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13 September 2012 Blue-emissive polymer light-emitting diodes through anode/cathode interfacial modification
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We report highly efficient blue polymer light-emitting diodes (PLEDs) achieved by introducing two nanoscale interfacial layer, made of poly(fluorine-co-triphrnylamine) [PFO-TPA] and cesium carbonate (Cs2CO3), between (1) the PEDOT:PSS and blue poly[9,9-diarylfluorene-co-2,5-Bisphenyl-1,3,4- oxadiazole] (P1)and (2) the aluminum cathode and the P1 emitter, individually. PFO-TPA with highest occupied molecular orbital level (-5.36 eV) lies between those of PEDOT:PSS (~5.0 ~ 5.2 eV) and P1 emitter (~5.54 eV), forming a stepwise energy ladder to facilitate the hole injection. For Cs2CO3, firstly, it enhances the injection of electrons by providing an lower electron injection barrier. Secondly, applied Cs2CO3 buffer decreases the PL intensity slowly down to ~96 % of the pristine P1 film, located at 422 nm, is less efficiency quenched than the Calcium (Ca). Therefore the overall electron injection attributed by Cs2CO3 buffer is higher. Thirdly, the device with Cs2CO3 buffer did not show the low-energy emission band originated from the fluorenone defects which are often introduced by Ca, thus stabilized blue emission from devices with high brightness can be demonstrated. Based on the hole-transporting PFO-TPA and the Cs2CO3/Al cathode, we obtained device efficiency and brightness as high as 13.99 cd A-1 and 35054 cd m-2, which is an improvement by two orders of magnitude higher over devices using Ca/Al as cathode and without hole-transporting PFO-TPA.
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Ming-Wei Lin, Chin-Hsin Yeh, Ten-Chin Wen, and Tzung-Fang Guo "Blue-emissive polymer light-emitting diodes through anode/cathode interfacial modification", Proc. SPIE 8476, Organic Light Emitting Materials and Devices XVI, 84760K (13 September 2012);

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