Site Isolation in Phosphorescent Bichromophoric Block Copolymers Designed for White Electroluminescence

Abstract

White organic light-emitting diodes (WOLEDs) have attracted great attention for their potential use in full color displays and solid-state lighting applications due to several advantages, such as low cost and flexibility. To date, the most efficient WOLEDs have used small phosphorescent molecules in multilayer structured devices prepared by high vacuum vapor deposition. The key issue in these systems is that the phosphorescent emission produced by each individual metal complex Ir(III) or Pt(II), [1,2] is narrow, thus requiring simultaneous emission from more than one color phosphor to illuminate across the visible region. Typically this is achieved through a combination of either three different chromophores emitting blue, green, and red, or of two different ones emitting green/blue and orange/red. If more than one phosphorescent emitter is present in a device, the electroluminescent color may be affected by the energy transfer (both Förster and Dexter) between emitters. Vapor deposition enables isolation of the various emitters to minimize the energy transfer and achieve the desired goal of multiple emission using techniques such as patterning, [3] stacking, [4,5] layered isolation, [6,7] and exciton management. [8]

Recent Advances in Polymer Organic Light-Emitting Diodes (PLED) Using Non-conjugated Polymers as the Emitting Layer and Contrasting Them with Conjugated Counterparts

“…The more advanced living radical polymerization such as nitroxide-mediated polymerization (NMP) allows polymers with much lower PDIs (1.2-1.4) to be synthesized. 36,46,51 Nucleophilic aromatic substitution (SNAr) polycondensation resulted in a low M n of 6.9 kDa yet with a decent PDI of 1.5 in a 55% yield 42 but the success of this polymerization method strongly depends on the degree of activation of the electrophilic carbon to be attacked, and; hence, its generality can be limited. 43 Vinyl addition polymerization of norbornene-based monomer offered a very high M n of 163 kDa, but suffered from a low yield of 37% and a high PDI of 2.8.…”

“…132 Both of these polymerization methods are able to produce block copolymers. 46,133 However, strongly nucleophilic organomagnesium species is inevitably present in GRIM, which severely limit the scope of monomers that can be polymerized, whereas radical polymerization is well known to have high functional group tolerance. 128,129 Interestingly, block copolymers are unpopular in PLED applications: only one non-conjugated block copolymer was reported recently 46 and conjugated block copolymer was absent.…”

“…46,133 However, strongly nucleophilic organomagnesium species is inevitably present in GRIM, which severely limit the scope of monomers that can be polymerized, whereas radical polymerization is well known to have high functional group tolerance. 128,129 Interestingly, block copolymers are unpopular in PLED applications: only one non-conjugated block copolymer was reported recently 46 and conjugated block copolymer was absent. 134,135 A critical parameter of OLED materials is purity because impurities can act as charge trapping sites and quenchers during device operation.…”

“…A similar phenomenon has been observed in DIr-P1-co-S in which the insulating styrene units limited the device efficiency as a result of compromised charge mobility. 29 Two strategies for white PLED have been discussed: biphasic Ir-P7(10B-co-xR-MW) for chromophore isolation 46 and concentration control of the high-energy and low-energy phosphors in (Ir-P5-x-y) for mutual emission. 44 The achieved EQEs were 1.5% and 7.1%, respectively.…”

“…14) to achieve site isolation in which energy transfer from the high-energy phosphor to the low-energy one could be controlled so that white emission could be generated. 46 The first block consisted of poly(TPA) randomly copolymerized with a small amount of blue phosphor (10 wt.%) while the second block contained poly(OXD) randomly copolymerized with an even smaller amount of red phosphor (0.1-2 wt.%). The two blocks were always of the same length to allow systematic study of how block length impacted the morphological properties of the polymer.…”