Metal Halide Perovskite/2D Material Heterostructures: Syntheses and Applications

Broken-gap type-III band alignment in monolayer halide perovskite/antiperovskite oxide van der Waals heterojunctions

The integration of halide perovskites with other functional materials provides a new platform for applications beyond photovoltaics, which has been realized in experiments. Here, through first-principles methods, we explore the possibility of constructing halide perovskite/antiperovskite oxide van der Waals heterostructures (vdWHs) for the first time with monolayers Rb₂CdCl₄ and Ba₄OSb₂ as representative compounds. Our calculation results reveal that the Rb₂CdCl₄/Ba₄OSb₂ vdWHs have negative binding energies and their most stable stacking possesses a rare type-III band alignment with a broken gap, which is highly promising for tunnel field-effect transistor (TFET) applications. Moreover, their electronic features can be further tuned by applying strain or an external electric field. Specifically, compressive strain can enlarge the tunneling window, while tensile strain can realize a type-III to type-II band alignment transformation. Therefore, our work provides fundamental insights into the electronic properties of Rb₂CdCl₄/Ba₄OSb₂ vdWHs and paves the way for the design and fabrication of future halide perovskite/antiperovskite-based TFETs.

Two-Dimensional Metal Halides for X-Ray Detection Applications

Metal halide perovskites have recently emerged as promising candidates for the next generation of X-ray detectors due to their excellent optoelectronic properties. Especially, two-dimensional (2D) perovskites afford many distinct properties, including remarkable structural diversity, high generation energy, and balanced large exciton binding energy. With the advantages of 2D materials and perovskites, it successfully reduces the decomposition and phase transition of perovskite and effectively suppresses ion migration. Meanwhile, the existence of a high hydrophobic spacer can block water molecules, thus making 2D perovskite obtain excellent stability. All of these advantages have attracted much attention in the field of X-ray detection. This review introduces the classification of 2D halide perovskites, summarizes the synthesis technology and performance characteristics of 2D perovskite X-ray direct detector, and briefly discusses the application of 2D perovskite in scintillators. Finally, this review also emphasizes the key challenges faced by 2D perovskite X-ray detectors in practical application and presents our views on its future development.

Feature Selection in Machine Learning for Perovskite Materials Design and Discovery

Perovskite materials have been one of the most important research objects in materials science due to their excellent photoelectric properties as well as correspondingly complex structures. Machine learning (ML) methods have been playing an important role in the design and discovery of perovskite materials, while feature selection as a dimensionality reduction method has occupied a crucial position in the ML workflow. In this review, we introduced the recent advances in the applications of feature selection in perovskite materials. First, the development tendency of publications about ML in perovskite materials was analyzed, and the ML workflow for materials was summarized. Then the commonly used feature selection methods were briefly introduced, and the applications of feature selection in inorganic perovskites, hybrid organic-inorganic perovskites (HOIPs), and double perovskites (DPs) were reviewed. Finally, we put forward some directions for the future development of feature selection in machine learning for perovskite material design.

Deciphering the Relevance of Quantum Confinement in the Optoelectronics of CsPbBr3 Perovskite Nanostructures

Perovskites (PVKs) have emerged as an exciting class of semiconducting materials owing to their magnificent photophysical properties and been used in solar cells, light-emitting diodes, photodetectors, etc. The growth of multidimensional nanostructures has revealed many exciting alterations in their optoelectronic properties compared to those of their bulk counterparts. In this work, we have spotlighted the influence of quantum confinement in CsPbBr₃ PVKs like the quantum dot (PQD), nanoplatelet (PNPL), and nanorod (PNR) on their charge transfer (CT) dynamics with 1,4-naphthoquinone (NPQ). The energy band alignment facilitates the transfer of both electrons and holes in the PNPL to NPQ, enhancing its CT rate, while only electron transfer in the PQD and PNR diminishes CT. The tunneling current across a metal-nanostructure-metal junction for the PNPL is observed to be higher than others. The higher exciton binding energy in the PNPL results in efficient charge transport by enhancing the mobility of the excited-state carrier and its lifetime compared to those of the PNR and PQD.

