Assembly Considerations for Dye-Sensitized Solar Modules with

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Assembly considerations for dye-sensitized solar modules with polymer gel electrolyte Marcelo K. Hirata, Jilian Nei Freitas, Thebano Emilio de Almeida Santos, Victor Pellegrini Mammana, and Ana F. Nogueira Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02181 • Publication Date (Web): 08 Sep 2016 Downloaded from http://pubs.acs.org on September 12, 2016

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Industrial & Engineering Chemistry Research

Assembly considerations for dye-sensitized solar modules with polymer gel electrolyte Marcelo K. Hirata‡,† , Jilian N. Freitas‡,*, Thebano E. A. Santos‡, Victor P. Mammana‡ and Ana F. Nogueira†,* ‡

Center for Information Technology Renato Archer – CTI, Rodovia D. Pedro I, Km 143,6,

13069-901, Campinas, SP, Brazil. †

Chemistry Institute, University of Campinas – UNICAMP, P. O. Box 6154, 13083-970,

Campinas, SP, Brazil. * [email protected] (A. F. Nogueira); [email protected] (J. N. Freitas)

ABSTRACT: Parallel, portable dye-sensitized solar cell modules with 5 x 5 cm2 of area containing either a polymer gel electrolyte or a standard liquid electrolyte were assembled and characterized as a function of time. For modules sealed with the thermoplastic Surlyn, a rapid loss of performance was observed, due to an insufficient protection of the metallic current collectors (silver grids) of the module and, sometimes, also associated with electrolyte leakage. Similar results were obtained using a glass frit layer as the only material for protective overcoat of the silver grids. Thus, the combination of the Surlyn with a glass frit was investigated. The process based on the combination of two materials allowed achieving a more effective sealing, with enhanced process yield and sample reproducibility. The modules assembled by this method,

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filled with either a polymer gel electrolyte or liquid electrolyte, showed an enhanced stability. Concomitantly, the formulation of the polymer gel electrolyte was also addressed, and had to be tuned to allow an easier filling of the modules.

KEYWORDS: Dye-sensitized solar cells, module, polymer gel electrolyte, sealing, stability.

1. INTRODUCTION Dye-sensitized solar cells (DSSCs) have been under intense investigation for the last two decades as a promising alternative for low cost, indoor or outdoor energy conversion. A life cycle assessment approach applied recently to these devices, looking at the synthesis of the main components, the fabrication of modules with different configurations and the operational phase of a roof-top photovoltaic system, indicated that DSSCs could compare similarly to other more mature thin film technologies1. The possibility of exploiting diffuse light and the possibility of making semi-transparent devices give DSSCs the potential to act on different niches of the market, for example, in the building-integrated photovoltaics sector or as chargers for laptops, mobile phones, etc., in indoor applications1-3. In lab scale, DSSCs (i.e., active area ˂ 1 cm-2) efficiencies of up to 13 % have been achieved4. Nevertheless, larger active areas and modules are required for the end-use and commercialization of DSSCs. Several types of DSSC module structures have been proposed, including structures with series or parallel connection (W-type, Z-type, monolithic series and parallel grid type cell). So far, the power conversion efficiencies reported for all kinds of DSSC modules have been lower than the record obtained in lab scale devices. Indeed, it has been demonstrated that the sheet resistance of the conducting glass substrate becomes a limiting factor as the area of the


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device is increased5-7. Because of this effect, metal current collectors (grids) are usually added to the surface of the conducting glass substrate of the modules. Amongst the candidates for metal grids, silver has been the most investigated material because of its low electrical resistance, low cost, and the possibility of deposition from solution processing techniques. Modules with efficiencies around 5-8 % have been reported using silver grids8-10. However, undesirable corrosion of silver by I-/I3--based electrolytes, that leads to a depression of the I3- concentration in the electrolyte due to formation of AgI

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, has been a

critical issue. To overcome this drawback, at least three approaches have been investigated: (i) use of alternative metals, such as Pt

