Understanding the Mechanisms of Methylammonium-Induced Thermal Instability in Mixed-FAMA Perovskites.

Despite a recent shift toward methylammonium (MA)-free lead-halide perovskites for perovskite solar cells, high-efficiency formamidinium lead iodide (FAPbI3) devices still often require methylammonium chloride (MACl) as an additive, which evaporates away during the annealing process. In this article, it is shown that the residual MA+, however, triggers thermal instability. To investigate the possibility of an optimal concentration of MA+ that may improve thermal stability, the intrinsic thermal stability of pure FA, FA-rich, MA-rich, and pure MA perovskite films (FA1-xMAxPbI3, FAMA) is studied. The results show that the thermal stability of FAMA perovskites decreases with more MA+, under degradation conditions that isolate the intrinsic thermal stability of the material (i.e., without moisture and oxygen effects). X-ray diffraction (XRD), proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS), photoluminescence (PL) and UV-visible spectroscopy, and depth-profiling X-ray Photoelectron Spectroscopy (XPS) are employed to show that the observed trend is mainly due to the decomposition of the MA+ cation, as opposed to other effects such as the precursor solvent and film morphologies. It is also found that the surfaces of these FAMA films are MA+ rich, although this phenomenon does not appear to affect thermal stability. Finally, it is demonstrated that this trend is unaffected by the presence of Spiro-OMeTAD atop the film, and thus solar cell devices should preserve this trend.

Retinomorphic Color Perception Based on Opponent Process Enabled by Perovskite Bipolar Photodetectors.

The ability to perceive color by the retina can be attributed to both its trichromatic photoreceptors and the antagonistic neural wiring known as the opponent process. While neuromorphic sensors have been shown to demonstrate memory and adaptation capabilities, color perception is still challenging due to the intrinsic lack of spectral selectivity in narrow bandgap semiconductors. Furthermore, research on emulating neural wiring is severely lacking. The combination of halide perovskite materials with a tunable bandgap and a novel bipolar photodetector design emulates the efficiency of the retina in processing color information. The stimuli-responsive material is also responsible for maintaining partial color constancy-an adaptation feature. Leveraging the unique enhancement of color contrasts, an in-sensor data compression and edge detection can also be demonstrated. The color perception, chromatic adaptation, and color contrast enhancement make perovskite bipolar photodetectors a unique example where the sensor and neural wiring can be co-developed in conjunction.

Stable and Highly Emissive Infrared Yb-Doped Perovskite Quantum Cutters Engineered by Machine Learning.

Quantum cutting (QC) allows the conversion of high-energy photons into lower-energy photons, exhibiting great potential for infrared communications. Yb-doped perovskite nanocrystals can achieve an efficient QC process with extremely high photoluminescence quantum yield (PLQY) thanks to the favorable Yb3+ incorporation in the perovskite structure. However, conventionally used oleic acid-oleylamine-based ligand pairs cause instability issues due to highly dynamic binding to surface states that have curbed their potential applications. Herein, zwitterionic type C3-sulfobetaine 3-(N,N-Dimethylpalmitylammonio)propanesulfonate molecule is utilized to build a strong binding state on the nanocrystals' surface through a new phosphine oxide synthesis route. Leveraging machine learning and Bayesian Optimization workflow to determine optimal synthesis conditions, near-infrared PLQY above 190% is achieved. The high PLQY is well maintained after over three months of aging, under high-flux continuous UV irradiation, and long continuous annealing. This is the first report of highly efficient and stable perovskite quantum cutters, which will drive the study of fundamental physics phenomena and near-infrared quantum communications.

Weakly Confined Organic-Inorganic Halide Perovskite Quantum Dots as High-Purity Room-Temperature Single Photon Sources.

Colloidal perovskite quantum dots (PQDs) have emerged as highly promising single photon emitters for quantum information applications. Presently, most strategies have focused on leveraging quantum confinement to increase the nonradiative Auger recombination (AR) rate to enhance single-photon (SP) purity in all-inorganic CsPbBr3 QDs. However, this also increases the fluorescence intermittency. Achieving high SP purity and blinking mitigation simultaneously remains a significant challenge. Here, we transcend this limitation with room-temperature synthesized weakly confined hybrid organic-inorganic perovskite (HOIP) QDs. Superior single photon purity with a low g(2)(0) < 0.07 ± 0.03 and a nearly blinking-free behavior (ON-state fraction >95%) in 11 nm FAPbBr3 QDs are achieved at room temperature, attributed to their long exciton lifetimes (τX) and short biexciton lifetimes (τXX). The significance of the organic A-cation is further validated using the mixed-cation FAxCs1-xPbBr3. Theoretical calculations utilizing a combination of the Bethe-Salpeter (BSE) and k·p approaches point toward the modulation of the dielectric constants by the organic cations. Importantly, our findings provide valuable insights into an additional lever for engineering facile-synthesized room-temperature PQD single photon sources.

