Improved light emission utilizing polyfluorene derivatives by thermal printing and solution process

Abstract

The emission properties of polymer light-emitting diode (PLEDs), using blue emissive poly(9,9-dioctylfluorene) (PFO) and yellow-green emissive poly[9,9-dioctylfluorenyl-2,7-diyl)-co-1, 4-benzo-{2,1'-3}- thiadiazole)] (F8BT) fabricated by the spin-coating method, the toluene vapor method and the thermal printing method, were investigated. poly(2,7-(9,9-din-octylfluorene)-alt-(1,4-phenylene-( (4-sec-butylphenyl)imino)-1,4-phenylene)) (TFB) is useful for buffer layer and a dopant when we use the spin-coating method. When we use TFB as interlayer of PLED, TFB acts as exciton-blocking layer, thus prevents luminescence quenching. When we use TFB with 2-(4-biphenylyl)-5-phenyl-1,3,4-oxadiazole (PBD) as dopants of PFO, better current efficiency was achieved, compared to PFO only device. This result derives from these materials working as hole and electron transporting molecules. The blue and yellow-green PLEDs fabricated by the spincoating method showed maximum efficiencies of approximately 1.1 and 1cd/A, respectively. The device with bis[1-(9,9-dimethyl-9H-fluoren-2-yl)-isoquinoline](acetylacetonate)iridium(III) (Ir(fliq)2acac) doped in PFO showed red-emission and a maximum efficiency of approximately 1cd/A. Current efficiencies of PLEDs with the β phase of PFO fabricated by the thermal printing method and the toluene vapor method were found to have better emission efficiency than that with the amorphous phase of PFO by the spin-coating method. The EL spectra of PLEDs using PFO and PFO:F8BT fabricated by the thermal printing method were polarized. The transient characteristics of PLEDs using β phase of PFO were better than amorphous phase of that. It is expected to improve the characteristics of PLEDs by the optimization of the thermal printing method. We demonstrated improved light emission of PLEDs with the high-quality β phase by the thermal printing method.