**2. Experimental methods**

100-nm-thick indium tin oxide (ITO) thin film coated glass substrates were commercially bought with a sheet resistance of 10-30 per square, used as the anode in the conventional OLEDs and as the cathode in the inverted OLEDs. After being cleaned in acetone, alcohol, and de-ionized water sequentially by an ultrasonic horn, the patterned ITO substrates were blown dry by nitrogen gun and then treated in UV-ozone for 15 min. The base vacuum pressures of the thin film and device fabrications were 110-5 to 410-4 Pa.

Copper phthalocyanine (CuPc, electron donor) and 3, 4, 9, 10 perylenetetracarboxylic dianhydride (PTCDA, electron acceptor) were used to form the interfaces of generating holes and electrons. Molybdenum oxide (MoO3) and NPB were adopted as hole injection and transport materials, respectively. Tris(8-quinolinolato) aluminum (Alq3) and BCP were chosen as the emissive and electron transport materials, respectively. The lithium carbonate was selected as an n-dopant to the PTCDA and BCP. 1,4,5,8-naphthalene-tetracarboxylicdianhydride (NTCDA) was also chosen to form an n-doped material with ITO.

The current versus voltage (I-V) characteristics of the devices were measured by a programmable Keithley 2400 sourcemeter or a Keithley electrometer 617, and the device luminance was recorded by an ST-86LA spot photometer. The optical absorption spectra of organic thin films were obtained using a Cary 300 spectrophotometer or an UV-3100 spectrophotometer. The X-ray diffraction (XRD) measurements were performed on an X-ray diffractometer (D/max-RB).
