Open Access
Add to BibSonomy
Abstract
Flexible glass has many applications including photovoltaics, organic light-emitting device (OLED) lighting, and displays. Its ability to be processed in a roll-to-roll facility enables high-throughput continuous manufacturing compared to conventional glass processing. For photovoltaic, OLED lighting, and display applications, transparent conductors are required with minimal optical reflection losses. Here, we demonstrate an anti-reflective coating (ARC) that incorporates a useful transparent conductor that is realizable on flexible substrates. This reduces the average reflectivity to less than 6% over the visible band from normal incidence to incident angles up to 60°. This ARC is designed by the average uniform algorithm method. The coating materials consist of a multilayer stack of an electrically functional conductive indium tin oxide with conductivity (31 Ω/□), and . The coatings showed modest changes in reflectivity and no delamination after 10,000 bending cycles. This demonstrates that effective conductive layers can be integrated into ARCs and can be realized on flexible glass substrates with proper design and process control.
© 2018 Optical Society of America
1. INTRODUCTION
Flexible glass, such as Corning Willow Glass, potentially enables high-throughput continuous manufacturing processes for a variety of applications [1]. Flexible substrates that can tolerate higher-temperature processing open the door to truly flexible solar cells, wraparound displays, and other applications that are currently difficult to achieve. These applications will still require the same functional layers as conventional glass devices, including transparent conductors and anti-reflective layers.
Many device concepts have been demonstrated to date with flexible glass substrates. Flexible glass cadmium telluride solar cells have been demonstrated with efficiency with no change in quantum efficiency whether flexed or flat [2], and flexible glass perovskite photovoltaics have been demonstrated with efficiency [3]. Organic light-emitting devices (OLEDs) have also been demonstrated [4], as well as large-area touch sensor devices [5]. Roll-to-roll (R2R) processing continues to mature and more complex and higher performance devices continue to be developed.
In this paper, another set of R2R processes necessary for the adoption of the technology into manufacturing are demonstrated. Devices such as solar cells, OLED lighting, and displays require both transparent conductive oxide (TCO) and antireflective coatings (ARCs) [6,7] to improve performance. Here, we fabricate combined anti-reflective coatings and functional transparent conductive layers on 100-μm-thick, flexible Willow Glass in a R2R process. The reflectivity is significantly reduced as compared to simple indium-tin-oxide (ITO)-coated Willow Glass, or even compared to completely uncoated Willow Glass, while providing a functional conductive interconnect layer. The resulting wafers are evaluated for both the conductivity, after the complete process, and their robustness under repeated bending stress.
Anti-reflective coatings are widely used in industry in applications such as photovoltaics and solar cells, electronic device displays, and general purpose glass such as eyeglasses and binoculars. The main purpose of ARC is to eliminate unwanted reflections and increase the overall transparency. For solar cells, OLED lighting, or displays, increased transparency translates to improved efficiency. While many transparent materials can be designed into an anti-reflective stack, for this application, a conductive layer is desired. Here we use a combination of annealed ITO with conductivity and (formed from sputtering a 2% Al-doped silicon target in an oxygen atmosphere). The Si target is doped with Al to improve conductivity to facilitate sputtering. The ITO provides a functional interconnect layer. The serves as a buffer layer sputtered between the flexible Willow Glass and the ITO layer to eliminate impurity caused by organic particles on the substrate surface.
The buffer layer is quite transparent and has sufficient refractive index contrast with ITO to realize a good coating.
First, the optical properties of the materials (- and -) are characterized with a Horiba ellipsometer. Using those properties, the anti-reflective coating is designed with the Average Uniform Algorithm (AUA). The AUA is a variation of a genetic algorithm in which a set of random coatings are used as a seed, and subsequent generations are based on the average of the best members of the sample set. In addition, it injects additional random coatings each generation to avoid being locked into a local minimum. This method has been previously proven as a robust method for anti-reflective coating design [8,9], which rapidly achieves a practical high-performance coating.
After design, the coating materials, and ITO, are sputtered in a R2R system [10] according to the developed recipes (see Fig. 1), and then annealed for 5 min at 500C. The final coating film stack is shown in Fig. 1.
Fig. 1. Annealed anti-reflective coating film stack. Each layer’s thickness is determined by the AUA method for an ARC coating on a flexible Willow Glass substrate.
The lowest layer of is the buffer layer, and the remaining layers are designed for minimum reflectivity with a functional conductive interconnect layer. The coated films are then characterized for reflection and transmission at a variety of angles, using an Agilent Cary 5000 UV-Vis-NIR spectrophotometer. Overall results are very promising, with average reflectivity less than 6% over the visible band from incident angle 0°–60° measured on double-sided coated flexible Willow Glass realized with quite conventional materials.
