Optimization of AZO films for integrating optically transparent antennas on photovoltaic

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1 Optimization of AZO films for integrating optically transparent antennas on photovoltaic M. E Zamudio, 1a M. Behzadirad, 2 C. Christodoulou 1 and, T. Busani 1,2 1 Electrical and Computer Department, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A 2 Center for High Technology Materials (CHTM), University of New Mexico, Albuquerque, New Mexico 87106, U.S.A The importance of having an optimal material for fabricating Optically Transparent Antennas (OTAs) is crucial for designing highly efficient antennas that can be integrated with photovoltaics. Transparent Conductor Oxides (TCOs) are promising for OTA fabrication due to their capability of being simultaneously transparent at optical frequencies and conductive within the radio frequency (RF) range. Here in this study, thin Aluminum and Zinc Oxide layers were co-sputtered onto Si and a polycrystalline photovoltaic cell and then annealed between 350 C and 450 C for 24 and 48 h in N 2 ambient. The annealing process ensured the formation of the Aluminum Zinc Oxide (AZO) with low resistivity 10 Ω transparency of 86% between 350 and 750 nm. The material was tested by performing RF characterization, and by fabricating and testing two different OTAs. The results of the optimization process and characterization show that the AZO material is feasible for OTAs fabrication. Research on OTAs has yielded different approaches for determining the most efficient way to achieve high optical transparency and high RF conductivity, simultaneously 1.One of the methods for designing OTAs is to use a metallic mesh, such as a copper mesh, to reach a high level of transparency. However, this method, yields the maximum reported antenna efficiency of 60% for a fixed transparency level of 90%. Another approach is to employ transparent conductor materials, such as AgHT, a film based on silver conductor 2. This material is usually coated with PET (polyethylene terephthalate) and has a conductive layer in between two layers of tin oxide 3. The sheet resistance was found to be of 4.2 Ω*cm 4. This particular film turned out to be very lossy and not suitable for optimal OTA s designs 5. A more efficient and cost effective method for designign OTAs is to employ transparent conductive oxides or TCOs 6. The TCOs are designed to have optical transparency >80% and a resistivity lower than 10 Ω 1. However, there is still a struggle to maintain low optical losses and high carrier mobility in TCOs. Indeed, to improve the conductivity of the TCO it is required to introduce metallic impurities. The impurities act as a dopant in a semiconductor, but at the same time they increase the absorption in the UV-VIS spectrum. TCOs based on tin oxides, doped with indium, Indium Tin Oxide (ITO), posses high conductivity due to their low resistivity, around 10 Ω 7, with record transmittance of 90.2% 8. Recently, researchers reported on using ITOs for optically transparent antennas integrated with photovoltaics 9 (PV). In their studies, the ITO was used as a film and as a mesh. It has been concluded that the transparency for the ITO is relatively 1 high, about 80% with low resistivity of Ω for frequencies over 10 GHz. Even though the ITO approach appears to be the most suitable material among the TCOs for PV and RF antennas, the growing scarcity of indium is increasing the cost of using the material. Additionally, the ITO is not stable and its electrical properties are affected under special atmospheric conditions where oxygen needs to be removed to prevent oxidation. These atmospheric conditions are called reduced atmosphere, and are used in annealing ovens where it is necessary to prevent corrosion on the metal by avoiding oxygen or any oxidizing vapor. Lastly, indium has a certain potential level of toxicity that represents a hazard to humans and the environment in general 10. To avoid hazards, high costs, and degradation in efficiency, studies have been done in employing different materials. Jiang et al 9 employed various oxides Zn 2 SnO 4, ZnSnO 3, MgIn 2 O 4, Zn 2 In 2 O 5; and ZnO multicomponent compounds( ZnO-In 2 O 3 and MgIn 2 O 4 - Zn 2 InO) showing that the properties of the materials can be controlled by altering their chemical composition. Also, they revealed that films based on Zinc Oxides (ZnO) show more stability, and are more cost effective than ITOs, with comparable optical properties. Among these composites, zinc oxide doped with aluminum Al: ZnO or AZO has demonstrated to have low resistivity and high transmittance comparable with ITOs. The AZO resistance is in the order of 10-4 Ω*cm, and its high transmittancy (T 90 % 11 ) makes AZO a good candidate to built antennas that can be integratd with photovoltaics. Although the material has not been reported for OTAs fabrication, AZO films have previously been studied for integration with photovoltaics as top contacts 12. In the mentioned

