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Two-Dimensional Organic-Inorganic Hybrid Perovskites: A New Platform for Optoelectronic Applications

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Two-Dimensional Organic-Inorganic Hybrid Perovskites: A New Platform for Optoelectronic Applications

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Abstract

2D perovskites are recently attracting a significant amount of attention, mainly due to their improved stability compared with their 3D counterpart, e.g., the archetypical MAPbI3. Interestingly, the first studies on 2D perovskites can be dated back to the 1980s. The most popular 2D perovskites have a general formula of (RNH3)2MAn−1MnX3n+1, where n represents the number of metal halide octahedrons between the insulating organic cation layers. The optoelectronic properties of 2D perovskites, e.g., band gap, are highly dependent on the thickness of the inorganic layers (i.e., the value of n). Herein, 2D perovskites are arbitrarily divided into three classes, strict 2D (n = 1), quasi‐2D (n = 2–5), and quasi‐3D (n > 5), and research progress is summarized following this classification. The majority of existing 2D perovskites only employ very simple organic cations (e.g., butyl ammonium or phenylethyl ammonium), which merely function as the supporting layer/insulating barrier to achieve the 2D structure. Thus, a particularly important research question is: can functional organic cations be designed for these 2D perovskites, where these functional organic cations would play an important role in dictating the optoelectronic properties of these organic–inorganic hybrid materials, leading to unique device performance or applications?

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... [4] Lead-based halide perovskites, materials with fascinating optoelectronic properties for highly efficient solar cells, light-emitting diodes (LEDs) and phosphors, [5] normally present narrow photoluminescence (PL) peaks that arise from the recombination of free excitons rather than STEs, due to their high electronic-dimensionality. [6] By dimensional reduction, broadband white-light emission has been observed in a series of layered Pb-based perovskites [7] due to the unique "sandwich structure" that provides low electronic-dimensionality, strong exciton-phonon coupling, and quantum confinement effects. [8] However, the environmental toxicity of Pb and low photoluminescence quantum efficiency (PLQE) are preventing them from practical application. ...
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Article
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2D materials with common hexagonal crystal structures, such as graphene, hexagonal boron nitride, and transition metal dichalcogenides have attracted great interest due to their novel physical and chemical properties. Pentagonal transition metal dichalcogenides (TMDs) exhibit distinct optical, electrical, and chemical properties, with valuable functionalities for various applications. This review highlights some of the most important developments in this field, with emphasis on their functionalities for neuromorphic computing, transistors, photodetection, catalysts, etc. Strategies for modifying their physical and chemical properties as well as device performance including defect engineering and interface engineering are presented. Finally, a forward‐looking outlook of pentagonal 2D materials is discussed.
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The performance of quasi‐2D perovskite light‐emitting devices (PeLEDs) has been greatly improved in the past several years. Characterizations such as transient absorption (TA) spectroscopy and absorption spectra have been widely adopted to confirm the phases in the quasi‐2D perovskite layers. However, all the above‐mentioned characterization techniques could only identify the n values of phases without providing information on the distribution of phases. Here, phase distributions on the quasi‐2D perovskite surfaces are identified by utilizing the charging effect during the scanning electron microscope (SEM) measurement. Then, compositional and antisolvent engineering are adopted to improve the quality of quasi‐2D perovskite films. Green PeLEDs with an external quantum efficiency (EQE) of >20% are achieved with more uniform condensed film morphology and proper phase distribution. The work shows that SEM measurement is a powerful tool for studying the phase distribution on the quasi‐2D perovskite surfaces. The charging effect during the scanning electron microscope measurement is used to identify the phase size, shape, and distributions on quasi‐2D perovskite surface. Then compositional and antisolvent engineering are adopted to improve the quality of quasi‐2D perovskite films. Green perovskite light‐emitting devices (PeLEDs) with an external quantum efficiency (EQE) of >20% are achieved.
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Two-dimensional Ruddlesden-Popper (RP) halide perovskites stand out as excellent layered materials with favorable optoelectronic properties for efficient light-emitting, spintronic, and other spin-related applications. However, properties often determined by defects are not well understood in these perovskite systems. This work investigates the ground state electronic structure of commonly formed defects in a typical RP perovskite structure by density functional theory. Our study reveals that these 2D perovskites generally retain their defect tolerance with limited perturbation of the electronic structure in the case of neutral-type point defects. In contrast, donor/acceptor defects induce deep midgap states, potentially causing harm to the material's electronic performance. To retain positive intrinsic properties, the halide vacancies and interstitial defects should be avoided. The observed strong electron localization results in trap states and consequently leads to reduced device performance. This understanding can guide experimental efforts that aim for improved 2D halide perovskite-based device performance.
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The bulk photovoltaic effect (BPVE) is a promising optoelectronic phenomenon that produces a steady-state photocurrent in homogeneous bulk materials without the application of an electric field. In principle, the BPVE is allowed to be observed in noncentrosymmetric systems. However, guidelines on the effectiveness of crystal symmetry for the BPVE, such as chirality and polarity, remain unknown because of the lack of systematically controllable materials. For this reason, three types of noncentrosymmetric two-dimensional organic-inorganic hybrid perovskites (2D-OIHPs) of lead iodides with the formula A2PbI4 and chiral ammonium cations (A+) were used in the experiments. These were classified as (i) achiral-polar systems, (ii) chiral-nonpolar systems, and (iii) chiral-polar systems. All systems showed a strong current enhancement under light irradiation and an electric field. However, only (i) and (iii) systems showed the BPVE. Furthermore, direction reversal of the detected zero-bias photocurrent (I0) was observed when the direction of the electric polarization was changed. The absence of the detected BPVE in the (ii) system can be attributed to the zero net electric polarization (P) of the crystal structure, whereas (i) and (iii) systems possess nonzero P. Based on these observations, the features of the BPVE in 2D-OIHP lead iodides demonstrated in this work could be interpreted using the shift current mechanism.
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By inducing π-conjugated organic molecule C 2 H 4 N 2 in group II–VI based CdSe network structure materials, the band structures and carrier transport of organic-inorganic hybrid superlattices Cd 2 Se 2 (C 2 H 4 N 2 ) 1/2 were investigated via first-principles calculations based on the density functional theory. With different stacking patterns, it is found that the carrier mobility can be modulated by 5-6 orders of magnitude. The physical mechanisms of the high carrier mobility in the hybrid structures have been revealed, which mean dipole organic layers realize electron delocalization via electrostatic potential difference and build-in electric field. Our calculations shown that the dipole organic layers originate from asymmetric π-conjugated organic molecules and charges move between molecules, while symmetric organic molecules tend to electrostatic balance. And although the electronic transport properties are highly restrained by flat bands of organic layers around Fermi energy in most structures, we found that the collective electrostatic effect can lead to very high electron mobility in AA1 and AA2 stacking systems, which might be attributed to the superposition of molecule electrostatic potential along with electrons transfer between molecules. Furthermore, it’s also found that the anisotropy of electron mobility can be tunable via the difference directions of dipole layers.