Influence of Ionic Additives in the PEDOT:PSS Hole Transport Layers for Efficient Blue Perovskite Light Emitting Diodes.

Ruddlesden-Popper (RP) perovskites have been gaining traction in the development of high-efficiency or blue-emitting perovskite light emitting diodes (PeLEDs) due to the unique energy funneling mechanism, which enhances photoluminescence intensity, and dimensional control, which enables spectral tuning. In a conventional p-i-n device structure, the quality of RP perovskite films, including grain morphology and defects, as well as device performance can be significantly influenced by the underlying hole-transport layer (HTL). Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is commonly used in several PeLEDs as an HTL because of its high electrical conductivity and optical transparency. Nonetheless, the energy level mismatch and exciton quenching caused by PEDOT:PSS often compromises PeLED performance. Herein, we investigate the mitigation of these effects through addition of work-function-tunable PSS Na to the PEDOT:PSS HTL and assess the impact on blue PeLED performance. Surface analysis of the modified PEDOT:PSS HTLs reveals a PSS-rich layer that alleviates exciton quenching at the HTL/perovskite interface. At an optimal concentration of 6% PSS Na addition, an improvement in the external quantum efficiency is observed, with champion blue and sky-blue PeLEDs achieving 4% (480 nm) and 6.36% (496 nm), respectively, while operation stability is prolonged by fourfold.

Molecular Locking with All-Organic Surface Modifiers Enables Stable and Efficient Slot-Die-Coated Methyl-Ammonium-Free Perovskite Solar Modules.

The power conversion efficiency (PCE) of the state-of-the-art large-area slot-die-coated perovskite solar cells (PSCs) is now over 19%, but issues with their stability persist owing to significant intrinsic point defects and a mass of surface imperfections introduced during the fabrication process. Herein, the utilization of a hydrophobic all-organic salt is reported to modify the top surface of large-area slot-die-coated methylammonium (MA)-free halide perovskite layers. Bearing two molecules, each of which is endowed with anchoring groups capable of exhibiting secondary interactions with the perovskite surfaces, the organic salt acts as a molecular lock by effectively binding to both anion and cation vacancies, substantially enhancing the materials' intrinsic stability against different stimuli. It not only reduces the ingression of external species such as oxygen and moisture, but also suppresses the egress of volatile organic components during the thermal stability testing. The treated PSCs demonstrate efficiency of 19.28% (active area of 58.5 cm2 ) and 17.62% (aperture area of 64 cm2 ) for the corresponding mini-module. More importantly, unencapsulated slot-die-coated mini-modules incorporating the all-organic surface modifier show ≈80% efficiency retention after 7500 h (313 days) of storage under 30% relative humidity (RH). They also remarkably retain more than 90% of the initial efficiency for over 850 h while being measured continuously.

Inorganic frameworks of low-dimensional perovskites dictate the performance and stability of mixed-dimensional perovskite solar cells.

Mixed-dimensional perovskites containing mixtures of organic cations hold great promise to deliver highly stable and efficient solar cells. However, although a plethora of relatively bulky organic cations have been reported for such purposes, a fundamental understanding of the materials' structure, composition, and phase, along with their correlated effects on the corresponding optoelectronic properties and degradation mechanism remains elusive. Herein, we systematically engineer the structures of bulky organic cations to template low-dimensional perovskites with contrasting inorganic framework dimensionality, connectivity, and coordination deformation. By combining X-ray single-crystal structural analysis with depth-profiling XPS, solid-state NMR, and femtosecond transient absorption, it is revealed that not all low-dimensional species work equally well as dopants. Instead, it was found that inorganic architectures with lesser structural distortion tend to yield less disordered energetic and defect landscapes in the resulting mixed-dimensional perovskites, augmented in materials with a longer photoluminescence (PL) lifetime, higher PL quantum yield (up to 11%), improved solar cell performance and enhanced thermal stability (T80 up to 1000 h, unencapsulated). Our study highlights the importance of designing templating organic cations that yield low-dimensional materials with much less structural distortion profiles to be used as additives in stable and efficient perovskite solar cells.

Defect Passivation Using a Phosphonic Acid Surface Modifier for Efficient RP Perovskite Blue-Light-Emitting Diodes.

