1. Institute of Polymer Optoelectronic Materials and Devices; State Key Laboratory of Luminescent Materials and Devices; South China University of Technology; Guangzhou 510640 P. R. China
2. Department of Physics and Astronomy; Shanghai Jiao Tong University; Shanghai 200240 P. R. China
3. Materials Sciences Division; Lawrence Berkeley National Lab; Berkeley CA 94720 USA
4. Polymer Science and Engineering Department; University of Massachusetts; Amherst MA 01003 USA