Direct observation of intersystem crossing in charge-pair states in polyfluorene polymer blends

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Abstract

In conjugated polymers, the process of interconversion between singlet and triplet bound charge-pairs is important in determining the efficiency limits both for light-emitting diodes (LEDs) and for photovoltaics. We probe the kinetics of spin-mixing in charge-pairs by studying the formation of triplet excitons after photoexcitation of blends of poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) with poly(9,9-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylene-diamine) (PFB) using excited-state transient absorption spectroscopy. We see clear evidence of triplet formation from long-lived charge-pairs during a period up to 100 ns after the excitation pulse, and we find that the intersystem crossing rate in these charge-pairs exceeds 2.0 × 106 s−1.

Graphical abstract

Triplet exciton formation occurs over 100 ns after photoexcitation in F8BT–PFB polymer blend films showing that intersystem crossing occurs in precursor charge-pairs.

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Acknowledgements

We are grateful to I.A. Howard and Professor R.H. Friend for useful discussions, and to the Engineering and Physical Sciences Research Council, UK for funding, including the Supergen programme ‘Excitonic Solar Cells’. H.O. is grateful for support from the Scientist Exchange Program between the Japan Society for the Promotion of Science (JSPS) and the Royal Society.

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