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polyaniline transparent counter electrode
Author’s Accepted Manuscript Bifacial Quasi-solid-state Dye-sensitized Solar Cells with Poly (vinyl pyrrolidone)/Polyaniline Transparent Counter Electrode Jing Gao, Ying Yang, Zheng Zhang, Jingyuan Yan, Zehua Lin, Xueyi Guo www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(16)30133-1 http://dx.doi.org/10.1016/j.nanoen.2016.05.010 NANOEN1275
To appear in: Nano Energy Received date: 19 December 2015 Revised date: 27 March 2016 Accepted date: 6 May 2016 Cite this article as: Jing Gao, Ying Yang, Zheng Zhang, Jingyuan Yan, Zehua Lin and Xueyi Guo, Bifacial Quasi-solid-state Dye-sensitized Solar Cells with Poly (vinyl pyrrolidone)/Polyaniline Transparent Counter Electrode, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2016.05.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Bifacial Quasi-solid-state Dye-sensitized Solar Cells with Poly (vinyl pyrrolidone)/Polyaniline Transparent Counter Electrode Jing Gao, Ying Yang, Zheng Zhang, Jingyuan Yan, Zehua Lin, Xueyi Guo School of Metallurgy and Environment, Central South University, Changsha 410083, China
Abstract: We synthesized poly (vinyl pyrrolidone) (PVP)/polyaniline (PANI) nanocomposites and applied them as transparent counter electrode (CE) for bifacial quasi-solid-state dye-sensitized solar cells (DSSCs). PVP surrounded with aniline acted as efficient steric stabilizer in the polymerization process. Moreover, the present of PVP in PANI CE facilitated the generation of active reaction-sites in the interface between counter electrode and electrolyte and further reduced the electron recombination. The DSSC fabricated with PVP (4 wt%)/PANI CE exhibited improved power conversion efficiency up to 5.45%, which was comparable to that of DSSC assembled with Pt CE (5.57%). In the case of rear-illumination, the PVP (4 wt%)/PANI based DSSC showed an efficiency of 4.66%, which was 85.5% of the efficiency obtained from front illumination and much higher than that of DSSC with Pt CE (1.14%). The bifacial DSSC with PVP/PANI counter electrode showed an optimal power efficiency of 5.81% under illumination from both front and rear sides, which is higher than that of Pt based DSSC (5.70%) under the same condition. Long-term stability tests indicated that the photovoltaic device with PVP/PANI counter electrode exhibited an enhanced durability than that of DSSC assembled with Pt CE.
Keywords: Polyaniline, Poly (vinyl pyrrolidone), Transparent counter electrode, Polymer electrolyte, Quasi-solid-state dye-sensitized solar cell
Corresponding authors. Tel.: +86 731 88877863; fax: +86 731 88836207.
E-mail address: [email protected]; [email protected]
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Introduction Currently, platinum (Pt) is widely exploited as a counter electrode (CE) material for dye-sensitized solar cells (DSSCs). It is notable that Pt CE utilized in DSSCs has issues such as complex fabrication, high price, and it could be dissolved in the electrolyte to form PtI4 and H2PtI6 due to the corrosive I-/I3- redox electrolyte [1, 2]. As a result various other materials are utilized for counter electrode to instead Pt in DSSCs, such as carbon-based materials including carbon nanotubes [3, 4], graphite [5], carbon black [6] and fullerenes C60 [7]; as well as conducting polymers containing poly(3,4-ethylenedioxythiophene) (PEDOT) [8-10], polypyrrole (PPy) [11] and polyaniline (PANI) [12, 13]. Among them, conductive polymers have shown promising performances due to its low cost, high electrical conductivity and good environmental stability. PANI film, owing to its high catalytic activity and excellent transparency in the visible region, attracts significant research interests in investigating DSSCs. Lee et al. [14] reported a highly porous PANI using carbon nanodots (CNDs) as a nucleating agent to improve the conversion efficiency of DSSCs. Bifacial DSSCs based on transparent counter electrode could increase the incident light harvest and improve the conversion efficiency of cells. Recently, Tai et al. [15] reported a bifacial DSSC with a transparent PANI CE, yielding a front-illuminated and rear-illuminated power conversion efficiency of 6.54% and 4.26% with liquid electrolyte, respectively. The more recently, Wu et al. [16] obtained an efficiency of 7.05% for bifacial DSSC with PANI CE irradiated from the both sides of photoanode and counter electrode. Although several investigations have been reported on high-quality transparent PANI films, most of them were applied in bifacial DSSCs with liquid electrolyte and none has reported the long-term stability of bifacial quasi-solid-state DSSCs with PANI counter electrode yet. Meanwhile, the best rear illumination power conversion efficiency is only 65% of the efficiency obtained from front illumination [15], which may be caused by the existence of PANI conglomerations during interfacial polymerization. Further improving the stability and the rear-illumination power conversion efficiency of bifacial DSSCs is significant for its application. In this work, we utilize Poly (vinyl pyrrolidone) (PVP)/PANI transparent film as the counter electrode to create bifacial quasi-solid-state DSSCs based on agarose polymer electrolyte as seen in Schematic 1. Poly (vinyl pyrrolidone) is an amphiphilic 2
water-soluble polymer which has excellent solubility, chemical stability, film-forming property and bonding capacity. Introducing PVP into PANI would enhance the uniformity of PANI nanoparticles and also improve the adhesion of the composite film to the substrate, thus can efficiently increase rear power efficiency. The microstructure and optical properties of PVP/PANI films are characterized. And the photovoltaic and electrochemical characteristics of quasi-solid-state DSSCs fabricated using these transparent counter electrodes are also studied and compared with those of the DSSCs with Pt CE. It is found that the rear illumination power conversion efficiency can reach 85.5% of the efficiency obtained from front illumination. A power efficiency of 5.81% is achieved for bifacial quasi-solid-state DSSC with PVP/PANI counter electrode by illumination from both front and rear sides, which is higher than that of Pt based DSSC (5.70%) under the same condition. The long-term stability measurement result shows that DSSCs with PVP/PANI counter electrode can improve the solar cell stability. Experimental Fabrication of PVP/PANI composite counter electrodes Different concentrations (0, 2, 3, 4, 5, 6 wt%) of Poly (vinyl pyrrolidone) (PVP, 99.8% purity, Urchem) were separately introduced to 50 mL of 0.5 M hydrochloric acid (HCl) containing 25 mM aniline (ANI, 99% purity, TCI), then another 50 mL 0.5 M HCl solution containing 25 mM ammonium persulfate (APS, 98% purity, TCI) was added, followed by immersing a piece of FTO glass (surface resistance =15 Ω sq-1, area = 1.5×2 cm2) into the aniline hydrochloride which is cleaned subsequently in acetone, deionized (DI) water and ethyl alcohol by ultrasonator for 15 min. After being kept for 20 min around -3 oC, the obtained HCl doped PANI films were washed repeatedly with DI water and 1 M HCl to remove the large deposited PANI grains and residual reagents. Finally, the resultant PVP/PANI films were immersed in 1 M HCl for 4 h and dried at 80 oC in vacuum for 30 min. Assembly of DSSCs The preparation of the dyed TiO2 photoanodes and the agarose polymer electrolytes were prepared according to the details in previous reports [17, 18]. For DSSC assembly, the polymer electrolyte solution was dropped on the dyed TiO2 film in an oven at 75 ℃. As soon as the polymer electrolyte became viscous, the obtained PVP/PANI and Pt counter electrode was placed on the TiO2/agarose electrolyte 3