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or Ni 5; (ii) use of alternative, less corrosive redox

mediators13,14; (iii) protection of the metal grid using a physical barrier, such as an overcoat layer. The first approach might bring a disadvantage related to enhanced cost of the material or processing, while the second one might bring the need for developing different dyes and/or counter electrodes. The last approach is expected to be the more straightforward one, particularly considering that any metal grid in contact with an electrolyte might lead to increased dark current through charge recombination. Consequently, even if new materials are to be considered, it is likely that a protective overcoat for the metal grid will be necessary in any case. To successfully add an overcoat layer on top of the metal grid, not only the processing method but also the material itself should be simple and cost effective. Several materials such as UVcurable or thermal-curable sealants and glass pastes have been used to investigate the protecting effect on the silver grid. However, most of them are not commercially available or easily available, and others do not achieve all the requirements of chemical compatibility or stability for long-term operation. Since stability is one of the main features for commercialization of DSSCs, all the components of the device have to be carefully selected, looking at the


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compatibility amongst them, the power generated and shelf-life. Several reports discuss stability tests for DSSCs carried out in outdoor conditions2,15-19 or with application of temperature20-22, and excellent reviews about DSSCs stability can be found elsewhere23,24. Overall, it is accepted that: high quality sealing is required to prevent physical degradation (i.e., electrolyte evaporation or leakage); effective isolation of metal grids from the electrolyte is necessary; and changes in the composition of the electrolyte, either via physical process (iodine sublimation, crystallization, etc.) or via unwanted chemical reactions17,19, might happen and have to be considered. This means that a careful tuning of the electrolyte composition is also a relevant step to achieve stability. Electrolyte composition may also be tuned to customize DSSCs for indoor and low-density artificial light applications25. Furthermore, it has been proposed that the substitution of the liquid electrolyte with a polymer or gel electrolyte might be beneficial for commercialization because these components could reduce the leakage problems. Detailed reviews about current research status of polymer- and polymer gel electrolyte-based DSSCs can be found elsewhere26,27. Particularly, some polymer-based electrolytes could be applied to cell or modules by simple printing techniques28,29. Screen printing is a versatile printing technology that is considered a suitable alternative for the confection of large-scale DSSC modules or panels30. Here, parallel DSSC modules with 5 x 5 cm2 of area containing either a polymer gel electrolyte or a standard liquid electrolyte were assembled and investigated as a function of time. Two major subjects were addressed: (i) the difficulties in achieving an effective sealing, which was addressed by tuning the characteristics of the protective overcoat for silver grid lines and their implications on the device stability; and (ii) the formulation of the polymer gel electrolyte, which was tuned to allow an easier filling of the modules.


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2. EXPERIMENTAL SECTION 2.1. Substrates FTO substrates (fluorine-doped tin oxide glass, TCO22-15, Solaronix) with a 5 cm x 5 cm size were cleaned in a sequence of ultrasonic baths and dried in nitrogen flow. Silver grid lines (width x length = 0.5 mm x 50 mm) were screen printed onto the FTO surface using a 200 mesh screen and silver paste (4081 BLA, Ticon), and then heated to 500 °C for 30 min. The distance between the metal grid lines was set to 8 mm. In some samples, a glass frit layer was screen printed on top of the silver grid lines using a 200 mesh screen and a mixture of Frit 500004, boric acid and GVD 09039 dispersant (Ferro Enamel do Brasil), and heated at 150 °C for 10 min, 300 °C for 10 min and 500 °C for 20 min. The glass frit and silver grid layers were characterized by scanning electron microscopy, revealing a thickness of ~ 5 µm. Morphological characterization of the silver grid and glass frit layer are presented in Figure S1 and S2 (Supporting Information). 2.2. Electrodes Photoelectrodes were obtained by screen printing TiO2 (DSL 18NR-T, DyeSol) onto the substrates, using a 200 mesh screen, followed by heating to 350 °C for 30 min and 450 °C for 30 min. This procedure was repeated twice, resulting in films with ~ 10 µm of thickness, as estimated with a ZYGO NewView 5000tm profilometer. The films geometry consisted of 5 stripes (width x length = 4 mm x 35 mm), giving a total module active area of 7 cm2. After cooling to 100 °C, the electrodes were immersed in a solution of the N719 dye (Solaronix) in ethanol for 20 h. The counter electrode consisted of a 100 nm Pt film deposited by sputtering on top of a FTO-glass plate.


