Highly efficient fluorescent-phosphorescent triplet-harvesting hybrid organic light-emitting diodes

Abstract

We demonstrate highly efficient white and nonwhite hybrid organic light-emitting diodes (OLEDs) in which singlet and triplet excited states, generated in the recombination zone, are utilized by fluorescence and phosphorescence, respectively. The excited states are formed at a blue fluorescent light-emitting layer (LEL), and the triplets diffuse through a spacer layer to one or more phosphorescent LEL(s). A key feature enabling the triplet diffusion in such OLEDs is the use of a blue fluorescent emitter with triplet energy above, or not much below, that of the fluorescent host. Additional material properties required for triplet harvesting are outlined. At 1000 cd/m2 a blue and yellow harvesting OLED shows 13.6% external quantum efficiency, 3.8 V, 30.1 lm/W, and color characteristics suitable for display application. High-efficiency harvesting R+G+B white, and B+G and B+R nonwhite OLEDs are also demonstrated. The triplet-harvesting mechanism was verified in all devices by physical methods including spectral analysis, time-resolved electroluminescence, magnetic field effect, and electron paramagnetic resonance.

Singlet Generation from Triplet Excitons in Fluorescent Organic Light-Emitting Diodes

“…For C 60 , it has been calculated that a quintet state would physically break a C-C bond [118] and calculations for DPA, diphenylanthracene, indicate that the quintet state is too energetic to form from two triplets 60 implies that the devices has been run until its output has fallen to 60% of its initial light output. [21]; in this scenario we would thus gain 0.2 singlets from TF as shown in Scheme 1. From Kondakov et al 's work on highly efficient fluorescent OLEDs using devices based on anthracene derivatives such as DPA as a host for blue fluorescence dyes (>0.95 quantum yield) sandwiched between hole and electron transport layers, very simple devices, they demonstrated better than 8% external quantum efficiency for these devices.…”

“…However, this limit only arises if the recombination process is independent of spin. If at any stage the recombination is effected directly or indirectly by the spin configuration of the intermediate CT states [53] or the polaron capture is affected by their spins [54], then it can follow that the 25% limit is broken and possibly more singlets could be produced [21].…”

“…There is still great potential to carry on using fluorescent blue emitters, for both display and white light applications where lifetime is the most demanding and critical parameter for the devices, but we would still require a way to make use of (75%) triplet excitons formed in the fluorescent emitters. Several ideas have emerged on triplet harvesting from a blue fluorescent emitter into red and green phosphors [19][20][21]; however, the device structures required to achieve this are very complex and involve exciton blocking layers of order 2 nm thick, totally impractical to manufacture at high yield over the large areas required for lighting. These complex device structures are not used in the current generation lighting panels or displays.…”

Efficient fluorescence/phosphorescence white organic light-emitting diodes with ultra high color stability and mild efficiency roll-off

“…[8][9][10] However, one bottleneck for further improving the performance (especially the lifetimes) of fully phosphorescent WOLEDs is the lack of stable and efficient blue phosphorescent emitters which lag far behind than green and red phosphorescent emitters. [11][12][13] Although many blue phosphorescent emitters have been reported, the lifetimes of the WOLEDs based on widely used blue phosphorescent emitters, iridium(III) bis [(4,6-…”

Highly efficient pin-type OLEDs

“…[43][44][45][46][47] This concept is based on the idea that an internal quantum effi ciency of 100% is still possible in hybrid white OLEDs if the triplets formed on the blue emitter are transferred to the phosphorescent red or green emitter (i.e. the triplets are 'harvested').…”