A bioinspired flexible neuromuscular system based thermal-annealing-free perovskite with passivation

Brain-inspired electronics aim to mimic the energy efficiency of the human brain, which vastly outperforms Von Neumann computers. Organic-inorganic hybrid perovskites (OHPs) are promising candidates for low-energy artificial synapses due to their low activation energy and mixed ionic/charge carrier conductivity. However, defects in perovskite films, formed during traditional high-temperature solution processing, hinder their performance and stability, increasing energy consumption. The creation of high-quality perovskite films at room temperature, without thermal annealing, is crucial for flexible electronics and bio-integrated systems. This research addresses these challenges by developing a room-temperature fabrication process for perovskite artificial synapses, coupled with defect passivation to achieve ultra-low energy consumption and high operating speeds. Furthermore, the integration of these synapses into a neuromuscular system addresses the need for physical intelligence (PI) in next-generation bio-inspired robots, enabling functions such as muscular fatigue warnings, enhancing safety and operational reliability.

Existing literature highlights the energy efficiency advantages of brain-inspired electronics compared to traditional computing architectures. Several studies have explored the use of organic-inorganic hybrid perovskites (OHPs) in artificial synapses due to their inherent properties. However, challenges remain in mitigating the formation of defects in perovskite films during fabrication, as these defects act as non-radiative recombination centers and increase energy consumption. While solution processing is advantageous for OHP synthesis, traditional methods often require high-temperature steps that are incompatible with flexible substrates. Previous attempts at creating artificial neuromuscular systems have focused primarily on unconditioned movements, lacking higher-level motion-status feedback. This research builds upon existing work by focusing on room-temperature fabrication and defect passivation to overcome limitations in energy efficiency and stability, also incorporating the development of an artificial neuromuscular system with integrated feedback mechanisms.

This study employed a room-temperature (RT) fabrication process for perovskite films. A low-boiling point solvent system (methylamine-ethanol and acetonitrile) was used to disperse the perovskite precursor. This solution was spin-coated onto a flexible substrate (PET-ITO), followed by passivation using a phenethyl ammonium iodide (PEAI) layer. The PEAI passivation layer was strategically applied to minimize defects and improve film quality. The resulting perovskite film, with a thickness of ~720 nm and a thin PEAI coating, was then used to construct artificial synaptic devices with an Au top electrode. The device fabrication was conducted in a nitrogen-filled glovebox to maintain an inert atmosphere. Various characterization techniques, including in-situ photoluminescence (PL), X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), time-resolved photoluminescence (TRPL), and space-charge-limited current (SCLC) measurements were used to analyze the material properties and device performance. Density Functional Theory (DFT) calculations were performed to understand ion migration pathways and activation energies. Operando Kelvin probe force microscopy (i-KPFM) and Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) were utilized to study ion migration dynamics under an applied bias. To assess mechanical flexibility, bending tests were conducted with a radius of curvature of 4.5 mm. The performance of the artificial synapse was evaluated through measurements of its response to various electrical stimuli, including paired-pulse facilitation (PPF), spike-duration dependent plasticity (SDDP), spike-rate dependent plasticity (SRDP), and long-term potentiation (LTP). Finally, a neuromuscular system was fabricated by integrating the perovskite synapse with an electrochemical artificial muscle (IPMC) to demonstrate a muscular-fatigue warning system.

The room-temperature fabrication method produced high-quality perovskite films with a strong [001] preferred orientation within 10 seconds. PEAI passivation significantly reduced defect density (from 3.14 × 10¹⁵ cm⁻³ to 1.43 × 10¹⁵ cm⁻³), increased charge carrier lifetime, and improved film morphology. The resulting artificial synapse exhibited various synaptic functions mimicking biological synapses, including paired-pulse facilitation (PPF), spike-duration-dependent plasticity (SDDP), and spike-rate-dependent plasticity (SRDP). Notably, the device achieved an unprecedentedly low energy consumption of 13.5 aJ per synaptic event, which is the lowest reported for two-terminal artificial synapses, combined with an extremely high response frequency up to 4.17 MHz. The device demonstrated excellent stability, showing only slight performance degradation after 2000 bending cycles, indicating great flexibility. The integrated neuromuscular system successfully demonstrated the ability to generate a muscular-fatigue warning signal based on the accumulated stimulation input. DFT calculations revealed the low activation energy of iodide ion migration (0.597 eV), corroborating the ultra-low energy consumption. The PEAI passivation was observed to fill both iodine and cation vacancies, suppressing ion migration.

This study successfully addressed the challenges associated with fabricating high-performance, energy-efficient perovskite-based artificial synapses. The room-temperature fabrication process, coupled with PEAI passivation, significantly improved device performance and stability while demonstrating the potential for flexible and bio-integrated applications. The extremely low energy consumption and high response frequency achieved represent a significant advancement in the field of neuromorphic computing. The demonstration of the artificial neuromuscular system with muscular fatigue warning capabilities opens exciting avenues for developing more sophisticated bio-inspired robots. The findings highlight the potential of perovskites as a highly promising material for next-generation neuromorphic devices. The success in mitigating defects and improving charge transport properties offers insights for future material design and device optimization. Further research could focus on exploring various passivation strategies and integration with other neuromorphic components to create more complex and functional systems.

This research presents a significant advancement in the development of artificial synapses for neuromorphic computing. The room-temperature fabrication of a highly efficient perovskite-based artificial synapse with ultra-low energy consumption (13.5 aJ) and high response frequency (4.17 MHz) showcases the potential of this technology. The integration of the synapse into a neuromuscular system capable of muscular-fatigue warning further demonstrates its versatility. Future studies could investigate the long-term stability of the device under continuous operation and explore its potential integration into complex neuromorphic circuits and bio-integrated systems.

While the study demonstrates remarkable performance, some limitations exist. The long-term stability of the device under extreme environmental conditions (e.g., high temperature, humidity) needs further investigation. Scaling up the fabrication process for mass production remains a challenge. Furthermore, the current neuromuscular system is a proof-of-concept demonstration and further refinements are required for practical applications.