Thermal behavior and indirect life test of large-area OLED lighting panels
© Pang et al.; licensee Springer. 2014
Received: 6 March 2014
Accepted: 31 March 2014
Published: 23 May 2014
In this work, we studied the thermal behavior and addressed the challenges of life testing of large area OLED devices. In particular, we developed an indirect method to accurately calculate the life time of large-area OLED lighting panels without physically life-testing the panels. Using small area OLEDs with structures identical with the tested panels, we performed the life tests at desired driving current densities at different temperatures and extracted the relationship between junction temperature and the lifetime for the particular device. By measuring the panel junction temperature during operation under the same current density and using the life time measured on small area test devices, we determine the lifetime of the panels based on the thermal dependence. We test this methodology by predicting the life time of white PHOLED panels and then physically testing the panels. The typical result for the lifetime to 80% of the initial luminance (LT80) of the panel at a constant dc current density of 10 mA/cm2 (3800 cd/m2), was predicted to be 526 hours in good agreement with the actual life-test at 10 mA/cm2 of 512 hrs. This good agreement, confirmed in different experiments, validates this novel technique as a practical life time predictor of large-area OLED lighting panels in a time saving manner.
Organic Lighting Emitting Devices (OLEDs) are considered promising candidates as next-generation solid state light sources due to their unique and exceptional features: thin form, lightweight, energy-efficient, low operational temperature, and large-area diffuse light sources with excellent visual quality [1–3]. Particularly, white OLED panels as future general illumination devices have developed rapidly over the past decades, and high-performance, long-lifetime and thin flexible white OLEDs have been demonstrated [4–9]. Today, commercial OLED lighting panels and luminaries are available in the market place with comparable performance to some LED lamps [10–17]. Impressive progress has been reached in the design of small-area devices with excellent power efficacy, good CIE and CRI numbers and long life times. A number of design challenges remain, however, to achieve similar parameters in large area panels at volume manufacturing scales. One technology challenge is the design of low resistance electrical connections while maintaining maximum aperture area. Additionally, achieving long lifetime at high luminance at the panel level is a challenge. Typically, the life time of the panels is reduced relative to small active area devices with the same structures. Poorer heat dissipation combined with larger Joule heating by the ITO substrates in the large-area devices are generally accepted explanations. The design work is typically focused on the above problems. Life testing of the panels is a necessary component of the design projects. Standard accelerated life testing of the large panels can be costly and time-consuming. The simplified panel test reported here is one approach to solving this problem.
Thermal behavior of OLED panels
Life test of small OLED pixels
where AF is the acceleration factor; t 2 is the extrapolated lifetime of the individual OLED device at an initial luminance level L 2 ; and t 1 is the measured lifetime with an initial luminance level L 1 .
where L is the luminance and the acceleration factor is 1.43. The linear dependence of LT80 on luminance (in semi-log scale) can be used to extrapolate the lifetime of the pixels at various luminance levels from the fitting equation. For instance, LT80 at an initial luminance of 1,000 cd/m2 is calculated to be 6,638 hrs.
The ALT process relies on the assumption that the degradation mechanism is essentially the same at the high current densities used to measure the acceleration factor as it is at the extrapolated lower current densities. However, this process is not suitable to accurately predict lifetimes of devices when different degradation mechanisms are introduced during the accelerated testing of a large-area OLED lighting panel. This is because, at high current density, even the most efficient large-area OLED panels generate heat over a large area that cannot be easily dissipated. This heat is typically generated by Joule heating in the bus lines, electrodes and non-emissive exciton recombination. This can lead to increased temperatures and/or temperature gradients across the OLED panel, which may result in faster and non-uniform aging . Therefore, the ALT method is not accurate in determining the lifetime of large-area OLED panels.
