Organic light-emitting diodes (OLEDs) are solid-state lighting sources that offer a number of unique advantages over their inorganic cousins, light-emitting diodes (LEDs). For example, OLEDs are diffuse light sources and low on glare. In addition, OLEDs are thin and can be made on flexible substrates, which means they can conform to almost any shape. However, OLED lighting is still in its infancy, and several research challenges – including reducing costs, improving reliability, and commercializing the high-efficacy performance that’s been demonstrated in the laboratory – still need to be overcome.
To provide information and analysis to the lighting industry, the U.S. Department of Energy (DOE) has released the latest in a series of reports about OLED lighting technology. The new report – Round 2 Update of Stress Testing Results for Organic Light-Emitting Diode Panels and Luminaires – builds on earlier DOE studies of OLED technology by updating information about previously benchmarked OLED products in accelerated stress tests (ASTs). In addition, the new report provides an initial analysis of the performance of the recently released Brite 3 OLED panels from OLEDWorks.
ACCELERATING THE AGING PROCESS
During the ASTs, products were operated continuously at mildly elevated ambient temperatures (either 35 °C or 45 °C) in order to accelerate the aging process, so that their degradation pathways could be studied in a reasonable period of time. As a control, some samples of each product were also operated continuously at room temperature. In addition, a population of Brite 3 panels have just begun testing in a more aggressive temperature-humidity environment of 65 °C and 90% relative humidity, to further accelerate aging.
Based on the stress testing, it is clear that the luminous flux maintenance (maintaining constant light output with age) of the current OLED products is improving but still remains lower than that of some mid-power LEDs made in 2011. Luminous flux degradation found in this testing can be modeled by using standing lighting industry methods such as a single-exponential decay function after an initial period. Improvements in the thermal management of OLED panels tended to produce gains in the luminous flux and the chromaticity maintenance of the panels. Differential loss of light emission from the OLEDs’ blue, green, and red light-emitting molecules produced chromaticity shifts that were significant in early products but have improved in later products that use more-stable materials and have better thermal management.
Mildly accelerated temperature conditions (10 °C to 20 °C above room temperature) were found to provide meaningful acceleration of OLED failure modes and can reproduce field failures in greatly reduced time periods. Abrupt failure of OLED panels generally occurs through a shorting mechanism that may be caused by the formation and growth of organic particles. The tendency for panels to fail abruptly is reduced in later products, making abrupt failures less likely, but dark spots still tend to occur over time. In addition, a steady increase in power provided by the driver was measured in most cases, with a concomitant decrease in efficiency.
CONTINUING TO IMPROVE
DOE testing shows that the performance of OLED panels continues to improve. The panel shorting and chromaticity maintenance issues that readily occurred in early products are less likely in more-recent ones. However, additional testing is needed to determine whether these failure modes have been completely eliminated, or if their probability for occurrence has been reduced. There are still issues with OLED lighting that must be addressed for it to provide high energy efficiency across products’ lifetime – such as achieving additional gains in luminous efficacy and doing something about the increasing power required for operation as the device ages. The gains in OLED lighting performance and reliability that have been achieved are encouraging and signal the possibility for OLEDs to become significant complements to LEDs in the future. Additional research focusing on new materials with improved performance and higher reliability will help to unlock the commercial potential of this technology in indoor lighting applications.