Transient Magnetic Field Effect of Photoexcitations in Donor-Acceptor Organic Semiconductors

We report transient photoinduced absorption (t-PA) and magnetic field (B)-dependent t-PA (t-MPA(B)) in a pristine low band gap pi-conjugated copolymer composed of donor and acceptor moieties, namely, the poly[[4,8-bis[(2-ethylhexyl)oxy]benzo [1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thi-eno[3,4-b]thiophenediyl]]) (or PTB7) used in photovoltaic applications. Unlike traditional pi-conjugated polymers in which the primary photoexcitations are singlet excitons (SE), in pristine PTB7 we find at short times coexistence of two primary photoexcitation species, namely, SE and triplet-triplet (TT) pair. Both species are photogenerated directly from the ground state and are spin-correlated. Although the TT pair decomposes into two separate triplet excitons (TEs) in similar to 100 ps, the separated TE spins are still entangled up to similar to 6 mu s. At longer times, the t-MPA(B) response of the surviving TEs shows transient narrowing effect, which is attributed to a distribution of the TE size.