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2.3. Electrolytes Two types of electrolytes were used to assemble the modules: a standard, commercially available I-/I3--based electrolyte (EL-HPE, DyeSol), and a polymer gel electrolyte. The basis for the recipe of the polymer gel electrolyte was previously developed31-34 and consists of a combination of a polyethylene oxide (PEO)-based copolymer with LiI, I2 and gammabutyrolactone (GBL). Here, the composition was further adjusted with addition of tertbutylpyridine (TBP) and 3-methoxypropionitrile (MPN). 2.4. Solar module assembly and characterization The sandwich-type module was assembled by sealing the photoelectrode and counter electrode together using a polymer thermoplastic (Surlyn, thickness ~30 µm). A Surlyn sheet cut in the appropriate shape was placed on the dye-sensitized photoelectrode, on top of the silver grid lines and around the active area of the module. In the samples containing a glass frit overcoat on top of the silver grid, the Surlyn sheet was placed on top of the glass frit layer. The sealing step was performed by heating the assembled modules to 120 °C for 4 min in an automatic press (MultiPress S, LPKF). Afterwards, the liquid or polymer gel electrolyte was introduced into the sealed module via predrilled holes in the counter electrode side, under vacuum. The holes in the counter electrode were then closed with an adhesive tape (Kapton, 3M). This temporary solution to close the holes was selected for preliminary tests and was considered tolerable for short timescale studies. The current-voltage (J-V) characteristics of the modules under irradiation of a solar simulator (SS-0.5K, Sciencetech) adjusted to 100 mW cm-2 were measured with a 2410C Keithley source meter. The devices were kept in ambient conditions and measured once a day. The open circuit


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voltage (Voc), short-circuit current (Isc), short-circuit current density (Jsc), fill factor (FF) and power conversion efficiency (PCE) parameters were extracted from the J-V curves and were evaluated as a function of time.

3. RESULTS AND DISCUSSION 3.1. Protection of the silver grid In previous works we demonstrated the application of a polymer gel electrolyte based on a PEO copolymer in a series connected DSSC module designed to deliver 8 V 35,36. Here, we aimed at demonstrating the application of a polymer gel electrolyte in a portable, parallel, gridtype DSSC module. The geometry and dimensions of the FTO-glass substrate, silver grids, TiO2 films and counter electrodes were selected based on the work of Ramasamy et al.10 In that work, the characteristics of a solar module assembled with screen-printing technology were evaluated for a period of 8 days, with a maximum PCE of 5.45%. That device architecture was considered interesting, mainly due to its simplicity, and was selected as basis for the investigations presented here. Simplicity of the fabrication process is a key factor when looking, for example, at opportunities related to low cost consumer goods applications. In our initial trials, the photoelectrodes and counter electrodes of the modules were fixed together by using a Surlyn layer, which acted as both the sealant and the spacer between the electrodes. A Surlyn layer was also placed on the top of the silver grid lines to act as a protective overcoat for silver and prevent its direct contact with the electrolyte. A comparison of the characteristics of modules sealed with Surlyn and filled with liquid electrolyte, assembled with or without silver grid lines is presented in Figure S4 and Table S1 (Supporting Information). The


