PECVD Synthesis of Flexible Optical Coatings for Renewable Energy ...

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for Renewable Energy Applications. Thomas F. Fuerst, Matthew O. Reese, Colin A. Wolden*. The design, fabrication, and evaluation of flexible, multilayer optical ...

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PECVD Synthesis of Flexible Optical Coatings for Renewable Energy Applications Thomas F. Fuerst, Matthew O. Reese, Colin A. Wolden* The design, fabrication, and evaluation of flexible, multilayer optical coatings deposited by plasma-enhanced chemical vapor deposition at low temperature are demonstrated using hybrid nanolaminates consisting of TiO2 and silicone (SiOxCyHz) as the high and low refractive index materials, respectively. A broadband anti-reflection coating was designed and deposited onto a variety of substrates including flexible polyethylene terephthalate (PET) and CdTe solar cells which was shown to increase absolute transmission by an average of 3% over 410–850 nm wavelengths and results in a commensurate increase in short circuit current density. An infrared reflector was designed and applied to PET which was found to provide 70% reflectance in the near-IR while maintaining >80% transmittance for visible light. The optical performance of these flexible coatings on PET remained unchanged after automated bend testing, and were shown to be robust with respect to humidity and thermal shock tests.

1. Introduction Multilayer coatings comprised of alternating high and low index materials are widely used to fabricate custom optical components, with TiO2 and SiO2 serving as commonly used materials.[1–8] These optical components are usually fabricated by physical vapor deposition techniques such as evaporation or sputtering[5,7] or sol-gel processing.[2,6] Prominent examples of coatings for energy efficient and renewable energy applications include anti-reflection (AR) coatings for solar cells[9] and infrared reflectors (IRR) for energy efficient windows.[8,10–14] Expanding these Prof. C. A. Wolden, T. F. Fuerst Department of Chemical and Biological Engineering, Colorado School of Mines, 1500 Illinois St. Golden, Colorado 80401 E-mail: [email protected] Dr. M. O. Reese National Renewable Energy Laboratory, 15013 Denver West Parkway Golden, Colorado 80401

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technologies to flexible polymer substrates may expand deployment through cost reductions associated with their low weight and through roll-to-roll manufacturing. One limitation of all-oxide coatings is their ability to maintain mechanical integrity when applied to a flexible substrate and subjected to compressive and/or tensile deformation.[15] A strategy to overcome this issue is to employ hybrid nanolaminates consisting of alternating polymer and oxide thin films. Such coatings offer improved flexibility and minimize the formation of defects such as cracks or pinholes which can compromise performance.[15,16] Other issues persist with using polymer substrates such as mechanical stability, UV stability, chemical resistance, and high permittivity to oxygen and water.[15,17] These deficiencies may be overcome through the application of appropriate coatings made from metal oxide materials that can protect the thin-film optoelectronics and increase device lifetimes.[18] However, these coatings must be durable to withstand a variety of environmental conditions