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Neuromorphic computing system, inspired by human brain, has the capability to break through the bottlenecks of conventional von Neumann architecture, which can improve the energy efficiency of data processing. Novel neuromorphic electronic components are the hardware foundation of efficient neuromorphic computation. Optoelectronic memristive device integrates the functions of sensing, storage and computing and has been considered as promising hardware candidate for neuromorphic vision. Herein, the recent advances of optoelectronic memristive device for in-sensor computing have been reviewed, including optoelectronic materials and mechanism, optoelectronic memristive device/characteristics as well as functionality and application of in-sensor computing. We first review the optoelectronic materials and corresponding memristive mechanism, which including photon-ion coupling and photon-electron coupling type. Then optoelelctronic and all-optical modulated memristive device are introduced according to the modulation mode. Moreover, we exhibit the application of optoelectronic device in the fields of cognitive function simulation, optoelectronic logic operation, neuromorphic vision and object tracking etc. Finally, we summarize the advantages/challenges of optoelectronic memristor and prospected the future development.
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Recently, all-inorganic CsPbX3 perovskite solar cells have stimulated enormous research interest due to their numerous merits including superior thermal and light stability and have become one of the most prominent research domains. In the past decade, the reported power conversion efficiency exhibited a rapid rise increasing from 2.9% to the present 20.8%. This review provides a summary of the recent exciting advancements in film quality optimization engineering and the investigations of the film formation mechanism of CsPbX3 perovskites, aiming to further promote the device performance towards the Shockley-Queisser efficiency limit. We begin with the introduction of the fundamental mechanism analysis of perovskite crystallization kinetics and evaluate the predominant pathways that contribute to non-radiative recombination losses. We subsequently summarize the promising strategies for fabricating high-quality CsPbX3 perovskite films with a particular focus on film formation regulation engineering and defect suppression approaches. Furthermore, we highlight state-of-the-art characterization techniques such as in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) to probe the film formation kinetics to fulfill the purpose of understanding the underlying film formation mechanism. Finally, we provide an outlook on the challenges and opportunities for further promoting the device performance and cover the issues relating to the commercial application of all-inorganic perovskite solar cells in the future.
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Solution‐processed 2D perovskite films generally contain mixed quantum wells (QWs) with multiple well width distribution, which seriously weakens the charge transfer. To achieve regulation of the QW width, strategies to optimize the crystallization dynamics of 2D perovskite films are urgently needed. In this review, systematic summary on QW distribution and guidelines for 2D perovskite phase regulation is provided, aiming to establish a general manual for preparing efficient 2D perovskite solar cells (PSCs). The factors affecting the distribution of multiple‐QWs in 2D perovskite films, including component engineering, additive engineering, process optimization, are first generalized. Then an extensive review of these factors that are widely used to reconstruct 2D perovskite crystallization process is conducted. Leveraging these insights, the effect of QWs distributions on 2D PSCs properties is also summarized. Similarly, considering the crystallization kinetics and device performance, the QWs width control of 2D perovskite films from the aspects of ligand engineering, precursor design, and fabrication optimization, is rationalized. Finally, an outlook on how to realize ordered QWs distribution in perovskite films for efficient 2D PSCs is proposed.
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Halide perovskites have attracted attention because of their potential in the fields of light-emitting diodes, lasers and detectors. Herein, a series of quasi-2D PBA2(MAPbBr3)n􏰀1PbBr4 perovskite films have been synthesized and the physical behavior behind their optoelectronic properties, which is dependent on the layer number (n), has been discussed in detail. The temperature-dependent photoluminescence (PL) test indicates that the perovskite film with n = 7 has outstanding PL characteristics, owing to its long radiative lifetime at different temperatures. Temperature-dependent amplified spontaneous emission (ASE) has been employed for the first time to scan the ASE threshold of our films, showing that the ASE threshold decreases from 29.91 mJ cm􏰀2 (3D perovskite film) to 19.23 mJ cm􏰀2 (n = 3). The current– voltage test with strong light bias takes the lead in pointing out that the detector based on the 3D perovskite film (n = p) exhibits excellent opto-current properties in comparison with other quasi-2D perovskite films. Our results provide a comprehensive insight into the optoelectronic properties of these quasi-2D perovskite films.
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Small-area metal–halide perovskite solar cells (PSCs) having power-conversion efficiencies (PCEs) of greater than 25% can be prepared by using a spin-coated perovskite layer, but this technique is not readily transferrable to large-scale manufacturing. Drop-casting is a simple alternative method for film formation that is more closely aligned to industry-relevant coating processes. In the present work, drop-casting was used to prepare films for screening two-dimensional Ruddlesden–Popper (2DRP) metal–halide perovskite formulations for potential utility in PSCs, without additional processing steps such as inert-gas blowing or application of antisolvent. The composition of the 2DRP formulation used for drop-casting was found to have a profound effect on optical, spectroscopic, morphological, and phase-distribution properties of the films as well as the photovoltaic performance of related PSC devices. This facile method for screening film quality greatly assists in speeding up the identification of perovskite formulations of interest. The optimal 2DRP perovskite formulation identified from screening was utilized for industry-relevant one-step roll-to-roll slot-die coating on a flexible plastic substrate, producing PSCs having PCEs of up to 8.8%. A mechanism describing film formation and phase distribution in the films is also proposed.
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Perovskite light‐emitting diodes (PeLEDs) have recently shown significant progress with external quantum efficiencies (EQEs) exceeding 20%. However, PeLEDs with pure‐red (620−660 nm) light emission, an essential part for full‐color displays, remain a great challenge. Herein, a general approach of spacer cation alloying is employed in Ruddlesden–Popper perovskites (RPPs) for efficient red PeLEDs with precisely tunable wavelengths. By simply tuning the alloying ratio of dual spacer cations, the thickness distribution of quantum wells in the RPP films can be precisely modulated without deteriorating their charge‐transport ability and energy funneling processes. Consequently, efficient PeLEDs with tunable emissions between pure red (626 nm) and deep red (671 nm) are achieved with peak EQEs up to 11.5%, representing the highest values among RPP‐based pure‐red PeLEDs. This work opens a new route for color tuning, which will spur future developments of pure‐red or even pure‐blue PeLEDs with high performance. A simple and general strategy of spacer cation alloying is developed to modulate the thickness distribution of quantum wells in Ruddlesden–Popper perovskite films. Based on this method, efficient perovskite light‐emitting diodes with tunable emissions between pure red (626 nm) and deep red (671 nm) are achieved with peak external quantum efficiencies up to 11.5%.
