Laser Sealing of Plastic to Metal

Organic semiconductor systems are becoming increasingly important for development of flexible photovoltaics and display systems. These organic materials are very sensitive to intrusion of air or moisture, so making a durable device requires hermetic seals. This challenge is made even more difficult by the fact that these devices are multi-layered structures composed of highly tailored materials. The chemistry of these materials is critical to their function, so the joining/sealing process must be custom designed to make a durable seal without changing the chemistry of the functional layer.

Seals for flexible photovoltaic systems are a critical need for protecting the photocell system from the environment whether an organic or inorganic system is used. The sealing materials cannot leach contaminants into the organic system or failure may result, therefore, highly specialized joining/sealing techniques are required.

Researchers at the OSU Dept. of Electrical & Computer Engineering have been working with organic photovoltaic devices on flexible substrates. The sensitivity of these structures to moisture and oxygen limited the assembly and testing of the devices in a controlled glovebox environment. OSU approached EWI to help design a sealing technique to encapsulate the active photovoltaic cells to enable testing the devices outside of a glovebox environment. Active cells were prepared by OSU and supplied to EWI for sealing. These cells consisted of polyethylene terephthalate (PET) substrate with In₂O₃ coated strips, spin-coated PV organic material, and over-plated aluminum coated strips. To encapsulate the photovoltaic device and create a hermetic seal, a polyethylene naphthenate (PEN) film was selected for its good barrier properties. A laser sealing approach was selected to precisely control the melt area with minimal damage to the anode and cathode. Laser sealing is a fast, non-contact process that requires a top component that transmits infrared light and an absorbing bottom component. If both components are transparent, an infrared absorber can be applied at the interface.

A diode laser and positioning system were used to create a perimeter seal between the PEN top layer and PET substrate. Because both films are transparent, an infrared absorber was applied along the weld area to convert the infrared energy to heat. The joint also required sealing the PEN film to the In₂O₃, aluminum coated strips and the photoactive layer. In Figure 1, the laser seal can be seen overlapping the photoactive layer. The seal is more visible at the In₂O₃ crossovers due to the additional absorption of the infrared radiation by the indium oxide. There is no apparent damage at the aluminum crossovers. Examination of the In₂O₃ crossovers at higher magnification reveals the photoactive layer in the sealing region and it can be seen that the appearance of the In₂O₃ is altered by the laser sealing process.

The open-circuit voltage and current were measured for the enclosed active cells. Immediately after sealing, samples were exposed to sunlight and the voltage and current were measured using a multimeter. The measured current was low and the voltage varied with sunlight exposure. Additional photovoltaic cell assemblies were sealed to be monitored for longevity testing. Although the photoactive layer showed signs of deterioration ten days after sealing, this was well beyond the previous exposure lifetime of about one hour.

This project successfully demonstrated the feasibility of using a diode laser source to seal flexible thermoplastic photovoltaic devices with anodes and cathodes, such as aluminum and In₂O₃. Throughout the study, moderate seal strengths were obtained for each material combination. Active photovoltaic cells were encapsulated and their operation was verified. Although the photoactive layer showed signs of deterioration ten days after sealing, this was well beyond the expected lifetime outside of a controlled glovebox environment.

There are many applications where it would be advantageous to seal polymer to other substrates such as MEMS, flexible displays, and consumer packaging. In many cases, dissimilar materials are not compatible for welding processes; however a polymer may offer sealing potential. This study found that hermetic seals could be obtained between PEN (thermoplastic) and aluminum and In₂O₃ stripes.

To learn more about Hermetic Sealing of Photovaltaic Cells, contact Sean Flowers at sflowers@ewi.org or call 614.688.5129.