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DOI: 10.1002/ppap.201500114

PECVD Synthesis of Flexible Optical Coatings for Renewable Energy Applications

such as water exposure, UV irradiation, and not delaminate to ensure the extended lifetime of the coated devices. Due to the outdoor application of these coating, integrating selfcleaning properties can reduce the maintenance costs of the devices and improve coating quality.[19,20] Kuhr et al.[21] demonstrated the promise of using silicone and TiO2 in multilayer optical coatings. In particular, they demonstrated AR coating on rigid polymer substrates polycarbonate (PC) and polymethylmethacrylate (PMMA) that exhibited both anti-scratch and easy-to-clean functionalities. In this work, we further demonstrate the potential of silicone-titania nanolaminates to serve as optical coatings on flexible substrates, specifically polyethylene terephtalate (PET). These multi-layer coatings are deposited sequentially by plasma-enhanced chemical vapor deposition (PECVD) at low temperature in a single reactor chamber. Low temperature is critical for depositing films on temperature sensitive polymer substrates. In our work, 808C was used as the substrate temperature which is well below the melting point of PET. Roll to roll processing of PET films is also typically conducted at temperatures less than 120 8C to minimize degradation of tensile performance. For example, it has been shown that PET can lose up to 2/3 of its flexibility at 120 8C.[22] For optical coatings a large contrast in refractive index between the high and low index material is desirable, as it minimizes the thickness and/or number of layers required to create the desired optical response. The refractive index of TiO2 can vary depending on deposition techniques, and high indices can be challenging to obtain by CVD at low temperature.[23] For example, Kuhr et al. obtained n ¼ 2.1 by using a proprietary pulsed PECVD technique developed at Schott.[21] Our group has developed low frequency pulsedPECVD as an alternative to ALD for self-limiting growth, and have applied this technique to numerous metal oxides including TiO2.[4,16,23–26] Like ALD, pulsed PECVD delivers a fixed amount of growth per cycle, providing digital control over layer thickness by programming the number of cycles. During the off step, the metal precursor adsorbs onto the substrate. During plasma exposure, the precursor is oxidized and ligands are removed, resulting in deposition of high quality oxides at low temperature. This technique delivers TiO2 with a refractive index of 2.5 as the high index material. TiO2 also provides UV protection and when used as the exterior layer it imparts durability and introduces its well-known self-cleaning functionality.[27,28] Thin films of silicone layers may be readily deposited by PECVD using precursors such as hexamethyldisiloxane (HMDSO, O[Si(CH3)3]2).[16,24,25] Silicone is a transparent polymer with an index of n ¼ 1.5 that possesses excellent thermal and mechanical stability. It has been used to impart anti-scratch functionality for optical coatings such as glasses.[21] Another benefit of HMDSO for fabricating hybrid nanolaminates is that through judicious addition of oxygen, the Plasma Process. Polym. 2016, 13, 184–190 ß 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

film composition may be graded from polymeric silicone to inorganic silica.[29] This feature has been exploited to create graded layers that improve adhesion or provide index matching,[21] though these features were not required in this study. In this work, we briefly describe the synthesis and characterization of the individual TiO2 and silicone layers. The capabilities of this system were demonstrated through the synthesis of several Bragg mirrors designed to transmit blue, green, and orange light and comparing their performance to theoretical predictions. A five layer, broadband AR coating was then designed and applied to polyethylene terephthalate (PET) films, fluorinated tin oxide (FTO) coated glass (TEC-15), and thin film CdTe solar cells built on TEC-15. In the final example, an IRR coating is designed and fabricated on PET films. The flexibility and robust nature of the coatings on PET was demonstrated by showing that the optical properties remained unchanged after 50 000 cycles of automated bend testing under both compressive and tensile stresses. The durability of both AR and IRR coatings was demonstrated qualitatively by a series of environmental exposure and adhesion tests.

2. Experimental Section TiO2 and silicone (SiOxCyHz) films were deposited sequentially using a parallel plate, capacitively coupled PECVD system that was evacuated by a mechanical pump to a base pressure 99.9%) and ultra high purity O2 (UHP, >99.999%). TiCl4 was transported using a bubbler at room temperature with UHP Ar carrier gas at 30 sccm. For the TiO2 deposition, O2 and Ar gases were delivered to the reactor chamber at 300 and 250 sccm, respectively, resulting in a chamber pressure of 800 mTorr. The plasma power was set at 50 W and pulsed using square wave modulation at 0.5 Hz using a LabView control, yielding a growth per cycle of 0.1 nm pulse1. Deposition of silicone layers was accomplished using HMDSO (Aldrich >98%) as the precursor. HMDSO was kept in a temperature-controlled bath set at 50 8C and was delivered directly into the reactor at a flow rate of 20 sccm which resulted in a chamber pressure of 78 mTorr. Continuous wave excitation at 60 W was used to obtain a silicone growth rate of 0.4 nm s1. The substrate temperature was kept at 80 8C by resistive heating for deposition of all films. A variety of substrates were used including Si wafers (B doped, 100 oriented, 625 mm thick, Siltronic), 127 mm

