Singlet and Triplet Energy Transfer in Phosphorescent Dye Doped Polymer Light Emitting Devices
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Singlet and Triplet Energy Transfer in Phosphorescent Dye Doped Polymer Light Emitting Devices Yong-Young Noh1, Chang-Lyoul Lee1, Hae Won Lee1, Hyun-Nam Cho2 and Jang-Joo Kim1 1 Department of Material Science and Engineering, Kwangju Institute of Science and Technology, 1 Oryong-Dong, Buk-Gu, Kwangju 500-712, South Korea 2 Polymer Materials Laboratory, Korea Institute of Science and Technology, P. O. Box 131, Cheongryang, Seoul 130-650, South Korea ABSTRACT Effect of host polymers on energy transfer in phosphorescent dye doped polymer light emitting devices has been investigated. Poly (N-vinylcarbazol) [PVK] and poly (9,9'-di-n-hexyl2,7-fluorene-alt-1,4(2,5dinhexyloxy) phenylene) [PFHP] were examined as the host materials for the phosphorescent dyes fac tris(2-phenypyridine) irdium( ) [Ir(ppy)3] and 2,3,7,8,12,13,17,18octaethyl-21H,23H-porphyrin platinum( ) [PtOEP]. The host and guest materials have the large spectrum overlap between the emission of the hosts and absorption of the guests. When the guests were doped in PVK, the singlet-singlet and triplet-triplet energy transfer took place efficiently. On the contrary, the energy transfer did not take place from π-conjugated polymer PFHP to the guests, even though common requirements for Förster and Dexter energy transfer were fulfilled. Host aggregation in PFHP based phosphorescent dye doped light emitting devices can play an undesired role obstructing efficient energy transfer.
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INTRODUCTION A phosphorescent dye doped polymer light emitting device has attracted attention because of the probability of high external EL quantum efficiency through easy processing [1-4]. For the productive light harvesting, singlet and triplet excited states of the host must participate in energy transfer to a guest via both of Förster and Dexter energy transfer or easy flow of charged elements into the guest to form singlet or triplet excitons on the dopant site directly [1]. Requirements for efficient Förster and Dexter energy transfer and charge confinement include: (1) large spectral overlap between absorption of guest and emission of host, (2) favorable location of HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) levels of host to those both of guest and (3) similar or higher location of the lowest excited triplet state (T1) of host relative to the guest. Thus adequate combination of host and guest is important in order to utilize all three pathways to achieve maximum efficiency. In this paper, we report the effect of host polymers on Förster and Dexter energy transfer to the phosphorescent dyes. EXPERIMENTAL DETAILS Ir(ppy)3 was synthesized according to the literature [5] and PFHP was synthesized by the Suzuki coupling process. PVK and PtOEP were purchased from Kanto and Porphyrin Products Inc. respectively. Films for room and low temperature PL (photoluminescence) measurements were formed on pre-cleaned quartz plate (Hellma). PVK and PFHP were dissolved in p-xylene and 1,2-dichloroethane, respectively, at a concentration of 0.01 g/1 g
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