As a fully functional device on flexible Willow Glass will need to be robust under flexure, the devices are characterized after repetitive bending stress. The devices are mounted on a bending tester connected at the ends to strips of paper for mechanical attachment. A load of 200 grams for tension was used, and they are bent through an angle of 20 deg (see Fig. 2), for 10,000 times in 4 h. After the structure is fabricated completely, including annealing, the top oxide is removed and the resistivity of the 103 nm ITO layer is characterized.
Fig. 2. Bending fatigue tester. The ARC sample was bent both sides with an angle of 20 deg and approximately 4 cm radius.
The outline of the paper is as follows. In Section 2, the details of extraction of optical parameters and the coating design are reported along with the coating process. Fatigue test processes are introduced in Section 3. The anti-reflective coated Willow Glass’s reflectance (R) and transmittance (T) are measured with a spectrophotometer and compared to calculations in Section 4. The proposed ARC work is summarized and concluded in Section 5.
2. EXPERIMENT
The extracted refractive index () and extinction coefficient () of coating materials and ITO, as well as the flexible Willow Glass substrate, are measured with a Horiba ellipsometer as described below (see Fig. 3). The extraction method involves fitting measured and against a model and obtaining a robust and ideally unique fit. In general, a classical dispersion model is used for model fitting of ITO on the substrate, where “Drude” and “Lorentzian” dispersion parameters of ITO/glass are , , , , , and . A default built-in dispersion model of can be used for model fitting . A fit is generally considered good and the fit data reliable when it has a very low value of error calculations, and reasonable agreement with otherwise known thicknesses.
Fig. 3. Annealed/unannealed , ITO, and Willow Glass refractive index and extinction coefficient were measured with an ellipsometer.
Values of - and - of both unannealed and annealed material were obtained. The unannealed material was tested as sputtered; the material was annealed for 5 min at 500C. The electrical and optical characteristics of the ITO were improved after annealing.
To obtain our material values, an uncoated 100-μm-thick Willow Glass substrate by itself was placed on the ellipsometer measurement stage with black tape used on the back side to reduce unwanted reflections. To obtain values for ITO and , thin layers of these substances were sputtered in our R2R process, annealed, and characterized additionally by the ellipsometer, using previously determined values for the glass. As can be seen from Fig. 3, the ellipsometry model qualitatively captured the features of the thin film on the flexible Willow Glass, and the extracted (- and -) values for our sputtered ITO and were reasonable and close to values reported in the literature for generic ITO and sputtered , respectively.
Using these measured values for the optical properties for our materials, the AUA algorithm is used to design the anti-reflective coating, constrained by the materials (ITO and ), the substrate, and the thickness (at least one ITO layer should be thick). The figure of merit, , to be minimized is given by
where represents the normalization factor, and is the reflectivity with respect to wavelength ranging from 350 to 750 nm, over a set of equally weighted reflection angles of 0°, 30°, and 60°. The first layer of serves a buffer layer and hence is a fixed layer in the design. The algorithm is arbitrarily allowed 10 layers, but converged to the five shown in Table 1. Further details about the AUA algorithm are given in [9,10]. The calculated values from the AUA algorithm will be compared to measurement in Section 4.
Table 1. ARC Layers Sputtering Parameters by R2R Process
The ARC Willow Glass is fabricated with R2R process equipment. Sputtering was done using the General Vacuum/Bobst Optilab unit (GVE) located at the Center for Advanced Microelectronic Manufacturing, Binghamton University. The GVE is equipped with two sputtering cathodes: one dual rotating cathode, which was used for the Al-SiOx deposition, and a DC pulse capable planar magnetron cathode, which was used for the ITO deposition. The GVE conveys webs while tension is held against a large central drum, which can be heated or cooled from up to 200C. The wafers processed were deposited at 125C for all films. As in typical R2R systems, the thickness was controlled by the web speed, not by the sputtering time. Discrete flexible Willow Glass wafer substrates were attached to a carrier web to enable early evaluation studies. Wafers were run through the sputtering zone first at 0.1 m/min, depositing approximately 138 nm of the dielectric on the surface of the Willow Glass. After the dielectric deposition was completed, the rotatable targets were turned off, the ITO target was turned on, and the pre-sputter step began. After pre-sputter, the wafers were then conveyed through the zone at a speed of 1.95 m/min to achieve the desired ITO thickness of 10 nm. After the ITO layer was deposited, the ITO target was turned off and the targets were turned back on, repeating the first deposition at a speed of 0.73 m/min to obtain the thickness of 27 nm. These steps were repeated using the speeds determined in Table 1 until the full anti-reflective stack was complete. After deposition, the wafers were carefully removed from the roll, flipped over, and placed back onto the polyimide for backside coating. The backside coating is identical to the front.