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polyethylene terephthalate (PET, McMaster-Carr), 1 mm precleaned glass slides (VWR), 3 mm FTO coated glass (TEC-15, Hartford Glass), and thin film CdTe solar cells built on TEC-15.[30] Note that no pretreatment was required to achieve strong adhesion to PET, which is a common problem associated with coatings on flexible polymer substrates.[31] Thickness and refractive indexes of the thin films were measured using spectroscopic ellipsometry (J. A. Woollman). The beam angle was set at an angle of 708, and scanned over the range of 400–1300 nm. The Cauchy model was used to fit the resulting data to determine thickness and refractive index. Refractive index is reported at 580 nm throughout this work. A Cary 5G UV-VIS-NIR spectrophotometer was used to obtain the transmission and reflectance measurements at normal incidence. Fourier transform infrared (FTIR) (Nicolet Nexus 870) spectroscopy was used to characterize the deposited films with scans in normal transmission mode, a resolution of 4 cm1, and range of 400–4000 cm1. Water contact angle (WCA) measurements were performed by immediately imaging a 2 ml droplet of DI water placed on the surface with a microsyringe and using ImageJ to determine the contact angle as described previously.[32] All WCA were measured within 1 min of droplet placement. The multilayer optical coatings were designed using commercial software TFCalc. The wavelength-dependent refractive indices measured by ellipsometry for each material were used as input. The designed coating was then optimized for layer thickness to meet the given transmittance/reflectance requirements. Automated bend tests were performed in a four-point configuration custom built system. The four-point configuration applies the maximum stress uniformly between the inner two points, which were separated by 32 mm for this study. The deflection was set such that the bend radius was 27 mm. For each sample, the coating experienced 25 000 bend cycles under tension and 25 000 bend cycles under compression, for a total of 50 000 bend cycles applied at a frequency of 1 Hz.

3. Results and Discussion

Figure 1. FTIR spectra of silicone (blue) and TiO2 (red) with representative peaks labeled.

XRD pattern from the same sample resulted in no noticeable peaks which lead us to conclude that the bulk TiO2 deposited is amorphous in nature. Refractive indices and thicknesses of TiO2 and silicone were determined by fitting the ellipsometry data using the Cauchy model. Figure 2 shows the refractive index for both materials as a function of wavelength. The refractive index of TiO2 decreases with wavelength and is measured to be 2.5 at 580 nm. Silicone’s refractive index remained nearly constant at 1.5 over the measured wavelength range. These values were used as inputs in designing the optical coatings described below.

3.1. Characterization of Thin Films First the composition and properties of the individual TiO2 and silicone (SiOxCyHz), films were analyzed using FTIR, ellipsometry, and water contact angle measurements. FTIR spectra were taken from 100 nm films deposited on Si, and the contributions of the substrate were background subtracted. Silicone was confirmed by prominent features assigned to Si-CHx vibrations at 800, 840, 1260, and 2960 cm1 shown in the FTIR pattern in Figure 1. In previous work, we observed that the polymeric content could be controlled by power as determined by the relative intensity of the Si-CHx deformation band at 1260 cm1,[29] and the spectrum shown in Fig. 1 is consistent with the optimal film composition observed in that work. The FTIR spectrum of TiO2 shown in Figure 1 displays evidence of Ti–O bonding characteristic of the anatase phase in the form of the only significant feature at 434 cm1; however, a

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Figure 2. Refractive index of TiO2 and silicone at wavelengths from 400 to 900 nm with 580 nm marked with the dashed line.

DOI: 10.1002/ppap.201500114

PECVD Synthesis of Flexible Optical Coatings for Renewable Energy Applications

Table 1. WCA photographs with measured values.

Material

Photographa

WCAa

TiO2

08

Silicone

1038

Photographb

WCAb

Photographc

08

42–748d





WCAc





a Immediately after deposition; b1 d after deposition in absence of light; cafter UV-ozone treatment; dspecific photograph has WCA of 748, but multiple samples were tested and range is reported.

