Interfacial Effects in Organic Semiconductor Layered Systems in an Optoelectronic Prosthetic Prototype

Recent advances in biomedical engineering have led to a growing worldwide effort to develop a retinal prosthesis designed to restore vision in people suffering from retinal dystrophies such as age-related macular degeneration (AMD) or retinitis pigmentosa (RP).[1-5] Some of this work has involved interfacing degenerative retinas with organic semiconductive inks to elicit an electrical response upon illumination in order to initiate a transduction of stimuli through the remaining retinal network, and thus restore vision. Previous work done by our group has demonstrated use of four materials to emulate the photoresponse in the rods and cones of the human eye (short, medium, long wavelengths in cones and a broadly based mid-range wavelength for rods).[6, 7] From Foerster’s early attempts to stimulate a nervous system in 1929 [8] to the commercially available Argus II system[9, 10] created by industry scientists at Second Sight Medical Products, prosthetics to date have been designed primarily as solid-state devices requiring transcranial wiring and associated ancillary components. Research this decade has capitalised on the electrical properties of carbon conjugated molecules and polymers to create arrays of miniature photovoltaic pixels to stimulate neurons in the retinal pathway. [4, 5, 11-15] If mounted on flexible and biocompatible substrates or injected directly into the eye via nanoparticle carriers,[16, 17] these materials may provide a straightforward and reliable basis for durable visual prosthetics. Significant challenges inherent in using carbon-based semiconductors are quantum efficiency and signal strength because, while carbon conjugated materials are well-known to form excitons under direct illumination, the resulting photo-response is notoriously low. Increasing power until a signal is discerned is a common work-around, however, while it might prove functional in a laboratory setting, the unnaturally powerful source may result in catastrophic damage to a device. Here, we manipulate signal strength and tune the balance between capacitive and Faradiac charge transfer using a range of interfacial layers with organic inks comprised of small molecules to improve signal strength as part of our development of an artificial retina prototype