Transparent conductive film by large area roll-to-roll processing

Thin Solid Films 544 (2013) 427–432

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Transparent conductive film by large area roll-to-roll processing Linda Y.L. Wu a,⁎, W.T. Kerk a, C.C. Wong b a b

Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore

a r t i c l e

i n f o

Available online 5 March 2013 Keywords: Roll-to-roll Slot die Conductive polymer Sheet resistance Silver nanowire

a b s t r a c t Sputtered indium tin oxide (ITO) coating on polyethylene terephthalate film has been used as the substrate for roll-to-roll fabrication of large area printed electronics devices, but it is expensive and could be cracked when bending, limiting its applications. Transparent conductive (TC) electrode made by roll-to-roll coating of transparent conductive ink on flexible substrate is an alternative, but both the ink material and the control of the coating quality are very crucial. The major challenges are the coating performance, coating uniformity and defect control during roll-to-roll processing. In this paper, we report the chemical synthesis of silver nanowires in preferred shape and size, the surface modification of the Ag nanowires for better dispersion into the commercial Poly(3,4-ethylenedioxythiophene) Poly(styrenesulfonate) (PEDOT:PSS) conductive polymer ink, and the controlled roll-to-roll coating process on flexible polyethylene terephthalate substrate by a one meter web-width roll-to-roll slot die coating system. We obtained high uniformity PEDOT:PSS coating with optical transmission higher than 80% and sheet resistance lower than 100 Ω/square, and silver containing coating with sheet resistance below 40 Ω/square and maintained optical transmittance. The slot die coating mechanism is investigated and the influencing factors for coating uniformity and defect are defined. The coated transparent conductive film has the same properties as the sputtered ITO and has been used as the TC electrode for printed lighting, whose performance has been proven by standard weathering test for 1000 h. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Currently used transparent conductive (TC) film for roll-to-roll process is sputtered indium-tin oxide (ITO) on polyethylene terephthalate (PET) film which is relatively expensive, can deteriorate in conductivity and/or be cracked when bending. In a recent report [1] for an organic solar cell, it was found that the process energy consumption of ITO coating accounted for almost 87% of the total process energy in the production steps. Therefore, there is a demand for the development of wet coating process using conductive polymers due to the potential low cost, large area roll-to-roll processability and higher performance than sputtered ITO film. The target sheet resistance is below 100 Ω/sq with visible light transmittance higher than 85%. For large area high quality coating, the control of roll-to-roll process parameters is essential to ensure optimum performance with highest process speed and defect free uniform coating layer. One of the potential applications of the roll-to-roll processing of thin film devices is polymer solar cells, which offer advantages over all photo-voltaic technologies in terms of the alleged possibility to be processed in large area on flexible substrate [2,3], entirely from solution by printing and coating techniques [4,5]. In these processes, vacuum coating steps for ITO [6,7] layer and other coating layers are ⁎ Corresponding author. Tel.: +65 6793 8999; fax: +65 6791 6377. E-mail address: [email protected] (L.Y.L. Wu). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.02.087

eliminated, and no high temperature is required. The current status is, however, far from this vision and to date most reports on polymer solar cells are for devices with a very small active area (b1 cm 2) on glass–ITO substrates. In addition, the film forming technique that is used almost exclusively is spin coating and most often the devices comprise one or two vacuum-coated layers. In order to process polymer solar cells in high volume and large area, there is no doubt that the methodology will have to involve roll-to-roll compatible techniques and the printing or coating technique will be required to have the capacity to pattern the printed or coated layer [8]. This implies that the majority of processing knowledge developed in laboratories so far (on spin coating, dip coating, and doctor blade) is not directly usable or transferrable to an industrially relevant process. The technology of coating TC film by wet chemical process has been reported by several industrial players such as Samsung, Konarka, and VTT in the printed electronics sector. However, the coating material and coating processes are mostly proprietary and largely kept as know-how within the company. In addition, each application requires different conductivity, transparency and other properties, which require process optimization in each coating process. Some available commercial transparent conductive inks can produce cheaper TC films and they do not crack when bending. However, the inks may not be simple to use for coating TC film in roll-to-roll process directly. It is expected that functional properties of the TC films can be improved by adding an appropriate level of silver nanowires. For

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example, silver grids can be printed on substrate and then a Poly(3,4-ethylenedioxythiophene) Poly(styrenesulfonate) (PEDOT:PSS) over-coat can be applied [9]. Several types of coating technologies are used in roll-to-roll processes, which can be broadly categorized as coating an even layer over the entire surface or coating with patterns. The former includes: knife-over-edge (similar to doctor blade in lab scale system), curtain coating system, roller coating, slide coating, and slot die coating; and the latter includes all the printing processes (ink jet printing, flexographic printing, gravure, and screen printing) and slot die coating [10]. Slot die coater is a versatile tool where the shim design allows both whole slot and patterned slot to fit the whole-layer coating and patterned coating respectively. In this paper, we report the chemical synthesis of silver nanoparticles and nanowires through control of catalyst and surface capping agents, the incorporation of the sliver nanowires into the PEDOT:PSS conductive polymer, and the rollto-roll coating process optimization for high quality TC film on PET substrates. 2. Experimental details

modification by a double amino-silane capping agent [14] was performed to provide water solubility of the silver nanowires to be miscible with the aqueous PEDOT:PSS inks. 2.3. Slot die coating and process optimization Slot die coating technique was adopted due to the suitability for mass production with enclosed ink feeding system, controllable speed and layer thickness, suitability for larger viscosity range, and both whole surface coating and patterned coating can be made by just changing the slot die shim. Coating experiments were carried out according to Design-Of-Experiment (DOE) designed parameters. The variable process parameters, fixed parameters, and output properties in the DOE process study are listed in Table 1. A matrix of 36 experiments was performed. In order to improve the electrical conductivity of the coated TC films, the PEDOT:PSS conductive polymer inks were modified by adding as-synthesized silver nanowires in solid contents of 10 to 20 wt.%. As a comparison, the two commercially available silver nanowires (NanoAmor and Seashell) were also added into the conductive polymer inks and tested in sheet resistance and light transmittance.

2.1. Silver nanowire synthesis and analyses 2.4. Coating characterization Polyol synthesis process of silver nanoparticles and nanowires was adopted in this study [11–13]. The chemical reagents used are: silver nitrate (AgNO3), ethylene glycol (EG, C2H6O2), poly(vinyl pyrrolidone) (PVP). Trace element of CuCl2 was used to induce high yield production of silver nanowires without the need to precisely control the injection interval by an injection pump. All materials are purchased from Sigma-Aldrich without further purification. Typical synthesis procedures are: a mixture of 120 μl 0.10 M AgNO3 solution and 15 ml of ethylene glycol is heated to 160 °C in a flask for initial nucleation of the silver seeds. After 10 min, 9 ml of 0.13 M PVP solution is added into the flask, no color change was observed. Five minutes later, another 9 ml of 0.10 M AgNO3 solution, which is the actual silver source, is added and the reaction mixture turns turbid with gray color. 0.5 ml of 9 × 10−4 M of CuCl2 in EG was added into the solution. The flask is heated for an additional 1 h to ensure that the growth is complete. Cooled-down solution is then centrifuged three times at 4000 rpm for 15 min each and washed with water (one time) and ethanol (two times) to remove excess solvent, PVP and other impurities in the supernatant. Precipitated silver nanowires can be redispersed in methanol. One drop of the dispersion is placed onto carbon tape and dried prior to Field Emission Scanning Electron Microscopy/Energy dispersive X-ray spectroscopy (FESEM/EDX, JSM 6340F, JEOL) measurement. The EDX analyses were carried out at 20 kV using an INCA quantitative analysis software. X-ray diffraction (XRD) measurements were performed on a Philips X'pert X-ray powder diffractometer in steps of 0.02° using Cu Kα radiation (with a wavelength of 1.54 Å) as the X-ray source. The 2θ angle scan range was set from 10°–80°. To prepare silver powder samples for XRD, a few drops of silver nanowire suspension were carefully applied to a microscope glass slide. The wet layer was allowed to dry slowly under ambient conditions to produce thin layer of silver nanowires for XRD analysis. Two other commercial silver nanowires from NanoAmor USA (wire diameter: 227 ± 80 nm, length: 6.1 ± 2.1 μm) and Seashell Tech (mean diameter: 67 nm, mean length: 7.4 μm) were also studied in terms of ink formulation and TC coating property. 2.2. Formulation of hybrid ink with PEDOT:PSS and silver nanowires Two types of PEDOT:PSS inks: Orgacon S305 (Agfa) and Clevios FE-T (Heraeus) were selected as the TC polymers for coatings. The three types of silver nanowires described above were incorporated into the TC polymers in 10% and 20% solid concentrations with proper adjustment of viscosity and polymer concentration. Suitable surface

