Inhibiting formation of Zn2+/N179 molecules insulation layer and degradation of ZnO-based dye-sensitized solar cells via quasi- solid-state electrolytes
Gang Li, Lei Sheng, Tingyu Li, Wendong Zhang, Kaiying Wang
PII: S0169-4332(19)31506-5
DOI: https://doi.org/10.1016/j.apsusc.2019.05.198
Reference: APSUSC 42785
To appear in: Applied Surface Science Received date: 15 February 2019 Revised date: 13 May 2019 Accepted date: 16 May 2019
Please cite this article as: G. Li, L. Sheng, T. Li, et al., Inhibiting formation of Zn2+/N179 molecules insulation layer and degradation of ZnO-based dye-sensitized solar cells via quasi-solid-state electrolytes, Applied Surface Science, https://doi.org/10.1016/
j.apsusc.2019.05.198
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Inhibiting formation of Zn
2+/N179 molecules insulation layer and degradation of ZnO-based dye-sensitized solar cells via
quasi-solid-state electrolytes
Gang Li1, Lei Sheng1, Tingyu Li1,*, Wendong Zhang1, Kaiying Wang1,2,*,
1MicroNano System Research Center, College of Information Engineering & Key Lab of Advanced
Transducers and Intelligent Control System (Ministry of Education), Taiyuan University of Technology, Taiyuan 030024, China
2Department of Microsystems-IMS, University of South-Eastern Norway, Horten 3184, Norway
* Email: [email protected], [email protected]
Abstract
ZnO dye-sensitized solar cells (DSSCs) have been deemed as one of promising solar devices.
However, ZnO DSSCs with liquid electrolyte always surfer from dissolution of ZnO film and formation of ZnO2+/Ru-based dye molecules insulation layer, thus weakens conversion efficiency and long-tern stability of the devices. To overcome these obstacles, quasi-solid-state iodine-based electrolytes based on PVDF-HFP, and filled with functionalized multi-walled carbon nanotube (FMWCNT) improve its ionic conductivity via increasing charge transport channels and free volume of iodine/tri-iodine. It is found that optimal conversion efficiency of 3.87% was achieved in ZnO nanosheets (NSs) DSSC with 0.5 wt%-FMWCNT quasi-solid-state electrolyte. Namely, it achieves approximate conversion efficiency of the DSSC with typical liquid iodine-based electrolyte (3.94%) under ~ half of ionic conductivity of liquid electrolyte. Moreover, this device remains 86.65% of original conversion efficiency after 1008h, which is higher than that of the device with liquid electrolyte (50.37%). The result confirms that quasi-solid-state electrolyte inhibits dissolution of ZnO film and formation of ZnO2+/N719 molecules insulation layer.
Keywords: quasi-solid-state electrolyte, PVDF-HFP, insulation layer, ZnO dye-sensitized solar cells.
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2 1. Introduction
Dye-sensitized solar cells (DSCs) have attracted much attention in the past decades because of their low cost and easy fabrication procedures [1-3]. Since TiO2-based dye-sensitized solar cell was reported by Grätzel in 1991, it has been investigated widely to promote its performance [4-7].
Up to now, TiO2 is the most high-efficiency semiconductor material for photoanode of DSSCs [8].
However, TiO2 seems to limit development of DSSCs due to its simple morphologies, difficult growth, so finding novel structured materials for photoanodes has been a research hotspot [9-12].
Among novel photoanode materials, ZnO has been considered as an alternative for TiO2, due to similar band gap energy, high electron mobility (115–155cm2.V-1.s-1), large excitation binding energy (60eV) against photocorrosion, low-cost availability and rich morphologies [13, 14].
Therefore, ZnO nanostructures have frequently been applied in photoanodes [15-17].
Unfortunately, efficiency of ZnO DSSCs with Ru-based sensitizer (one of the most efficiency commercial sensitizer) are weakened due to dissolution of ZnO and formation of agglomerates of Zn2+ and dye molecules in liquid iodine electrolyte, which blocks the electrons injection to the semiconductor and decreases conversion efficiency as well as long-term stability of DSSCs [18, 19].
