Abstract
The efficiency of electrofluorescent polymer light-emitting diodes is determined by singlet exciton fraction (χS) formation and its value still remains controversial. In this work, χS in spiropolyfluorene (SPF) is determined by analyzing transient emission of phosphor-dopant probe. The χS is found to range from 50% to 76%, depending on applied voltage. Higher applied voltage gives larger χS. Besides, more rapid increment in χS with applied voltage is observed in the higher-molecular-weight polymer. The voltage or molecular weight dependence of χS suggests the probability of singlet exciton (SE) generation through triplet-triplet annihilation (TTA) is enhanced due to higher triplet exciton (TE) concentration at higher applied voltage or accommodation of more TEs in a polymer chain with high molecular weight, thereby increasing probability of TTA. At lower applied voltage, χS is contributed by charge recombination. Its value (χS ~50%) higher than the statistical limit 25% is in agreement with efficient interconversion between triplet and singlet polaron pairs (PP) and with larger formation rate of SE relative to that of TE.
Highlights
From simple quantum statistical consideration, charge recombination of electron and hole results in one singlet exciton (SE) and three triplet exciton (TE) as was confirmed for OLED9, 10
The triplet energies (ETs) of SPF and Ir(btp)2acac are 2.1 eV40 and 2.0 eV41, respectively, the exothermic triplet-triplet energy transfer (ET) from SPF to Ir(btp)2acac is expected. These results indicate that Ir(btp)2acac can act as an effective energy (SE and TE) harvester for the SPF
The model compounds normally used in photophysical studies, such as poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenyle-nevinylene) (MEH-PPV) or polyfluorene (PFO), were not adopted here for the following reasons
Summary
From simple quantum statistical consideration, charge recombination of electron and hole results in one SE and three TEs as was confirmed for OLED9, 10. Some studies adopted poly(p-phenylene vinylene) (PPV) derivatives with alkoxy side chains as investigated system[10,11,12, 15, 18], which tends to form aggregates (the emitting species which is due to intermolecularly mutual interaction between two or more lumophores in the ground state by extending the delocalization of π-electrons over these conjugated segments)[20]. These aggregates can act as energy harvesters[20] or recombination centers[21] and will affect χS value obtained. Careful design of molecules with a small energy gap (ΔEST) between singlet state and triplet state (ΔEST ≦ 0.2 eV)[36] should be fulfilled for obtaining highly luminescent TADF materials
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