The optical transmittance of the TC coating was recorded using UV–VIS–NIR Spectrophotometer (UV3101PC, Shimadzu). The morphology of silver nanowires was determined by FESEM at 5 kV. Sheet resistance of the TC film was measured using a four point probe resistivity mapping instrument (Brand: CDE ResMap Resistivity Solutions). Thickness of the coating was determined by the Dektak 3D Profilometer. 3. Results and discussion 3.1. Silver nanowire characterization and ink formulation By varying the molar ratio of PVP to AgNO3, the Cl− ionic content, and the injection speed of the surface capping agent, silver nanoparticles and nanowires were achieved. It was found that the trace amount of CuCl2 had significant influence on the silver product shape and size. Fig. 1 shows the silver products after 90 min of reaction at 9 μM trace amount of CuCl2 with comparison to the silver product without addition of CuCl2.

Table 1 DOE parameters. Process variables Gap between coater and web Pump feeding rate (%) Web speed (m/min)

40 μm, 60 μm 30, 50, 80, 100 2, 3, 4, 5, 6, 7, 8, 10

Fixed parameters Ink type Slot die shim Substrate Web width Coating length each ID Drying temperature R2R machine used Gear pump feeding rate Plasma treatment

Agfa 305 100 μm Coated PET 500 mm approximately 20 m 130 °C Coatema CC06 1 m R2R 54 ml/min (at 100%) O2 and Ar gas

Output properties Sheet resistance Rs Light transmission T Defects density Intermediate output

Along 20 m length Same location as Rs Dr. Schenk statistic Wet coating thickness

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Fig. 1. Polyol synthesized silver nanoparticles with (a) 9 μM CuCl2 added, with comparison to the silver product (b) without addition of CuCl2.

A mixture of truncated cubes and tetrahedron shape of silver nanoparticles of 100–120 nm in size was obtained with 9 μM CuCl2 added as shown in Fig. 1(a). Whereas, without the addition of CuCl2, irregular shaped silver nanoparticles were formed as shown in Fig. 1(b). When the trace amount of CuCl2 was increased to 0.9 mM, a progressive growth from nanoparticles to nanowires at different reaction times was observed. With timely addition of the double amino-silane capping agent, the silver particles could be capped and stabilized in solvent solution, and FE-SEM analysis was carried out. Fig. 2 shows the images of the silver nanoparticles and nanowires after reaction for 15, 35, and 90 min. It is seen that at 15 min, nucleated particles around 30 nm in size were formed, followed by the growth of particles to about 80 nm at 35 min, when a starting of nanowires being formed along the pentagonal facets of the silver crystals. At the reaction time of 90 min, high

yield of silver nanowires with face diagonal diameter less than 100 nm with length ranging from 3 to 30 μm was obtained. Contrary to the previous result with lower CuCl2 concentration, single crystal particle was noticeably absent. PVP is an effective stabilizer in the synthesis of silver nanoparticles. Interaction between PVP and the formed silver nanoparticles has been reported [15]. It showed that nitrogen atom on PVP interacted with silver and formed a protected shell to control the growth. In our study, we found that without adding PVP, bulky precipitation at the bottom of the reaction flask was formed. When PVP was added, a dispersed opaque suspension was always obtained. Molar ratios in the range of 1.0–3.0 were tested. At the molar ratio higher than 2.7, the main product was particles. At the ratio of ~1.5, nanowires were formed. These results suggest that PVP is necessary for the formation of silver nanowires;

Fig. 2. Progressive growth of silver nanoparticles to nanowires after reaction for (a) 15 min, (b) 35 min, and (c) 90 min.

