Improving the performance of organic light-emitting diodes

Abstract

Organic light-emitting diodes (OLEDs) are promising candidates for large-area full-color flat panel displays due to their ease of fabrication and convenience for many applications.1 OLEDs work through the passage of an electric current across a fluorescent or phosphorescent organic layer resulting in an excitation/emission profile of the material used. With OLEDs, the injection efficiency of electrons is a critical parameter and depends to a great extent on the work function (the minimum energy needed to move an electron out of a substance) of the electrode. A thin hole-injection layer (HIL) or an anode buffer layer (ABL) between the indium tin oxide (ITO) anode and the organic emitting layer are usually adopted to enhance the performance of the hole-injection process.2–6 Thus, current electroluminescent devices typically have the following layered configuration: ITO anode/HIL or ABL/organic emitting layer/tris(8-hydroxyquinoline) aluminum (Alq3)/lithium fluoride (LiF)/aluminum cathode. Our recent work suggests that either an HIL composed of metal phthalocyanine (MPc) or an ABL of Li-doped zinc oxide (LZO) should improve the holeinjection efficiency.7–9 The organic, inorganic, and Al layers of our test device were successively deposited using vacuum vapor evaporation at room temperature. The LZO powders with a doped concentration of 5% Li were prepared by sintering a mixture of ZnO and Li2CO3 powders in air. Various MPc layers were tested for their effect on injection efficiency (see Figure 1). The turn-on voltage of the devices decreases from 5.3V to 4.3V when CoPc or CuPc layers are inserted: see Figure 1(b). Compared to the non-MPc device, higher emission efficiency was observed in all MPc devices. The CuPc device achieved the highest efficiency as shown in Figure 1(c). For the same emission intensity, the higher efficiency suggests that a much lower current density is required. Figure 1. Characteristics of organic LEDs with a variety of MPc hole-injection layers: (a)current density versus voltage (J-V), (b) luminescence versus voltage (L-V), and (c) luminescence versus current density (L-J). All the devices consisted of a stacked structure of: ITO/MPc(10nm)/NPB(60nm)/Alq3(75nm)/LiF(1nm)/Al(200nm). NPB: N,N-bis(naphthalen-1-yl)-N,N-bis(phenyl)benzidine, a hole transport material.