Perovskite light-emitting diodes based on spontaneously formed submicrometre-scale structures.

Light-emitting diodes (LEDs), which convert electricity to light, are widely used in modern society-for example, in lighting, flat-panel displays, medical devices and many other situations. Generally, the efficiency of LEDs is limited by nonradiative recombination (whereby charge carriers recombine without releasing photons) and light trapping1-3. In planar LEDs, such as organic LEDs, around 70 to 80 per cent of the light generated from the emitters is trapped in the device4,5, leaving considerable opportunity for improvements in efficiency. Many methods, including the use of diffraction gratings, low-index grids and buckling patterns, have been used to extract the light trapped in LEDs6-9. However, these methods usually involve complicated fabrication processes and can distort the light-output spectrum and directionality6,7. Here we demonstrate efficient and high-brightness electroluminescence from solution-processed perovskites that spontaneously form submicrometre-scale structures, which can efficiently extract light from the device and retain wavelength- and viewing-angle-independent electroluminescence. These perovskites are formed simply by introducing amino-acid additives into the perovskite precursor solutions. Moreover, the additives can effectively passivate perovskite surface defects and reduce nonradiative recombination. Perovskite LEDs with a peak external quantum efficiency of 20.7 per cent (at a current density of 18 milliamperes per square centimetre) and an energy-conversion efficiency of 12 per cent (at a high current density of 100 milliamperes per square centimetre) can be achieved-values that approach those of the best-performing organic LEDs.

Myth behind Metastable and Stable n-Hexylammonium Bromide-Based Low-Dimensional Perovskites.

In perovskite solar cells, passivating the surface or interface that contains a high concentration of defects, specifically deep-level defects, is one of the most important topics to substantially enhance the power conversion efficiency and stability of the devices. Long-chain alkylammonium bromides have been widely and commonly adapted for passivation treatment. However, the mechanism behind is still not well explored as the formation route and the exact structure of these alkylammonium bromide-based low-dimensional perovskites are unclear. Herein, we investigate the physical and chemical properties of an n-hexylammonium bromide (HABr)-based low-dimensional perovskite including both thin films and single crystals. First of all, the HA2PbBr4 perovskite film and aged single crystal demonstrate different X-ray diffraction patterns from those of the fresh as-prepared single crystal. We found that the fresh HA2PbBr4 single crystal exhibits a metastable phase as its structure changes with aging due to the relaxation of crystal lattice strains, whereas the HA2PbBr4 perovskite film is pretty stable as the aged single crystal. Upon reacting with FAPbI3, HABr can be intercalated into the FAPbI3 lattice to form a mixed-cation perovskite of HAFAPbI3Br, which is in a dynamic equilibrium of decomposition and formation. In contrast, the reaction of HABr with excess PbI2 forms a stable HA2PbI2Br2 perovskite. Based on such findings, we rationally develop a HA2PbI2Br2-passivated FACs-based perovskite by reacting HABr with excess PbI2, the photovoltaics based on which are more stable and efficient than those passivated by the HAFAPbI3Br perovskite. Our discovery paves way for a more in-depth study of bromide-containing low-dimensional perovskites and their optoelectronic applications.

Surface Termination on Unstable Methylammonium-based Perovskite Using a Steric Barrier for Improved Perovskite Solar Cells.

Compared to widely adopted low-dimensional/three-dimensional (LD/3D) heterostructure, functional organic cation based surface termination on perovskite can not only realize advantage of defect passivation but also prevent potential disadvantage of the heterostructure induced intercalation into 3D perovskite. However, it is still very challenging to controllably construct surface termination on organic-inorganic hybrid perovskite because the functional organic cations' substitution reaction is easy to form LD/3D heterostructure. Here, we report using a novel benzyltrimethylammonium (BTA) functional cation with rational designed steric hindrance to effectively surface terminate onto methylammonium lead triiodide (MAPbI3 ) perovskite, which is composed of the most unstable MA cations. The BTA cation is difficult to form a specific 1.5-dimensional perovskite of BTA4 Pb3 I10 by cation substitution with MA cation, which then provides a wide processing window (around 10 minutes) for surface terminating on MAPbI3 films. Moreover, the BTAI surface terminated BTAI-MAPbI3 shows better passivation effect than BTA4 Pb3 I10 -MAPbI3 heterojunction. Finally, BTAI surface terminated solar cell (0.085 cm2 ) and mini-module (11.52 cm2 ) obtained the efficiencies of 22.03 % and 18.57 %, which are among the highest efficiencies for MAPbI3 based ones.

