Lead-free

Researchers at the University of New South Wales, University of Queensland, RMIT University, University of Sydney, Australian National University, Southeast University in China, and the Hong Kong Polytechnic University, have developed a fully light-driven optoelectronic synapse based on a lead-free tin halide perovskite that strengthens under visible light and weakens under near-infrared light, entirely without electrical erasure, at an energy cost approaching that of biological synapses.

Visible light strengthens the synapse (excitation), near-infrared light weakens it (inhibition), via charge transfer between the perovskite and C₆₀ layers - enabling tasks like traffic sign recognition and motion tracking. Credit: Mei et al., Science Advances (2026)

Neuromorphic vision systems aim to combine sensing, memory, and processing in a single device to avoid the energy cost of shuttling data between separate components, an approach inspired by how biological neurons balance excitatory and inhibitory synaptic signals to process visual information efficiently. Most reported optoelectronic synapses, though, have focused on emulating only excitatory plasticity, the strengthening response that most photodetector materials naturally produce under illumination, while giving comparatively little attention to inhibitory plasticity, the suppression response that helps biological vision filter out redundant input and sharpen attention. Because most devices rely on light to build up photocarriers and boost conductance, achieving the opposite response, a controlled reduction in conductance, has typically required a separate electrical erasure step rather than optical control, adding complexity and undercutting the low-power, all-optical appeal of the approach.

Researchers at Sophia University in Tokyo, led by Professor Yuko Takeoka of the Faculty of Science and Technology's Department of Materials and Life Sciences, together with contributing researchers from the National Institute for Materials Science (NIMS), have developed a multifunctional molecular passivation strategy for tin-based perovskite solar cells that improves both efficiency and long-term durability. Adding a small heteroatom molecule, 2-aminobenzothiazole (2-ABZ), to quasi-two-dimensional Ruddlesden-Popper tin perovskite devices raised power conversion efficiency to 9.07%, up from 6.60% for untreated control devices, while improving how well the cells hold up over time.

Tin-based perovskites are widely seen as the leading lead-free alternative to conventional lead halide perovskite solar cells, offering a route around the toxicity concerns associated with lead, but they have historically lagged behind on both efficiency and stability, limiting their commercial viability. The Sophia University and NIMS team addressed this by incorporating 2-ABZ, a molecule containing nitrogen, carbon, sulfur, and hydrogen, into the perovskite layer as what the researchers describe as a multifunctional molecular stabilizer. Rather than targeting a single failure mode, 2-ABZ acts across the device architecture: it regulates crystallization, reduces trap formation, prevents ion migration, inhibits tin oxidation, and improves interfacial energy alignment.

Researchers at the University of Electro-Communications (UEC) in Tokyo and Madan Mohan Malviya University of Technology (MMMUT) in India have developed a new solvent and additive system that enables solution-processed, lead-free germanium-tin (Ge-Sn, 1:1) perovskite solar cells, reaching 4.56% efficiency, which the team describes as the highest performance achieved with any solution-based method for this composition to date.

Germanium and tin are both being explored as isoelectronic, lower-toxicity alternatives to lead in halide perovskite solar cells, but Ge-Sn alloyed perovskites have been difficult to process from solution: the team found that dimethyl sulfoxide (DMSO), a solvent commonly used in perovskite fabrication, reacts too rapidly with the germanium precursor GeI₂, preventing the formation of clean, pure-phase films. To get around this, the researchers developed a DMSO-free solvent system based on a mixture of N,N-dimethylformamide, N,N′-dimethylpropyleneurea, and 4-tert-butylpyridine, which suppresses that unwanted reactivity and allows Ge-rich perovskite films to be wet-coated while forming a pure-phase Ge-Sn (1:1) structure, confirmed by X-ray diffraction.

Researchers at National Cheng Kung University in Taiwan have developed a lead-free, oxide-perovskite resistive random-access memory (RRAM) device that combines a Na_0.5K_0.5NbO₃ (NKN) perovskite layer with hafnium oxide (HfO₂), demonstrating forming-free switching, high endurance, and highly symmetric synaptic behavior suited to neuromorphic computing hardware.

RRAM, or memristor, devices are widely studied as artificial synapses for neuromorphic computing because their resistance can be tuned continuously to mimic the strengthening and weakening of biological synaptic connections, a property known as spike-timing-dependent plasticity (STDP). A persistent obstacle in the field has been achieving STDP that is symmetric between potentiation (strengthening) and depression (weakening), since imbalanced synaptic response introduces systematic bias into the weight updates a neural network relies on for learning, degrading accuracy over repeated training cycles.

Researchers at the University of Wisconsin–Madison, the National Laboratory of the Rockies, the University of Toledo, and the University of Colorado Boulder, have designed a tin-based perovskite with built-in protection against air and moisture, addressing a major obstacle to achieving practical lead-free perovskite photovoltaics. Devices built with the new material reached 16.2% power conversion efficiency, among the best reported for tin perovskite solar cells, while showing markedly improved durability.

Tin-based perovskites are considered the leading lead-free alternative thanks to favorable light-absorbing and electronic properties, but they are especially vulnerable to oxygen and moisture, which rapidly degrade the material and its performance, limiting practical deployment. To address this, the team examined how the organic components of these hybrid materials assemble and designed new tin perovskites intended to impede damage from oxygen and water. They synthesized and compared a family of closely related organic cations substituted with different halogen atoms (fluorine, chlorine, and bromine), and found that the chlorinated version, 4-chloro-phenethylammonium (4ClPEA), packed most tightly into the crystal structure.