Currently, various reports have indicated great potential of quasi-solid-state electrolyte in DSSCs [20-22]. Therefore, employing quasi-solid-state electrolytes as alternative for liquid electrolyte is a promising method to overcome the issues from liquid electrolyte DSSCs based on ZnO and enhance its performance [23]. Among quasi-solid-state electrolytes, PVDF-HFP based electrolytes are a common option due to its excellent ionic conductivity, high plasticity, long-term stability and economy [24, 25]. However, compared with liquid electrolyte, pure PVDF-HFP based electrolytes remain low ionic conductivity and diffusion coefficient, which decrease conversion efficiency of DSSCs. Hence, various nanomaterials, such as TiO2, ZnO, SiO2, Al2O3
nanoparticles, clay, graphene nanosheets and carbon nanotubes composite electrolyte, have been employed widely to synthesize composite electrolyte for improving the performance of DSSCs [26-32]. It is well known that functionalized multi-walled carbon nanotube (FMWCNT) possesses higher conductivity, dispersibility, charge transport and strong capability to increase ionic free volume, but it have been rarely integrated with PVDF-HFP for composited electrolyte [33, 34].
In this work, various quasi-solid-state electrolytes based on PVDF-HFP and FMWCNT have
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been synthesized, and employed to overcome the obstacles appearing in ZnO NSs-based DSSCs with liquid electrolyte.
2. Experiment
2.1. Reagents and materials
The reagents, such as poly vinylidenefluoride-hexafluoro propylene copolymer (PVDF-HFP), nitric acid (HNO3), sulfuric acid (H2SO4), chloroplatinic acid (H2PtCl6), zinc acetate dihydrate (Zn(CH3COO)2·2H2O), potassium chloride (KCl), propylene carbonate (PC), anhydrous lithium iodide (LiI), iodine (I2), 4-tert-butylpyridine (TBP), 1-Methyl-3-propylimidazolium iodide (PMII), and N, N-dimethyl formyl (DMF), were all in analyzed purity, purchased from purchased from Sinopharm Chemical Reagent Co., Ltd. Other materials include Multi-walled carbon nanotubes (MWCNT) and ITO glass (6Ω/sq).
2.2. Preparation of FMWCNT
0.5 g MWCNT was added to the mixed acid of concentrated nitric acid and sulfuric acid (HNO3:H2SO4= 3:1), and ultra-sonicated for 30 min. Subsequently, the suspension is kept at 110 ℃ for 48 h with continuous stirring. After cooling, the mixture was washed by deionized water and centrifuged, and repeated several times until it is neutral. The obtained solid were dried at 70 ℃ for 12 h [35].
2.3. Preparation of electrolytes
Typical liquid electrolyte (LE) was prepared via adding 0.1 M LiI, 0.05 M I2, 0.6 M PMII and 0.5 M TBP into the mixed solution of acetonitrile and valeronitrile (v/v= 85/15). To fabricate quasi-solid-state electrolytes (QSS) with high ionic conductivity, 0 wt%, 0.25 wt%, 0.5 wt%, 0.75 wt% and 1.0 wt% FMWCNT were added to the mixed solution consisted of 0.1 g TBP, 0.3 g LiI and 0.25 g I2, 10ml DMF and 10 ml PC respectively, and then ultra-sonicated for 1 h. After addition of 1 g PVDF-HFP, the mixture was maintained at 80 ℃ for 4 h with vigorous stirring.
At 0.5 h before the end, 0.07 g PMII and 1ml LE was added into the mixture. The obtained QSS were denoted as 0%-QSS, 0.25%-QSS, 0.5%-QSS, 0.75%-QSS, 1.0%-QSS electrolyte, respectively.
