Exploring a Fatigue-Free Layered Hybrid Perovskite Ferroelectric for Photovoltaic Non-Volatile Memories

Dual polarization-enabled ultrafast bulk photovoltaic response in van der Waals heterostructures

The bulk photovoltaic effect (BPVE) originating from spontaneous charge polarizations can reach high conversion efficiency exceeding the Shockley-Queisser limit. Emerging van der Waals (vdW) heterostructures provide the ideal platform for BPVE due to interfacial interactions naturally breaking the crystal symmetries of the individual constituents and thus inducing charge polarizations. Here, we show an approach to obtain ultrafast BPVE by taking advantage of dual interfacial polarizations in vdW heterostructures. While the in-plane polarization gives rise to the BPVE in the overlayer, the charge carrier transfer assisted by the out-of-plane polarization further accelerates the interlayer electronic transport and enhances the BPVE. We illustrate the concept in MoS2/black phosphorus heterostructures, where the experimentally observed intrinsic BPVE response time achieves 26 ps, orders of magnitude faster than that of conventional non-centrosymmetric materials. Moreover, the heterostructure device possesses an extrinsic response time of approximately 2.2 ns and a bulk photovoltaic coefficient of 0.6 V-1, which is among the highest values for vdW BPV devices reported so far. Our study thus points to an effective way of designing ultrafast BPVE for high-speed photodetection.

The past 10 years of molecular ferroelectrics: structures, design, and properties

Ferroelectricity, which has diverse important applications such as memory elements, capacitors, and sensors, was first discovered in a molecular compound, Rochelle salt, in 1920 by Valasek. Owing to their superiorities of lightweight, biocompatibility, structural tunability, mechanical flexibility, etc., the past decade has witnessed the renaissance of molecular ferroelectrics as promising complementary materials to commercial inorganic ferroelectrics. Thus, on the 100th anniversary of ferroelectricity, it is an opportune time to look into the future, specifically into how to push the boundaries of material design in molecular ferroelectric systems and finally overcome the hurdles to their commercialization. Herein, we present a comprehensive and accessible review of the appealing development of molecular ferroelectrics over the past 10 years, with an emphasis on their structural diversity, chemical design, exceptional properties, and potential applications. We believe that it will inspire intense, combined research efforts to enrich the family of high-performance molecular ferroelectrics and attract widespread interest from physicists and chemists to better understand the structure-function relationships governing improved applied functional device engineering.

Ferroelectricity and Thermochromism in a 2D Dion-Jacobson Organic-Inorganic Hybrid Perovskite

2D organic-inorganic hybrid perovskites (OIHPs) have become one of the hottest research topics due to their excellent environmental stability and unique optoelectronic properties. Recently, the ferroelectricity and thermochromism of 2D OIHPs have attracted increasing interests. Integrating ferroelectricity and thermochromism into perovskites can significantly promote the development of multichannel intelligent devices. Here, a novel 2D Dion-Jacobson OIHP of the formula (3AMP)PbI4 (where 3AMP is 3-(aminomethyl)pyridinium) is reported, which has a remarkable spontaneous polarization value (Ps) of 15.6 µC cm-2 and interesting thermochromism. As far it is known, such a large Ps value is the highest for 2D OIHPs recorded so far. These findings will inspire further exploration and application of multifunctional perovskites.

Efficient Charge Transport in Inverted Perovskite Solar Cells via 2D/3D Ferroelectric Heterojunction

While the 2D/3D heterojunction is an effective method to improve the power conversion efficiency (PCE) of perovskite solar cells (PSCs), carriers are often confined in the quantum wells (QWs) due to the unique structure of 2D perovskite, which makes the charge transport along the out-of-plane direction difficult. Here, a 2D/3D ferroelectric heterojunction formed by 4,4-difluoropiperidine hydrochloride (2FPD) in inverted PSCs is reported. The enriched 2D perovskite (2FPD)2PbI4 layer with n = 1 on the perovskite surface exhibits ferroelectric response and has oriented dipoles along the out-of-plane direction. The ferroelectricity of the oriented dipole layer facilitates the enhancement of the built-in electric field (1.06 V) and the delay of the cooling process of hot carriers, reflected in the high carrier temperature (above 1400 K) and the prolonged photobleach recovery time (139.85 fs, measured at bandgap), improving the out-of-plane conductivity. In addition, the alignment of energy levels is optimized and exciton binding energy (32.8 meV) is reduced by changing the dielectric environment of the surface. Finally, the 2FPD-treated PSCs achieve a PCE of 24.82% (certified: 24.38%) with the synergistic effect of ferroelectricity and defect passivation, while maintaining over 90% of their initial efficiency after 1000 h of maximum power point tracking.

