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Stability of dye-sensitized solar cells under light soaking test
Journal of Non-Crystalline Solids 356 (2010) 2049–2052
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Stability of dye-sensitized solar cells under light soaking test Enrico Leonardi, Stefano Penna, Thomas M. Brown, Aldo Di Carlo, Andrea Reale ⁎ CHOSE — Centre for Hybrid and Organic Solar Energy, Via G. Peroni 400/402, 00131 Rome, Italy Electronic Engineering Department, University of Rome Tor Vergata, Via del Politecnico 1, 00133 Rome, Italy
a r t i c l e
i n f o
Article history: Received 26 May 2010 Available online 2 July 2010 Keyword: Dye Solar Cells
a b s t r a c t Long term stability of Dye Solar Cells (DSC) is still an open issue to be addressed to obtain affordable photovoltaic devices for practical use. Continuous light soaking of DSC devices is a useful technique to induce an accelerated degradation mechanism that would allow evaluating the lifetime of a device. In this paper the results of 2280 h of continuous light soaking at 0.8 sun of DSC devices working under different operating conditions have been reported. The time-dependence of electrical parameters of the devices has been combined to the electro-impedance analysis in order to provide a physical explanation of the degradation mechanism. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the last fifteen years, the Dye Solar Cells (DSC) have attracted much interest of the scientific and commercial communities, mainly due to the high energy conversion efficiency, on the one hand [1,2], and the low fabrication cost combined with a relatively simple manufacturing process, on the other hand. A DSC device is basically made of three components: large band gap mesoporous titania (TiO2) as the working electrode that is sensitized with an organic material (dye) aimed at enlarging the absorption band of the device, a catalyst such as platinum or carbon as the counter electrode, and lastly, an electrolyte solution based on a redox couple, typically iodide/triiodide (I−/I3−) [1]. The mesoporous TiO2 film is considered to be the heart of the device [3]. The titania particles with a large refraction index enhance the light absorption of the photovoltaic electrode as they can induce strong light scattering [4]. However, the larger the particles, the lower the effective surface area of the TiO2 film for absorbance. Therefore, the aim is to find an optimum equilibrium between a large surface area and efficient light scattering. Many studies have been carried out to find this relationship [5,6]. Currently, the efficiency values of DSC devices are considered to be sufficient to supply low power consumption devices. Nevertheless, the lifetime of the device and the stability of the materials involved are still too poor to allow for a practical application of photovoltaic cells. Therefore, a still unreached goal is to ensure device lifetimes that are suitable to the market needs. Individual components of a solar cell have been extensively studied and are currently able to provide a lifetime of tens of years without noticeable degradation [7,8].
⁎ Corresponding author. Electronic Engineering Department, University of Rome Tor Vergata, Via del Politecnico 1, 00133 Rome, Italy. E-mail address: [email protected] (A. Reale). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.05.072
Several studies performed on ageing tests of DSC solar cells have identified the following as the main causes of loss in performance: 1) electrolyte contamination by environmental agents (oxygen and water) [9], 2) desorption of dye from titania [9] and 3) degradation of organic compounds due to the ultraviolet component of the solar spectrum [10,11]. Therefore, it is clear that a fundamental role is played by sealants that must ensure that contaminants, such as oxygen and water, do not affect the inner organic compounds of the DSC. There are some previous studies on the DSC stability [12,13], but further experiments and analyses are needed in order to better understand the degradation mechanism of DSC, both in standard conditions of testing and under different types of stress. 2. Experimental and methods 2.1. Fabrication method and materials The cells were fabricated using commercial 2 cm×4 cm fluorinated tin oxide glass (TEC-8 Pilkington) as a substrate, 3 mm in thickness and with 8 Ω/square resistivity. The TiO2 layer was obtained using DSL 18 NRT paste (Dyesol) deposited by a screen-printing facility (Baccini) and fired in a fan oven at 525 °C for 30 min. Three active areas of 0.5 cm ×0.5 cm each (0.25 cm2), TiO2 layers, 10 μm thick, were deposited on each glass substrate. The TiO2 layer was sensitized by a 15 hour dip coating process in a 0.5 mM solution of N719 dye (Dyesol) in pure ethanol (Carlo Erba). Platinum paste PT-1 (Dyesol) was deposited by screen-printing as the counter electrode. The wet Platinum layer was fired in a fan oven at 425 °C for 15 min. An HSE electrolyte (Dyesol) solution was injected into the device with a vacuum back filling method. Four samples with three devices on each substrate were fabricated. All the cells were primary sealed using a thermoplastic SX1170-PF (Solaronix) film, applied by heat-assisted pressure, and secondary sealed
2050
E. Leonardi et al. / Journal of Non-Crystalline Solids 356 (2010) 2049–2052
Fig. 1. Simplified circuit for TiO2 in conductive state.
