Singlet-Triplet Exciton Annihilation Nearly Suppressed in Organic Semiconductor Laser Materials Using Oxygen as a Triplet Quencher

We report on the use of oxygen as triplet quencher to reduce singlet-triplet annihilation in light-emitting organic solid films and solvent-free molecular semiconducting liquids. For this purpose, a fluorescent heptafluorene derivative is dispersed either in a wide bandgap 4,4′-Bis (N-carbazolyl)-1,1′-biphenyl (CBP) host or in a solvent-free liquid matrix based on 9-(2-ethylhexyl)carbazole (EHCz). To introduce oxygen in the samples, a modified cold isostatic pressure technique is used in the case of spin-coated CBP blends while oxygen is bubbled in the liquids. The influence of the oxygenation on their photophysical and amplified spontaneous emission properties is examined. Both solid blend films and liquids showed before and after the introduction of oxygen a photoluminescence quantum yield higher than 80% and an amplified spontaneous emission threshold lower than 0.4 μJ/cm
²
. While oxygen does not quench significantly the singlet excitons in these systems, singlet-triplet annihilation is strongly reduced in solid thin films and nearly suppressed in the liquid layers. This study demonstrates that liquid organic semiconductors are promising candidates for optically pumped continuous wave lasers and provides important insights for a more effective triplet management in organic semiconductor lasers.