Unique Perovskitizer N─Pb Bond Switching Induced Polar Photovoltaic Effect in Trilayered Hybrid Perovskite

Polar photovoltaic effect (PPE) has attracted great attention in regulating desired optoelectronic properties, which can be driven by order-disorder and displacive phase transitions. Bond-switching is also a feasible method to induce PPE, but such investigation is very rare. Lead-halide hybrid perovskite (LHHP) is an outstanding photodetection material; lead atoms possess rich coordination modes to provide possibilities to construct switchable bonds. Here, a unique perovskitizer N─Pb bond-switching is disclosed to induce polar photovoltage in the emerging LHHP, PA2MHy2Pb3Br10 (1, PA = n-propylamine, MHy = methylhydrazine). Interestingly, the perovskitizer MHy+ provides 2s2 lone pair while the Pb atom affords empty d orbitals, which coordinate with each other to generate a flexible N─Pb bond. Further, the introduction of N─Pb bonds results in a high distortion of the PbBr6 octahedron to form local polarity and further orientation to induce spontaneous polarization. More importantly, such a flexible N─Pb bond switching mechanism drives a notable PPE and controllable polarized photo-response, a polarization ratio up to 9.7 at the polar phase in striking contrast with the non-polar phase (1.03). The work provides the first demonstration of bond-switching to induce polar phase transition and polar photovoltage in the photoconductive hybrid perovskites for photoelectric applications.

Dion-Jacobson to Alternating-Cations-Interaction Reconstruction toward Narrow Bandgap 2D Aromatic Hybrid Perovskite

The 2D aromatic Dion-Jacobson (DJ) hybrid perovskites combining advantages of high stability, enhanced light absorption, and favorable charge transport, are regarded as a kind of very promising materials for high-performance optoelectronic applications. However, due to the rigidity and large size of the aromatic ring, how to further reduce the interlayer distance to achieve better carrier transport and wider light response window still remain extremely challenging. Here, an interesting DJ-to-ACI (alternating-cations-interaction) reconstruction in 2D aromatic perovskite is first realized by inserting MA+ cations into (4-AP)PbI4 (1, 4-AP = 4-amidinopyridinium), successfully constructing an unprecedented ACI perovskite of (4-AP)(MA)2 Pb2 I8 (2, MA = methylamine). Remarkably, such a DJ-to-ACI reconstruction not only effectively reduces the interlayer spacing from 3.89 to 3.15 Å but also alleviates the structural distortion, which jointly causes a significant bandgap narrowing from 2.22 to 1.95 eV (smaller than all current 2D monolayered DJ perovskites), hence achieving a broad photodetection window over 660 nm. This work reports a novel narrow bandgap 2D ACI perovskite derived from the aromatic DJ motif, which sheds light on future regulations on the structure and properties of hybrid perovskites.

Bulk Photovoltaic Effect in Two-Dimensional Distorted MoTe2

In future solar cell technologies, the thermodynamic Shockley-Queisser limit for solar-to-current conversion in traditional p-n junctions could potentially be overcome with a bulk photovoltaic effect by creating an inversion broken symmetry in piezoelectric or ferroelectric materials. Here, we unveiled mechanical distortion-induced bulk photovoltaic behavior in a two-dimensional (2D) material, MoTe2, caused by the phase transition and broken inversion symmetry in MoTe2. The phase transition from single-crystalline semiconducting 2H-MoTe2 to semimetallic 1T'-MoTe2 was confirmed using X-ray photoelectron spectroscopy (XPS). We used a micrometer-scale system to measure the absorption of energy, which reduced from 800 to 63 meV during phase transformation from hexagonal to distorted octahedral and revealed a smaller bandgap semimetallic behavior. Experimentally, a large bulk photovoltaic response is anticipated with the maximum photovoltage VOC = 16 mV and a positive signal of the ISC = 60 μA (400 nm, 90.4 Wcm-2) in the absence of an external electric field. The maximum values of both R and EQE were found to be 98 mAW-1 and 30%, respectively. Our findings are distinctive features of the photocurrent responses caused by in-plane polarity and its potential from a wide pool of established TMD-based nanomaterials and a cutting-edge approach to optimize the efficiency in converting photons-to-electricity for power harvesting optoelectronics devices.