with high strength UV epoxy (Dymax) cured under a UV Lamp with 252 mW/cm2 intensity (Dymax) for less than 10 s. Finally, a commercial silver dag (RS) was used to improve the electron collection at the external contacts. Different measurements were performed with a UV–Vis spectrophotometer UV-2550 (Shimadzu) to verify possible damage induced by UV exposure on the compounds involved in the cell, in particular, organic compounds. More in detail, the N719 solution absorbance in pure ethanol, HSE electrolyte and titania layers, both not dyed and dyed, were separately measured before and after exposure to the UV lamp for 45 s. In order to understand more deeply how a DSC behaves under stressing conditions, two sample devices were exposed to a C class solar spectrum in a light soaking chamber LSC 1.3 (Dyesol) at 0.8 sun for 2280 h, whereas two nominally equal devices were kept in the dark in order to evaluate the different contributions to degradation of
performance in DSC devices. A software interface developed on the IEEE 488 standard (National Instruments Labview v8.5) was used to control the equipment, monitor the data acquisition and to update the measurement settings. The three active areas of the four devices under light soaking were left working under different operating conditions: one active area per device was forced to work at the maximum power point (Pmax), whose value was periodically updated to balance the performance variations, whereas the remaining active areas were left at open circuit voltage (Voc). The two cells in dark conditions were left at Voc. A B class sun simulator (KHS Technical Lighting) was used as the light source (1 sun intensity, AM 1.5 solar spectrum) to evaluate the actual degradation of the devices and an E5262A Analyzer (Agilent) was used to acquire the electrical parameters. The data were mediated through a repeated interrogation of the instrument for each working point acquired, the average error was estimated to be less than 1%. Finally, the electrochemical behavior of the samples during the testing period was observed with an Autolab PGSTAT302N (Metrohm) potentiostat. The experimental data was fitted to the model represented by the equivalent circuit shown in Fig. 1. In this figure, the TiO2 network representation was simplified, following what was presented by Fabregat et al. [14]. In such an equivalent circuit, the series resistance element Rs is affected by the resistance of TCO electrodes and by the electrolyte conductivity, the Zelec element represents the ionic diffusion within the electrolyte, and the two Randles circuits, that are the working electrode and the counter electrode, represent an interface of the electrolyte liquid with the titania layer and the Platinum layer, respectively. The equivalent circuit elements have the following meaning: • Cte is the chemical capacitance that stands for the change of electron density as a function of the Fermi level [15]; • Rte is the charge-transfer resistance related to a recombination of electrons at the TiO2/electrolyte interface; • Rs is the series resistance introduced by the TCO;
Fig. 2. Comparison of absorbance before and after UV exposure for a) N719 dye in ethanol, b) HSE electrolyte solution, c) titania film, and d) dye sensitized titania film.
