Mask-Free Patterned Perovskite Microcavity Arrays via Inkjet Printing Targeting Laser Emission.

Perovskite materials are promising candidates for the implementation of electrically pumped lasers considering the enhanced performance of perovskite-based light-emitting diodes. Nonetheless, current methods of fabricating perovskite optical microcavities require complex patterning technologies to build suitable resonant cavities for perovskite laser emission, burdening the device structure design. To address this issue, we applied inkjet printing, a maskless patterning technique, to directly create spontaneous formations of polycrystalline perovskite microcavity arrays to explore their laser-emitting action. The substrate surface tension was tuned to modulate the perovskite crystallization process in combination with optimization of printing ink recipes. As a result, polycrystalline perovskite microcavity arrays were achieved, contributing to the laser emission at 528 nm with a lasing threshold of 1.37 mJ/cm2, while simultaneously achieving high-definition patterning of flexible display. These results clearly illustrate the efficiency of inkjet printing technology in the preparation of polycrystalline perovskite optical microcavities and promote the development of flexible laser arrayed displays, providing a facile process toward the realization of perovskite-cavity laser devices.

Unraveling Deposition Atmosphere Impact on Reproducibility of Perovskite Light-Emitting Diodes.

Manipulating the crystallization dynamics of perovskite emitters is an effective strategy for preparing high-performance perovskite light-emitting diodes (PeLEDs). In general, amorphous-like thermodynamically stable intermediates are desirable for a retarded and controllable crystallization process of perovskite emitters. Despite a variety of well-demonstrated strategies for crystallization control, it has been generally realized that perovskite thin-film emitters show problematic reproducibility. Here, we unraveled that the coordinating solvent vapor residues could raise deleterious influences on the formation of amorphous intermediate phases, which thus leads to varying crystal qualities from batch to batch. We demonstrated that undesirable crystalline intermediate phases tend to form with a strong coordination solvent vapor atmosphere, which alters the crystallization process and thus brings about additional ionic defects. By applying an inert gas flush strategy, this detrimental effect could be effectively mitigated, enabling PeLEDs with high reproducibility. This work provides new insight into the fabrication of efficient and reproducible perovskite optoelectronics.

Protocol for efficient and self-healing near-infrared perovskite light-emitting diodes.

Preparation of highly efficient and stable perovskite light-emitting diodes (PeLEDs) with reproducible device performance is challenging. This protocol describes steps for fabrication of high-performance and self-healing PeLEDs. These include instructions for synthesis of charge-transporting zinc oxide (ZnO) nanocrystals, step-by-step device fabrication, and control over self-healing of the degraded devices. For complete details on the use and execution of this protocol, please refer to Teng et al. (2021).

A Hygroscopic Janus Heterojunction for Continuous Moisture-Triggered Electricity Generators.

Moisture-triggered electricity generator (MEG) harvesting energy from the ubiquity of atmospheric moisture is one of the promising potential candidates for renewable power demand. However, MEG device performance is strongly dependent on the moisture concentration, which results in its large fluctuation of the electrical output. Here, a Janus heterojunction MEG device consisting of nanostructured silicon and hygroscopic polyelectrolyte incorporating hydrophilic carbon nanotube mesh is proposed to enable ambient moisture harvesting and continuous stable electrical output delivery. The nanostructured silicon with a large surface/volume ratio provides strong coupling interaction with water molecules for charge generation. A polyelectrolyte of polydiallyl dimethylammonium chloride (PDDA) can facilitate charge selective transporting and enhance the effectiveness of moisture-absorbing in an arid environment simultaneously. The conductive, porous, and hydrophilic carbon nanotube mesh allows water to be ripped through as well as the generated charges being collected timely. As such, any generated charge carriers in the Janus heterojunction can be efficiently swept toward their respective electrodes, because of the device asymmetric contact. A MEG device continuously delivers an open-circuit voltage of 1.0 V, short-circuit current density of 8.2 µA/cm2, and output power density of 2.2 µW/cm2 under an ambient environment (60% relative humidity, 25 °C), which is a record value over the previously reported values. Furthermore, the infrared thermal measurements also reveal that the moisture-triggered electricity generation power is likely ascribed to surrounding thermal energy collected by the MEG device. Our results provide an insightful rationale for the design of device structure and understanding of the working mechanism of MEG, which is of great importance to promote the efficient electricity conversion induced by moisture in the atmosphere.

