Photovoltaic (PV) are simple and sustainable, but when installed in semi-arid and desert areas dust deposition problems are encountered. There are several methods for cleaning PV panels. One of these methods is the electrodynamic screen (EDS) where very high voltage passes through electrodes placed on the upper surface of the panels, which repels the dust off the surface. In this research, an improvement of the cleaning capability of the EDS is proposed. This is done by placing a cover at the backside of the electrodes. It is found that adding the back cover increases the electrostatic field at the upper side of the electrodes, consequently improves the cleaning capability of the electrodes. The change in the electric field above the electrodes depends on the material of the back cover. The objective of this study is to categorize the back-cover materials based on their capability to store and conduct an electric charge. A new physical term has been developed in this research, which is called the "electrical diffusivity, alpha electrical", and it is analogues to the thermal diffusivity in the science of heat transfer. The electrical diffusivity is defined as the ability of the material to conduct electrical charge relative to its ability to store the charge. The units of the "electrical diffusivity" is m2/s, which is the same as that of the thermal diffusivity. It is found that the electrical diffusivity of wood is 3 orders and 2 orders of magnitude less than those of air and glass, respectively, which indicates that wood resists the diffusion of charge more than glass and air. The electric field due to using wood, glass and air as surface insulators for electrodes has been measured using an ultra-sensitive sensor. It is found that the electric field strength above the electrodes in case of using wood as a back cover is higher than in case of glass and air, which is in line with the low electrical diffusivity of wood. It is concluded that the newly developed term "electrical diffusivity" can be used to categorize the diffusivity of the electric charge in different materials, and can be used as a design parameter for material selection especially in applications of electric field shielding.