structure to form a sandwich structure. The Pt CE used here was obtained by sputtering 10 nm Ti and 50 nm Pt on FTO glass. Then the whole device was dried at 75 ℃ for another 2.5 h to remove the solvent and create a quasi-solid-state DSSC. Characterization and measurements The surface morphologies of PVP/PANI films were investigated by a Scanning Electron Microscope (SEM, JSM-6360LV, JEOL, Japan). Ultraviolet-visible spectrophotometer (UV-Vis, Hitachi, Tokyo, Japan) was used to measure the absorption and transmittance of the PVP/PANI film. Fourier transform infrared (FT-IR) spectrum was obtained by using Fourier transform infrared spectroscopy (Thermo Nicolet Corporation, Nexus 670, America) to confirm the reaction between PVP and PANI. Electrochemical impedance spectra (EIS) measurement of the DSSCs was performed with an electrochemical analyzer (CHI604D, Chenhua, Shanghai, China) under illumination of 96.3 mW/cm2 with a bias of -0.8 V, a frequency range of 105-10-1 Hz and a perturbation voltage of 10 mV. With the same instrument, Tafel polarization plots were obtained using symmetrical cells of various CEs, controlling the scan rate at 10 mV/s. Photon-to-current conversion efficiencies of the DSSCs were evaluated by measuring the current-voltage (J-V) curves under irradiation from a Xe lamp (CHF–XM500, Trust–tech, China) with electrochemical analyzer (CHI604D) as recorder. The intensity of the incident light was calibrated by a Si-1787 photodiode (spectral response range 320-730 nm). The incident light intensity was 96.3 mW/cm2 with the active cell area of 0.25 cm2. Results and discussion Reaction mechanism for preparing PVP/PANI composite CE The reaction mechanism of preparing PVP/PANI composite counter electrode is schematically described in Schematic 2. The carbonyl group (C=O) of lactam in PVP receives protons to form a resonance structure of carbon positive ions (Formula 1), and the –NH2 group of ANI attacks the central carbon atom of carbon positive ions (Formula 2). Then the resultant grafts to the lactam and carbonyl groups convert to alcoholic hydroxyl (–OH) (Formula 3). In an acidic environment, the alcoholic hydroxyl group form an imine bond (C=N) by intramolecular dehydration (Formula 4) [19, 20]. When APS is added, the free aniline is oxidized and converts to a radical cation (ANI+) (Formula 5). Similarly, the benzene ring that connects with the PVP 4
chains losses electrons under the oxidation of APS and form a radical cations with strong activity (Formula 6). The polymerization between cation radicals and PVP side chains (Formula 7) happens autocatalytic [21] on the active sites of stabilizer and forms PVP/PANI composite film on FTO as seen in Schematic 2 [22]. FT-IR spectrum of PVP/PANI CEs To ascertain the interaction between PVP and the synthesized PANI, the FT-IR spectrum was carried out. Figure 1 shows FTIR of pure PVP (Figure 1a), pure PANI film and PVP (4 wt%)/PANI composite film (Figure 1b). The characteristic absorption bands at 3440 cm-1, 2958 cm-1, 1645 cm-1, 1450 cm-1, 1290 cm-1 in Figure 1a correspond to the O-H, CH2 antisymmetric stretching mode, C=O, CH2 in-plane swing vibration, and C-N stretching in PVP, respectively [23]. The bands around 1655 cm-1, 1580 cm-1, 1115 cm-1 in Figure1b are assigned to the C=N, C=C, C-H stretching mode of the quinoid ring, and 1479 cm-1, 1290 cm-1, 795 cm-1 are assigned to the C=C of benzoid rings, C-N stretching of the secondary aromatic amine, and an aromatic C-H out-of-plane bending vibration on 1,4-ring of PANI, respectively [24]. While the ∙
stretching mode of C-N+ polarons also appears at 1240 cm-1, indicating that the PANI polymer film has doping characteristic. In addition, the intensity of 1240 cm-1 peak ∙
increases after adding PVP which is caused by more C-N+ polaron formed at conjugated PANI chains. Meanwhile, compare the spectrum of PANI film with PVP (4 wt%)/PANI film, it can be found that the intensity of peak around 1645 cm-1 for C=N stretching increases obviously, and get slight red-shift to 1655 cm-1 after adding PVP. This is attributed to much more C=N bonds and conjugation structures produced by the reaction between carbonyl and amine group [25]. Morphologies of PVP/PANI CEs Figure 2a ~ f show the SEM images of PVP/PANI composite films with different PVP concentrations (0, 2, 3, 4, 5 and 6 wt%). It is seen that there are irregular PANI macroscopic precipitations observed on the PANI film without adding PVP, which results in a rough and heterogeneous surface. Increasing PVP concentration to 4 wt% leads to a relatively smooth and homogeneous morphology as shown in Figure 2b ~ d. Further adding PVP up to 5 wt% attributes to the formation of PANI aggregates (Figure 2e ~ f). 5
It has been confirmed that the C=N group at PVP side chains provides a new reactive site for the polymerization of the adsorbed aniline oligomers and promotes the PANI growing in a more ordered manner compared with that without stabilizer [26, 27], and more active ANI+ or oligomeric intermediates grafted to PVP side chains to form homogeneous film as increasing PVP concentration. However, irregular aggregations can be seen in samples with PVP content above 4 wt% in Figure 2e ~ f, which may be attributed to the increased solution viscosity caused by the decreased solubility of PVP, leading to tangled molecular and aggregation. UV-Vis absorption analysis of PVP/PANI CEs Figure 3 shows the UV-Vis absorption spectra of PVP/PANI films with different PVP concentrations. The peak around 345 nm (λ1) represents the characteristic absorption peak of π-π* transition that is typical to benzene ring of PANI polymer [28], and the peaks around 402 nm (λ2) and 840 nm (λ3) are assigned to the excitation transition of polarons and bipolarons of doping PANI [29], which may be caused by the interband charge transfer from benzenoid to quinoid rings of the protonated PANI molecule. The wavelength and intensity of the absorption peaks for different PVP/PANI films are listed in Table 1. The wavelengths of λ1 and λ3 show slightly red-shift as PVP content increases from 0 to 4 wt%. This could be attributed to the decreased π-π* and bipolaron transition band gap caused by the increased conjugation structures formed at PANI chains after adding PVP, respectively. Consequently, the decreased transition band gap means there are more transitions of π-electrons and bipolarons, which leads to the increased absorption intensity of λ1 and λ3 [30]. However, the reduced λ1 and λ3 intensity (PVP > 4 wt%) could result from the overmuch PVP long chains intertwine with each other and lead to a decreased conjugation degree. In addition, due to the main factor that influences the transition band gap of polaron is the conjugated molecule type of polymer structure, the positon of polaron transition peak λ2 around 402 nm almost remains unchanged under different PVP concentrations. Front-illuminated photovoltaic and electrochemical performances of DSSCs based on Pt and PVP/PANI CEs The current density-voltage (J-V) curves for DSSCs fabricated with Pt CE and PVP/PANI CEs with different PVP content under front illumination are plotted in 6
Figure 4a. The derived photovoltaic parameters, including the short-circuit current density (Jsc), open circuit voltage (Voc), and power conversion efficiency (η) are presented in Figure 4b, respectively. It can be observed that Jsc increases after introducing PVP and reaches the maximum when the PVP concentration of 4 wt%. The Voc of the DSSCs with PVP/PANI CEs changes from 0.53 to 0.58 V as PVP content increases. Pt CE-based DSSC shows much higher Voc (0.66 V) compared to that of all the PVP/PANI based devices. The optimum overall power conversion efficiency 5.45% is obtained with the device based on PVP (4 wt%)/PANI CEs, which is comparable with the DSSC assembled with Pt CE (η = 5.57%) under the same conditions. To further analyze the effect of PVP content on the electrochemical performances of these DSSCs, electrochemical impedance spectroscopy (EIS) measurement was carried out. Figure 5a shows Nyquist plots of the EIS spectra. There are three arcs can be observed, which corresponds to the charge transfer process at CEs/electrolyte interface at high frequency (105-103 Hz), the charge recombination process at the TiO2/dye/electrolyte interface at middle frequency (103-1 Hz), and the ion diffusion process occurring in the electrolyte at low frequency (1-10-1 Hz), respectively [31, 32]. Figure 5b shows the change of R1, R2 and R3 with PVP contents, where R1 represents the charge transfer resistance at the PVP/PANI CE/electrolyte interface, R2 represents the recombination resistance at the dyed-TiO2 /electrolyte interface, and R3 represents the ion diffusion resistance in the DSSC [33]. In Figure 5b, R1 decreases from 15.41 Ω to 12.39 Ω as the PVP content increases from 0 to 4 wt%, indicating that the reduction of triiodide ions at PANI CE/electrolyte interface is promoted after introducing PVP. This decrease in R1 could result from the improved uniform PANI morphology and larger conjugation degree as discussed in Figure 2 ~ 3, which is benefit to the connection between counter electrodes and electrolyte and accelerates electron transport to facilitate triiodide reduction process (I3- + e = 3I-) [32]. The decreased R1 would result in the improved Jsc as seen in Figure 4b [34]. The recombination resistance R2 increases from 180.91 Ω to 216.31 Ω with PVP content increases from 0 to 4 wt%, suggesting that the electron recombination process is efficiently suppressed by introducing PVP [32]. This could be attributed to the improved electro-catalytic ability of PVP/PANI CE which facilitates the reduction of I3- on the surface of PVP/PANI CEs, leading to reduced electron recombination. However, R2 decreases when PVP concentration exceeds 4 wt%, which probably 7
