Fabrication of stable organometallic halide perovskite NWs based optoelectronic devices

Geometric Shape Recognition with an Ultra-High Density Perovskite Nanowire Array-Based Artificial Vision System

Artificial vision systems (AVS) have potential applications in visual prosthetics and artificially intelligent robotics, and they require a preprocessor and a processor to mimic human vision. Halide perovskite (HP) is a promising preprocessor and processor due to its excellent photoresponse, ubiquitous charge migration pathways, and innate hysteresis. However, the material instability associated with HP thin films hinders their utilization in physical AVSs. Herein, we have developed ultrahigh-density arrays of robust HP nanowires (NWs) rooted in a porous alumina membrane (PAM) as the active layer for an AVS. The NW devices exhibit gradual photocurrent change, responding to changes in light pulse duration, intensity, and number, and allow contrast enhancement of visual inputs with a device lifetime of over 5 months. The NW-based processor possesses temporally stable conductance states with retention >105 s and jitter <10%. The physical AVS demonstrated 100% accuracy in recognizing different shapes, establishing HP as a reliable material for neuromorphic vision systems.

Image processing with a multi-level ultra-fast three dimensionally integrated perovskite nanowire array

Besides its ubiquitous applications in optoelectronics, halide-perovskites (HPs) have also carved a niche in the domain of resistive switching memories (Re-RAMs). However owing to the material and electrical instability challenges faced by HP thin-films, rarely perovskite Re-RAMs are used to experimentally demonstrate data processing which is a fundamental requirement for neuromorphic applications. Here, for the first time, lead-free, ultrahigh density HP nanowire (NW) array Re-RAM has been utilized to demonstrate image processing via design of convolutional kernels. The devices exhibited superior switching characteristics including a high endurance of 5 × 10⁶ cycles, an ultra-fast erasing and writing speed of 900 ps and 2 ns, respectively, and a retention time >5 × 10⁴ s for the resistances. The work is bolstered by an in-depth mechanistic study and first-principles simulations which provide evidence of electrochemical metallization triggering the switching. Employing the robust multi-level switching behaviour, image processing functions of embossing, outlining and sharpening were successfully implemented.

Down-Scalable and Ultra-fast Memristors with Ultra-high Density Three-Dimensional Arrays of Perovskite Quantum Wires

With strikingly high speed, data retention ability and storage density, resistive RAMs have emerged as a forerunning nonvolatile memory. Here we developed a Re-RAM with ultra-high density array of monocrystalline perovskite quantum wires (QWs) as the switching matrix with a metallic silver conducting pathway. The devices demonstrated high ON/OFF ratio of ∼10⁷ and ultra-fast switching speed of ∼100 ps which is among the fastest in literature. The devices also possess long retention time of over 2 years and record high endurance of ∼6 × 10⁶ cycles for all perovskite Re-RAMs reported. As a concept proof, we have also successfully demonstrated a flexible Re-RAM crossbar array device with a metal-semiconductor-insulator-metal design for sneaky path mitigation, which can store information with long retention. Aggressive downscaling to ∼14 nm lateral dimension produced an ultra-small cell effectively having 76.5 nm² area for single bit storage. Furthermore, the devices also exhibited unique optical programmability among the low resistance states.

Optically tunable ultra-fast resistive switching in lead-free methyl-ammonium bismuth iodide perovskite films

Resistive RAMs (Re-RAMs) have come to the fore as a rising star among the next generation non-volatile memories with fast operational speed, excellent endurance and prolonged data retention capabilities. Re-RAMs are being profusely used as storage and processing modules in neuromorphic hardware and high frequency switches in radio-frequency (RF) circuits. Owing to its intrinsic hysteresis and abundance of charge migration pathways, lead halide perovskites have emerged as a promising switching medium in Re-RAMs besides their ubiquitous usage in optoelectronic devices. Here, we adopted a lead-free eco-friendly methyl-ammonium bismuth iodide (MA₃Bi₂I₉) perovskite (prepared by solvent-free engineering) as the switching medium sandwiched between copper (Cu) and indium doped tin oxide (ITO) electrodes. The devices exhibited a 10⁴ high ON/OFF ratio that provided a large window for the multi-bit data storage in a single cell with good accuracy. Robust endurance of 1730 cycles and good data retention ability of >3 × 10⁵ s were also observed. Careful switching speed measurements showed the devices can operate with an ultra-fast speed of 10 ns for writing and erasing respectively. The devices responded to light illumination and the prolonged retention of the opto-electrically tuned resistance states paved the way for image memorization.

Three-Dimensional Perovskite Nanophotonic Wire Array-Based Light-Emitting Diodes with Significantly Improved Efficiency and Stability

Hybrid perovskites have emerged as promising candidates for highly efficient light-emitting diodes in the past few years due to their excellent crystallinity, high color purity, wide-range bandgap tunability, and solution processability. However, the reported device external quantum efficiency has not reached the level on par with that of conventional inorganic and organic light-emitting diodes. Moreover, device stability still needs substantial improvement. In this work, we demonstrate the fabrication of perovskite nanophotonic wire array-based light-emitting diodes with a capillary-effect-assisted template method. Compared with the planar control device, the nanostructured device demonstrates 45% improvement of external quantum efficiency from 11% to 16% owing to substantial enhancement on device light extraction efficiency verified by optical modeling. Intriguingly, it is also discovered that the nanostructured device possesses 3.89 times lifetime compared to the planar control device, due to effective template passivation. The results here have clearly shown that with a proper photonic device structure design, both the device performance and lifetime can be significantly improved.

