Organic Systems

We are interested in organic semiconductors both for its atomic structures (e.g., stacking) and electronic properties. The systems might be polymers, small molecule crystals, or monolayer of oligomers. The potential applications for such organic semiconductors include organic solar cell, organic light emitting diode, and organic field effect transistor. We have developed several methods to calculate the electronic structures of such systems, while the atomic structure studies are mostly carried out using classical force field methods. One approach is the charge patching method, which was initially developed for inorganic system, but has been extended to organic systems. The Hamiltonian of the charge patching method will then be diagonalized by the folded spectrum method. This diagonalization can also be done using the overlapping fragment method. This method uses the fragment wave functions as the basis set to diagonalize the overall Hamiltonian of the system. We have also developed methods to calculate the electron-phonon coupling constants and used time-domain method to study the charge transport in the organic systems.

The properties we are interested include the electronic state, the localization of the states, the density of state of the electron, the effect of thermo fluctuation, the carrier transport properties.

A few examples are given here for the systems studied in this group.

These are the hole wave functions for a disordered P3HT blend. The atomic structures are calculated by force field methods, while the charge density is provided by the charge patching method, and the electronic states are calculated by the folded spectrum method.

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The tail density of state in P3HT disordered blend and the state localization in the tail density of state. Such density of state is important to be used in phenomenological calculations.

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The stacking structure of P3HT polymers, calculated using classical forcefields and ab initio methods.