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The practical utilization of electronic and optoelectronic materials such as perovskites not only relies on their good device performance and moisture stability but also requires stability under other operating conditions. Herein, the operational stability of the Dion–Jacobson perovskite (DJP) photodetector under different harsh conditions is carefully studied and compared with that of its widely studied counterpart, i.e., the Ruddlesden–Popper perovskite (RPP) photodetector. The DJP‐based photodetector can maintain its responsivity after storage in humid environments for two months and can ensure stable operation without any photocurrent degradation under continuous light illumination for 20 000 s. Moreover, the DJP film does not exhibit any changes in its absorbance spectrum and surface morphology when it is heated at 100 °C for 18 h, and the film is hardly affected by high‐energy electron beam irradiation. In addition, the mechanical stability of the DJP film is also found to be superior based on the cyclic bending measurements of the fabricated flexible photodetectors. The excellent performance and stability of the DJP‐based photodetector are found to be the result of the elimination of weak van der Waals bonds among the octahedral PbI4 layers of DJP films, which is typically observed in RPP films. The Dion–Jacobson perovskite (DJP)‐based photodetector performs better than the Ruddlesden–Popper perovskite (RPP)‐based photodetector in terms of device performance; moisture, illumination, and thermal stabilities; resistance to electron beam irradiation; and mechanical robustness. The excellent performance and stability of the DJP‐based photodetector are found to be the result of the elimination of weak van der Waals bonds; the bond elimination minimizes material deterioration, which is typically observed in RPP films.
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Metal halide perovskites are of fundamental interest in the research of modern thin-film optoelectronic devices, owing to their widely tunable optoelectronic properties and solution processability. To obtain high-quality perovskite films and ultimately high-performance perovskite devices, it is crucial to understand the film formation mechanisms, which, however, remains a great challenge, due to the complexity of perovskite composition, dimensionality, and processing conditions. Nevertheless, the state-of-the-art in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) technique enables one to bridge the complex information with device performance by revealing the crystallization pathways during the perovskite film formation process. In this review, the authors illustrate how to obtain and understand in situ GIWAXS data, summarize and assess recent results of in situ GIWAXS studies on versatile perovskite photovoltaic systems, aiming at elucidating the distinct features and common ground of film formation mechanisms, and shedding light on future opportunities of employing in situ GIWAXS to study the fundamental working mechanisms of highly efficient and stable perovskite solar cells toward mass production.
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Lead halide based perovskite semiconductors self-assemble with distinct organic cations in natural multi-quantum-well structures. The emerging electronic properties of these two-dimensional (2D) materials can be controlled by the combination of the halide content and choice of chromophore in the organic layer. Understanding the photophysics of the perovskite semiconductor materials is critical for the optimization of stable and efficient optoelectronic devices. We use femtosecond transient absorption spectroscopy (fs-TAS) to study the mechanism of energy transfer between the organic and inorganic layers in a series of three lead-based mixed-halide perovskites such as benzylammonium (BA), 1-naphthylmethylammonium (NMA), and 1-pyrenemethylammonium (PMA) cations in 2D-lead-based perovskite thin films under similar experimental conditions. After optical excitation of the 2D-confined exciton in the lead halide layer, ultrafast energy transfer is observed to organic singlet and triplet states of the incorporated chromophores. This is explained by an effective Dexter energy transfer, which operates via a correlated electron exchange between the donating 2D-confined exciton and the accepting chromophore under spin conservation.
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Recently, two-dimensional organic–inorganic hybrid perovskites have attracted great attention for their outstanding performances in solar energy conversion devices. By using first principles calculations, we explored the structural, electronic and optical properties of recently synthesized (PEA)2PbI4 and (PEA)2SnI4 organic–inorganic hybrid perovskites to understand the photovoltaic performances of these systems. Our study reveals that both the perovskites are direct band gap semiconductors and possess desirable band gap for solar energy absorption. We have further extended our study to fluoro-, chloro-, and bromo-functionalized phenethylammonium (PEA) cations based [X(X = F, Cl, Br)PEA]2A(A = Pb, Sn)I4 perovskite materials. The halogenated benzene moiety confers an ultrahydrophobic character and protects the perovskites from ambient moisture. The halogen functionalized perovskites remain direct band gap semiconductors and all the perovskites show very strong optical absorption (∼7 × 10⁵ cm⁻¹) across UV–visible region. We have further calculated the photo-conversion efficiency (PCE) of both arene and functionalized arene based perovskites. The halogen-functionalized PEA-based perovskites also exhibit high PCE as like pristine ones and finally achieve high PCE of up to 24.30%, making them competitive with other previously reported perovskite-based photovoltaic devices.
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A quasi-two-dimensional ferroelectric (CH3OC3H9N)2CsPb2Br7 was successfully obtained by dimensionality reconstruction of a pure-two-dimensional monolayered perovskite (CH3OC3H9N)2PbBr4, which enables unprecedented passive X-ray detection with an impressive sensitivity up to 410 μC Gy⁻¹ cm⁻². Abstract Halide hybrid perovskites are attracting considerable attention as highly promising candidates for directly sensing X-ray radiation, but it is challenging to realize passive X-ray detection without an external power supply. However, the bulk photovoltaic effect (BPVE) in ferroelectrics promotes the independent separation of photoexcited carriers. Herein, by dimensionality reconstruction of a pure-two-dimensional (P-2D) monolayered perovskite (CH3OC3H9N)2PbBr4, we obtained a quasi-two-dimensional (Q-2D) ferroelectric (CH3OC3H9N)2CsPb2Br7. Converting P-2D into Q-2D perovskite stimulates a significant BPVE associated with robust ferroelectricity, as well as an enhanced mobility lifetime product. These features show the promise of the fabrication of the first passive X-ray detector based on ferroelectrics with an impressive sensitivity up to 410 μC Gy⁻¹ cm⁻² at zero bias, which is even superior to the value of the state-of-the-art α-Se detector operated at relatively high bias.
