Development Of Lifetime Test System For Organic Light-emitting Diode And Its Aging Mechanisms

Organic Light-Emitting Diode(OLED) has gained great favor of colleges and enterprises and rapid development because of its excellent performance these years. OLED is considered as the next generation of display and lighting technology because its self-illumination and low power consumption for realizing full-colour display and solid-state lighting. Up to now, researchers pay more attention to improve the luminous efficiency of OLED, but ignore the importance of lifetime. Actually, one of the critical issues that limit the commercial use of OLED is lifetime. As the factors affecting the lifetime of OLED is various and complicated, OLED degradation can’t only attribute to a certain kind of aging mechanism, but the combined results of various aging mechanisms. So it’s really difficult to simultaneously solve a variety of aging mechanisms to improve OLED lifetime dramatically. At present, OLED lifetime test equipment is limited and basically monopolized by South Korea, and it’s hard to meet the demands of lifetime test of mass production because of its sole function and low automation, seriously restricting OLED industrialization processIn our work, 512-channel OLED lifetime test system which is researched and developed independently in our lab is presented first. This system is developed based on Lab VIEW development platform, and the flexible software design combined with modular hardware make it convenient to build up any channel OLED lifetime test system. The multi-thread parallel architecture and reusable code of the system realize the lifetime test of 512-channel OLEDs simultaneously. Meanwhile, the system can operate continuously and steadily for more than 5000 h with 512-channel OLEDs under test, which meets the requirements of a long time testing. The system has a broad range of test conditions with high precision, working current from 0 to 100 m A and working voltage from 0 to 20 V. The system also provides three kinds of test mode, constant-current, constant-voltage and constant-brightness, which can be used for further analyzing OLED aging mechanisms.In general, OLED lifetime test and the study of aging mechanisms proceed under constant-current test mode. Under constant-current test mode, the continuous decay of brightness is disadvantageous for future application of OLED in lighting panel and display screen. For lighting panel, brightness decay not only affects the service lifetime and lighting effects, but also reduces the power efficiency. For display, brightness decay causes the image sticking effect and colour deviation, which seriously affects the user’s view experience. So it’s really necessary to realize the constant-brightness driving mode of OLED.Here, we use the home-made 512-channel OLED lifetime test system to carry out OLED constant-brightness lifetime test, and the dependence of the current density and voltage on time is obtained, which provides the guidance for the design of the circuits to realize the constant brightness. From the curve of current density over time, the increase of the current density can be divided into three stages. In order to maintain the constant brightness, the current density of OLED increases linearly and slowly for a long period at the beginning of the lifetime test, which is defined as the first aging stage. At the second aging stage, the current density rises exponentially with shorter time. During the third aging stage, the current density vertically rises, featuring the catastrophic failure of the device within several minutes. We also define the time of the first aging stage as the lifetime of the constant-brightness driving mode, which fits the lifetime-brightness relation found in the constant-current driving mode, i.e. Ln×Tth=Const., where L is the brightness and n is the acceleration exponent. Comparing the current density-voltage-brightness characteristic and PL efficiency of the device before and after constant-brightness driving, it can be seen that the aging of OLED is the combined results of the reduction of the charge injection efficiency and the degradation of the bulk active layer. Meanwhile, the thermal effect during the constant-brightness driving mode indicates the power consumption of OLED raises the junction temperature, eventually leading to catastrophic failure of the device. We also prove that the nature of the aging of the constant-brightness driving mode is the quenching of the emissive centers.This article also analyzes the influence of temperature on the aging mechanism of OLED. It is found that the degradation of OLED can also be considered as the combined results of coulombic decay and thermal decay. By comparing the constant-brightness driving mode with the constant-current and constant-voltage driving mode, it is found the constant-brightness driving mode is also a kind of accelerated aging process. However, the constant-brightness driving mode not only gets a good lifetime, but also provides the users with better view experience, which meets the commercial demands of OLED.