On the basis of multi-phonon transition theory, the origin of the fast component(100 fs) and the slow component (ns) in photo-excitation dynamics is explained and their evolution time is calculated. From these results, it is seen that the fast component is due to the formation of neutral bipolaron from free electron-hole pair, and the slow one is due to the non-radiative decay of bipolaron. The equations motion of the lattice are also integrated in real time to simulate the formation of bipolaron.