accounts for the increased defects caused by the destroyed homogeneous structure of PANI CE with higher PVP contents. These defects are in favor of electron recombination. In addition, it’s obvious that the recombination resistance (R2) of Pt CE-based DSSC is much higher than that of DSSCs assembled with PVP/PANI CEs (Figure 5b), which may result from the more compact structures of Pt CE obtained by sputtering method and suppress the charge recombination at the interface between Pt CE and electrolyte, leading to improved Voc of Pt CE-based device as seen in Figure 4. The ion diffusion resistance R3 decreases after adding PVP until 4 wt%, which attributes to the increased conjugation degree as discussed above and thus improves I-/I3- diffusion in the electrolyte. Photovoltaic performances of DSSCs based on PVP/PANI and Pt CEs under front and rear side illumination In Figure 6a, the thin PVP (4 wt%)/PANI film shows excellent transparency from 450-700 nm and obtains its maximum transmittance of 65% at 536 nm, while the transmittance of Pt CE is as low as 15% from 420-840 nm. The transmittance of CE affects greatly on the incident light harvest and the rear illuminated conversion efficiency of bifacial DSSC. Figure 6b compares the J-V curves of the bifacial DSSCs based on PVP (4 wt%)/PANI and Pt CEs irradiated from the front side, rear side and both front and rear sides. To enhance the rear side light harvesting of bifacial DSSCs, a mirror is placed under the rear side of the device to reflect the light from the light source, which is defined as the bifacial DSSCs that illuminated from both front and rear sides. The results of photovoltaic parameters of Pt and PVP (4 wt%)/PANI based DSSCs under different illumination conditions are summarized in Table 2. When irradiated from the rear side, the device based on PVP (4 wt%)/PANI CE exhibits an efficiency of 4.66%, which is nearly 85.5% of that from front illumination (5.45%). This comparable efficiency under rear illumination may be caused by the improved uniformity of PANI counter electrode after introducing PVP in PANI. However, the device based on Pt CE only shows as low as 1.14%, which is due to its low transmittance of light under rear side illumination. From Table 2, it also can be confirmed that illumination from both front and rear sides can effectively enhance the power conversion efficiency of bifacial DSSCs with PVP/PANI CE from 5.45% (front illumination) to 5.81% (both front and rear illumination), which is higher than that of Pt based DSSC (5.70%) under the same illumination condition. The incident 8
light irradiation intensities from the front and the rear sides are 96.3 and 45.6 mW/cm2, respectively. Tafel polarization plots of PVP/PANI and Pt CEs Further, to confirm the electro-catalytic activity of the counter electrodes, the Tafel polarization plots were measured as seen in Figure 7. Generally, there are three divided zones in Tafel plot: (1) polarization curves (|V|<120 mV), (2) Tafel zone (120 mV<|V|<400 mV), and (3) diffusion zone (|V|>400 mV) [35]. In the Tafel zone, the larger slope means a higher exchange current density (J0) and better catalytic activity [36]. And in the diffusion zone, the limiting diffusion current density (Jlim) is the intersection of the cathodic branch with the Y-axis, which determins the diffusion properties of the redox couple and the CE catalysts. As seen in Figure 7a, the slope for PANI CEs increases with PVP concentrations increasing and reaches the maximum at PVP content of 4 wt%, indicating that the best catalytic activity of PVP(4 wt%)/PANI CE. Moreover, the RTafel value of counter electrodes can be measured via the following equation [37]:
Obviously, the RTafel is in inversely proportional to J0, meaning the RTafel values follow the same trend as the R1-EIS values as seen in Figure 5b. In addition, the Jlim of PVP(4 wt%)/PANI CE is greatly larger than that of Pt CE, suggesting a higher electron diffusion velocity for the redox couple in the electrolyte, so the faster electron transporting circulation may be one reason for the higher Jsc of PVP(4 wt%)/PANI, as seen in J-V curves of DSSCs (Figure 4a). Stability of DSSCs based on PVP/PANI and Pt CEs Figure 8 shows the normalized current density (Jsc), open circuit voltage (Voc) and power conversion efficiency () as a function of aging time for the DSSCs based on Pt and PVP (4 wt%)/PANI CEs. The normalized efficiency of the DSSC with Pt CE exhibits a ~74.9% decrease after 240 h testing (without sealing), while the cells with PVP (4 wt%)/PANI CE show slow degradation (~49.1%) compared with that of Pt CE under the same condition. This improvement in DSSC long-term stability may be attributed to the more conjugated and uniform porous film provided by PVP, leading to an improved interfacial connection between polymer electrolyte and counter 9
electrode. It is found that Jsc of both DSSCs with Pt and PVP/PANI CEs rapidly decreases as the aging time increases, which may be caused by the evaporation of the solvent in polymer electrolyte during the storage without sealing and can be avoided with well sealing. The evaporation of solvent destroys the cross–linking networks in polymer electrolyte and impedes the carrier transport, resulting in inferior Jsc as aging time increases. The Voc of the DSSCs with PVP (4 wt%)/PANI CE remains almost unchanged during the time goes by, indicating the electron recombination process is almost unchanged in DSSC. Conclusions In summary, in this work we developed a PVP-incorporated PANI counter electrode for quasi-solid-state DSSC applications. After adding PVP as a steric stabilize with proper concentration, the PVP/PANI counter electrode exhibited the improved conjugation degree, which was benefit to the connection between polymer electrolyte and PANI counter electrode and efficiently enhanced DSSC performances. EIS characterization further revealed the improvement on electron recombination and redox transport process in the device. Meanwhile, the stability tests indicated that the PVP (4 wt%)/PANI CE showed better durability than that of Pt CE-based DSSC. This kind of PVP/PANI counter electrode could be a potential substitute for noble metal electrode for both DSSC and perovskite solar cells. Acknowledgments. We gratefully acknowledge the financial support of this work by the Third Innovation Driven Project of Central South University (2016CX022); the Teacher Research Fund of Central South University (905010104); Scientific Research Foundation for the Returned Overseas Chinese Scholar, State Education Ministry and the projects of innovation for graduate student in CSU (201510533205, CX2015237). References [1] E. Olsen, G. Hagen, S.E. Lindquist, Sol. Energ. Mat. Sol. C. 63 (2000) 267-273. [2] S. Huang, Q. He, W. Chen, J. Zai, Q. Qiao, X. Qian, Nano Energy 15 (2015) 205-215. [3] P. Poudel, A. Thapa, H. Elbohy, Q. Q. Qian, Nano Energy 5 (2014) 116-121. [4] P. Poudel. Q. Q. Qiao, Nano Energy 4 (2014) 157-175. [5] I.Y. Jeon, H. M. Kim, I. T. Choi, K. Lim, J. Ko, J. C. Kim, H, J. Choi, M. J. Ju, J. J. Lee, H.K. Kim, J. B. Baek, Nano Energy 13 (2015) 336-345. 10
