Comparison of ultraviolet- and charge-induced degradation phenomena

Comparison of ultraviolet- and charge-induced degradation phenomena

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Comparison of ultraviolet- and charge-induced degradation phenomena in blue Fuorescent organic light emitting diodes Ruben Seifert, a ! Sebastian Scholz, Björn Lüssem, and Karl Leo b ! Institut für Angewandte Photophysik, Technische Universität Dresden, D-01062 Dresden, Germany s Received 22 April 2010; accepted 9 June 2010; published online 9 July 2010 d We compare the degradation of organic light emitting diodes s OLEDs d by UV light and by electrical driving. We prove that the exponential dependence of the half-lifetime on the current density known from electrical aging is also valid for UV-degradation. The inFuence of excitons on the degradation of OLEDs is determined and we experimentally distinguish between the inFuence of singlet and triplet excitons. We conclude that singlet excitons are the main cause of degradation for Spiro-DPVBi s 2,2 8 ,7,7 8 -tetrakis s 2,2-diphenylvinyl d spiro-9,9 8 -biFuorene d -based OLEDs by a comparison of the degradation of electrically driven and UV-excited OLEDs. © 2010 American Institute of Physics . f doi: 10.1063/1.3460285 g Since the invention of the ±rst ef±cient organic light emitting diode s OLED d by Tang and VanSlyke 1 in 1987, there has been intense research in the ±eld of degradation phenomena of OLEDs. However, the lifetime of OLEDs is still a challenge, in particular for blue devices. While red OLEDs achieve lifetimes of < 1 000 000 h s Ref. 2 d at 1 300 cd / m 2 and green OLEDs 100 000 h s Ref. 3 d at 500 cd / m 2 , blue OLEDs reach lifetimes of only 21 000 h s Ref. 4 d at 1 000 cd / m 2 . Various results show that excitons play a major role in degradation phenomena of organic materials. Scholz et al. 5 demonstrated that excited 4,4 8 ,4 9 -tris s carbazol-9-yl d - triphenylamine and iridium s III d bis s 2-methyldibenzo- f f,h g chinoxalin ds acetylacetonat d molecules 6 dissociate and cause a chemical degradation process. However, it is un- known if this process is caused by a singlet or a triplet exci- tation. Kondakov 7 showed by calculations that the lowest excited singlet state is able to lead to a bond dissociation of N,N 8 -diphenyl-N,N 8 -bis s 1-naphthyl d -1,1 8 - biphenyl-4,4 8 - diamine and 1,1-bis s di-4-tolylaminophenyl d cyclohexane. ²or 4,4 8 -bis s Ncarbazolyl d biphenyl s CBP d it has been shown 8 that the formation of the degradation product corresponds to the absorption of the ±rst singlet excited state of CBP. How- ever, experiments which directly relate electrical and photo- aging have not been reported so far. In this paper, we investigate the inFuence of different UV-intensities on the degradation of OLEDs using the ef±- cient Fuorescent blue emitter 8 8 -tetrakis s 2,2- diphenylvinyl d 8 -biFuorene s Spiro-DPVBi Ref. 9 d as sample material and prove that the same equations can describe UV-degradation and electrical degradation at differ- ent current densities. To separately study the inFuence of singlet and triplet excitons, UV-degradation is used to excite singlet excitons only. By comparing electrical and UV aged samples, singlet and triplet induced degradation can be dis- tinguished.
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This note was uploaded on 03/27/2011 for the course CHEM 2211L taught by Professor T.a. during the Spring '08 term at University of Georgia Athens.

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Comparison of ultraviolet- and charge-induced degradation phenomena

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