Optimizing photodiode arrays for the use as retinal implants

The basic function of photoreceptors in the human eye is very similar to that of solar cells, namely delivering potential changes upon illumination. Solar cell or photodiode arrays may therefore serve for replacing natural photoreceptors, the progressive degeneration of which often results in a complete loss of vision in diseases like retinitis pigmentosa. Based on amorphous silicon, we have recently developed ultrathin and flexible microphotodiode arrays for subretinal implantation. We now report on the functional optimization of these devices, finally aiming at the recovery of vision in favour of the handicapped patients. The main topic of this study is the enhancement of stimulation power by the additional conversion of near infrared radiation, because the visible light alone will not be able to generate a sufficiently high charge transfer to the retina cells. Our concept is to use an amorphous silicon photoconductor for local light-induced enabling of the stimulation current, which is driven by a crystalline silicon solar cell acting as an infrared receiver. A proof of this concept is presented. First results indicate how the performance of our devices can be improved by minimizing the band discontinuities between amorphous and crystalline silicon, and by a proper optimization of doping levels and buffer layers.