1. An image sensor comprising: a semiconductor substrate; a plurality of pixel regions, each of the plurality of pixel regions comprising an optically sensitive material over the substrate, the optically sensitive material positioned to receive light; a bias electrode for each of the plurality of pixel regions, the bias electrode configured to provide a bias voltage to the optically sensitive material of the respective pixel region; a pixel circuit for each of the plurality of pixel regions, each pixel circuit comprising a charge store formed on the semiconductor substrate and a read out circuit, the charge store in electrical communication with the optically sensitive material of the respective pixel region, the charge store being a diode that is characterized by a leakage current; the pixel circuit configured to reset the voltage on the charge store to a reset voltage during a reset period; the pixel circuit configured to discharge charge from the charge store to the optically sensitive material during an integration period; and the pixel circuit configured to read out a signal from the charge store during a read out period; wherein the bias voltage is configured to change from a first level during the reset period to a second level during the integration period; wherein the bias voltage is configured to change from the second level during the integration period to a third level during the read out period; and wherein the choice of the second level bias voltage during the integration period is to lead to a lower charge store leakage current during the integration period compared to during the reset period.