into poly(ethylene oxide)/poly(vinylidene fluoride-hexafluoropropylene) composite polymer electrolyte, Electrochim. Acta 108 (2013) 757–762. W.M.N.M.B. Wanninayake, K. Premaratne, G.R.A. Kumara, R.M.G. Rajapakse, Use of lithium iodide and tetrapropylammonium iodide in gel electrolytes for improved performance of quasi-solid-state dye-sensitized solar cells: recording an efficiency of 6.40%, Electrochim. Acta 191 (2016) 1037–1043. E. Chatzivasiloglou, T. Stergiopoulos, A.G. Kontos, N. Alexis, M. Prodromidis, P. Falaras, The influence of the metal cation and the filler on the performance of dye-sensitized solar cells using polymer-gel redox electrolytes, J. Photochem. Photobiol. A Chem. 192 (1) (2007) 49–55. H.T. Chou, H.C. Hsu, C.H. Lien, S.T. Chen, The effect of various concentrations of PVDF-HFP polymer gel electrolyte for dye-sensitized solar cell, Microelectron. Reliab. 55 (11) (2015) 2174–2177. K. Kakiage, Y. Aoyama, T. Yano, K. Oya, J.I. Fujisawa, M. Hanaya, Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes, Chem. Commun. 51 (88) (2015) 15894–15897.