The WCA of each material was measured immediately after deposition to further confirm the identity of the species. Table 1 shows representative photographs and summarizes the results of these WCA measurements. The as-deposited TiO2 films are hydrophilic, as evidenced by the completed wetting of the applied water droplet and a negligible contact angle. However, the WCA increased to 42–748 after the samples were stored in the absence of light for several days, the specific angle was dependent on how long the sample was stored since deposition. This phenomenon is attributed to carbon contaminants such as dust forming a layer on the surface and altering the surface properties, however, the hydrophilicity can be restored under UV light treatment.[33] We show that after 20 min of UV-ozone treatment hydrophilic properties were restored. The silicone layer is characterized by a hydrophobic surface with a WCA of 1038, reflecting its organic nature as described by the FTIR results. This result is consistent with the contact angle of silicone thin films reported in the literature.[21,34] In contrast to TiO2 it was observed that the WCA of silicone did not significantly change after being stored in the absence of light.

other wavelengths in the visible spectrum as shown in the transmittance (%T) in Figure 3a, thus resulting in a pure blue coating shown in Figure 3c. During the synthesis of all multilayer structures, surrogate Si wafers were introduced into the reactor to measure the properties of the individual layers. After deposition of each layer, the coated Si wafers were removed and replaced with new wafers. The reactor was then given an hour to pump down to base pressure and ensure that the substrate temperature returned to 80 8C before initiating the deposition of the subsequent layer. The table in Figure 3d compares the thickness measured by ellipsometry with the target value shown in parenthesises. The majority of the individual layers were within 2% of the target values. Figure 3a compares the measured transmission spectrum for BM #1 with the theoretical model. The two spectra are in near perfect agreement with exception that the experimental values are uniformly 5% lower than the TFCalc model. This can be explained partially by the in

3.2. Flexible Multilayer Optical Coatings The optical tunability and nanoscale control of this system was demonstrated by designing three Bragg mirrors (BM) using TFCalc, fabricating them on 1 mm microscope slide glass, and comparing experimental performance with design specifications. The three layer BMs were fabricated using TiO2 as the first and third layer sandwiching silicone as depicted in Figure 3a. BM #1 was designed to transmit wavelengths 400–450 nm and reflect Plasma Process. Polym. 2016, 13, 184–190 ß 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 3. (A) Comparison of the modeled and measured transmission spectra for BM #1, whose structure is shown in the inset. (B) Transmission spectra obtained from the three BMs on glass. (C) Picture of the three BMs on 1 mm microscope slide glass. (D) Table showing the measured and target values of the individual layers in each coating.

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house model in TFCalc for glass which predicts 96% transmittance while the measured value for the 1 mm glass was 92%. BM #2 was designed to transmit green light by maximizing transmittance from 450 to 600 nm while reflecting other visible light. The transmittance for BM #2 can be seen in Figure 3b, which resulted in the coatings shown in Figure 3c, and the thickness of the layers are shown in Figure 3d. Finally, BM #3 was fabricated to produce an orange coating. This was achieved by maximizing transmittance from 500 to 700 nm shown in Figure 3b. Both BM #2 and #3 coatings show control over film thickness, with similar results to BM #1. These Bragg mirrors demonstrated that our PECVD technique provides sufficient control to fabricate optical coatings that meet their target specifications. Multilayer AR coatings are a well-developed technology,[9] and coatings made from TiO2 and SiO2 show great promise as a broadband AR coating for the visible spectrum.[5] In this work, a five layer coatings for use in thin film solar cells was developed. The band gap of CdTe (1.5 eV) requires the coatings to have broadband AR properties over the visible wavelengths up to 850 nm. The AR coating was designed to maximize transmittance for this range on TEC-15 glass, and then it was applied to CdTe solar cells that employed TEC-15 as a superstrate.[30] The basic AR coating was based on the four layer coatings developed by Mazur et al.,[5] however, a thin 7 nm TiO2 cap was added. The goal of this cap layer was to improve scratch resistance and provide the hydrophilic and self-cleaning properties of TiO2 as the outer layer. Figure 4a compares the thickness of the specific layers in the optical design. Figure 4b compares the transmission spectra of TEC-15 before and after the application of the coating, which shows the absolute transmission was enhanced by an average of 3% over the wavelength range of 410–850 nm. As expected,

