A flat panel X-ray imager using an amorphous selenium detector which uses a polymer grid structure within the X-ray conversion layer to form an interface between the X-ray conversion layer and the pixel electrodes. The grid structure provides an additional physical barrier between pixel electrodes that reduces electrical bridging due to crystallization of the amorphous selenium due to temperature increases while also reducing the delamination temperature by providing a good anchor to the selenium, as well as a "cushion" for thermal expansion or contraction of the pixel electrodes. The grid can further include one or more grid electrodes for effecting avalanche-gain within the X-ray conversion layer while minimizing dark current entry into the selenium layer from the pixel electrode.