Improved Long-term Stability of Dye-Sensitized Solar Cell by Zeolite Additive in Electrolyte

Accepted Manuscript Title: Improved Long-term Stability of Dye-Sensitized Solar Cell by Zeolite Additive in Electrolyte Authors: Saad Sarwar, Kwan-Woo Ko, Jisu Han, Chi-Hwan Han, Yongseok Jun, Sungjun Hong PII: DOI: Reference:

S0013-4686(17)31225-2 http://dx.doi.org/doi:10.1016/j.electacta.2017.05.191 EA 29626

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

28-2-2017 25-5-2017 29-5-2017

Please cite this article as: Saad Sarwar, Kwan-Woo Ko, Jisu Han, ChiHwan Han, Yongseok Jun, Sungjun Hong, Improved Long-term Stability of Dye-Sensitized Solar Cell by Zeolite Additive in Electrolyte, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.05.191 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 proof before it is published in its final 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.

Improved Long-term Stability of Dye-Sensitized Solar Cell by Zeolite Additive in Electrolyte Saad Sarwara,b, Kwan-Woo Koa, Jisu Hana,b, Chi-Hwan Hana,*, Yongseok Junc,**, Sungjun Honga,*** a

Korea Institute of Energy Research, 71-2, Jangdong, Yuseong, Daejeon 305-343, Republic of Korea

b

University of Science & Technology, 217, Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea

c

Department of Materials Chemistry and Engineering, Konkuk University, Seoul 143-701, Republic of Korea

*Co-Corresponding author: Tel: +82. 2. 860. 3061 **Co-Corresponding author: Tel: +82. 2. 450. 0440 ***Corresponding author: Tel:+82. 42. 860. 3205

E-mail addresses: [email protected] (S. Sarwar), [email protected] (K.-W. Ko), [email protected] (J. Han), [email protected] (C.-H. Han), [email protected] (Y. Jun), [email protected] (S. Hong).

HIGHLIGHTS  Novel Nano-ZSM based electrolyte is prepared for DSSCs.  Nano-ZSM additive acts as both incident light scatter and moisture scavenger.  Nano-ZSM additive improves long-term thermal stability of DSSCs.

Abstract In this study, we demonstrate dye-sensitized solar cells showing an improved long-term stability at a high temperature by incorporating Zeolite molecular sieve in ionic liquid based electrolytes. The short-circuit photocurrent density of devices with 5wt% of Zeolite molecular sieve increases by 17% on average relative to that of device without any additive, mainly owing to its light scattering effect. Moreover, the devices containing Zeolite molecular sieve show remarkable enhancement of the thermal stability at 60°C for a period of 1200 hours under dark condition with a marginal variation of performance. On the contrary, the performance of devices without additives is continuously deteriorated during this period because of adverse effects of trace waters present in electrolytes, which can lead to dye molecules' detachment or degradation. This research study will pave a new way to fabricate thermally stable dye-sensitized solar cells with high efficiency. 1

Keywords:Dye-sensitized solar cells, electrolytes, nano molecular sieve, long-term stability

1. Introduction After the COP21 (Paris agreement)[1], it is a critical time to focus the research on renewable energies if the goals of the agreement are to be achieved ever. Solar energy is the most abundant form of renewable energy with the sun providing the annual needs of planet earth in just a few hours theoretically[2]. Therefore, photovoltaic devices are attractive solutions for the energy applications. Dye-Sensitized Solar Cells (DSSCs)[3] have many advantages among photovoltaic devices such as a simple fabrication process, benign condition for the fabrication, and its possible applications as Building Integrated Photovoltaics (BIPV)[3–7] based on semi-transparencies with colors. After 25 years of intensive research, DSSCs are on the verge of commercialization as the submodule efficiencies exceed 8.8%[8]. Nevertheless, a long-term stability of DSSCs at high temperature has remained as a major bottleneck to their penetration into commercial sectors[9] as solar cells are subjected to harsh temperature and humidity when operating in the real conditions[10]. There are several factors influencing the long-term stability of DSSCs. Among them, the incorporation of liquid electrolytes in DSSC always possesses possibilities of the leakage and volatilization of solvents which results in a gradual performance deterioration of DSSC[11–13]. In addition, water intrusion in DSSCs is another hindrance to achieve long-term stability which causes the dye molecules to detach from photoanode[14] via a hydrolysis due to weak TiO2-dye interaction[15–20]. In one study, it was reported that water resulted in the loss of iodine thus slowing down the reaction kinetics. The presence of trace amount of water is inevitable in a DSSC unless the cell assembly is done in a completely inert environment that would increase the manufacturing cost. Even in that case, it is possible that water seeps into the cell through sealing during the operation. To support this scenario, one study reported that there might be as much as 10wt% of water in the electrolyte after just 1-year outdoor operation of a flexible DSSC[21]. Zeolite molecular sieves are highly porous crystalline materials with an interconnected 3D structure with specific pore sizes[22,23]. Owing to their characteristics, these molecular sieves have been used as selective adsorbents and catalysts[24–26]. In particular, uniform pore size and 2

large surface area make them ideal for selective adsorption application. This property can also be applied to DSSC while assembling and handling the chemicals without any special care. Furthermore, these sieves are stable at high temperature and incorporating them in the DSSC will make the system more thermally stable. Recently, Yan et. al reported that an electronically insulating layer of molecular sieve having 0.3 nm pore size on photoelectrode improved the performance and water stability of DSSCs[27] without checking the long-term stability of devices. Moreover, the approach might significantly reduce the transmittance of DSSCs which make them difficult for BIPV application. Thus, we studied the effects of zeolite additives in electrolytes on the performance and long-term stability of DSSCs at 60oC for 1200 hours. The addition of zeolite into electrolytes serves two purposes. Firstly, the crystalline nature of Zeolite induces the light scattering in DSSCs which assists in trapping the light and hence increasing the JSC of the cells. The need of an extra non-transparent scattering layer in state-ofthe-art DSSCs which may hinder their application as window in BIPV can be thus omitted, which would reduce the manufacturing cost, as well as the sacrificial reduction in performance[28] can be avoided via a good contact between electrolytes and photoanode. Because of these beneficial effects of Zeolite, the thickness of TiO2 layer in this study can be reduced to 5–6 μm with a marginal decrease of device’s transmittance by zeolite addition. Secondly, Zeolite can adsorb the trace amount of water present in DSSCs. To confirm this idea, we employed a commercially available nano-meter sized Zeolite as an additive in electrolytes which has the pore size of 0.5 nm and adsorbs up to 26wt% of water.

2. Experimental 2.1. Preparation of photoanode films Fluorine-doped SnO2 conducting glass (FTO glass, Pilkington, TEC 8, 2.3 mm) was sonicated in a bath of deionized water, ethanol, and acetone for 5 minutes each to degrease. About 6–7 micrometer TiO2 paste, particle size 20 nm (ENB Korea) was printed on the cleaned glass pieces by doctor blade technique. The printed area was 25 mm2 (5 mm  5 mm). The film was then sintered at 550°C in ambient air using a muffle furnace. 2.2. Preparation of electrolytes

3

All the chemicals except BMII (1-butyl-3-methylimidazaolium iodide) were purchased from Sigma-Aldrich and were used without further purification. Ionic liquid BMII was purchased from C-TRI Korea. Reference electrolyte was prepared by dissolving 0.05 M I2, 0.1 M LiI (Lithium Iodide, 99.9%), 0.48M tBP(4-tert-butylpyridine, 96%), 0.6 M BMII and 0.12 M NaSCN (Sodium thiocyanate, >99.99%) in MPN (3-Methoxypropionitrile, >98%). The electrolyte was stirred for 24 hours prior to use. Zeolite powder (Nano ZSM-5, P-26, ACS Materials, average particle size ~ 300 nm) was ground in Agate Mortar for 30 minutes before mixing with the reference electrolyte at various weight percent from 0 to 20 and then stirred again until the solution was homogeneous to make Zeolite additive electrolyte. 2.3. Fabrication of dye-sensitized solar cells The TiO2 printed FTO glass was immersed in 0.3 mM of Ruthenium dye (N719) solution in anhydrous ethanol for 21 hours at 40°C. After the dye adsorption, the substrates were removed and rinsed with ethanol to remove excess dyes from the TiO2 films. Counter electrodes were prepared by screen printing the commercial Pt paste (Dyesol) on cleaned and hole drilled FTO glass and dried in an oven for 10 minutes at 70°C before sintering in a muffle furnace for 30 min at 400°C. Both electrodes were assembled and separated by thermoplastic adhesive polymer film (Surlyn, thickness 60 µm) 2.4. Characterizations Photocurrent-voltage measurements of the as-prepared DSSCs were performed on a digital source meter (Potentiostat/Galvanostat Model 273A, EG&G) by applying external potential bias to the device and recording the generated photocurrent. The solar light was produced with a solar simulator (Oriel 91192, USA) at 100 mW cm−2 (AM 1.5, global) which was calibrated with a reference Si cell. The IPCE was measured in the range of 300–800 nm using a specially designed IPCE system for dye-sensitized solar cells (HS Technologies, Korea) in a DC mode. The monochromatic beam was generated by a 150W Xe lamp as the light source while the system was calibrated by a Si photodiode. The photovoltaic performance was characterized by using the parameters VOC, JSC, and FF; the overall efficiency (η) was characterized by using the current

4

density-voltage properties of the cells with maintaining the aperture area of the cells to be 0.5  0.5 cm2 by the use of a square black mask.

3. Results and discussion DSSCs were fabricated with and without the addition of Nano-ZSM in an electrolyte in the range of 0 to 10wt% and their photovoltaic performances characteristics measured at 1 sun (100 mW cm−2) are shown in Table 1. The average performance of three devices with standard deviation for each condition is provided. While the photovoltaic parameters of DSSC without Nano-ZSM were, JSC = 11.13 mA cm−2, VOC = 0.79 V, FF = 0.63 and  = 5.52%, the JSC and overall efficiency was improved when Nano-ZSM was added in an electrolyte. In particular, Nano-ZSM additive significantly improved the JSC by ca. 17% with 5wt% addition in the electrolyte as shown in Figure 1. Incident photon to current conversion efficiencies (IPCEs) of devices with 0wt% and 5wt% Nano-ZSM in the electrolyte are measured as shown in Figure 2. The Jsc obtained by integrating the curves are slightly lower but in accordance with those obtained by J-V curves. The percentage increase of 20% is almost same as that in the case of J-V curves.

The increase in JSC could be attributed to the enhanced light absorbance due to the crystalline nature of Nano-ZSM. Besides the increase in light absorbance via enhanced dye adsorption, an alternate method to improving JSC is normally manipulating the photon pathways by light trapping/scattering within the active material or in an electrolyte. Scattering within the active material is often done by employing a scattering layer over the active layer. However, this method possesses some drawbacks such as extra heating treatment to attach the scattering layer on absorption layer at the expense of loss of device’s transmittance. Another way to achieve the analogous scattering effect is to use particles suspended in the electrolyte, which can obviate such disadvantage. In this study, the scattering is Mie Scattering as the average crystallite size of Nano-ZSM, i.e. 300 nm is comparable to the wavelength of visible light[29]. Simulations have shown that the scattering is maximum if there are 5vol % 250–300 nm particles in a mix with smaller particles[30]. Figure 3 shows SEM images of Nano-ZSM particles used in this study.

5

They consisted of primary zeolite nano particles of size less than a few tens nanometers that were aggregated into secondary particles having a broad ranges of particle sizes over hundred nanometers, which resulted in average particle size of ca. 300 nm as specified by the manufacturer. To confirm the scattering by Nano-ZSM additives, dummy cells with no TiO2 layer were fabricated with various amount of Nano-ZSMs in the electrolyte and analyzed by UV-Vis transmittance. The UV-Vis transmittance curves of these cells with the electrolyte containing 0, 2, 5, 10, and 20wt% of Nano-ZSM were shown in Figure 4. As the amount of Nano-ZSM increased, the transmittance of cells decreased as anticipated, especially in the range of 400 and 530 nm. This result clearly indicates that the light scattering is proportional to the amount of Nano-ZSM present in the electrolyte. This result is in a good agreement with the improvement of JSC with Nano-ZSM additive in DSSCs. To balance the performance enhancement and transmittance modulation of DSSCs with Nano-ZSM additive, we set 5wt % of Nano-ZSM as an optimum amount in the following experiments. The additional role played by Nano-ZSM in this study is to capture the trace amount of water present in the electrolyte in order to mitigate the adverse effects of moisture on the long-term performance of DSSCs. In order to evaluate the thermal behavior of Nano-ZSM additives, we conducted their thermal gravimetric analyses (TGA) at a heating rate of 10oC min-1 under dry nitrogen flow from 20 to 800oC. The as-received Nano-ZSM showed the weight loss of ca 5% in the range from 20 to 300oC corresponding to the slow dehydration of water molecules. When Nano-ZSM additives were pre-heated for 30min at 150oC, the corresponding weight loss of water reduced to ca. 2.5%. Even though they were fully dried for 12 hours at 200oC under vacuum, the sample rapidly re-adsorbed the atmospheric moisture as shown by the blue curve in Figure 5. Thus, we prepared the electrolytes containing Nano-ZSM additive which were fully dried just prior to use. To check effects of Nano-ZSM additive on the long term thermal stability of DSSCs, they were kept at 60C in the dark and their photovoltaic performances were measured periodically for 1200 hours. As shown in Figure 6, the thermal stability of DSSCs containing Nano-ZSM additive were significantly improved compared to the reference case. The current density of DSSCs with 5wt% of ZSM-nano additive gradually decreased from 13.02 to 9.66 mA cm−2 up to 6

300 hours and remained stable at ca. 9 mA cm−2 even up to 1200 hours. In contrast, DSSCs without Nano-ZSM additive showed the initial increase of JSC followed by a continuous decrease. This result can be interpreted as follows. Zhu et al recently reported the effect of water intrusion on DSSC performance in which added water resulted in enhancement of JSC due to a downward shift of the TiO2 conduction band edge and a 4–5 fold decrease in recombination without any adverse effect on transport[31]. Thus, the initial increase of short-circuit photocurrent density of DSSCs without Nano-ZSM additive can be ascribed to the trace amount of water present in devices because all manufacturing process of devices was done under ambient conditions. However, long-period storage of devices at a high temperature eventually decreases JSC mostly owing to desorption of dye molecules from TiO2 surface or exchange of thiocyanate ligand of dyes with water. On the other hand, these effects can be mitigated by adding water adsorbent such as Nano-ZSM. In general, the adsorption of water with ZSM-nano is based on weak forces such as hydrogen bonding. In other words, this interaction is at thermodynamic equilibrium at a specific condition. The initial decrease of JSC of devices with ZSM-nano can be ascribed to the water molecules desorbed from ZSM-nano at high temperature and then the thermodynamic equilibrium of the interaction between trace water and ZSM-nano reached which resulted in the stabilized value of JSC.

4. Conclusion We investigated the influence of the Zeolite addition in DSSC electrolyte on the efficiency and long-term stability. The DSSC with 5 wt% Zeolite additive in electrolyte results in an increase of Jsc without affecting other parameters, thus increasing the overall efficiency. The Jsc increases by 17% on average through incorporating the new electrolyte in DSSC because of the increased optical path length in Zeolite added DSSC. The increase in the diffused light is due to the Zeolite crystal structure which helps in light scattering. Furthermore, the DSSC showed sustained efficiency during 1200 hours of subjection to 60°C in dark conditions as compared to the reference DSSC which degraded continuously. The stability is attributed to the water scavenging effect of porous Zeolite additive at a high temperature.

7

Acknowledgement The authors acknowledge financial support from the Ministry of Trade, Industry and Energy(MOTIE), Korean Institute for Advancement of Technology through the International Coopperative R&D program(NP2015-0008/B6-7602).

8

References [1]

Paris

Agreement

-

European

Commission,

(n.d.).

http://ec.europa.eu/clima/policies/international/negotiations/paris/index_en.htm. [2]

J.

Tsao,

N.

Lewis,

G.

Crabtree,

Solar

FAQs,

Sandia.gov.

(n.d.).

http://www.sandia.gov/~jytsao/Solar FAQs.pdf (accessed September 26, 2016). [3]

B. O’Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films, Nature. 353 (1991) 737–740. doi:10.1038/353737a0.

[4]

M. Pagliaro, R. Ciriminna, G. Palmisano, Flexible Solar Cells, ChemSusChem. 1 (2008) 880–891. doi:10.1002/cssc.200800127.

[5]

P. Joshi, Y. Xie, M. Ropp, D. Galipeau, S. Bailey, Q. Qiao, Dye-sensitized solar cells based on low cost nanoscale carbon/TiO2 composite counter electrode, Energy Environ. Sci. 2 (2009) 426. doi:10.1039/b815947p.

[6]

G. Li, F. Wang, Q. Jiang, X. Gao, P. Shen, Carbon Nanotubes with Titanium Nitride as a Low-Cost Counter-Electrode Material for Dye-Sensitized Solar Cells, Angew. Chemie Int. Ed. 49 (2010) 3653–3656. doi:10.1002/anie.201000659.