[6] B. Lei, W. Fang, Y. Hou, J. Liao, D. Kuang, C. Su, J. Photochem. Photobiol. A: Chemistry 216 (2010) 8-14. [7] G.S. Liu, W.W. Zhang, Z.Q. Hu, J. Feng, Journal of Dalian Polytechnic University (2011) 50-53. [8] P. Y. Chen, C.T. Li, C. P. Lee, R. Vittal, C. C. Ho, Nano Energy 12 (2015) 374-385. [9] K. Lee, W. Chiu, H. Wei, C. Hu, V. Suryanarayanan, W. Hsieh, K. Ho, Thin Solid Films 518 (2010) 1716-1721. [10] C. P. Lee, C. A. Lin, T. C. Wei, M. L. Tsai, Y. Meng, C. T. Li, K. C. Ho, C. I. Wu, S. P. Lau, J. H. H. He, Nano Energy 18 (2015) 109-117. [11] T. Makris, V. Dracopoulos, T. Stergiopoulos, P. Lianos, Electrochim. Acta 56 (2011) 2004-2008. [12] S. Ameen, M.S. Akhtar, Y.S. Kim, O. Yang, H. Shin, J. Phys. Chem. C 114 (2010) 4760-4764. [13] Y. Xiao, G. Han, Y. Li, M. Li, Y. Chang, J. Mater. Chem. A 2 (2014) 3452-3460. [14] K. Lee, S. Cho, M. Kim, J. Kim, J. Ryu, K. Shin, J. Jang, J. Mater. Chem. A 3 (2015) 19018-19026. [15] Q. Tai, B. Chen, F. Guo, S. Xu, H. Hu, B. Sebo, X. Zhao, Acs Nano 5 (2011) 3795-3799. [16] J. Wu, Y. Li, Q. Tang, G. Yue, J. Lin, M. Huang, L. Meng, Scientific Reports 4 (2014). [17] Y. Yang, P. Yi, C. Zhou, J. Cui, X. Zheng, S. Xiao, X. Guo, W. Wang, J. Power Sources 243 (2013) 919-924. [18] S. Xiao, J. Cui, P. Yi, Y. Yang, X. Guo, Electrochim. Acta 144 (2014) 221-227. [19] H.B. Gao, Organic Chemistry, third ed., Higher Education Press, Beijing, (1993) 145-199. [20] Q.Y. Xin, R.Q. Xu, Basic Organic Chemistry, second ed., Higher Education Press, Beijing, (1993) 45-80. [21] M.M. Ayad, M.A. Shenashin, Eur. Polym. J. 39 (2003) 1319-1324. [22] J. Stejskal, I. Sapurina, J. Colloid Interface Sci. 274 (2004) 489-495. [23] S.F. Wen, Fourier Transform Infrared Spectrometer, Chemical Industry Press, Beijing, (2005) 33-68. [24] J. Tang, X. Jing, B. Wang, F. Wang, Synth. Met. 24 (1988) 231-238. [25] H. Huang, Z.C. Guo, The Preparation and Application of Conductive Polyaniline, Science Press, Beijing, (2010) 23-61. [26] I. Sapurina, A. Riede, J. Stejskal, Synth. Met. 123 (2001) 503-507. [27] K.W. Oh, S.H. Kim, E. Kim, J. Appl. Polym. Sci. 81 (2001) 684-694. [28] S. Ebrahim, M. Soliman, T.M. Abdel-Fattah, J. Electron. Mater. 40 (2011) 2033-2041. [29] F. Hua, E. Ruckenstein, J. Polym. Sci., Part A: Polym. Chem. 42 (2004) 2179-2191. [30] H. Wang, School of National University of Defense Technology, (2009) 20-52. [31] N. Fuke, A. Fukui, R. Komiya, A. Islam, Y. Chiba, M. Yanagida, R. Yamanaka, L. Han, Chem. Mater. 20 (2008) 4974-4979. [32] M. Wang, Y. Lin, X. Zhou, X. Xiao, L. Yang, S. Feng, X. Li, Mater. Chem. Phys. 107 (2008) 61-66. [33] K. Fredin, J. Nissfolk, G. Boschloo, A. Hagfeldt, J. Electroanal. Chem. 609 (2007) 55-60. [34] M. Wang, Y. Lin, X. Zhou, X. Xiao, L. Yang, S. Feng, X. Li, Mater. Chem. Phys. 107 (2008) 61-66. 11
[35] P. Chen, C. Li, C. Lee, R. Vittal, K. Ho, Nano Energy 12 (2015) 374-385. [36] Q. Li, X. Chen, Q. Tang, H. Xu, B. He, Y. Qin, J. Mater. Chem. A 1 (2013) 8055-8060. [37] A.J. Bard, L.R. Faulkner, Electrochemical Methods, 2nd ed., Wiley: New York (2001).
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Author biography Jing Gao is currently pursuing her Ph.D. degree under the supervision of Prof. Ying Yang in the School of Metallurgy and Environment, Central South University. Her research interests are mainly focused on bifacial dye-sensitized solar cells with low cost transparent counter electrode. Ying Yang received the Ph.D. degree in School of Physics and Technology, Wuhan University in 2009. She was a Postdoctoral Researcher in University of Wyoming, USA, from 2013-2014. Now she is an associate Professor in the School of Metallurgy and Environment, Central South University. Her research interests focus on new energy materials and devices. Zheng Zhang received his B.S. degree in School of Metallurgical engineer at Kunming University of science and technology. Now he is a master candidate of Metallurgical Engineer at Central South University. His research interests are dye-sensitized solar cells and the application of quantum dot in perovskite solar cells. Jingyuan Yan is an undergraduate majoring New Energy Materials and Devices in the School of Metallurgy and Environment, Central South University. She is now participating in Jing Gao’s project focusing on bifacial dye-sensitized solar cells.
Zehua Lin is an undergraduate majoring New Energy Material and Devices in the School of Metallurgy and Environment, Central South University. His reserch interests are focus on photovoltaic performance of quantum dot dye-sensitized solar cells. Xueyi Guo received his Ph.D. degree in Nonferrous Metals from Central South University in 1995. He was a visiting research scholar in the University of Tokyo, Japan, from 2000-2003. Now he is a Full Professor in the School of Metallurgy and Environment, Central South University. His research interests focus on advanced technology of resources recycling and eco-materials.
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Table 1. The wavelength and intensity of absorption peaks for PVP/PANI films with different PVP concentrations, derived from the UV-Vis spectra as shown in Figure 3. λ1-Abs
λ2-Abs
λ3-Abs
Samples
λ1 (nm)
(a.u.)
λ2 (nm)
(a.u.)
λ3 (nm)
(a.u.)
PANI-0 wt% PVP
336
0.782
402
0.699
833
0.769
PANI-2 wt% PVP
341
0.859
403
0.806
838
0.889
PANI-3 wt% PVP
347
0.878
402
0.860
842
1.001
PANI-4 wt% PVP
351
0.908
403
0.883
845
1.098
PANI-5 wt% PVP
343
0.840
401
0.786
841
0.869
PANI-6 wt% PVP
339
0.825
402
0.771
836
0.854
Table 2. Photovoltaic parameters of Pt and PVP (4 wt%)/PANI based DSSCs under different illumination conditions, derived from the J-V curves as shown in Figure 6b. Illumination Front Rear Front+Rear
CEs
Jsc (mA/cm2)
Voc (V)
FF
η (%)
PVP(4w%)/PANI Pt PVP(4w%)/PANI Pt
20.32 14.96 19.08 3.26
0.57 0.66 0.55 0.54
0.46 0.55 0.43 0.62
5.45 5.57 4.66 1.14
PVP(4w%)/PANI Pt
25.20 17.10
0.54 0.58
0.44 0.56
5.81 5.70
14
CAPTIONS Schematic 1 Schematic of quasi-solid-sate dye-sensitized solar cell based on PVP/PANI as transparent counter electrode. Schematic 2 Reaction mechanism for preparing PVP/PANI counter electrode. Figure 1 FT-IR spectrum of (a) pure PVP; (b) pure PANI and PVP (4 wt%)/PANI films. Figure 2 The surface morphologies of PANI counter electrodes with different concentrations of PVP (wt%) (a) 0; (b) 2; (c) 3; (d) 4; (e) 5; (f) 6. Figure 3 UV-Vis absorption spectra of PVP/PANI counter electrodes with different PVP contents. Figure 4 (a) J-V curves of DSSCs based on Pt CE and PVP/PANI CEs with different PVP concentrations under front illumination (96.3 mW/cm2); (b) The short-circuit current density (Jsc), open circuit voltage (Voc), and power conversion efficiency (η) of DSSCs with Pt CE and PANI CEs with different PVP concentrations. Figure 5 (a) Nyquist plots of the EIS spectra of the DSSCs based on Pt and PANI CEs with different PVP concentrations; (b) The charge transfer resistance R1, charge recombination resistance R2, and ion diffusion resistance R3 of DSSCs with Pt CE and PANI CEs with different PVP concentrations. The EIS measurement was carried out under 96.3 mW/cm2 simulated by a Xe lamp and the forward bias was -0.8 V. Figure 6 (a) UV-Vis transmittance spectra of Pt and PVP (4 wt%)/PANI CEs; (b) J-V curves of DSSCs based on Pt and PVP (4 wt%)/PANI CEs under front, rear and both sides illuminations. (The incident light irradiation intensities from the front and the rear sides are 96.3 and 45.6 mW/cm2, respectively.) Figure 7 (a) Tafel plots of the symmetrical cells based on PVP/PANI CEs with different PVP concentrations and (b) Pt CE and PVP (4 wt%)/PANI CE. Figure 8 Stability test of Jsc, Voc and η variations with aging time for DSSCs based on PVP (4 wt%)/PANI and Pt CEs under illumination of 96.3 mW/cm2.
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Schematic 1
16
Schematic 2
17
Fig. 1
Fig. 2
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Fig. 3
Fig. 4
19
Fig. 5
Fig. 6
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Fig. 7
Fig. 8
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Highlights
Incorporating
PVP
to
PANI
obviously
improves
the
electrochemical
performances of transparent counter electrode. The rear-illuminated power conversion efficiency (4.66%) reaches 85.5% of that obtained from front side illumination (5.45%) for bifacial quasi-solid-state DSSC. An optimal efficiency of 5.81%, higher than that of Pt-based DSSC (5.7%), is achieved under illumination from both front and rear sides. DSSCs based on PVP/PANI transparent counter electrode exhibits better durability than that based on Pt.
