Organic light-emitting diodes (OLEDs) have recently become attractive for solid-state lighting applications.1,2 They can be as efficient as compact fluorescent lamps, but they turn on instantly, are continuously dimmable, have better color-rendering capabilities, and a large, potentially flexible, emitting surface (see Figure 1). This can make them one of the lighting solutions for the 21st century. Unfortunately, organic semiconductors (OS) used in OLEDs are usually rather bad electrical conductors as they have low free-charge-carrier densities. Thus, undoped intrinsic organic layers have a low conductivity and often present large injection barriers for charge carriers from adjacent electrodes. In OLEDs, these properties decrease the efficiency as they cause a large operational voltage and a difference between the number of holes and electrons required (the charge carrier balance).3 The introduction of p- and n-doped charge carrier injection layers can reduce these effects and improve the OLED's efficiency.
Figure 1. A large-area, white organic LED.
For a long time, the injection barrier for electrons from the OLED's cathode into the electron transport level (lowest unoccupied molecular orbital: LUMO) of the first organic layer was lowered by reactive metals with low work function Φ (e.g., barium, calcium, or magnesium) or a thin interlayer of a salt (e.g., lithium fluoride).4–6 This does not, however, improve the conductivity of the organic layer. It is not as difficult to inject holes from the transparent indium tin oxide (ITO), acting as the anode, because its work function is often closer to the hole transport level (highest occupied molecular orbital: HOMO) of the organics, resulting in only a small barrier. We have developed a new p-type doping system with good hole transport and injection characteristics. It also has the advantage of being cheap, easily processable and almost transparent to visible light. We have also investigated n-doping which shows analogous behavior.
We co-evaporated a typical hole transport material N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'biphenyl-4,4”diamine (α-NPD) and the nearly colorless rhenium heptoxide (Re2O7) from two thermal evaporation sources, forming a nearly transparent 150nm thick OS layer on an ITO-covered glass substrate. A 200nm aluminum layer was evaporated on top as a counter electrode. The co-evaporation of the Re2O7 led to a p-doped OS, increasing the free-charge-carrier density and thus its conductivity σ. This is the result of an electron transfer from the α-NPD to the Re2O7 via a charge transfer complex (leaving a free hole on the α-NPD).6 Compared to a pure 150nm α-NPD layer with σ≈10−15S/cm, a doping concentration of 10% Re2O7 increased the conductivity by seven orders of magnitude, and a concentration of 50% Re2O7 by ten orders of magnitude. The doped layers in an OLED have much lower electrical resistances than the intrinsic light-emitting layers, and so they can be quite thick without a significant increase in the OLED's operational voltage. Therefore, the transport layer thickness can be easily adjusted to tune the optical microcavity for optimum light extraction.
The other important advantage of doped transport layers is the improved injection of charge carriers from electrodes into the organics. This is important for the charge carrier balance within a white OLED (which affects the device efficiency and color). Due to the increased free-charge-carrier density of the doped layers, their HOMO and LUMO levels are strongly bent at the Schottky barriers between the organics and the electrodes. This allows charge carriers to be injected more easily via a tunneling process (thermionic field emission).
To investigate the effect on the hole injection process, we realized another four devices by evaporation (see Figures 2 and 3 for the device layouts and corresponding current density-voltage curves). Hole injection from ITO (Φ=−5.0eV) into the intrinsic α-NPD (HOMO =−5.4eV) is already quite good and only needs around 1V to start a significant current. However, without a p-doped layer between the intrinsic α-NPD and the aluminum (Φ=−4.2eV) electrode, there is hardly any hole injection from the aluminum as its work function is too low (i and p-i). But even for aluminum, good hole injection can be reached with a doped transport layer (i-p and p-i-p). This allows the use of nearly any electrode material for charge carrier injection into organic devices, regardless of its work function.
Figure 2. Device layout for investigation of charge carrier injection. p: p-doped layer; i: intrinsic layer.
Current density-voltage curves of the devices shown in Figure 2
In summary, we used the easily evaporable Re2O7 to show the p-doping effects on the organic semiconductor α-NPD. A mixed layer from these materials shows low light absorbance. It has a much higher conductivity than regular α-NPD, and it also allows easier charge carrier injection from adjacent electrodes. In complete OLED devices, p- and n-doped layers next to the anode and cathode, respectively, are often used to reach highest efficiency, making doped layers with low absorption loss especially important. Such OLEDs have already been shown by several groups, including our own. We will continue our research efforts to identify inexpensive, long-living doping materials for less absorbant doped transport layers in commercial OLEDs.
Ralf Krause, Frank Steinbacher, Jan Hauke Wemken, Günter Schmid, Arvid Hunze