Effect of aging conditions on the performance of dip coated platinum counter electrode based dye sensitized solar cells

Abstract

Platinum is the most widely accepted excellent catalytic material for counter electrodes in dye sensitized solar cells (DSSCs). The problems associated with platinum are high cost and low corrosion resistance against the iodide/triiodide redox couple. Use of platinum nanoparticles for counter electrodes resolves the cost issue. The present work shows the effect of different aging conditions on the performance of DSSCs fabricated using platinum nanoparticle based counter electrodes. Two conditions – room temperature under darkness (R-D) and heating (60 °C) under illumination (H-I) – were chosen for the aging studies. Platinum nanoparticles with a size of ∼2 nm were synthesized and a dip coating process was followed to fabricate counter electrodes. Electrolyte (iodide/triiodide) and counter electrode interaction were studied by fabricating symmetric cells composed of two platinized electrodes. It was found that platinum almost entirely reacted with the electrolyte within 51 days for the H-I condition aged symmetric cell, whereas R-D condition aging was relatively ineffective in the dissolution of platinum. Interestingly, in DSSCs aged under the H-I condition for 3 months, photoconversion activity was still observed, which was later attributed to the presence of platinum on the counter electrode. So, in short circuit conditions under illumination, the dissolution of platinum by the electrolyte is delayed. The R-D condition aged cell showed a smaller decrease in performance compared to the H-I condition aged cell. Other components of the DSSCs such as titania film and dye were also characterized. Finally, it was found that apart from dissolution of platinum from the counter electrode, desorption of additives or ions of electrolyte from titania are also responsible for the decay in DSSC performance with aging.

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1. Introduction

Dye sensitized solar cells (DSSCs) have emerged as a suitable alternative to thin film solar cells. Due to their low cost and easy fabrication technique, research in this field has gained immense interest from researchers across the world. Improving the efficiency and stability are the major challenges in this type of solar cell. An efficiency of 12.3% has been achieved in liquid electrolyte based DSSCs.¹ However with liquid electrolytes, the stability of the cell remains a major problem. Leakage of electrolytes acts as a barrier towards long term stability. Effective sealing can prevent the leakage of volatile matter from DSSCs and ensure long term confinement of the liquid electrolyte. Reaction of the liquid electrolyte with the counter electrode leads to a decrease in performance of the cell over time. Till now, platinum has served as the most efficient counter electrode material but cost is a major issue with platinum based counter electrodes. Normally, platinum based counter electrodes are prepared by sputtering or thermal deposition.² To reduce the usage of platinum without a significant compromise in performance, nano platinum based counter electrodes based on dip coating³ and electrophoretic deposition⁴ have been demonstrated. Almost similar DSSC efficiencies have been achieved with lower loading of platinum on the counter electrode prepared by dip coating process as compared to sputter deposited platinum based counter electrode having higher amount of deposited platinum.⁵ Another issue with platinum is its reaction with corrosive iodide/triiodide based redox electrolyte. Olsen et al. have studied the dissolution of a vapour deposited platinum counter electrode in an iodide/triiodide redox electrolyte.⁶ The electrode lost its catalytic activity within 48 h. Hauch et al. have pointed out the poisoning effect of platinum based electrodes.⁷ Storage in air for a few months decreased the catalytic activity of the electrode. Cells fabricated from that electrode regained their activity up to a certain extent after storage for a few weeks. Syrrokostas et al. investigated the effect of air and electrolyte storage on electrodeposited and thermal deposited platinum based electrodes.⁸ In both the storage conditions performance decreased with time. Storage in the electrolyte caused a drop in catalytic activity mainly due to the dissolution of platinum rather than passivation of platinum by iodine. Atomic iodine absorbed onto the electrode surface combines with platinum and leads to its dissolution. Regeneration of catalytic activity of an aged counter electrode by heat or acid treatment was ineffective. Kitamura et al. have demonstrated that the presence of water in an electrolyte can cause faster corrosion of the platinum electrode.⁹ Sputter coated platinum electrodes changed to a porous structure in a short time when dipped in an electrolyte containing water and stored at 85 °C.

Studies on the degradation of platinum based counter electrodes have been carried out by several groups, but most of these studies were based on electrodes, excluding DSSCs. In a cell under operation, platinum dissolution from the electrode might be different than with an electrode dipped in electrolyte solution. The present study investigates the stability of nano platinum based counter electrodes prepared by a dip coating method in DSSCs under different aging conditions. Each component of cell has been analyzed after the aging study to show the effect of aging on different components.

