Image sensors include a second photoelectric conversion device disposed in a lower portion of a substrate and a first photoelectric conversion device extending between the secondary photoelectric conversion device and a light receiving surface of the substrate. Electrical isolation between the first and second photoelectric conversion devices is provided by a photoelectron barrier, which may be an optically transparent electrically insulating material. MOS transistors may be utilized to transfer photoelectrons generated within the first and second photoelectric conversion devices to a floating diffusion region within the image sensor. These transistors may represent one example of means for transferring photoelectrons generated in the first and second photoelectric conversion devices to a floating diffusion region in the substrate, in response to first and second gating signals, respectively. The first and second gating signals may be active during non-overlapping time intervals.