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′-dihexy-lfluorene)-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/m² at 17 V.