Graphical abstract
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PII: DOI: Reference:
S2211-2855(16)30133-1 http://dx.doi.org/10.1016/j.nanoen.2016.05.010 NANOEN1275
To appear in: Nano Energy Received date: 19 December 2015 Revised date: 27 March 2016 Accepted date: 6 May 2016 Cite this article as: Jing Gao, Ying Yang, Zheng Zhang, Jingyuan Yan, Zehua Lin and Xueyi Guo, Bifacial Quasi-solid-state Dye-sensitized Solar Cells with Poly (vinyl pyrrolidone)/Polyaniline Transparent Counter Electrode, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2016.05.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Bifacial Quasi-solid-state Dye-sensitized Solar Cells with Poly (vinyl pyrrolidone)/Polyaniline Transparent Counter Electrode Jing Gao, Ying Yang, Zheng Zhang, Jingyuan Yan, Zehua Lin, Xueyi Guo School of Metallurgy and Environment, Central South University, Changsha 410083, China
Abstract: We synthesized poly (vinyl pyrrolidone) (PVP)/polyaniline (PANI) nanocomposites and applied them as transparent counter electrode (CE) for bifacial quasi-solid-state dye-sensitized solar cells (DSSCs). PVP surrounded with aniline acted as efficient steric stabilizer in the polymerization process. Moreover, the present of PVP in PANI CE facilitated the generation of active reaction-sites in the interface between counter electrode and electrolyte and further reduced the electron recombination. The DSSC fabricated with PVP (4 wt%)/PANI CE exhibited improved power conversion efficiency up to 5.45%, which was comparable to that of DSSC assembled with Pt CE (5.57%). In the case of rear-illumination, the PVP (4 wt%)/PANI based DSSC showed an efficiency of 4.66%, which was 85.5% of the efficiency obtained from front illumination and much higher than that of DSSC with Pt CE (1.14%). The bifacial DSSC with PVP/PANI counter electrode showed an optimal power efficiency of 5.81% under illumination from both front and rear sides, which is higher than that of Pt based DSSC (5.70%) under the same condition. Long-term stability tests indicated that the photovoltaic device with PVP/PANI counter electrode exhibited an enhanced durability than that of DSSC assembled with Pt CE.
Keywords: Polyaniline, Poly (vinyl pyrrolidone), Transparent counter electrode, Polymer electrolyte, Quasi-solid-state dye-sensitized solar cell
Corresponding authors. Tel.: +86 731 88877863; fax: +86 731 88836207.
E-mail address: [email protected]; [email protected]
1
Introduction Currently, platinum (Pt) is widely exploited as a counter electrode (CE) material for dye-sensitized solar cells (DSSCs). It is notable that Pt CE utilized in DSSCs has issues such as complex fabrication, high price, and it could be dissolved in the electrolyte to form PtI4 and H2PtI6 due to the corrosive I-/I3- redox electrolyte [1, 2]. As a result various other materials are utilized for counter electrode to instead Pt in DSSCs, such as carbon-based materials including carbon nanotubes [3, 4], graphite [5], carbon black [6] and fullerenes C60 [7]; as well as conducting polymers containing poly(3,4-ethylenedioxythiophene) (PEDOT) [8-10], polypyrrole (PPy) [11] and polyaniline (PANI) [12, 13]. Among them, conductive polymers have shown promising performances due to its low cost, high electrical conductivity and good environmental stability. PANI film, owing to its high catalytic activity and excellent transparency in the visible region, attracts significant research interests in investigating DSSCs. Lee et al. [14] reported a highly porous PANI using carbon nanodots (CNDs) as a nucleating agent to improve the conversion efficiency of DSSCs. Bifacial DSSCs based on transparent counter electrode could increase the incident light harvest and improve the conversion efficiency of cells. Recently, Tai et al. [15] reported a bifacial DSSC with a transparent PANI CE, yielding a front-illuminated and rear-illuminated power conversion efficiency of 6.54% and 4.26% with liquid electrolyte, respectively. The more recently, Wu et al. [16] obtained an efficiency of 7.05% for bifacial DSSC with PANI CE irradiated from the both sides of photoanode and counter electrode. Although several investigations have been reported on high-quality transparent PANI films, most of them were applied in bifacial DSSCs with liquid electrolyte and none has reported the long-term stability of bifacial quasi-solid-state DSSCs with PANI counter electrode yet. Meanwhile, the best rear illumination power conversion efficiency is only 65% of the efficiency obtained from front illumination [15], which may be caused by the existence of PANI conglomerations during interfacial polymerization. Further improving the stability and the rear-illumination power conversion efficiency of bifacial DSSCs is significant for its application. In this work, we utilize Poly (vinyl pyrrolidone) (PVP)/PANI transparent film as the counter electrode to create bifacial quasi-solid-state DSSCs based on agarose polymer electrolyte as seen in Schematic 1. Poly (vinyl pyrrolidone) is an amphiphilic 2
water-soluble polymer which has excellent solubility, chemical stability, film-forming property and bonding capacity. Introducing PVP into PANI would enhance the uniformity of PANI nanoparticles and also improve the adhesion of the composite film to the substrate, thus can efficiently increase rear power efficiency. The microstructure and optical properties of PVP/PANI films are characterized. And the photovoltaic and electrochemical characteristics of quasi-solid-state DSSCs fabricated using these transparent counter electrodes are also studied and compared with those of the DSSCs with Pt CE. It is found that the rear illumination power conversion efficiency can reach 85.5% of the efficiency obtained from front illumination. A power efficiency of 5.81% is achieved for bifacial quasi-solid-state DSSC with PVP/PANI counter electrode by illumination from both front and rear sides, which is higher than that of Pt based DSSC (5.70%) under the same condition. The long-term stability measurement result shows that DSSCs with PVP/PANI counter electrode can improve the solar cell stability. Experimental Fabrication of PVP/PANI composite counter electrodes Different concentrations (0, 2, 3, 4, 5, 6 wt%) of Poly (vinyl pyrrolidone) (PVP, 99.8% purity, Urchem) were separately introduced to 50 mL of 0.5 M hydrochloric acid (HCl) containing 25 mM aniline (ANI, 99% purity, TCI), then another 50 mL 0.5 M HCl solution containing 25 mM ammonium persulfate (APS, 98% purity, TCI) was added, followed by immersing a piece of FTO glass (surface resistance =15 Ω sq-1, area = 1.5×2 cm2) into the aniline hydrochloride which is cleaned subsequently in acetone, deionized (DI) water and ethyl alcohol by ultrasonator for 15 min. After being kept for 20 min around -3 oC, the obtained HCl doped PANI films were washed repeatedly with DI water and 1 M HCl to remove the large deposited PANI grains and residual reagents. Finally, the resultant PVP/PANI films were immersed in 1 M HCl for 4 h and dried at 80 oC in vacuum for 30 min. Assembly of DSSCs The preparation of the dyed TiO2 photoanodes and the agarose polymer electrolytes were prepared according to the details in previous reports [17, 18]. For DSSC assembly, the polymer electrolyte solution was dropped on the dyed TiO2 film in an oven at 75 ℃. As soon as the polymer electrolyte became viscous, the obtained PVP/PANI and Pt counter electrode was placed on the TiO2/agarose electrolyte 3