Organic Tetrabutylammonium Cation Intercalation to Heal Inorganic CsPbI3 Perovskite.

The in situ formation of reduced dimensional perovskite layer via post-synthesis ion exchange has been an effective way of passivating organic-inorganic hybrid perovskites. In contrast, cesium ions in Cs-based inorganic perovskite with strong ionic binding energy cannot exchange with those well-known organic cations to form reduced dimensional perovskite. Herein, we demonstrate that tetrabutylammonium (TBA+ ) cation can intercalate into CsPbI3 to effectively substitute the Cs cation and to form one-dimensional (1D) TBAPbI3 layer in the post-synthesis TBAI treatment. Such TBA cation intercalation leads to in situ formation of TBAPbI3 protective layer to heal defects at the surface of inorganic CsPbI3 perovskite. The TBAPbI3 -CsPbI3 perovskite exhibited enhanced stability and lower defect density, and the corresponding perovskite solar cell devices achieved an improved efficiency up to 18.32 % compared to 15.85 % of the control one.

Detection of volatile organic compounds released by wood furniture based on a cataluminescence test system.

Wood furniture is an important source of indoor air pollution. To date, the detection of harmful substances in wood furniture has relied on the control of a single formaldehyde component, therefore the detection and evaluation of pollutants released by wood furniture are necessary. A novel method based on a cataluminescence (CTL) sensor system generated on the surface of nano-3TiO2-2BiVO4 was proposed for the simultaneous detection of pollutants released by wood furniture. Formaldehyde and benzene were selected as a model to investigate the CTL-sensing properties of the sensor system. Field emission scanning electronic microscopy (FESEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) were employed to characterize the as-prepared samples. The results showed that the as-prepared test system exhibited outstanding CTL properties such as stable intensity, a high signal-to-noise ratio, and short response and recovery times. In addition, the limit of detection for formaldehyde and benzene was below the standard permitted concentrations. Moreover, the sensor system showed outstanding selectivity for formaldehyde and benzene compared with eight other common volatile organic compounds (VOCs). The performance of the sensor system will enable furniture VOC limit emissions standards to be promulgated as soon as possible.

An evaluation model for interactive gaming furniture design based on parent-child behavior.

This study takes the parent-child game behavior of children aged 3~6 and their parents as the research object, and extracts and summarizes the user behavioral needs of parents and children when they use game-based furniture together by using the questionnaire research method, observation method, and interview method. Based on the KJ method, 16 behavioral demand indicators were compiled by five furniture design students to construct a user behavioral demand system. In addition, AHP and entropy weight method were used to solve the user behavioral demand weights from subjective and objective perspectives in this study. Twenty experts and designers in this research field scored the indicators two by two and solved the subjective weights of user behavioral requirements according to the AHP algorithm. A seven-level Likert scale was used to design the questionnaire and distribute it to the parents of children aged 3-6 to fill in, and the 121 valid questionnaires obtained were used as raw data for entropy weighting to obtain the objective weights of user behavioral needs representing the opinions of interactive game-based furniture users. Finally, with 0.4 as the proportion coefficient of subjective weights, the subjective and objective weights were weighted to get the comprehensive weight value of each demand. The results show that the eight items with higher weights for user behavioral needs include: firm and stable, safe in use, comfortable for both parents and children, holding behavior by human-machine dimensions, able to sit on the ground and play, able to play face-to-face, easy to find for picking up, and sufficient operating space. In general, parent-child interactive game furniture firstly needs to meet the user's needs for safety and comfort, and secondly needs to meet the user's needs for the state of the game posture and the furniture size to meet the needs of the fetching and storage posture and the game space. The fuzzy comprehensive evaluation model established based on these needs can take into account the opinions of design experts and users at the same time and put the needs of children and parents in an equally important position so that the design of children's play furniture can tend to meet the needs of parents and children when they use it together, and to promote parent-child interaction and the healthy growth of children.