2 investigations, the authors show the desired properties that AZO films must have to be integrated with solar cells. In this work we present AZO films sputtered on top of poly-si solar cells which can serve as both transparent antennas and top contact in solar cells. The AZO film was prepared by sputtering thin layers of zinc oxide and aluminum oxide on a silicon (100) wafer using a PVD 75 Kurt J Lesker system operating at 13.7 MHz. The typical pressure prior deposition of the Al and the ZnO was with a typical power of 110 W for Al, and 150 W for ZnO. Argon gas was introduced in the chamber with a partial pressure of The deposition rate, and the percentage of each one of the materials involved in the AZO was controlled by employing different targets for each element in the film (Aluminum and Zinc Oxide). The use of different targets also contributed to regulate the material thicknesses, and to minimize the light trapping mechanism due to the surface roughness. We optimized the deposition rate for the ZnO and Al to be and nm/min, respectively. A total of 4 layers of Al and ZnO was deposited; each layer had a thickness of 5nm for ZnO, and 1nm for Al. After the deposition process, the samples were annealed to allow the Al to dope the ZnO by thermal diffusion. To establish the optimal fabrication procedure for AZO films, four different samples were annealed using an horizontal furnace with a constant N 2 gas flowing. Two of the samples were heated at C for 24 and 48 hours, and other two were heated at 450 C for 24 and 48 hours, respectively. The sheet resistance of the samples was measured next by using the four-probe technique. In this technique, a current was passed by the two outer probes and the voltage was measured through the remaining two inner probes. By employing the measured I/V, the sheet resistivity was obtained, hence the DC conductivity can be measured 13. The lowest resistivity of Ω was measure for the sample annealed for 48 hours at 450 C. The transmittance was measured with the Cary UV- VIS NIR 6000i spectrometer, at room temperature between 350 and 750 nm, which is the best quantum efficiency spectral region of the solar cell. Data for the different samples studied is presented in Figure 1 and compared with room temperature deposited ZnO. For this purpose the AZO samples were prepared on a quartz substrate. The quartz trasmittance was recorded prior to 2 the Al and ZnO deposition and annealing. The quartz trasmittance spectra was then substracted from the AZO and ZnO trasmittance spectra. We noticed that the most transparent film is the one annealed at 450 C for 48 hours, which presents a transmittance of 86% (see Figure 1). Subsequently, the RF characterization was done in a wide frequency range of 1GHz to 50 GHz by employing a Tx/Rx method 14. In this method, the transmission and reflection coefficients of the samples were measured by making strong contact of an open ended coaxial probe and the sample and measuring the scattering parameters with an Agilent PNA-X Network Analyzer. Figure 1 Transmittance for annealed AZO samples and for nonannealed ZnO sample. The RF characterization was planned and carry out considering the results obtained for the entire optimization process. A resistivity of Ω was obtained for the sample with thickness of 24, annealed at 450 C during 48 hours. All subsequent experiments were done on this material. For the RF characterization, it was decided to also vary the AZO thickness to verify the thickness effect on the transmittance and the RF conductivity. This variation helped to determine which thickness was optimal for transparent antenna fabrication purposes. Table 1 shows a comparison of the RF conductivity as a function of the thickness for five different samples with AZO deposited on a 5mm glass substrate. The thicknesses were chosen to be 12nm, 24nm, 48 nm, 72nm and 98nm. In Table 1 we can see the maximum (Max) and the average values (A.V.) of the measured conductivity for the five different thicknesses. The measurements were taken before annealing (B.A.) and after annealing (A.N.). A total of 4 samples were considered for each thickness and the error was measured to be Ω.

3 The results showed that the resistivity decreased after annealing, as expected. Moreover, the average values for the annealed samples are more uniform compared to the non-annealed samples. This is an indication that he Al doping is working as expected and the Al has homogenously diffused thought the ZnO layers. The comparison of the sample thicknesses suggests the use of the sample with thickness 96 whose resistivity reached the minimum value. Still, as seen in figure 2, the transmittance of this sample only reached 77%; therefore, it was concluded, that the sample of thickness 24 is the most optimal because its transmittance reached 83%, and the average value for resistivity reached Ω. Table 1 Resistivity values for five different AZO samples with different thicknesses (all units [Ω ]). Max-B.A. is the maximum value of the samples before annealing; Max-A.N. is the maximum values of the samples after annealing; A.V.-B.A. is the average values of the samples before annealing; and A.V.-A.N. is the average values of the samples after annealing. Thickness Max-B.A. Max-A.N. A.V.-B.A. A.V.-A.N. 12nm nm nm nm nm All designed and simulated with CST (Computer Simulation Technology); the model was fabricated with AZO film deposited on top of a polysilicon solar cell by employing the optimization process previously described. For the patch antenna fabrication a photolithography process using the AZ5214 EIR negative photoresistor was employed. The optimized AZO film was covered with the negative photoresistor. The exposure using UV ligth, at 365 nm trough a quartz mask, allowed the definition of the antenna shape pattern. The exposure time was 1.95 seconds. After exposure, the photoresistor was developed with the developer AZ400 1:4 for 50 seconds. The next step was to etch the antenna into the AZO film using a buffered oxide etchant (BOE) solution for 20 seconds. During the etching process, the areas that were not protected by the mask were etched from the wafer resulting in the prototype shown in Figure 3a. Figure 3b shows the return loss of the antenna represented by the scattering parameter S11. When this parameter is lower than - 10dB, means that the antenna has a good input impedance match, the losses are at minimum, and enough power is delivered to the antenna for radiation purposes. Figure 2 Percentage of Transmittance for different AZO thicknesses, the 24nm sample obtained 83% transmittance with a resistivity of Ω As a proof of concept, an inset fed patch transparent antenna with total dimensions 20mm by 24mm was 3 Figure 3 (a) fabricated prototype and (b) S-parameters simulated and measured The discrepamcy in Figure 3 between the measured and simulated resulrs are due to the impurities present