E. Leonardi et al. / Journal of Non-Crystalline Solids 356 (2010) 2049–2052
• Zelec is the impedance of diffusion of the electrolyte redox species; • Rce is the charge-transfer resistance at the counter electrode/ electrolyte interface; • Cce is the interfacial capacitance at the counter electrode/electrolyte interface. The ageing process and the performance evaluation were all carried out at 25 °C, corresponding to the temperature fixed by the STC (Standard Temperature Conditions) defined in the IEC 61215 standard for ageing of photovoltaic panels based on silicon, both crystalline and amorphous. 3. Results and discussion Fig. 2 shows the analysis of the effect of 45 s of UV light soaking on the dye, electrolyte, titania dyed and not dyed. No degradation or decreasing of the absorbance was observed in the samples for such an observation time that is more than four times longer than required by the UV-curing of Dymax resins. The UV degradation effect was tested with the same boundary conditions that had been used for manufacturing the cells (a protective layer using glass with TCO, the same substrate used for manufacturing the devices). The variations of the electrical parameters of the cells, i.e. efficiency, short circuit current density, open circuit voltage and fill factor, related to both the above mentioned working conditions, are shown in Fig. 3. It can be observed that the performance values continue to be stable with time, showing a similar trend for both the efficiency, the short circuit current density and the fill factor. The efficiency graph (Fig. 3a) shows an
2051
overall degradation of 15% at the end of the period of observation with respect to the beginning. The short circuit current density (Fig. 3b) presents slightly different decay rates, with 3% degradation for the Voc working cell and no degradation for the cell working at Pmax. Similar behavior was observed on the fill factor (Fig. 3c), with a 7% degradation for the cell working at Voc and 4% for the one working at Pmax. The open circuit voltage curves (Fig. 3d) show slopes with a more remarkable difference, corresponding to different degradation rates, between the Pmax working cell and the Voc working cell. In particular, stressing the cell at Pmax seems to induce a faster reduction of the Voc value (12%), especially in the first 200 h of observation, with respect to the cell working at Voc (7%). It is evident for all the graphs in Fig. 3, as the first hundreds of hours of observation, namely corresponding to one week of constant lighting, strongly affect the performance of DSC devices resulting in a higher degradation rate. From the above shown results, it can be noted that different working conditions mainly affect the open circuit voltage parameter's behavior (Fig. 3c), with no meaningful difference between the other parameters. The cells were observed with an electro-impedance spectroscopy (EIS) technique to analyze the degradation effects of different working conditions from a physical point of view. In Fig. 4 the Nyquist diagrams related to the samples at the beginning and at the end of the ageing period are shown. It can be observed that the cells working at Voc, both in light and in dark conditions (Fig. 4a and b), show similar behavior at 25 °C. The most remarkable effect can be observed in the improvement of the catalysis at the counter-electrode, showing faster dynamics of reaction that is due to the lower charge transfer resistance. Such an improvement is more evident in the samples observed after 2280 h of
Fig. 3. Comparison of time dependence of DSC parameters during lifetime testing under different working conditions (maximum power point and open circuit): a) mean of efficiencies with standard deviation, b) mean of short circuit current densities with standard deviation, c) mean of fill factors with standard deviation and d) mean of open circuit voltages with standard deviation.
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E. Leonardi et al. / Journal of Non-Crystalline Solids 356 (2010) 2049–2052
the longer time needed for ion diffusion within the electrolytic solution, is more evident for the cells working at Pmax (Fig. 4c). A higher diffusion resistance can be due to a lower concentration of triiodide ions in the electrolyte, resulting in a variation of the redox potential. Since the redox potential affects the open circuit voltage value, this can explain the different behavior of the Voc parameter observed between Voc and Pmax working conditions. The not reversible depletion of triiodide ions in the electrolyte liquid under illumination has been already observed in previous works [16]. 4. Conclusions Analyzing the data of the ageing test for 2280 h under light soaking at 0.8 sun at 25 °C it comes out that similar effects of degradation are induced on such electrical parameters as short circuit current density, the fill factor and efficiency, both under open circuit voltage and under maximum power point working conditions. A more evident difference was noticed in the behavior of open circuit voltage with a stronger degradation observed in the cells working at the maximum power point. This was confirmed by the electro-analytic measurements performed on the test cells. We also noticed that, for both working conditions, the most remarkable variations of performance, related to different inner equilibrium of the device, took place during the first 200 h of testing. This can suggest that a more affordable evaluation of the actual performances of the DSC devices should take into consideration this period as an “annealing time”, similarly to what is required by the amorphous silicon panels. Other studies are currently in progress to reduce the effects of degradation. A strong improvement can be supplied by the application of UV filters that may protect the organic compounds involved in DSC samples. Furthermore, improvement of primary and secondary sealing can contribute to reducing the contamination effects induced by such environmental agents as oxygen and moisture. This study also shows that the UV curable technology can be used in the DSC application without any appreciable deterioration of the cell elements by UV rays. Acknowledgements The help of Ms. Tiziana Lopardo is gratefully acknowledged. The authors acknowledge the financial support for this work by Regione Lazio “Polo Solare Organico”. References
Fig. 4. Comparison of Nyquist diagrams from electroimpedance spectroscopy of DSC samples at the beginning and after testing for 2000 h in different working conditions: a) not connected device under continuous lighting at 0.8 sun, b) not connected device under dark condition and c) device working at maximum power point under continuous lighting at 0.8 sun.