2.4. Fabrication of ZnO-based photoanode
ZnO NSs were grown on ITO glass (ZnO NSs/ITO) with 0.8×0.8cm2 via electrochemical deposition in the aqueous solution consisting of 0.05M (Zn(CH3COO)2·2H2O) and 0.1 M KCl for
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15 min. ITO glass, Pt mesh and saturated calomel electrode (SCE) was worked as working, counter, reference electrodes, respectively. The constant potential of -1.1 V was implemented between working and reference electrode. The ZnO NSs/ITO was annealed at 450℃ for 2 h, and then placed in 0.3 mM dye solution (N719) for 24 h. Subsequently, the samples were washed with deionized water and dried at 40℃ for 1h.
2.5. Assembly of DSSCs
The Pt counter electrode was prepared by three-electrode electrochemical deposition in aqueous solution containing 1.3 wt% chloroplatinic acid and 0.75 wt% hydrochloric acid. The prepared photoanodes, gel electrolytes and counter electrodes were assembled for DSSCs with sandwich structure. Moreover, similar DSSC with liquid electrolyte were assembled.
2.6. Characteristic and measure
Morphology of ZnO NSs was investigated by scanning electron microscopy (FE-SEM, SU-3500). Crystalline phases of the samples were studied by X-ray diffraction (XRD, ARL EQUINOX 1000) under monochrome Cu Ka (λ=0.154145 nm). Electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry of electrolytes were obtained by electrochemical workstation for calculating ionic conductivity and diffusion coefficient. The photocurrent density-voltage (J-V) curves of DSSCs with gel electrolyte were measured under AM 1.5G (100 mW/cm2) illumination by Keithley 2400 digital source meter.
3. Result and discussion 3.1. Micro morphology
In the experiment, the ZnO NSs was grown on ITO glass via electrochemical deposition. The morphology of ZnO NSs was shown in Fig. 1(a-c). It can be seen from images that the ZnO exhibits irregular nanosheets morphology. The XRD pattern of ZnO is present in Fig. 1(d). In the XRD pattern, there are (111), (100), (002), (200) and (101) diffraction peaks. Among peaks, (100), (002) and (101) corresponded to the phase ZnO, and indicate highly crystalline structure.
Undoubtedly, the ZnO NSs has been synthesized on ITO successfully. The (111) and (200) with low intensity represented the potassium, which indicated that there is a small quantity of potassium in the ZnO NSs film. The potassium derived from KCl in solution and is unavoidable.
Subsequently, these ZnO NSs growing on ITO are fabricated for DSSCs, and then measured.
3.2. Ionic conductivity of electrolytes
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Ionic conductivity, an important parameter of electrolyte, is used to evaluate the performance of various electrolytes. The electrochemical impendence spectroscopy (EIS) was carried on Cu symmetrical cell to obtain Nyquist plot, and then the ionic conductivity can be calculated via the data from Nyquist plot. The ionic conductivity (σ) can be calculated by following formula (1) directly [36].
(1)
where t is the thickness of gel electrolyte, Rb refers to the volume resistance of electrolyte, A represents the contact area of electrolyte with parallel electrode.
The volume resistance (Rb) is necessary for calculation of ionic conductivity, defined as the radius of first arc of Nyquist plot [37]. The Nyquist plots of Cu symmetrical cell with different electrolytes are shown in Fig. 2(a). In order to obtain accurate Rb, Nyquist plots of different electrolytes are fitted by Zview software, and then the accurate ionic conductivity can be calculated.
Fig. 2(b) exhibits the ionic conductivities of different electrolytes, and corresponding parameters are summarized in Table 1. It is found that 0%-QSS electrolyte exhibits the lowest ionic conductivity of 2.12×10-4 s/cm, liquid (LE) electrolyte possesses the highest ionic conductivity of 9.89×10-4s/cm. The ionic conductivity of 0.5%-QSS composite electrolyte reaches 4.57×10-4 s/cm, which was obviously higher than that of 0%-QSS electrolyte and close to a half of that of LE. The results indicate that loading FMWCNT enhance ionic conductivity of the quasi-solid-state electrolyte based on PVDF-HFP. The 0 ionic conductivities of 0.75 wt%, 1.0 wt%-QSS composite electrolytes are 4.28×10-4s/cm, 4.07×10-4s/cm, respectively. There is a slight reduction in ionic conductivity, compared with 0.5%-QSS electrolyte.
