Failure Mode Investigation for Blue Polymer EL Device
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0937-M07-40
Failure Mode Investigation for Blue Polymer EL Device Mu-Gyeom Kim, Sang Yeol Kim, Tae Woo Lee, Sang Hun Park, Ju Cheol Park, Jong Jin Park, and Lyong Sun Pu Samsung Advanced Institute of Technology, Yongin-Si, Gyeonggi-Do, 449-712, Korea, Republic of
ABSTRACT We report the failure mode observed in polymer electroluminescent (EL) blue devices based on SB1 (from Samsung Advanced Institute of Technology). Such modes were analyzed using nondestructive and destructive methods. Using nondestructive methods, we investigated the changes in hole and electron mobility and measured transient EL at various times during device life test. We also observed compositional and morphological variation using TEM-EDX, STM, FT-IR, TOF-SIMS and reverse engineering as destructive methods. Electron mobility was reduced by nearly two orders of magnitude during the device lifetime to half initial luminance and was related to the formation of an insoluble layer inside the emitting layer on the anode side. This insoluble layer showed relatively ordered surface morphology and might be a cross-linked layer through the C-O-C bond cleavage process during EL operation. But, contrary to the sulfur migration mechanism into the insoluble layer suggested by CDT, we confirmed no obvious difference of sulfur composition between insoluble and emitting layers. Rather, we observed some degree of Ba diffusion into the emitting layer from decomposition of BaF2, however we do not believe this had a major effect on device degradation. INTRODUCTION Since the first report of polymer based electroluminescent (EL) devices in 1990 [1], there have been great efforts poured into improving device performance. After more than a decade of such efforts, we are now on the verge of real application for display. But the future of displays using polymer materials is unclear, especially because of the relatively short lifetime of blue compared to red and green. Considering the above, we investigated the cause of the short lifetime in polymer EL blue devices. There have been many reports related to lifetime, mostly for green [2] and red polymer devices. [3] Despite reduced numbers of multiple layers, polymer devices bear quite complicated failure modes inside. The degrading mechanism depends highly on materials and device structures. In this work, we investigated the failure mechanisms/modes of blue devices currently under development in SAIT (Samsung Advanced Institute of Technology). EXPERIMENT Initially, we prepared polymer EL blue devices with structure: ITO/PEDOT 50nm/SB1 80nm/BaF2 5nm/Ca 3nm/Al 200nm, where SB1 is a SAIT blue material. We approached the
failure mode investigation two ways. The nondestructive method was to measure both electron and hole carrier mobilities from transient EL during device life test. As a destructive way, we investigated cross-sectional /compositional analysis using EDX-filtered TEM, morphology by STM, FT-IR, and TOF-SIMS on the insoluble layer defined below after reverse engineering.
RESULTS AND DISCUSSION At the beginning o
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