The ‘double dendron’ approach to host free phosphorescent poly(dendrimer) OLEDs

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

“…PLED performances based on conjugated polymers have been found to depend on their PDIs. 33,34 In line with the results obtained from polystyrene analogues (Ir-P1, DIr-P1 and DDIr-P1), 31 it has been shown a clear positive correlation between the number of dendrons on the ligands and the U PL of the polymers both in solution and neat films. For example, U PL of Ir-P3, DIr-P3 and DDIr-P3 in DCM solutions were 48%, 65%, and 71%, respectively.…”

“…31 They contrasted three polymers Ir-P1, DIr-P1 and DDIr-P1 (Fig. 7) to investigate how the number of dendrons on the central iridium(III) phosphor impacted the photophysical properties and device performances of the polymers.…”

“…32 The key merit of these polymers was their narrower polydispersity index (PDI) of ca. 1.4 than those with polystyrene backbone (Ir-P1, DIr-P1 and DDIr-P1) 31 prepared by free-radical copolymerization (e.g., DDIr-P1 had a PDI as high as 4). PLED performances based on conjugated polymers have been found to depend on their PDIs.…”

“…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.…”

“…For example, the highest EQE obtained by DDIr-P1-co-VK copolymer was 14.7%, 30 nearly 50% higher than that of DDIr-P1 homopolymer (EQE: 9.2%). 31 One likely reason is that the 1,3-bis(2-ethylhexyloxyphenyl)phenyl dendron was neither a p-type nor a n-type moiety, and; hence, charge transport in the emitting layer was hampered. 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.…”