Efficient Phosphorescence by Reducing Intrachain Chromophore Interactions in Dendrimer-Containing Polymers

Abstract

Poly(dendrimers) comprised of a poly(styrene) backbone with dendrimer side chains containing an iridium(III) complex core, first-generation biphenyl dendrons, and (2-ethylhexyl)oxy surface groups show increased viscosity compared to their individual dendrimer components, which is important for inkjet printing processes. However, intrachain interchromophore interactions lead to lower photoluminescence quantum yields even in solution relative to the simple isolated dendrimers. We demonstrate that the phosphorescence efficiency of a polymer can be enhanced by incorporating the dendrimer monomer unit(s) into a copolymer with poly(styrene) spacer units. The poly(styrene) spacer units remove the intrachain interchromophore interactions between the chromophores in solution. The copolymer gives a >50% increase in solution photoluminescence quantum yields (to 94%) and an improved organic light-emitting diode performance with an external quantum efficiency of 6.7% at 100 cd/m2 at 11 V when compared to the homopolymer with the same dendrimer side chain.

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

“…7, vide infra), 31 implying efficient energy transfer from carbazole moieties to the phosphors. Their U PL values were found to be 69%, 64%, and 64%, respectively, which were lower than DIr-P1-co-S (U PL : 94%) 29 due to back triplet energy transfer from the phosphors to the carbazole units (poly(Nvinylcarbazole has a low triplet energy of 2.5 eV). .…”

“…29 They prepared a random copolymer (DIr-P1-co-S, Fig. 5) between DIr-P1 and polystyrene by free-radical copolymerization.…”

“…35 They found that the PLQYs of the polymer neat films decreased with increasing Ir(ppy) 3 phosphor concentrations (0.5-29 mol.%), suggesting the presence of undesirable interchromophore interactions in the film when the phosphor content became more concentrated, which is consistent with the observations reported in the aforementioned studies. 27,29,31 Although the copolymer with 0.5 mol.% Ir(ppy) 3 phosphor exhibited the best PLQY of 81% among the series, a minimum of 3 mol.% phosphor was required to complete the energy transfer from the host to the dopant. Thanks to the bipolar characteristics of the host to facilitate both hole and electron injections and transport, a single-layered PLED [ITO/AQ1200/Poly(M6-MAco-Ir-2C-MA) ($30 nm)/LiQ (2 nm)/Al (100 nm)] was fabricated, where a 6 mol.% phosphor was found to show the best compromise between PLQY and charge transport in the polymeric emitting layer.…”

“…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.…”