Highly efficient hybrid inorganic-organic light-emitting diodes by using air-stable metal oxides and a thick emitting layer.

During the last two decades, considerable progress has been made with the device efficiency and lifetime of organic lightemitting diodes (OLEDs), enabling their application in flatpanel displays and solid-state lighting. To realize high-performance, full-color OLED devices as commercial products, it is necessary to further improve the power efficiency and lifetime of the devices. To date, research in this direction has focused on the fabrication of multilayered devices with the goal of overcoming low device efficiencies and short operation lifetimes in small-molecule-based OLEDs and polymer light-emitting diodes (PLEDs). However, production costs are critical for competition with existing panel displays, such as liquid-crystal displays, and current lighting devices, such as fluorescent lamps. In this regard, the simplification of OLEDs without loss of the optimized performance becomes very important. In addition, air-stable solution-processed LEDs without air-sensitive charge injection materials are important for realizing rollto-roll mass production of flexible OLEDs, because it is difficult to apply conventional encapsulation techniques to the production of flexible OLEDs. Strategies for fabricating simplified air-stable devices without a thin low-work-function metal (i.e. , Ca or Ba) beneath the Al electrode have proven to be flawed when applied to conventional structures (Figure 1a), because most of the holes can recombine with electrons near the Al electrode, leading to significant quenching of the excitons. Therefore, it is of particular importance to incorporate air-stable, efficient charge injection contacts into the devices. Although several approaches using ionic polymers that are soluble in polar solvents have been employed, they have required conventional encapsulation techniques for stable operation because of their strong susceptibility to moisture. A recent promising approach towards overcoming these issues is to use inorganic metal oxides as charge injection contacts (Figure 1b). These are attractive because of their good transparency in the visible range of the spectrum, good charge transport properties, and excellent air stability. Most recent studies have reported hybrid inverted PLEDs in which n-type metal oxides, such as titanium oxide (TiO2), [8–11] zirconium oxide (ZrO2), [12] or zinc oxide (ZnO), 11,13, 14] are employed as bottom electron injection contacts on top of indium tin oxide (ITO) or fluorine-doped tin oxide (FTO), and thermally evaporated molybdenum trioxide (MoO3) is used as the top hole injection layer beneath a Au positive electrode (Figure 1b, with or without a Cs2CO3 layer). [8–14] The problem with hybrid PLEDs has been their relatively low luminous efficiency, attributed to the high current density flowing through the device without radiative electron–hole recombination. Because MoO3 provides effective p-type doping at an interface with emitting layers, barrierless hole injection can be achieved in these devices. As shown in Table 1, Mori et al. reported the fabrication of hybrid PLEDs using poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) as emitting layer and TiO2 as electron injection contact. These devices showed a maximum brightness of 700 cdm 2 at 6 V and a maximum luminous efficiency on the Figure 1. Schematic conventional and hybrid inverted PLED device structures. a) Conventional PLED with a thin emitting layer (50–200 nm), using PEDOT:PSS as hole injection layer and Ca as electron injection contact layer. b) Hybrid inverted PLED with a thin emitting layer (50–200 nm), using ZnO as electron injection contact layer and MoO3 as hole injection layer. c) Conventional PLED with a thick emitting layer (1450 nm), using PEDOT:PSS as hole injection layer and Ca as electron injection contact layer. d) Hybrid inverted PLED with a thick emitting layer (1450 nm), using ZnO as electron injection contact layer and MoO3 as hole injection layer.