Revealing a Zinc Oxide/Perovskite Luminescence Quenching Mechanism Targeting Low-Roll-off Light-Emitting Diodes.

Balanced charge injection is key to achieving perovskite light-emitting diodes (PeLEDs) with a low efficiency roll-off at a high brightness. The use of zinc oxide (ZnO) with a high electron mobility as the charge transport layers is desirable; however, photoluminescence (PL) quenching of a perovskite on ZnO always occurs. Here, a quasi-two-dimensional perovskite on ZnO is explored to uncover the PL quenching mechanism, mainly ascribed to the deprotonation of ammonium cations on the ZnO film in association with the decomposition of low-dimensional perovskite phases. Surprisingly, crystal plane-dependent PL quenching results indicate that the deprotonation rate strongly correlates with the crystal orientation of the ZnO surface. We developed a strategy for suppressing perovskite PL quenching by incorporating an atomic layer deposited Al2O3 onto the ZnO film. Consequently, an efficient inverted PeLED was achieved with a maximum external quantum efficiency of 17.7% and a less discernible efficiency roll-off at a high current density.

Unveiling the critical role of ammonium bromide in blue emissive perovskite films.

Implementation of ammonium halides to trigger low-dimensional perovskite formation has been intensively investigated to achieve blue perovskite light-emitting diodes (PeLEDs). However, the general roles of the incorporated ammonium cations on the quality of the perovskite films, as well as device performance, are still unclear. It is indispensable to build a guideline to rationalize ammonium halides for decent blue emissive films. Here, by thoroughly investigating a series of ammonium cations containing the different number of ammonium groups and ionic radius, we reveal that the mechanism beyond the tunable emission wavelength, crystallization kinetics, and spectral stability of the obtained blue perovskite films is highly relevant to the molecular structure of the ammonium cations. In parallel with reducing the dimensionality to form normal Ruddlesden-Popper phases, the incorporated ammonium cations also likely modulate the Pb-Br orbit coupling through A-site engineering and generate either Dion-Jacobson or "hollow" perovskites, providing alternative routes to achieve efficient and stable blue emissive films. Our work paves a way to rationalize ammonium halides to develop prevailing active layers for further improving the performance of blue PeLEDs.

Manipulating crystallization dynamics through chelating molecules for bright perovskite emitters.

Molecular additives are widely utilized to minimize non-radiative recombination in metal halide perovskite emitters due to their passivation effects from chemical bonds with ionic defects. However, a general and puzzling observation that can hardly be rationalized by passivation alone is that most of the molecular additives enabling high-efficiency perovskite light-emitting diodes (PeLEDs) are chelating (multidentate) molecules, while their respective monodentate counterparts receive limited attention. Here, we reveal the largely ignored yet critical role of the chelate effect on governing crystallization dynamics of perovskite emitters and mitigating trap-mediated non-radiative losses. Specifically, we discover that the chelate effect enhances lead-additive coordination affinity, enabling the formation of thermodynamically stable intermediate phases and inhibiting halide coordination-driven perovskite nucleation. The retarded perovskite nucleation and crystal growth are key to high crystal quality and thus efficient electroluminescence. Our work elucidates the full effects of molecular additives on PeLEDs by uncovering the chelate effect as an important feature within perovskite crystallization. As such, we open new prospects for the rationalized screening of highly effective molecular additives.

Unveiling Crystal Orientation in Quasi-2D Perovskite Films by In Situ GIWAXS for High-Performance Photovoltaics.