Although in-situ direct life testing can be applied to large-area OLED panels [29, 30], it can be very time-consuming and the test may fail due to catastrophic panel failure. Electrical shorting is a typical failure mode, which may cause an undesired abortion of the test. Therefore, it is essential to develop a reliable method to accurately and efficiently measure the lifetime of large-area OLED panels.
New life testing method
Fabricate a series of small-area test pixels with an equivalent organic stack to the one used in the large-area OLED lighting panel whose life time is to be determined.
Life-test pixels at target current densities (i.e. luminance levels) at a range of ambient temperatures in a thermally controlled environment.
Plot lifetime (LT) vs. reciprocal ambient temperature in semi-log scale at target current density that provides the target luminance L o .
Provide the large-area OLED light panel that needs to be measured.
Place the panel into a thermally controlled environment and measure voltage V as a function of ambient temperature T at a non-Joule heating current density J low (e.g. 0.1 mA/cm2), such that T ≈ T j .
- (f)Plot voltage vs. ambient temperature from step (e). The gradient of this plot gives 1/K, where K is the “K-factor” for the given OLED device stack and the testing large-area panel layout, shown in Equation 3 below.(3)
T j is then be used alongside (c) to determine lifetime for the OLED device stack in the given panel architecture.
Results and discussions
Voltage at a range of ambient temperatures measured at J low = 0.1 mA/cm 2 for the white OLED light panel with an equivalent organic stack to the test pixels
Ambient temperature [°C]
Voltage [V] Jlow = 0.1 mA/cm2
Luminance, and ΔV measured for the OLED light panel at J high = 2, 5, 10 and 20 mA/cm 2 , where, ΔV = V1 − V4 calculated ΔT j and T j based on the K factor. Also listed is extrapolated T j from Figure 7 using V 4
Δ V = V 1 − V 4
The final step in this process is to use the junction temperature to predict the lifetime of the OLED light panel. As shown in Figure 9, at 10 mA/cm2, the junction temperature calculated for the panel is T j ≈ 34°C ≈ 307 K. If we substitute this temperature into the equation for the line of best fit from Figure 6, we extrapolated LT80 = 526 hrs. This is the lifetime which we have predicted for the large area OLED light panel at room temperature at 10 mA/cm2. Similarly, lifetime at other luminance levels (i.e. current densities) can also be calculated from this method.
Summary of LT80 of a white OLED lighting panel and an equivalent small-area test pixel at 10 mA/cm 2 by prediction and by direct measurement
LT80 [hrs] at 10 mA/cm2
Pixel 2 mm2
Panel 19.11 cm2
997 (ALT result)
As can be seen from Table 3, a 13°C temperature difference results in 1.9x lifetime reduction. Improved thermal management of large-area OLED panels can enhance lifetime. Various approaches have been evaluated and developed. For example, metal shunting grids may be placed on the anode to improve heat distribution [28, 35]; an additional metal plate may be attached to the substrate as a heat sink or a thicker cathode may be used ; and replacing the conventional cover glass with thin film encapsulation also enhances heat dissipation [37, 38]. In general, employing highly efficient PHOLEDs to reduce non-emissive exciton decay and optimizing the layout design to minimize Joule heating are the two key factors that enable low temperature operation of large-area OLED lighting panels, and long lifetime.
In this work, we proposed a new method to address the challenge in life testing large-area OLED lighting panels. Based on the thermal dependence of OLED lifetime and by measuring the junction temperature of a panel under test at a target luminance, we could determine the panel lifetime according to that temperature. The method was validated on a 1911 mm2 large-area white OLED panel and the calculated LT80 ≈ 526 hours, agreed well with the life test result LT80 = 512 hours. The advantage of this indirect method is to avoid driving panels at an accelerated condition which induces degradation, as well as to save time life testing. We believe this novel method offers a reliable and practical way of predicting lifetime for large-area OLED panels.
The authors would like to thank Dr. Peter A. Levermore for his contribution in this work. This work is partially supported by U.S. Department of Energy under contract DE-SC0004281.
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