White organic light-emitting devices (WOLEDs) based on phosphorescent blue and yellow emitters were fabricated, while p-type di-(4-(N,N-ditolyl-amino)- phenyl)cyclohexane (TAPC) and n-type 2,2′,2″-(1,3,5-benzenetriyl)- tris(1-phenyl-1-H-benzimidazole) (TPBi) were separately utilized as a homogeneous host for both blue and yellow emissive layers (EMLs). Then, various spacers were inserted between the two EMLs for performance characterization. The results showed that for the TAPC-host devices, a device using 4,7-diphenyl-1,10-phenanthroline (Bphen) as the spacer had a maximum current efficiency (CE) of 11.3 cd/A, while stable white light emission with Commission Internationale del'Eclairage (CIE) coordinates of (0.394, 0.435) at a bias of 5 V was observed. Similarly, among the TPBi-host devices, a device using 4,4′-bis(carbazol-9-yl)biphenyl (CBP) as the spacer exhibited a maximum CE of 18.1 cd/A, accompanied by negligible color variation with the CIE coordinates of (0.284,0.333) at 5 V. For the double-EML devices, the improved device efficiency and color stability by introducing proper spacer was attributed to broadened recombination region and efficient energy transfer between the EMLs. © 2011 Elsevier B.V. All rights reserved.
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