During sputtering, the deposition drum in the R2R system was kept at 125C and the chamber pressure at 0.006 mbar. The films was formed by sputtering a Si target doped with a 2% Al dopant, in an atmosphere of 20% oxygen/80% Argon, at a target power of 7 KW, and the ITO was sputtered with a 1.5% O2/98.5% Ar ratio at a target power of 2.8 KW and a chamber pressure of 0.006 mbar.
A scanning electron micrograph (SEM) of a preliminary set of layers fabricated on Willow Glass is shown in Fig. 4. There was sufficient contrast to allow the SEM to be used to hone the conveyance speed and perfect the recipe.
Shown in Fig. 5 are three pictures through coated and uncoated Willow Glass substrates: an ITO-coated Willow Glass, a double-sided AR coated Willow Glass, and a bare uncoated Willow Glass substrate, respectively. As can be clearly seen, the AR coated Willow Glass shows much reduced reflection and much increased transmission compared to the other samples. After deposition and annealing of the anti-reflective layers, the reflectance and transmittance of the fabricated ARC Willow Glass are measured by spectrophotometer over a variety of angles of incidence. The calculated reflectance and transmittance of our optimized coating at three different angles (0°, 30°, and 60°), as compared to the measured reflectance and transmittance by spectrophotometer, are shown at Section 4. Note that the calculation is only for specular reflection, while the measurement in the spectrophotometer includes both specular and diffuse reflections. In this case, there is minimal difference.
Fig. 5. Three photos taken outdoors at approximately normal angle of incidence for comparisons of an ITO-coated Willow Glass, ARC Willow Glass, and a bare uncoated Willow Glass.
3. CONDUCTIVITY AND FATIGUE TESTING PROCESSES
After characterization of reflectivity and transmissivity, the durability of the layers was tested as follows. The sample was mounted on a bend tester using regular writing paper strips with a load of 200 grams mandrel for tension in the strips. The AR coated Willow Glass was bent to 20° for 10,000 cycles in four hours in a cyclic loading test. The reflectance and transmittance are only modestly changed (see Fig. 6) as compared to unbent AR coated Willow Glass. No ARC delamination is observed, and the final reflectance shows a minimal change from 2.29% to 2.94% at 550 nm.
Fig. 6. Comparison of reflectance and transmittance for 10,000 cycles fatigue-tested ARC sample with the unbent ARC sample.
In order to evaluate the use of the 103 nm ITO layer as an interconnect layer, the overlying layer was removed by three minutes of dry etching process, and the conductivity of underlying ITO layer was measured. The conductivity was determined to be with a corresponding sheet resistance of 31 Ω/□.
4. RESULTS AND DISCUSSION
The thicknesses of the ARC evaluated by the SEM were approximately 77 nm/103 nm/23 nm/9 nm/110 nm from the topmost ARC layer to the buffer layer (see Fig. 4), measured by SEM (which is not that precise at these thicknesses with a cut cross-sectional segment). A best fit by an improved random phase method considering the incoherent substrate was done to obtain the best fits for actual thicknesses, determined to be 79 nm/103 nm/31 nm/10 nm/136 nm. The ARC thicknesses were generally within 10% of the target thickness and gave a reasonable qualitative fit.
The coating performance is largely insensitive to the buffer layer thickness, since the index contrast between the Willow Glass and is small.
Figure 7(a) shows the measured reflectance of a bare flexible Willow Glass substrate, an ITO-coated Willow Glass, and double-sided annealed ARC Willow Glass, as well as the calculated reflectance at a variety of angles (0°, 30°, and 60°). There is generally reasonably good agreement with the calculated reflectance and the measured reflectance. Transmittance [see Fig. 7(b)] was also calculated from the air into the Willow Glass and had even better agreement with calculation. We note this result was achieved with materials which have the same or higher index than the substrate (and the ITO is also somewhat absorbing). With the annealed AR coating, the transmission was increased from 91.91% to 97.2%. The measured reflectance of the annealed AR coating was reduced from 7.93% to 0.28% as compared to an uncoated Willow Glass substrate at 0° at a wavelength of 550 nm. Reflection and transmittance measurements at 550 nm from our standard bare Willow Glass, double-sided annealed AR coated Willow Glass, and a sample ITO-coated (275 nm) Willow Glass are shown in Table 2.
Fig. 7. (a) Comparison of reflectance of annealed ARC by AUA calculation based on measured thicknesses and spectrophotometer measurement at 0°, 30°, and 60°. (b) Comparison of transmittance of annealed ARC by AUA calculation based on measured thicknesses and spectrophotometer measurement at 0°, 30°, and 60°.
Table 2. Table of Reflectance and Transmittance Measured at 550 nm by Agilent Cary 5000 UV-Vis-NIR Spectrophotometer
As can be seen, the annealed AR-coated wafers are optically better than the bare Willow Glass and much better than the standard ITO glass wafer.