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Halide hybrid perovskites are attracting considerable attention as highly promising candidates for directly sensing X-ray radiation, but it remains great challenge to realize passive X-ray detection without external power supply. Bulk photovoltaic effect (BPVE) in ferroelectrics promotes the independent separation of photoexcited carriers, which offer an unprecedented opportunity in this regard. Herein, by dimensionality reconstruction of a pure-two-dimensional (P-2D) monolayered perovskite (CH 3 OC 3 H 9 N) 2 PbBr 4 ( 1 ), we successfully obtained a quasi-two-dimensional (Q-2D) ferroelectric (CH 3 OC 3 H 9 N) 2 CsPb 2 Br 7 ( 2 ), which is composed of bilayered perovskites sheets of corner-sharing PbBr 6 octahedra. Emphatically, converting P-2D into Q-2D perovskite arouses a significant BPVE associated with robust ferroelectricity, as well as enhanced mobility lifetime product. These features promise the fabrication of the first passive X-ray detector based on ferroelectrics with an impressive sensitivity up to 410 µC Gy -1 cm -2 at zero bias, which is even much superior to the value of state-of-the-art α -Se detector operated at relatively high bias. Our work demonstrates vast perspectives for the application of the ferroelectric semiconductors for photovoltaic electronic devices.
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Perovskite light-emitting diodes (LEDs) are attracting great attention due to their efficient and narrow emission. Quasi-two-dimensional perovskites with Ruddlesden-Popper-type layered structures can enlarge exciton binding energy and confine charge carriers and are considered good candidate materials for efficient LEDs. However, these materials usually contain a mixture of phases and the phase impurity could cause low emission efficiency. In addition, converting three-dimensional into quasi-two-dimensional perovskite introduces more defects on the surface or at the grain boundaries due to the reduction of crystal sizes. Both factors limit the emission efficiency of LEDs. Here, firstly, through composition and phase engineering, optimal quasi-two-dimensional perovskites are selected. Secondly, surface passivation is carried out by coating organic small molecule trioctylphosphine oxide on the perovskite thin film surface. Accordingly, green LEDs based on quasi-two-dimensional perovskite reach a current efficiency of 62.4 cd A-1 and external quantum efficiency of 14.36%.
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Halide perovskites with reduced-dimensionality (e.g. quasi-2D, Q-2D) are promising to be stable while remaining the high performance as compared to their three-dimensional counterpart. Generally, they are obtained in (A1)2(A2)n-1PbnI3n+1 thin films by adjusting A site cations, however, the underlying crystallization kinetics mechanism is less explored. In this manuscript, we employed ternary cations halides perovskite (BA)2(MA,FA)3Pb4I13 Q-2D perovskites as an archetypal model, to understand the principles that links the crystal orientation to the carrier behavior in the polycrystalline film. We reveal that appropriate FA+ incorporation can effectively control the perovskite crystallization kinetics, which reduces non-radiative recombination centers to acquire high-quality film with limited non-orientated phase. We further developed an in situ photoluminescence technique to observe that the Q-2D phase (n = 2, 3, 4) was formed first followed by the generation of n=∞ perovskite in Q-2D perovskites. These findings substantially benefit the understanding of doping behavior in Q-2D perovskites crystal growth, and ultimately leads to the highest efficiency of 12.81% in (BA)2(MA,FA)3Pb4I13 Q-2D perovskites based photovoltaic devices.
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Organometal halide perovskites have recently emerged as a highly promising class of functional materials for a variety of applications. The exceptional structural tunability enables these materials to possess three- (3D), two- (2D), one- (1D), and zero-dimensional (0D) structures at the molecular level. Remarkable progress has been realized in the research of perovskites in recent years, focusing mainly on 3D and 2D structures but leaving low-dimensional 1D and 0D structures significantly underexplored. Here we offer our perspective on the most exciting developments in the low-dimensional organometal halide perovskites. Due to the strong quantum confinement and site isolation, 1D and 0D perovskites exhibit remarkable and useful properties that are significantly different from those of 3D and 2D perovskites. The excitement about the recent developments lies not only in the specific achievements but also in what these materials represent in terms of a new paradigm in materials design.
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Halide-based perovskites have been found to be promising semiconductor materials for a variety of electronic and optoelectronic devices with high performance, including solar cells, light emitting diodes, photodetectors, etc. An ultra-long charge carrier lifetime/diffusion length, a high photoluminescence quantum efficiency, and a defect tolerant nature are essential in achieving high efficiency devices. Fundamental understanding of the structure–property–performance relationship of the halide perovskite at the nanoscale is currently attracting a significant amount of interest. In particular, an important and urgent topic is to bring the halide perovskites into the two-dimensional (2D) materials family not only for device miniaturization but also for exploring their physical properties under quantum confinement. Here, we highlight recent breakthroughs in the synthesis, characterization, and device application of atomically thin 2D halide perovskites. Moreover, future directions and challenges will be discussed to shed light on this promising field.
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Two-dimensional (2D) organic–inorganic perovskites have recently emerged as one of the most important thin-film solar cell materials owing to their excellent environmental stability. The remaining major pitfall is relatively poor photovoltaic performance in contrast to 3D perovskites. In this work we demonstrate cesium cation (Cs+) doped 2D (BA)2(MA)3Pb4I13 perovskite solar cells giving power conversion efficiency (PCE) as high as 13.7%, the highest among reported 2D devices, with excellent humidity resistance. The enhanced efficiency from 12.3% (without Cs+) to 13.7% (with 5% Cs+) is attributed to perfectly controlled crystal orientation, increased grain size of 2D planes, superior surface quality, reduced trap-state density, enhanced charge-carrier mobility and charge-transfer kinetics. Surprisingly, it is found that the Cs+ doping yields superior stability for the 2D perovskite solar cells when subjected to high humidity environment without encapsulation. The device doped using 5% Cs+ degrades only ca. 10% after 1400 hours exposure in 30% relative humidity (RH), and exhibits significantly improved stability under heating and high moisture environment. Our results provide an important step toward air-stable and fully printable low dimensional perovskite as a next-generation renewable energy source.
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Perovskite solar cells are remarkably efficient; however, they are prone to degradation in water, oxygen and ultraviolet light. Cation engineering in 3D perovskite absorbers has led to reduced degradation. Alternatively, 2D Ruddlesden–Popper layered perovskites exhibit improved stability, but have not delivered efficient solar cells so far. Here, we introduce n-butylammonium cations into a mixed-cation lead mixed-halide FA0.83Cs0.17Pb(IyBr1−y)3 3D perovskite. We observe the formation of 2D perovskite platelets, interspersed between highly orientated 3D perovskite grains, which suppress non-radiative charge recombination. We investigate the relationship between thin-film composition, crystal alignment and device performance. Solar cells with an optimal butylammonium content exhibit average stabilized power conversion efficiency of 17.5 ± 1.3% with a 1.61-eV-bandgap perovskite and 15.8 ± 0.8% with a 1.72-eV-bandgap perovskite. The stability under simulated sunlight is also enhanced. Cells sustain 80% of their ‘post burn-in’ efficiency after 1,000 h in air, and close to 4,000 h when encapsulated.
