Triphenylamine‐Dendronized Pure Red Iridium Phosphors with Superior OLED Efficiency/Color Purity Trade‐Offs

Abstract

Since the first synthesis of red-phosphorescent metal complexes for use in highly efficient organic light-emitting diodes (OLEDs), [1] the scope and diversity of studies on metalorganic phosphors in color tuning have continued to expand at an exponential rate. [2,3] While great success has been achieved in green-light phosphors, the design and synthesis of efficient red emitters is intrinsically more difficult, in accordance with the energy-gap law.[4] Many red organic dyes currently in use do not show a good compromise between device efficiency and color purity because of the nature of red-light emitters.[4] Typically, efficient and bright dopants are not red enough, and red-enough dopants are not efficient and bright. Optimization of the OLED efficiency/color purity trade-offs is therefore a key consideration for the realization of highly efficient phosphors of good color purity for pure red OLEDs.Recent research endeavors in the field of OLED devices have focused on solution-processable phosphorescent dendrimers and polymers owing to their advantages as low-cost, largearea displays and lighting sources.[5] In this regard, however, only a limited number of examples of phosphorescent dendrimers have been developed, [6] and their solution-processed OLEDs show a maximum external quantum efficiency (h ext ) of 5.7 % for red light.[6c] In such cases, the first-generation molecules are not very effective in relieving molecular interactions among the emissive cores, [6e] although higher-generation dendrimers can usually reduce the carrier mobility substantially. [7] Despite this situation, the apparently poorer performance of the OLEDs fabricated by spin-coating than that of their vacuum-deposited counterparts suggests the need for more efforts in developing highly amorphous Ir III complexes with new dendritic frameworks. Generally, a large hole-injection (HI) barrier for organic materials often limits the device efficiency. Since most of the hole-transporting (HT) materials reported are derived from aromatic amines, [8] the incorporation of arylamine units into the dendritic wedges should improve the HI/HT properties and the morphological stability of the Ir III phosphors. In light of this, pure red-emitting bifunctional Ir III complexes with triphenylamine dendrons (Ir-G1 and Ir-G2) have been synthesized and compared with Ir-G0. The new dendrimers were isolated as highly amorphous (T g % 145-220 8C) and thermally stable (decomposition starts above 417 8C) solids. The T g value increases gradually with dendron size.The Ir III dendrimers show two major absorption bands in their UV/Vis spectra (Figure 1). The intense bands below 380 nm are assigned to the spin-allowed intraligand 1 p-p* transitions. The weaker, lower-energy features in the visible region are due to the 1 MLCT and 3 MLCT (metal-ligand charge transfer) excitations induced by the strong spin-orbital coupling between the singlet and triplet manifolds. Upon light irradiation at 420 nm, all Ir complexes show deep-red photo-

Organic Optoelectronic Materials

“…This was attributed to the strong π-stacking along the b axis in the α-phase. Mas-Torrent et al had developed a series of TTF derivatives [180,181]. Because the long alkyl chains promoted intermolecular π-π overlapping because of their extremely closely packed nature, TTF-4SC18, 149 easily assembled into one-dimensional stacks and can be easily solution-processed by zone-casting.…”

Highly efficient pure white polymer light-emitting devices based on poly(N-vinylcarbazole) doped with blue and red phosphorescent dyes

“…Moreover, OXD-7 was also added to facilitate electron transport in the emissive layer, and its high triplet energy level (2.7 eV) can avoid the quenching of the excited states of the phosphorescent materials. Two phosphorescent iridium complexes were used in this work: the blue emitting Ir complex, iridium(III) bis(2-(4,6-difluorophenyl)-pyri-dinato-N,C2') picolinate (FIrpic) was purchased from the American Dye Sources Inc., and the pure red-emitting iridium complexes Ir-G2 were synthesized according to the literature [12] (Figure 1). The electron injection material poly [(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorenen)-alt-2,7-(9,9-dioctyfluorene)] (PFN) was also synthesized in our laboratory [14,15].…”

“…In particular, PLEDs based on polymeric materials or solution-processed technology [3][4][5][6] are of special interest due to their unique merits over the vacuumdeposited counterparts, such as low-cost of manufacturing, easy-processibility over large area size, compatibility with flexible substrates, etc. So far, the peak efficiency of WOLEDs based on vacuum-deposited technology has already far exceeded that of incandescent light bulbs (12)(13)(14)(15)(16)(17) lm W 1 ) [7] and very recently has reached or exceeded that of fluorescent lamps (40-70 lm W 1 ) [8].…”

Optoelectronic properties of new functionalized heteroleptic iridium complex

“…The dendritic structure allows the independent modification of the core (light emission), branching groups (charge transport) and surface groups (processing properties), which combines high efficiency, good color tunability and solution processibility. The dendritic structure played a major role in the device performance and this was explained partially by a reduction in the intermolecular interactions between the emissive species [16].…”