oxidation susceptibility of organic

Degradation/oxidation susceptibility of organic photovoltaic cells in aqueous solutions K. Habib, A. Husain, and A. Al-Hazza Citation: Review of Scientific Instruments 86, 124101 (2015); doi: 10.1063/1.4936593 View online: http://dx.doi.org/10.1063/1.4936593 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/86/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Multiple-interface tracking of degradation process in organic photovoltaics AIP Advances 3, 102121 (2013); 10.1063/1.4826586 Synthesis and characterization of specific electrode materials for solar cells and supercapacitors J. Renewable Sustainable Energy 5, 041816 (2013); 10.1063/1.4817716 Electrochemical analysis of transparent oxide-less photovoltaic cell with perforation patterned metal substrate Appl. Phys. Lett. 102, 183904 (2013); 10.1063/1.4804987 Transient photovoltaic behavior of air-stable, inverted organic solar cells with solution-processed electron transport layer Appl. Phys. Lett. 94, 113302 (2009); 10.1063/1.3099947 Semitransparent organic photovoltaic cells Appl. Phys. Lett. 88, 233502 (2006); 10.1063/1.2209176

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REVIEW OF SCIENTIFIC INSTRUMENTS 86, 124101 (2015)

Degradation/oxidation susceptibility of organic photovoltaic cells in aqueous solutions K. Habib,a) A. Husain, and A. Al-Hazza Materials Science and Photo-Electronics Laboratory, Innovative Resources Energy (IRE) Program, Energy and Building Research (EBR) Center, KISR, P.O. Box 24885 SAFAT, Kuwait 13109, Kuwait

(Received 23 July 2015; accepted 13 November 2015; published online 4 December 2015) A criterion of the degradation/oxidation susceptibility of organic photovoltaic (OPV) cells in aqueous solutions was proposed for the first time. The criterion was derived based on calculating the limit of the ratio value of the polarization resistance of an OPV cell in aqueous solution (Rps) to the polarization resistance of the OPV cell in air (Rpair). In other words, the criterion lim(Rps/Rpair) = 1 was applied to determine the degradation/oxidation of the OPV cell in the aqueous solution when Rpair became equal (increased) to Rps as a function of time of the exposure of the OPV cell to the aqueous solution. This criterion was not only used to determine the degradation/oxidation of different OPV cells in a simulated operational environment but also it was used to determine the electrochemical behavior of OPV cells in deionized water and a polluted water with fine particles of sand. The values of Rps were determined by the electrochemical impedance spectroscopy at low frequency. In addition, the criterion can be applied under diverse test conditions with a predetermined period of OPV operations. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4936593]

I. INTRODUCTION

Despite of intensive studies on the cause of the failure of organic photovoltaic (OPV) cells, the degradation/oxidation of OPV cells in aqueous solutions has not been well understood.1–3 The reason of the intensive studies on improving the power conversion efficiency (PCE) of OPV cells was due to the economic and environmental needs of replacing their inorganic counterparts in the near future (1). The commercialization of the OPV cells is dependent on the improvement of the PCE of the cells compared to the inorganic cells, i.e., crystalline silicon solar cells. Nowadays, the crystalline silicon solar cells have by far dominated 99% of photovoltaic markets4 with a maximum PCE of 24%.5,6 On the contrary, it has been recently reported2 that the PCE of the OPV has been gradually improved from 2.5%,7 6%,8–13 6.5%,14 and 10%15 throughout the years of the development. Therefore, the development of functional OPV cells with an energy efficient life is more favorable than the inorganic cells. This is true because the OPV cells should be as easily be spread as a plastic sheet or a combination of plastic/foil over any surface. In fact, OPV cells require much less material in comparison to the inorganic cells. The material of the OPV cells consists mainly of a conductive organic material, i.e., poly (3,4-ethylenedioxythiophene):polystyrene (PEDOT:PSS) that is sandwiched between two metallic electrodes.6 One metallic electrode has a low-workfuction, i.e., aluminium (Al). In contrast, the other electrode has a highworkfunction, i.e., Indium–tin-oxide (ITO). The electrode with the high-workfunction is usually a transparent/a semitransparent thin film, in which the light can be harvested directly from a light source, i.e., sun, through the thin film.

a)E-mail: [email protected]. Tel.: 965-97956296. Fax: 965-25430239.

