Polyfluorene based blends for white light emission

Abstract

Single layer white light emitting diodes were constructed with blends containing as blue emitter and matrix host poly(9,9-dihexyl-2,7-fluorene) (LaPPS10), and the green and red emitters poly[(9,9-dihexyl-9H-fluorene-2,7-diyl)-1,2-ethenediyl-1,4-phenylene-1,2-ethenediyl] (LaPPS16) and poly[2-methoxy-5-(2-ethylhexoxy)-1,4-phenylene vinylene] (MEH-PPV), respectively, as guests components. Two blends were studied: LaPPS10:LaPPS16:MEH-PPV = 100:0.01:0.20 (w/w) (JF14) and another with the same components and a second blue emitter: a copolymer of poly(methyl methacrylate- co-methyl antracenyl methacrylate), P(MMA- co-MMAnt), with the composition LaPPS10:P(MMA- co-MMAnt):LaPPS16:MEH-PPV = 100:40:0.01:0.20 (w/w) (JF17). The PL spectra of JF14 and JF17 in the higher energy edge are characteristic of the matrix (LaPPS 10). The morphological characterization showed the formation of a biphasic structure with a matrix of LaPPS10 and segregated domains of MEH-PPV in the case of JF14 and of P(MMA- co-MMAnt) in the case of JF17. Significant differences between EL and PL spectra were observed for the blends indicating that the mechanism for the excited states or quenching processes are very different in both cases. In addition, the EL emissions suggested that the cascade mechanism for the charge migration or charge recombination or energy transfer processes are also incomplete allowing more than one type of emission. EL spectra were red-shifted in relation to PL, which was partly attributed to the trapping of energy carriers that may occur in EL, apart from energy transfer. The CIE coordinates for EL emission of JF14 blend the coordinates are (0.30, 0.32) and for JF17 blend are (0.29, 0.38) corresponding to white light emission. Spectroscopic data brought evidence of the formation of the β phase in the polyfluorene matrix, and due to differences in band gap, it was concluded that hole carriers tend to be injected into that phase rather than into the disordered one, leading to the proposal that EL emission was predominantly originated from the β phase.