1. A photodetector comprising: an integrated circuit; and at least two optically sensitive layers, a first optically sensitive layer and a second optically sensitive layer, the first optically sensitive layer over at least a portion of the integrated circuit and the second optically sensitive layer over the first optically sensitive layer; wherein each optically sensitive layer is interposed between two electrodes, the electrodes including a respective first electrode and a respective second electrode; wherein the integrated circuit selectively applies a bias to the electrodes and reads signals from the optically sensitive layers, wherein the signal is related to the number of photons received by the respective optically sensitive layer.