Light-emitting diode panels produce even brightness in large areas

In recent years, there has been increasing interest in using white organic light-emitting diodes (OLEDs) for lighting applications.^1–5 A principle reason for this is that OLEDs are highly efficient phosphorescent sources. In contrast to LEDs, which are high-intensity point sources, OLEDs provide an inherently diffuse light of high quality and with high color rendering index (CRI) that is naturally adapted to large-area lighting needs. In addition, OLED lighting panels can be transparent and flexible, which opens up exciting new lighting uses and design concepts.

For broad commercial success, however, key technical challenges related to the fabrication and optimization of large-area OLED light panels must be solved. First, using only standard transparent conductive oxide anodes with high sheet resistance, such as indium tin oxide (ITO), producing a large-area panel with uniform luminance is difficult. Second, in a large-area OLED light panel, Joule heating causes significant temperature elevation as the charge is transported from the electrode contacts to the emissive region. Careful panel design is, therefore, required to minimize panel temperature, especially at high-luminance operation. This is particularly important for maximizing panel life because organic materials typically degrade more quickly at elevated temperatures. Finally, the shorting defect is particularly problematic for large-area devices. An OLED device has films of organic material placed between parallel electrodes. As the organic layers are very thin (approximately a few hundred nanometers in thickness), excess rough surface features, asperities, or particles can cause the two electrodes to touch, creating a short circuit. When this happens, all the current flows through the shorted spot because of its much lower resistance, and, therefore, no current can flow through the organic layers to generate light. For large-area OLED lighting panels, such shorting defects can cause dysfunction in a significant portion of the area.

Figure 1. Equivalent circuit of organic light-emitting diode (OLED) lighting panel under (a) normal conditions and (b) where one lighting pixel has a shorting defect. V+: Voltage. GND: Ground. I: Current. R: Resistance.

Figure 2. The structure of OLED lighting pixel with a short-reduction layer (SRL): (a) cross section and (b) top view. ITO: Indium tin oxide.

We introduced an effective method to reduce these shorts. In our design of the OLED lighting backplane, we divided the the large lighting area into several individual pixels united by a metal grid, and then connected each pixel to one short-reduction layer (SRL, composed of ITO) in a series (see Figure 1). When there is a shorting defect in one lighting pixel, the current passing through the defective pixel can be substantially limited at a certain point because of the constant resistance of the SRL. Consequently, these shorting pixels no longer adversely affect the normal pixels. This method also reduces undesirable overheating that large shorting currents cause.

We also show our design schematically (see Figure 2) illustrating the structure of one lighting pixel in which one ITO layer (ITO-1) functions as an SRL and another (ITO-2) as an anode. The sheet resistance of ITO-1 and ITO-2 is 30ohm/square. ITO-1 is covered by a passivation layer and is electrically connected to ITO-2 via the through hole. Furthermore, to improve the brightness uniformity of OLED lighting panels, low-resistance metal grids composed of titanium/aluminim/titanium are embedded on ITO-1 as auxiliary electrodes. The advantage of this design is that the effective resistance of the SRL (i.e., ITO-1) can easily be determined by defining its shape (including width and length) during the photolithography process. Thus, of most importance, we can adjust the effective resistance of the SRL for different pixel sizes.

Figure 3. Image of 129×129mm²and 50×50mm²OLED lighting panels.

On the basis of this design, we successfully fabricated large-area OLED lighting panels with active areas measuring 129×129mm² with light-extraction technology. The OLED lighting panel can achieve efficacy of 40lm/W, correlated color temperature of 2800K, and CRI of >80 at a practical brightness of 1000cd/m². The aperture ratio of this lighting panel was as high as 90% owing to the high transmittance of ITO and the passivation layer, and the variation of brightness ([max.−min.]/[max.+min.]) was <20%. In addition, we have produced 50×50mm² OLED lighting panels with various emitting colors using our innovative backplane design (see Figure 3).

To summarize, we demonstrated a novel design for large-area OLED lighting panels that feature low electrode resistance and complete short reduction. At the same time, these panels produce uniformly high brightness and product yield with low power-efficiency loss. In the near future, we plan to develop large-area flexible OLED lighting with an eye-catching luminaire design.