Optimizing photodiode arrays for the use as retinal implants

The basic function of photoreceptors in the human eye is very similar to th at of solar cells, namely delivering potential changes upon illumination. S olar cell or photodiode arrays may therefore serve for replacing natural ph otoreceptors, the progressive degeneration of which often results in a comp lete loss of vision in diseases like retinitis pigmentosa. Based on amorpho us silicon, we have recently developed ultrathin and flexible microphotodio de arrays for subretinal implantation. We now report on the functional opti mization of these devices, finally aiming at the recovery of vision in favo ur of the handicapped patients. The main topic of this study is the enhance ment of stimulation power by the additional conversion of near infrared rad iation, because the visible light alone will not be able to generate a suff iciently high charge transfer to the retina cells. Our concept is to use an amorphous silicon photoconductor for local light-induced enabling of the s timulation current, which is driven by a crystalline silicon solar cell act ing as an infrared receiver. A proof of this concept is presented. First re sults indicate how the performance of our devices can be improved by minimi zing the band discontinuities between amorphous and crystalline silicon, an d by a proper optimization of doping levels and buffer layers. (C) 1999 Els evier Science S.A. All rights reserved.