1. Department of Physics and Centre
for Plastic Electronics, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom
2. Institut de Ciència de Materials de Barcelona, UAB Campus, 08193 Bellaterra,
Spain
3. Key Laboratory of Luminescence
and Optical Information, Ministry of Education and Institute of Optoelectronics
Technology, Beijing Jiaotong University, Beijing 100044, People’s Republic of China