An artificial electronic retina that can “see” in a similar way to the human vision system and can recognize hand­written digits has been built by KAUST researchers as they seek to develop better options for computer vision appli­cations. Mani Teja Vijjapu, an electrical engineering Ph.D. student, Khaled Nabil Salama and coworkers have designed and fabricated an array of photo­receptors that detect the intensity of visible light via a change in electrical capa­citance, mimicking the behavior of the eye’s rod retina cells. When the array was connected to an electronic CMOS-sensing circuit and a spiking neural network (a single-layer network with 100 output neurons), it was able to recognize hand­written numbers with an accuracy of around 70 percent.

“The ultimate goal of our research in this area is to develop efficient neuromorphic vision sensors to build efficient cameras for computer vision appli­cations,” explained Salama. “Existing systems use photo­detectors that require power for their operation and thus consume a lot of energy, even on standby. In contrast, our proposed photo­receptors are capacitive devices that don’t consume static power for their operation.” 

The photo­receptor array is made by sandwiching a material with suitable optical and dielectric properties between a bottom aluminum electrode and a patterned top electrode of indium tin oxide to form a pixelated array of miniature light-sensitive metal-insulator-metal capacitors. The array is made on a thin substrate of polyimide so that it is flexible and can be curved as desired, including a hemi­spherical shape mimicking the human eye.

In selecting materials for their photo­receptor, the team used a hybrid material of perovskite (methylammonium lead bromide) nano­crystals embedded in terpolymer poly­vinylidene fluoride tri­fluoro­ethylene-chloro­fluoro­ethylene (PVDF-TrFE-CEF). Already of great interest in solar cell research, the perovskite is a strong absorber of visible light, while PVDF-TrFE-CEF has a high dielectric constant. “We chose hybrid perovs­kites because of their exceptional photo­electronic properties, such as excellent light absorption, long carrier lifetime and high carrier mobility,” explained Vijjapu.

Tests with a 4x4 array and LED illumination of different visible colors indicate that the optical response of the array mimics the response of the human eye with a maximum sensi­tivity to green light. Importantly, the photo­receptors are also found to be highly stable, with no change in response even after being stored for 129 weeks in ambient condi­tions. Future plans for the team include building larger arrays of photo­receptors, optimizing the interface circuit design and employing a multilayered neural network to improve the accuracy of the recog­nition functionality. (Source: KAUST)

Reference: M. T. Vijjapu et al.: A flexible capacitive photoreceptor for the biomimetic retina, Light: Sci. Appl. 11, 3 (2022); DOI: 10.1038/s41377-021-00686-4

Link: Sensors Lab, Advanced Membranes and Porous Materials Center, Computer, Electrical and Mathematical Science and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal, Kingdom of Saudi Arabia