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Two-dimensional (2D) layered hybrid organic-inorganic halide perovskite semiconductors form natural “multiple quantum wells” that have strong spin-orbit coupling due to the heavy elements in their building blocks. This may lead to “Rashba splitting” close to the extrema in the electron bands. We have used a plethora of ultrafast transient, nonlinear optical spectroscopies and theoretical calculations to study the primary (excitons) and long-lived (free carriers) photoexcitations in thin films of 2D perovskite, namely, (C6H5C2H4NH3)2PbI4. The density functional theory calculation shows the occurrence of Rashba splitting in the plane perpendicular to the 2D barrier. From the electroabsorption spectrum and photoinduced absorption spectra from excitons and free carriers, we obtain a giant Rashba splitting in this compound, with energy splitting of (40 ± 5) meV and Rashba parameter of (1.6 ± 0.1) eV·Å, which are among the highest Rashba splitting size parameters reported so far. This finding shows that 2D hybrid perovskites have great promise for potential applications in spintronics.
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Unintentional self-doping in semiconductors through shallow defects is detrimental to optoelectronic device performance. It adversely affects junction properties and it introduces electronic noise. This is especially acute for solution-processed semiconductors, including hybrid perovskites, which are usually high in defects due to rapid crystallization. Here, we uncover extremely low self-doping concentrations in single crystals of (C6H5C2H4NH3)2PbI4・(CH3NH3PbI3)n-1 (n=1, 2, and 3)—over three orders of magnitude lower than those of typical three-dimensional hybrid perovskites—by analyzing their conductivity behavior. We propose that crystallization of hybrid perovskites containing large organic cations suppresses defect formation and thus favors a low self-doping level. To exemplify the benefits of this effect, we demonstrate extraordinarily high light-detectivity (1013 Jones) in (C6H5C2H4NH3)2PbI4・(CH3NH3PbI3)n-1 photoconductors due to the reduced electronic noise, which makes them particularly attractive for the detection of weak light signals. Furthermore, the low self-doping concentration reduces the equilibrium charge carrier concentration in (C6H5C2H4NH3)2PbI4・(CH3NH3PbI3)n-1, advantageous in the design of p-i-n heterojunction solar cells by optimizing band alignment and promoting carrier depletion in the intrinsic perovskite layer, thereby enhancing charge extraction.
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Organic-inorganic hybrid metal halide perovskites, an emerging class of solution processable photoactive materials, welcome a new member with a one-dimensional structure. Herein we report the synthesis, crystal structure and photophysical properties of one-dimensional organic lead bromide perovskites, C4N2H14PbBr4, in which the edge sharing octahedral lead bromide chains [PbBr4 ²⁻]∞ are surrounded by the organic cations C4N2H14 ²⁺ to form the bulk assembly of core-shell quantum wires. This unique one-dimensional structure enables strong quantum confinement with the formation of self-trapped excited states that give efficient bluish white-light emissions with photoluminescence quantum efficiencies of approximately 20% for the bulk single crystals and 12% for the microscale crystals. This work verifies once again that one-dimensional systems are favourable for exciton self-trapping to produce highly efficient below-gap broadband luminescence, and opens up a new route towards superior light emitters based on bulk quantum materials.
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Planar structures for halide perovskite solar cells have recently garnered attention, due to their simple and low-temperature device fabrication processing. Unfortunately, planar structures typically show I–V hysteresis and lower stable device efficiency compared with mesoporous structures, especially for TiO2-based n-i-p devices. SnO2, which has a deeper conduction band and higher electron mobility compared with traditional TiO2, could enhance charge transfer from perovskite to electron transport layers, and reduce charge accumulation at the interface. Here we report low-temperature solution-processed SnO2 nanoparticles as an efficient electron transport layer for perovskite solar cells. Our SnO2-based devices are almost free of hysteresis, which we propose is due to the enhancement of electron extraction. By introducing a PbI2 passivation phase in the perovskite layer, we obtain a 19.9 ± 0.6% certified efficiency. The devices can be easily processed under low temperature (150 ∘C), offering an efficient method for the large-scale production of perovskite solar cells.
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In this work, different from the commonly explored strategy of incorporating a smaller cation, MA+ and Cs+ into FAPbI3 lattice to improve efficiency and stability, it is revealed that the introduction of phenylethylammonium iodide (PEAI) into FAPbI3 perovksite to form mixed cation FAxPEA1?xPbI3 can effectively enhance both phase and ambient stability of FAPbI3 as well as the resulting performance of the derived devices. From our experimental and theoretical calculation results, it is proposed that the larger PEA cation is capable of assembling on both the lattice surface and grain boundaries to form quais-3D perovskite structures. The surrounding of PEA+ ions at the crystal grain boundaries not only can serve as molecular locks to tighten FAPbI3 domains but also passivate the surface defects to improve both phase and moisture stablity. Consequently, a high-performance (PCE:17.7%) and ambient stable FAPbI3 solar cell could be developed.
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The notion that halide perovskite crystals (ABX3, where X is a halide) exhibit unique structural and optoelectronic behavior deserves serious scrutiny. After decades of steady and half a decade of intense research, the question which attributes of these materials are unusual, is discussed, with an emphasis on the identification of the most important remaining issues. The goal is to stimulate discussion rather than to merely present a community consensus. Halide perovskites are fascinating crystalline materials for optoelectronic devices and can yield efficient solar cells. A critical review of their structural and optoelectronic characteristics is provided and it is found that they exhibit a unique combination of properties. Based on this notion, open questions about these materials are addressed.
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Two-dimensional (2D) hybrid perovskites are stoichiometric compounds consisting of alternating inorganic metal-halide sheets and organoammonium cationic layers. This materials class is widely tailorable in composition, structure, and dimensionality and is providing an intriguing playground for the solid-state chemistry and physics communities to uncover structure-property relationships. In this Perspective, we describe semiconducting 2D perovskites containing lead and tin halide inorganic frameworks. In these 2D perovskites, charges are typically confined to the inorganic framework because of strong quantum and dielectric confinement effects, and exciton binding energies are many times greater than kT at room temperature. We describe the role of the heavy atoms in the inorganic framework, the geometry and chemistry of organic cations, and the “softness” of the organic-inorganic lattice on the electronic structure and dynamics of electrons, excitons, and phonons that govern the physical properties of these materials.
