Long-Term Stability of Dye-Sensitized Solar Cells Assembled with

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Long-Term Stability of Dye-Sensitized Solar Cells Assembled With Cobalt Polymer Gel Electrolyte Gabriela Gava Sonai, Armi Tiihonen, Kati Elina Miettunen, Peter D. Lund, and Ana Flavia Nogueira J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03865 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on July 30, 2017

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The Journal of Physical Chemistry

Long-Term Stability of Dye-Sensitized Solar Cells Assembled with Cobalt Polymer Gel Electrolyte

Gabriela G. Sonai1, Armi Tiihonen2, Kati Miettunen2, Peter D. Lund2, Ana F. Nogueira1*

1

Laboratory of Nanotechnology and Solar Energy, Chemistry Institute, University of Campinas – UNICAMP, P.O. Box 6154, 13083-970, Campinas, SP, Brazil. 2

New Energy Technologies, Department of Applied Physics, Aalto University, 00076 Aalto, Espoo, Finland.

*Corresponding author: Phone: +55 19 35213022/9 E-mail address: [email protected]

1 ACS Paragon Plus Environment


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Abstract The long-term stability of a DSSC is a key issue for up-scaling and commercialization of this technology. It is well known that gel electrolytes can improve the long-term stability and allow easy DSSC manufacturing. However, there is limited knowledge on the long-term stability of cobalt-based gel electrolytes and also how this stability is affected when applying different dye sensitizers. Moreover, long-term stability studies have been done with no, or an imperfect, sealing. In this work we investigated the performance and the stability of cobalt-based polymer gel electrolytes using devices properly sealed. Here, two different dyes, an organic and a ruthenium dye, were selected to investigate the device’s performance. The cobalt liquid electrolyte was gelled with a PEO based terpolymer (PEO-EM-AGE) and compared to its liquid counterpart. After 1000 hours, the efficiencies of the liquid and gel based solar cells with the ruthenium dye were statistically similar to each other. On the other hand, the DSSCs using the organic dye performed similarly by statistical analysis only up to 500 h. Our findings suggest that the choice of the dye has an important impact on the long-term stability of DSSCs and must be considered a key factor in the degradation mechanism.


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1. Introduction In recent years, redox mediators based on cobalt complexes have been applied in dyesensitized solar cells (DSSCs) promising to replace the widely used iodide/tri-iodide mediators. 1, 2, 3

One of the most attractive properties of cobalt redox couples is the possibility to develop

efficient devices using chemicals that are much less corrosive and that can deliver higher opencircuit potential values in comparison with the traditional use of iodine/iodide mediators.

4, 5, 6

The current efficiency record of DSSCs is 14.3%, combining cobalt redox couples co-sensitized with ADEKA-1 (carbazole-based dye) and LEG4 (triphenylamine-based dye). 7 Stability is one of the main issues for the commercialization of DSSCs, and therefore long-term stability tests are also required for cobalt-based DSSCs. In general, stability tests are run for 1000 h under 1 Sun illumination, which is equivalent to one year outdoor exposure in Northern European Latitudes. 8 DSSCs are devices consisting of many components and, consequently, several factors can affect their long-term stability. 9 Usually, the dye and the electrolyte are the components most affected with respect to stability in iodine/iodide based DSSCs;

10

the same holds for cobalt-

based devices. Previous investigations focused on the influence of some additives on device stability have shown that using tBP (tert-butylpyridine) in small amounts can improve the longterm stability. 11 On the other hand, the presence of lithium salt can negatively affect the stability of DSSCs.

11

Other additives, as N-methylbenzimidazole (NMBI), were also found to increase

the instability in DSCs. 12 Cobalt complexes with bipyridine ligand can undergo dissociation inside the electrolyte, and to overcome this problem Kashif et al

12

have prepared a new cobalt complex with a

hexadentate pyridyl ligand to enhance the stability of these complexes. Despite the fact that [Co(bpy)3]2+/3+ complexes with bi-dented ligands are considered unstable under certain conditions, stability studies have reported that devices can survive for 2000 h retaining about 90% of their initial efficiency, under 1 Sun illumination. 13 Thermal studies have also confirmed the stability of this mediator at 70 ºC under dark conditions. 14



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Gel polymer electrolytes can improve the long-term stability of DSSCs and also present other attractive properties, such as ease of preparation, low cost, excellent contact with electrodes and the capability of trapping the liquid electrolyte efficiently.

