Star‐Shaped Boron‐Containing Asymmetric Host Materials for Solution‐Processable Phosphorescent Organic Light‐Emitting Diodes

Abstract

Boron‐containing compounds have attracted considerable attention because of their electron‐accepting properties, and they are widely used in a variety of fields. However, due to the essential requirement to protect the empty pz‐orbital of the boron atom using large steric hindrance or rigid groups, borane derivatives generally show poor solubility and are rarely reported as acceptor units to construct bipolar host materials for phosphorescent organic light‐emitting diodes (PhOLEDs). Here, a combined star‐shaped and asymmetric donor–acceptor molecular design strategy to improve the solubility and fine tune the optical and electronic properties of boron‐containing materials is presented. High thermal stability, solvent solubility, solution processability, and triplet energy are achieved simultaneously. With the thus‐designed boron‐containing bipolar molecules as host materials, the solution‐processed PhOLEDs exhibit high device performances, which are comparable to the vacuum‐processed counterparts, showing high external quantum efficiencies up to 18.5% and 14.5% in blue and white PhOLEDs, respectively. These results demonstrate the great potential of the star‐shaped and symmetry‐breaking borane derivatives in solution‐processable organic optoelectronic devices.

The Strategies for High‐Performance Single‐Emissive‐Layer White Organic Light‐Emitting Diodes

“…Besides the device fabrication with thermal evaporation, many all‐phosphorescence SEL‐WOLEDs were prepared by solution‐process due to the unique merits of simple fabrication procedure and the easy control of the emitter doping concentration. [ 55,116–120 ] The detailed EL characteristics of the solution‐processed SEL‐WOLEDs are summarized in Table 3 . To obtain high‐efficiency solution‐processed WOLEDs, the morphological control and balanced charge injection/transport are the key factors.…”

Multiple σ–π Conjugated Molecules with Selectively Enhanced Electrical Performance for Efficient Solution‐Processed Blue Electrophosphorescence

“…The basic metrics of solution‐processed hosts are excellent electrical property for endowing efficient and balanced carrier injection and flux, and high triplet energy ( E T ) levels for maintaining exothermic energy migration from the hosts to dopants and confining the triplet excitons in EML . Furthermore, additional requirements of hosts have to be simultaneously fulfilled to support high‐efficiency solution‐processed PhOLEDs: i) good solubility in common organic solvent to fabricate stable, uniform and pin‐hole free films, ii) high thermal and chemical stability to endow high temperature thermal annealing for eliminating organic solvents and, iii) excellent compatibility with the dopants to reduce the phase separation and elongate the device operation lifetime. Therefore, it is a daunting challenge to exploit solution‐processed host matrixes for high‐performance blue PhOLEDs, since the blue hosts should simultaneously possess wide bandgap and high E T over 2.8 eV for exothermic energy transfer, and efficient carrier injection and flux for improving charge balance and broadening the recombination zone …”

Phosphine Sulfide-Based Bipolar Host Materials for Blue Phosphorescent Organic Light-Emitting Diodes

“…Bipolar hosts, generally prepared by combining the donor (D) and acceptor (A) units into a molecular skeleton (i.e., D–A molecules), are capable of achieving a broad recombination zone and balanced charge in the emitting layer (EML), so they are considered one of the most promising types of hosts to realize highly efficient PhOLEDs [ 6 , 7 , 8 ]. Up until the present, the bipolar hosts for green, yellow, and red phosphorescent emitters have made commendable progress [ 9 , 10 , 11 , 12 ], while it is still challenging to develop superior blue hosts because the blue phosphors inherently possess high triplet energy ( E T ) levels [ 13 , 14 , 15 ], thus the E T of the hosts should be accordingly elevated to prevent the reverse energy transfer from the guests back to the hosts and to effectively confine triplet excitons on the guests. However, the non-negligible intramolecular interactions in D–A molecules would produce a low energy charge transfer (CT) state, which brings down the E T levels [ 16 ].…”