An image sensor including at least one pixel for collecting charge in its photodiode is provided. The image sensor comprises: a substrate having a first surface on a front side and a second surface on a back side, a photodetector formed in the silicon substrate and having a light-receiving surface on the second surface, and a first layer with positive charges disposed on the second surface, the first layer being configured to form an electron accumulation region at the light-receiving surface of the photodetector for suppressing a dark current at a back side interface of the image sensor. A method for fabricating an image sensor including a first layer with positive charges is also provided.