Polymeric electrets have been widely used as a sensor due to their light weight and low cost. The long term performance and relaibility of the sensors rely on the charge stability. Surface potential measurement is one of the most useful tools to gauge electrical properties of materials. However, the surface charges (or surface potential) tend to decay over a period of time, therefore, affects the lifetime of the sensor. For corona charged sample, it has been observed that the potential of sample with an initial high surface potential decays faster than that with an initial lower surface potential, known as cross-over phenomenon. This mechanism is also affected by room temperature, humidity and initial charging time. The phenomenon has been found a few decades ago and various theories and models have been proposed. The common feature of the existing models is that they are all based on single charge carrier injection from corona charged surface. With our recent space charge measurement results on corona charged sample, double injection from both electrodes has been verified. Based on this new fact, a new model based on bipolar charge injection has been proposed. In this research, the details of the new model will be tested both experimentally and numerically. If fully verified, it is expected that a new set of parameters related to the material can be extracted and these parameters can be useful in assess the material. Having understood the charge transport mechanisms, another objective of the present project is to improve the stability of the charge. The improvement could be achieved by using nano particles loaded polymers.