Quasi-2D perovskites are enchanting alternative materials for solar cells due to their intrinsic stability. The manipulation of crystal orientation of quasi-2D perovskites is indispensable to target efficient devices, however, the origin of orientation during the film fabrication process still lacks in-depth understanding and convincing evidence yet, which hinders further boosting the performance of photovoltaic devices. Herein, the crystallizing processes during spin-coating and annealing are probed by in situ grazing-incidence wide-angle X-ray scattering (GIWAXS), and the incident-angle-dependent GIWAXS is conducted to unveil the phase distribution in the films. It is found that undesirable lead iodide sol-gel formed intermediate phase would disturb oriented crystalline growth, resulting in random crystal orientation in poor quasi-2D films. A general strategy is developed via simple additive agent incorporation to suppress the formation of the intermediate phase. Accordingly, highly oriented perovskite films with reduced trap density and higher carrier mobility are obtained, which enables the demonstration of optimized quasi-2D perovskite solar cells with a power conversion efficiency of 15.2% as well as improved stability. This work paves a promising way to manipulate the quasi-2D perovskites nucleation and crystallization processes via tuning nucleation stage.

Efficient and Bright Pure-Blue All-Inorganic Perovskite Light-Emitting Diodes from an Ecofriendly Alloy.

Metal halide perovskite light-emitting diodes (PeLEDs) have been regarded as alternative candidates for full-color display applications with rapid progress to surge the external quantum efficiencies (EQEs) over 20%. However, in contrast to the high efficiencies of green, red, and near-infrared PeLEDs, the performance of their blue cousins is still lagging behind, especially the pure-blue one. Obtaining blue perovskite films with negligible nonradiative recombination loss and high stability is of great importance to realize efficient and spectrally stable blue PeLEDs. In this work, through partially replacing the toxic lead ions (Pb2+) with ecofriendly strontium ions (Sr2+) to tune the emission wavelength along with using passivation strategies, all-inorganic pure-blue perovskite films with a high photoluminescence quantum yield of 60.7% were achieved, which then delivered PeLEDs with a luminance of 510 cd m-2 and an EQE of 1.43%. The device yields a record radiance among the most efficient PeLEDs at 467 nm. In addition, the resultant PeLEDs displayed exceptional spectral stability during the electrical bias operation. Our work provides a promising avenue to develop environmentally friendly perovskite materials for efficient and spectrally stable pure-blue PeLEDs and beyond.

High-Efficiency Perovskite Light-Emitting Diodes with Improved Interfacial Contact.

Unbalanced charge injection is one of the major issues that hampers the efficiency of perovskite light-emitting diodes (PeLEDs). Through engineering the device structure with multiple hole transport layers (HTLs), i.e., poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)diphenylamine) (TFB)/poly(9-vinylcarbazole) (PVK) and nickel oxide (NiOx)/TFB/PVK, efficient PeLED devices have been successfully demonstrated. However, in a typical solution-processed PeLED with multiple HTLs, the underlying conjugated HTL could be easily redissolved by the ink of the following one, which not only dramatically deteriorates the electrical property of HTLs but also influences the quality of the top perovskite films. In this work, through inserting a thin atomic layer-deposited aluminum oxide (Al2O3) layer between HTLs and the perovskite layer, an improved interfacial contact can be achieved, which enables us to obtain perovskite films with enhanced characteristics and balanced charge injection in the resultant PeLEDs. In addition, because of the proper refractive index (r), the presence of the Al2O3 layer also favors the light out-coupling of PeLEDs. As a result, we fabricate green PeLEDs with good repeatability and external quantum efficiency of 17.0%, which is approximately 60% higher than that of the control device without Al2O3. Our work provides a promising avenue to enhance interfacial contact between the charge transport layer and perovskite for efficient perovskite-based optoelectronic devices.

Dual Functionalization of Electron Transport Layer via Tailoring Molecular Structure for High-Performance Perovskite Light-Emitting Diodes.

