Researchers at the National Tsing Hua University in Taiwan, led by Professor Hao-Wu Lin, have demonstrated that high-performance filter-less artificial human photoreceptors can be realized by integrating a novel optical metal/dielectric/metal microcavity structure with vacuum-deposited perovskite photoresponse devices.
Sensory substitution with flexible electronics is one of the intriguing fields of research. Scientists already fabricate electronic devices that can replicate, to a certain degree degree, some of the human senses – touch (electronic skin – e-skin), smell (e-nose), and taste (e-tongue). E-versions of the eye's photoreceptors (e-retinas) could potentially be used in a wide range of applications from robotic humanoid vision to artificial retina implantation for vision restoration or even vision extension into a wider range of wavelength.
"The monolithic realization of an artificial retina that can successfully mimic the spectral responses of rods and of all three types of cones, and can provide compatible weak-light sensitivity, short response time, and large dynamic range is very challenging," Professor Hao-Wu Lin explains.
Special optical layers, called color filters, have been utilized to facilitate the spectral response tuning of photosensors. The integration of these optical filters, though, is expensive and complicated, especially in flexible and curved devices.
"We successfully combined high-performance vacuum-deposited perovskite photoresponse devices with vacuum-deposited optical microcavity structures that, for the first time, realize multi-color photoreceptors on a single substrate with a single pump-down vacuum process," says Lin. "These easy-to-fabricate artificial photoresponse devices truthfully mimic the spectral responses of human color cones and rods."
The team shows that the artificial photoreceptors exhibit excellent performance, similar to human eyes, such as high dynamic range and low noise. Some characteristics, like response time, even out-perform biological human photoreceptors. The devices exhibited a high stability and retained their original photocurrent values under continuous illumination of 5000 lux for 48 hours.
Lin mentions that the resolution of the reproduced image was determined and limited by a mask of a size of 0.5 × 0.5 mm2, which can be significantly improved using a higher resolution fine shadow mask technology (such as developed for high-resolution vacuum-deposited OLED displays) and fine pixelization of the device.
Lin also says that, owing to the versatile vacuum-deposited microcavity structure and the high-performance vacuum-deposited perovskite devices, it is quite easy to fabricate artificial photoreceptors of other animals like birds (birds have four types of cone cells that extend the response to the ultraviolet range).
"Those devices may one day help us understand mechanisms of the wide variety of the animal vision," he adds.
Although there is still a long way to go before this kind of e-retina could be interfaced with a human brain, the researchers believe that their devices are very suitable to be used in robotic eyes. The flexible nature of the devices ensures the possibility of a curved image sensing array that can easily provide a non-distorted image with a single lens system (just like the human eye).