structure to form a sandwich structure. The Pt CE used here was obtained by sputtering 10 nm Ti and 50 nm Pt on FTO glass. Then the whole device was dried at 75 ℃ for another 2.5 h to remove the solvent and create a quasi-solid-state DSSC. Characterization and measurements The surface morphologies of PVP/PANI films were investigated by a Scanning Electron Microscope (SEM, JSM-6360LV, JEOL, Japan). Ultraviolet-visible spectrophotometer (UV-Vis, Hitachi, Tokyo, Japan) was used to measure the absorption and transmittance of the PVP/PANI film. Fourier transform infrared (FT-IR) spectrum was obtained by using Fourier transform infrared spectroscopy (Thermo Nicolet Corporation, Nexus 670, America) to confirm the reaction between PVP and PANI. Electrochemical impedance spectra (EIS) measurement of the DSSCs was performed with an electrochemical analyzer (CHI604D, Chenhua, Shanghai, China) under illumination of 96.3 mW/cm2 with a bias of -0.8 V, a frequency range of 105-10-1 Hz and a perturbation voltage of 10 mV. With the same instrument, Tafel polarization plots were obtained using symmetrical cells of various CEs, controlling the scan rate at 10 mV/s. Photon-to-current conversion efficiencies of the DSSCs were evaluated by measuring the current-voltage (J-V) curves under irradiation from a Xe lamp (CHF–XM500, Trust–tech, China) with electrochemical analyzer (CHI604D) as recorder. The intensity of the incident light was calibrated by a Si-1787 photodiode (spectral response range 320-730 nm). The incident light intensity was 96.3 mW/cm2 with the active cell area of 0.25 cm2. Results and discussion Reaction mechanism for preparing PVP/PANI composite CE The reaction mechanism of preparing PVP/PANI composite counter electrode is schematically described in Schematic 2. The carbonyl group (C=O) of lactam in PVP receives protons to form a resonance structure of carbon positive ions (Formula 1), and the –NH2 group of ANI attacks the central carbon atom of carbon positive ions (Formula 2). Then the resultant grafts to the lactam and carbonyl groups convert to alcoholic hydroxyl (–OH) (Formula 3). In an acidic environment, the alcoholic hydroxyl group form an imine bond (C=N) by intramolecular dehydration (Formula 4) [19, 20]. When APS is added, the free aniline is oxidized and converts to a radical cation (ANI+) (Formula 5). Similarly, the benzene ring that connects with the PVP 4
chains losses electrons under the oxidation of APS and form a radical cations with strong activity (Formula 6). The polymerization between cation radicals and PVP side chains (Formula 7) happens autocatalytic [21] on the active sites of stabilizer and forms PVP/PANI composite film on FTO as seen in Schematic 2 [22]. FT-IR spectrum of PVP/PANI CEs To ascertain the interaction between PVP and the synthesized PANI, the FT-IR spectrum was carried out. Figure 1 shows FTIR of pure PVP (Figure 1a), pure PANI film and PVP (4 wt%)/PANI composite film (Figure 1b). The characteristic absorption bands at 3440 cm-1, 2958 cm-1, 1645 cm-1, 1450 cm-1, 1290 cm-1 in Figure 1a correspond to the O-H, CH2 antisymmetric stretching mode, C=O, CH2 in-plane swing vibration, and C-N stretching in PVP, respectively [23]. The bands around 1655 cm-1, 1580 cm-1, 1115 cm-1 in Figure1b are assigned to the C=N, C=C, C-H stretching mode of the quinoid ring, and 1479 cm-1, 1290 cm-1, 795 cm-1 are assigned to the C=C of benzoid rings, C-N stretching of the secondary aromatic amine, and an aromatic C-H out-of-plane bending vibration on 1,4-ring of PANI, respectively [24]. While the ∙
stretching mode of C-N+ polarons also appears at 1240 cm-1, indicating that the PANI polymer film has doping characteristic. In addition, the intensity of 1240 cm-1 peak ∙
increases after adding PVP which is caused by more C-N+ polaron formed at conjugated PANI chains. Meanwhile, compare the spectrum of PANI film with PVP (4 wt%)/PANI film, it can be found that the intensity of peak around 1645 cm-1 for C=N stretching increases obviously, and get slight red-shift to 1655 cm-1 after adding PVP. This is attributed to much more C=N bonds and conjugation structures produced by the reaction between carbonyl and amine group [25]. Morphologies of PVP/PANI CEs Figure 2a ~ f show the SEM images of PVP/PANI composite films with different PVP concentrations (0, 2, 3, 4, 5 and 6 wt%). It is seen that there are irregular PANI macroscopic precipitations observed on the PANI film without adding PVP, which results in a rough and heterogeneous surface. Increasing PVP concentration to 4 wt% leads to a relatively smooth and homogeneous morphology as shown in Figure 2b ~ d. Further adding PVP up to 5 wt% attributes to the formation of PANI aggregates (Figure 2e ~ f). 5
It has been confirmed that the C=N group at PVP side chains provides a new reactive site for the polymerization of the adsorbed aniline oligomers and promotes the PANI growing in a more ordered manner compared with that without stabilizer [26, 27], and more active ANI+ or oligomeric intermediates grafted to PVP side chains to form homogeneous film as increasing PVP concentration. However, irregular aggregations can be seen in samples with PVP content above 4 wt% in Figure 2e ~ f, which may be attributed to the increased solution viscosity caused by the decreased solubility of PVP, leading to tangled molecular and aggregation. UV-Vis absorption analysis of PVP/PANI CEs Figure 3 shows the UV-Vis absorption spectra of PVP/PANI films with different PVP concentrations. The peak around 345 nm (λ1) represents the characteristic absorption peak of π-π* transition that is typical to benzene ring of PANI polymer [28], and the peaks around 402 nm (λ2) and 840 nm (λ3) are assigned to the excitation transition of polarons and bipolarons of doping PANI [29], which may be caused by the interband charge transfer from benzenoid to quinoid rings of the protonated PANI molecule. The wavelength and intensity of the absorption peaks for different PVP/PANI films are listed in Table 1. The wavelengths of λ1 and λ3 show slightly red-shift as PVP content increases from 0 to 4 wt%. This could be attributed to the decreased π-π* and bipolaron transition band gap caused by the increased conjugation structures formed at PANI chains after adding PVP, respectively. Consequently, the decreased transition band gap means there are more transitions of π-electrons and bipolarons, which leads to the increased absorption intensity of λ1 and λ3 [30]. However, the reduced λ1 and λ3 intensity (PVP > 4 wt%) could result from the overmuch PVP long chains intertwine with each other and lead to a decreased conjugation degree. In addition, due to the main factor that influences the transition band gap of polaron is the conjugated molecule type of polymer structure, the positon of polaron transition peak λ2 around 402 nm almost remains unchanged under different PVP concentrations. Front-illuminated photovoltaic and electrochemical performances of DSSCs based on Pt and PVP/PANI CEs The current density-voltage (J-V) curves for DSSCs fabricated with Pt CE and PVP/PANI CEs with different PVP content under front illumination are plotted in 6
Figure 4a. The derived photovoltaic parameters, including the short-circuit current density (Jsc), open circuit voltage (Voc), and power conversion efficiency (η) are presented in Figure 4b, respectively. It can be observed that Jsc increases after introducing PVP and reaches the maximum when the PVP concentration of 4 wt%. The Voc of the DSSCs with PVP/PANI CEs changes from 0.53 to 0.58 V as PVP content increases. Pt CE-based DSSC shows much higher Voc (0.66 V) compared to that of all the PVP/PANI based devices. The optimum overall power conversion efficiency 5.45% is obtained with the device based on PVP (4 wt%)/PANI CEs, which is comparable with the DSSC assembled with Pt CE (η = 5.57%) under the same conditions. To further analyze the effect of PVP content on the electrochemical performances of these DSSCs, electrochemical impedance spectroscopy (EIS) measurement was carried out. Figure 5a shows Nyquist plots of the EIS spectra. There are three arcs can be observed, which corresponds to the charge transfer process at CEs/electrolyte interface at high frequency (105-103 Hz), the charge recombination process at the TiO2/dye/electrolyte interface at middle frequency (103-1 Hz), and the ion diffusion process occurring in the electrolyte at low frequency (1-10-1 Hz), respectively [31, 32]. Figure 5b shows the change of R1, R2 and R3 with PVP contents, where R1 represents the charge transfer resistance at the PVP/PANI CE/electrolyte interface, R2 represents the recombination resistance at the dyed-TiO2 /electrolyte interface, and R3 represents the ion diffusion resistance in the DSSC [33]. In Figure 5b, R1 decreases from 15.41 Ω to 12.39 Ω as the PVP content increases from 0 to 4 wt%, indicating that the reduction of triiodide ions at PANI CE/electrolyte interface is promoted after introducing PVP. This decrease in R1 could result from the improved uniform PANI morphology and larger conjugation degree as discussed in Figure 2 ~ 3, which is benefit to the connection between counter electrodes and electrolyte and accelerates electron transport to facilitate triiodide reduction process (I3- + e = 3I-) [32]. The decreased R1 would result in the improved Jsc as seen in Figure 4b [34]. The recombination resistance R2 increases from 180.91 Ω to 216.31 Ω with PVP content increases from 0 to 4 wt%, suggesting that the electron recombination process is efficiently suppressed by introducing PVP [32]. This could be attributed to the improved electro-catalytic ability of PVP/PANI CE which facilitates the reduction of I3- on the surface of PVP/PANI CEs, leading to reduced electron recombination. However, R2 decreases when PVP concentration exceeds 4 wt%, which probably 7
