Light emission properties of a cross-conjugated fluorene polymer: Demonstration of its use in electro-luminescence and lasing devices

Abstract

Light emission properties of a fluorene cross-conjugated polymer (PF-1) based on the monomer 4,7-bis[2-(9,9-dimethyl)fluorenyl] benzo[1,2,5]thiadiazole are reported. This polymer exhibits solubility at high concentrations, good processability into thin solid films of good quality and a broad emission band with a fluorescence quantum yield of approximately 1. Based on these features, in this paper we implemented the use of PF-1 as an active layer in polymer light-emitting diodes (PLEDs) and as a laser gain medium in solution. To get insight on the conducting properties of PF-1, two different electron injectors, poly [(9,9-bis(31-(N, N-dimethylamino) propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) and lithium fluoride (LiF), were used in a simple PLED architecture. PLEDs with the PFN film were found to exhibit better performance with a maximum luminous efficiency of 40 cd/A, a turn-on voltage (Von) of approximately 4.5 V and a luminance maximum of 878 cd/m2 at 5.5 V, with a current density of 20 A/m2. For the lasing properties of PF-1, we found a lasing threshold of around 75 μJ and a tunability of 20 nm. These values are comparable with those of rhodamine 6G, a well-known laser dye.