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results clearly indicate the superior performance of the modules containing silver grids and reinforce the need for using metallic current collectors in the photoelectrodes of DSSC modules. Figure 1 presents pictures of modules containing silver grid lines protected and sealed with a layer of Surlyn. For this kind of module, it was observed that the grid was not properly isolated from the electrolyte, as it was frequently identified a disruption of the metal lines in a very short time after assembly. It was also observed that the sealing process was sometimes unsatisfactory, with the formation of bubbles and other defective sites in the melted thermoplastic layer, which could be easily identified by visual inspection immediately after assembly (Figure 1a, inset). Modules with such sealing flaws frequently presented electrolyte leakage or evaporation (Figure 1b), due to localized disruptions of the sealing layer, regardless of the type of electrolyte used. It had been previously argued that the application of vacuum during the hot pressing sealing step is a key process to achieve effective protection of silver lines coated with a 30 µm Surlyn film37. Here, the available experimental setup operated only in ambient conditions. We performed sealing tests with Surlyn in a large number of modules, using three different press equipment and a variety of temperature/time/pressure conditions. Perfectly sealed samples were occasionally obtained, but that was not an easily controlled or systematic process. A large fraction of the modules presented defects and unsatisfactory sealing. Despite the fact that Surlyn is routinely employed as sealant for DSSCs and that we were able to obtain high quality results using Surlyn alone in lab-scale devices (active area ≤ 1 cm2), that was not the case for our modules. The lack of reproducibility and low process yield motivated the selection of a different material/process to be used as protection layer for the silver grids in our modules.


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Figure 1. Typical photos of modules sealed with Surlyn and using only this material as protective overcoat for the silver grids, taken: (a) immediately after assembly or (b) 2 days after assembly. Then inset shows the presence of bubbles in the Surlyn layer after the sealing step.

Glass frit was selected as candidate for the following trials, due to its availability, compatibility with other cell components, and because it can be applied by a simple, low cost screen printing technique. Thus, modules were assembled with the same configuration described earlier, but substituting the Surlyn thermoplastic with a glass frit layer as protective overcoat for the silver grids. Figure 2 displays typical pictures obtained for this kind of module. A disruption of the silver grid integrity, accompanied by loss of the electrolyte characteristic color, began to happen after ~ 2 to 5 days for most of the samples. After a period of ~ 10 to 20 days, the module integrity was completely compromised for all samples. These results clearly indicate that the


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glass frit layer used here was not sufficient to guarantee the necessary protection of the silver grids. Microscopy images revealed the presence of small holes in the glass frit layer (Figure S3, Supporting Information), through which the electrolyte may have come in contact with the metal grid underlying layer.

Figure 2. Images of modules containing a glass frit layer as overcoat for the silver grid lines: (a) immediately after module assembly, (b) after 10 days and (c) after 20 days.

The use of glass frit as protective layer for silver grids in DSSCs containing liquid-based electrolytes had been investigated in other works38-40. While Yeon et al.38 found satisfactory results for the stability of devices containing I-/I3--based liquid electrolyte during a period of 10 days, Matsui et al.40 found that the use of a glass frit layer alone was not sufficient to maintain the performance of a DSSC based on an ionic liquid-based electrolyte, and that the electrochemical properties of the redox couple were quickly lost. These authors also demonstrated that the use of a protective overcoat for silver grids based on two layers - a bottom layer of sintered glass frit with a top layer of a thermostable polymer – rendered DSSCs with satisfactory stability in the presence of ionic liquid-based electrolytes40.


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Considering the aforementioned results and findings in the literature, our next trial comprised a combination of the two materials previously investigated. A glass frit layer was deposited on the top of the silver grid lines via screen printing and then, during the sealing step, a layer of Surlyn was also placed on the top the glass frit layer. Figure 3a presents a comparison of the J-V curves measured immediately after assembly for modules containing different protective overcoats for the silver grids: (i) a single layer of glass frit or (ii) a bottom layer of glass frit covered with a layer of Surlyn. The dark J-V curves for these modules are presented in Figure S5 (Supporting Information).