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With nearly 20% of global electricity consumed by lighting, more efficient illumination sources can enable massive energy savings. However, effectively creating the high-quality white light required for indoor illumination remains a challenge. To accurately represent color, the illumination source must provide photons with all the energies visible to our eye. Such a broad emission is difficult to achieve from a single material. In commercial white-light sources, one or more light-emitting diodes, coated by one or more phosphors, yield a combined emission that appears white. However, combining emitters leads to changes in the emission color over time due to the unequal degradation rates of the emitters and efficiency losses due to overlapping absorption and emission energies of the different components. A single material that emits broadband white light (a continuous emission spanning 400–700 nm) would obviate these problems.
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This study presents a combined experimental and theoretical study of the electronic structure of two 2D metal halide perovskite films. Ultraviolet and inverse photoemission spectroscopies are performed on solution-processed thin films of the n = 1 layered perovskite butylammonium lead iodide and bromide, BA2PbI4 and BA2PbBr4, characterized by optical absorption and X-ray diffraction, to determine their valence and conduction band densities of states, transport gaps, and exciton binding energies. The electron spectroscopy results are compared with the densities of states determined by density functional theory calculations. The remarkable agreement between experiment and calculation enables a detailed identification and analysis of the organic and inorganic contributions to the valence and conduction bands of these two hybrid perovskites. The electron affinity and ionization energies are found to be 3.1 and 5.8 eV for BA2PbI4, and 3.1 and 6.5 eV for BA2PbBr4. The exciton binding energies are estimated to be 260 and 300 meV for the two materials, respectively. The 2D lead iodide and bromide perovskites exhibit significantly less band dispersion and a larger density of states at the band edges than the 3D analogs. The effects of using various organic ligands are also discussed.
Article
The construction of hybrid perovskites (both 2 dimension (2D) and 3 dimension (3D)) has attracted intensive research interest recently. Here, a facile, two-step consecutive deposition method was developed for the first time to grow a hierarchical quasi-2D/3D perovskite superstructure, with oriented quasi-2D ((BA)2(MA)n−1PbnI3n+1) perovskite nanosheet (NS) perpendicular aligned on 3D perovskites. The superstructure are found to be the mixture of multiple perovskite phases, with n = 2, 3, 4 and 3D perovskite, however, the n value was naturally increased from top to the bottom that is distinct from many other work. We found that the concentration gradient, namely, the initial ratio and amount of BAI/MAI, collectively contributing the spatially confined nucleation and growth of oriented quasi-2D superstructure perovskite on 3D perovskites. An efficient charge carrier transfer was demonstrated from small-n to large-n phases in this perovskite superstructure, indicating a different type of energy funnel from top to the bottom.
Article
Metal halide perovskites have achieved photovoltaic efficiencies exceeding 22%, but their widespread use is hindered by their instability in the presence of water and oxygen. To bolster stability, low-dimensional perovskite analogues have been developed wherein bulky organic ligands terminate the perovskite lattice, forming quantum wells (QWs) that are protected by the organic layers. In thin films, the width of these QWs exhibits a distribution that results in a spread of bandgaps of the materials arising due to varying degrees of quantum confinement across the population. Means to achieve refined control over this QW width distribution, and to examine and understand its influence on photovoltaic performance, are therefore of intense interest. Here we show here that moving to the ligand allylammonium enables a narrower distribution of QW widths, creating a flattened energy landscape that leads to x1.4 and x1.9 longer diffusion lengths for electrons and holes, respectively. We attribute this to the diminishment of ultrafast shallow hole trapping that originates from the most strongly confined QWs that do not form in a narrow distribution. We observe an increased PCE of 14.4% for allylammonium-based perovskite QW photovoltaics, compared to 11-12% PCEs obtained for analogous devices using phenethylammonium and butylammonium ligands. We then optimize the devices using mixed-cation strategies, achieving 16.5% PCE for allylammonium devices. The devices retain 90% of their initial PCEs after >650 hours when stored in ambient atmospheric conditions.
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This perspective paper focuses on the dimensionality of organic-inorganic halide perovskite and its relevant advantages over 3D perovskite. The charges in two-dimensional (2D) materials are restricted in their movement to the two-dimensional plane; however, their confined structure allows one to tune the optical and electronic properties by varying their thickness. Here we focus on the main advantages of 2D halide perovskite including its enhanced stability; several recent reports are summarized discussing this point. It was observed that the 2D perovskites exhibit enhanced moisture resistivity, starting from film but also in a complete solar cell. The power conversion efficiency of photovoltaic solar cells, based on 2D- and quasi-2D perovskite (2D+3D perovskite) as a light harvester, is increasing but still not as much as 3D perovskite. However, their other advantages (e.g., stability, tuning of optical and electronic properties, and the large amount of possible organic cations that can be integrated into their structure) makes them a stable alternative for efficient and large-scale module solar cells, which may constitute the next step for commercializing perovskite-based solar cells. To sum up several strategies of how to improve the photovoltaic performance of low dimensional perovskite are discussed.
Article
Organic-inorganic hybrid metal halide perovskites have emerged as a highly promising class of light emitters, which can be used as phosphor for optically pumped white light emitting diodes (WLEDs). By controlling the structural dimensionality, metal halide perovskites can exhibit tunable narrow and broadband emissions from the free exciton and self-trapped excited states, respectively. Here we report a highly efficient broadband yellow light emitter based on zero-dimensional (0D) tin mixed-halide perovskite (C4N2H14Br)4SnBrxI6-x (x = 3). This rare-earth free ionically bonded crystalline material possesses a perfect host-dopant structure, in which the light emitting metal halide species (SnBrxI6-x4-, x = 3) are completely isolated from each other and embedded in the wide band gap organic matrix composed of C4N2H14Br-. The strongly Stokes shifted broadband yellow emission peaked at 582 nm from this phopshor, a result of excited state structural reorganization, has an extremely large full width at half maximum (FWHM) of 126 nm and high photoluminescence quantum efficiency (PLQE) of ~ 85% at room temperature. UV pumped WLEDs fabricated using this yellow emitter together with a commercial europium-doped barium magnesium aluminate blue phosphor (BaMgAl10O17:Eu2+) can exhibit high color rendering indexes (CRIs) of up to 85.
Article
Solution processed organic-inorganic hybrid perovskites are emerging as a new generation materials for optoelectronics. However, the electroluminescence is highly limited in light emitting diodes (LED) due to the low exciton binding energy and the great challenge in stability. Here, we demonstrate a super air stable quasi-two dimensional perovskite film employing hydrophobic fluorine-containing organics as barrier layers, which can store in ambient for more than 4 months with no change. The dramatically reduced grain size of the perovskite crystal in contrast to three dimensional (3D) perovskites was achieved. Together with the natural quantum well of quasi-two dimensional perovskite confining the excitons to recombination, the LED exhibited the maximum luminance of 1.2 × 10³ cd/m² and current efficiency up to 0.3 cd/A, which is twenty fold enhancement than that of LED based on 3D analogues under the same condition.
