Princeton University

School of Engineering & Applied Science

Encapsulation of Organic Light Emitting Diodes

Bhadrinarayana Lalgudi Visweswaran
Engineering Quadrangle J401
Wednesday, September 24, 2014 - 2:00pm to 3:30pm

Organic Light Emitting Diodes (OLEDs) have been introduced in flat panel displays, handheld devices, phones and lighting. OLEDs are very attractive semiconductors in terms of visual appearance, efficiency and mechanical flexibility but they have the drawback of being unstable upon exposure to moisture and air.  To obtain a flexible OLED device, a flexible thin-film barrier encapsulation with low permeability for water is necessary. This thin-film barrier coats the plastic substrate on which the displays are made and it prevents water from reaching the OLED. The lack of a perfect flexible barrier film has prevented the realization of a flexible display.
Water permeates through 4 modes in the barrier film, namely, through microcracks, through the bulk of the barrier film by diffusion, through particle induced cracks and sideways through permeable grid layer. The development of a reliable barrier film has been difficult owing to multiple modes of failure acting simultaneously. We have managed to separate these failure modes and have addressed them one at a time.
Quantitative techniques for evaluating the diffusion coefficient and solubility of barrier materials by measuring electrical capacitance and mechanical have been described. These techniques are novel as they circumvent the effect of particles and defects, which the existing techniques are incapable of doing.
Systematic study of particle encapsulation with fixed size of artificial particles was performed, and a three layer barrier film made of the hybrid material was designed. As a demonstration of performance of the barrier film against particle contamination, an OLED test coupon lasted 500 hours at 85 °C and 85% relative humidity even though 5 µm size glass beads had been spread over the surface of the OLED prior to the barrier film deposition.
We have shown a systematic approach to address all forms of failure due to water permeation that is observed in an OLED encapsulated with a thin film barrier. Emulating these steps, the research and development of barrier films can be sped up. Using these steps, we have demonstrated a reliable hybrid barrier film that can provide necessary protection to provide an OLED lifetime of over 10 years.