Optical gain properties of interfacial material PFN-Br and its application potentials in future electrically pumped organic lasers

In this paper, the optical gain properties of the water/alcohol soluble conjugated polyelectrolyte (Poly[(9,9-bis(3′-((N,N-dimethyl)-N-ethylammonium)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]) (PFN-Br) and its potential applications in future electrically pumped organic lasers are revealed and systematically studied. To the best of our knowledge, no studies on the optical gain properties of PFN-Br or its prototype, poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] have been reported before. These conjugated polyelectrolytes are widely used as the interlayers in organic light emitting diodes or organic solar cells. The thickness of such an interlayer is usually less than 10 nm, which is considered not sufficient for supporting light waveguiding. Therefore, the thickness of the PFN-Br layer used in this work is increased to more than 100 nm. Through careful study, the polymer is found to possess a low threshold of amplified spontaneous emission (ASE) (~11 μJ/cm²) and a small ASE cutoff thickness (<50 nm). It is an efficient blue emission (~456 nm) gain medium. The ASE peak of the PFN-Br film is red-shifted as the thickness increases from 50 to 220 nm. By utilizing the great resistance of PFN-Br against the organic solvent, such as toluene, PFN-Br/F8BT bilayer devices on quartz and PFN-Br/MEH-PPV bilayer devices on ITO glass are fabricated and characterized. In the PFN-Br/F8BT bilayer devices, it is found that the PFN-Br interlayer has very limited influence on F8BT. The ASE threshold of F8BT increases only twice, compared with that of F8BT monolayer device, when 100-nm-thick PFN-Br layer is introduced beneath the F8BT film. No significant change in optical gain or loss is observed. Most of the extra losses in F8BT due to the introduction of PFN-Br are attributed to the larger refractive index of PFN-Br than that of quartz substrate. Furthermore, in the PFN-Br/MEH-PPV bilayer devices on ITO glass, introducing PFN-Br interlayer resulting in optimal ASE performance of MEH-PPV compared with that on bare ITO surface. The ASE threshold of MEH-PPV is reduced as much as 60% (from 402 μJ/cm² to 160 μJ/cm²) while the PFN-Br layer is sandwiched between ITO and MEH-PPV. The PFN-Br layer modifies the waveguiding modes, and reduces the interaction between excitons and ITO electrodes. As a result, the ASE performance of MEH-PPV is improved. The findings of this report indicate that the PFN-Br is not only a good carrier transport material but also a highly-efficient gain medium. PFN-Br, combined with its advantages in different fields, is expected to play various roles in future organic electrically pumped lasers.