Article
Extremely low energy consumption neuromorphic computing is required to achieve massively parallel information processing on par with the human brain. To achieve this goal, resistive memories based on materials with ionic transport and extremely low operating current are required. Extremely low operating current allows for low power operation by minimizing the program, erase and read currents. However, materials currently used in resistive memories, such as defective HfOx, AlOx, TaOx, etc, cannot suppress electronic transport (i.e. leakage current) while allowing good ionic transport. Here, we show that 2D Ruddlesden-Popper phase hybrid lead bromide perovskite single crystals are promising materials for low operating current nanodevice applications because of their mixed electronic and ionic transport and ease of fabrication. Ionic transport in the exfoliated 2D perovskite layer is evident via the migration of bromide ions. Filaments with diameter of approximately 20 nm are visualized, and resistive memories with extremely low program current down to 10 pA are achieved, a value at least one order of magnitude lower than conventional materials. The ionic migration and diffusion as an artificial synapse is realized in the 2D layered perovskites at the pA level, which can enable extremely low energy neuromorphic computing.
Article
Conventional 3D organic–inorganic halide perovskites have recently undergone unprecedented rapid development. Yet, their inherent instabilities over moisture, light, and heat remain a crucial challenge prior to the realization of commercialization. By contrast, the emerging 2D Ruddlesden−Popper-type perovskites have recently attracted increasing attention owing to their great environmental stability. However, the research of 2D perovskites is just in their infancy. In comparison to 3D analogues, they are natural quantum wells with a much larger exciton binding energy. Moreover, their inner structural, dielectric, optical, and excitonic properties remain to be largely explored, limiting further applications. This review begins with an introduction to 2D perovskites, along with a detailed comparison to 3D counterparts. Then, a discussion of the organic spacer cation engineering of 2D perovskites is presented. Next, quasi-2D perovskites that fall between 3D and 2D perovskites are reviewed and compared. The unique excitonic properties, electron–phonon coupling, and polarons of 2D perovskites are then be revealed. A range of their (opto)electronic applications is highlighted in each section. Finally, a summary is given, and the strategies toward structural design, growth control, and photophysics studies of 2D perovskites for high-performance electronic devices are rationalized.
Article
Two-dimensional (2D) organolead halide perovskites are promising candidates for many optoelectronic applications in view of their improved moisture-resistance and tunability in bandgap energy as compared to their 3D counterparts. Herein, we demonstrate the use of cation exchange between A and B in (A)2(B)n-1PbnI3n+1 (A = CH3(CH2)3NH3+, B = CH3NH3+) to fabricate hybrid mixed 2D perovskites with well-controlled n-values of perovskite constituents as well as their relative amounts. We find that the perovskite components in thin films can be systematically tuned from pure 3D to 2D hybrid mixture (containing multiple 2D components), and to pure single-phase 2D (n = 1) by simply adjusting the cation-exchange time. Extensive static and transient spectroscopic measurements show the internal electron-hole separation in the hybrid mixed 2D perovskite film due to the ordered band alignment between different perovskite components. This engineered directional charge flow can be particularly helpful for the photovoltaic application of 2D perovskites.
Article
Pure green light emitting diodes (LEDs) are essential to realize an ultra-wide color gamut in the next-generation displays, as is defined by the Rec. 2020 standard. However, because the human eye is more sensitive to the green spectral region, it is not yet possible to achieve an ultra-pure green electroluminescence (EL) with sufficiently narrow bandwidth that covers >95% of the Rec. 2020 standard in the CIE 1931 color space. Here, we demonstrate efficient, ultra-pure green EL based on the colloidal two-dimensional (2D) formamidinium lead bromide (FAPbBr3) hybrid perovskites. Through the dielectric-quantum-well (DQW) engineering, the quantum-confined 2D FAPbBr3 perovskites exhibit a high exciton binding energy of 162 meV, resulting in a high photoluminescence quantum yield (PLQY) of ~92% in the spin-coated films. Our optimized LED devices show a maximum current efficiency (ηCE) of 13.02 cd A-1 and the CIE 1931 color coordinates of (0.168, 0.773). The color gamut covers 97% and 99% of the Rec. 2020 standard in the CIE 1931 and the CIE 1976 color space, respectively, representing the “greenest” LEDs ever reported. Moreover, the device shows only a ~ 10% roll-off in ηCE (11.3 cd A-1) at 1000 cd m-2. We further demonstrate large-area (3 cm2) and ultra-flexible (bending radius of 2 mm) LEDs based on the 2D perovskites.
Article
Molecular piezoelectrics are highly desirable for their easy and environment-friendly processing, light weight, low processing temperature, and mechanical flexibility. However, although 136 years have passed since the discovery in 1880 of the piezoelectric effect, molecular piezoelectrics with a piezoelectric coefficient d33 comparable with piezoceramics such as barium titanate (BTO; ~190 picocoulombs per newton) have not been found. We show that trimethylchloromethyl ammonium trichloromanganese(II), an organic-inorganic perovskite ferroelectric crystal processed from aqueous solution, has a large d33 of 185 picocoulombs per newton and a high phase-transition temperature of 406 kelvin (K) (16 K above that of BTO). This makes it a competitive candidate for medical, micromechanical, and biomechanical applications.
Article
Hybrid organic–inorganic perovskites have emerged as a new class of semiconductors that exhibit excellent performance as active layers in photovoltaic solar cells. These compounds are also highly promising materials for the field of spintronics due to their large and tunable spin–orbit coupling, spin-dependent optical selection rules, and their predicted electrically tunable Rashba spin splitting. Here we demonstrate the optical orientation of excitons and optical detection of spin-polarized exciton quantum beating in polycrystalline films of the hybrid perovskite CH3NH3PbClxI3−x. Time-resolved Faraday rotation measurement in zero magnetic field reveals unexpectedly long spin lifetimes exceeding 1 ns at 4 K, despite the large spin–orbit couplings of the heavy lead and iodine atoms. The quantum beating of exciton states in transverse magnetic fields shows two distinct frequencies, corresponding to two g-factors of 2.63 and −0.33, which we assign to electrons and holes, respectively. These results provide a basic picture of the exciton states in hybrid perovskites, and suggest they hold potential for spintronic applications.
