What happen when Photon bang the the electron in DSSC cell??

The short answer is that when an electron in the valance band absorbs energy from a photon to become a conduction band (mobile) electron, both that electron as well as the hole "left behind" in the valence band can participate in an electric current. Thus, it is said, an electron-hole pair is created.
Where there was no mobile charge carrier, there are now two; where there was no electron-hole pair, this is now a pair. Remember, a hole is, in reality, a vacancy in the valance band. When this hole is due a valance electron "jumping" into the conduction band, the hole is part of an electron-hole pair.