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Fig. 3. SEM image, EDX spectrum, and XRD pattern of the as-synthesized silver nanowires in diameter of 100 nm, and length of 7–8 μm.

however, too high concentration of PVP might induce tightly covering of all surfaces of silver nanoparticles by PVP resulting in isotropic growth mode. Fig. 3 shows the SEM image obtained at operating voltage of 5 kV, EDX spectrum obtained at operating voltage of 10 kV and XRD pattern of the as-synthesized silver nanowires using powder diffraction. Strong diffraction peaks in XRD pattern can be assigned to the face-centered-cubic (fcc) phase of silver (JCPDS 01-071-4613). The four sharp peaks at 2θ = 38.1°, 44.3°, 64.5° and 77.4° were indexed to (111), (200), (220) and (311) reflection lines. The higher intensities of the (111) and (200) peaks implied the enrichment of (111) crystalline planes in the nanowires. No impurity species such as AgCl or AgO were detected. It is confirmed that pure phase silver material was formed. With the surface capping by double amino-silane groups, the nanowires were readily dispersible in the aqueous PEDOT:PSS inks without any agglomeration and sediment, therefore, uniform coating material was formed. Malynych et al. [16] demonstrated the use of PVP as a universal surface modifier for immobilization on nanoparticles due to the strong binding of the pyridine groups of PVP to metals. In addition, the pyridyl groups of PVP could also interact with various non-metallic polar surfaces especially those terminated with amines, carboxyl, hydroxyl and other groups capable to form hydrogen bonding. In our case, we noticed easy binding of amino-silane capping to the silver surface and render the nanowires good dispersion in PEDOT:PSS inks. 3.2. Slot die coating process optimization Several trend curves were obtained from the DOE experimental results. Generally, both transmittance (T) and sheet resistance (Rs) increase with increasing web speed, which should be due to the thinner coating layer with less conductive species. Since our target was to obtain Figure-Of-Merit (FOM is defined as T/Rs) higher than 0.8 with Rs lower than 100 Ω/sq, we list the best three DOE parameters and the results in Table 2. From the correlation and prediction equations, the optimum parameters and the predicted results are also listed at the bottom row of Table 2. It is seen that higher pump feeding rate of 145% is required. Fig. 4 shows the dependence of T and Rs on the wet coating thickness. It is seen that T and R are contradicting targets, because higher T requires thinner layer, but thinner layer leads to

higher Rs. The correlation equations obtained from the DOE study are listed below: T ¼ −0:2195W þ 91:212

ð1Þ

−1:015

Rs ¼ 6247:3W

ð2Þ

−0:0291

FOM ¼ 0:0146W

ð3Þ

W ¼ 22:1690–3:9875Web speed þ 0:3374Pump rate–0:0675Gap ð4Þ where W is the wet coating thickness in μm. Web speed is in m/min, pump feed rate is in ml/min, and gap is in μm. It is seen that the most influential process parameters, in descending order, are: web speed, pump feeding rate, and coater-web gap. By controlling the combination of these parameters, the wet coating thickness can be precisely controlled, whereby the optimum dry coating thickness and target coating performance can be achieved. For each coating material, a separate DOE needs to be performed and these correlation equations are to be obtained. From the equations, the target coating properties can be predicted by controlling the wet coating thickness, which is determined by the combined process parameters. 3.3. Performance of various inks 3.3.1. Orgacon S305 (Agfa) ink A study was carried out for this ink by varying the process parameters leading to different wet coating thicknesses. Similar to the Table 2 The best process parameters based on DOE and predicted results. ID

Gap (μm)

Pump rate (%)

Web speed (m/min)

Ave T (%)

Ave Rs (Ω/sq)

FOM

Defect (D/m)

16 28 34

40 60 60

100 80 100

2 2 2

77.4 81.0 77.4

92.1 114.5 91.6

0.8406 0.7077 0.8449

2.2 0.7 0.7

145

2.4

80

0.8294

–

Prediction 40

99.95

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431

Fig. 4. Transmission and sheet resistance of pure PEDOT:PSS coated film plotted against wet coating thickness from the DOE study.

