Double-Layer Structured WPLEDs Based on Three Primary RGB Luminescent Polymers: Toward High Luminous Efficiency, Color Purity, and Stability

Efficient white polymer light-emitting diodes (WPLEDs) were fabricated on the basis of three kinds of luminescent polymers: poly{2-[3‘,5‘-bis(2‘ ‘-ethylhexyloxy)benzyloxy]-1,4 phenylenevinylene}- co -poly(2-methoxy-5-(2‘-ethyl-hexy loxy)-1,4-phenylene vinylene) (BE-co-MEH-PPV), red; poly(6,6‘-bi-(9,9‘-dihexylfluorene)- co -(9,9‘-dihexylfluorene-3-thiophene-5‘-yl)) (PFT), green; and poly(9,9-dioctylfluorene-2,7-diyl) (PFO), blue. In order to overcome the complicated cascade energy transfer and deep trapping process, the two narrow band gap red and green light-emitting polymers are separated into two layers. In the bottom emission layer, hole-transporting material poly( N,N ‘ -bis(4-butylphenyl)- N,N ‘ -bis(phenyl)benzidine (poly-TPD) and electron-blocking material poly( N -vinylcarbazole) (PVK) were chosen to work together as binary-host; concomitantly, the novel red light-emitting polymer BE-co-MEH-PPV was employed as guest. In addition, in order to avoid phase separation in the top emission layer, two fluorene-derivative polymers PFO and PFT were used as host and guest, respectively. Through these improvements, the resulting WPLEDs showed stable white light emission with a maximum luminescence efficiency of 4.4 cd/A and a maximum brightness of 4420 cd/m 2 at 17 V.