The activation mechanism in phosphorous implanted silicon under excimer laser irradiation is investigated. The activation efficiency in the solid phase has been measured in a wide range of irradiation conditions, tuning the laser fluence in the sub-, partial, and total melting regime. Moreover, fixing the fluence, the activation as a function of the shot number has been analyzed. The total active fraction varies by several orders of magnitude and shows a complex trend depending on the process conditions. Our model, based on the interaction between defects and the active/inactive impurities, explains this scenario. In particular, it predicts experimental P active profiles, thus demonstrating that the status of the defect system rules the activation phenomenon, where the coupling between dopant and defect clusters at the early irradiation stage plays a crucial role.