Then, the light is absorbed by the conductive organic material. The conductive organic material has two regions of molecular orbits. One called highest occupied molecular orbits (HOMO) and the other called lowest unoccupied molecular orbits (LUMO). So, when the light is absorbed by the conductive organic material, an electron is promoted from the HOMO to the LUMO, leaving behind a hole in the HOMO. As a result, an exciton is formed. Subsequently, an exciton disassociation occurs due to the difference between the workfunction, i.e., electric field, between the metallic electrodes. This leads to the electron collection at the metallic electrode with the low-workfunction, i.e., Al, and the collection of the holes at the metallic electrode with the high-workfunction, i.e., ITO. Even though the OPV cell seems to have a simple structure of a thin film, however, the film has a heterostructure that is made of at least three different materials (heterojunction). This sort for hetero-structure would be electrochemically active especially when the film operates in the surrounding environment, where access of oxygen, water, and pollutants is present. In other words, a successful operation of OPV cells in the future will be dependent on the integrity assessment of the heterojunction of the film in the surrounding environment. Naturally, the success of the operation of OPV cells can be measured by the produced PCE and the operational life of the cells. There are a number of investigations on OPV cells that have been conducted in order to determine the cause of the degradation of the organic material and the oxidation of metallic electrodes of the OPV cells. A review on different methods of investigating the cause of the degradation/oxidation of the cells has been given elsewhere.3 Among the methods, the electrochemical impedance spectroscopy (EIS) was used16–18 to provide physical/chemical information on the reason of the decrease of the photoelectric current of the cells. The conductivity of the heterojunction was found to vary between the bulk of the heterojunction

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and the contact region between the metallic electrodes of organic material16,17 in the presence of ambient environment. The conductivity of the contact region between the metallic electrodes and the organic material was found much lower than that in the bulk of the heterojunction. This observation was interpreted due to the corrosion of the Al electrode to Al2O3, in the presence of O2 or a relative humidity, in which the dielectric constant of the region was observed to increase by the Al electrode depletion. Consequently, the photoelectronic current was recorded to decrease.16,17 Another investigation by EIS has suggested to implement a better design for the heterojunction in order to exhibit a large decrease in the dielectric constant under illumination in ambient environment.18 In general, the OPV cells require a cleaning (maintenance) on a frequent basis in actual operational environment. The cleaning is essential due to contaminations of the surrounding environment. Therefore, a detailed study on the electrochemical behavior of the OPV cell in a simulated operational environment is necessary for understanding the cause of the degradation of the organic material and the oxidation (corrosion) of the metallic electrodes of the OPV cell. In the present study, the EIS will be used to measure parameters such as the alternating current (AC) impedance (Z) of the OPV cell at a low frequency in a deionized water and a polluted water, with fine particles of sand. Then, the polarization resistance of the OPV cell at the open circuit potential of the OPV cell will be determined from the value of Z in the deionized water and the polluted water. The polarization resistance is wildly used to determine the susceptibility of metallic electrodes, covered by organic thin films, to corrosion.19–21 The reason of testing the OPV cell in the deionized water and the polluted water is to simulate the surrounding operational environment of cell for producing a sustainable energy source in an arid/wet climate of Kuwait. Then, the obtained parameters of the EIS tests will be compared with those of the OPV cells in air. As a result, the variation of the Rp of the OPV cell will be determined in different conditions. The PCE will be correlated to Rp based on the variation of the Rp of the OPV cell in different solutions compared to the one in air. In the present work, a criterion of the degradation/oxidation of the OPV cells was proposed. The proposed criterion, lim(Rps/Rpair) = 1, will be correlated to the PCE of the OPV cell in aqueous solutions when Rpair became equal (increased) to Rps as a function of time of the exposure of the OPV cell to the aqueous solution. Then, the criterion was plotted based on the obtained Rp values by EIS versus the predetermined operation time of the OPV cells, 1000 h. The Rp value of the OPV cell can be measured by the EIS at low frequency, f = 0.16 Hz, as the following:20,21 Rp = |Z| = ρU/A,

