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(3'-(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 (V_on) 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.

Keywords: fluorene cross-conjugated polymer; lasing properties; polymer light emitting diodes (PLEDs).

Figure 1

Chemical structure of PFN, conjugated polymer PF–1, and PEDOT:PSS; and PLEDs architecture.

Figure 2

Experimental setup for lasing characterization.

Figure 3

(a) Normalized absorption (red triangles), fluorescence emission (black circles), both in solution of chlorobenzene, and electroluminescence emission (purple stars) in solid state of PF–1; (b) AFM image of a PF–1 thin film surface (R_α of the order of 1.4 nm).

Figure 4

(a) Current density (red symbols) and luminous density (black symbols) as a function of applied voltage for PLEDs containing PFN (circles) and LiF (stars) as electron injectors, respectively. Schematic energy diagram for HOMO and LUMO levels for PLEDs with (b) PFN and (c) LiF.

Figure 5

Luminous efficiency of PLEDs based on PF–1 polymer with the electron injectors PFN and LiF as a function of (a) applied voltage, and (b) current density.

Figure 6

(a) Fluorescence (circles) and lasing (narrow line) of PF–1 in chlorobenzene solution. Inset, tunability of PF–1 lasing; (b) FWHM and lasing intensity of 566 nm emission for a solution of PF–1 as a function of the pump energy; (c) PF–1 fluorescence lifetime.