Emerging Halide Perovskite Ferroelectrics

Halide perovskites have gained tremendous attention in the past decade owing to their excellent properties in optoelectronics. Recently, a fascinating property, ferroelectricity, has been discovered in halide perovskites and quickly attracted widespread interest. Compared with traditional perovskite oxide ferroelectrics, halide perovskites display natural advantages such as structural softness, low weight, and easy processing, which are highly desirable in applications pursuing miniaturization and flexibility. This review focuses on the current research progress in halide perovskite ferroelectrics, encompassing the emerging materials systems and their potential applications in ferroelectric photovoltaics, self-powered photodetection, and X-ray detection. The main challenges and possible solutions in the future development of halide perovskite ferroelectric materials are also attempted to be pointed out.

Strain-Driven Solid-Solid Crystal Conversion in Chiral Hybrid Pseudo-Perovskites with Paramagnetic-to-Ferromagnetic Transition

Hybrid organic-inorganic perovskites (HOIPs) are promising stimuli-responsive materials (SPMs) owing to their molecular softness and tailorable structural dimensionality. The design of mechanically responsive HOIPs requires an in-depth understanding of how lattice strain induces intermolecular rearrangement that impacts physical properties. While chirality transfer from an organic cation to an inorganic lattice is known to influence chiral-optical properties, its effect on strain-induced phase conversion has not been explored. As opposed to achiral or racemic organic cations, chiral organic cations can potentially afford a new dimension in strain-responsive structural change. Herein, we demonstrate that mechanical strain induces a solid phase crystal conversion in chiral halide pseudo-perovskite single crystals (R/S)-(FE)₂CuCl₄ (FE = (4-Fluorophenyl)ethylamine) from a 0D isolated CuCl₄ tetrahedral to 1D corner-sharing CuFCl₅ octahedral framework via the incorporation of Cu···F interaction and N-H···F hydrogen bonding. This strain-induced crystal-to-crystal conversion involves the connection of neighboring 0D CuCl₄ tetrahedra via Cu²⁺-Cl^--Cu²⁺ linkages as well as the incorporation of a F-terminated organic cation as one of the X atoms in BX₆ octahedra, leading to a reduced band gap and paramagnetic-to-ferromagnetic conversion. Control experiments using nonchiral or racemic perovskite analogs show the absence of such solid phase conversion. To demonstrate pressure-sensitive properties, the 0D phase is dispersed in water-soluble poly(vinyl alcohol) (PVA) polymer, which can be applied to a large-scale pressure-induced array display on fibrous Spandex substrates via a screen-printing method.

Synthetic approaches for perovskite thin films and single-crystals

Halide perovskites are compelling candidates for the next generation of photovoltaic technologies owing to an unprecedented increase in power conversion efficiency and their low cost, facile fabrication and outstanding semiconductor properties.

Unprecedented Self-Powered Visible-Infrared Dual-Modal Photodetection Induced by a Bulk Photovoltaic Effect in a Polar Perovskite

Visible-infrared dual-modal light harvesting is crucial for various optoelectronic devices, particularly for solar cells and photodetectors. Hybrid metal-halide perovskites are recently emerging for visible-infrared dual-modal photodetection owing to their prominent multiphoton absorption and carrier transport performances. However, they work relying on an applied external power source or complicated heterostructures. It is still a difficult task to realize visible-infrared dual-modal self-powered photoresponse induced by a bulk photovoltaic effect (BPVE) in a single material. In this work, we constructed a polar multilayered perovskite, (Br-BA)₂(EA)₂Pb₃Br₁₀ (BEP; EA⁺ = ethylammonium, and Br-BA⁺ = 4-brombutylammonium). Notably, the polar feature endows BEP with a BPVE. In addition, BEP presents a distinctive two-photon activity arising from the layered quantum-well structure. Benefitting from these striking characteristics, self-powered visible-infrared dual-modal photodetection is realized, and a direct self-powered detection of 800 nm light with a photocurrent of 2.1 nA cm^-2 is achieved. This work will inspire the design of desired photoelectric materials with a BPVE for high-performance self-powered visible-infrared dual-modal photodetection.

Post-synthetic modification within MOFs: a valuable strategy for modulating their ferroelectric performance

It is a great route designing new MOF ferroelectrics to enrich the scope of ferroelectrics or improving the ferroelectric performance to enhance the opportunity of applications through the strategy of post-synthetic modification (PSM).