Degradation mechanisms in organic light-emitting diodes

Abstract

Recent discoveries of degradation mechanisms in organic light emitting diodes (OLEDs) are reviewed. One common observation regarding the OLED reliability is the gradual increase of the diode driving voltage under forward bias. This trend reverses when the bias polarity is reversed, and the initial driving voltage can be recovered almost completely. The mobile ions are proposed to explain the phenomenon. By solving a system of transient equations governing the mobile ion motion under an external field, we obtained the transient mobile ion distributions and their contribution to the driving voltage. We found that the mobile ion model with reasonable assumptions could very well explain the experimental results. Another advancement in the field is the utilization of a single organic alloy layer to improve the device lifetime. A typical organic alloy layer consists of a mixture of hole and electron transport materials. Significant lifetime improvement over heterojunction structures is achieved with comparable efficiency. The significant improvement in the device reliability is attributed to the elimination of the heterojunction interface and the minimization of the formation of Alq3 cations. The high efficiency is achieved due to the balanced carrier injection, transport and confined recombination in the device.

Degradation Mechanisms and Reactions in Organic Light-Emitting Devices

“…306,307 These results suggest that there is more than one mechanism involved in the degradation of the investigated devices. The ion mobility model itself cannot explain the incomplete recovery of the OLED performance, especially the behavior of the luminance.…”

“…295,306,307 Sato et al mentioned that luminance loss and voltage increase during device aging may be independent from each other. 15 Of course, it is obvious that the overall efficiency of a device strongly depends on the power consumption; therefore, the voltage increase has a significant influence on the device characteristics over time.…”

“…306,307,309 Usually, voltage rise is attributed to an increased injection barrier at some interface and/or a decrease in conductivity of some transport layer, for example, because of accumulation of traps. 23 Multiple interfaces and transport layers may contribute to this effect.…”

A New AMOLED Pixel Circuit With Pulsed Drive and Reverse Bias to Alleviate OLED Degradation

“…The extrinsic causes are, for example, particle contamination and inevitable humidification [7] in the fabrication process. As for the intrinsics, they are due to accumulative holes [8], the impurity of movable ions [9], [10], and the Indium diffusion [11] induced by high-density large currents under long-time driving. To tackle the OLED degradation due to long-time current drives, some compensation methods were developed in a few past works [12].…”

Frequency Dependency of Multi-layer OLED Current Density-voltage Shift and Its Application to Digitally-driven AMOLED

“…As J OLED -V OLED characteristics shift with operation time, lifetime of the digitally-driven AMOLED will be further reduced. Therefore, in order to suppress J OLED -V OLED shifts, pulsed driving or negative biasing methods [7][8][9] can be additionally combined with digital driving methods because it is well known that AC driving or negative biasing improves J OLED -V OLED characteristics [10][11][12] by affecting redistribution of space charge, movement of ionic impurities, and rotations of permanent dipoles in the typical multilayer OLEDs [13][14][15]. In addition, for thin film devices where charge trapping and defect creation are dominant degradation mechanisms, improvement of the operation stability has been reported depending on the bias stress frequencies although rather low frequencies ranging from 1 to 4 Hz were used [16].…”