(1)

where Z is the AC impedance of the OPV cell, Ω; Rp is the direct current (DC) resistance of the OPV cell, Ω; A is the exposed surface area of the OPV cell to solution, 2.83 cm2; U is the thickness of the OPV cell, 121.1 µm, measured by Eddy current principle,22 based on an average of 10 readings; and ρ is the electrical resistivity of the OPV cell, Ω cm. Therefore, a criterion of the degradation/oxidation of the OPV cell can be derived from Eq. (1) as follows:

lim(Rps/Rpair) = 1,

(2)

Rpair − Rps, where Rps is the polarization resistance of the OPV cell in aqueous solution, Ω and Rpair is the polarization resistance of the OPV cell in air, Ω. Equation (2) states that when Rpair becomes equal (increases) to Rps as a function of the operational time of the OPV cell to the aqueous solution, the OPV cell is deteriorating by a degradation of the organic material or by an oxidation (corrosion) of the metallic electrodes of the OPV cell in the surrounding environment. II. EXPERIMENTAL WORKS

In this investigation, Eq. (2) was used for the first time to determine the susceptibility of OPV cells to degradation/oxidation by the deionized water and the polluted water with fine particles of sand. The chemical composition of OPV cell is ZnO/PEDOT:PSS/Ag. The transparent ZnO was the high-workfunction electrode, facing the light source. The PEDOT:PSS was the active organic material. The Ag was the low-workfunction electrode, in which the Rp of the pure Ag and the Ag2O in air will be calculated and the Rp of the Ag2O in the deionized water and the polluted water will be measured by the EIS. The thickness of the cell was 121.1 µm, in a thin film form. The corrosion is expected to take place at the contact region between the active organic material (PEDOT:PSS) and the Ag electrode in the presence of water in aerated environment. The OPV cell is a newly developed and supplied by National Laboratory for Sustainable Energy, Technical University of Denmark, Frederiksborgvej 399, DK4000 Roskilde, Denmark.23 EIS measurements were performed against a saturated calomel electrode (SCE) according to standard procedures described elsewhere.24–26 A standard electrochemical cell of three electrodes was used. The cell is made of a 1000 cm3 flask, a reference electrode (the SCE), a counter electrode (made of high density graphite bar), and a working electrode of the OPV cell. The exposed surface area of all samples was 2.82 cm2. In addition, EIS measurements were conducted using a potentiostat/galvanostat and the iviumstat brand made by IVIUM Technologies, in Eidenhoven, The Netherland. The iviumstat was used in order to obtain impedance spectra of the OPV cell in aqueous solution. The EIS spectra of all investigated samples were determined in the deionized water and the polluted water, saturated with fine particles of sand. The chemical composition of the sand is a typical of that of silicon dioxide (SiO2). The reason of testing the OPV cell in the deionized water and the polluted water is to simulate the surrounding environment of cell during the operation in arid/wet climate of Kuwait. A typical light intensity of 80 W/m2 was used during the EIS tests. The EIS spectra24–26 were basically the complex plane plots (Nyquist plots) and the Bode plots. The complex plane plots (Nyquist plots) are basically the imaginary impedance (Zimag) versus the real impedance (Zreal). Also, values of the AC impedance were obtained from Bode plots at a low frequency for all investigated samples. Values of the AC impedance were