Fig. 5. Sheet resistance and transmission of various silver (NanoAmor, Seashell, and self-synthesized) containing inks compared to plain FE-T ink.

above-described DOE study, the sheet resistance (Rs) and light transmission (T%) in relation to wet coating thickness were obtained as:

3.4. Application in printed lighting

T ¼ −0:202  W þ 89:516 ð−1:465Þ

Rs ¼ 25127  W^

:

ð5Þ ð6Þ

From these equations, it was predicted that the wet coating thickness between 43.5 and 47 μm, T% will be higher than 80% and Rs will be lower than 100 Ω/sq. 3.3.2. Clevios FE-T (Heraeus) ink Based on the DOE study, the correlation between wet coating thickness and T and Rs were expressed as: T ¼ −0:4408  W þ 93:208 ð1:294Þ

Rs ¼ 6163:5  W^

:

ð7Þ ð8Þ

From these equations, it was predicted that with thickness between 24.5 and 29 μm, T% will be higher than 80% and Rs will be lower than 100 Ω/sq. The optimum viscosity of this ink was 26.9 mPa.s with solid content of 1.85 wt.%. Since the ink material was fixed for the process study, dry film thickness is determined by the solid contents in the wet layer, therefore, is directly proportional to the wet coating thickness. Using this ink and optimized process parameters, the dry film thickness was measured to be 300 nm with wet coating thickness of 25 μm. The dry film thickness determines both the electrical conductivity and the optical transmittance, and must be optimized. 3.3.3. Silver nanowire modified inks As-synthesized silver nanowires were incorporated into the FE-T ink in 10 and 20 wt.% solid contents. Similarly, NanoAmor and Seashell silver nanowires were also added into the FE-T ink in the same solid fractions. All coatings were applied by a roll-to-roll coating machine on PET substrates with optimized settings. The sheet resistance and light transmittance were measured on all the coated films and plotted in Fig. 5. It is seen that the sheet resistance was reduced to below 40 Ω/Sq from the original 150 Ω/Sq of the plain FE-T ink, and the light transmittance was affected by the silver, especially with 20 wt.%. Comparing the as-synthesized silver nanowire with the two commercial grades, self synthesized silver resulted in highest transmittance (78%) with the same sheet resistance. This is due to the smaller wire diameter and good dispersion of nanowires in the polymer ink. The two commercial silver products have the same performance, and they are more expensive compared to self synthesized silver by our low cost high yield automatic synthesis process.

The coated TC film using the optimized roll-to-roll process was subsequently used as the electrode for printed lighting. Fig. 6(a) shows the coated roll with plain FE-T ink and 20 wt.% silver nanowire containing ink. Similar light transmission at 80% was detected. The morphology and uniformity of the silver containing layer are comparable to the PEDOT:PSS coated surface without silver. Both surfaces had the roughness below 10 nm and defect free as seen on the two coated rolls shown in Fig. 6(a). This is because silver nanowires are aligned parallel to the substrate within the matrix of PEDOT:PSS, and they tend to settle at the bottom of the coated layer due to the higher density of silver compared to polymer. Fig. 6(b) shows a printed electro-luminance (EL) lighting device. Both the plain PEDOT:PSS ink coated TC film and silver containing ink coated TC film were used as the transparent electrode. Both devices showed good lighting uniformity of higher than 90% with the brightness level for a blue EL ranging from 100 to 500 lx depending on the operating condition. The devices have been functioning well for several months. A weathering test was performed for the TC electrode using a Xenon Weathering Test Chamber (ATLAS Ci3000+) according to ISO 11341 standard method [17]. The coated TC films were covered by a PET sheet with edge sealed, and put into the Xenon weathering chamber under continuous radiation with standard cycle of 18 min water spray plus 102 min dry at relative humidity of 65 +/− 5%. After every 100 h, the PET cover was opened and sheet resistance (Rs) and light transmission (T%) of the TC layer were measured. After 1000 h of test, the light transmission dropped 5% (still above 78%) and sheet resistance increased about 10%. The ITO reference film was also tested in the same batch. After 1000 h, the ITO film became yellow, light transmission dropped 5% (still at 80%) and sheet resistance increased 20%. This indicates that our silver containing PEDOT:PSS coated film has comparable performance as the vacuum processed ITO film, and the roll-to-roll coating process offers high quality coating without the need of using high cost vacuum coating process. With the proven property in this study, our coated TC electrode could be used for many other printed electronics applications such as solar films, touch screen, and sensors. 4. Conclusion Polyol synthesis method with trace amount of CuCl2 enabled the control of shape and size of silver nanoparticles and nanowires. With proper surface capping, silver containing inks in PEDOT:PSS polymer were obtained and high quality transparent conductive coatings have been made on PET substrate. Through the DOE study, the large area roll-to-roll coating process was optimized. The most influential parameters, in descending order, are: web speed, pump feeding rate and