Highly Crystalized Cl-Doped SnO2 Nanocrystals for Stable Aqueous Dispersion Toward High-Performance Perovskite Photovoltaics.

Tin dioxide (SnO2 ) with high conductivity and low photocatalytic activity has been reported as one of the best candidates for highly efficient electron transport layer (ETL) in perovskite solar cell (PSC). The state-of-the-art SnO2 layer is achieved by chemical bath deposition with tunable properties, while the commercial SnO2 nanocrystals (NCs) with low tunability still face the necessity of further improvement. Here, a kind of highly crystallized Cl-doped SnO2 NCs is reported that can form very stable aqueous dispersion with shelf life up to one year without any stabilizer, which can facilitate the fabrication of PSCs with satisfactory performance. Compared to the commercial SnO2 NCs regardless of the extrinsic Cl-doping conditions, the intrinsic Cl-doped SnO2 NCs effectively suppress the energy barrier and reduces the trap state density at the buried interface between perovskite and ETL. Consequently, stable PSCs based on such Cl-doped SnO2 NCs achieve a champion efficiency up to ≈25% for small cell (0.085 cm2 ) and ≈20% for mini-module (12.125 cm2 ), indicating its potential as a promising candidate for ETL in high-performance perovskite photovoltaics.

Functional organic cation induced 3D-to-0D phase transformation and surface reconstruction of CsPbI3 inorganic perovskite.

Efficiency and stability are the main research focuses for perovskite solar cells. Inorganic perovskites like CsPbI3 possess higher chemical stability than those with organic A-site cations, while they also exhibit higher defect density. Nonetheless, it is highly challenging to induce orderly secondary arrangement or reconstruction of inorganic perovskites with reduced defects because of their unique chemical properties. In this work, in-situ three-dimension-to-zero-dimension (3D-to-0D) phase transformation and surface reconstruction on CsPbI3 film is achieved as induced by a functional organic cation, benzyldodecyldimethylammonium (BDA), a process of which that is similar to phase-transfer catalysis. With the help of BDABr salt treatment, 0D Cs4PbI6 perovskites are secondarily formed along CsPbI3 grain boundaries with Cs-related cationic defects passivated, yielding structures of higher stability. The BDA-CsPbI3 films exhibit reduced non-radiative recombination and promoted charge transfer, leading to inorganic perovskite solar cells with a high power conversion efficiency of 20.63% and good operational stability.

Photovoltaic-driven electrocatalytic upcycling poly(ethylene terephthalate) plastic waste coupled with hydrogen generation.

The electrochemical upconversion of plastic wastes has been demonstrated as an attractive alternative to the sluggish OER process to simultaneously produce valued chemicals and reduce the energy consumption. Herein, we report a photovoltaic-driven electrocatalytic strategy to upcycle poly(ethylene terephthalate) (PET) into value-added formic acid products and co-produce green hydrogen. The waste PET was dissolved by KOH and then directly pumped into an electrochemical flow reactor (EFR) including CuO nanowires (NWs) anode and Pt/C 20% cathode (PV-EFR) and driven by the commercial silicon photovoltaic (PV) panels. This PV-EFR system exhibits a solar-to-chemical (STC) efficiency of 32.6% under AM 1.5 G simulated sunlight (100 mW cm-2), and high Faradaic efficiencies (FE, ∼ 67% for formic acid, and ∼90% for green hydrogen) with exceptional 120 h long-term stability in the STC mode. Such a photovoltaic-driven electrocatalytic strategy exhibits great potential for the rational utilization of renewable energy sources to produce high-value chemicals and fuels by upconversion of waste plastics.

Post-Treatment-Free Dual-Interface Passivation via Facile 1D/3D Perovskite Heterojunction Construction.

