Heterostructure Photovoltaic Cells for Prosthetic Retina Applications

Abstract

Abstract: : Objective To develop an aluminum gallium arsenide (AlGaAs) electronic retinal implant and to compare the stability of this implant to the silicon electronic retinal implant in an animal model in vivo. Additionally, to examine whether coating the AlGaAs implant with titanium isolates its components (Aluminum, Gallium, Arsenic) from the intraocular environment. Methods: Fabrication The p–n junction solar cell structures were grown by molecular beam epitaxy (MBE). The epilayers were grown on n+ semi–insulating gallium arsenide (GaAs) substrates. Test cells were fabricated on mesas 7µm×7µm while the actual implants were fabricated on 10µm dots using standard lithography and electron beam evaporation for metallization. The AlGaAs/GaAs hetero–junction micro–solar cells of the implants have active areas of 8×10–7cm². Implantation An AlGaAs electronic implant without the titanium coating and a silicon electronic implant were placed into the vitreous humor of separate living Northern tree frogs (Ranas Pipiens) for a maximum of 96 hours. The implants were then removed and examined using high powered light microscopy for stability. Titanium coated implants were then fabricated and two of the titanium coated implants were placed into the vitreous humor of two tree frogs for a total of 2 weeks. Examination of those implants for stability is pending. Results: Under AM1 simulated conditions the cells exhibit conversion efficiency as high as 20.88%, short–circuit current density of 36mA/cm2, open circuit voltage of 0.72V, and fill factor of 0.73. Examination of the implants after the 96 hour implantation period revealed that the outer coating of the AlGaAs implant without the titanium coating underwent partial decomposition in the intraocular environment of the tree frogs. Conclusions: Retinitis pigmentosa and other ocular diseases that result in photoreceptor degeneration, sparing the inner retinal layers continue to be a problem without a solution. An electronic implant that can act as a photo–transducer converting light to suitable electrical signals that can be interpreted by the remaining viable neurons may provide a viable solution. Research previously performed using silicon semiconductor implants show promise but a viable solution is still not available. The aluminum gallium arsenide micro–solar cell is superior to the silicon micro–solar cell because the former has a more favorable spectral response and open circuit voltage. Without a suitable coating the AlGaAs implants are not stable in the intraocular environment.

Keywords: retina: distal (photoreceptors, horizontal cells, bipolar cells) • retinal degenerations: hereditary • retinal degenerations: hereditary