Great progress in modification and optimization of emission layer (EML) in perovskite light-emitting diodes (PeLEDs) results in a significant improvement in device efficiency. However, so far, less attention has been paid to the exploration of hole/electron injection and transporting layers to maximize the utilization of charge carriers for efficient and stable PeLEDs. At present, low electron mobility of electron transport layer (ETL) causes an unbalanced charge injection, and the defects at the ETL/perovskite interface limit the formation and utilization of generated excitons. Here, a series of compounds (BPBiTP, BPBiPN, and BPBiPA) flanked by diphenyl-1H-benzo[d]imidazole end groups have been developed as ETL materials, where the bridging units (benzene, naphthalene, anthracene) are manipulated to achieve dual functionality, namely, the high charge carrier mobility and effective passivation of perovskite surface. The coordinating end groups effectively reduce the trap state at the interface of ETL and EML due to their strong nucleophilic quality. H-aggregation of anthracene units and large transfer integral in BPBiPA lead to its superior electron mobility of 8.4 × 10-4 cm2 V-1 s-1 in the solid state, over 1 order of magnitude higher than that of the typical one (TPBi). Consequently, green PeLEDs with a maximum external quantum efficiency (EQE) of 19.7%, reduced efficiency roll-off, as well as extended operational lifetime have been achieved without any outcoupling technique. Our result demonstrated that optimization of ETL materials via improving both passivation capability and electron mobility is a powerful strategy for producing high-performance PeLEDs.

High-Performance Perovskite Light-Emitting Diode with Enhanced Operational Stability Using Lithium Halide Passivation.

Defect passivation has been demonstrated to be effective in improving the radiative recombination of charge carriers in perovskites, and consequently, the device performance of the resultant perovskite light-emitting diodes (LEDs). State-of-the-art useful passivation agents in perovskite LEDs are mostly organic chelating molecules that, however, simultaneously sacrifice the charge-transport properties and thermal stability of the resultant perovskite emissive layers, thereby deteriorating performance, and especially the operational stability of the devices. We demonstrate that lithium halides can efficiently passivate the defects generated by halide vacancies and reduce trap state density, thereby suppressing ion migration in perovskite films. Efficient green perovskite LEDs based on all-inorganic CsPbBr3 perovskite with a peak external quantum efficiency of 16.2 %, as well as a high maximum brightness of 50 270 cd m-2 , are achieved. Moreover, the device shows decent stability even under a brightness of 104  cd m-2 . We highlight the universal applicability of defect passivation using lithium halides, which enabled us to improve the efficiency of blue and red perovskite LEDs.

Author Correction: Solution-processed perovskite light emitting diodes with efficiency exceeding 15% through additive-controlled nanostructure tailoring.

The original version of this Article contained an error in the spelling of the author Dan Credgington, which was incorrectly given as Dan Credington. This has now been corrected in both the PDF and HTML versions of the Article.

Alternative Type Two-Dimensional-Three-Dimensional Lead Halide Perovskite with Inorganic Sodium Ions as a Spacer for High-Performance Light-Emitting Diodes.

Two-dimensional (2D) lead halide perovskites with long-chain ammonium halides display high photoluminescence quantum yields (PLQYs), because of their size and dielectric confinement, which hold promise for a high-efficiency and low-cost light-emitting diode (LED). However, the presence of an insulating organic long-chain spacer cation (L) dramatically deteriorates the charge transport properties along the out-of-plane nanoplatelet direction or adjacent nanocrystals, which would limit the device performance of the LED. To overcome this issue, we successfully incorporate small alkaline ions such as sodium (Na+) to replace the long organic molecule. Grazing incidence X-ray diffraction measurements verify 2D layer formation with a preferred crystallite orientation. In addition, the incorporated sodium salt also generates amorphous sodium lead bromide (NaPbBr3) in perovskite as spacers to form a nanocrystal-like halide perovskite film. The PLQY is dramatically improved in the sodium-incorporated film because of its enhanced photoluminescence lifetime. Upon incorporation of a low concentration of an organic additive, this two-dimensional-three-dimensional (2D-3D) perovskite can achieve a compact and uniform film. Therefore, a 2D-3D perovskite achieves a high external quantum efficiency of 15.9% with good operational stability. We develop a type of 2D-3D halide perovskite with various inorganic ions as spacers for promising high-performance optoelectronic devices.

