Singlet Exciton Fraction in Electroluminescence from Conjugated Polymer

The efficiency of electrofluorescent polymer light-emitting diodes is determined by singlet exciton fraction (χ(S)) formation and its value still remains controversial. In this work, χ(S) in spiropolyfluorene (SPF) is determined by analyzing transient emission of phosphor-dopant probe. The χ(S) is found to range from 50% to 76%, depending on applied voltage. Higher applied voltage gives larger χ(S). Besides, more rapid increment in χ(S) with applied voltage is observed in the higher-molecular-weight polymer. The voltage or molecular weight dependence of χ(S) 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, χ(S) is contributed by charge recombination. Its value (χ(S) ~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.