2. Experimental

Materials

Hexachloroplatinic acid hexahydrate (Puriss, Spectrochem), polyvinylpyrrolidone [PVP] K30 (Puriss, Spectrochem) and 1-propanol (Synthesis grade, Merck) were used to synthesize platinum nanoparticles. 1-Methyl-3-propylimidazolium iodide [PMII] (≥98% Aldrich), iodine (LR, Thomas Baker), 4-tert-butylpyridine [TBP] (96% Aldrich) and 3-methoxypropionitrile [MPN] (≥98% Aldrich) were used to prepare the electrolyte. Fluorine-doped tin oxide (FTO) glass substrates (TEC7, sheet resistance 8–9 Ω □⁻¹, Pilkington), TiO₂ nanopowder (P25, Degussa), polyethylene glycol (M_w = 600) [PEG 600] (Thomas Baker), and RuL₂(NCS)₂ (L = 2,2′-bipyridyl-4,4′-dicarboxylic acid) [known as N3 dye] (dyesol) were used for fabrication of the cell. Hydrochloric acid (37%, Emparta, Merck) was used for preparation of the samples to quantify platinum in solution.

Synthesis of platinum nanoparticles

First 1.44 g of PVP was dissolved in 60 ml of 1-propanol by stirring and then 6 ml of 6 mM platinum precursor solution in distilled water was added to it. The solution was stirred for 15 minutes and then refluxed at 110 °C for 1 h. The colour of the solution changed to black, indicating the formation of platinum nanoparticles. The prepared platinum sol was stable due to PVP, which acts as a capping agent.

Fabrication of dye sensitized solar cells

A dip coating process was followed to fabricate platinum based counter electrodes. Two holes were drilled on each FTO-glass counter electrode. After cleaning, the substrates were dipped in the platinum nanoparticle suspension for 10 minutes. Then the electrodes were heated at 450 °C for 1 h in air to remove organics. Titania slurry used for fabricating photoanodes was prepared by roller milling TiO₂ nanopowder, PEG 600 and ethanol for 24 h. The slurry was then coated onto the cleaned FTO-glass substrates and sintering was carried out at 450 °C for 1 h. Then the electrodes were cooled to 80 °C and dipped into a 0.3 mM solution of N3 dye in ethanol. After 24 h, the electrodes were taken out of the dye solution, rinsed with ethanol and dried. To seal both the electrodes together, a 25 micron spacer (SX1170-25 PF, Solaronix) was used and the assembly was heated at 115 °C for 15 minutes. An electrolyte, with a composition 0.6 M PMII, 0.1 M iodine and 0.5 M TBP in MPN, was filled into the empty space between the electrodes through the holes. The holes were sealed using cover glass and the same thermoplastic polymer film used to seal the electrodes. The active area of each cell was ∼1 cm².

Characterization

The particle size of the synthesized platinum nanoparticles was observed by field emission gun-transmission electron microscopy (FEG-TEM; JEOL JEM 2100F). Sealed symmetric cells were prepared by using nano platinum based counter electrodes. Characterization of the symmetric cells was done by PGSTAT (AUTOLAB 302N). Cyclic voltammetry (CV) was carried out using such cells at a scan rate of 10 mV s⁻¹ from −1 to +1 V and impedance measurements were performed over a frequency range of 0.1 MHz to 0.1 Hz with an amplitude of 10 mV. Photocurrent–voltage (I–V) characteristics of the DSSCs were determined by using a Keithley model 2420 source measure unit. A 150 W xenon lamp on a Newport solar simulator with AM 1.5G was used for illuminating the cells. Impedance measurements on the DSSCs were carried out over a frequency range from 0.1 MHz to 0.1 Hz with an amplitude of 10 mV at open circuit potential under illumination or at −0.75 V bias potential under darkness. Symmetric cells and DSSCs were aged at room temperature (∼27 °C)-dark (R-D) or 60 °C heating along with illumination (H-I) from a LED light source (24 W, 3600 Lumens, Lucifer Lights Ltd) in short circuit conditions. The light source was intentionally chosen as it has an emission maxima in the region of visible light absorption maxima of N3 dye. Both symmetric cells and DSSCs were cooled to room temperature before the measurements were carried out. After a certain aging period, symmetric cells and DSSCs were disassembled and the components were characterized. The morphology of the electrodes was observed by field emission gun-scanning electron microscopy (FEG-SEM; JEOL JSM 7600F). The adsorbed dye on the electrode was desorbed using a 1 N NaOH solution and the absorption spectrum was recorded using a UV-Visible spectrometer (Jasco V650). Dye loading was determined using a calibrated curve. Nanoindentation studies on titania of the photoanodes were carried out using a TriboIntender TI900 (Hysitron Inc., USA). CV of the counter electrodes was carried out at a scan rate of 50 mV s⁻¹ from −0.6 to +1.2 V in an electrolyte solution composed of 5 mM LiI, 0.5 mM iodine and 0.1 M LiClO₄ in acetonitrile. Platinum wire was used as the counter electrode and the reference electrode was Ag/AgCl (3 M KCl). Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) (Model ARCOS, Spectro, Germany) was performed with the electrolyte extracted from the aged symmetric cells to check the presence of platinum. Samples were prepared by dissolving 5 mg of electrolyte in 5 ml of dilute HCl (10 vol%).