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Fig. 6. Coated TC films (a) by PEDOT:PSS ink (left) and self-synthesized silver containing ink (right), and (b) the printed EL lighting using the coated TC electrode.

coater-web gap. By controlling the combination of these parameters, the wet coating thickness can be precisely controlled, whereby the target coating performance can be achieved. High uniformity coating was obtained using Clevios FE-T ink with optimized properties of 80% light transmission and 100 Ω/sq sheet resistance. Using the silver nanowire modified inks, sheet resistance below 40 Ω/sq with the same transmittance was achieved. The coated TC film has been used as the electrode for printed light device, whose performance has been proven by weathering test for 1000 h. Acknowledgment We acknowledge the financial support by Agency for Science, Technology and Research of Singapore (A*STAR) SERC Nanofabrication, Processing and Characterisation (SnFPC) Initiative. References [1] N. Espinosa, R. Garcı´a-Valverde, A. Urbina, F.C. Krebs, Sol. Energy Mater. Sol. Cells 95 (2011) 1293. [2] F.C. Krebs, Sol. Energy Mater. Sol. Cells 93 (2009) 394.

[3] F.C. Krebs, H. Spanggaard, T. Kjaer, M. Biancardo, J. Alstrup, Mater. Sci. Eng. B 138 (2007) 106. [4] F.C. Krebs, M. Jørgensen, K. Norman, O. Hagemann, J. Alstrup, T.D. Nielsen, J. Fyenbo, K. Larsen, J. Kristensen, Sol. Energy Mater. Sol. Cells 93 (2009) 422. [5] F.C. Krebs, Sol. Energy Mater. Sol. Cells 92 (2008) 715. [6] T. Aernouts, P. Vanlaeke, W. Geens, J. Poortmans, P. Heremans, S. Borghs, R. Mertens, R. Andriessen, L. Leenders, Thin Solid Films 451 (2004) 22. [7] M. Strange, D. Okackett, M. Kaasgaard, F.C. Krebs, Sol. Energy Mater. Sol. Cells 92 (2008) 805. [8] L. Blankenburg, K. Schultheis, H. Schache, S. Sensfuss, M. Schrodner, Sol. Energy Mater. Sol. Cells 93 (2009) 476. [9] J.-S. Yu, I. Kim, J.-S. Kim, J. Jo, T.T. Larsen-Olsen, R.R. Søndergaard, M. Hösel, D. Angmo, M. Jørgensen, F.C. Krebs, Nanoscale 4 (2012) 6032. [10] R. Sondergaard, M. Hosel, D. Angmo, T.T. Larsen-Olsen, F.C. Krebs, Mater. Today 15 (2012) 36. [11] K.K. Caswell, C.M. Bender, C.J. Murphy, Nano Lett. 3 (5) (2003) 667. [12] B.J. Wiley, Y. Sun, Y. Xia, Langmuir 21 (2005) 8077. [13] J. Jiu, K. Murai, D. Kim, K. Kim, K. Suganuma, Mater. Chem. Phys. 114 (2009) 333. [14] Y.L. Wu, A.I.Y. Tok, F.Y.C. Boey, X.T. Zeng, X.H. Zhang, Appl. Surf. Sci. 253 (2007) 5473. [15] H. Wang, X. Qiao, J. Chen, X. Wang, S. Ding, Mater. Chem. Phys. 94 (2005) 449. [16] S. Malynych, I. Luzinov, G. Chumanov, J. Phys. Chem. B 106 (2002) 1280. [17] International standard ISO 11341:2004, Paint and Varnishes — Artificial Weathering and Exposure to Artificial Radiation — Exposure to Filtered Xenon-arc Radiation, 2004.