the increase in transmittance coincided with a 3% decrease in average absolute reflection from 8.3 to 5.4% over the same wavelength range. The AR coating was then deposited on a thin film solar cell that was built on the fluorine-doped tin oxide (FTO) side of the TEC-15 glass. Figure 4c compares the short-circuit current density from a number of solar cells before and after the coating. The median current density increased from 21.5 to 22.75 mA cm2 which translated to overall increase in solar cell efficiency. The enhancement in current density is greater than the average transmission increase because the majority of current is generated in the wavelength range of 500–700 nm where quantum efficiency is highest and the transmission enhancement is 4–5% in this range. IRR coatings or heat mirrors are of keen interest for energy efficient window applications. Typically these are created by thin coats (25 nm) of a metal such as silver,[11,13] however, designing a Bragg mirror for this application is also possible.[8,14] The objective was to maximize reflectance of the near-IR radiation (800–2500 nm) while maximizing transmittance of the visible wavelengths by using a five layer coating of TiO2 and silicone. However, Bragg mirrors are not capable of creating a broadband reflectance of the whole near-IR spectrum. Therefore, our goal was reduced to reflecting wavelengths 800–1200 nm, which accounts for 68% of the NIR insolation in the AM1.5 global spectrum. Figure 5a displays the thickness of the designed and deposited IRR coatings on PET and Figure 5b shows the optical performance of coating with an average absolute transmission in the visible light region of 81% and an average absolute reflection of 70% in the near-IR region of interest. There is a small decrease in the visible region relative to the 88% transmission of uncoated PET. In addition, Figure 5b demonstrates the UV protection instilled by using TiO2 as the high index material. The

Figure 4. (A) Schematic showing the design of five layer AR coating; (B) comparison of the transmission spectra of TEC15 glass with and without the AR coating; (C) box-plot of the short circuit current density before and after the application of the AR coating.

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DOI: 10.1002/ppap.201500114

PECVD Synthesis of Flexible Optical Coatings for Renewable Energy Applications

Figure 6b shows the transmission spectrum of the IRR coating both before and after bend testing. The difference between the two spectra for the IRR is 0.6%, thus demonstrating the flexible nature of these optical coatings. The coatings were examined by optical microscopy and scanning electron microscopy at the point of maximum curvature before and after bend testing. No evidence of cracks or defect formation was observed on either sample, further confirming the robust nature of these coatings. These results are attributed to the use of silicone layer in the multilayer Figure 5. (A) Diagram of IRR coating with corresponding thickness of each nanolaminate structure. layer. (B) Transmission spectrum is shown by the black line and reflection spectra by the Durability is an important factor in the gray line of the IRR coating on PET. feasibility of these coatings. A tape test was conducted on the BM, AR, and IRR coatings applied to band gap of the deposited amorphous TiO2 was measured PET. Scotch Tape was applied and then removed at a 908 to be 3.4 eV using a Tauc plot with indirect band gap angle from the coated samples. No sign of flaking or analysis.[35] The energy is comparable to the literature removal of the coatings were detected, confirming the good reports for the band gap of amorphous TiO2[36] or its level of adhesion between the layers and to the PET phase pure species.[35] The band gap results in the steep substrate for these coatings. A humidity exposure test was drop in transmittance at 365 nm wavelengths and 0% also conducted which exposed the coatings to 100% absolute transmittance below 330 nm. This protects the humidity at 50 8C for 24 h. No sign of degradation was substrate and subsequent layers from harmful UV radiation visible after exposure. A thermal shock test was also while not limiting visible light transmittance. Similar conducted where coatings were exposed to an environUV protection is employed into AR and BM coatings by mental temperature of 80 8C for 2 h and the warmed to utilizing TiO2 as the high index material. 70 8C over a 4 h period and then stored for additional 2 h Multilayer optical coatings are routinely applied to rigid before cooling back to room temperature. These conditions substrates such as those described above. The goal of using emulate the harsh environment these coatings could be these hybrid nanolaminates was to extend this technology exposed to in an extended lifetime test. Complete flaking or to flexible substrates. To this end, the identical AR coating peeling of similar coatings on PMMA has been observed described in Figure 4A was applied to 127 mm, flexible PET in the literature after thermal cycling,[21] however, our foils 2 cm by 10 cm in dimension. After deposition, the coatings showed no deformation after being exposed to coating underwent a severe bend test of 50 000 bend cycles the varied temperatures. These series of tests suggest that of bend radius 2.7 cm with 25 000 cycles applying tensile stress after which the sample was turned over and subjected to compressive stress for the remaining 25 000 cycles. Figure 6a compares the transmission spectrum of the pristine PET to that with the AR coating, both before and after automated bend testing. Similar to the TEC-15 AR coating, the PET coating increases the average transmission by 3.8% over the range of 425– 850 nm wavelengths, and the performance remains nominally unchanged after the extensive bend test. The difference Figure 6. (A) Comparison of transmission spectra of pristine PET with AR coated PET between the before and after spectra is both before and after 50 000 cycles of automated bend testing. (B) Transmission less than 0.2%. Similar flexibility is shown spectra of PET coating with the multilayer IR reflector before and after 50 000 by the IRR coating depicted in Figure 5a. cycles of automated bend testing. Plasma Process. Polym. 2016, 13, 184–190 ß 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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our deposited coatings are durable and adhesive, while also providing good flexible optical performance.

