Largely improving the lifetime by inserting a Yb metal layer into the charge generation layer in tandem organic light-emitting diodes

Compared to the single-unit organic light-emitting device (OLED), the tandem OLED connects multiple light-emitting units in series through the charge generation layer (CGL), thereby improving the device's current efficiency and lifetime. In this paper, we found that an ultra-thin Yb metal layer with low work function, high transmittance, and large atomic radii can be added to the CGL as an ion-blocking layer to prolong tandem OLEDs' lifetime. A green light tandem OLED utilizing Bphen:Cs2CO3/Yb/HAT-CN/NPB as the CGL exhibited an impressively longer lifetime (T90) of 308 h at an initial brightness of 10000 cd/m2, which is 362 times longer than that of a device without a Yb layer inserted (T90 = 0.85 h) and is 6.3 times greater than that of a single-unit OLED (T90 = 49 h). In addition, the tandem OLED with Yb can achieve a maximum current efficiency (CEmax) of 113.5 cd/A and a maximum power efficiency (PEmax) of 68.6 lm/W, which are 2.3 times and 1.28 times higher, respectively, compared to those achieved by a single-unit OLED. The insertion of the Yb metal layer promotes the injection of electrons from HAT-CN to Bphen:Cs2CO3 and can well curb the performance degradation caused by the diffusion of the two materials at this interface, thereby greatly improving the device lifetime. In particular, the performance and lifetime of the tandem OLED, composed of this CGL, remain largely unaffected when varying the Yb thickness within the range of 2∼4 nm, indicating that the device has a wide process window. Lastly, a high-performance white light tandem OLED with a Yb layer was obtained, achieving a CEmax of 75.2 cd/A, a PEmax of 40.9 lm/W, and a T90 lifetime of 65 h at the initial brightness of 10000 cd/m2. The results indicate that metal Yb is suitable for the evaporation process of organic materials and can withstand a significant margin of error, showing great potential in the production of tandem OLEDs.