White electroluminescence from a single polymer system: phenothiazine derivatives as a red emissive dopant and polyfluorene as a blue host

Abstract

Herein, the synthesis of a benzothiadiazole and phenothiazine based luminogen, (2Z,2′Z)-3,3′-(7,7′-(benzo[c][1,2,5]thiadiazole-4,7-diyl)bis(10-(2-ethylhexyl)-10H-phenothiazine-7,3-diyl))bis(2-(4-bromophenyl)acrylonitrile) (BTZPZP), exhibiting red emission in a solution as well as a thin film, is reported. This monomer unit was copolymerized with 9,9-dihexyl fluorene using a palladium catalysed Suzuki-cross coupling reaction wherein the feed ratio of the latter is varied from 0.125 to 5.0 mol% to fine tune the optoelectronic properties. White light emission is demonstrated in copolymer BTZPZP-0.25 through incomplete energy transfer. The co-polymers of all films examined in the photoluminescence (PL) regions at about 430 and 570–595 nm displayed peaks originating from fluorene segments and the BTZPZP chromophore, respectively. The copolymer BTZPZP-0.25 device fabricated with (ITO/PEDOT; PSS/P(BTZPZP)s/Al) showed white electroluminescence with Commission Internationale de l'Eclairage (CIE) coordinates of (0.32, 0.37). The maximum current efficiency, power efficiency and brightness of the white light emitting copolymer of BTZPZP 0.25% are 4.5 cd A⁻¹, 4.2 lm W⁻¹ and 9224 cd m⁻², respectively. The superior performance of the WPLED copolymer BTZPZP-0.25 is attributed to the presence of a phenothiazine group in the BTZPZP monomer, which results in a stable electroluminescence spectrum when the voltage level is varied from 4 V to 9 V.