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obtained at the low frequency based on the extrapolation of the intersection line at a frequency equal to 0.16 Hz from the x-coordinate in Bode plots to the y-coordinate in Bode plot. Bode plots are basically the logarithm of impedance (Z) (Ycoordinate) and the phase (θ) (Y-coordinate) plotted versus the logarithm of the frequency (X-coordinate). All electrochemical parameters of the OPV cell were determined by using the IviumSoft Electrochemistry Software, using an equivalent circuit model of a painted sample.24–26 Also, in order to plot the complex plane (Nyquist) and Bode plots, the frequency was chosen to be ranged between 0.1 and 100 000 Hz with amplitude of ±5 mV. The AC impedance (Z) value of the samples was determined from Bode plots24–26 at a frequency equivalent to f = 0.16 Hz (of angular velocity ω = 1 rad/s), where ω = 2πf. From Eq. (2), the value of (Rps/Rpair) was calculated based on the obtained data of |Z| from the EIS tests of the OPV cells in the deionized water and the polluted water. The values of Rpair of the OPV cell in air, for the pure Ag and for the Ag2O, were obtained elsewhere.27 Figures 1(a) and 1(b) are examples of the Bode plot (a) and the complex plane plot (b) (Nyquist plot) of the OPV cell in deionized water. The obtained data of |Z| and the calculated parameters of Rpair, Rps, and (Rps/Rpair) are given in Table I. The obtained data of |Z| were based on an average of two readings with

TABLE I. Calculated parameters of the OPV cell in air and the measured values in deionized water and the polluted water.

Materials/solution Ag in air27 Ag2O in air27 Ag2O in deionized water Ag2O in polluted water

AC impedance |Z| = Rp polarization resistance (Ω)

Ratio of (Rps/Rpair)

6.8 × 10−9 3.42 × 10−6 5.15 × 102 1.65 × 103

1 5 × 104 8.0 × 1010 2.44 × 1011

a standard deviation of ±0.63 and an error bar of 1.3%. The values of Rpair of OPV cell in air were considered as standard values with respect to the rest of the obtained data in this investigation. Also, the values of Rpair of a pure Ag and of Ag2O27 were calculated in air for a comparison with the other values of Ag2O samples in the deionized water and the polluted water. The calculation of Rair for the pure Ag and Ag2O was based on the resistivity values of Ag (ρ = 15.9 × 10−9 Ω m) and the resistivity value of Ag2O (ρ = 6.8 × 10−6 Ω m)27 by using Eq. (1) with A = 2.83 cm2 and U = 121.1 µm. In other words, the values of Rpair and (Rps/Rpair) of Ag and Ag2O electrodes in air represent the ideal bulk value of polarization resistance of OPV cell in operations. III. RESULTS AND DISCUSSION

It is obvious from Table I that the calculated value of Rpair of Ag (6.8 × 10−9 Ω) is three folds less than that of the Ag2O (3.42 × 10−6 Ω) in air. This is the reason behind the corrosion susceptibility of pure Ag to corrosion in the operational environment, as a metallic electrode, in the OPV cell. On the contrary, the measured value of the Rps of Ag2O in the polluted water (1.65 × 103 Ω) was found higher than that in the deionized water (5.15 × 102 Ω). This explains the material depletion of the electrode with low-workfunction at the contact area of the organic material and the electrode, leading to the increase of the dielectric constant of the electrode in question16,17 and decreasing the photoelecronic current of the OPV cell. Furthermore, the contrast between the values of Rps of Ag2O in the polluted water (1.65 × 103 Ω) and the one in the deionized water (5.15 × 102 Ω), in comparison to the calculated values of Rpair of the pure Ag and Ag2O, would reflect a considerable consequence on PCE of the OPV cells in such polluted medium.