Nanoplatelet modulation in 2D/3D perovskite targeting efficient light-emitting diodes.

Light-emitting diodes (LEDs) based on two-dimensional (2D) perovskite nanoplatelets exhibit high electroluminescence (EL) efficiency because of the quantum confinement effect, which increases electron-hole recombination to promote radiative emission. It is well-known that a 2D nanoplatelet structure (〈n〉 = 1) is detrimental for luminescence efficiency due to possible thermal quenching of excitons at room temperature. Here, a simple strategy is developed to suppress growth of NMA2PbBr4 (〈n〉 = 1) nanoplatelets by carefully tuning the precursor ratio of cesium bromide (CsBr), formamidinium bromide (FABr) and 1-naphthylmethylammonium bromide (NMABr). The sub-domain size of the perovskite crystal decreases as the long-chain ligand NMABr ratio increases, leading to enhanced photoluminescence quantum yields (PLQY) due to size confinement effect when the NMABr ratio is below 60%. Unfortunately, the NMA2PbBr4 component in 2D/3D perovskites also grows with increasing NMABr ratio, which results in poor EL efficiency. FABr incorporation can provide additional control over suppression of NMA2PbBr4 growth in 2D/3D perovskites. A compact and uniform perovskite film with reduced NMA2PbBr4 content achieves PLQY of ∼61%. Benefiting from these features, a green perovskite LED yields current efficiency of 46.8 cd A-1 with an external quantum efficiency of 14.9%. This study paves a new way to modulate the crystal structure in perovskites via a simple and effective method for high-performance LEDs.

Long-lived polarization memory in the electronic states of lead-halide perovskites from local structural dynamics.

Anharmonic crystal lattice dynamics have been observed in lead halide perovskites on picosecond timescales. Here, we report that the soft nature of the perovskite crystal lattice gives rise to dynamic fluctuations in the electronic properties of excited states. We use linear polarization selective transient absorption spectroscopy to study the charge carrier relaxation dynamics in lead-halide perovskite films and nanocrystals. We find that photo-excited charge carriers maintain an initial polarization anisotropy for several picoseconds, independent of crystallite size and composition, and well beyond the reported timescales of carrier scattering. First-principles calculations find intrinsic anisotropies in the transition dipole moment, which depend on the orientation of light polarization and the polar distortion of the local crystal lattice. Lattice dynamics are imprinted in the optical transitions and anisotropies arise on the time-scales of structural motion. The strong coupling between electronic states and structural dynamics requires a unique interpretation of recombination and transport mechanisms.

Solution-processed perovskite light emitting diodes with efficiency exceeding 15% through additive-controlled nanostructure tailoring.

Organometal halide perovskites (OHP) are promising materials for low-cost, high-efficiency light-emitting diodes. In films with a distribution of two-dimensional OHP nanosheets and small three-dimensional nanocrystals, an energy funnel can be realized that concentrates the excitations in highly efficient radiative recombination centers. However, this energy funnel is likely to contain inefficient pathways as the size distribution of nanocrystals, the phase separation between the OHP and the organic phase. Here, we demonstrate that the OHP crystallite distribution and phase separation can be precisely controlled by adding a molecule that suppresses crystallization of the organic phase. We use these improved material properties to achieve OHP light-emitting diodes with an external quantum efficiency of 15.5%. Our results demonstrate that through the addition of judiciously selected molecular additives, sufficient carrier confinement with first-order recombination characteristics, and efficient suppression of non-radiative recombination can be achieved while retaining efficient charge transport characteristics.