Figure 3. Comparison of the characteristics of modules: (a) J-V curves measured immediately after assembly of modules containing protective overcoat based on (-□-) a layer of glass frit or (-●-) a bottom layer of glass frit covered with a layer of Surlyn; (b) photos taken 5 days after


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assembly, showing the typical behavior observed for modules containing protective overcoat based on (top) glass frit or (bottom) glass frit and Surlyn. Interestingly, within the first days after assembly the modules performed similarly. A slightly lower photocurrent observed for the module containing both frit and Surlyn as protective overcoat might be related to a larger spacing between the photoelectrode and counter electrode, caused by the stacking of the materials, which increases the path for ionic transport in the electrolyte. However, due to the lack of stability of the devices containing only a single layer of glass frit as protective overcoat, after ~5 days only the module assembled with the combination of glass frit and Surlyn showed an appreciable response. The use of combined layers solved the issue related to the corrosion of the silver grid, as illustrated in Figure 3b and Figure S6 (Supporting Information), without significantly compromising the performance of the module. Using this strategy, most of the modules assembled could be characterized and presented enhanced stability with time. Although there is an intrinsic disadvantage to the proposed approach, related to the use of a two-step sealing process instead of a single-step process (based on Surlyn alone, for example), we believe that the gain concerning a higher process yield and reproducibility is an advantage, which balances the disadvantage of adding more steps to the manufacturing process. We also note that the FF of both modules presented in Figure 3 is low (~50-55%). This related to a high series resistance, mainly attributed to the resistivity of the substrate. As presented in Table S1 (Supporting Information), the introduction of the silver grids in the module leads to an enhancement of FF from 43% to 49%, while PCE is enhanced from 1.56% to 2.37%. Other reports found more expressive enhancements of FF (from 48% to 65% or 72%, for Pt or Ag grids, respectively)12 and PCE (from 0.3% to 4.3% for Ni grids)5 after the addition of metal grids


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on the FTO substrate, using different geometries than the one used here. We believe that the configuration of the silver grid has to be further optimized, to allow a more expressive reduction of the substrate resistivity and, therefore, a more expressive increase of FF. To further evaluate the effectiveness of the proposed approach, the evolution of the photovoltaic parameters as a function of time for a module assembled with glass frit and Surlyn and filled with liquid electrolyte was evaluated during 60 days, as shown in Figure 4. Despite the fact that no visual changes in the metal grid were identified, a decrease of Jsc with time was still observed (Figure 4b). This loss became more apparent after ~20 days from assembly and was attributed to slow electrolyte leakage or evaporation through the Kapton-sealed holes in the counter electrode. Upon visual inspection, it was observed a lack of electrolyte in some parts of the device, due to the leakage, while the appearance of the grid lines remained the same as immediately after assembly, keeping its integrity. To support that the loss in Jsc is related to a leakage of the electrolyte via the electrode holes, and not to a degradation of cell components, a module was refilled with the liquid electrolyte after visual observation of signals indicative of leakage and loss of performance, and was evaluated again. Figure S7 (Supporting Information) displays the photovoltaic characteristics of the refilled module, showing that a similar behavior as that presented immediately after the first assembly could be restored. The external part of the module also requires a more robust, permanent sealing solution, instead of the Kapton tape used here. Nevertheless, this closure was performed as a tolerable temporary solution for short times studies. We found that a more permanent solution, such as sealing the outer layer of the device with another glass piece and Surlyn, was not easily accomplished. Snaps or cracks formed in the electrodes during the process to couple an extra glass piece to the module and compromised the


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device performance. Tests involving different materials and methods to seal the electrode holes are still under development in our laboratory and will not be addressed here.

Figure 4. Evolution of the photovoltaic parameters as a function of time for a module assembled with glass frit and Surlyn as protective overcoat for the silver grids and filled with liquid electrolyte. All the parameters have been normalized in relation the value measured immediately after assembly.