FIG. 1. (a) The Bode plot of the OPV cell in deionized water. The upper plot is the logarithm of impedance (Z) versus the logarithm of frequency and the lower plot is the phase angle (θ) versus the logarithm of frequency. (b) The complex plane plot (Nyquist plot) of OPV cell in the deionized water. The plot is basically the imaginary impedance (Zimag) versus the real FIG. 2. lim(Rps/Rpair) versus time of exposure of the OPV cell in air. impedance (Zreal). This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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IV. CONCLUSION REMARKS

FIG. 3. lim(Rps/Rpair) versus time of exposure of the OPV cell in the deionized water.

Figures 2–4 illustrate the lim(Rps/Rpair) versus the time of exposure of the OPV cell in air, the deionized water, and the polluted water, respectively. Figs. 2–4 were plotted at time of exposure = 0, (Rps/Rpair) = 5 × 1014, 8 × 1010, and 2.44 × 1011 for Ag2O in air, distilled water, and polluted water, respectively. Furthermore, at time of exposure = 1000 h, Figs. 2–4 were plotted; (Rps/Rpair) = 1, 1, and 1 for the Ag2O in air, distilled water, and polluted water, respectively. The operational period of the OPV cell was assumed to last for 1000 h. Figures 2–4 are divided into two regions. One region is above the red line in the figures, in which the OPV cell is not susceptible to corrosion of the Ag electrode with respect to the proposed criterion of Eq. (2). The other region is below the red line in the figures, in which the OPV cell is susceptible to corrosion of the Ag electrode, with respect to the proposed criterion of Eq. (2). In this case, a maintenance (cleaning) of the material is essential for better PCE of the OPV cell. The scale of the susceptibility of Ag electrode to corrosion to Ag2O is the lowest when the pure Ag is directly in contact with the air (Fig. 2) and the highest when the Ag of the OPV cell is in the polluted water (Fig. 4). The PCE of the OPV cell can be assessed by monitoring the behavior of the lim(Rps/Rpair) on a frequent basis during a predetermined operational time of the OPV cell. Then, the obtained value of lim(Rps/Rpair) can be compared with a standard plot of lim(Rps/Rpair) like that of Figs. 2–4 with a specific time of operation. So, Figs. 2–4 can be standard plots of the corrosion susceptibility of the OPV cell for different electrode materials.

A criterion of the degradation/oxidation of the OPV cell materials was developed. The criterion was derived based on the limit of the ratio value of the polarization resistance of an OPV cell in aqueous solution (Rps) to the polarization resistance of the OPV cell in air (Rpair). Plots of the lim(Rps/Rpair) versus the time of exposure of the OPV cell were obtained in air, in the deionized water and the polluted water. The ratio of (Rps/Rpair) as a function of operational time was observed to vary for the Ag electrode of the OPV cell from one medium to another. The ratio of (Rps/Rpair) of the OPV cell was found the highest (2.44 × 1011) for the Ag2O in the polluted water and the lower for the Ag2O in the deionized water (8 × 1010), with respect to the Ag2O electrode in air, 5 × 104. This implies that a direct contact between the Ag and the surrounding media would lead to the most damage to the OPV cell, with respect to the ideal situation of the OPV cell in air. Therefore, plots of the lim(Rps/Rpair) versus the time of exposure, like those of Figs. 2–4, can be standard plots for the assessment of the deterioration of different optoelectronic materials that are commonly used in the OPV cells. 1T. Nguyen, C. Renaud, F. Reisdorffer, and L. Wang, “Degradation of phenyl