3. Results and discussion

Fig. 1 shows the TEM image of the synthesized platinum nanoparticles. Particles with a size of ∼2 nm can be observed from the image. Monodisperse and agglomerate free particles confirm that capping of platinum nanoparticles by PVP has effectively occurred.

Fig. 1 FEG-TEM image of PVP capped platinum nanoparticles.

In order to investigate the stability of platinum nanoparticle based counter electrodes, symmetric cells were subjected to aging under both conditions (H-I and R-D) as DSSCs. Illumination or darkness have no major role to play in the case of symmetric cells, as no current will flow through the cell. So, temperature will be the deciding factor in both the conditions. It can be observed that the limiting current density (J_Lim) remains stable for symmetric cells stored in the R-D condition but it decreases for the H-I condition on aging (Fig. 2a). The cyclic voltammogram obtained from the symmetric cells stored in the R-D condition shows negligible variation after aging for 51 days but for the H-I condition, an increase in over potential for the redox reaction can be observed (Fig. 2b). Platinum acts as a catalyst for the redox reaction but FTO is not a suitable catalyst and the redox reaction can only occur on FTO by applying a high potential compared to platinum (Fig. 2d). This explains the observed increase in over potential for the redox reaction in the H-I condition on aging. Also there is an increase in the charge transfer resistance (R_CT) at the electrode/electrolyte interface in the H-I condition due to aging (Fig. 2c). All of these indicate that dissolution of platinum from electrodes occurs faster under the H-I condition and electrodes remain stable under the R-D condition. Also, the concentration of redox species would change due to the reaction with platinum and that is also responsible for such changes. Fig. 3 supports the argument of dissolution of platinum nanoparticles from electrodes under the H-I condition upon aging. Platinum nanoparticles are almost absent in electrodes stored in the H-I condition, whereas particles can be observed for electrodes subjected to the R-D condition. Dissolution of platinum was further confirmed by ICP-AES analysis of electrolytes extracted from symmetric cells aged for 90 days. For the H-I and R-D condition aged symmetric cells, 0.143 ppm and 0.046 ppm of platinum were detected respectively. Such results confirm the faster reaction of platinum with the electrolyte in the H-I condition as compared to the R-D condition. Detachment of some particles from the electrode is also possible, but the major factor for deterioration in performance of the platinum based electrode is reaction with the electrolyte. Dissolution of platinum has been reported by soaking the electrodes in electrolyte under low or room temperature conditions, but in the present study a significant change occurs only at a high temperature.^6,8 This might be due to the use of symmetric cells, which contain a limited amount of electrolyte to react with platinum.

Fig. 2 Variations in symmetric cell parameters with aging under H-I and R-D conditions: (a) limiting current, (b) cyclic voltammogram and (c) charge transfer resistance. (d) Cyclic voltammograms of FTO-glass and dip coated Pt on FTO-glass obtained using a three electrode system.

Fig. 3 FEG-SEM images of electrodes used in symmetric cells after aging in (a) H-I and (b) R-D conditions.

Fig. 4 shows the effect of different aging conditions on the photovoltaic parameters (normalized) of the DSSCs and the actual values for the different parameters are provided in Table 1. It can be observed that the cell aged under the R-D condition performs superior to the cell aged under the H-I condition. Due to the dissolution of platinum from the counter electrode, the regeneration of triiodide would be affected and also the ratio of triiodide to iodide changes due to the reaction of iodine with platinum.^6,8 The rate of reaction between the electrolyte and platinum would be enhanced by application of heat or in the H-I condition. This leads to a decrease in performance of the H-I condition aged cell over time. But such a cell effectively compares with DSSCs fabricated with a sputter deposited platinum (∼5 nm thickness) based counter electrode aged under similar conditions (Fig. 5). The initial increase in the R-D condition aged cell might be due to better adsorption of additives or cations from the electrolyte, which would prevent the back transfer of electrons from titania to the electrolyte. This can be explained on the basis of lower dark current observed in the R-D condition aged cell, as compared to the H-I condition aged cell (Fig. 4e). A decrease in dark current over time was observed for the R-D aged cell, whereas dark current decreased for a few days after which it increased in the H-I aged cells. Correlating such trends to the V_OC of respective cells shows a correlation only for the R-D aged cell, whereas V_OC decreases for the H-I aged cell. This might be due to a change in the concentration of electrolyte, which would interfere with dye regeneration. Further investigations were carried out by electrochemical impedance analysis of such cells.