It is widely accepted that the ionic mobility in the gel polymer electrolyte depends on ions diffusion and charge transport in the polymer matrix. The 0%-QSS electrolyte forms long polymer chain, which inhibits the charge transport and ionic diffusion in QSS electrolyte. So it has the low ionic conductivity. FMWCNT structure is concentric circular tube of hexagonal carbon atoms, which increases the ionic free volume in gel polymer electrolyte to improve ionic diffusion magnificently. Moreover, these functional groups (majorly -COOH) derived from functionalization process facilitates the charge transport in nanotubes, restraining the formation of
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long polymer chain, and promoting the uniform distribution of FMWCNT in polymer, which further improves the charge transport [33]. Therefore, the ionic conductivity of 0.5%-QSS composite electrolyte increases from 2.12×10-4 s/cm to 4.57×10-4 s/cm. However, increasing amount of FMWCNT increases non-uniform dispersion and aggregation of FMWCNT, which adheres polymer blocks and weakens ionic diffusion as well as charge transport. In addition, the interaction of -COOH and Li+ has been enhanced with increasing FMWCNT. The interaction also decreases ionic conductivity of electrolyte [38]. Therefore, the ionic conductivity of 1.0 wt%-QSS composite electrolyte appears to a slight decline (4.07×10-4s/cm).
3.3. The apparent diffusion coefficient of
Linear sweep voltammetry (LSV) was implemented on three electrodes system, in which Pt ultra-microelectrode, Pt wire and Ag/AgCl electrode are used as working electrode, counter electrode, reference electrode, respectively. The steady-state voltammograms of electrolytes with different amount of FMWCNT are shown in Fig. 3. The apparent diffusion coefficient ( and ) of the gel electrolytes were obtained from the LSV voltammograms. Namely, the apparent diffusion coefficient (Dapp) of can be calculated by the following Eq (2) [39].
ISS = 4nRFCDapp (2)
where ISS is the steady-state limited current of various electrolytes, n refers to the number of electrons involved in each reaction, F corresponds to the Faraday constant, R is the radius of ultra-microelectrode, and C represents the concentration of electro active species.
The apparent diffusion coefficient of various electrolytes is summarized in Table 2. It can be seen that, the Dapp of 0wt%-QSS electrolyte is 1.166×10-7cm2/s. The Dapp of 0.25wt%-QSS composite electrolyte increases to 1.761×10-7 cm2/s, which confirms that FMWCNT can improve ionic diffusion of electrolyte. With increase of FMWCNT, the Dapp of 0.5%-QSS composite electrolyte is 2.211×10-7 cm2/s, which reaches 78.48% of that of Liquid electrolyte. However, the Dapp of 0.75wt%-QSS composite electrolyte doesn’t increase, but declines to 1.864×10-7 cm2/s.
Even，the Dapp of 1.0 wt%-QSS composite electrolyte has decreased to 1.523×10-7 cm2/s. The low ionic diffusion coefficient of electrolyte might result from the insufficient charged ionic conductivity. It is clear that these results are substantially influenced by the ionic conductivity of the gel electrolytes, so exhibits similar tendency with the ionic conductivity [32].
3.4. Photovoltaic performance of DSSCs
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ZnO NSs-based DSSCs fabricated with the five quasi-solid-state electrolytes and the typical LE were measured under the AM 1.5G simulated sunlight (100 mW/cm−2). Their photocurrent density-voltage (J-V) curves are exhibited Fig. 4(a), and corresponding performance parameters, such as short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF) and conversion efficiency (η) are summarized in Table 3. The devices with 0wt%-, 0.25wt%-, 0.5wt%-, 0.75wt%-, 1.0wt%-QSS and liquid iodine-based electrolyte were denoted as device 1, device 2, device 3, device 4, device 5 and device 6, respectively.