3.2. Electrolyte composition For modules sealed with Surlyn, the electrolyte filling is usually performed via holes predrilled in the counter electrode. This procedure is widely accepted and is usually easy and fast when


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liquid electrolytes are used. For polymer or gel electrolytes, on the other hand, the process may be slow and may need assistance of temperature or vacuum. Indeed, it has been shown that vacuum application is a powerful tool to improve the penetration of polymer electrolytes into the photoelectrode of monolithic dye-sensitized solar cells41,42. For the modules investigated here, it was found that electrolyte filling was only effectively achieved with assistance of a vacuum system. For the liquid electrolyte, the filling was completed in approximately 5 min. However, when a polymer gel electrolyte containing 85 wt% of GBL as solvent was used, the filling process took many hours to be completed (i.e., took overnight to deliver modules completely filled, without air bubbles). While the filling of polymer gel electrolyte with such composition was found to be very easy and fast for lab scale devices (active area ˂ 1 cm2), this was not the case for the modules. Thus, adjustments in the electrolyte had to be performed, to change its fluidity and make it more suitable for the filling of large area, sealed devices. The composition adjustment was achieved by adding the solvent MPN to the polymer gel electrolyte, at 1:1 GBL:MPN weight ratio. The total solvent concentration (GBL + MPN) in the gel electrolyte was fixed at 90 wt% in relation to the polymer weight. TBP was also added, to compensate for the Voc loss that could originate from the decrease of the polymer concentration in the media. Previous studies found that electrolytes with high concentration of PEO-based copolymers originated DSSCs with high Voc values, because of the basic nature of these polymers32,43. The formulation of this polymer gel electrolyte, including the selection of MPN and TBP, was based on previous studies about the development of polymer electrolytes for lab scale DSSCs31-34,44. Other compositions, with slight variations of the polymer to GBL to MPN ratios, were also tested in the modules, but the best results were obtained with the composition


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described here. The electrolyte containing 1:1 GBL:MPN composition allowed reaching a similar filling time as observed for the liquid electrolyte, being suitable for the module assembly. Table 2 displays the performance of modules assembled with polymer gel electrolyte before and after addition of TBP and MPN, in comparison with a module assembled with the standard liquid electrolyte. All devices were characterized immediately after assembly. It was observed that the presence of the TBP leads to higher Voc in comparison to the gel electrolyte prepared without this additive, as expected. It was also seen that the performance of the module did not change significantly after addition of the MPN. No further improvements on Jsc were obtained by addition of this solvent, which was related to a high salt dissociation and ionic mobility achieved in the polymer gel electrolyte based solely on GBL and PEO-based copolymers, as discussed in previous reports32,34,36. Regardless of the fact that no significant changes of the device performance were observed after addition of MPN, one should consider that the addition of this solvent was crucial to make the electrolyte filling process viable for these modules.

Table 2. Comparison of the photovoltaic parameters for DSSC modules assembled with glass frit and Surlyn, filled with electrolytes of different compositions, measured immediately after assembly. Electrolyte

Voc (V)

Isc (mA)

Jsc (mA/cm2)

FF

PCE (%)