Researchers at the University of Moratuwa in Sri Lanka and the Sri Lanka Institute of Information Technology have built a machine-learning framework that screens oxide perovskites for band gaps suited to solar absorbers, narrowing a starting pool of over half a million candidate compositions down to 38 lead- and cadmium-free contenders worth further study.

Oxide perovskites are attractive for photovoltaics because their electronic and optical properties can be tuned through composition, but most known compositions are either insulating or have band gaps too wide for efficient light absorption. Identifying the minority with moderate, solar-relevant band gaps has traditionally meant either slow experimental trial and error or computationally expensive density functional theory (DFT) calculations, which become impractical once thousands of candidate compositions are involved.

Researchers at Yale University, working with the U.S. Department of Energy's Argonne National Laboratory and Brookhaven National Laboratory, and with contributing researchers from the University of Toledo, the University of Pennsylvania and Cornell University, have identified a previously unknown structural state in the lead-free double perovskite Cs₂AgInCl₆, a material valued for its ability to emit warm white light across the visible spectrum for LED applications. Triggered by photoexcitation, the new state forms when silver and indium ions swap positions within the crystal lattice, in under a nanosecond, but the material takes several milliseconds to relax back to its ordered ground state. 

Cs₂AgInCl₆'s white-light emission relies on self-trapped excitons (STEs): photoexcited charge carriers that distort the crystal lattice around them and release their energy as broadband light rather than a narrow emission line. While STE formation itself has been studied before, the researchers found that earlier measurements, typically limited to nanosecond timescales, had missed a slower process hiding underneath it. Using transient optical spectroscopy extended out to microsecond and millisecond delay times, they detected a broadband photoinduced absorption signal that persisted long after the microsecond-scale STE signal had fully decayed, pointing to a distinct, unidentified structural state activated by light.

Researchers at Northeastern University and Shijiazhuang Tiedao University have published a review that examines how a "high-entropy" design strategy can be used to improve perovskite-structured, lead-free ferroelectric ceramics for dielectric energy storage - components used in pulsed-power systems, EVs and smart grids.

Dielectric capacitors made from these ceramics store energy fast and cycle reliably, but conventional lead-free formulations have long trailed lead-based ones on energy storage density, efficiency and stability, while lead-based systems carry the toxicity concerns endemic to lead-containing materials generally. High-entropy perovskites, formed by combining five or more principal elements into a single-phase solid solution, offer a way to close that gap. The review attributes the improvement to four coupled mechanisms: a high-entropy stabilization effect that suppresses unwanted secondary phases by raising the material's configurational entropy; a lattice distortion effect that disrupts long-range ferroelectric order through local strain fields, producing the narrow hysteresis loops and low remanent polarization that relaxor ferroelectrics need for efficient energy storage; a sluggish diffusion effect that slows atomic migration during processing, refining grain size and raising breakdown strength; and a "cocktail effect," in which the combined elements produce properties beyond what any of them would contribute individually. In these materials, the authors note, high-entropy induced lattice distortion has been shown to stabilize polar nanoregions as small as about 1 nanometer, cutting polarization hysteresis without sacrificing peak polarization.

Researchers at Liaoning Technical University in China have used device simulation to show that a lanthanum ferrite (LaFeO₃) hole transport layer could substantially improve the performance of solar cells based on Cs₂AgSbBr₆, a lead-free double perovskite absorber. Using the Solar Cell Capacitance Simulator (SCAPS) together with material parameters measured from an experimentally synthesized LaFeO₃ thin film, the team modeled a complete device and predicted a power conversion efficiency (PCE) of 25.01%.

Cs₂AgSbBr₆ is considered a promising lead-free alternative to conventional lead halide perovskites, combining a near-ideal bandgap (around 1.6 eV) with strong thermal stability (up to 500°C) and resilience under ambient conditions. Its performance has been held back, however, by a mismatch with conventional hole transport layers: the material's deep valence band maximum creates a substantial energy offset with common HTLs such as Spiro-OMeTAD and PEDOT:PSS, hindering charge extraction, promoting recombination, and limiting achievable open-circuit voltage. Existing commercial HTL options also tend to be costly and comparatively unstable.

Researchers from Pohang University of Science and Technology, University of Electronic Science and Technology of China and Sungkyunkwan University have developed a volatile-assisted surface reconstruction strategy that addresses one of the most critical limitations of tin (Sn²⁺) halide perovskites: their intrinsic redox instability and rapid degradation under ambient conditions.

Sn²⁺-based halide perovskites are widely considered promising lead-free semiconductors due to their tunable bandgaps, low effective mass, and efficient charge transport. However, their practical deployment has been hindered by the presence of undercoordinated Sn²⁺ surface sites, which are highly susceptible to oxidation. This process induces uncontrolled self-p-doping and generates trap states, ultimately destabilizing device operation and severely limiting performance reproducibility. To overcome this, the researchers introduced a volatile-assisted coordination mechanism based on potassium acetate (KAc). When applied to CsSnI₃ films, acetate ions transiently coordinate with undercoordinated Sn²⁺ sites, forming tin acetate (Sn(Ac)₂), a volatile intermediate that is removed during mild annealing. This process effectively eliminates reactive surface tin species.