Hindrance-Functionalized π-Stacked Polymer Host Materials of the Cardo-Type Carbazole–Fluorene Hybrid for Solution-Processable Blue Electrophosphorescent Devices

Abstract

b S Supporting Information ' INTRODUCTIONPhosphorescent organic light-emitting devices (PHOLEDs) have attracted tremendous research interests owing to the spinÀ orbit coupling interactions of heavy metals, resulting in the harvest of both singlet and triplet excitons to give the approaching 100% internal quantum efficiency theoretically. 1À4 However, it is a prerequisite for high-performance PHOLEDs to blend the triplet emitters of heavy-metal complexes into host matrixes to reduce their concentration-quenching effect. Recently, considerable progresses have been achieved in small molecular host materials based blue PHOLEDs in terms of brightness, power efficiency, and durability. 5À8 However, small-molecule based devices require complex coevaporation techniques, high vacuum, and tedious and precise control process during device fabrication, thus greatly hindering the success of product development and commercialization. In contrast, solution process methods, such as spin-coating, inkjet printing, or screen printing, show more advantages of easier fabrication process and lower cost with phosphorescent polymer light-emitting diodes (PPLEDs). 9 In this context, the key issue is to develop the excellent polymer host materials for PPLEDs, especially for blue PPLEDs.Similar to small molecular host materials, high triplet energy (E T ) levels are crucial for suitable polymer host materials to effectively prevent reverse energy transfer from emitters to hosts and confine triplet excitons on the triplet emitters in PPLEDs. 10À12 In general, π-conjugated polymers usually have low E T levels, which are not suitable as host materials for blue and green phosphorescent heavy-metal complexes, 13,14 such as poly(2,7-fluorene)s and its derivatives which are the widely used π-conjugated polymer host materials possess low E T of 2.15À2.3 eV. 15,16 One effective method to increase the E T levels is to design the nonlinear π-conjugated polymers via the method, limiting the delocalization length of carriers and excitons. However, the obtained π-conjugated polymers, such as poly(3,6-carbazole)s (E T = 2.53À2.6 eV), 17,18 poly(3,6-fluorene)s (E T = 2.58 eV), 19 poly(3,6-silafluorene)s (E T = 2.55 eV), 20 and poly(m-phenylene) derivatives (E T = 2.64 eV) 21 still have too low E T levels to act as efficient host materials for typical blue electrophosphorescent emitters, such as bis[(4,6-difluorophenyl)pyridinato-N, C 2 (picolinato)iridium(III) (FIrpic) (E T = 2.65 eV). Other π-conjugation-interrupted technique also provides a useful strategy to improve the E T levels of polymer host materials through the introduction of sp 3 -hybrized carbon, silicon, or oxygen. 22 However, there are still rare polymer host materials for blue electrophosphorescent emitters up to date.With respect to π-conjugated polymers, π-stacked polymers as supramolecular semiconductors exhibit the unique carrier and exciton features in the applications of various polymer semiconducting devices. 23,24 The design of π-stacked polymers opens another way to develop the ...

Carbazole-based polymers as hosts for solution-processed organic light-emitting diodes: Simplicity, efficacy

“…Using this strategy, the blend composed of P32, 30 wt% of tert-BuPBD as the electron-transporting material and 8 wt% of Ir(mppy) 3 gave remarkable performances with a luminous efficiency of about 35.1 cd/A at 100 cd/m 2 , a brightness of 6700 cd/m 2 at 10 V. While searching other side-groups that could further improved performances, only the phenothiazine-functionalized polymer P33 showed performances approaching those obtained with P32 (29 cd/A at 100 cd/m 2 , 6100 cd/m 2 at 10 V). Hole-dominated transportation of PVK can be easily counterbalanced upon functionalization of the carbazole moiety, what was done with P34 [128]. Thus, P34 with its E T of 2.80 eV, its improved electron transport ability compared to that of PVK and its bulky substituent that efficiently hinders p-p stacking in the solid state and then dimerization of carbazoles could efficiently host FIrpic, furnishing blue PLEDs with a maximum CE of 14.2 cd/A and a maximum PE of 2.8 lm/W.…”

Organic Flexible Electronics

“…For conjugated‐polymer emitters, polyphenylenevinylene (PPV), polyfluorene (PF), and poly( p ‐phenylene) (PPP) are typical examples that emit in the visible region . In addition, π‐stacked poly(vinylcarbazole) (PVK) is a typical nonconjugated polymer with a relatively high triplet energy state along with excellent hole‐transporting ability, which is also used extensively as a host material . On the other hand, the design of π‐stacked polymers opens another avenue to the development of polymer hosts for red, green, and blue emitters.…”

Flexible Metal‐Free Memory Electronic Made of π‐Conjugation‐Interrupted Hyperbranched Polymer Switch and Reduced Graphene Oxide Electrodes

“…These devices exhibit not only excellent thermal stability owing to the rigid carbon‐rich aromatic structures, but also high throughout fabrication due to their outstanding solubility by means of hyperbranched structures and alkyl side chains. [ 17–19 ] This structural design provides a meaningful reference for highly stable polymer memory materials.…”

“…While the PCzPF‐PF exhibits weaker red‐shift (16 nm) and green emission in thin film state, indicating that π–π stacking alignments of Cz groups are partly blocked by the bulky PF units owing to the hindrance effect. [ 19,20 ] The green emission band might be attributed to the fluorenone defects induced by the degradation of fluorene‐based PCzPF and PCzPF‐PF. [ 21 ]…”