light soaking (Fig. 4a) with respect to the sample aged in dark conditions. A similar improvement of the catalytic activity of the counter-electrode is visible in the cells working at Pmax (Fig. 4c), which also shows a higher recombination resistance at the working electrode that results in a higher probability for charges to be collected at the external contacts [14]. The increase in the electrolyte diffusion resistance that is related to
[1] B. O'Regan, M. Graetzel, Nature 353 (1991) 737. [2] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Muler, P. Liska, N. Vlachopoulos, M. Graetzel, J. Am. Chem. Soc. 115 (1993) 6382. [3] A. Hagfeldt, M. Graetzel, Acc. Chem. Res. 33 (2000) 269. [4] A. Usami, Chem. Phys. Lett. 277 (1997) 105. [5] J. Ferber, J. Luther, Sol. Energy Mater. Sol. Cells 58 (1999) 321. [6] Y. Lin, Y.T. Ma, L. Yang, X.R. Xiao, X.W. Zhou, X.P. Li, J. Electroanal. Chem. 588 (2006) 51. [7] Xiaobo Chen, Samuel S. Mao, Chem. Rev. 107 (2007) 2891. [8] M. Amirnasr, Thermochim. Acta 348 (2000) 105. [9] A. Hinsch, J.M. Kroon, R. Kern, I. Uhlendorf, J. Holzbock, A. Meyer, J. Ferber, Prog. Photovolt: Res. Appl. 9 (2001) 425. [10] H. Arakawa, 3rd Dye Solar Cells Industrialization Conference, April 22–24, 2009 Nara (Japan), 2009. [11] H. Matsui, 3rd Dye Solar Cells Industrialization Conference, April 22–24, 2009 Nara (Japan), 2009. [12] P.M. Sommeling, J. Photochem. Photobiol. A Chem. 164 (2004) 137. [13] M. Graetzel, C. R. Chimie 9 (2006) 578. [14] F. Fabregat-Santiago, Sol. Energy Mater. Sol. Cells 87 (2005) 117. [15] J. Bisquert, Phys. Chem. Chem. Phys. 5 (2003) 5360. [16] H. Tributsch, Coord. Chem. Rev. 248 (2004) 1511.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Stability of dye-sensitized solar cells under light soaking test Enrico Leonardi, Stefano Penna, Thomas M. Brown, Aldo Di Carlo, Andrea Reale ⁎ CHOSE — Centre for Hybrid and Organic Solar Energy, Via G. Peroni 400/402, 00131 Rome, Italy Electronic Engineering Department, University of Rome Tor Vergata, Via del Politecnico 1, 00133 Rome, Italy
a r t i c l e
i n f o
Article history: Received 26 May 2010 Available online 2 July 2010 Keyword: Dye Solar Cells
a b s t r a c t Long term stability of Dye Solar Cells (DSC) is still an open issue to be addressed to obtain affordable photovoltaic devices for practical use. Continuous light soaking of DSC devices is a useful technique to induce an accelerated degradation mechanism that would allow evaluating the lifetime of a device. In this paper the results of 2280 h of continuous light soaking at 0.8 sun of DSC devices working under different operating conditions have been reported. The time-dependence of electrical parameters of the devices has been combined to the electro-impedance analysis in order to provide a physical explanation of the degradation mechanism. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the last fifteen years, the Dye Solar Cells (DSC) have attracted much interest of the scientific and commercial communities, mainly due to the high energy conversion efficiency, on the one hand [1,2], and the low fabrication cost combined with a relatively simple manufacturing process, on the other hand. A DSC device is basically made of three components: large band gap mesoporous titania (TiO2) as the working electrode that is sensitized with an organic material (dye) aimed at enlarging the absorption band of the device, a catalyst such as platinum or carbon as the counter electrode, and lastly, an electrolyte solution based on a redox couple, typically iodide/triiodide (I−/I3−) [1]. The mesoporous TiO2 film is considered to be the heart of the device [3]. The titania particles with a large refraction index enhance the light absorption of the photovoltaic electrode as they can induce strong light scattering [4]. However, the larger the particles, the lower the effective surface area of the TiO2 film for absorbance. Therefore, the aim is to find an optimum equilibrium between a large surface area and efficient light scattering. Many studies have been carried out to find this relationship [5,6]. Currently, the efficiency values of DSC devices are considered to be sufficient to supply low power consumption devices. Nevertheless, the lifetime of the device and the stability of the materials involved are still too poor to allow for a practical application of photovoltaic cells. Therefore, a still unreached goal is to ensure device lifetimes that are suitable to the market needs. Individual components of a solar cell have been extensively studied and are currently able to provide a lifetime of tens of years without noticeable degradation [7,8].