It can be observed that device 6, with typical LE, achieves the highest Jsc (10.54 mA/cm2) and thus the highest conversion efficiency of 3.94% among the six devices. In quasi-solid-state DSSCs (device1~device5), the device 1 with 0wt%-QSS electrolyte shows the lowest Jsc of 6.62mA/cm2 and conversion efficiency of 2.48%, which results from possibly the low ionic conductivity. The device 3 with 0.5%-QSS composite electrolyte obtained Jsc of 10.06mA/cm2 and conversion efficiency of 3.87%, which is close to the performance of device 6. In terms of the fact that 0.5wt%-QSS composite electrolyte just has ionic conductivity of 4.57×10-4s/cm and the liquid electrolyte possesses ionic conductivity of 9.89×10-4s/cm, the situation is likely to result from strong electron injection and low electron recombination [40] . The device 4 and 5 present Jsc of 9.48 mA/cm2 and 9.12 mA/cm2 respectively, which displays a small reduction comparing with device 3. IPCE curves (exhibited in Fig. 4(b)) of different devices also verify the situation above.
Moreover, DSSCs employed with QSS electrolytes show higher Voc than that of DSSC with liquid electrolyte. The explanation can be described as following. In DSSCs, the Voc is determined by the difference between the conductive band potential of photoanode and the standard reduction potential of the redox couple [39]. Normally, the cations, such as Li+ and H+, adsorbed at electrolyte/ZnO interface, result in a positive shift of conductive band potential of ZnO [41-43].
The higher cation concentration could cause a larger positive shift, which decreases the Voc of DSSCs. For LE, the cations have higher mobility and solubility than that in QSS. Furthermore, it is easier for the cations in LE to get into the voids between the ZnO nanosheets than that in QSS.
Therefore, more cations accumulated at the LE/ZnO interface, resulting in a higher cation concentration and further a lower Voc [44].
3.5. Dark current density-voltage curves
Fig. 5 shows the dark current density-voltage curves of various devices. It is observed from
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the curve that the dark current of device with QSS electrolyte is lower than that of device 6 with LE. It is widely acceptable that the dark current of DSSCs can indicate the intensity of electron recombination at semiconductor film/electrolyte interface and the contact interface between substrate and electrolyte [45]. Therefore, it is clear that in the device 3 with 0.5%-QSS electrolyte has low electron recombination intensity compared with the device 6 with LE. This result imply that QSS composite electrolyte can inhibit the formation of insulation layer consisting Zn2+ and N719 molecules, and thus enhance electron injection.
3.5. EIS analysis of DSSCs
The EIS of DSSC with different electrolytes (exhibited in Fig. 6) are fitted with equivalent circuit (insert image of Fig. 6), and charge transfer resistance of DSSCs are summarized in Table 4.
In EIS spectra, Rs refers to the series resistance of the electrolytes and electric contacts in the DSSCs. R1, R2 and R3 correspond to the charge transfer processes occurring at the counter electrode/electrolyte (first arc), photoanode/electrolyte interface (second arc) and the Warburg element of the ionic diffusion for the redox couple ( and ) in the electrolyte (third arc) respectively. Therefore, the values of R2 are used to evaluate the electron injection of ZnO [46].
It is found from Table 4 that, the device 1 with 0wt%-QSS electrolyte has the highest R2 of 29.67Ω, which is due to its low ionic conductivity possibly. The device 3 with 0.5%-QSS composite electrolyte shows the lowest R2 (22.44 Ω). The device 2, 4 and 5 possesses R2 of 27.45 Ω, 26.81 Ω and 28.38 Ω, respectively. Compared with R2 of the device 6 with LE (29.05 Ω), the R2 of devices with QSS composite electrolytes are lower, which indicates that quasi-solid-state ZnO-NSs DSSCs have stronger electron injection than device 6 [47]. Considering the fact that 0.5wt%-QSS composite electrolyte owns half of ionic conductivity of LE, and the DSSC with 0.5%-QSS composite electrolyte harvests smaller dark current density and approximate conversion efficiency, it can be confirmed that QSS composite electrolyte inhibits the formation of insulation layer consisting of Zn2+ and N719 molecules and achieve strong electron injection.
