Deep localized distortion of alternating bonds and reduced transport of charged carriers in conjugated polymers under photoexcitation.

In a real bulk heterojunction polymer solar cell, after exciton separation in the heterojunction, the resulting negatively-charged carrier, a polaron, moves along the polymer chain of the acceptor, which is believed to be of significance for the charged carrier transport properties in a polymer solar cell. During the negative polaron transport, due to the external light field, the polaron, which is re-excited and induces deep localization, also forms a new local distortion of the alternating bonds. It is revealed that the excited polaron moves more slowly than the ground-state polaron. Furthermore, the velocity of the polaron moving along the polymer chain is crucially dependent on the photoexcitation. With an increase in the intensity of the optical field, the localization of the excited polaron will be deepened, with a decrease of the polaron's velocity. It is discovered that, for a charged carrier, photoexcitation is a significant factor in reducing the efficiency during the charged carrier transport in polymer solar cells. Mostly, the deep trapping effect of charged carrier in composite conjugated polymer solar cell presents an opportunity for the future application in nanoscale memory and imaging devices.

Localization and relaxation of singlet exciton formation in conjugated polymers under photoexcitation.

This paper employs a molecular dynamics approach to uncover the time profile of exciton formation, which can be divided into two stages: localization of electron-hole pairs and relaxation process (nuclear and electronic). Under photoexcitation, an electron-hole pair is formed by an electronic transition, and the pair in turn becomes localized through the electron-lattice interaction, which triggers the total energy to shift violently and oscillate. The oscillation during the first 40 fs induces the excitation to step into the second stage, i.e., relaxation. After the relaxation process of about 850 fs, the total energy, lattice energy, and electron energy reach certain values whereas the lattice configuration and electron remain localized, indicating the formation of a singlet exciton.

Electronic Two-Transition-Induced Enhancement of Emission Efficiency in Polymer Light-Emitting Diodes.

With the development of experimental techniques, effective injection and transportation of electrons is proven as a way to obtain polymer light-emitting diodes (PLEDs) with high quantum efficiency. This paper reveals a valid mechanism for the enhancement of quantum efficiency in PLEDs. When an external electric field is applied, the interaction between a negative polaron and triplet exciton leads to an electronic two-transition process, which induces the exciton to emit light and thus improve the emission efficiency of PLEDs.

Carrier-collision-induced formation of charged excitons and ultrafast dynamics fluorescence spectra.

After a hole injection layer is inserted into a polymer light-emitting material, the injection of positive charge not only easily causes distortion in the conjugated polymer chain but also produces positive polarons. The ultrafast dynamics shows that, when the positive polaron approaches and collides with the triplet exciton, that exciton will become charged, whereby the non-emissive triplet exciton becomes radiative and emits light. Furthermore, the lifetime of the charged triplet exciton is longer than the singlet exciton. This paper explicitly depicts the dynamic fluorescence spectra of the radiative transition of the charged triplet exciton occurring during the decay of the charged exciton, and also exhibits the difference between traditional adiabatic dynamics and non-adiabatic dynamics.

Fluorescence dynamics and dipole moment evolution of singlet exciton decay in conjugated polymers.

Both fluorescence dynamics and time-dependent electron transitions are introduced within a previously developed molecule dynamics approach for treating conjugated polymers. This is able to provide a panoramic view of luminescence dynamics during singlet exciton decay, in which the fluorescence dynamics is largely determined by the electron population and the evolution of the dipole moment. The fluorescence intensity is weakened due to a reduced dipole moment and diminished decay rate of the electron, which validates a previous assumption based on experimental studies. The lifetime of the singlet exciton in a conjugated polymer is found to be 1.2 ns, and the calculated time profile of the fluorescence intensity is in agreement with recent experimental results.