Liquid electrolyte

0.67

39.55

5.65

0.55

2.08

Polymer gel electrolyte

0.58

52.89

7.56

0.30

1.32

Polymer gel with TBP

0.68

39.47

5.64

0.43

1.65

Polymer gel with TBP and MPN

0.69

39.41

5.63

0.43

1.68


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Despite the fact that the Jsc and Voc obtained with polymer gel electrolyte containing TBP, or TBP and MPN, are very similar to the values obtained with the liquid electrolyte, the PCE for these modules is lower than the PCE of the liquid-based module. This was related to a significantly lower FF observed for the modules containing any of the gel polymer electrolytes investigated here. We correlate this to a hindered penetration of the electrolyte into the TiO2 porous electrode, due to its more viscous nature. Such phenomenon has been acknowledged in previous studies of lab scale devices by our group33 and others42. Considering the characteristics of the TiO2 film, it is reasonable to assume that such phenomenon is happening here. Different anode structures, with larger pores, may be more suitable for DSSCs containing polymer electrolytes45 and will be explored in the future. The performance of modules assembled with the combination of glass frit and Surlyn, filled with the three kinds of polymer gel electrolytes was evaluated during a period of 20 days. Figure 5 shows the variation of the parameters extracted from the J-V curves as a function of time. A slight improvement of the parameters was observed a few days after assembly. This might be related to some sort of accommodation of the polymer chains in the electrolyte, by swallowing of the liquid component for example, which could give room to further improvements of the penetration of the TiO2 pores by the electrolyte. A maximum PCE of 1.9 % and 2.0 % were achieved for the modules containing the polymer gel with TBP, and polymer gel with TBP and MPN, respectively. These values are close to those obtained for the DSSCs modules assembled with the standard liquid electrolyte. For all modules the performance was mostly maintained for about 20 days: FF and Voc displayed no significant changes or some increment, while a slow decrease of the Jsc with time was observed in all cases, similar to the behavior of the module filled with liquid electrolyte. For


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lab scale solar cells (˂ 1 cm2) containing polymer-based electrolytes with different compositions from the ones investigated here, it has been demonstrated a certain stability of in time scales of about 10-40 days28,29, or a decrease in Jsc for DSSCs assembled with polymer electrolytes in similar time-scale46.

Figure 5. Comparison of the photovoltaic parameters as a function of time for DSSC modules assembled with glass frit and Surlyn, filled with electrolytes of different compositions: (--) polymer gel electrolyte, (-◊-) polymer gel electrolyte with TBP, and (-●-) polymer gel electrolyte with TBP and MPN.


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Besides the unwanted chemical reaction between the electrolyte and the silver grid, there are other possible causes for the degradation of the device performance. If there is moisture intrusion into the cell, for example, it might enhance desorption or degradation of the dye. In such case, a color change in the electrode is expected, besides the loss of efficiency. Alternatively, a change of ingredients of the electrolyte due to contamination with water would cause a strong impact on the parameters, with significant decrease of both Voc and Jsc. In the time scale of the experiments presented here, a slight increase of Voc and decrease of Jsc were observed. A possible explanation for this time dependent behavior of the module is the formation (and growth upon ageing) of iodine crystals. The decrease of I3- concentration due to crystallization of iodine may restrain its diffusion, or cause a downshift in the redox potential of the electrolyte, leading to an increase of Voc. The formation of iodine crystals has been demonstrated for DSSCs containing ionic liquid-based electrolytes15,17. It is also important to note that some degradation mechanisms may be triggered or accelerated under different stress conditions, like high temperature, humidity, light intensity, or depending on the device operation regime, for example. Therefore, stability tests carried out under other stress conditions will have to be considered in the future, to provide complimentary information for the results discussed here. Overall, the efficiencies presented here are lower than the efficiencies reported for other modules with liquid or polymer gel electrolytes. We believe the efficiency of our modules may be enhanced by tuning the characteristics of the photoelectrode, such as the TiO2 film thickness and porosity, addition of a blocking layer and a light scattering layer. The configuration of the photoelectrode, including the size and shape of the metal grid and active layer could be further optimized by using different geometries, as has been demonstrated elsewhere37. Nevertheless, the


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approach developed here is expected to be easily applied to optimized photoelectrodes, to allow an enhanced confection yield of parallel grid-type modules with improved power conversion efficiency and stability.

