Vertical-junction photodiodes for smaller pixels in retinal prostheses.

Area of Science:

• Biomedical Engineering
• Ophthalmology
• Materials Science

Background:

• Current photovoltaic pixels for vision restoration in age-related macular degeneration (AMD) are limited by pixel size, impacting visual acuity.
• Smaller pixels face challenges with decreased electric field penetration and photodiode responsivity, reducing device efficiency.
• Novel designs are needed to enable smaller pixels for improved prosthetic vision.

Purpose of the Study:

• To develop and validate novel photodiodes with vertical p-n junctions for enhanced subretinal stimulation.
• To investigate the performance of smaller photovoltaic pixels (down to 20μm) for vision restoration.
• To assess the impact of a vertically oriented electric field on retinal stimulation thresholds.

Main Methods:

• Fabrication of silicon photodiode arrays with vertical p-n junctions in sizes ranging from 20μm to 55μm.
• Characterization of optical and electronic performance (dark current, responsivity, quantum efficiency) in vitro.
• Measurement of retinal stimulation thresholds using visually evoked potentials in rats with retinal degeneration, employing a vertical electric field.

Main Results:

• Photodiodes demonstrated high responsivity (0.51 A W⁻¹) and 85% internal quantum efficiency, independent of pixel size.
• Low dark current (<10 pA) and uniform pixel performance across arrays were observed.
• The stimulation threshold was consistently low (0.057±0.029mW mm⁻²) and independent of pixel size.

Conclusions:

• Vertical p-n junction photodiodes achieve excellent charge collection efficiency, scalable down to 20μm pixel sizes.
• A vertically oriented electric field ensures stimulation thresholds are independent of pixel size.
• These findings represent a critical step towards validating smaller photovoltaic pixels for subretinal implantation.