Energy due to the important role that plays in human activities, is an important part in human life. With the increase in world population, demand for energy has led to an inevitable increase in the consumption of fossil fuels, oil and gas. But today, due to the non-renewable resources and their environmental impacts, the use of clean and renewable energy sources has been on the agenda. One type of renewable energy, is solar energy that with using solar cells can be converted it into electrical energy. There are different types of solar cells. One of them, dye solar cell which is inexpensive and has good efficiency. One of dye solar systems, are systems that based on TiO 2 . In this thesis, with using density functional tight binding approximation (DFTB) and time-dependent density functional tight-binding approximation in real time (Time domain TD- DFTB) methods, we investigated TiO 2 surface and Dye/TiO 2 . Calculations have been done by DFTB+ code. We've used the DFTB method because it is faster than the conventional methods of density functional theory. It also enables us to look larger systems. So by DFTB and Time domain TD-DFTB methods we can do the simulations for the systems with the many atoms, and thus obtain more realistic results. In this thesis, at first we investigated different surface of TiO 2 and showed that we can choose the appropriate clusters of TiO 2 that are appropriate to illustrate the properties of TiO 2 surface. T hen we investigate the adsorption of Dye molecules on TiO 2 and at the end we considered the optical properties of Dye/TiO 2 system. Results obtained in all stages compared with the results of other methods and experience and good agreement has been observed.