4. CONCLUSIONS In this work we investigated portable, parallel, grid-type DSSC modules containing polymer gel electrolyte or a standard liquid electrolyte. Two aspects of module assembly were considered: one related to the use of a protective overcoat layer and sealing step, and the other related the electrolyte filling step. Firstly, we discussed modules containing different protective overcoats for the silver grids: a single layer of Surlyn, a single layer of glass frit, or a combination of the two materials (bottom layer of glass frit covered with a layer of Surlyn). It was found that, immediately after assembly, all modules performed similarly. However, the devices containing a single material as protective overcoat presented a lack of stability after ~2-5 days. A disruption of the silver grid integrity, accompanied by loss of the electrolyte characteristic color was usually observed for these modules. This process was sometimes accompanied by unsatisfactory sealing (presence of bubbles and defects in the Surlyn layer), which ultimately leads to electrolyte evaporation or leakage. The use of a combination of glass frit and Surlyn, on the other hand, was proven to minimize those issues. This approach allowed the evaluation of the evolution of the photovoltaic parameters as a function of time for a module filled with a standard liquid electrolyte during 60 days, revealing a preservation of the cell component integrity. Secondly, we addressed electrolyte filling related issues. It was found that electrolyte filling was only effectively achieved with assistance of a vacuum system. When using a liquid electrolyte, the filling was completed in approximately 5 min. However, for a polymer gel electrolyte the filling


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process was usually very slow, taking overnight to deliver modules completely filled and without air bubbles. Thus, we found that the composition of the polymer gel electrolyte had to be tuned, which was achieved by addition of an extra solvent and an additive, to enhance the electrolyte fluidity and to compensate for the Voc loss that could originate from the decrease of the polymer concentration in the media, respectively. Modules assembled with the combination of glass frit and Surly and filled with the polymer gel electrolyte were evaluated during a period of 20 days, displaying a similar behavior to the module filled with liquid electrolyte. Overall, the efficiency of our modules is expected to be further enhanced by tuning the characteristics of the photoelectrode (optimized TiO2 layer, changes in the size and shape of the metal grid, etc.). Regardless, the approach discussed here should be readily applicable to optimized photoelectrodes, to allow the production of parallel grid-type modules with improved confection yield, power conversion efficiency and stability.

Supporting Information. Microscopy images of silver grids and glass frit layer. Comparison of the performance of DSSCs modules with and without silver grids. Dark J-V curves. Pictures of the modules before and after ageing. Performance of a module refilled with liquid electrolyte after leakage. This information is available free of charge via the Internet at http://pubs.acs.org. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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The authors thank Grupo Rede de Distribuição de Energia and CNPq for financial support. MKH thanks PCI/CTI/MCTI for scholarship. The authors also thank the Microfabrication Laboratory and the Electron Microscopy Laboratory at LNNano/CNPEM for sputtering deposition of Pt films and SEM-FEG analysis

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Figure Captions Figure 1. Typical photos of modules sealed with Surlyn and using only this material as protective overcoat for the silver grids, taken: (a) immediately after assembly or (b) 2 days after assembly. Then inset shows the presence of bubbles in the Surlyn layer after the sealing step. Figure 2. Images of modules containing a glass frit layer as overcoat for the silver grid lines: (a) immediately after module assembly, (b) after 10 days and (c) after 20 days. Figure 3. Comparison of the characteristics of modules: (a) J-V curves measured immediately after assembly of modules containing protective overcoat based on (-□-) a layer of glass frit or (○-) a bottom layer of glass frit covered with a layer of Surlyn; (b) photos taken 5 days after assembly, showing the typical behavior observed for modules containing protective overcoat based on (top) glass frit or (bottom) glass frit and Surlyn. Figure 4. Evolution of the photovoltaic parameters as a function of time for a module assembled with glass frit and Surlyn as protective overcoat for the silver grids and filled with liquid electrolyte. All the parameters have been normalized in relation the value measured immediately after assembly. Figure 5. Comparison of the photovoltaic parameters as a function of time for DSSC modules assembled with glass frit and Surlyn, filled with electrolytes of different compositions: (--) polymer gel electrolyte, (-◊-) polymer gel electrolyte with TBP, and (-●-) polymer gel electrolyte with TBP and MPN.


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Assembly Considerations for Dye-Sensitized Solar Modules with

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