Two-Dimensional Halide Perovskites: Approaches to Improve Optoelectronic Properties

Multifunctional Lead-Free Halide Perovskite Based Poly(vinylidene fluoride) Composites for Biomechanical Energy Harvesting and Self-Powered Piezo-Optoelectronic Applications

Lead-free halide perovskite (LFHP) materials have recently received a lot of attention in optoelectronic applications due to their low toxicity and outstanding optical characteristics. Simultaneously, the increased thrust for flexible, wearable, and lightweight optoelectronic devices is driving improvements in sensor and actuator technology. In this context, flexible piezoelectric polymer composites based on LFHPs are gaining popularity due to their exceptional piezoelectric, pyroelectric, ferroelectric, and optical traits. Thus, this investigation presents long-term stable lead-free rubidium copper chloride (Rb2CuCl3)-based poly(vinylidene fluoride) composites. The optimized PVDF/Rb2CuCl3 composite yields ∼92.4% of the electroactive phase of the PVDF. Interfacial interactions between PVDF and Rb2CuCl3 have played a pivotal role in the electroactive β-phase transformation, resulting in improved long-term stability. A piezoelectric nanogenerator (PENG) has been fabricated employing the PVDF/Rb2CuCl3 composite for mechanical energy harvesting and biophysiological motion monitoring, demonstrating potential applications in the healthcare industry. The Piezoelectric Energy Harvester (PEH) with the PRCC_2.5 composite (PVDF composite of 2.5 wt % Rb2CuCl3) outperformed other composites, with a maximum open-circuit voltage (Voc) of ∼51.7 V and a short-circuit current (Isc) of ∼4.6 μA. The pristine PVDF-based device (PEH 0) had inferior performance, with a Voc of ∼12 V and an Isc of ∼0.5 μA. PEH 2.5 device exhibited a charge of ∼126 nC, which is far higher than the PEH 0 for which the corresponding charge was ∼7 nC. Furthermore, during the periodic application of the force of ∼5 N, the stability and durability of the PEH 2.5 device were evaluated. 10,250 compression cycles were used to measure the electrical output of the PEH 2.5 device. Remarkably, following the 10,250 cycles, there was no discernible drop in the output voltage (∼16 V). In addition, a photodetector has been developed to investigate the piezo-phototronic effect, displaying quick photoswitching behavior with rise and decay periods of ∼3.22 and ∼5.48 s, respectively. These findings demonstrate that the flexible PVDF/Rb2CuCl3 composites have significant potential as an optical signal-modulated piezoresponsive wearable sensor.

Fast, Highly Stable, and Low-Bandgap 2D Halide Perovskite Photodetectors Based on Short-Chained Fluorinated Piperidinium as a Spacer

Ruddlesden-Popper (RP) two-dimensional (2D) halide perovskite (HP), with attractive structural and optoelectronic properties, has shown great potential in optoelectrical devices. However, the relatively wide bandgap (Eg) and stability, which cause inferior efficiency, prevent its feasibility from further applications. To tackle these issues, for the first time, a novel fluorine-containing piperidinium spacer, (3-HCF2CF2CH2OCH2-PPH+), abbreviated as (4FH-PPH+), has been designed for the stable and efficient n = 1 2D HPs. Its fluorophobicity can significantly enhance the noncovalent interactions between cations and [PbI6]4- octahedra. The observed Eg of the fluorinated (4FH-PPH)2PbI4 perovskite is found to be 2.22 eV, which is the lowest value among all fluorinated perovskites reported so far. Interestingly, this fluorinated film-based 2D perovskite photodetector (PD) exhibits the outstanding responsivity of 502 mA W-1, photodetectivity of 5.73 × 1010 cm Hz1/2 W-1, and impressive response/recovery time of 42/46 ms under 450 nm at 20 V. To the best of our knowledge, it is found for the first time that the 2D HP, with the fluorinated short-chained segment included into the organic spacer, shows remarkable stability for up to 49 days. These results strongly demonstrate the potential of the 2D fluorinated short-chained (4FH-PPH)2PbI4 HP with a low Eg as a promising candidate for next-generation optoelectronic devices.

Self-Assembly of Delta-Formamidinium Lead Iodide Nanoparticles to Nanorods: Study of Memristor Properties and Resistive Switching Mechanism

In the quest for advanced memristor technologies, this study introduces the synthesis of delta-formamidinium lead iodide (δ-FAPbI3) nanoparticles (NPs) and their self-assembly into nanorods (NRs). The formation of these NRs is facilitated by iodide vacancies, promoting the fusion of individual NPs at higher concentrations. Notably, these NRs exhibit robust stability under ambient conditions, a distinctive advantage attributed to the presence of capping ligands and a crystal lattice structured around face-sharing octahedra. When employed as the active layer in resistive random-access memory devices, these NRs demonstrate exceptional bipolar switching properties. A remarkable on/off ratio (105) is achieved, surpassing the performances of previously reported low-dimensional perovskite derivatives and α-FAPbI3 NP-based devices. This enhanced performance is attributed to the low off-state current owing to the reduced number of halide vacancies, intrinsic low dimensionality, and the parallel alignment of NRs on the FTO substrate. This study not only provides significant insights into the development of superior materials for memristor applications but also opens new avenues for exploring low-dimensional perovskite derivatives in advanced electronic devices.

Tuning Spin-Polarized Lifetime at High Carrier Density through Deformation Potential in Dion-Jacobson-Phase Perovskites

The control of spin relaxation mechanisms is of great importance for spintronics applications as well as for fundamental studies. Layered metal-halide perovskites represent an emerging class of semiconductors with rich optical spin physics, showing potential for spintronic applications. However, a major hurdle arises in layered metal-halide perovskites with strong spin-orbit coupling, where the spin lifetime becomes extremely short due to D'yakonov-Perel' scattering and Bir-Aronov-Pikus at high carrier density. Using the circularly polarized pump-probe transient reflection technique, we experimentally reveal the important scattering for spin relaxation beyond the electron-hole exchange strength in the Dion-Jacobson (DJ)-type 2D perovskites (3AMP)(MA)n-1PbnI3n+1 [3AMP = 3-(aminomethyl)piperidinium, n = 1-4]. Despite a more than 10-fold increase in carrier concentration, the spin lifetimes for n = 3 and 4 are effectively maintained. We reveal neutral impurity and polar optical phonon scatterings as significant contributors to the momentum relaxation rate. Furthermore, we show that more octahedral distortions induce a larger deformation potential which is reflected on the acoustic phonon properties. Coherent acoustic phonon analysis indicates that the polaronic effect is crucial in achieving control over the scattering mechanism and ensuring spin lifetime protection, highlighting the potential of DJ-phase perovskites for spintronic applications.