Tandem near-infrared organic photodiodes (OPDs) for future application in artificial retinal implants (Conference Presentation)

Abstract

Age-related retinal diseases lead to blindness due to progressive loss of image-capturing photoreceptor neural cells, which are responsible for the conversion of light entering the eye into electrical signals that are sent to the brain. Retinitis pigmentosa and age-related macular degeneration are two leading causes of severe visual losses in adult individuals, affecting over 1 million people worldwide. In these diseases, rod and cone photoreceptor cells are progressively lost while neural cells in the retinal network remain functional. Electronic retinal prostheses have great potential to restore sight by electrical stimulation of the surviving neurons. In this project, we aim at developing an artificial retinal implant based on organic photodetectors (OPDs). Neural stimulation is provided by an OPD pixel array processed on ultrathin plastic foil. Upon illumination with near-infrared (NIR) light, photo-generated electrical charge in each OPD pixel is delivered to the biological tissue via stimulating electrodes. Flexibility and softness of organic materials allow to interface intimately with neurons so that electrical signals generated by the implant are translated into bio-signals. Firstly, we investigated the photovoltaic performance of NIR-sensitive OPDs based on polymer:fullerene bulk heterojunction. We used PDPP3T:PCBM photoactive layer to detect NIR light up to 930 nm. We showed that by going from a single to a tandem OPD the open-circuit voltage can be doubled and the charge threshold for neural stimulation can be reached at lower light intensities. This results in a more efficient cellular stimulation while maintaining high implant resolution. Furthermore, we analysed the stimulating electrode behavior in a biological-like environment. We characterized the double-layer capacitance of gold, platinum and titanium nitrite (TiN) electrodes by pulsed-voltage measurements in phosphate buffered saline solution (PBS). We observed that TiN exhibits the highest charge capacitance due to the high surface roughness, which enables to maximize the charge injection into the electrolyte. Using a combination of experimental work and modeling we studied the process of charge injection into the biological tissue. We simulated the injected charge during 1 ms NIR light pulse under different illumination conditions. We compared the performance of retinal implants based on single or tandem OPD pixels coupled to 20 µm stimulating electrodes. As we expected, tandem OPD pixels always maximize charge injection into the electrolyte due to their higher photo-voltage. Moreover, the high double-layer capacitance of TiN electrodes metal electrodes results in sufficient charge injection levels for neural stimulation, which is generally achieved between 1 and 4 nC. In conclusion, we predicted that neural activity can be triggered using organic photovoltaic pixels in combination with high charge capacitance electrode materials. These findings are paving way to the development of a high-resolution retinal prostheses based on organic soft materials.