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.

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.

Experiments have shown that intensive charge injections are able to greatly enhance the efficiency of polymer light emitting diodes (PLEDs). Yet, under strong external electric filed/high voltages, there is a certain degree of efficiency roll-off. For this paradox, this article reveals the following underlying mechanism: Under a low electric field, a charged polaron is driven to the potential well created by an exciton, where they fuse together to form a "charged exciton". The carrier fusion induces the triplet state to fluoresce, which greatly enhances the performance of the device (ideally, the internal quantum efficiency can exceed 95%). As long as the external field continues to increase and surpasses 4.5 × 10 4 V/cm, the above confinement is broken, and the polaron also steps out of the potential well, which leads to a major setback of the overall device efficiency. Then, when the electric field reaches as high as 0.8 MV/cm, the original exciton is dissociated. For achieving highly efficient fluorescent PLEDs, it is seen that the appropriate electric field magnitude ranges from 5 × 10 2 to 2 × 10 4 V/cm.

The injection of charge carriers from the electron/hole injection or transport layers in polymer light-emitting diodes potentially increases the device efficiency not by changing of charge intensity but by lattice distortion variation and quasi-particle interactions. From the low-dimensional condensed matter physics perspective, a valid mechanism is proposed to bring a type of novel channels that, under a proper external electric field, transitionforbidden triplet excitons are transformed and partially charged by charge carriers (polarons/bipolarons), thus are able to emit light and to enhance fluorescence greatly.