4 during the measurement, and to the soldering of the SMA connector creating impedance mismatch as the frequency varies.. For this rectangular patch antenna, the resonance frequency was 10.84GHz, whereas the simulation yielded a resonance at 10.89GHz. results comparison is still satisfactory, and the antenna presents a good behavior overall. It is important to note that this antenna works at very high frequencies (45 GHz). The results show that the AZO material is still feasible for very high frequency antennas. The results of this antenna show that our optimized AZO film has good properties for fabricating an efficient antenna. This means that the film possesses good RF conductivity making it feasible to fabricate, and to integrate efficient optically transparent antennas on top of photovoltaics. To test the proposed material in higher frequencies, a different antenna, mainly a CPW (Coplanar Wave Guide)-fed antenna with total dimensions 10mm by 10.68mm, for frequencies up to 50 GHz, was designed. Since the SMA connector introduced some losses on the rectangular patch antenna, the CPW antenna was designed to have the ground plane and feeding line in the same surface. This made the S-parameter measurements possible using a probe station, without having to use an SMA connector and introduce any additional losses. The measurements were done by means of a probe station Cascade Microtech MPS150. The specific probe for these measurements has a spacing of 500 µm. To perform the calibration it was neccesary to employ an Impedance Standard Substrate (ISS) 15 with the specific size and requirements for a 50 Ω load, and for short contacts. The open part of the calibration was done by separating the probe from the ISS at least 200 µm. Figure 4 Fabricated CPW antenna, probe, and results Figure 4 shows that the measured and the simulated results have a good agreement at the resonance frequency. The peaks appearing in the measured results are due to some impurities that appear on the wafer during measurement. The measured and simulated 4 In summary, we studied the fabrication and characterization of OTA onto Si using AZO films and their integration with photovoltaic cell. The AZO material was directly deposited on a silicon substrate to represent the solar cell material. The optimization process of the material resulted in a very efficient conductive film operating at RF frequencies and this developed material is capable of presenting high levels of conductivity at RF and excellent transmittance at optical frequencies. This work was performed as part of the (Sustainable Energy Pathways Through Education and Technology) grant funded by the NSF (National Science Foundation), and the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Los Alamos National Laboratory (Contract DE-AC52-06NA25396) and Sandia National Laboratories (Contract DE-AC04-94AL85000) J.R Saberin, Optically Transparent Antennas for Small Satellites. (Master Thesis University of Utah, Salt Lake City, 2010). A Katsonaurus, Y Hao, N Collings, and W A Crossland, Electronics Letters 45 (14) (2009). Thomas P, Transparent UWB Antenna for Wireless Applications and Energy Harvesting. (Brunel University, London, 2012). Hautcoeur J, F Colombel, X Castel, M Himdi, and E Motta Cruz, Progress In Electromagnetics Research 22, 259 (2011). M E Zamudio, Y Tawk, J Costantine, F Ayoub, and C Christodoulou, presented at the IEEE Antennas and Propagation Symposium, Vancouver, Ca, 2015 (unpublished). N. Guan, H. Furuya, D. Delaune, and K. Ito, PIERS ONLINE 4 (1) (2008). Kentaro Utsumi, Osamu Matsunaga, and Tsutomu Takahata, Elsevier Science S.A (334), 30 (1998). Z Chen, W Li, R Li, Y Zhang, G Xu, and H Cheng, in American Chemical Society (2013), pp Tursunjan Yasin, (State University of Utah, Lugan, Utah, 2013). X Jiang, L Wong, M K Fung, and S T Lee, in Applied Physics Letters (2003), p / H Zhou, D Yi, Zh Yu, L Xiao, and J Li, in Thin Solid Films (2007), pp Vladimir Tvarozek, Pavol Sutta, Sona Flickyngerova, Ivan Novotny, Pavol Gaspierik, Marie Netrvalova, and Erik Vavrinsky, in InTech (2010), pp. ISBN: 978. R. Ellingson and M. Heben, (The University of Toledo, 2011), Vol J Y Chung, N K Nahar, L Zhang, Y Bayram, K Sertel, and J L Volakis, IET Microwaves, Antennas & Propagation 6 (4), 371 (2012). in Cascade Microtech (2016). a Electronic Mail: mzamudio@unm.edu christos@unm.edu busanit@unm.edu

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