C61 butyric acid methyl ester:(3-hexylthiophene) organic photovoltaic cells and structure changes as determined by defect investigations,” J. Photonics Energy 2(1), 021013 (2012). 2M. Girtan and M. Rusu, “Role of ITO and PEDOT: PSS in stability/degradation of polymer: Fullerene bulk heterojunctions solar cells,” Sol. Energy Mater. Sol. Cells 94, 446–450 (2010). 3M. Jørgensen, K. Norrman, and F. Krebs, “Review: Stability/degradation of polymer solar cells,” Sol. Energy Mater. Sol. Cells 92, 686–714 (2008). 4A. Goetzberger, C. Hebling, and H. Chock, “Photovoltaic materials, history, status and outlook,” Mater. Sci. Eng., R 40, 1 (2003). 5M. Green, K. Emery, D. King, S. Igari, and W. Warta, “Solar cell efficiency tables (version 23),” Prog. Photovoltaics 11, 347 (2003). 6H. Spanggaard and F. Krebs, “A brief history of the development of organic and polymeric photovoltaics,” Sol. Energy Mater. Sol. Cells 83, 125–146 (2004). 7S. E. Shaheen, C. J. Brabec, N. S. Sariciftci, F. Padinger, T. Fromherz, and J. C. Hummelen, “2.5% efficient organic plastic solar cells,” Appl. Phys. Lett. 78(6), 841–843 (2001). 8M. Reyes-Reyes, K. Kim, and D. L. Caroll, “High-efficiency photovoltaic devices based on annealed poly(3hexylthiophene) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6) C61 blends,” Appl. Phys. Lett. 87(6), 083506-1–083506-3 (2005). 9W. Ma, C. Y. Yang, X. Gong, K. Lee, and A. Hegeer, “Thermally stable, efficient polymer solar cells with nanoscale control of the interpenetrating network morphology,” Adv. Funct. Mater. 15, 1617–1622 (2005). 10G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, and Y. Yang, “High-efficiency solution processable polymer photovoltaic cells by selforganization of polymer blends,” Nat. Mater. 4, 864–868 (2005). 11Y. Kim, S. Cook, S. M. Tuladhar, S. A. Choulis, J. Nelson, J. R. Durrant, D. D. C. Bradley, M. Ree et al., “A strong regioregularity effect in selforganizing conjugated polymer films and high-efficiency polythiophene: Fullerene solar cells,” Nat. Mater. 5, 197–203 (2006). 12J. Y. Kim, S. H. Kim, H.-H. Lee, K. Lee, W. Ma, X. Gong, and A. J. Heeger, “New architecture for high-efficiency polymer photovoltaic cells using solution-based titanium oxide as an optical spacer,” Adv. Mater. 18, 572–576 (2006). 13S. H. Park, A. Roy, S. Beaupre, S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee, and A. J. Heeger, “Bulk heterojunction solar cells with internal quantum efficiency approaching 100%,” Nat. Photonics 3(5), 297–303 (2009). 14J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T. Q. Nguyen, M. Dante, and A. J. Heeger, “Efficient tandem polymer solar cells fabricated by all-solution processing,” Science 317, 222–225 (2007). 15G. Li, R. Zhu, and Y. Yang, “Polymer solar cells,” Nat. Photonics 6(3), FIG. 4. lim(Rpair/Rps) versus time of exposure of the OPV cell in the polluted water. 153–161 (2012). This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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Tector 6000 made by Defelsko, Rchmond, ON, Canada. Hosel, D. Angmo, R. Sondergaad, G. Dos Reis Benatto, J. Carle, M. Jorgensen, and F. Krebs, “High-volume processed, ITO-free superstrates and substrates for roll-to-roll development of organic electronics,” Adv. Sci. 1(1) (2014). 24EG&G, Application Note AC-1 EG & G (Princeton Applied Research, Princeton, NJ, 1982). 25R. Boboian, Electrochemical Technique for Corrosion Engineering (National Association of Corrosion Engineering (NACE), Houston, TX, 1986). 26American Society for Testing Materials (ASTM) B457-67, p. 179 (ASTM, West Conshohocken, PA1994). 27For the value of the resistivity see http://en.wikipedia.org/wiki/Electrical_ resistivity_and_conductivity for the pure silver (Ag) and Oxide silver (Ag2O) in air. 23M.
