We investigate the properties of long-lived species in F8BT films through time-resolved photoluminescence (PL) measurements at room temperature and 10 K. The kinetics consist of an initial exponential decay (τ = 2.26 ns) followed by a weak power-law decay (I(t) [proportionality] t(-1)) up to at least 1 ms, both of which depend weakly on temperature. From fluence-dependent PL and ultrafast transient absorption (TA) measurements, we confirm that this emission arises from the recombination of geminate charge-pairs generated through singlet-singlet annihilation. This behavior is a consequence of the donor-acceptor nature of this polymer, which enhances singlet-singlet annihilation and facilitates the formation of long-lived geminate-pairs from energetic singlet states.
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