Amorphous silicon photodiodes for replacing degenerated photoreceptors in the human eye

An ambitious neurotechnology program has been started in Germany in 1995, in the long run aiming at the realization of visual prostheses for blind people. A broad technological approach has been chosen which besides crystalline silicon microelectronics also involves amorphous silicon photodiodes for subretinal implantation, because thin film technology may be able to offer better solutions to some of the complex problems. Special topics we report on here include the development of low temperature deposition techniques for enabling the use of flexible plastic or bio-degradable substrate foils, the study of protective and biocompatible coatings, as well as novel contacting and energy supply schemes. The key issue for stimulating retina cells by the use of technically generated photocurrents is an optimum capacitive coupling to these cells. For this purpose we study several contact layers (p-doped a-Si:H, microcrystalline Si, metal-induced crystallization) which provide high perpendicular but at the same time low lateral conductivity, thereby greatly reducing parasitic losses to the surrounding tissue. The photovoltaic mode of operation of the implanted photodiodes may be limited due to shortcomings in the charge transfer to the nerve cells, in which case additional infrared energy has to be coupled into the devices. Local light-induced stimulation can then be realised by using an a-Si:H i-layer as a photoresistor on top of the IR-sensitive solar cell.