Effects of Charge Generation Layer Parameters on Luminance Balance Between Bottom and Top Emitting Units in Top Emitting Tandem Organic Light Emitting Diodes

A top emission tandem organic light-emitting diode (tandem OLED) consists of two vertically stacked emission units, referred to as the bottom and the top sub-cell. In principle, this architecture can achieve twice the luminance at the same current density compared with a single-unit OLED. However, it remains unclear whether the luminance and emission spectra of the two sub-cells change synchronously with the applied bias. The interplay between layer-specific parameters and the modulation effects of the charge generation layer (CGL) makes this behavior highly complex. In this study, we focus on structural optimization and performance enhancement of top emitting tandem OLEDs, with particular emphasis on the influence of CGL and electron injection layer parameters. Through systematic optimization, the emission spectra of the bottom and top sub-cells are engineered to evolve synchronously with the applied bias. Top emitting devices were fabricated on both glass and stainless-steel substrates with the following structure: reflector anode (reflective Aluminum) / bottom blue emission unit / Liq / Al / CGL / top orange emission unit / Liq / cathode (half transparent Al/Ag). In this work, both the material and the thickness of the CGL were varied. A comparison was made between HATCN/TAPC and HATCN/NPB configurations, and the results indicate that NPB exhibits better energy-level alignment with HATCN, thereby facilitating more efficient hole generation. Under an applied bias of 15 V, the current density increased from 16.8 mA/cm2 (HATCN/TAPC) to 28.8 mA/cm2 (HATCN/NPB), while the luminance improved from 41.45 cd/m2 to 371.7 cd/m2. Furthermore, increasing the HATCN thickness from 10 nm to 15 nm enhanced the hole injection efficiency into the top emission unit, leading to an additional increase in device luminance. To improve the electron injection efficiency of the bottom emission unit, the thickness of the aluminum layer on the left side of the CGL was adjusted from 4 nm to 6 nm. The device with the 6 nm layer exhibited a luminance of 714 cd/m2 at 15 V, along with a pronounced enhancement of blue emission in the electroluminescence spectrum, thereby yielding comparable emission intensities from the bottom and top units. The optimized top emitting tandem OLED structure was subsequently implanted on stainless-steel substrates. Under a current density of 100 mA/cm2, the luminance of the single blue and single orange OLEDs was 13 cd/m2 and 75 cd/m2, respectively, whereas the tandem OLED achieved approximately 250 cd/m2. At 130 mA/cm2, the luminance of the tandem OLED further increased to 305 cd/m2, significantly surpassing that of the single-unit devices. The turn-on voltage also verified the tandem configuration, with values of approximately 10.5 V for the tandem OLED compared with 4.5 V and 6 V for the single blue and orange devices, respectively. The color stable demonstrating the advantages of tandem OLEDs and highlighting their potential for applications in flexible displays and solid-state lighting. INDEX TERMS Organic light emitting diodes, tandem, charge generation layer, electron injection layer.