Article
The formation of a dense and uniform thin layer on the substrates is crucial for the fabrication of high-performance perovskite solar cells (PSCs) containing formamidinium with multiple cations and mixed halide anions. The concentration of defect states, which reduce a cell’s performance by decreasing the open-circuit voltage and short-circuit current density, needs to be as low as possible. We show that the introduction of additional iodide ions into the organic cation solution, which are used to form the perovskite layers through an intramolecular exchanging process, decreases the concentration of deep-level defects. The defect-engineered thin perovskite layers enable the fabrication of PSCs with a certified power conversion efficiency of 22.1% in small cells and 19.7% in 1-square-centimeter cells.
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This study depicts the influence of the Rashba effect on the band-edge exciton processes in all-inorganic CsPbBr3 perovskite single colloidal nanocrystal (NC). The study is based on magneto-optical measurements carried out at cryogenic temperatures under various magnetic field strengths, in which, discrete excitonic transitions were detected by linearly- and circularly-polarized measurements. Interestingly, the experiments show a nonlinear energy splitting between polarized transitions versus magnetic field strength, indicating a crossover between a Rashba effect (at the lowest fields) to a Zeeman effect at fields above 4 T. We postulate that the Rashba effect emanates from a lattice distortion induced by the Cs⁺ motion degree of freedom, or due to a surface effect in nanoscale NCs. The unusual magneto-optical properties shown here underscore the importance of the Rashba effect in the implementation of such perovskite materials in various optical and spin-based devices.
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Despite the impressive photovoltaic performances with power conversion efficiency beyond 22%, perovskite solar cells are poorly stable under operation, failing by far the market requirements. Various technological approaches have been proposed to overcome the instability problem, which, while delivering appreciable incremental improvements, are still far from a market-proof solution. Here we show one-year stable perovskite devices by engineering an ultra-stable 2D/3D (HOOC(CH2)4NH3)2PbI4/CH3NH3PbI3 perovskite junction. The 2D/3D forms an exceptional gradually-organized multi-dimensional interface that yields up to 12.9% efficiency in a carbon-based architecture, and 14.6% in standard mesoporous solar cells. To demonstrate the up-scale potential of our technology, we fabricate 10 × 10 cm² solar modules by a fully printable industrial-scale process, delivering 11.2% efficiency stable for >10,000 h with zero loss in performances measured under controlled standard conditions. This innovative stable and low-cost architecture will enable the timely commercialization of perovskite solar cells.
Article
Ion migration, which occurs in regular three-dimensional perovskites, is shown to be suppressed in low dimensional perovskites both in the dark and under illumination, an indication of better stability of these materials for solar cells and light emitting diodes.
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Perovskite solar cells (PSCs) exceeding a power conversion efficiency (PCE) of 20% have mainly been demonstrated by using mesoporous titanium dioxide (mp-TiO2) as an electron-transporting layer. However, TiO2 can reduce the stability of PSCs under illumination (including ultraviolet light). Lanthanum (La)-doped BaSnO3 (LBSO) perovskite would be an ideal replacement given its electron mobility and electronic structure, but LBSO cannot be synthesized as well-dispersible fine particles or crystallized below 500?C. We report a superoxide colloidal solution route for preparing an LBSO electrode under very mild conditions (below 300?C). The PSCs fabricated with LBSO and methylammonium lead iodide (MAPbI3) show a steady-state power conversion efficiency of 21.2%, versus 19.7% for a mp-TiO2 device. The LBSO-based PSCs could retain 93% of its initial performance after 1000 hours of full Sun illumination.
Article
Grazing-incidence X-ray scattering (GIXS) methods have proven to be a valuable asset for investigating the morphology of thin films at different length scales. Consequently, GIXS has been applied to the fast-progressing field of organometal halide perovskites. This exciting class of materials has propelled research in the areas of cheap and sustainable photovoltaics, light emitting devices, and optoelectronics in general. Especially, perovskite solar cells (PSC) have seen a remarkable rise in power conversion efficiencies, crossing the 20% mark after only five years of research. This research news outlines GIXS studies focusing on the most challenging research topics in the perovskite field today: Current–voltage hysteresis, device reproducibility, and long-term stability of PSC are inherently linked to perovskite film morphology. On the other hand, film formation depends on the choice of precursors and processing parameters; understanding their interdependence opens possibilities to tailor film morphologies. Owing to their tunability and moisture resistance, 2D perovskites have recently attracted attention. Examples of GIXS studies with different measurement and data analysis techniques are presented, highlighting especially in-situ investigations on the many kinetic processes involved. Thus, an overview on the toolbox of GIXS techniques is linked to the specific needs of research into organometal halide perovskite optoelectronics.
Article
Here, we present the fifth member (n = 5) of the Ruddlesden-Popper (CH3(CH2)3NH3)2(CH3NH3)n−1PbnI3n+1 family, which we successfully synthesized in high yield and purity. Phase purity could be clearly determined from its X-ray powder diffraction patterns, which feature the (0k0) Bragg reflections at low 2θ angles. The obtained pure n = 5 compound was confirmed to be a direct band-gap semiconductor with Eg = 1.83 eV. The direct nature of the band gap is supported by density functional theory calculations. Intense photoluminescence was observed at room temperature at 678 nm arising from the band edge of the material. High-quality thin films can be prepared by the hot-casting method from solutions with a pure-phase compound as a precursor. The planar solar cells fabricated with n = 5 thin films demonstrate excellent power-conversion efficiency of 8.71% with an impressive open-circuit voltage of ∼1 V. Our results point to the use of layered perovskites with higher n numbers and pure chemical composition.
Article
Halide perovskites have high light absorption coefficients, long charge carrier diffusion lengths, intense photoluminescence, and slow rates of non-radiative charge recombination. Thus, they are attractive photoactive materials for developing high-performance optoelectronic devices. These devices are also cheap and easy to be fabricated. To realize the optimal performances of halide perovskite-based optoelectronic devices (HPODs), perovskite photoactive layers should work effectively with other functional materials such as electrodes, interfacial layers and encapsulating films. Conventional two-dimensional (2D) materials are promising candidates for this purpose because of their unique structures and/or interesting optoelectronic properties. Here, we comprehensively summarize the recent advancements in the applications of conventional 2D materials for halide perovskite-based photodetectors, solar cells and light-emitting diodes. The examples of these 2D materials are graphene and its derivatives, mono- and few-layer transition metal dichalcogenides (TMDs), graphdiyne and metal nanosheets, etc. The research related to 2D nanostructured perovskites and 2D Ruddlesden-Popper perovskites as efficient and stable photoactive layers is also outlined. The syntheses, functions and working mechanisms of relevant 2D materials are introduced, and the challenges to achieving practical applications of HPODs using 2D materials are also discussed.