Flexible Photodetector Arrays Based on Patterned CH3 NH3 PbI3- x Clx Perovskite Film for Real-Time Photosensing and Imaging

The quest for novel deformable image sensors with outstanding optoelectronic properties and large-scale integration becomes a great impetus to exploit more advanced flexible photodetector (PD) arrays. Here, 10 × 10 flexible PD arrays with a resolution of 63.5 dpi are demonstrated based on as-prepared perovskite arrays for photosensing and imaging. Large-scale growth controllable CH₃ NH₃ PbI_{3- x} Cl_x arrays are synthesized on a poly(ethylene terephthalate) substrate by using a two-step sequential deposition method with the developed Al₂ O₃ -assisted hydrophilic-hydrophobic surface treatment process. The flexible PD arrays with high detectivity (9.4 × 10¹¹ Jones), large on/off current ratio (up to 1.2 × 10³ ), and broad spectral response exhibit excellent electrical stability under large bending angle (θ = 150°) and superior folding endurance after hundreds of bending cycles. In addition, the device can execute the functions of capturing a real-time light trajectory and detecting a multipoint light distribution, indicating that it has widespread potential in photosensing and imaging for optical communication, digital display, and artificial electronic skin applications.

Large and Ultrastable All-Inorganic CsPbBr3 Monocrystalline Films: Low-Temperature Growth and Application for High-Performance Photodetectors

Stability is a key problem that hinders the practical application of lead halide perovskite. Therefore, all-inorganic perovskite CsPbX₃ monocrystalline films are urgently needed to fabricate photoelectric devices. Herein, a low-temperature and substrate-independent growth method is demonstrated to grow millimeter-level inorganic perovskite monocrystalline thin films. These films present good optical and electrical properties comparable to bulk ones. What is more, they exhibit excellent long-term stability toward humidity and thermal treatment. The as-grown CsPbBr₃ monocrystalline films are then fabricated into photodetectors with high photodetection performance. These results demonstrate that the CsPbBr₃ monocrystalline films have potential in fabricating high-performance optoelectronic devices.

Nanotextured Spikes of α-Fe2O3/NiFe2O4 Composite for Efficient Photoelectrochemical Oxidation of Water

We demonstrate for the first time the application of p-NiFe₂O₄/n-Fe₂O₃ composite thin films as anode materials for light-assisted electrolysis of water. The p-NiFe₂O₄/n-Fe₂O₃ composite thin films were deposited on planar fluorinated tin oxide (FTO)-coated glass as well as on 3D array of nanospike (NSP) substrates. The effect of substrate (planar FTO and 3D-NSP) and percentage change of each component (i.e., NiFe₂O₄ and Fe₂O₃) of composite was studied on photoelectrochemical (PEC) water oxidation reaction. This work also includes the performance comparison of p-NiFe₂O₄/n-Fe₂O₃ composite (planar and NSP) devices with pure hematite for PEC water oxidation. Overall, the nanostructured p-NiFe₂O₄/n-Fe₂O₃ device with equal molar 1:1 ratio of NiFe₂O₄ and Fe₂O₃ was found to be highly efficient for PEC water oxidation as compared with pure hematite, 1:2 and 1:3 molar ratios of composite. The photocurrent density of 1:1 composite thin film on planar substrate was equal to 1.07 mA/cm² at 1.23 V_RHE, which was 1.7 times higher current density as compared with pure hematite device (0.63 mA/cm² at 1.23 V_RHE). The performance of p-NiFe₂O₄/n-Fe₂O₃ composites in PEC water oxidation was further enhanced by their deposition over 3D-NSP substrate. The highest photocurrent density of 2.1 mA/cm² at 1.23 V_RHE was obtained for the 1:1 molar ratio p-NiFe₂O₄/n-Fe₂O₃ composite on NSP (NF1-NSP), which was 3.3 times more photocurrent density than pure hematite. The measured applied bias photon-to-current efficiency (ABPE) value of NF1-NSP (0.206%) was found to be 1.87 times higher than that of NF1-P (0.11%) and 4.7 times higher than that of pure hematite deposited on FTO-coated glass (0.044%). The higher PEC water oxidation activity of p-NiFe₂O₄/n-Fe₂O₃ composite thin film as compared with pure hematite is attributed to the Z-path scheme and better separation of electrons and holes. The increased surface area and greater light absorption capabilities of 3D-NSP devices result in further improvement in catalytic activities.

All Inorganic Cesium Lead Iodide Perovskite Nanowires with Stabilized Cubic Phase at Room Temperature and Nanowire Array-Based Photodetectors

Alluring optical and electronic properties have made organometallic halide perovskites attractive candidates for optoelectronics. Among all perovskite materials, inorganic CsPbX₃ (X is halide) in black cubic phase has triggered enormous attention recently owing to its comparable photovoltaic performance and high stability as compared to organic and hybrid perovskites. However, cubic phase stabilization at room temperature for CsPbI₃ still survives as a challenge. Herein we report all inorganic three-dimensional vertical CsPbI₃ perovskite nanowires (NWs) synthesized inside anodic alumina membrane (AAM) by chemical vapor deposition (CVD) method. It was discovered that the as-grown NWs have stable cubic phase at room temperature. This significant improvement on phase stability can be attributed to the effective encapsulation of NWs by AAM and large specific area of these NWs. To demonstrate device application of these NWs, photodetectors based on these high density CsPbI₃ NWs were fabricated demonstrating decent performance. Our discovery suggests a novel and practical approach to stabilize the cubic phase of CsPbI₃ material, which will have broad applications for optoelectronics in the visible wavelength range.