Defect management strategies are vital for enhancing the performance of perovskite-based optoelectronic devices, such as perovskite-based light-emitting diodes (PeLEDs). As additives can fucntion both as acrystallization modifier and/or defect passivator, a thorough study on the roles of additives is essential, especially for blue emissive Pe-LEDs, where the emission is strictly controlled by the n-domain distribution of the Ruddlesden-Popper (RP, L2An-1PbnX3n+1, where L refers to a bulky cation, while A and X are monovalent cation, and halide anion, respectively) perovskite films. Of the various additives that are available, octyl phosphonic acid (OPA) is of immense interest because of its ability to bind with uncoordinated Pb2+ ( notorious for nonradiative recombination) and therefore passivates them. Here, with the help of various spectroscopic techniques, such as X-ray photon-spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and photoluminescence quantum yield (PLQY) measurements, we demonstrate the capability of OPA to bind and passivate unpaired Pb2+ defect sites. Modification to crystallization promoting higher n-domain formation is also observed from steady-state and transient absorption (TA) measurements. With OPA treatment, both the PLQY and EQE of the corresponding PeLED showed improvements up to 53% and 3.7% at peak emission wavelength of 485 nm, respectively.

Reversible Photochromism in ⟨110⟩ Oriented Layered Halide Perovskite.

Extending halide perovskites' optoelectronic properties to stimuli-responsive chromism enables switchable optoelectronics, information display, and smart window applications. Here, we demonstrate a band gap tunability (chromism) via crystal structure transformation from three-dimensional FAPbBr3 to a ⟨110⟩ oriented FAn+2PbnBr3n+2 structure using a mono-halide/cation composition (FA/Pb) tuning. Furthermore, we illustrate reversible photochromism in halide perovskite by modulating the intermediate n phase in the FAn+2PbnBr3n+2 structure, enabling greater control of the optical band gap and luminescence of a ⟨110⟩ oriented mono-halide/cation perovskite. Proton transfer reaction-mass spectroscopy carried out to precisely quantify the decomposition product reveals that the organic solvent in the film is a key contributor to the structural transformation and, therefore, the chromism in the ⟨110⟩ structure. These intermediate n phases (2 ≤ n ≤ ∞) stabilize in metastable states in the FAn+2PbnBr3n+2 system, which is accessible via strain or optical or thermal input. The structure reversibility in the ⟨110⟩ perovskite allowed us to demonstrate a class of photochromic sensors capable of self-adaptation to lighting.

Halide Perovskite Solar Cells for Building Integrated Photovoltaics: Transforming Building Façades into Power Generators.

The rapid emergence of organic-inorganic lead halide perovskites for low-cost and high-efficiency photovoltaics promises to impact new photovoltaic concepts. Their high power conversion efficiencies, ability to coat perovskite layers on glass via various scalable deposition techniques, excellent optoelectronic properties, and synthetic versatility for modulating transparency and color allow perovskite solar cells (PSCs) to be an ideal solution for building-integrated photovoltaics (BIPVs), which transforms windows or façades into electric power generators. In this review, the unique features and properties of PSCs for BIPV application are accessed. Device engineering and optical management strategies of active layers, interlayers, and electrodes for semitransparent, bifacial, and colorful PSCs are also discussed. The performance of PSCs under conditions that are relevant for BIPV such as different operational temperature, light intensity, and light incident angle are also reviewed. Recent outdoor stability testing of PSCs in different countries and other demonstration of scalability and deployment of PSCs are also spotlighted. Finally, the current challenges and future opportunities for realizing perovskite-based BIPV are discussed.

One-Pot Synthesis and Structural Evolution of Colloidal Cesium Lead Halide-Lead Sulfide Heterostructure Nanocrystals for Optoelectronic Applications.

Heterostructures, combining perovskite nanocrystals (PNC) and chalcogenide quantum dots, could pave a path to optoelectronic device applications by enabling absorption in the near-infrared region, tailorable electronic properties, and stable crystal structures. Ideally, the heterostructure host material requires a similar lattice constant as the guest which is also constrained by the synthesis protocol and materials selectivity. Herein, we present an efficient one-pot hot-injection method to synthesize colloidal all-inorganic cesium lead halide-lead sulfide (CsPbX3 (X = Cl, Br, I)-PbS) heterostructure nanocrystals (HNCs) via the epitaxial growth of the perovskite onto the presynthesized PbS nanocrystals (NCs). Optical and structural characterization evidenced the formation of heterostructures. The embedding of PbS NCs into CsPbX3 perovskite allows the tuning of the absorption and emission from 400 to 1100 nm by tuning the size and composition of perovskite HNCs. The CsPbI3-PbS HNCs show enhanced stability in ambient conditions. The stability, tunable optical properties, and variable band alignments accessible in this system would have implications in the design of novel optoelectronic applications such as light-emitting diodes, photodetectors, photocatalysis, and photovoltaics.