Regulating the coordination geometry of polyhedra in zero-dimensional metal halides for tunable emission

Although self-trapped exciton (STE) emissions in zero-dimensional metal halides have been intensively investigated, the understanding of the relationship between the coordination geometries of the metal halides and their photophysical properties is still lacking. In this work, we successfully synthesized single crystals, with strong STE emissions, of the bimetallic materials (Bmpip)₉[Pb₃Br₁₁](ZnBr₄)₂ (PbZn-Br) and (Bmpip)₉[Pb₃Br₁₁](MnBr₄)₂ (PbMn-Br), where Bmpip⁺ is 1-butyl-1-methyl-piperidinium (C₁₀H₂₂N⁺), via a facile anti-solvent crystallization strategy. With respect to the monometallic material, (Bmpip)₂[PbBr₄] (Pb-Br), the introduction of Zn²⁺ and Mn²⁺ effectively alters the coordination geometry of the lead bromide polyhedral configuration from a PbBr₄^2- tetrahedron to a Pb₃Br₁₁^5- trimer. As a result, the maximum emission peak of PbZn-Br exhibits an obvious red shift and the full width at half maximum is almost two-fold wider than that of Pb-Br due to stronger electron-phonon coupling. Moreover, due to the intrinsic emission of the Mn²⁺ ions, an intriguing tunable emission was achieved in PbMn-Br with an impressively high photoluminescence quantum yield of up to 67%. The ultra-stable PbMn-Br single crystals show potential as an ideal down-conversion phosphor for use in UV-pumped white light-emitting diode devices.

Hybrid mixed-dimensional WTe2/CsPbI3perovskite heterojunction for high-performance photodetectors

Perovskites have showed significant potential for the application in photodetectors due to their outstanding electrical and optical properties. Integrating two-dimensional (2D) materials with perovskites can make full use of the high carrier mobility of 2D materials and strong light absorption of perovskite to realize excellent optoelectrical properties. Here, we demonstrate a photodetector based on the WTe₂/CsPbI₃heterostructure. The quenching and the shortened lifetime of photoluminescence (PL) for CsPbI₃perovskite confirms the efficient charge transfer at the WTe₂/CsPbI₃heterojunction. After coupled with WTe₂, the photoresponsivity of the CsPbI₃photodetector is improved by almost two orders of magnitude due to the high-gain photogating effect. The WTe₂/CsPbI₃heterojunction photodetector reveals a large responsivity of 1157 A W^-1and a high detectivity of 2.1 × 10¹³Jones. The results pave the way for the development of high-performance optoelectronic devices based on 2D materials/perovskite heterojunctions.

2D Ruddlesden-Popper perovskite sensitized SnP2S6 ultraviolet photodetector enabling high responsivity and fast speed

As the newly developed wide-bandgap semiconductors, two-dimensional layered metal phosphorus chalcogenides (2D LMPCs) exhibit enormous potential applications in ultraviolet (UV) photodetection due to their superior optoelectronic performance. However, 2D LMPC-based UV photodetectors generally suffer from low responsivity and slow response speed, which hinder their practical applications. Here, we present an effective strategy of sensitizing 2D LMPC UV photodetectors with a 2D Ruddlesden-Popper (RP) perovskite to enable high responsivity and fast response speed. As a demonstration, a hybrid heterojunction composed of RP perovskite (PEA)₂PbI₄ and a 2D SnP₂S₆ flake is fabricated by spin-coating method. Benefitting from the strong optical absorption of (PEA)₂PbI₄ and the efficient interfacial charge transfer caused by the favorable type-II energy band alignment, the as-fabricated 2D SnP₂S₆/(PEA)₂PbI₄ hybrid heterojunction photodetectors show high responsivity (67.1 A W^-1), large detectivity (2.8 × 10¹¹ Jones), fast rise/delay time (30/120 μs) and excellent external quantum efficiency (22825%) at 365 nm. Under field-effect modulation, the responsivity of the heterojunction photodetector can reach up to 239.4 A W^-1, which is attributed to the photogating mechanism and reduced Schottky barriers. Owing to the excellent photodetection performance, the heterojunction device further shows superior imaging capability. This work provides an effective strategy for designing high-performance UV photodetectors toward future applications.