[7]

S. Yoon, S. Tak, J. Kim, Y. Jun, K. Kang, J. Park, Application of transparent dyesensitized solar cells to building integrated photovoltaic systems, Build. Environ. 46 (2011) 1899–1904. doi:10.1016/j.buildenv.2011.03.010.

[8]

M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Solar cell efficiency tables (Version 45), Prog. Photovoltaics Res. Appl. 23 (2015) 1–9. doi:10.1002/pip.2573.

[9]

H.-S. Lee, S.-H. Bae, Y. Jo, K.-J. Kim, Y. Jun, C.-H. Han, A high temperature stable electrolyte system for dye-sensitized solar cells, Electrochim. Acta. 55 (2010) 7159–7165. doi:10.1016/j.electacta.2010.07.011.

[10] H. Yang, C. Yu, Q. Song, Y. Xia, F. Li, Zhigang Chen, X. Li, A. Tao Yi, C. Huang, HighTemperature and Long-Term Stable Solid-State Electrolyte for Dye-Sensitized Solar Cells by Self-assembly, (2006). doi:10.1021/CM061112D. 9

[11] J. Wu, Z. Lan, S. Hao, P. Li, J. Lin, M. Huang, L. Fang, Y. Huang, Progress on the electrolytes for dye-sensitized solar cells, Pure Appl. Chem. 80 (2008) 2241–2258. doi:10.1351/pac200880112241. [12] M. Ye, X. Wen, M. Wang, J. Iocozzia, N. Zhang, C. Lin, Z. Lin, Recent advances in dyesensitized solar cells: From photoanodes, sensitizers and electrolytes to counter electrodes, Mater. Today. 18 (2015) 155–162. doi:10.1016/j.mattod.2014.09.001. [13] M.K. Hirata, J.N. Freitas, T.E. de A. Santos, V.P. Mammana, A.F. Nogueira, Assembly considerations for dye-sensitized solar modules with polymer gel electrolyte, Ind. Eng. Chem. Res. (2016) acs.iecr.6b02181. doi:10.1021/acs.iecr.6b02181. [14] Y.-S. Jung, B. Yoo, M.K. Lim, S.Y. Lee, K.-J. Kim, Effect of Triton X-100 in wateradded electrolytes on the performance of dye-sensitized solar cells, Electrochim. Acta. 54 (2009) 6286–6291. doi:10.1016/j.electacta.2009.06.006. [15] Y. Liu, A. Hagfeldt, X.-R. Xiao, S.-E. Lindquist, Investigation of influence of redox species on the interfacial energetics of a dye-sensitized nanoporous TiO2 solar cell, Sol. Energy Mater. Sol. Cells. 55 (1998) 267–281. doi:10.1016/S0927-0248(98)00111-1. [16] H. Pettersson, T. Gruszecki, Long-term stability of low-power dye-sensitised solar cells prepared by industrial methods, Sol. Energy Mater. Sol. Cells. 70 (2001) 203–212. doi:10.1016/S0927-0248(01)00025-3. [17] B. Macht, M. Turrión, A. Barkschat, P. Salvador, K. Ellmer, H. Tributsch, Patterns of efficiency and degradation in dye sensitization solar cells measured with imaging techniques, Sol. Energy Mater. Sol. Cells. 73 (2002) 163–173. doi:10.1016/S09270248(01)00121-0. [18] S.J. Park, K. Yoo, J.-Y. Kim, J.Y. Kim, D.-K. Lee, B. Kim, H. Kim, J.H. Kim, J. Cho, M.J. Ko, Water-Based Thixotropic Polymer Gel Electrolyte for Dye-Sensitized Solar Cells, ACS Nano. 7 (2013) 4050–4056. doi:10.1021/nn4001269. [19] H.-L. Lu, Y.-H. Lee, S.-T. Huang, C. Su, T.C.-K. Yang, Influences of water in bisbenzimidazole-derivative electrolyte additives to the degradation of the dye-sensitized 10

solar

cells,

Sol.

Energy

Mater.

Sol.

Cells.