For achieving high-efficiency perovskite solar cells, it is almost always necessary to substantially passivate defects and protect the perovskite structure at its interfaces with charge transport layers. Such a modification generally involves the post-treatment of the deposited perovskite film by spin coating, which cannot meet the technical demands of scaling up the production of perovskite photovoltaics. In this work, we demonstrate one-step construction of buried and capped double 1D/3D heterojunctions without the need for any post-treatment, which is achieved through facile tetraethylammonium trifluoroacetate (TEATFA) prefunctionalization on the SnO2 substrate. The functional TEATFA salt is first deposited onto the SnO2 substrate and reacts on this buried interface. Once the FAPbI3 perovskite precursor solution is dripped, a portion of the TEA+ cations spontaneously diffuse to the top surface over film crystallization. The TEATFA-based water-resistant 1D/3D TEAPbI3/FAPbI3 heterojunctions at both the buried and capped interfaces lead to much better photovoltaic performance and higher operational stability. Since this approach saves the need for any postsynthesis passivation, its feasibility for the fabrication of large-area perovskite photovoltaics is also showcased. Compared to ∼15% for a pristine 5 cm × 5 cm FAPbI3 mini-module without postsynthesis passivation, over 20% efficiency is achieved following the proposed route, demonstrating its great potential for larger-scale fabrication with fewer processing steps.

Zwitterion-Functionalized SnO2 Substrate Induced Sequential Deposition of Black-Phase FAPbI3 with Rearranged PbI2 Residue.

Black-phase formamidinium lead iodide (FAPbI3 ) with narrow bandgap and high thermal stability has emerged as the most promising candidate for highly efficient and stable perovskite photovoltaics. In order to overcome the intrinsic difficulty of black-phase crystallization and to eliminate the lead iodide (PbI2 ) residue, most sequential deposition methods of FAPbI3 -based perovskite will introduce external ions like methylammonium (MA+ ), cesium (Cs+ ), and bromide (Br- ) ions to the perovskite structure. Here a zwitterion-functionalized tin(IV) oxide (SnO2 ) is introduced as the electron-transport layer (ETL) to induce the crystallization of high-quality black-phase FAPbI3 . The SnO2 ETL treated with the zwitterion of formamidine sulfinic acid (FSA) can help rearrange the stack direction, orientation, and distribution of residual PbI2 in the perovskite layer, which reduces the side effect of the residual PbI2 . Besides, the FSA functionalization also modifies SnO2 ETL to suppress deep-level defects at the perovskite/SnO2 interface. As a result, the FSA-FAPbI3 -based perovskite solar cells (PSCs) exhibit an excellent power conversion efficiency of up to 24.1% with 1000 h long-term operational stability. These findings provide a new interface engineering strategy on the sequential fabrication of black-phase FAPbI3 PSCs with improved optoelectronic performance.

Stable Pure Iodide MA0.95Cs0.05PbI3 Perovskite toward Efficient 1.6 eV Bandgap Photovoltaics.

Perovskite photovoltaics with the advantages of facile fabrication and high efficiency have been the rising star in the field for a decade. Methylammonium lead triiodide (MAPbI3) was the first widely studied perovskite to initiate the boom of perovskite photovoltaics, but it was later considered thermodynamically instable for commercialization. Here, we demonstrate that simple cesium (Cs) doping without any complicated process can form a stable MA-based perovskite with a widened bandgap, which may broaden the application of MA-based perovskites in tandem solar cells. A record-high efficiency of ≤22% is thus achieved for a 1.6 eV bandgap perovskite solar cell. This work not only provides a new stable and efficient pure iodide candidate as a 1.6 eV bandgap perovskite but also reveals that Cs incorporation can help improve the efficiency and stability of MA-based perovskites.

Decoupling engineering of formamidinium-cesium perovskites for efficient photovoltaics.

