F8BT Oligomers for Organic Solid-State Lasers

The yellow-green emissive poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) polymer is widely used because of its suitability for a variety of applications. However, we have found that F8BT shows huge performance variations that depend on the chemical supplier, with photoluminescence quantum yields (PLQYs) ranging from 7 to 60% in neat films. Polymers generally face problems including purity, polydispersity, and reproducibility, which also affect F8BT polymers. Therefore, to overcome these problems, we investigated oligomers of F8BT, which can easily be purified and can thus be obtained in a high-purity form. In the three oligomers (M1-M3) that we synthesized, the PLQYs were much higher than those of conventional F8BT (>80% in their neat films) although their PL spectra were nearly the same as that of F8BT, and their amplified spontaneous emission (ASE) thresholds were lower than that of the polymer (e.g., 1.9 μJ cm-2 for M3 and 2.7 μJ cm-2 for F8BT) because of a higher net gain and better film morphology. Furthermore, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energies of the oligomers were found to be similar to those of F8BT, making them candidate materials for use as hosts in light-emitting devices. The ASE using a near-infrared laser emitter doped in F8BT and oligomer hosts showed a clear difference despite nearly the same properties for steady-state emission.