High Efficiency Organic Photovoltaics Based on Non-Fullerene Acceptors (PCE10:ITIC:Y6) for Retinal Prosthesis

Retinal prostheses are a promising option to provide artificial vision to individuals blinded by damaged photoreceptors in the retina. For the electrical stimulation of surviving retinal neurons, early commercialized retinal prosthetic devices have used intra-/extra-ocular cables for power/data transmission, however, these components introduce operation difficulties and/or increase the chance of post-operative complications. To address these issues, several studies have demonstrated the use of photovoltaics and photodiodes, including organic photovoltaic materials, which can generate electrical current without any external physical connections.<br/>In the present work, we demonstrate the use of a high-efficiency, non-fullerene-acceptor (NFA) based organic photovoltaic layer (PCE10:ITIC:Y6) blend for retinal prosthetic applications. Among the three materials, PCE10 was selected because of its deep highest occupied molecular orbital (HOMO) which provides an improved electronic band structure compared to other organic materials (such as P3HT). Both ITIC and Y6 are recently developed NFAs which have been shown to provide superior photovoltaic performance with greater than 15% power conversion efficiency as well as broad spectral response and outstanding stability, making them excellent candidates for investigation as retinal prostheses.<br/>We fabricated a retinal prosthesis devices using PCE10:ITIC:Y6/Au (100/20 nm) layers on ITO-coated glass substrates. The PCE10:ITIC:Y6 blend was spin-coated in an N₂-filled glove box. Then, Au was deposited as the top electrode using an e-beam evaporator. Lastly, polydimethylsiloxane (PDMS) was applied to encapsulate all parts but the center of the top electrode.<br/>Photostimulation was conducted using a 450 nm LED (M450LP1, Thorlabs) which illuminated the light in the optical power density of 36.19 μW/mm² on the surface of the top electrode. The photocurrent of the fabricated device was evaluated in a recording chamber filled with Ames’ medium using patch pipettes in voltage-clamping mode. We located the patch electrode close (~10 μm) to the device surface, and then measured electrical artifacts resulted from the photovoltaic currents in response to 5-sec-long light stimuli. The photocurrents were indirectly calculated in comparison with electric artifacts measured in response to known current amplitudes delivered by a stimulus generator (STG2004, Multi-Channel Systems). As the light intensity increased from 20 to 100 % (36.19 μW/mm²), the peak photocurrent amplitude increased linearly from 53.58 to 242.94 nA/mm². Our previous study reported that electrical currents in similar amplitudes can evoke spiking activities in retinal ganglion cells (RGCs), suggesting that these photovoltaic devices can generate sufficient photocurrent for use as prosthetic retina.<br/>To characterize the relationship between the stimulation duration and photocurrent, we illuminated devices for variable amounts of time ranging from 5 to 1 sec. As the irradiation time decreased, the photocurrent peak amplitude decreased exponentially from 242.94 to 110.94 nA/mm². We also tested the reproducibility of photocurrent response upon repetitive illumination. When devices were illuminated with 5-sec-long light pulses delivered at 0.1 Hz (20 repetitions), the photocurrent produced at the end of the sequence decreased by only ~10% compared to the initial response.<br/>This study characterized the photo-response of a high-efficiency, NFA based organic photovoltaic layer (PCE10:ITIC:Y6) for retinal prosthetic use. Our results showed that fabricated devices were able to generate suitable photocurrent to stimulate RGCs; future experiments will investigate their spiking responses in RGCs. In addition to the wireless nature of the photovoltaic device, we expect the red-shift in stimulation wavelength of the blend results in less phototoxicity.<br/>*Hyunsun Song and Hyeonhee Roh equally contributed to this work.