Vacuum-Processed Metal Halide Perovskite Light-Emitting Diodes: Prospects and Challenges.

In less than a decade, organic-inorganic metal halide perovskites (MHPs) have shown tremendous progress in the field of light-emitting applications. Perovskite light-emitting diodes (PeLEDs) have reached external quantum efficiencies (EQE) exceeding 20 % and they have been recognized as a potential contender of the commercial display technologies. However, perovskite thin films in PeLEDs are generally deposited via a spin-coating process, which is not favourable for large area device fabrication. Despite the great success of solution-processed PeLEDs, very few articles have been reported on vacuum processed PeLEDs and the improvements in their optoelctronic performances are also progressing slowly. On the other hand, vacuum processing techniques are mostly used in organic LED technology as they can guarantee (i) the absence of solvent during thin-film growth, (ii) process scalability over large area substrates, and (iii) precise thin-film thickness control. This thin-film growth process is suitable for application in the advancement of a large variety of display technologies. In this Review, we present an overview of current research advances in the field of perovskite thin films grown via vacuum techniques, a study of their photophysical properties, and integration in PeLEDs for the generation of different colors. We also highlight the current challenges and future prospects for the further development of vacuum processed PeLEDs.

Colorful Perovskite Solar Cells: Progress, Strategies, and Potentials.

In the past few years, a large variety of perovskite solar cells (PSCs) with vivid and well-distinguished color hues have been demonstrated. In this Perspective, we compare different strategies employed to realize colorful PSCs both in opaque and semitransparent designs. The approaches used to modulate the PSCs' colorful appearance can be divided into two main categories: the first one based on the modifications of their internal layers (i.e., absorber, electron- and/or hole-transporting layers, and electrodes), while the second is based on the addition of external colored or nanostructured films to the standard PSCs. The advantages and bottlenecks of each strategy are discussed in terms of PSCs' color tunability, transparency, photovoltaic performances, fabrication processes feasibility, and scalability, in view of suitable applications in an urban context for building-integrated photovoltaics.

Effects of All-Organic Interlayer Surface Modifiers on the Efficiency and Stability of Perovskite Solar Cells.

Surface imperfections created during fabrication of halide perovskite (HP) films could induce formation of various defect sites that affect device performance and stability. In this work, all-organic surface modifiers consisting of alkylammonium cations and alkanoate anions are introduced on top of the HP layer to passivate interfacial vacancies and improve moisture tolerance. Passivation using alkylammonium alkanoate does not induce formation of low-dimensional perovskites species. Instead, the organic species only passivate the perovskite's surface and grain boundaries, which results in enhanced hydrophobic character of the HP films. In terms of photovoltaic application, passivation with alkylammonium alkanoate allows significant reduction in recombination losses and enhancement of open-circuit voltage. Alongside unchanged short-circuit current density, power conversion efficiencies of more than 18.5 % can be obtained from the treated sample. Furthermore, the unencapsulated device retains 85 % of its initial PCE upon treatment, whereas the standard 3D perovskite device loses 50 % of its original PCE when both are subjected to ambient environment over 1500 h.

Room temperature synthesis of low-dimensional rubidium copper halide colloidal nanocrystals with near unity photoluminescence quantum yield.

Metal lead halide perovskite nanocrystals have emerged as promising candidates for optoelectronic applications. However, the inclusion of toxic lead is a major concern for the commercial viability of these materials. Herein, we introduce a new family of non-toxic reduced dimension Rb2CuX3 (X = Br, Cl) colloidal nanocrystals with one-dimensional crystal structure consisting [CuX4]3- ribbons isolated by Rb+ cations. These nanocrystals were synthesised using a room-temperature method under ambient conditions, which makes them cost effective and scalable. Phase purity quantification was confirmed by Rietveld refinement of powder X-ray diffraction and corroborated by 87Rb MAS NMR technique. Both samples also exhibited high thermal stability up to 500 °C, which is essential for optoelectronic applications. Rb2CuBr3 and Rb2CuCl3 display PL emission peaks at 387 nm and 400 nm with high PLQYs of ∼100% and ∼49%, respectively. Lastly, the first colloidal synthesis of quantum-confined rubidium copper halide-based nanocrystals opens up a new avenue to exploit their optical properties in lighting technology as well as water sterilisation and air purification.

