1. Institute of Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Straße 24-25, Potsdam-Golm D-14476, Germany
2. Optoelectronics of Organic Semiconductors, 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
3. Department of Physics, Swansea University, Singleton Park, Swansea SA2 8PP Wales, U.K.