Although pure formamidinium iodide perovskite (FAPbI3) possesses an optimal gap for photovoltaics, their poor phase stability limits the long-term operational stability of the devices. A promising approach to enhance their phase stability is to incorporate cesium into FAPbI3. However, state-of-the-art formamidinium-cesium (FA-Cs) iodide perovskites demonstrate much worse efficiency compared with FAPbI3, limited by the different crystallization dynamics of formamidinium and cesium, which result in poor composition homogeneity and high trap densities. We develop a novel strategy of crystallization decoupling processes of formamidinium and cesium via a sequential cesium incorporation approach. As such, we obtain highly reproducible, highly efficient and stable solar cells based on FA1 - x Cs x PbI3 (x = 0.05-0.16) films with uniform composition distribution in the nanoscale and low defect densities. We also revealed a new stabilization mechanism for Cs doping to stabilize FAPbI3, i.e. the incorporation of Cs into FAPbI3 significantly reduces the electron-phonon coupling strength to suppress ionic migration, thereby improving the stability of FA-Cs-based devices.

High-Brightness Perovskite Microcrystalline Light-Emitting Diodes.

Here a high-brightness perovskite microcrystalline light-emitting diode (LED) is reported, in which the perovskite microcrystals were grown directly on the conductive substrate and a simple metal-insulator-semiconductor structure was adopted. A peak external quantum efficiency of 0.46% was obtained, which is high for perovskite microcrystalline LEDs. Importantly, the maximum luminance of the device reaches 8848.4 cd m-2, indicating an ultrahigh brightness of >1.2 × 106 cd m-2 for the microcrystals (corresponding to an ultrahigh current density of 80.9 A cm-2), because the light-emitting area of the microcrystals accounts for only ∼0.7% of the device area. In addition, we have studied the degradation of the device at a high current density by in situ microscopic observation and found that a severe Joule heating effect at large injection is the primary problem to be solved to realize electrically pumped perovskite microcrystal lasing.

The Chemical Design in High-Performance Lead Halide Perovskite: Additive vs Dopant?

Metal halide perovskite solar cells (PSCs) have attracted enormous attention as one of the most promising candidates for next-generation photovoltaics in the past few years. During the development of PSCs, various chemicals have been added to improve film quality and device performance. However, there are still debates about whether these chemicals are additives as removed from the final film or dopants incorporated into the crystal lattice. It is important to clarify whether these added chemicals are additives or dopants when designed for high-quality perovskite films' fabrications. Herein, we summarized several commonly used chemicals for hybrid and all-inorganic perovskites, such as MACl, DMAI, MAAc, and alkali metal cations. The underlying mechanism and their roles during the formation of perovskite films were discussed. In the end, we proposed some conclusive important factors to clarify additives and dopants, which would be helpful for the further chemical design for improving high-performance perovskite devices.

Using steric hindrance to manipulate and stabilize metal halide perovskites for optoelectronics.

The chemical instability of metal halide perovskite materials can be ascribed to their unique properties of softness, in which the chemical bonding between metal halide octahedral frameworks and cations is the weak ionic and hydrogen bonding as in most perovskite structures. Therefore, various strategies have been developed to stabilize the cations and metal halide frameworks, which include incorporating additives, developing two-dimensional perovskites and perovskite nanocrystals, etc. Recently, the important role of utilizing steric hindrance for stabilizing and passivating perovskites has been demonstrated. In this perspective, we summarize the applications of steric hindrance in manipulating and stabilizing perovskites. We will also discuss how steric hindrance influences the fundamental kinetics of perovskite crystallization and film formation processes. The similarities and differences of the steric hindrance between perovskite solar cells and perovskite light emission diodes are also discussed. In all, utilizing steric hindrance is a promising strategy to manipulate and stabilize metal halide perovskites for optoelectronics.

Deep-Red Perovskite Light-Emitting Diodes Based on One-Step-Formed γ-CsPbI3 Cuboid Crystallites.