⁎ Corresponding author. Electronic Engineering Department, University of Rome Tor Vergata, Via del Politecnico 1, 00133 Rome, Italy. E-mail address: [email protected] (A. Reale). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.05.072
Several studies performed on ageing tests of DSC solar cells have identified the following as the main causes of loss in performance: 1) electrolyte contamination by environmental agents (oxygen and water) [9], 2) desorption of dye from titania [9] and 3) degradation of organic compounds due to the ultraviolet component of the solar spectrum [10,11]. Therefore, it is clear that a fundamental role is played by sealants that must ensure that contaminants, such as oxygen and water, do not affect the inner organic compounds of the DSC. There are some previous studies on the DSC stability [12,13], but further experiments and analyses are needed in order to better understand the degradation mechanism of DSC, both in standard conditions of testing and under different types of stress. 2. Experimental and methods 2.1. Fabrication method and materials The cells were fabricated using commercial 2 cm×4 cm fluorinated tin oxide glass (TEC-8 Pilkington) as a substrate, 3 mm in thickness and with 8 Ω/square resistivity. The TiO2 layer was obtained using DSL 18 NRT paste (Dyesol) deposited by a screen-printing facility (Baccini) and fired in a fan oven at 525 °C for 30 min. Three active areas of 0.5 cm ×0.5 cm each (0.25 cm2), TiO2 layers, 10 μm thick, were deposited on each glass substrate. The TiO2 layer was sensitized by a 15 hour dip coating process in a 0.5 mM solution of N719 dye (Dyesol) in pure ethanol (Carlo Erba). Platinum paste PT-1 (Dyesol) was deposited by screen-printing as the counter electrode. The wet Platinum layer was fired in a fan oven at 425 °C for 15 min. An HSE electrolyte (Dyesol) solution was injected into the device with a vacuum back filling method. Four samples with three devices on each substrate were fabricated. All the cells were primary sealed using a thermoplastic SX1170-PF (Solaronix) film, applied by heat-assisted pressure, and secondary sealed
2050
E. Leonardi et al. / Journal of Non-Crystalline Solids 356 (2010) 2049–2052
Fig. 1. Simplified circuit for TiO2 in conductive state.
with high strength UV epoxy (Dymax) cured under a UV Lamp with 252 mW/cm2 intensity (Dymax) for less than 10 s. Finally, a commercial silver dag (RS) was used to improve the electron collection at the external contacts. Different measurements were performed with a UV–Vis spectrophotometer UV-2550 (Shimadzu) to verify possible damage induced by UV exposure on the compounds involved in the cell, in particular, organic compounds. More in detail, the N719 solution absorbance in pure ethanol, HSE electrolyte and titania layers, both not dyed and dyed, were separately measured before and after exposure to the UV lamp for 45 s. In order to understand more deeply how a DSC behaves under stressing conditions, two sample devices were exposed to a C class solar spectrum in a light soaking chamber LSC 1.3 (Dyesol) at 0.8 sun for 2280 h, whereas two nominally equal devices were kept in the dark in order to evaluate the different contributions to degradation of
performance in DSC devices. A software interface developed on the IEEE 488 standard (National Instruments Labview v8.5) was used to control the equipment, monitor the data acquisition and to update the measurement settings. The three active areas of the four devices under light soaking were left working under different operating conditions: one active area per device was forced to work at the maximum power point (Pmax), whose value was periodically updated to balance the performance variations, whereas the remaining active areas were left at open circuit voltage (Voc). The two cells in dark conditions were left at Voc. A B class sun simulator (KHS Technical Lighting) was used as the light source (1 sun intensity, AM 1.5 solar spectrum) to evaluate the actual degradation of the devices and an E5262A Analyzer (Agilent) was used to acquire the electrical parameters. The data were mediated through a repeated interrogation of the instrument for each working point acquired, the average error was estimated to be less than 1%. Finally, the electrochemical behavior of the samples during the testing period was observed with an Autolab PGSTAT302N (Metrohm) potentiostat. The experimental data was fitted to the model represented by the equivalent circuit shown in Fig. 1. In this figure, the TiO2 network representation was simplified, following what was presented by Fabregat et al. [14]. In such an equivalent circuit, the series resistance element Rs is affected by the resistance of TCO electrodes and by the electrolyte conductivity, the Zelec element represents the ionic diffusion within the electrolyte, and the two Randles circuits, that are the working electrode and the counter electrode, represent an interface of the electrolyte liquid with the titania layer and the Platinum layer, respectively. The equivalent circuit elements have the following meaning: • Cte is the chemical capacitance that stands for the change of electron density as a function of the Fermi level [15]; • Rte is the charge-transfer resistance related to a recombination of electrons at the TiO2/electrolyte interface; • Rs is the series resistance introduced by the TCO;
Fig. 2. Comparison of absorbance before and after UV exposure for a) N719 dye in ethanol, b) HSE electrolyte solution, c) titania film, and d) dye sensitized titania film.
