Efficient three-color white light-emitting diodes from a single polymer with PFN/Al bilayer cathode

In this work, a bilayer cathode, composed by alcohol-soluble poly[(9,9-dioctyl-2,7-fluorene)-alt-(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)] (PFN) and high work-function Al, was utilized for efficient electron injection in non-doped white light-emitting diodes (WPLEDs) based on an electroluminescent single conjugated polymer PFO-R005-G010 that could show red, green, and blue (RGB) emissions simultaneously. The non-doped WPLEDs with the bilayer cathode could display higher luminous efficiency (LE) and better white light CIE coordinates, than those of well-known Ba/Al cathode. A pure white light (CIE coordinates of (0.33, 0.33)) containing spectral ranges of both deep blue and deep red as well as a good forward-viewing maximum LE of 6.62 cd/A at luminance of 200 cd/m2 could be achieved for a device with a PFN thickness of 10 nm and a thermal annealing process before Al deposition. The WPLED could show a maximum luminance of 9500 cd/m2 and good spectral stability at high luminance range. The results suggest that material systems for high efficiency three color WPLEDs, already realized by Ca/Al or Ba/Al cathode, could be the promising candidates for WPLEDs with a bilayer cathode.