Fig. 4 Variations in photovoltaic parameters with aging under H-I and R-D conditions (a) J_SC, (b) V_OC, (c) fill factor and (d) efficiency. The variation of dark current measured at ∼500 mV for both conditions is shown in (e).

Table 1 Photovoltaic parameters at different time intervals of DSSCs aged under H-I and R-D conditions

Cell-days	JSC (mA cm^−2)	VOC (mV)	FF (%)	η (%)
H-I-0	5.7	709	63.2	2.6
1	5.6	729	63.4	2.6
7	4.8	699	62.2	2.1
15	4.3	678	61.8	1.8
30	3.9	678	63.2	1.7
45	3.6	668	61.2	1.5
60	3.6	648	61.2	1.4
90	3.6	628	57.1	1.3
R-D-0	5.3	729	65.6	2.5
1	5.3	749	66.1	2.6
7	5.7	759	65.2	2.8
15	5.6	759	65.0	2.8
30	5.2	759	67.8	2.7
45	5.1	759	67.3	2.6
60	4.8	749	68.1	2.5
90	4.5	739	67.8	2.2

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Fig. 5 Variations in photovoltaic parameters of DSSCs fabricated with dip coated platinum (DC-Pt) and sputter deposited platinum (SD-Pt) counter electrodes with aging under H-I condition (a) J_SC, (b) V_OC, (c) fill factor and (d) efficiency. Variation of dark current measured at ∼500 mV for both conditions is shown in (e).

Fig. 6 shows the variation of impedance parameters of DSSCs with aging. It can be observed that under illumination, at open circuit potential, the cell aged under the R-D condition has almost stable parameters, which also correlates well with the observed photovoltaic parameters (Fig. 6a). However, in the case of the H-I condition aged cell, both Pt/electrolyte charge transfer resistance (R_CT1) and titania/electrolyte charge transfer resistance (R_CT2) increase with time. Dissolution of platinum would decrease the catalytic activity of triiodide reduction and increases the charge transfer resistance. Increase in R_CT2 may be due to a decrease in the concentration of triiodide or retardation of dye regeneration. During aging, continuous changes in the open circuit potential were observed over time. So, in order to have suitable clarification regarding the charge transfer processes, impedance measurements were carried out in darkness at a bias of −0.75 V. Similar to the measurement at the open circuit condition under illumination, stable parameters were obtained for the R-D condition aged cell. In the case of the H-I condition aged cell R_CT1 increased with time, suggesting that platinum dissolution is a major factor. But R_CT2 decreases with time as compared to the increase in magnitude with aging time under illumination. This could be due to the desorption of additives or ions of the electrolyte from the surface of titania, which would increase the rate of recombination. It also explains the decrease in capacitance or constant phase element value (Y₀) over time. So, from both the measurements it can be concluded that platinum dissolution, the decrease in triiodide concentration and the desorption of electrolyte additives or ions are major causes for the decrease in cell performance.

Fig. 6 Change in impedance parameters of DSSCs with aging under H-I and R-D conditions under (a) illumination at open circuit potential and (b) darkness at a bias of −0.75 V.

Fig. 7 shows the absorption spectra of N3 dye desorbed from photoanodes after aging under H-I and R-D conditions. It can be observed that both the curves are almost similar. Even though N3 is known to have a maxima at ∼534 nm, a maxima occurs below 500 nm due to its presence in the NaOH solution, which has also been observed in dyes desorbed from fresh films in a separate study carried out in the lab. The absorption maxima for N3 dye from H-I and R-D aged films were 495 nm and 498 nm respectively. So, a small blue shift of absorption occurs due to aging under the H-I condition. For N719 based dye, which has some similarity with N3 dye used in the present study, it has been found that the blue shift occurs due to the reaction of triiodide with the dye molecule.¹⁰ Iodide substitutes the isothiocyanate groups present in the dye and such a reaction increases the difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) level of the dye. So, in the present study, the isothiocyanate groups present in the N3 dye might be replaced by iodide from the electrolyte, leading to a blue shift and such a reaction would be faster in the H-I condition as compared to the R-D condition. The amount of dye present in both the films was almost similar. For the H-I and R-D conditions, dye loading of 9.6 × 10⁻⁸ moles cm⁻² and 9.7 × 10⁻⁸ moles cm⁻² were obtained, respectively. So, due to chemical change in the dye, the photoconversion efficiency of the cell decreases along with other factors during aging under the H-I condition. Due to the lack of experimental facilities for dye behavior, other phenomena of the dye are excluded in the present study.

