ISCST 2012

1. Advantages of TCO via Ultrasonic Spray Under Atmospheric Conditions ISCST 2012 Authors: Robb W Engle, Director of Technical Services, Sono-Tek Corporation, Milton, NY Morgan Dart, Engineering Assistant, Materials Science, Sono-Tek Corporation Presented at the 16th International Coating Science & Technology Symposium, September 11, 2012, Atlanta, GA

2. Define TCO TCO, or Transparent Conductive Oxide layers are typically metal oxides, often doped, that are used on the optical surfaces of PV panels, glass, and interactive consumer devices. TCO is measured by how well it conducts current versus how little it blocks light (photon) transmittance.

3. History of TCO Deposition Spray Pyrolysis was first used in development of TCO layer technology. This was difficult at the time, to grow into manufacturing processes.

4. Sputtering In sputtering, a “target” containing the metallic particles is bombarded from the back with ions. Collisions with the target of the correct potential cause atoms of the metal to be freed from the opposite side. These “freed” atoms, under vacuum, coat the surfaces available to them, including the substrate. Capitol equipment and operating expenditures are quite high in that: the process typically runs in the fractions of millitorrs of vacuum, the electricity used in the process is quite high, including serious cooling challenges, and finally the transfer efficiency of the process is often low, typically below 25%.

5. Spray Pyrolysis From the Greek for “fire” and “separating”, this processes uses high heat to catalyze a reaction. Many recognize four different styles of spray pyrolysis: carrier removal contact change plume manipulation plume and contact change (in order from coolest to hottest). Non uniform droplet sizes wreak havoc on the consistency and reliability of spray pyrolysis.

6. Ultrasonic Nozzle Design Ultrasonic nozzles create high frequency mechanical vibrations by stimulating piezoelectric transducers with high frequency oscillating electrical energy. When liquid is fed through the nozzle, it is atomized at the nozzle tip and emerges as a fine mist of mathematically defined micron-sized droplets. Titanium Rear Horn Atomized liquid is generated As a soft, low-velocity spray Resulting in minimal overspray Active Electrode Rear Housing Ground Electrode Front Housing Ground Lug O-Ring Seal Liquid Feed Channel O-Ring Seal Atomizing Surface Titanium Amplifying Section (Front Horn) O-Ring Seal Electrical Connector to the Broadband Ultrasonic Generator Piezoelectric Transducers

7. Define Ultrasonic Atomization Droplets forming on nozzle tip • When liquid is added to a resonating nozzle, waves are formed on the atomizing surface perpendicular to the axis of vibration underneath. • Increasing the power of electrical energy causes the wave peaks to get so high that droplets fall off the tips of the wave. These droplets have a mathematically definable size. • Nozzle tip vibration, wave formation and ultrasonic atomization:

8. Uniform Drop Sizes Drop size is governed by the frequency at which the nozzle vibrates, and by the surface tension and density of the liquid being atomized. Frequency is the predominant factor. Ultrasonic nozzles have a tight drop distribution curve compared to pressure nozzles, contributing to a much higher uniformity of spray. Drop distribution comparison = 60 kHz Sono-Tek Ultrasonic Nozzle = Pressure Nozzle

9. Process Challenges High Temperature Chemical Compatibility

10. High Temperature The temperatures required for methods 2,3 and 4 of spray pyrolysis are quite hot, often ranging from 150 to 1000 degrees C on the substrate. Temperatures this hot require considerable engineering to ensure capitol equipment isn’t damaged. Ultrasonic atomization equipment must be cooled to surface this environment, as must many other components such as motors and solenoid coils.

11. Chemical Compatibility Much more challenging than the temperature aspect is the chemistry used to dissolve metals. At a minimum, acids such as Nitric or Hydrochloric are commonly used. More common are processes using Fluorine doped oxide layers. HF represents one of the most difficult materials in engineering, even more so when it is heated. Corrosive resistant ultrasonic nozzle

12. Advantages of Ultrasonic Process Various tests have been documented and published demonstrating the feasibility of atmospheric TCO layers (not under vacuum).

13. Uniform Droplet Size The first key advantage to the ultrasonic atomization process is the uniform size of the droplets produced. As they are produced from a mathematically defined waveform (fig.1), the droplets are significantly uniform in size. These similarly sized droplets all require the same amount of energy to achieve the next target state, and all reach the target state at the same time in the process. This eliminates the challenge of too small droplets having their chemistry “overcooked” and too large droplets reaching the substrate too wet or “undercooked.” Fig 1 – Drop size calculation equations

14. Plume Velocity versus Pyrolysis State Since the process engineers have direct control over the speed of the plume, they can allow the plumes to fall in a more predictable, controllable rate. This gives key control to the pyrolysis reaction, not otherwise available in high velocity plumes, such as those from dual fluid atomization equipment.

15. Summary There is a profound and impending market for efficiently produced TCO in advanced energy and consumer products. The costs of starting up a production facility and operating a sputtering line are prohibitive. Ultrasonic atomization has been proven to distinguish the process through two key advantages. Recent improvements in the technology are now overcoming the two main process challenges.