accounts for the increased defects caused by the destroyed homogeneous structure of PANI CE with higher PVP contents. These defects are in favor of electron recombination. In addition, it’s obvious that the recombination resistance (R2) of Pt CE-based DSSC is much higher than that of DSSCs assembled with PVP/PANI CEs (Figure 5b), which may result from the more compact structures of Pt CE obtained by sputtering method and suppress the charge recombination at the interface between Pt CE and electrolyte, leading to improved Voc of Pt CE-based device as seen in Figure 4. The ion diffusion resistance R3 decreases after adding PVP until 4 wt%, which attributes to the increased conjugation degree as discussed above and thus improves I-/I3- diffusion in the electrolyte. Photovoltaic performances of DSSCs based on PVP/PANI and Pt CEs under front and rear side illumination In Figure 6a, the thin PVP (4 wt%)/PANI film shows excellent transparency from 450-700 nm and obtains its maximum transmittance of 65% at 536 nm, while the transmittance of Pt CE is as low as 15% from 420-840 nm. The transmittance of CE affects greatly on the incident light harvest and the rear illuminated conversion efficiency of bifacial DSSC. Figure 6b compares the J-V curves of the bifacial DSSCs based on PVP (4 wt%)/PANI and Pt CEs irradiated from the front side, rear side and both front and rear sides. To enhance the rear side light harvesting of bifacial DSSCs, a mirror is placed under the rear side of the device to reflect the light from the light source, which is defined as the bifacial DSSCs that illuminated from both front and rear sides. The results of photovoltaic parameters of Pt and PVP (4 wt%)/PANI based DSSCs under different illumination conditions are summarized in Table 2. When irradiated from the rear side, the device based on PVP (4 wt%)/PANI CE exhibits an efficiency of 4.66%, which is nearly 85.5% of that from front illumination (5.45%). This comparable efficiency under rear illumination may be caused by the improved uniformity of PANI counter electrode after introducing PVP in PANI. However, the device based on Pt CE only shows as low as 1.14%, which is due to its low transmittance of light under rear side illumination. From Table 2, it also can be confirmed that illumination from both front and rear sides can effectively enhance the power conversion efficiency of bifacial DSSCs with PVP/PANI CE from 5.45% (front illumination) to 5.81% (both front and rear illumination), which is higher than that of Pt based DSSC (5.70%) under the same illumination condition. The incident 8
light irradiation intensities from the front and the rear sides are 96.3 and 45.6 mW/cm2, respectively. Tafel polarization plots of PVP/PANI and Pt CEs Further, to confirm the electro-catalytic activity of the counter electrodes, the Tafel polarization plots were measured as seen in Figure 7. Generally, there are three divided zones in Tafel plot: (1) polarization curves (|V|<120 mV), (2) Tafel zone (120 mV<|V|<400 mV), and (3) diffusion zone (|V|>400 mV) [35]. In the Tafel zone, the larger slope means a higher exchange current density (J0) and better catalytic activity [36]. And in the diffusion zone, the limiting diffusion current density (Jlim) is the intersection of the cathodic branch with the Y-axis, which determins the diffusion properties of the redox couple and the CE catalysts. As seen in Figure 7a, the slope for PANI CEs increases with PVP concentrations increasing and reaches the maximum at PVP content of 4 wt%, indicating that the best catalytic activity of PVP(4 wt%)/PANI CE. Moreover, the RTafel value of counter electrodes can be measured via the following equation [37]:
Obviously, the RTafel is in inversely proportional to J0, meaning the RTafel values follow the same trend as the R1-EIS values as seen in Figure 5b. In addition, the Jlim of PVP(4 wt%)/PANI CE is greatly larger than that of Pt CE, suggesting a higher electron diffusion velocity for the redox couple in the electrolyte, so the faster electron transporting circulation may be one reason for the higher Jsc of PVP(4 wt%)/PANI, as seen in J-V curves of DSSCs (Figure 4a). Stability of DSSCs based on PVP/PANI and Pt CEs Figure 8 shows the normalized current density (Jsc), open circuit voltage (Voc) and power conversion efficiency () as a function of aging time for the DSSCs based on Pt and PVP (4 wt%)/PANI CEs. The normalized efficiency of the DSSC with Pt CE exhibits a ~74.9% decrease after 240 h testing (without sealing), while the cells with PVP (4 wt%)/PANI CE show slow degradation (~49.1%) compared with that of Pt CE under the same condition. This improvement in DSSC long-term stability may be attributed to the more conjugated and uniform porous film provided by PVP, leading to an improved interfacial connection between polymer electrolyte and counter 9
electrode. It is found that Jsc of both DSSCs with Pt and PVP/PANI CEs rapidly decreases as the aging time increases, which may be caused by the evaporation of the solvent in polymer electrolyte during the storage without sealing and can be avoided with well sealing. The evaporation of solvent destroys the cross–linking networks in polymer electrolyte and impedes the carrier transport, resulting in inferior Jsc as aging time increases. The Voc of the DSSCs with PVP (4 wt%)/PANI CE remains almost unchanged during the time goes by, indicating the electron recombination process is almost unchanged in DSSC. Conclusions In summary, in this work we developed a PVP-incorporated PANI counter electrode for quasi-solid-state DSSC applications. After adding PVP as a steric stabilize with proper concentration, the PVP/PANI counter electrode exhibited the improved conjugation degree, which was benefit to the connection between polymer electrolyte and PANI counter electrode and efficiently enhanced DSSC performances. EIS characterization further revealed the improvement on electron recombination and redox transport process in the device. Meanwhile, the stability tests indicated that the PVP (4 wt%)/PANI CE showed better durability than that of Pt CE-based DSSC. This kind of PVP/PANI counter electrode could be a potential substitute for noble metal electrode for both DSSC and perovskite solar cells. Acknowledgments. We gratefully acknowledge the financial support of this work by the Third Innovation Driven Project of Central South University (2016CX022); the Teacher Research Fund of Central South University (905010104); Scientific Research Foundation for the Returned Overseas Chinese Scholar, State Education Ministry and the projects of innovation for graduate student in CSU (201510533205, CX2015237). References [1] E. Olsen, G. Hagen, S.E. Lindquist, Sol. Energ. Mat. Sol. C. 63 (2000) 267-273. [2] S. Huang, Q. He, W. Chen, J. Zai, Q. Qiao, X. Qian, Nano Energy 15 (2015) 205-215. [3] P. Poudel, A. Thapa, H. Elbohy, Q. Q. Qian, Nano Energy 5 (2014) 116-121. [4] P. Poudel. Q. Q. Qiao, Nano Energy 4 (2014) 157-175. [5] I.Y. Jeon, H. M. Kim, I. T. Choi, K. Lim, J. Ko, J. C. Kim, H, J. Choi, M. J. Ju, J. J. Lee, H.K. Kim, J. B. Baek, Nano Energy 13 (2015) 336-345. 10