Fig. 7 Absorption spectra of N3 dye desorbed from the photoanodes of DSSCs subjected to aging under H-I and R-D conditions.

Nanoindentation studies on a fresh titania electrode and electrodes subjected to aging, show that fresh film exhibits the highest resistance to penetration of indentor under low load conditions (Fig. 8). During aging, the titania film remains in contact with the electrolyte, which reacts with the film making the interparticle contact weaker. Such a reaction would be promoted by the application of heat, so films from DSSCs, subjected to the H-I condition, show maximum penetration of indentor under low load conditions and films from the R-D stored DSSCs remain in between fresh and the H-I curves.

Fig. 8 Force–displacement curves of titania films without aging and with aging under H-I and R-D conditions.

Fig. 9 shows the FEG-SEM images of titania films of DSSCs subjected to different aging conditions. Even though small differences were observed in the nanoindentation curves, no such differences were observed in the microstuctures of the films. So, it is confirmed that the interparticle bonding is only affected by aging due to contact of the film with the electrolyte. No change to the morphology of the particle and films occurs with aging.

Fig. 9 FEG-SEM images of titania films of DSSCs aged under (a) H-I and (b) R-D conditions.

Fig. 10 shows the FEG-SEM images of counter electrodes of DSSCs aged under H-I and R-D conditions. Interestingly, platinum particles were observed on electrodes subjected to the H-I condition as compared to almost no platinum under similar conditions in the symmetric cell. This confirms that under static conditions in the symmetric cell, the redox electrolyte reacts with platinum more efficiently. However, in DSSCs under a short circuit condition, the process of triiodide formation at the photoanode and reduction at the counter electrode occurs continuously. Hence, the chance of a reaction with platinum at the counter electrode decreases. Still, due to some dissolution of platinum, the performance decreases with time under the H-I condition.

Fig. 10 FEG-SEM images of counter electrodes of DSSCs aged under (a) H-I and (b) R-D conditions.

Counter electrodes were subjected to CV in a redox electrolyte in order to observe their electrocatalytic activity. The peak at a negative potential corresponds to the reduction of triiodide, which remains of interest for DSSC application. It can be observed that the triiodide reduction potential is shifted to higher potentials in aged electrodes compared to fresh electrodes (Fig. 11). Electrodes aged under the H-I condition show a maximum shift and also the peak current is the minimum for it. So, there is dissolution of platinum from both the aged electrodes and the maximum dissolution is observed for the H-I condition aged electrode. Some platinum still remains and exhibits catalytic activity. Due to this, the DSSC continues to show photoconversion after 3 months of aging.

Fig. 11 Cyclic voltammograms of the fresh counter electrode and counter electrodes of DSSCs aged under different conditions.

4. Conclusions

Aging studies on nano platinum based counter electrodes were performed by utilizing them in symmetric cells and DSSCs under two different conditions. In the case of symmetric cells, it was observed that dissolution of platinum occurs much faster under heat and illumination. However, in the case of DSSCs, such faster dissolution under similar conditions never occurs as the cell is under operation at short circuit condition. A small change occurs to the hardness of the titania films upon aging as the liquid electrolyte remains in contact with titania. No change is observed in the morphology of the titania film. A minor blue shift occurs to the absorption spectrum of the dye upon aging under heat and illumination. Finally, it can be concluded that nano platinum from dip coating still remains even after the DSSCs were aged for 3 months at 60 °C under illumination. The decrease in performance is not only due to the dissolution of platinum leading to poor catalytic activity, but also partly due to a change in the concentration of redox species due to the reaction of iodine with platinum, which might lead to poor dye regeneration. Desorption of additives and ions of the electrolyte from the bare surface of titania also adds to the change in performance of the DSSC with aging. The chemical change of the dye molecules is also responsible for the decrease in performance of the cells aged under heat and illumination.

Acknowledgements

The authors acknowledge Corning Inc. USA and DST India (Project codes-10DST030 and 11DST073) for financial support. We are thankful to Sophisticated Analytical Instruments Facility (SAIF), IIT Bombay for providing the ICP-AES, FEG-SEM and FEG-TEM facilities. Industrial Research & Consultancy Centre (IRCC), IIT Bombay is also acknowledged for providing the Nanoindenter facility.

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