Operational Windows for TADF Gain Media in Organic Solid-State Lasers: An Exciton-Evacuation Framework
Abstract
TADF solid-state lasers demand gain media that achieve stimulated emission without generating steady-state triplet excitons. Delayed fluorescent materials that exhibit reverse intersystem crossing are ideal since they convert triplet excitons into emissive singlets but neither a small energy gap nor low optical threshold are necessary criteria for laser potential. The important issue considered in this work is the question of whether TADF gain records can be divided into practical operational windows when considering threshold, photoluminescence quantum yield, delayed-emission lifetime, prompt-emission lifetime, effective triplet-harvesting energy gap, gain-wavelength and condensed-phase structure all within one record. Sixteen emitter state records have been included in this data set, comprising molecules 1a, 1b, 2a, 2b, 3, 4a-4d, 5, 6, 7, and 8 dispersed in mCBP film, polymer microspheres, mCBP microplate, mCBP microrings and neat microcrystals. Optical utility for TADF laser operation is assessed by an exciton evacuation score which favors high radiative yield and gain accessibility but discourages long delayed-fluorescence decay times, long prompt lifetime and large triplet-harvesting energy gap. Film record 2a in mCBP film is the top overall performer with $\PLQY=0.93$, $\taud=0.8~\mu$s, and $\Pth=3.3~\mu$J cm$^{-2}$. Green resonator record 4b in neat microcrystals is the top performing resonator since it offers $\PLQY=0.80$, $\taud=2.0~\mu$s, $\DeltaEff=0.02$ eV and $\Pth=5.0~\mu$J cm$^{-2}$. Compound 4c yields the red microcrystal route with $\Pth=3.0~\mu$J cm$^{-2}$, whereas compound 7 in mCBP microplates establishes the near-infrared lasing window at $740$ nm with $\Pth=3.6~\mu$J cm$^{-2}$. TADF lasers whose threshold is high even when the radiative yield, exciton removal, and morphology are all good are thus more convincing than those whose threshold is low even when taken on its own.