[6] B. Lei, W. Fang, Y. Hou, J. Liao, D. Kuang, C. Su, J. Photochem. Photobiol. A: Chemistry 216 (2010) 8-14. [7] G.S. Liu, W.W. Zhang, Z.Q. Hu, J. Feng, Journal of Dalian Polytechnic University (2011) 50-53. [8] P. Y. Chen, C.T. Li, C. P. Lee, R. Vittal, C. C. Ho, Nano Energy 12 (2015) 374-385. [9] K. Lee, W. Chiu, H. Wei, C. Hu, V. Suryanarayanan, W. Hsieh, K. Ho, Thin Solid Films 518 (2010) 1716-1721. [10] C. P. Lee, C. A. Lin, T. C. Wei, M. L. Tsai, Y. Meng, C. T. Li, K. C. Ho, C. I. Wu, S. P. Lau, J. H. H. He, Nano Energy 18 (2015) 109-117. [11] T. Makris, V. Dracopoulos, T. Stergiopoulos, P. Lianos, Electrochim. Acta 56 (2011) 2004-2008. [12] S. Ameen, M.S. Akhtar, Y.S. Kim, O. Yang, H. Shin, J. Phys. Chem. C 114 (2010) 4760-4764. [13] Y. Xiao, G. Han, Y. Li, M. Li, Y. Chang, J. Mater. Chem. A 2 (2014) 3452-3460. [14] K. Lee, S. Cho, M. Kim, J. Kim, J. Ryu, K. Shin, J. Jang, J. Mater. Chem. A 3 (2015) 19018-19026. [15] Q. Tai, B. Chen, F. Guo, S. Xu, H. Hu, B. Sebo, X. Zhao, Acs Nano 5 (2011) 3795-3799. [16] J. Wu, Y. Li, Q. Tang, G. Yue, J. Lin, M. Huang, L. Meng, Scientific Reports 4 (2014). [17] Y. Yang, P. Yi, C. Zhou, J. Cui, X. Zheng, S. Xiao, X. Guo, W. Wang, J. Power Sources 243 (2013) 919-924. [18] S. Xiao, J. Cui, P. Yi, Y. Yang, X. Guo, Electrochim. Acta 144 (2014) 221-227. [19] H.B. Gao, Organic Chemistry, third ed., Higher Education Press, Beijing, (1993) 145-199. [20] Q.Y. Xin, R.Q. Xu, Basic Organic Chemistry, second ed., Higher Education Press, Beijing, (1993) 45-80. [21] M.M. Ayad, M.A. Shenashin, Eur. Polym. J. 39 (2003) 1319-1324. [22] J. Stejskal, I. Sapurina, J. Colloid Interface Sci. 274 (2004) 489-495. [23] S.F. Wen, Fourier Transform Infrared Spectrometer, Chemical Industry Press, Beijing, (2005) 33-68. [24] J. Tang, X. Jing, B. Wang, F. Wang, Synth. Met. 24 (1988) 231-238. [25] H. Huang, Z.C. Guo, The Preparation and Application of Conductive Polyaniline, Science Press, Beijing, (2010) 23-61. [26] I. Sapurina, A. Riede, J. Stejskal, Synth. Met. 123 (2001) 503-507. [27] K.W. Oh, S.H. Kim, E. Kim, J. Appl. Polym. Sci. 81 (2001) 684-694. [28] S. Ebrahim, M. Soliman, T.M. Abdel-Fattah, J. Electron. Mater. 40 (2011) 2033-2041. [29] F. Hua, E. Ruckenstein, J. Polym. Sci., Part A: Polym. Chem. 42 (2004) 2179-2191. [30] H. Wang, School of National University of Defense Technology, (2009) 20-52. [31] N. Fuke, A. Fukui, R. Komiya, A. Islam, Y. Chiba, M. Yanagida, R. Yamanaka, L. Han, Chem. Mater. 20 (2008) 4974-4979. [32] M. Wang, Y. Lin, X. Zhou, X. Xiao, L. Yang, S. Feng, X. Li, Mater. Chem. Phys. 107 (2008) 61-66. [33] K. Fredin, J. Nissfolk, G. Boschloo, A. Hagfeldt, J. Electroanal. Chem. 609 (2007) 55-60. [34] M. Wang, Y. Lin, X. Zhou, X. Xiao, L. Yang, S. Feng, X. Li, Mater. Chem. Phys. 107 (2008) 61-66. 11
[35] P. Chen, C. Li, C. Lee, R. Vittal, K. Ho, Nano Energy 12 (2015) 374-385. [36] Q. Li, X. Chen, Q. Tang, H. Xu, B. He, Y. Qin, J. Mater. Chem. A 1 (2013) 8055-8060. [37] A.J. Bard, L.R. Faulkner, Electrochemical Methods, 2nd ed., Wiley: New York (2001).
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Author biography Jing Gao is currently pursuing her Ph.D. degree under the supervision of Prof. Ying Yang in the School of Metallurgy and Environment, Central South University. Her research interests are mainly focused on bifacial dye-sensitized solar cells with low cost transparent counter electrode. Ying Yang received the Ph.D. degree in School of Physics and Technology, Wuhan University in 2009. She was a Postdoctoral Researcher in University of Wyoming, USA, from 2013-2014. Now she is an associate Professor in the School of Metallurgy and Environment, Central South University. Her research interests focus on new energy materials and devices. Zheng Zhang received his B.S. degree in School of Metallurgical engineer at Kunming University of science and technology. Now he is a master candidate of Metallurgical Engineer at Central South University. His research interests are dye-sensitized solar cells and the application of quantum dot in perovskite solar cells. Jingyuan Yan is an undergraduate majoring New Energy Materials and Devices in the School of Metallurgy and Environment, Central South University. She is now participating in Jing Gao’s project focusing on bifacial dye-sensitized solar cells.
Zehua Lin is an undergraduate majoring New Energy Material and Devices in the School of Metallurgy and Environment, Central South University. His reserch interests are focus on photovoltaic performance of quantum dot dye-sensitized solar cells. Xueyi Guo received his Ph.D. degree in Nonferrous Metals from Central South University in 1995. He was a visiting research scholar in the University of Tokyo, Japan, from 2000-2003. Now he is a Full Professor in the School of Metallurgy and Environment, Central South University. His research interests focus on advanced technology of resources recycling and eco-materials.
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Table 1. The wavelength and intensity of absorption peaks for PVP/PANI films with different PVP concentrations, derived from the UV-Vis spectra as shown in Figure 3. λ1-Abs
λ2-Abs
λ3-Abs
Samples
λ1 (nm)
(a.u.)
λ2 (nm)
(a.u.)
λ3 (nm)
(a.u.)
PANI-0 wt% PVP
336
0.782
402
0.699
833
0.769
PANI-2 wt% PVP
341
0.859
403
0.806
838
0.889
PANI-3 wt% PVP
347
0.878
402
0.860
842
1.001
PANI-4 wt% PVP
351
0.908
403
0.883
845
1.098
PANI-5 wt% PVP
343
0.840
401
0.786
841
0.869
PANI-6 wt% PVP
339
0.825
402
0.771
836
0.854
Table 2. Photovoltaic parameters of Pt and PVP (4 wt%)/PANI based DSSCs under different illumination conditions, derived from the J-V curves as shown in Figure 6b. Illumination Front Rear Front+Rear
CEs
Jsc (mA/cm2)
Voc (V)
FF
η (%)
PVP(4w%)/PANI Pt PVP(4w%)/PANI Pt
20.32 14.96 19.08 3.26
0.57 0.66 0.55 0.54
0.46 0.55 0.43 0.62
5.45 5.57 4.66 1.14
PVP(4w%)/PANI Pt
25.20 17.10
0.54 0.58
0.44 0.56
5.81 5.70
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CAPTIONS Schematic 1 Schematic of quasi-solid-sate dye-sensitized solar cell based on PVP/PANI as transparent counter electrode. Schematic 2 Reaction mechanism for preparing PVP/PANI counter electrode. Figure 1 FT-IR spectrum of (a) pure PVP; (b) pure PANI and PVP (4 wt%)/PANI films. Figure 2 The surface morphologies of PANI counter electrodes with different concentrations of PVP (wt%) (a) 0; (b) 2; (c) 3; (d) 4; (e) 5; (f) 6. Figure 3 UV-Vis absorption spectra of PVP/PANI counter electrodes with different PVP contents. Figure 4 (a) J-V curves of DSSCs based on Pt CE and PVP/PANI CEs with different PVP concentrations under front illumination (96.3 mW/cm2); (b) The short-circuit current density (Jsc), open circuit voltage (Voc), and power conversion efficiency (η) of DSSCs with Pt CE and PANI CEs with different PVP concentrations. Figure 5 (a) Nyquist plots of the EIS spectra of the DSSCs based on Pt and PANI CEs with different PVP concentrations; (b) The charge transfer resistance R1, charge recombination resistance R2, and ion diffusion resistance R3 of DSSCs with Pt CE and PANI CEs with different PVP concentrations. The EIS measurement was carried out under 96.3 mW/cm2 simulated by a Xe lamp and the forward bias was -0.8 V. Figure 6 (a) UV-Vis transmittance spectra of Pt and PVP (4 wt%)/PANI CEs; (b) J-V curves of DSSCs based on Pt and PVP (4 wt%)/PANI CEs under front, rear and both sides illuminations. (The incident light irradiation intensities from the front and the rear sides are 96.3 and 45.6 mW/cm2, respectively.) Figure 7 (a) Tafel plots of the symmetrical cells based on PVP/PANI CEs with different PVP concentrations and (b) Pt CE and PVP (4 wt%)/PANI CE. Figure 8 Stability test of Jsc, Voc and η variations with aging time for DSSCs based on PVP (4 wt%)/PANI and Pt CEs under illumination of 96.3 mW/cm2.
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Schematic 1
16
Schematic 2
17
Fig. 1
Fig. 2
18
Fig. 3
Fig. 4
19
Fig. 5
Fig. 6
20
Fig. 7
Fig. 8
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Highlights
Incorporating
PVP
to
PANI
obviously
improves
the
electrochemical
performances of transparent counter electrode. The rear-illuminated power conversion efficiency (4.66%) reaches 85.5% of that obtained from front side illumination (5.45%) for bifacial quasi-solid-state DSSC. An optimal efficiency of 5.81%, higher than that of Pt-based DSSC (5.7%), is achieved under illumination from both front and rear sides. DSSCs based on PVP/PANI transparent counter electrode exhibits better durability than that based on Pt.
Graphical abstract
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