Narrow Bandwidth Polymers and Metal Oxide Interlayers to Improve an Artificial Retina

Recent advances in biomedical engineering have led to a growing worldwide effort to develop a retinal prosthesis designed to restore vision to people suffering from retinal dystrophies such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP). Some of this work has involved interfacing degenerative retinas with organic-semiconductor based devices to elicit an electrical response upon illumination in order to initiate a transduction of stimuli through the remaining retinal network, and thus provide a capability to restore vision. Previous work done by our group has demonstrated several conjugated polymers and small molecules with different bandgaps used towards a device prototype for potential vision restoration in full colour by emulating the each photoresponse from the rods and cones of the human eye.[1]<br/>Commercially available prosthetics to date have been designed primarily as solid-state devices requiring transcranial wiring and associated ancillary optical and power components. Recent research into less invasive and more comfortable devices has turned to organic photovoltaic materials that are lightweight, flexible, and relatively low-cost. Moreover, using organics capitalises on their potential biocompatibility and their electrical properties to create arrays of miniature photovoltaic pixels to stimulate neurons in the retinal pathway and may provide a straightforward and reliable basis for durable visual prosthetics.<br/>However, challenges in using carbon-based semiconductors to elicit enough photocurrent to stimulate a cell in the retinal network remain. Firstly, the power conversion efficiency of thin film organic photovoltaics has only recently exceeded 12%,[2-4] a value that is woefully small. Increasing excitation power until a photocurrent is detectable 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 prosthetic device in a human body. It would be much more favourable to utilise the electronic properties of additional interfacial materials to improve device performance.<br/>In this study, we report on the characteristics of devices fabricated with a range of organic semiconductor polymers for active layers as part of our development of an artificial retina prototype. Previous work has identified a range of promising small molecules that emulate trichromatic vision (rods and three cones); here we describe improvements made with “off the shelf” polymers with narrow absorption bands that suitably match those of human photoreceptors. Importantly, we employ electron-transporting / hole-blocking metal oxide interlayers at the current collecting electrode to elicit capacitive charge transfer of sufficient magnitude for neuronal stimulation. By reducing the difference in work functions of the electrode and LUMO level of the active materials, a significantly higher photoinduced current can be produced for all of the molecules in this study, and the resulting capacitive nature of charge transfer can help to dramatically reduce peroxide-induced Faradiac reactions in device-neuron interfaces.[5]<br/>1. Shkunov, M., et al., Pixelated full-colour small molecule semiconductor devices towards artificial retinas. The Journal of Materials Chemistry C, 2021.<br/>2. Meng, L., et al., Organic and solution-processed tandem solar cells with 17.3% efficiency. Science, 2018. 361(6407): p. 1094-1098.<br/>3. Cui, Y., et al., Over 16% efficiency organic photovoltaic cells enabled by a chlorinated acceptor with increased open-circuit voltages. Nature Communications, 2019. 10(1).<br/>4. Armin, A., et al., A History and Perspective of Non-Fullerene Electron Acceptors for Organic Solar Cells. Advanced Energy Materials, 2021. 11(15): p. 2003570.<br/>5. Ehlich, J., et al. (2022). "Direct measurement of oxygen reduction reactions at neurostimulation electrodes." Journal of Neural Engineering.