Carrier transfer and luminescence characteristics of thickness‐dependent organic light‐emitting diodes using transporting material as the host of emitting layer

Abstract

We have systematically investigated hole/electron distribution and exciton dynamics in blue fluorescent and yellow phosphorescent organic light‐emitting diodes (OLEDs). We demonstrate that the importance of appropriate charge carrier confinement and exciton management for the realization of highly efficient and stable OLEDs. For yellow phosphorescent device based on Ir(MePQ)2(acac), we have studied the behavior of various intrinsic emission zones on the characteristics of OLEDs with the hole/electron‐transporting matrix. The current efficiency increases from 15.7 to 31.7 cd A−1 with the rise in electron‐transporting matrix from 20 to 80 nm. We attributed the improved performance to the strong hole‐transfer capabilities and relatively weak electron‐donating abilities of Ir‐complex doped transporting matrix. In contrast, the efficiencies of blue devices decreased with increasing emitting layer (EML), suggesting that the rate of hole‐electron pair recombination continuously decrease with the increasing EML thickness due to blue fluorescent dopant hindering hole transfer. In addition, the complementary‐color white OLEDs (WOLEDs) with various EML thickness were also demonstrated. Based on their performance, we have found that the use of thin EML in WOLEDs can significantly improve optoelectronic conversion efficiency, whereas deteriorated device properties reversely.

Reliability of organic light-emitting diodes in low-temperature environment*

“…The main reason for this is probably the presence of unbalanced hole and electron transport in devices at low temperature, which results in the change in recombination zone, thereby affecting the EL spectra. [26,27] We further studied the operation lifetime of the OLED devices at different low temperatures of 25 • C, 0 • C, and −20 • C. For typical fluorescent OLEDs, T 80 is usually chosen as the comparison index, which refers to the time when the luminescence drops to 80% of the initial luminescence at a constant current density. [28][29][30] Figure 6 shows the time evolution of the luminance, and the change in voltage from its initial value, ∆V = |V (t = 0) −V (t)|, all devices were measured under conditions of vacuum chamber (10 −1 Pa).…”