Multiple exciton generation regions in phosphorescent white organic light emitting devices

We demonstrate a white electrophosphorescent organic light emitting device (WOLED) with a three-section emission layer (EML) where excitons are formed in the multiple emission regions. The EML consists of a stepped progression of highest occupied and lowest unoccupied molecular orbital energies of the ambipolar hosts. Analysis shows that (36 +/- 6)% of the excitons form in the blue emitting region, while (64 +/- 6)% form in the green emitting region at 100 mA/cm(2). The doping of the red, green and blue phosphors, each in its own host, allows for efficient utilization of excitons formed in these multiple regions. Based on this architecture, the WOLED has an internal quantum efficiency close to unity, The WOLED has total external quantum and power efficiencies of eta(ext,t) = (26 +/- 1)% and eta(p,t) = (63 +/- 3) lm/W at 12 cd/m(2), decreasing to eta(ext,t) = (23 +/- 1)% and eta(p,t) = (37 +/- 2) lm/W at 500 cd/m(2). When an undoped electron transport layer is used, the peak efficiency is eta(ext,t) = (28 +/- 1)%. Due to the distributed exciton formation in the EML, the WOLED exhibits higher total efficiency than monochromatic devices employing the same red, green and blue dopant-host combinations. (c) 2008 Elsevier B.V. All rights reserved.