A comparative study of encapsulation structures for OLEDs

Encapsulation is the final and most important step in the fabrication of organic light emitting diodes (OLEDs). An OLED operated in air can have a lifetime, defined as the time it takes for the luminance to decrease to half of its initial value, up to a few hours or less [1] due to degradation mechanisms induced by water vapour and oxygen. For emerging niche applications, such as OLED fluorescence biosensors [2] for routine laboratory analysis, the OLED is operated for a short period of time and then it must be disposed of. In this case OLEDs lifetimes of the order of a few hundreds of hours at initial luminance values in the range (500 1000) cd/m2 can be considered acceptable. To keep the low cost profile required for these disposable applications a fast, simple and inexpensive encapsulation method is highly desirable. For this reason, we report two different encapsulation structures for bilayer AlQ3-based OLEDs and compare their effectiveness in increasing device lifetime. The first encapsulation structure (package A) is the simplest and consists of a conformal coating layer of polydimethylsiloxane (GELEST, Inc.) deposited by spinning on a glass slide cover that is then applied to the device surface by a light pressure. The encapsulation is performed in air straight after taking the OLEDs sample out from the vacuum chamber. The resin is allowed to fully cure at room temperature for 12 h before lifetime measurements are carried out. The second encapsulation method is performed in an acrylic glove box under an overpressure of high purity dry N2. The glove box and its load lock chamber are purged with N2 for few hours. Again the OLEDs sample is immediately transferred from the vacuum chamber to the glove box. The encapsulation structure (package B) consists of an Al cap placed on top of the device using a thin bead of UV-curing resin applied around the edge of the cover. To absorb residual molecules of water vapour, a desiccant (Dryflex®, SAES Getters) is inserted into the package. Finally the resin is cured under an UV lamp for 1 min. Accelerated lifetime tests are performed at fixed DC current corresponding to an initial luminance L0 of 1070 cd/m2 and 1023 cd/m2 for OLEDs, respectively, in package A and in package B. Fig. 1 shows normalized luminance and voltage vs. time. In both cases a decay of luminance and a simultaneous voltage increase with time is evident. The package B is much more effective than package A in increasing device lifetime. This is 230 h for OLED in package B, corresponding to a projected lifetime of 2350 h at an initial luminance of 100 cd/m2. The peculiarity of package B is the absence of dark spots in the OLED at the end of the lifetime test, demonstrating its excellent barrier properties for oxygen and water vapour. Both OLEDs exhibit a strong luminance decay within the first hour of operation that we ascribe to electric field driven molecular dipoles orientation and ionic impurities migration [3] within the organic layers. OLED in package A has the highest early decay, probably due to more pronounced Joule heating effects arising from limited heat dissipation capability of package A structure. However, after the first few hours of operation, the luminance decays at a much slower rate with no Al cathode delamination at the end of the lifetime test. Further details will be given at the conference. [1] P. Cusumano, F. Buttitta, A. Di Cristofalo, and C. Calì , Synth. Met., 139 657 (2003) [2] B. Choudhury, R. Shinar, and J. Shinar , J. Appl. Phys., 96 2949 (2004) [3] M. Nakai, H. Fujii, T. Tsujioka, Y. Hamada, and H. Takahashi, Jpn. J. Appl. Phys., 41 881 (2002)