E. Leonardi et al. / Journal of Non-Crystalline Solids 356 (2010) 2049–2052
• Zelec is the impedance of diffusion of the electrolyte redox species; • Rce is the charge-transfer resistance at the counter electrode/ electrolyte interface; • Cce is the interfacial capacitance at the counter electrode/electrolyte interface. The ageing process and the performance evaluation were all carried out at 25 °C, corresponding to the temperature fixed by the STC (Standard Temperature Conditions) defined in the IEC 61215 standard for ageing of photovoltaic panels based on silicon, both crystalline and amorphous. 3. Results and discussion Fig. 2 shows the analysis of the effect of 45 s of UV light soaking on the dye, electrolyte, titania dyed and not dyed. No degradation or decreasing of the absorbance was observed in the samples for such an observation time that is more than four times longer than required by the UV-curing of Dymax resins. The UV degradation effect was tested with the same boundary conditions that had been used for manufacturing the cells (a protective layer using glass with TCO, the same substrate used for manufacturing the devices). The variations of the electrical parameters of the cells, i.e. efficiency, short circuit current density, open circuit voltage and fill factor, related to both the above mentioned working conditions, are shown in Fig. 3. It can be observed that the performance values continue to be stable with time, showing a similar trend for both the efficiency, the short circuit current density and the fill factor. The efficiency graph (Fig. 3a) shows an
2051
overall degradation of 15% at the end of the period of observation with respect to the beginning. The short circuit current density (Fig. 3b) presents slightly different decay rates, with 3% degradation for the Voc working cell and no degradation for the cell working at Pmax. Similar behavior was observed on the fill factor (Fig. 3c), with a 7% degradation for the cell working at Voc and 4% for the one working at Pmax. The open circuit voltage curves (Fig. 3d) show slopes with a more remarkable difference, corresponding to different degradation rates, between the Pmax working cell and the Voc working cell. In particular, stressing the cell at Pmax seems to induce a faster reduction of the Voc value (12%), especially in the first 200 h of observation, with respect to the cell working at Voc (7%). It is evident for all the graphs in Fig. 3, as the first hundreds of hours of observation, namely corresponding to one week of constant lighting, strongly affect the performance of DSC devices resulting in a higher degradation rate. From the above shown results, it can be noted that different working conditions mainly affect the open circuit voltage parameter's behavior (Fig. 3c), with no meaningful difference between the other parameters. The cells were observed with an electro-impedance spectroscopy (EIS) technique to analyze the degradation effects of different working conditions from a physical point of view. In Fig. 4 the Nyquist diagrams related to the samples at the beginning and at the end of the ageing period are shown. It can be observed that the cells working at Voc, both in light and in dark conditions (Fig. 4a and b), show similar behavior at 25 °C. The most remarkable effect can be observed in the improvement of the catalysis at the counter-electrode, showing faster dynamics of reaction that is due to the lower charge transfer resistance. Such an improvement is more evident in the samples observed after 2280 h of
Fig. 3. Comparison of time dependence of DSC parameters during lifetime testing under different working conditions (maximum power point and open circuit): a) mean of efficiencies with standard deviation, b) mean of short circuit current densities with standard deviation, c) mean of fill factors with standard deviation and d) mean of open circuit voltages with standard deviation.