15

PVDF-HFP

copolymer was applied as gel electrolyte in cobalt-based DSSCs in the pioneer work of Xiang et. al

16

, resulting in an efficiency of 8.7%. The devices also remained stable after 700 h of

continuous illumination. Recently, a quasi-solid electrolyte based on in situ polymerization of a dimethylmethacrylate oligomer (BEMA) with poly(ethylene glycol) methyl ether methacrylate (PEGMA) was applied effectively in cobalt-based DSSCs. The devices presented a good stability after 1500 h, exhibiting a decay of only 5% of their initial efficiency. 17 Although there is a general consensus that gel electrolytes improve the device lifetime of DSSCs, in most investigations the devices were assembled with an imperfect encapsulation or even with none, resulting in an enhancement of stability commonly related to solvent evaporation retardation. These studies have provided information regarding improperly sealed devices, but it is crucial also to investigate gel electrolyte stability with effective sealing. Such investigation may provide information about the chemical effects of gelling on device stability. The use of polymers as gelling agents is of great interest because polymers can give the desired mechanical characteristics of the electrolyte to be deposited by a variety of methods (e.g. printing or injection methods). Printing is the most suitable method for larger-scale production, and for printing, the gel electrolyte should have high viscosity rather than a jelly structure that can be destroyed in the printing process. 18 PEO-based polymers are a good option as polymer matrix because they present important characteristics compatible with the printing process (unlike PVDF-HFP). The purpose of this work is to investigate the long-term stability of DSSCs based on cobalt gel electrolytes prepared with poly(ethylene oxide-co-2-(2-methoxyethoxy) ethyl glycidyl etherco-allyl glycidyl ether), (PEO-EM-AGE) terpolymer. To gain a deeper understanding of the mechanism involved in the degradation of these devices, we also selected two different types of


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dyes: an organic dye and a ruthenium dye, to evaluate how the stability and performance may vary depending on the type of the dye used.

2. Experimental 2.1. Materials and Reagents The cobalt complexes Co(bpy)3(TFSI)2 and Co(bpy)3(TFSI)3 (bpy = 2,2’bipyridine, TFSI = bis(trifluoromethylsulfonyl)-imide) (Figure S1) were prepared based on previous literature. 19

16,

The detailed synthesis of cobalt complexes is presented in Supporting Information. The

following reagents and solvents were purchased from Sigma-Aldrich and were used without further

purification:

nitrosonium

tetrafluoroborate

(NOBF4),

lithium

bis(trifluoromethylsulfonyl)-imide (LiTFSI), 4-tert butylpyridine (tBP), CoCl2.6H2O, MK2 dye (Figure 1-a), Z907 dye (Figure 1-b). The polymer used was poly(ethylene oxide-co-2-(2methoxyethoxy) ethyl glycidyl ether-co-allyl glycidyl ether (PEO-EM-AGE, Daiso) with monomer composition of (78-20-2). The total molecular mass of the polymer was 2.15 ×10-6 g mol-1 (Figure S2).

Figure 1: (a) organic dye (MK2) and (b) ruthenium-based dye (Z907).

2.2. Fabrication of Solar Cells



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Firstly, fluorine-doped tin oxide (FTO) coated glass substrates (4.0 mm TEC-15 FTO) were cleaned with detergent, acetone and ethanol, followed by UV-O3 treatment for 20 min. Titanium tetrachloride treatment was performed by dipping the substrate in a 40 mM TiCl4 aqueous solution at 70 ºC for 30 min. Then, the substrate was washed with water and ethanol. The TiO2 layers were deposited on the substrate using an automated screen printer. The first layer was the transparent TiO2 (DSL 18NR-T, Dyesol) and the second layer was TiO2 scattering layer (WER2-O, Dyesol). The photoelectrodes were sintered at 450 ºC for 30 min. The TiCl4 treatment was applied again on photoelectrode, followed by another sintering process under the same conditions used previously. The photoelectrodes had a thickness of 8 µm and active area of 0.4 cm2. They were cooled down to 80 ºC and immersed for 18 h in the desired dye solution: either

0.2

mM

(2-cyano-3-[5‴-(9-ethyl-9H-carbazol-3-yl)-3′,3″,3‴,4-tetra-n-hexyl-

[2,2′,5′,2″,5″,2‴]-quarter thiophen-5-yl] acrylic acid) (MK-2) dye solution prepared with toluene:acetonitrile

