Femtosecond probing of exciton relaxation and transport dynamics in polybithiophene

Abstract

The relaxation and transport dynamics of singlet excitons in 100 nm thick, electrochemically prepared polybithiophene films were investigated by monitoring the time evolution of photoinduced bleaching of the S0–S1 absorption and photoinduced absorption (S1–Sn) with femtosecond resolved transient absorption spectroscopy. The decay dynamics of both photoinduced bleaching and photoinduced absorption in the spectral range from 500 to 800 nm are observed to be independent of the pump pulse wavelength and can be fitted by a double exponential, the time constants of which are τ1=120±20 fs and τ2=2±0.3 ps. The fast decay process with ∼τ1=120 fs was ascribed to the transport dynamics of the initially generated free excitons migrating over parallel aligned polymer segments to structural defects acting as traps. The competing process, occurring predominantly in structurally disordered regions, is structural relaxation of the initially generated free excitons to self-trapped excitons decaying with a lifetime of about 2 ps.

Fluorescence Up-Conversion Study of Excitation Energy Transport Dynamics in Oligothiophene−Fullerene Linked Dyads

“…As shown in Figure for 12T-C 60 , a longer π-conjugated segment (LT) is located near the C 60 moiety in type A, whereas a shorter π-conjugated segment (ST) is located near the C 60 moiety in type B; furthermore, a relatively long π-conjugative unit is located in the center position of 12T in type C. Figure depicts the schematic energy diagrams accompanying the photoinduced energy-transfer processes for 12T-C 60 . Our kinetic simulations need the following three postulations: (1) the EET process within oligothiophenes takes place via the Förster incoherent energy hopping mechanism, similar to polythiophenes, − (2) the time constants (rate constants) are ensemble averages of the segments with trifling distributions, and (3) the population of the S 1 state of 12T goes back to the ground state with EET to C 60 within 15 ps as seen in Figure a, that is, no participation of torsional relaxation dynamics of the S 1 state with the lifetime of 600 ps for 12T-ref. Then, time-dependent total population ( P total ( t )) of the 1 12T* moiety in 12T-C 60 moiety is given by eq 3…”

“…In polythiophene, the π-conjugation is thought to be separated by conformational defects . As many groups already discussed, the excitation-energy transfer (EET) mechanism among the segments is described on the basis of the resonance energy transfer model between the weakly coupled chromophores. − The initial photoexcitation dynamics of conjugated polymers/oligomers were well-investigated experimentally and theoretically. − Recently, Sundström and co-workers reported the exciton transfer in a polythiophene in solution revealed by femtosecond transient absorption measurements. ,, The observed dynamic Stokes shift of the stimulated emission was discussed on the basis of Monte Carlo simulations of resonant energy transfer by line-dipole Förster energy transfer model. − The vibrational relaxation takes place within ∼100 fs, because the exciton self-trapping strongly couples with the phonon mode of polythiophenes. − Within 1 ps, the EET occurs from higher-energy oligothiophene segments to lower-energy segments (downhill excitation energy transfer). In addition, the energy migration also occurs between isoenergetic oligothiophene segments in the time range from a few picoseconds to several tens of picoseconds. − Sundström and co-workers also observed the spectral red-shift due to the torsional relaxation process of polythiophenes .…”

Intensity modulated photocurrent spectroscopy (IMPS) of solid-state polybithiophene-based solar cells

“…Specific examples are exciton generation and dissociation, as well as photoinduced electron transfer in donor-acceptor polymer materials (see, e.g., Refs. [2][3][4][5]). At the same time, there have been practically no studies of the photocurrent dynamics in organic solar cells on the intermediate (microsecond to millisecond) time scale.…”

“…At the same time, there have been practically no studies of the photocurrent dynamics in organic solar cells on the intermediate (microsecond to millisecond) time scale. 2 Intensity modulated photocurrent spectroscopy (IMPS) is a powerful tool for studying the mechanism and kinetics of various photoelectrochemical processes [6] including those in dye-sensitized solar cells [7,8]. IMPS use a sinusoidal modulation of the intensity of incident light shone on, in this case, a solar cell.…”

Long-Range Energy Migration in Photoexcited Polymers

“…22 A fast decay dynamics with the time constant for the exciton transport was also observed to be 120 fs in polybithiophene with the excess energy of 200 meV. 23 Since the initial excess energy of PVK excitations in our study can be estimated to be about 800 meV, the excitation migration time in PVK bulk is expected to be shorter than 100 fs.…”