Inorganic CsPbI3 perovskite with high chemical stability is attractive for efficient deep-red perovskite light-emitting diodes (PeLEDs) with high color purity. Compared to PeLEDs based on ex-situ-synthesized CsPbI3 nanocrystals/quantum dots suffering from low conductivity and efficiency droop under high current densities, in situ deposited 3D CsPbI3 films from precursor solutions can maintain high conductivity but show high trap density. Here, it is demonstrated that introducing diammonium iodide can increase the size of colloids in the precursor solution, retard the phase-transition rate, and passivate trap states of the in-situ-formed cuboid crystallites. The PeLED based on the one-step-formed 3D CsPbI3 cuboid crystallite films shows a peak external quantum efficiency (EQE) value up to 15.03% because of the high conductivity and reduced trap states. Furthermore, this one-step method also has a wide processing window, which is attractive for flow-line production of large-area PeLED modules. The fabrication of a 9 cm2 PeLED that exhibits a peak EQE of 10.30% is successfully demonstrated.

Incorporation of Two-Dimensional WSe2 into MAPbI3 Perovskite for Efficient and Stable Photovoltaics.

Achieving reduced defect density and efficient charge carrier extraction plays a vital role for efficient and stable perovskite solar cells (PSCs). Over the course of technical development, it is desired to use one single material or approach to synergistically passivate defects and enhance the charge extraction. In this work, we developed an effective strategy for obtaining efficient and stable PSCs via incorporating quasi-monolayer two-dimensional WSe2 into the MAPbI3 perovskite layer. The addition of WSe2 helps with the formation of perovskite film with higher quality and also passivates the Pb-related defects through Pb-Se coordination bonding. MAPbI3/WSe2 shows a more matched energy-level alignment between the perovskite layer and hole transport layer for accelerated hole extraction. Consequently, the performances of PSCs significantly improved with power conversion efficiency increase from 19.2% to 21.2% after the incorporation of WSe2. Accordingly, the MAPbI3/WSe2-based PSCs exhibit well-improved photostability with suppression of Pb0 defect formation.

MA Cation-Induced Diffusional Growth of Low-Bandgap FA-Cs Perovskites Driven by Natural Gradient Annealing.

Low-bandgap formamidinium-cesium (FA-Cs) perovskites of FA1-x Cs x PbI3 (x < 0.1) are promising candidates for efficient and robust perovskite solar cells, but their black-phase crystallization is very sensitive to annealing temperature. Unfortunately, the low heat conductivity of the glass substrate builds up a temperature gradient within from bottom to top and makes the initial annealing temperature of the perovskite film lower than the black-phase crystallization point (~150°C). Herein, we take advantage of such temperature gradient for the diffusional growth of high-quality FA-Cs perovskites by introducing a thermally unstable MA+ cation, which would firstly form α-phase FA-MA-Cs mixed perovskites with low formation energy at the hot bottom of the perovskite films in the early annealing stage. The natural gradient annealing temperature and the thermally unstable MA+ cation then lead to the bottom-to-top diffusional growth of highly orientated α-phase FA-Cs perovskite, which exhibits 10-fold of enhanced crystallinity and reduced trap density (~3.85 × 1015 cm-3). Eventually, such FA-Cs perovskite films were fabricated into stable solar cell devices with champion efficiency up to 23.11%, among the highest efficiency of MA-free perovskite solar cells.

5-Ammonium Valeric Acid Iodide to Stabilize MAPbI3 via a Mixed-Cation Perovskite with Reduced Dimension.

5-Ammonium valeric acid iodide (AVAI) has been widely known as a stabilizer to enhance the stability of MAPbI3 perovskite, but its role and function is still under exploration. The typical 2D perovskites of AVA2PbI4 have been proposed as the capping layer for stabilization. Here, a novel AVA-MA mixed-cation perovskite of AVAMAPbI4 is found to show a more even and compact coverage than the typical 2D perovskite of AVA2PbI4. A simple post-treatment on MAPbI3 films by using AVAI isopropanol solution can fabricate such a mixed-cation 2D perovskite capping layer on the MAPbI3 sample. This AVAMAPbI4 capping layer effectively passivates surface defects of MAPbI3 perovskite films and reduces the charge-carrier recombination, enabling AVAI-MAPbI3 perovskite films to exhibit improved stability against thermal and moisture stress. Finally, the AVAI-MAPbI3-based perovskite solar cells also show an enhanced photovoltaic performance with a champion PCE up to 20.05% with enhanced stability.