Improving the performance of organic light-emitting diodes

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 hole-injection efficiency.^7-9

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.

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.

This may decrease the joule heating during operation and thus increase the durability of the device. Combined with photoelectron emission measurements, we also find that the turn-on voltage of the devices decreases significantly with the increase of the highest occupied molecular orbital levels against a vacuum of the MPc films. This indicates that the turn-on voltage is predominantly determined by the energy barrier for hole injection at the MPc/NPB interface and not that at the ITO/MPc interface. The OLED with a CuPc layer emits a bright and uniformly green light with clearly defined pixels (see Figure 2).

Figure 2. Flexible patterned OLED with some of the pixels on and some off in different ITO lines.

The threshold voltage increased with the insertion of the LZO layer. However, it decreased markedly when the LZO layer was treated with UV-ozone (a dry cleaning and surface modification method) for 20min (see Figure 3). Similarly, the urn-on voltage increased from 4V to 6V when a 1nm-thick LZO layer was deposited on ITO, but decreased to 3V when the LZO layer was treated with UV-ozone. The decrease in turn-on voltage is a reflection of improved hole-injection efficiency. The operation voltage required to produce a luminance of 100cd/m² was 6V for ITO, 8V for LZO/ITO and 5V for UV-ozone-treated LZO/ITO. The maximum luminance in the ITO device was 16780cd/m², but increased to 24850cd/m² for the UV-ozone-treated LZO/ITO device. The power efficiencies at 100cd/m² are calculated as 2.7lm/W for ITO, 0.7lm/W for LZO/ITO, and 4.3lm/W for UV-ozone-treated LZO/ITO. In addition, we think that holes were effectively injected from the anode to the organic layer, promoting the power efficiency. Photoelectron emission measurements indicated that the injection barrier for holes from ITO to NPB decreased from 0.64eV for UV-ozone-treated ITO anodes to 0.03eV for UV-ozone-treated LZO/ITO anodes. This again reduces the turn-on voltage and increases the power efficiency of OLEDs.

Figure 3. Characteristics of OLEDs with different anodes, including (a) the current density versus voltage and (b) luminance versus voltage.

This study demonstrates the effect of inserting an MPc or an LZO layer between the anode and organic emitting layers on the electro-optical properties of OLEDs. We determined that the origins of the improved turn-on voltage of the devices can be attributed to the reduction of the injection barrier at the ITO/NPB interface. Our next step is to develop high efficiency white OLEDs using these novel layers. White OLEDs with lower turn-on voltages and higher power efficiency will advance the industrialization of OLEDs displays.