Quantum dot (QD) light-emitting diodes (QLEDs) with an inverted architecture suffer from charge-injection imbalance and severe QD charging, which degrade device performance. Blocking excess electron injection into QDs is crucial for efficient inverted QLEDs. It is observed that polyethylenimine (PEI) has two opposite effects on electron injection: one is blocking electron injection by its intrinsic insulativity and the other one is promoting electron injection by reducing the work function of ZnO/PEI. In this work, the insulating nature of PEI has been dominantly utilized to reduce electron injection and the charge-injection balance is realized when PEI becomes thicker and blocks more excess electrons. Furthermore, PEI contributes to QD charging suppression and results in a smoother surface morphology than that of ZnO nanoparticles, which is beneficial for leakage current reduction and QD deposition. As a result, the optimized QLED with 15 nm PEI shows a 2.5 times improved efficiency compared to that of the QLED without PEI. Also, the QLED possesses the maximum external quantum efficiency and current efficiency of 16.5% and 18.8 cd/A, respectively, accompanied with a low efficiency roll-off of 15% at 1000 cd/m, which is comparable to that of the reported inverted red QLED with the highest efficiency.

By conjugating carbazole moiety to the different positions of the rigid skeleton 1,2-diphenyl-1Hphenanthro [9,10-d]imidazole, a series of hybrid bipolar phosphorescent hosts was synthesized, and their photophysical properties were investigated. The introduction of a rigid phenanthroimidazole moiety greatly improves their morphological stability, with high decomposition temperatures (T d ) and high glass transition temperatures (T g ) in the range of 394−417 and 113−243 °C, respectively. The highly efficient green and orange phosphorescent organic light-emitting diodes (PhOLEDs) have been achieved by employing these compounds as the phosphorescent hosts. For the device of ITO/MoO 3 (10 nm)/NPB (80 nm)/TCTA (5 nm)/mPhBINCP:9 wt % Ir(ppy) 3 (20 nm)/TmPyPB (45 nm)/LiF (1 nm)/Al (100 nm), a maximum luminous efficiency (η c,max ) of 77.6 cd/A, maximum power efficiency (η p,max ) of 80.3 lm/W, and maximum external quantum efficiency (η EQE,max ) of 21% were obtained. Furthermore, these hosts are also applicable for the orange phosphorescent emitter (fbi) 2 Ir(acac), a yellow PhOLEDs with pPhBICP as host, for which a performance of η c,max of 57.2 cd/A, η EQE,max of 19.3%, and η p,max of 59.8 lm/W was achieved. These results demonstrated that the phenanthroimidazole unit is an excellent electron-transporting group for constructing the bipolar phosphorescent host.

All‐inorganic perovskite has attracted much attention because of the higher stability. Many organic additives such as alkyl chain ammonium and polymers are usually introduced into perovskite to improve their performance. However, the long chain ammonium cations in perovskite may restrain the carrier transfer ability and ultimately deteriorate the performance of light‐emitting diodes (LEDs). In this work, the CsPbBr3 nanoparticles (NPs) are in situ fabricated by the synergistic effect of poly(ethylene oxide) and phenethylammonium bromide (PEABr). Particularly, sodium bromide (NaBr) with better conductivity is successfully introduced into CsPbBr3 NPs to substitute PEA partially, ultimately to passivate the defect and promote the carrier transfer ability. Besides, the addition of NaBr results in a better promotion for electron mobility than for hole mobility leading to a more balanced charge transport in devices. It enables NaBr based CsPbBr3 NPs green LEDs to exhibit a maximum external quantum efficiency (EQEmax) of 17.4%, which presents obvious enhancement compared to the LEDs without NaBr (EQEmax = 12%). Further, NaBr based CsPbBr3 NPs LEDs with a large area of 108 mm2 still show a high maximum EQE of 10.2%. Above all, this work provides a feasible way of adding metal additive in perovskite films to improve the performance of perovskite LEDs.