Energy-Efficient Neuromorphic Synapses with Lead-Free Cs₃Bi₂I₉ Memristors

Neuromorphic hardware demands memory devices that switch reliably at ultralow power—yet most high-performance memristors rely on toxic lead-based perovskites. A breakthrough uses lead-free Cs₃Bi₂I₉ perovskite, fabricated in a flexible Au/PMMA/Cs₃Bi₂I₉/PMMA/Au crossbar, to deliver robust resistive switching and synaptic behavior at femtojoule energy scales. In this post, we’ll explore the materials, device architecture, synaptic performance, and why Navson’s precision spin coaters are key to reproducible, scalable fabrication.

Pioneering Lead-Free Perovskite Memristors

Lead-free Cs₃Bi₂I₉ replaces toxic Pb with bismuth, retaining favorable band structure for resistive switching. When sandwiched between PMMA buffer layers and Au electrodes on flexible PET, these memristors:

  • Achieve on/off ratios of ~10⁶
  • Endure >10⁴ switching cycles
  • Retain data beyond 10³ seconds at elevated temperatures

Materials & Device Fabrication

  1. Bottom Au Electrode: Thermal evaporation through a shadow mask
  2. PMMA Buffer Layer: Spin-coat + anneal for dielectric stabilization
  3. Cs₃Bi₂I₉ Film: Spin-coat precursor in DMF/DMSO, anneal to crystallize
  4. Top PMMA/Au Electrode: Final spin-coat and metallization

Navson’s spin coater ensures uniform thickness and crack-free morphology at each step.

Characterization Highlights

  • Structural & Chemical: XRD confirms hexagonal Cs₃Bi₂I₉; XPS verifies Bi, Cs, I core levels; SEM shows smooth films
  • Electrical Switching: Vₛₑₜ ≈ +0.20 V, Vᵣₑₛₑₜ ≈ –0.28 V with low cycle-to-cycle variance
  • Conduction Mechanisms: HRS follows space-charge-limited conduction, LRS is ohmic
  • Thermal Stability: Activation energy from retention tests indicates stable data well beyond 10³ s

Emulating Biological Synapses

Cs₃Bi₂I₉ memristors mimic synaptic functions with:

  • Paired-Pulse Facilitation: PPF index >1.2 at 50 ns–10 µs intervals
  • Long-Term Potentiation/Depression: Stable weight updates over 60 pulses
  • Ultralow Energy: ≈25 aJ per switching event—among the lowest for lead-free devices

Benchmarking Against Lead-Based Devices

Compared to Pb-perovskite memristors, Cs₃Bi₂I₉ crossbars match or exceed:

  • On/off ratio
  • Endurance
  • Retention time
  • Multilevel state control
  • Switching speed

—all while eliminating lead toxicity.

Why Navson Spin Coaters Matter

  • Stable Speed Control: Consistent film thickness for reproducible devices
  • Recipe Flexibility: Tailor spin profiles for perovskite/PMMA chemistries
  • Rapid Throughput: Fast spin-coat and anneal cycles accelerate development

Acknowledgements

  • Dr. Sujaya Kumar Vishwanath (sujayav@iisc.ac.in), Dr. Shubham Sahay (ssahay@iitk.ac.in), and Dr. Aditya Sadhanala (sadhanala@iisc.ac.in) – corresponding authors
  • Chaya Karkera and Tauheed Mohammad – equal contributions to experiments and analysis
  • Pritish Sharma, Rantej Naik Badavathu, Upanya Khandelwal, Anil Kanwat, Poulomi Chakrabarty, Devamrutha Suresh – materials preparation, device fabrication, and characterization
  • Centre for Nanoscience and Engineering, IISc Bangalore; Dept. of Electrical Engineering, IIT Kanpur; Energy Research Institute @ NTU Singapore – institutional support and facility access

Conclusion & Next Steps
Lead-free Cs₃Bi₂I₉ perovskite memristors, enabled by precision spin coating, pave the way for sustainable, ultralow-power neuromorphic hardware. With on/off ratios of 10⁶, endurance >10⁴ cycles, and synaptic energy as low as 25 aJ, they offer a viable replacement for lead-based alternatives.

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Reference
S. Research et al., “PBA-MoS₂ Composites for Transparent Counter Electrodes in DSSCs,” Solar Energy Materials & Solar Cells, 2025.
https://pubs.acs.org/doi/abs/10.1021/acsenergylett.5c00411

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