title: Singlet fission of hot excitons in π-conjugated polymers.
abstract: We used steady-state photoinduced absorption (PA), excitation dependence (EXPA(ω)) spectrum of the triplet exciton PA band, and its magneto-PA (MPA(B)) response to investigate singlet fission (SF) of hot excitons into two separated triplet excitons, in two luminescent and non-luminescent π-conjugated polymers. From the high energy step in the triplet EXPA(ω) spectrum of the luminescent polymer poly(dioctyloxy)phenylenevinylene (DOO-PPV) films, we identified a hot-exciton SF (HE-SF) process having threshold energy at E≈2E(T) (=2.8 eV, where ET is the energy of the lowest lying triplet exciton), which is about 0.8 eV above the lowest singlet exciton energy. The HE-SF process was confirmed by the triplet MPA(B) response for excitation at E>2E(T), which shows typical SF response. This process is missing in DOO-PPV solution, showing that it is predominantly interchain in nature. By contrast, the triplet EXPA(ω) spectrum in the non-luminescent polymer polydiacetylene (PDA) is flat with an onset at E=E(g) (≈2.25 eV). From this, we infer that intrachain SF that involves a triplet-triplet pair state, also known as the 'dark' 2A(g) exciton, dominates the triplet photogeneration in PDA polymer as E(g)>2E(T). The intrachain SF process was also identified from the MPA(B) response of the triplet PA band in PDA. Our work shows that the SF process in π-conjugated polymers is a much more general process than thought previously.
authors.affiliation: Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA Materials Research Science and Engineering Center (MRSEC), University of Utah, Salt Lake City, UT 84112, USA.
authors.forename: Yaxin
authors.initials: Y
authors.lastname: Zhai
authors.affiliation: Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA.
authors.forename: Chuanxiang
authors.initials: C
authors.lastname: Sheng
authors.affiliation: Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA Materials Research Science and Engineering Center (MRSEC), University of Utah, Salt Lake City, UT 84112, USA val@physics.utah.edu.
authors.forename: Z Valy
authors.initials: ZV
authors.lastname: Vardeny
keywords: organic photovoltaic; singlet fission; π-conjugated polymers