95

(2011)

158–162.

doi:10.1016/j.solmat.2010.02.018. [20] R. Harikisun, H. Desilvestro, Long-term stability of dye solar cells, Sol. Energy. 85 (2011) 1179–1188. doi:10.1016/j.solener.2010.10.016. [21] C. Law, S.C. Pathirana, X. Li, A.Y. Anderson, P.R.F. Barnes, A. Listorti, T.H. Ghaddar, B.C. ORegan, Water-based electrolytes for dye-sensitized solar cells, Adv. Mater. 22 (2010) 4505–4509. doi:10.1002/adma.201001703. [22] D.W. Breck, Zeolite molecular sieves: structure, chemistry, and use, Wiley, 1973. [23] M.E. Davis, Ordered porous materials for emerging applications, Nature. 417 (2002) 813– 821. doi:10.1038/nature00785. [24] F. Akhtar, L. Andersson, S. Ogunwumi, N. Hedin, L. Bergström, Structuring adsorbents and catalysts by processing of porous powders, J. Eur. Ceram. Soc. 34 (2014) 1643–1666. doi:10.1016/j.jeurceramsoc.2014.01.008. [25] H. LEE, Applied Aspects of Zeolite Adsorbents, in: 1973: pp. 311–318. doi:10.1021/ba1973-0121.ch028. [26] J.M. Thomas, R. Raja, G. Sankar, R.G. Bell, Molecular-sieve catalysts for the selective oxidation of linear alkanes by molecular oxygen, Nature. 398 (1999) 227–230. doi:10.1038/18417. [27] Y. Yan, J. Wang, Q. Yu, Y. Huang, Q. Chang, C. Hao, S. Jiao, S. Gao, H. Li, D. Wang, Application of 3A molecular sieve layer in dye-sensitized solar cells, Appl. Phys. Lett. 105 (2014) 83907. doi:10.1063/1.4894183. [28] D.M. Han, K.-W. Ko, C.-H. Han, Y.S. Kim, Facile formation of a micro-crater structure for light scattering in quasi-solid state dye-sensitized solar cells, RSC Adv. 4 (2014) 28133. doi:10.1039/c4ra03128h.

11

[29] J.A. Dorman, L. Schmidt-Mende, Chapter 5. The Role of Nanostructured Metal Oxides in Hybrid Solar Cells, in: Unconv. Thin Film Photovoltaics, Royal Society of Chemistry, Cambridge, n.d.: pp. 141–176. doi:10.1039/9781782624066-00141. [30] J. Ferber, J. Luther, Computer simulations of light scattering and absorption in dyesensitized solar cells, Sol. Energy Mater. Sol. Cells. 54 (1998) 265–275. doi:10.1016/S0927-0248(98)00078-6. [31] K. Zhu, S.-R. Jang, A.J. Frank, Effects of water intrusion on the charge-carrier dynamics, performance, and stability of dye-sensitized solar cells, Energy Environ. Sci. 5 (2012) 9492. doi:10.1039/c2ee22178k.

12

Figure 1. J-V curves of the best-performing DSSCs depending on the amount of Zeolite added in electrolyte. The light intensity is 100 mW cm−2.

Figure 2. IPCE curves of Z0 (without Zeolite in the electrolyte) and Z5.0 (5wt% Zeolite loading). The corresponding Jsc was calculated to be 10.17 and 12.8 mA cm−2 respectively.

13

Figure 3. SEM images of Nano-ZSM additives. Inset is the magnified image showing the primary particles and secondary aggregates of Zeolite additives.

Figure 4. Transmittance data of dummy cells with Zeolite additive in the electrolyte (Z0= no Nano-ZSM and 2, 5, 10, 20wt% of Nano-ZSM).

14

Figure 5. TGA curves of Nano-ZSM additives as received (black square), after heating in oven for 30 mins at 150oC (red circle) and after heating in vacuum oven for 12 hours at 200oC (blue triangle)

15

Figure 6. Variation of average Jsc and efficiency values of three independent DSSCs during long-term thermal stability test at 60°C in dark.

16

Table 1 Average photovoltaic parameters of three DSSCs with Zeolite added electrolytes. Sample

Jsc/mA cm−2

VOC/V

FF

Efficiency/%

RS/

Z0

11.13±0.05

0.79±0.006

0.63±0.004

5.52±0.06

56.51±0.698

Z2.5

11.44±0.08

0.78±0.0004

0.63±0.005

5.60±0.02

55.34±1.046

Z5.0

13.02±0.44

0.76±0.005

0.60±0.015

5.95±0.01

52.45±2.369

Z7.5

12.2±0.29

0.77±0.004

0.62±0.022

5.80±0.07

49.78±2.089

Z10.0

11.78±0.044

0.78±0.007

0.62±0.009

5.72±0.01

54.14±2.291

17