Concentration effects on charge transport in dye-doped polymer light-emitting diodes

Abstract

Light emitting diodes made of a single spin-coated layer of poly(9-vinylcarbazole) doped with coumarin-5 15 dye have been studied. The influence of dye concentration on emission and electrical characteristics is evidenced. Two different regimes are identified. At low concentrations, apparent hole injection barrier raises, holes are trapped and mobility decreases. External quantum efficiency increases with concentration. At concentrations larger than 10 %wt, coumarin aggregates, photoluminescence yield drops and electroluminescence yield increases up to 0. 1 %-photons per electron. We show that using coumarin dye in a single-layer diode improves electron-hole injection and recombination balance more than using an additional hole-blocking layer. INTRODUCTIONPolymeric semiconductor devices are receiving increasing attention in view of potential applications requiring lowcost processing over large areas. In this respect, unlike with evaporated molecules, the wet-processing capability of polymers offers total compatibility with other complementary tecimologies. In a strategy oriented towards the development of low-cost electroluminescent devices for display applications, we chose single-layer structures made of available polymer materials. In this respect, single-layer blue light-emitting-diodes were prepared using poly(9-vinylcarbazole) (PVK), a commercially available polymer with large glass-transition temperature Tg 200°C, high stability and well documented hole-transport properties.[1] Polymer was doped with a highly fluorescent organic laser dye : coumarin-515 (Figure 1), with a fluorescence yield close to one. Such device which has been studied by many authors, [2][3][4][5] owing to its ease of processing, may also be considered as a model for electroluminescence in non-conjugated polymers. Indeed, this soluble polymer is readily spincoated from stable solutions and all device measurements can be performed under room-atmosphere conditions, provided that stable metal electrodes are used. In this paper, we discuss the effect of fluorescent dye concentration on the electroluminescence behavior. Results are analyzed in view of time of flight and photoluminescence behaviors. Effect of charge injection balance is addressed using additional hole injecting or blocking layers. The electron injection and hole trapping behaviors of coumarin are evidenced. Figure 1: molecular structure of PVK (a), coumarin-5 15 (b) and PMMA-PBD polymer (c). 302 SPIE Vol. 3281 . 0277-786X/98/$1O.O0 N a) (H3C)2N N1 N CH CH2 b) H3C' Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/24/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx