1. Department of Electrical and Electronic Engineering, Research Institute for Smart Energy (RISE), Photonics Research Institute (PRI), The Hong Kong Polytechnic University 1 , Hung Hom, Kowloon, Hong Kong, People's Republic of China
2. Department of Materials Science and Engineering, Center of Super-Diamond and Advanced Films (COSDAF), Hong Kong Institute of Clean Energy (HKICE), City University of Hong Kong 2 , Hong Kong, People's Republic of China
3. Department of Physics, Research Centre of Excellence for Organic Electronics, Institute of Advanced Materials, Hong Kong Baptist University 3 , Kowloon Tong, Hong Kong, China