Abstract
In this thesis, we employ a time-independent and time-dependent density functional theory (TI-DFT and TD-DFT, respectively) to calculate the performance for polymer light-emitting diodes (PLEDs). Electronic and optical properties of PLEDs emission materials are analyzed and investigated. PLEDs are one kind of “Organic Electroluminescenc”, OEL, devices. The materials of polythiophene (PT) are used to fabricate the emitting layer in PLEDs. Therefore, it is of prime importance to understand the characteristics of the emitting layer materials which play an important role in PLED research. The first step of this research is to set-up-thiophene (T1) monomer and 2, 2′-Bithiophene (T2, dimer), T3 (trimer), T4 (tetramer)…, T10, T15, T20 molecular models and then calculate their band gap. We can determine the best exchange-correlation functional and basis set for thiophene and thiophene derivatives by using linear extrapolation techniques to determine these band gap values and then performing a comparsion with experimental data. Then, the optimized structures, bond length, band gap, molecular orbital, ionization energies, electron affinities and reorganization energy were calculated by using the DFT. The corresponding UV/Vis spectrum and absorption/excitation energies can be evaluated from the TD-DFT simulation results. The Stokes shift can be observed by comparing the difference between absorption and emission spectrum. In the experimental part, we took the 2, 2′-Bithiophene (dimer, T2) molecule, and its UV/Vis experimental spectrum data closely matches the simulation results. This is a good indication that we can use B3LYP/6-31G(d) to simulate other molecular units (T3, T4…, T10, T15, T20) and thiophene derivatives. It can be concluded that the derivatives, T_∞Br, may display a better performance than the polythiophene (PT) counterpart and can serve as the design reference for new emission materials of PLEDs devices.