3.6. Long-term stability
The variation of conversion efficiency of different DSSCs with time is shown in Fig. 7. After 1008 h, the DSSCs with LE, 0wt%-QSS and 0.5%-QSS composite electrolyte maintained 50.37%, 76.27%, 86.65% of their initial conversion efficiency, respectively. Therefore, dissolution of ZnO results in low long-term stability of the device with LE [48]. The DSSCs with 0.5%-QSS
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composite electrolyte shows higher long-term stability than that of the device with liquid electrolyte, which indicates that composite electrolyte can weaken the dissolution ZnO and enhance its long-term stability magnificently.
4. Conclusion
In conclusion, six DSSCs were successfully prepared with five QSS electrolytes and one typical liquid electrolyte. It was found that the ionic conductivity of 0.5wt%-QSS composite electrolyte (4.48×10-4 s/cm) is about a half of ionic conductivity of typical liquid electrolyte. And, the DSSC with this electrolyte achieved approximate conversion efficiency (3.87%) for the device with LE (3.94%), smaller dark current density, lower R2 (the resistance for charge transport at ZnO/electrolyte interface). Moreover, the device employed 0.5%-QSS composite electrolyte kept 86.65% of the initial efficiency after 1000 hours, which is higher than that of the device with typical liquid electrolyte. The results point out that QSS composite electrolyte can weaken dissolution of ZnO and formation of Zn2+/N719 molecules, thereby improving the conversion efficiency and long-term stability of ZnO DSSC.
Acknowledgement
This research was supported by the National Natural Science Foundation of China (61674113, 51622507, and 61471255), Natural Science Foundation of Shanxi Province, China (2016011040), and Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi Province, China (2016138).
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Table 1. Ionic conductivity of different electrolytes
Electrolytes Rb (Ω) σ (s/cm)
0wt%-QSS 104.35 2.12×10-4
0.25wt%-QSS 62.84 3.52×10-4
0.5wt%-QSS 48.41 4.57×10-4
0.75wt%-QSS 51.69 4.28×10-4
1.0wt%-QSS 54.35 4.07×10-4
Liquid electrolyte (LE) 22.37 9.89×10-4
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Table 2. Apparent diffusion coefficients of different electrolytes.
Electrolytes ISS of (mA) Dapp of (cm2/s)
0wt%-QSS 0.0531 1.166×10-7
0.25wt%-QSS 0.0802 1.761×10-7
0.5wt%-QSS 0.1007 2.211×10-7
0.75wt%-QSS 0.0849 1.864×10-7
1.0wt%-QSS 0.0694 1.523×10-7
LE 0.1283 2.8172×10-7
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Table 3. Parameters of DSSCs with different electrolytes.
DSSCs Electrolytes Voc (V) Jsc
(mA/cm2) FF η (%)
Device 1 0wt%-QSS 0.675 6.62 0.55 2.48±0.02
Device 2 0.25wt%-QSS 0.671 8.05 0.57 3.09±0.03
Device 3 0.5wt%-QSS 0.667 10.06 0.58 3.87±0.02
Device 4 0.75wt%-QSS 0.653 9.48 0.58 3.62±0.03
Device 5 1.0wt%-QSS 0.655 9.12 0.57 3.45±0.03
Device 6 LE 0.643 10.54 0.58 3.94±0.02
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Table 4. Electrochemical parameters of DSSCs with different electrolytes obtained from EIS.
DSSCs Rs/Ω R1/Ω R2/Ω R3/Ω
Device 1 7.42 13.56 29.67 9.55
Device 2 8.41 11.36 27.45 7.35
Device 3 7.80 10.91 22.44 5.91
Device 4 7.98 11.18 26.81 8.56
Device 5 6.97 11.35 28.38 9.23
Device 6 6.83 7.91 29.05 3.82
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Highlights
ZnO NSs combined with quasi-solid-state iodine-based electrolyte were used for DSSCs.
Quasi-solid-state electrolyte filled with FMWCNT was optimized for high ionic conductivity.
Dissolution and degradation of ZnO NSs were weakened for long-term stability of DSSCs.
