Acknowledgments
We are thankful to the former students of Qi Wang and Yonghua Chen who partially contributed to the research work on hybrid WOLEDs in our group. This work was financially supported by National Basic Research Program of China (973 Program, No. 2013CB834805, 2009CB623604), National Natural Science Foundation of China (21161160442, 51333007) and Foundation of Jilin Research Council (2012ZDGG001, 20130206003GX).