Ultrafast dynamics of singlet-singlet and singlet-triplet exciton annihilation in poly(3-2′-methoxy-5′octylphenyl)thiophene films

Abstract

Singlet-singlet ͑S-S͒ and singlet-triplet ͑S-T͒ exciton annihilation was studied in poly͑3-2Ј-methoxy5Јoctylphenyl͒thiophene films. For the S-S exciton annihilation studies, transient absorption spectroscopy at excitation laser pulse fluences of 1.2ϫ 10 13 -4.4ϫ 10 14 photons/ cm 2 and 2.5 kHz pulse repetition rate was applied. The obtained kinetics demonstrate a typical nonexponential character with intensity-dependent amplitudes and lifetimes. In time-resolved fluorescence experiments, low excitation pulse fluences of 1.6 ϫ 10 9 -2.2ϫ 10 12 photons/ cm 2 at high repetition rates of 0.4, 0.8, 4, and 81 MHz lead to S-T exciton annihilation as a result of triplet exciton accumulation. S-T annihilation kinetics results in monoexponential decay of the fluorescence kinetics and manifests itself as a decrease of the singlet exciton lifetime. The calculated time-independent S-S and S-T exciton annihilation rates strongly support the conclusion that the processes are controlled by the interchain diffusion of singlet excitons. Despite the low efficiency of S-T annihilation compared to that of S-S annihilation, it has a substantial effect on the singlet exciton lifetime due to a relatively long triplet lifetime ͑60 s͒. Thus, even optical excitation with low fluence at high pulse repetition rate creates a significant concentration of triplet states that efficiently quenches singlet excitons.

Singlet Annihilation in Films of Regioregular Poly(3-hexylthiophene): Estimates for Singlet Diffusion Lengths and the Correlation between Singlet Annihilation Rates and Spectral Relaxation

“…To give an idea of the precision of the fit, we include dotted lines that correspond to annihilation constants of 6 × 10 -8 (below) and 1.5 × 10 -8 cm 3 s -1 (above), respectively, roughly double or half the best fit value. This value of ∼3 × 10 -8 cm 3 s -1 for the annihilation constant is, therefore, a reasonable precise value and is furthermore comparable to thatreportedforotherconjugatedpolymersintheliterature, 31,39,41,42,[50][51][52][53][54][55] where annihilation constants on the order of 10 -8 cm 3 s -1 are typical.…”

Excitation Energy Transfer in Organic Materials: From Fundamentals to Optoelectronic Devices

“…the density of quenching sites is virtually constant as well, the fluorescence dynamics prevail a mono-exponential decay but the obtained kinetics may not represent the truly intrinsic fluorescence lifetimes anymore. It should be noted that the results of Zaushitsyn et al [45] are in contrast to the observations of King et al who have reported the absence of singlet-triplet annihilation in a prototypical polyfluorene. [37] Singlet Energy Transfer in Blends of Conjugated Polymers…”

“…Recently, earlier results on the origin of DF in MeLPPP have been revisited and it has been proposed that TTA is the only origin of DF in MeLPPP. [44] In bulk samples triplet states can accumulate under continuous wave (cw) or quasi-cw excitation at high laser repetition rates as recently demonstrated by Zaushitsyn et al [45] This opens additional quenching channels for singlet states, if singlet-triplet annihilation is turned on upon excited triplet state accumulation. The effect of singlet-triplet annihilation on the lifetime of the excited singlet state can be quite drastic leading to shortening of the measured fluorescence lifetime with increasing repetition rate of the pump laser.…”

Polyfluorene Photophysics

“…However, although theoretically possible, once again the Förster radius is very small. In the case of S-T annihilation there have been only a few reports of triplets efficiently quenching singlet states in polyfluorene, either in the solid state and in single molecule studies and the efficiency of the observed process is much lower than singletsinglet annihilation [85,86]. The efficiency of S-S annihilation is enhanced by the migration of the singlets, but the migration does not seem to dramatically enhance the efficiency of S-T annihilation.…”