Electroluminescence efficiency, triplet

The /3-diketonate [Nd(dbm)3bath] (see figs. 41 and 117) has a photoluminescence quantum efficiency of 0.33% in dmso-7r, solution at a 1 mM concentration. It has been introduced as the active 20-nm thick layer into an OLED having an ITO electrode with a sheet resistance of 40 il cm-2, TPD as hole transporting layer with a thickness of 40 nm, and bathocuproine (BCP) (40 nm) as the electron injection and transporting layer (see fig. 117). The electroluminescence spectrum is identical to the photoluminescence emission the luminescence intensity at 1.07 pm versus current density curve deviates from linearity from approximately 10 mA cm-2 on, due to triplet-triplet annihilation. Near-IR electroluminescent efficiency <2el has been determined by comparison with [Eu(dbm)3bath] for which the total photoluminescence quantum yield in dmso-tig at a concentration of 1 mM is Dpi, = 6% upon ligand excitation, while its external electroluminescence efficiency is 0.14% (3.2 cdm-2 at 1 mAcm-2) ... 

LUO 10] Luo Y., Aziz H., Correlation between triplet-triplet annihilation and electroluminescence efficiency in doped fluorescent organic light-emitting de ices ,AdvancedFunctionalMaterials, vol. 20, no. 8, pp. 1285-1293, 2010. 

In electroluminescent applications, electrons and holes are injected from opposite electrodes into the conjugated polymers to form excitons. Due to the spin symmetry, only the antisymmetric excitons known as singlets could induce fluorescent emission. The spin-symmetric excitons known as triplets could not decay radiatively to the ground state in most organic molecules [65], Spin statistics predicts that the maximum internal quantum efficiency for EL cannot exceed 25% of the PL efficiency, since the ratio of triplets to singlets is 3 1. This was confirmed by the performance data obtained from OLEDs made with fluorescent organic... 

If the lowest energy elementary excitations are bound singlet and triplet excitons, the theoretical maximum EL quantum efficiency is 25% of the photoluminescence (PL) quantum efficiency. On the contrary, if separated electron and hole polarons are the elementary excitations, the maximum theoretical EL efficiency can approach unity. Thus, a quantitative comparison of the quantum efficiencies for electroluminescence and photoluminescence can, in principle, provide fundamental information on the nature of the excited states. 

In addition to the incorporation of fluorescent chromophores in PF, phosphorescent moieties have been widely used to copolymerize with fluorene or as pendent group in PFs for color tuning (via the selection of ligands chelated to heavy metals) and performance improvement. The ability to enhance device performance via phosphors is because strong spin-orbit coupling (resulting from the inclusion of heavy metal atoms in the phosphor structure) can efficiently utilize triplet excitons for electroluminescence and theoretically there are three times as many as triplet as singlet excitons. 

---