High-efficiency inverted quantum dot light-emitting diodes with enhanced hole injection

Abstract

Hybrid MoO/HAT-CN is employed as a hole injection layer (HIL) in green inverted colloidal quantum dot light-emitting devices (QLEDs). The hybrid HILs can be easily prepared and have been found to effectively improve the electroluminescent properties. The best performance device had an HIL of 1.5 nm-thick MoO/2.5 nm-thick HAT-CN and showed a turn-on voltage of 1.9 V, a maximum current efficiency (CE) of 41.3 cd A, and maximum external quantum efficiency of 9.72%. Compared to the corresponding devices with the single MoO or HAT-CN interlayer, the CE of the hole-only devices was improved by 1.6 or 1.5 times, respectively. The measured electrical performance shows that hole-only devices with hybrid HILs have a smaller leakage current density at low driving voltage and much enhanced hole injection current than the devices with single interlayers. It indicates that much improved electroluminescent efficiency in green inverted QLEDs with hybrid MoO/HAT-CN orginates from the significant enhancement of hole injection efficiency and suppression of space charge accumulation in the quantum dot-emitting region due to the improved balance of the charge carriers. The hybrid HILs can be extended to other color inverted QLEDs, which are favorable to achieve bright, highly efficient, and color saturation devices for display applications.

The influence of the hole transport layers on the performance of blue and color tunable quantum dot light‐emitting diodes

“…Figure shows the schematic device structure and the energy band diagram of blue QLEDs. Various organic materials such as poly[N,N′‐bis(4‐butylphenyl)‐N,N′‐bis(phenyl)‐benzidine] (poly‐TPD); 4,4′‐bis(carbazole‐9‐yl)biphenyl (CBP); 4,4′,4″‐tri(9‐carbazoyl)‐triphenylamine (TcTa); and N,N′‐dicarbazolyl‐3,5‐benzene (mCP) have been used as HTLs for QLEDs . For appropriate HTLs, the materials should have deep‐lying HOMO levels and high hole mobility so that hole injection is efficient.…”

“…The mismatching of energy levels leads to the inefficient hole injection thereby causes the imbalance of charge injection, which further promotes the nonradiative Auger recombination . In addition, the excess electrons that cannot recombine with the holes could run away and finally captured by the anode, forming the overflow or leakage current that results in the reduction of the device efficiency …”

“…15 In addition, the excess electrons that cannot recombine with the holes could run away and finally captured by the anode, forming the overflow or leakage current that results in the reduction of the device efficiency. 16 To solve these problems, the hole injection should be effectively enhanced by using HTLs with deep-lying HOMO levels and high hole mobility. Among all reported HTLs, poly(9-vinylcarbazole) (PVK) 4,14,[17][18][19] and poly[9,9dioctylfluorene-co-N-(4-(3-methylpropyl))diphenylamine] (TFB) 8,14,20 are the two most widely used materials due to their advantages of deep-lying HOMO levels and high hole mobility, respectively.…”

“…Various organic materials such as poly[N,N 0 -bis(4-butylphenyl)-N,N 0 -bis(phenyl)-benzidine] (poly-TPD); 4,4 0 -bis(carbazole-9-yl)biphenyl (CBP); 4,4 0 ,4″tri(9-carbazoyl)-triphenylamine (TcTa); and N,N 0dicarbazolyl-3,5-benzene (mCP) have been used as HTLs for QLEDs. 14,16,[24][25][26][27] For appropriate HTLs, the materials should have deep-lying HOMO levels and high hole mobility so that hole injection is efficient. In addition, the HTLs also function as electron blocking layer, and therefore, they should have low LUMO levels to block the excess electrons overflowing from the QDs.…”

Stability of Quantum Dots, Quantum Dot Films, and Quantum Dot Light‐Emitting Diodes for Display Applications

“…Various metal oxides have been utilized in the hole‐injection layer (HIL) and/or HTL of QLEDs, including tungsten oxide (WO x ), molybdenum oxide (MoO x ), nickel oxide (NiO), copper oxide (Cu x O), and vanadium oxide (VO x ) . These metal oxides have excellent stability and suitable energy levels for efficient charge injection/transport, and can also prevent the penetration of water or oxygen molecules .…”