Realizing Reduced Imperfections via Quantum Dots Interdiffusion in High Efficiency Perovskite Solar Cells.

Realization of reduced ionic (cationic and anionic) defects at the surface and grain boundaries (GBs) of perovskite films is vital to boost the power conversion efficiency of organic-inorganic halide perovskite (OIHP) solar cells. Although numerous strategies have been developed, effective passivation still remains a great challenge due to the complexity and diversity of these defects. Herein, a solid-state interdiffusion process using multi-cation hybrid halide perovskite quantum dots (QDs) is introduced as a strategy to heal the ionic defects at the surface and GBs. It is found that the solid-state interdiffusion process leads to a reduction in OIHP shallow defects. In addition, Cs+ distribution in QDs greatly influences the effectiveness of ionic defect passivation with significant enhancement to all photovoltaic performance characteristics observed on treating the solar cells with Cs0.05 (MA0.17 FA0.83 )0.95 PbBr3 (abbreviated as QDs-Cs5). This enables power conversion efficiency (PCE) exceeding 21% to be achieved with more than 90% of its initial PCE retained on exposure to continuous illumination of more than 550 h.

Publisher Correction: Mixed-Dimensional Naphthylmethylammonium-Methylammonium Lead Iodide Perovskites with Improved Thermal Stability.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Hot Carriers in Halide Perovskites: How Hot Truly?

Slow hot carrier cooling in halide perovskites holds the key to the development of hot carrier (HC) perovskite solar cells. For accurate modeling and pragmatic design of HC materials and devices, it is essential that HC temperatures are reliably determined. A common approach involves fitting the high-energy tail of the main photobleaching peak in a transient absorption spectrum with a Maxwell-Boltzmann distribution. However, this approach is problematic because of complications from the overlap of several photophysical phenomena and a lack of consensus in the community on the fitting procedures. Herein, we propose a simple approach that circumvents these challenges. Through tracking the broadband spectral evolution and accounting for bandgap renormalization and spectral line width broadening effects, our method extracts not only accurate and consistent carrier temperatures but also other important parameters such as the quasi-Fermi levels, bandgap renormalization constant, etc. Establishing a reliable method for the carrier temperature determination is a step forward in the study of HCs for next-generation perovskite optoelectronics.

Mixed-Dimensional Naphthylmethylammoinium-Methylammonium Lead Iodide Perovskites with Improved Thermal Stability.

Metal halide perovskite solar cells, despite achieving high power conversion efficiency (PCE), need to demonstrate high stability prior to be considered for industrialization. Prolonged exposure to heat, light, and moisture is known to deteriorate the perovskite material owing to the breakdown of the crystal structure into its non-photoactive components. In this study, we show that by combining the organic ligand 1-naphthylmethylammoinium iodide (NMAI) with methylammonium (MA) to form a mixed dimensional (NMA)2(MA)n-1PbnI3n+1 perovskite the optical, crystallographic and morphological properties of the newly formed mixed dimensional perovskite films under thermal ageing can be retained. Indeed, under thermal ageing at 85 °C, the best performing (NMA)2(MA)n-1PbnI3n+1 perovskites films show a stable morphology, a low PbI2 formation rate and a significantly reduced variation of both MA-specific vibrational modes and fluorescence lifetimes as compared to the pristine MAPbI3 films. These results highlight the role of the bulky NMA+ organic cation in mixed dimensional perovskites to both inhibit the MA+ diffusion and reduce the material defects, which act as non-radiative recombination centres. As a result, the thermal stability of metal halide perovskites has been substantially improved.

Cubic NaSbS2 as an Ionic-Electronic Coupled Semiconductor for Switchable Photovoltaic and Neuromorphic Device Applications.

The recent emergence of lead halide perovskites as ionic-electronic coupled semiconductors motivates the investigation of alternative solution-processable materials with similar modulatable ionic and electronic transport properties. Here, a novel semiconductor-cubic NaSbS2 -for ionic-electronic coupled transport is investigated through a combined theoretical and experimental approach. The material exhibits mixed ionic-electronic conductivity in inert atmosphere and superionic conductivity in humid air. It is shown that post deposition electronic reconfigurability in this material enabled by an electric field induces ionic segregation enabling a switchable photovoltaic effect. Utilizing post-perturbation of the ionic composition of the material via electrical biasing and persistent photoconductivity, multistate memristive synapses with higher-order weight modulations are realized for neuromorphic computing, opening up novel applications with such ionic-electronic coupled materials.