© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com COMMUNICATION Harvesting All Electrogenerated Excitons through Metal Assisted Delayed Fluorescent Materials Zhi-Qiang Zhu, Tyler Fleetham, Eric Turner, and Jian Li* Dr. Z.-Q. Zhu, Dr. T. Fleetham, Dr. E. Turner, Prof. J. Li Material Science and Engineering Arizona State University Tempe, AZ 85287, USA E-mail: Jian.Li.1@asu.edu DOI: 10.1002/adma.201401772 are close, the triplet excitons can decay radiatively through the combination of intersystem crossing ( T 1 →S 1 ) and delayed fluo- rescence ( S 1 →S 0 ) processes. This approach has the benefit of being able to achieve a higher energy emission for a given tri- plet energy which enables the incorporation of these emitters into known stable host and transport materials unlike many deep blue Ir or Pt emitters. [9] Nevertheless, this process is nec- essarily endothermic and a portion of the triplet excitons will decay nonradiatively due to the absence of an efficient phos- phorescent emission process, thus high efficiencies can only be achieved for very small S 1 – T 1 energy splitting. [10] In this communication, we demonstrate another mechanism of utilizing electrogenerated excitons, denoted metal-assisted delayed fluorescence (MADF) process, where a heavy metal ion will be incorporated into the complex system to ensure both efficient phosphorescence and delayed fluorescent processes. As shown in Figure 1d, when the energy levels of the T 1 state and the S 1 state are reasonably close, the two radiative decay process, i.e., phosphorescence ( T 1 →S 0 ) and thermally activated delayed fluorescence ( S 1 →S 0 ) can potentially occur simultane- ously. Due to its efficient triplet emission process, the MADF emitters can harvest all of singlet and triplet excitons regard- less of comparably larger energy difference between the T 1 and S 1 states. We have synthesized (Supporting Information) and characterized two green emitting palladium complexes, i.e., PdN3N and PdN3O, which exhibit both an efficient phospho- rescent and delayed fluorescent processes with various ratios. Devices of PdN3N achieved nearly 21% peak external quantum efficiency (EQE) and also demonstrated remarkable device operational stability to 90% initial luminance (LT90) estimated at over 20 000 h at 100 cd m -2 . The absorption spectra for PdN3N as well as the N3N ligand are shown in Figure 2. Both the complex and ligand exhibit very strong absorption bands below 400 nm ( ε > 10 4 cm -1 ) assigned to 1 π–π* transitions, localized on the cyclometalating ligands. The small shift to lower energy of these transitions in the complex is attributed to the preferable planar molecular geometry of the ligand when covalently bonded to the Pd ion as well as the anionic nature of the ligand in the complex. The intense bands in the 400–500 nm region ( ε ≈ 10 3 –10 4 cm –1 M -1 ) are redshifted relative to all the absorption bands attributed to the ligand and are assigned to singlet metal to ligand charge transfer ( 1 MLCT) transitions. The 77 K photoluminescence (PL) emission spectrum shows a narrow primary emission peak at 522 nm with small vibronic peaks characteristic of many phosphorescent emit- ters. The emission at 522 nm is attributed T 1 →S 0 transition on the basis of the large Stokes shift from the absorption cut-off. This is a higher wavelength than many existing phenyl-pyridine complexes due to its extended conjugation through the Organic light emitting diodes (OLEDs) are widely touted as a leading candidate for next generation displays and solid state lighting technologies. [1] Through diligent device and materials design, OLEDs emitting efficiently across the visible spectrum have been achieved. [2] Nevertheless, a number of challenges remain, particularly, the development of stable and efficient blue emitters remains a substantial deficit for the on-going efforts in the field of organic displays and lighting. [3] While a handful of deep blue emitters have achieved emission efficien- cies comparable to their analogs emitting in “green” and “red” region, they have demonstrated much lower operational sta- bility than their counterparts. [4] Moreover, it has been speculated that the formation of triplet excitons tends to directly facilitate the dissociation of σ-bonds, as has been demonstrated for Si Si in polysilane materials and other material systems, indi- cating a greater challenge for developing stable deep blue triplet emitters. [5] Thus, from the energy standpoint, it will be ideal to develop efficient blue emitters with triplet energy in “green” or “red” region which can also harvest all of “blue” singlet and tri- plet excitons. Such requirements have exceeded the individual capability of existing blue fluorescent emitters, which cannot harvest the triplet excitons, or green phosphorescent emitters, which cannot emit in the blue region. Thus, a special and inno- vative molecular design will be needed to achieve such a goal. The investigations on the detailed mechanisms of harvesting electrogenerated excitons inside of organic electrolumines- cent devices have been well documented in the past two dec- ades. [6] For most organic fluorescent emitters, fluorescence ( Figure 1a) is the main pathway for their radiative decay process where phosphorescence is severely suppressed due to its sym- metry forbidden character. On the other hand, cyclometalated Ir and Pt complexes have fast intersystem crossing and rapid phosphorescence process (Figure 1b) due to strong spin–orbit coupling, enabling themselves to harvest both electrogenerated singlet and triplet excitons, resulting in a theoretical 100% elec- tron to photon conversion efficiency. [7] Recent studies on car- bazolyl-dicyanobenzene derivatives and copper(I) based metal complexes, characterized as thermal activated delayed fluores- cent (TADF) emitters, have also demonstrated high emission quantum yield at the room temperature and can be utilized in the device settings to harvest both singlet and triplet excitons. [8] As illustrated in Figure 1c, when the energy levels of the lowest triplet excited state ( T 1 ) and the lowest singlet excited state ( S 1 ) Adv. Mater. 2015, DOI: 10.1002/adma.201401772 www.advmat.de www.MaterialsViews.com