Study of Carrier Generation in Phthalocyanines by Time-Resolved Fluorescence

Breaking of lattice potential well-induced confinement of carriers in conjugated polymers

Recent experimental research has reported that a surface electric field on the polymer solar cell can restrain the recombination of the resultant charged carriers [23]. Based on this, this article reveals an underlying mechanism: If a surface electric field below 4.5 × 10⁴ V/cm is applied to the polymer layer, the electric field drives the charged polaron to transport. Once the polaron approaches and collides with the exciton, it is easily trapped by the potential well produced by the exciton and then forms a charged exciton. The decay of the resultant charged exciton rapidly reduces the number of excitons. However, once the external field surpasses the threshold value of 4.5 × 10⁴ V/cm, the charged polaron absorbs momentum from the external electric field and shakes off the trapping of the exciton. It finally steps out of the original lattice potential well, where the appropriate electric field magnitude ranges from 5.5 × 10⁴ V/cm to 8 × 10⁵ V/cm. After a collision of 300 fs, apart from a phase shift, the exciton still exists. Then, the originally carriers is dissociated when the electric field reaches 0.8 MV/cm. The applied surface field is able to effectively keep the excitons from fusion with the transporting charged polarons, which provides a valid and easily manufactured approach to yield higher efficiency of polymer solar cells.

Forbidden singlet exciton transitions induced by localization in polymer light-emitting diodes in a strong electric field

Through combining the electron transition process and dipole moment evolution as well as electron-phonon coupling, molecular dynamics calculations show that the radiative decay of singlet excitons in a conjugated polymer, such as a polymer light-emitting diode (PLED), is largely determined by the evolution of the dipole moment. Without an electric field, the decay life of a singlet exciton is about 1 ns. Once an electric field is applied and exceeds a critical value, with electron-phonon coupling, the original lattice structure evolves into two new localized lattice distortions, consistent with the experimental results. Owing to the new lattice structure and self-trapping, the dipole moment rapidly decreases to zero within 5 fs, eliminating the radiative decay of the singlet exciton.

Electric field effects on photoinduced electron transfer processes of methylene-linked compounds of pyrene and N,N-dimethylaniline in a polymer film

Time-resolved measurements of the electric-field-induced change in fluorescence intensity have been made for methylene-linked compounds of pyrene and N,N-dimethylaniline (DMA) doped in a polymer film. The lifetime of the fluorescence emitted from the locally excited state of pyrene chromophore becomes shorter in the presence of electric field (F), when the dopant concentration is high. The lifetime of the excipelx fluorescence resulting from the photoinduced electron transfer (PIET) from DMA to the excited state of pyrene chromophore between different molecules also becomes shorter in the presence of F. Based on the simulation of the electric field effect on fluorescence decay, the mechanism of intermolecular PIET between DMA and pyrene chromophore in a polymer film is discussed.