The sheet resistance of the 103-nm ITO functional layer in the proposed ARC’s stack is 31 Ω/□. This sheet resistance is suitable for interconnects in some display or touch sensor array applications. For transparent conductors in solar cell applications, a sheet resistance of 10 Ω/□ is needed and can be achieved with a 275-nm-thick ITO layer with contacts through an etched top layer of the stack. For transparent antenna applications [11], a sheet resistance of or a thickness of 650 nm ITO is needed, and again electrical contact can be made by etching the top layer. The thicknesses of the other layers in the stack need to be adjusted accordingly to meet requirements for reflectivity and transmissivity.
5. CONCLUSIONS
In this paper, we have demonstrated a multilayer ARC designed with an AUA algorithm and fabricated in R2R processes equipment using ITO and layers. The resultant coating realized good optical performance and included a functional interconnect layer. Very low reflectance and high transmittance in the visible wavelength range were realized as compared to the Willow Glass substrate with ITO and even the uncoated Willow Glass. The coating was robust over 10,000 bending cycles, showing modest changes and no delamination. Using this sort of integrated anti-reflective coating, devices such as solar cells, displays, or highly transparent antennas can be easily and inexpensively fabricated on R2R processes with conventional materials.
Funding
Science and Technology Center, Corning Incorporated Foundation; The Center for Advanced Microelectronics Manufacturing, Binghamton University; Advanced Diagnostic Laboratory, Binghamton University.
REFERENCES
1. S. Garner, S. Glaesemann, and X. Li, “Ultra-slim flexible glass for roll-to-roll electronic device fabrication,” Appl. Phys. A 116, 403–407 (2014). [CrossRef]
2. H. P. Mahabaduges, W. L. Rance, J. M. Burst, D. M. Meysing, C. A. Wolden, M. O. Reese, T. A. Gessert, W. K. Metzger, S. Garner, P. Cimo, and T. M. Barnes, “High-efficiency, flexible CdTe solar cells on ultra-thin glass substrates,” Appl. Phys. Lett. 106, 133501 (2015). [CrossRef]
3. B. Dou, E. M. Miller, J. A. Christians, E. M. Sanehira, T. R. Klein, F. S. Barnes, S. E. Shaheen, S. M. Garner, S. Ghosh, A. Mallick, D. Basak, and M. F. A. M. van Hest, “High-performance flexible perovskite solar cells on ultrathin glass: implications of the TCO,” J. Phys. Chem. Lett. 8, 4960–4966 (2017). [CrossRef]
4. K. A. Knauer, E. Najafabadi, W. Haske, M. P. Gaj, K. C. Davis, C. Fuentes-Hernandez, U. Carrasco, and B. Kippelen, “Stacked inverted top-emitting green electro phosphorescent organic light-emitting diodes on glass and flexible glass substrates,” Org. Electron. 14, 2418–2423 (2013). [CrossRef]
5. J. Chen and C. T. Liu, “Technology advances in flexible displays and substrates,” IEEE Access 1, 150–158 (2013). [CrossRef]
6. C. S. Thompson, “Investigation of moth-eye antireflection coatings for photovoltaic cover glass using fdtd modeling method,” in Photovoltaic Specialist Conference (PVSC), June , 2014, pp. 2273–2275.
7. W. F. Wu and B. S. Chiou, “Mechanical and optical properties of ITO films with anti-reflective and anti-wear coatings,” Appl. Surf. Sci. 115, 96–102 (1997). [CrossRef]
8. A. Allazam, E. Yuzgec, and H. W. Lewis III, “A new optimization algorithm for non-convex problems,” in Industrial and Systems Engineering Research Conference (2013).
9. P. P. Rajbhandari, A. Emrani, T. P. Dhakal, C. R. Westgate, and D. J. Klotzkin, “Antireflection coatings designed by the average uniform algorithm for thin film solar cells,” Appl. Opt. 53, 8001–8006 (2014). [CrossRef]
10. R. Malay, A. Nandur, J. Hewlett, R. Vaddi, and B. E. White Jr., M. D. Poliks, S. M. Garner, M. H. Huang, and S. C. Pollard, “Active and passive integration on flexible glass substrates: subtractive single micron metal interposers and high performance IGZO thin film transistors,” in Electronic Components & Technology Conference, 2015 IEEE 65th, May , 2015, pp. 691–699.
11. M. D. Poliks, Y. L. Sung, J. P. Lombardi, R. E. Malay, J. M. Dederick, C. R. Westgate, M. H. Huang, S. Garner, S. Polland, and C. Daly, “Transparent antennas for wireless systems based on patterned indium tin oxide and flexible glass,” in IEEE Electronic Components and Technology Conference (ECTC), May , 2017.
