Recently, tandem white organic light-emitting diodes are attracting a great deal of interest due to their potential illumination applications. A tandem structure is a mean two or more electroluminescence units that joined together through connecting units in series. So, we can make sense that the connecting unit has play an important role in this particular structure. In my paper research, the functions of connecting units are usually being considered with two aspects: charge transporting and charge generation. But, it was rarely discussed which function is the key point. Three devices were fabricated in this study : (¢¹) unit device, (¢º) tandem device 1 with connecting layers of Alq3:Li/MoO3 and (¢») tandem device 2 with connecting layers of Alq3:Li/HAT-CN. Base on this two connecting unit structure, we can get a quantifiable data from adopting, carrier transporting and carrier generation and figure out which function of connecting unit in tandem device is the main factor. The unit device has outstanding performance comparing to others fluorescence white OLED. It exhibits a maximum luminance of 49820 cd/m2 at 1755 mA/cm2, the power efficiency of 2.52 lm/W at 20 mA/cm2 and CIE (0.33, 0.33). But the power efficiency decreased to 2.07 lm/W at 20 mA/cm2 in tandem device 1. In order to improve device performance, we choose new material HAT-CN to replace to MoO3. From our result, the tandem device 2 have increase power efficiency from 2.07 lm/W to 2.24 lm/W at 20 mA/cm2, and the maximum luminance reach 71790 cd/m2 at 650 mA/cm2 with CIE(0.30, 0.39). We want to figure out why the power efficiency increased after using HAT-CN in tandem device. So we further designed and fabricated the hole only device, electron only device and capacitance device to figure out the charge transporting and charge generation behavior between these two connecting unit constructed based on Alq3:Li/MoO3 and Alq3:Li/HAT-CN. After analyzing the results of these devices, we found that the charge generating plays a major role in tandem OLED¡¦s performance. In other words, if the more carriers can be generated in the connecting layers, the higher power efficiency in tandem device can be realized.