Recent progress in use of MXene in perovskite solar cells: for interfacial modification, work-function tuning and additive engineering

The use of perovskites in photovoltaic and related industries has achieved tremendous success over the last decade. However, there are still obstacles to overcome in terms of boosting their performance and resolving stability issues for future commercialization. The introduction of a new 2D material of halide perovskites is now the key advancement in boosting the solar energy conversion efficiency. The implication of a new 2D material (MXene) in perovskite solar cells has been initiated since its first report in 2018, showing excellent transparency, electrical conductivity, carrier mobility, superior mechanical strength, and tunable work function. Based on distinctive features at the hetero-interface, halide perovskite and MXene heterostructures (HPs/Mx) have recently exhibited exceptional improvements in both the performance and stability of perovskite solar cells. Furthermore, the wide families of HPs and MXene materials allow playing with the composition and functionalities of HP/Mx interfaces by applying rational designing and alterations. In this review a comprehensive study of implementing MXenes in perovskite solar cells is presented. First, the implementation of MXenes in perovskites as an additive, and then in charge extraction layers (HTL/ETL), is described in detail. It is worth noting that still only Ti₃C₂T_x, Nb₂CT_x,V₂CT_x MXene is being incorporated into perovskite photovoltaics. Finally, the present obstacles in the use of MXenes in PSCS are discussed, along with the future research potential. This review is expected to provide a complete and in-depth description of the current state of research and to open up new opportunities for the study of other MXenes in PSCs.

Screening of Excitons by Organic Cations in Quasi-Two-Dimensional Organic-Inorganic Lead-Halide Perovskites

Interlayer organic cations in quasi-two-dimensional halide perovskites are a versatile tuning vehicle for the optoelectronic properties of these complex systems, but chemical intuition for this design route is yet to be established. Here, we use density functional theory, the GW approximation, and the Bethe-Salpeter equation approach to understand the contribution of the organic cation to the quasiparticle band gap and exciton binding energy of layered perovskites. We show that organic cations in quasi-two-dimensional perovskites contribute significantly to the dielectric screening in these systems, countering quantum confinement effects on the quasiparticle band gap and the exciton binding energy. Using a simple electrostatics model inspired by parallel-plate capacitors, we decouple the organic cation and inorganic layer contributions to the effective dielectric constants and show that dielectric properties of layered perovskites are broadly tunable via the interlayer cation, providing a direct means of tuning photophysical properties for a variety of applications.

2D Material and Perovskite Heterostructure for Optoelectronic Applications

Optoelectronic devices are key building blocks for sustainable energy, imaging applications, and optical communications in modern society. Two-dimensional materials and perovskites have been considered promising candidates in this research area due to their fascinating material properties. Despite the significant progress achieved in the past decades, challenges still remain to further improve the performance of devices based on 2D materials or perovskites and to solve stability issues for their reliability. Recently, a novel concept of 2D material/perovskite heterostructure has demonstrated remarkable achievements by taking advantage of both materials. The diverse fabrication techniques and large families of 2D materials and perovskites open up great opportunities for structure modification, interface engineering, and composition tuning in state-of-the-art optoelectronics. In this review, we present comprehensive information on the synthesis methods, material properties of 2D materials and perovskites, and the research progress of optoelectronic devices, particularly solar cells and photodetectors which are based on 2D materials, perovskites, and 2D material/perovskite heterostructures with future perspectives.

High responsivity of hybrid MoTe2/perovskite heterojunction photodetectors

Two-dimensional (2D) van der Waals heterojunction offers alternative facile platforms for many optoelectronic devices due to no-dangling bonds and steep interface carrier gradient. Here, we demonstrate a 2D heterojunction device, which combines the benefits of high carrier mobility of 2D MoTe₂and strong light absorption of perovskite, to achieve excellent responsivity. This device architecture is constructed based on the charge carriers separation and transfer with the high-gain photogating effect at the interface of the heterojunction. The device exhibits high responsivity of 334.6 A W^-1, impressive detectivity of 6.2 × 10¹⁰Jones. All the results provide the insight into the benefits of interfacial carriers transfer for designing hybrid perovskite-2D materials based optoelectronic devices.

Solution-processed two-dimensional materials for next-generation photovoltaics

In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting/recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to "on-demand" tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing/coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic-inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components/additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices.