1. An image sensor comprising: a semiconductor substrate; a plurality of pixel regions, each pixel region comprising an optically sensitive material over the substrate, the optically sensitive material positioned to receive light; a bias electrode for each pixel region, the bias electrode configured to provide a bias voltage to the optically sensitive material of the respective pixel region; a pixel circuit for each pixel region, 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 pixel circuit configured to reset the voltage on the charge store to a reset voltage during a reset period; the pixel circuit configured to integrate charge from the optically sensitive material to the charge store during an integration period; the pixel circuit configured to read out a signal from the charge store during a read out period; and the pixel circuit including a reference voltage node configured to be coupled to the charge store during the reset period and the read out circuit during the read out period; wherein a reference voltage applied to the reference voltage node is configured to be varied during the operation of the pixel circuit.