Keywords: nanolayers; optical coatings; plasma-enhanced chemical vapor deposition (PECVD); silicone

4. Conclusion

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Hybrid multilayers utilizing TiO2 and silicone as the high and low refractive index materials are demonstrated to produce optical coatings with the fidelity of all oxide multilayers while introducing flexibility to allow for roll-toroll manufacturing. The multilayer coatings were deposited using a single chamber PECVD process at a substrate temperature of 80 8C. Proof of the tunable optical properties, nanoscale PECVD system control, and comparison to theoretical models were achieved by designing and fabricating three layered Bragg mirrors tuned to various colors in the visible. The applicability of this system for energy efficient and renewable energy applications was demonstrated by designing and fabricating multi-layer AR and IRR coatings. The AR coating was specifically designed and applied to improve the efficiency of CdTe solar cells by maximizing transmittance of incident light to the cell. The AR coating on TEC-15 and PET substrates increased the average transmittance by 3–4% over the wavelengths 425– 850 nm, and enhanced the current density of CdTe solar cells. IRR coatings on PET achieved 70% reflectance in the near-IR range of 800–1200 nm while retaining of 81% absolute transmittance in the visible region. 50 000 cycles of automated bend testing of the AR and IRR coatings on PET resulted in no change in optical performance. Moreover these coatings displayed excellent adhesion and were shown to be robust upon exposure to humidity and thermal shock tests. This work clearly shows that pairing TiO2 and silicone enables the design of well-behaved and effective optical coatings that may both be plasma-deposited without breaking vacuum. Furthermore, these coatings are both mechanically robust to bending as well as stable to temperature/humidity shocks. Further studies, such as increased number of layers as well as probing the limits of the film’s durability, can be explored for more specific or demanding applications.

Acknowledgements: We would like to thank the Colorado Office of Economic Development and International Trade for financial support of this work. We greatly appreciate Ms. Jiaojiao Li for providing solar cells and conducting J-V measurements through the support of the Bay Area Photovoltaic Consortium. MR was supported by the U.S. Department of Energy through the SunShot Foundational Program to Advance Cell Efficiency (F-PACE) under Contract No. DE-AC36-08-GO28308.

Received: June 27, 2015; Revised: July 31, 2015; Accepted: August 31, 2015; DOI: 10.1002/ppap.201500114

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DOI: 10.1002/ppap.201500114