(1:1)

or

0.2

dicarboxylato)(4,4’-di-nonyl-2’-bipyridyl)

mM

(cis-bis(isothiocyanato)(2,2’-bipyridyl-4,4’-

ruthenium(II))

(Z907)

dye

solution

with

t-

butanol:acetonitrile (1:1). The platinum counter electrodes were prepared by spreading 4µL of 5mM H2PtCl6 solution over the pre-drilled FTO glass pieces and heated at 390 ºC for 20 min before use. The glass substrates were previously cleaned in a similar way than the photoelectrode substrate. The solar cells were assembled using a 25 µm thick frame (Surlyn, Dupont) and the gel electrolytes were vacuum backfilled. The electrolyte composition consisted of 0.2 M [Co(bpy)3]2+, 0.07 M [Co(bpy)3]3+, 0.05 M Li+, 0.2 M tBP in acetonitrile as liquid electrolyte and 5% of the polymer PEO-EM-AGE (poly(ethylene oxide-co-2-(2-methoxyethoxy) ethyl glycidyl ether-co-allyl glycidyl ether) added to the liquid electrolyte mixture (Table 1).


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Table 1: Codification of group of cells according with the dye used to sensitize the photoelectrode (MK2 or Z907) and the presence or absence of polymer to obtain liquid (L) or gel (G) electrolytes.

2.3. Characterizations The photovoltaic measurements were acquired in a solar simulator (PEC-L01 Portable Solar Simulator, Peccell Technologies, Inc) containing xenon lamps calibrated with an official calibration solar cell (PV Measurements) to a state equivalent to 1000 W m−2 AM 1.5G (1 Sun) illumination spectrum in the visible range. Current-voltage (IV) curves under 1 Sun were measured with a Keithley 2420 (SourceMeter). A black mask was applied on top of the cells to avoid the overestimation of measured parameters caused by the scattered illumination from the edge area of the cells. Incident photon conversion efficiency was measured with a QEX7 Solar Cell Spectral Response Measurement System (PV Measurements) in DC mode with no bias light, from 300 to 900 nm. EIS in the dark and under illumination was performed with a potentiostat (Zahner Zennium) at the frequency range of 1 × 10−1 to 1 × 106 Hz swept back and forth with 10 mV amplitude. The EIS data was fitted using ZView (Scribner Associates Inc.) program with the equivalent circuit presented in Figure S3. The EIS under illumination was obtained at VOC under 1 Sun, while the EIS in dark was measured in the voltage range of 0 to 0.8 V at intervals of 0.1 V. The IV curves were measured also as a function of varying light



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intensity. These analyses were performed with a potentiostat (Autolab) combined with a white LED light source built in-house. The ageing test was performed using a light-soaking device built in-house containing halogen lamps (Philips projection lamp, type 13117) calibrated for 1 Sun illumination in the visible range. The lamps emitted approximately 20% of the UV in the AM 1.5G spectrum and because of that, a 400 nm UV cut-off filter (Asmetec GmbH) was applied to suppress the UV degradation of the devices. During the ageing, automatic IV measurements were performed with a Solar Cell Ageing Test Unit (SCATU) built in-house using a potentiostat (Bio-Logic SP-150). The solar cells were photographed weekly until 1000 h of ageing was completed, and the images were used for quantitative monitoring of changes in the color of electrolytes. The method is described in the literature, 20 and the system consisted of a color-sensitive camera and a set of LEDs mounted in a black chamber. The contact angle was measured by dropping a small amount of the desired solvent/electrolyte (water/ acetonitrile/ liquid or gel electrolyte) on top of TiO2 films sensitized with MK2 or Z907 dye. Images were obtained with a Theta Optical Tensiometer (Attension). Initially, the measurements were performed for five cells of each group. Possible outliers were identified using Dixon’s Q-test. One outlier was found: one MK2-L device that presented a clearly abnormal low current density value after the ageing. Additionally, one Z907-L device that leaked during ageing was discarded from the analysis. As a result, the statistical analysis was performed in quintuplicate and quadruplicate for DSSCs containing gel and liquid electrolytes, respectively. The data was recorded as mean ± standard deviations and analyzed by Minitab 17. Paired t-tests were conducted to compare the photovoltaic parameters of the same cells before and after ageing. Additionally, unpaired t-tests were carried out to compare the results obtained for liquid and gel electrolytes for DSSCs sensitized with the same dye. Differences between mean values at 5% (P

Long-Term Stability of Dye-Sensitized Solar Cells Assembled with

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