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E. Leonardi et al. / Journal of Non-Crystalline Solids 356 (2010) 2049–2052
the longer time needed for ion diffusion within the electrolytic solution, is more evident for the cells working at Pmax (Fig. 4c). A higher diffusion resistance can be due to a lower concentration of triiodide ions in the electrolyte, resulting in a variation of the redox potential. Since the redox potential affects the open circuit voltage value, this can explain the different behavior of the Voc parameter observed between Voc and Pmax working conditions. The not reversible depletion of triiodide ions in the electrolyte liquid under illumination has been already observed in previous works [16]. 4. Conclusions Analyzing the data of the ageing test for 2280 h under light soaking at 0.8 sun at 25 °C it comes out that similar effects of degradation are induced on such electrical parameters as short circuit current density, the fill factor and efficiency, both under open circuit voltage and under maximum power point working conditions. A more evident difference was noticed in the behavior of open circuit voltage with a stronger degradation observed in the cells working at the maximum power point. This was confirmed by the electro-analytic measurements performed on the test cells. We also noticed that, for both working conditions, the most remarkable variations of performance, related to different inner equilibrium of the device, took place during the first 200 h of testing. This can suggest that a more affordable evaluation of the actual performances of the DSC devices should take into consideration this period as an “annealing time”, similarly to what is required by the amorphous silicon panels. Other studies are currently in progress to reduce the effects of degradation. A strong improvement can be supplied by the application of UV filters that may protect the organic compounds involved in DSC samples. Furthermore, improvement of primary and secondary sealing can contribute to reducing the contamination effects induced by such environmental agents as oxygen and moisture. This study also shows that the UV curable technology can be used in the DSC application without any appreciable deterioration of the cell elements by UV rays. Acknowledgements The help of Ms. Tiziana Lopardo is gratefully acknowledged. The authors acknowledge the financial support for this work by Regione Lazio “Polo Solare Organico”. References
Fig. 4. Comparison of Nyquist diagrams from electroimpedance spectroscopy of DSC samples at the beginning and after testing for 2000 h in different working conditions: a) not connected device under continuous lighting at 0.8 sun, b) not connected device under dark condition and c) device working at maximum power point under continuous lighting at 0.8 sun.
light soaking (Fig. 4a) with respect to the sample aged in dark conditions. A similar improvement of the catalytic activity of the counter-electrode is visible in the cells working at Pmax (Fig. 4c), which also shows a higher recombination resistance at the working electrode that results in a higher probability for charges to be collected at the external contacts [14]. The increase in the electrolyte diffusion resistance that is related to
[1] B. O'Regan, M. Graetzel, Nature 353 (1991) 737. [2] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Muler, P. Liska, N. Vlachopoulos, M. Graetzel, J. Am. Chem. Soc. 115 (1993) 6382. [3] A. Hagfeldt, M. Graetzel, Acc. Chem. Res. 33 (2000) 269. [4] A. Usami, Chem. Phys. Lett. 277 (1997) 105. [5] J. Ferber, J. Luther, Sol. Energy Mater. Sol. Cells 58 (1999) 321. [6] Y. Lin, Y.T. Ma, L. Yang, X.R. Xiao, X.W. Zhou, X.P. Li, J. Electroanal. Chem. 588 (2006) 51. [7] Xiaobo Chen, Samuel S. Mao, Chem. Rev. 107 (2007) 2891. [8] M. Amirnasr, Thermochim. Acta 348 (2000) 105. [9] A. Hinsch, J.M. Kroon, R. Kern, I. Uhlendorf, J. Holzbock, A. Meyer, J. Ferber, Prog. Photovolt: Res. Appl. 9 (2001) 425. [10] H. Arakawa, 3rd Dye Solar Cells Industrialization Conference, April 22–24, 2009 Nara (Japan), 2009. [11] H. Matsui, 3rd Dye Solar Cells Industrialization Conference, April 22–24, 2009 Nara (Japan), 2009. [12] P.M. Sommeling, J. Photochem. Photobiol. A Chem. 164 (2004) 137. [13] M. Graetzel, C. R. Chimie 9 (2006) 578. [14] F. Fabregat-Santiago, Sol. Energy Mater. Sol. Cells 87 (2005) 117. [15] J. Bisquert, Phys. Chem. Chem. Phys. 5 (2003) 5360. [16] H. Tributsch, Coord. Chem. Rev. 248 (2004) 1511.