Boosting Perovskite Light-Emitting Diode Performance via Tailoring Interfacial Contact.

Solution-processed perovskite light-emitting diodes (LEDs) have attracted wide attention in the past several years. However, the overall efficiency and stability of perovskite-based LEDs remain inferior to those of organic or quantum dot LEDs. Nonradiative charge recombination and the unbalanced charge injection are two critical factors that limit the device efficiency and operational stability of perovskite LEDs. Here, we develop a strategy to modify the interface between the hole transport layer and the perovskite emissive layer with an amphiphilic conjugated polymer of poly[(9,9-bis(3'-( N, N-dimethylamino)propyl)-2,7-fluorene)- alt-2,7-(9,9-dioctylfluorene)] (PFN). We show evidences that PFN improves the quality of the perovskite film, which effectively suppresses nonradiative recombination. By further improving the charge injection balance rate, a green perovskite LED with a champion current efficiency of 45.2 cd/A, corresponding to an external quantum efficiency of 14.4%, is achieved. In addition, the device based on the PFN layer exhibits improved operational lifetime. Our work paves a facile way for the development of efficient and stable perovskite LEDs.

Highly Luminescent and Stable Perovskite Nanocrystals with Octylphosphonic Acid as a Ligand for Efficient Light-Emitting Diodes.

All inorganic perovskite nanocrystals (NCs) of CsPbX3 (X = Cl, Br, I, or their mixture) are regarded as promising candidates for high-performance light-emitting diode (LED) owing to their high photoluminescence (PL) quantum yield (QY) and easy synthetic process. However, CsPbX3 NCs synthesized by the existing methods, where oleic acid (OA) and oleylamine (OLA) are generally used as surface-chelating ligands, suffer from poor stability due to the ligand loss, which drastically deteriorates their PL QY, as well as dispersibility in solvents. Herein, the OA/OLA ligands are replaced with octylphosphonic acid (OPA), which dramatically enhances the CsPbX3 stability. Owing to a strong interaction between OPA and lead atoms, the OPA-capped CsPbX3 (OPA-CsPbX3) NCs not only preserve their high PL QY (>90%) but also achieve a high-quality dispersion in solvents after multiple purification processes. Moreover, the organic residue in purified OPA-CsPbBr3 is only ∼4.6%, which is much lower than ∼29.7% in OA/OLA-CsPbBr3. Thereby, a uniform and compact OPA-CsPbBr3 film is obtained for LED application. A green LED with a current efficiency of 18.13 cd A-1, corresponding to an external quantum efficiency of 6.5%, is obtained. Our research provides a path to prepare high-quality perovskite NCs for high-performance optoelectronic devices.

Crosslinked conjugated polymers as hole transport layers in high-performance quantum dot light-emitting diodes.

Film morphologies of functional layers in all-solution-processed quantum dot light-emitting diodes (QLEDs) play a crucial role in device performance. Solvents for adjacent layers should be strictly orthogonal to prevent the preceding layer being redissolved by the processing solvent of the next layer. Herein, we use a photochemical crosslinking method to obtain solvent-resistant hole transport layers (HTLs) with photoinitiator bifunctional bis-benzophenone (BP-BP). With this method, ultra-smooth quantum dot (QD) layers can be fabricated using toluene as solvent, which is known to be a nonorthogonal solvent in common non-crosslinked HTLs. A green QLED device based on crosslinked HTLs exhibits a high external quantum efficiency of 8.93%, which is 1.9-fold higher than that of the non-crosslinked device. The improved device performance is ascribed to the well preserved film morphology of crosslinked HTLs and the prevention of QDs intermixing with HTLs during the QD deposition in toluene. This crosslinking strategy avoids high-temperature annealing, allowing the fabrication of flexible devices on plastic substrates. Moreover, it broadens the range of applicable solvents for solution-processed multilayer optoelectronic devices because non-orthogonal solvents can be used after crosslinking preceding layers.