Micropatterned 2D Hybrid Perovskite Thin Films with Enhanced Photoluminescence Lifetimes

Mimicking nature to develop halide perovskite semiconductors from proteins and metal carbonates

Halide perovskite (HPs) nanostructures have recently gained extensive worldwide attentions because of their remarkable optoelectronic properties and fast developments. However, intrinsic instability against environmental factors-i.e., temperature, humidity, illumination, and oxygen-restricted their real-life applications. HPs are typically synthesized as colloids by employing organic solvents and ligands. Consequently, the precise control and tuning of complex 3D perovskite morphologies are challenging and have hardly been achieved by conventional fabrication methods. Here, we combine the benefits of self-assembly of biomolecules and an ion exchange reaction (IER) approach to customize HPs spatial shapes and composition. Initially, we apply a biomineralization approach, using biological templates (such as biopolymers, proteins, or protein assemblies), modulating the morphology of MCO3 (M = Ca2+, Ba2+) nano/microstructures. We then show that the morphology of the materials can be maintained throughout an IER process to form surface HPs with a wide variety of morphologies. The fabricated core-shell structures of metal carbonates and HPs introduce nano/microcomposites that can be sculpted into a wide diversity of 3D architectures suitable for various potential applications such as sensors, detectors, catalysis, etc. As a prototype, we fabricate disposable humidity sensors with an 11-95% detection range by casting the formed bio-templated nano/micro-composites on paper substrate.

Template-Assisted Synthesis of 2D Perovskite Grating Single Crystal Films at Low Temperatures for UV Polarization-Sensitive Photodetectors

2D perovskites have attracted tremendous attention due to their superior optoelectronic properties and potential applications in optoelectronic devices. Especially, the larger bandgap of 2D perovskite means that they are suitable for UV photodetection. However, the layered structure of 2D perovskites hinders the interlayer carrier transport, which limits the improvement of device performance. Therefore, nanoscale structures are normally used to enhance the light absorption ability, which is an effective strategy to improve the photocurrent in 2D perovskite-based photodetectors. Herein, a template-assisted low-temperature method is proposed to fabricate 2D perovskite ((C6 H5 C2 H4 NH3 )2 PbBr4 , (PEA)2 PbBr4 ) grating single crystal films (GSCFs). The crystallinity of the (PEA)2 PbBr4 GSCFs is significantly improved due to the slow evaporation of the precursor solution under low temperatures. Based on this high crystalline quality and extremely ordered microstructures, the metal-semiconductor-metal photodetectors are assembled. Finite-different time-domain (FDTD) simulation and experiment indicate that the GSCF-based photodetectors exhibit significantly improved performance in comparison with the plane devices. The optimized 2D perovskite photodetectors are sensitive to UV light and demonstrate a responsivity and detectivity of 28.6 mA W-1 and 2.4 × 1011 Jones, respectively. Interestingly, the photocurrent of this photodetector varies as the angle of the incident polarized light, resulting in a high polarization ratio of 1.12.

Sub-30 nm 2D Perovskites Patterns via Block Copolymer Guided Self-Assembly for Color Conversion Optical Polarizer

Despite the remarkable advances made in the development of 2D perovskites suitable for various high-performance devices, the development of sub-30 nm nanopatterns of 2D perovskites with anisotropic photoelectronic properties remains challenging. Herein, a simple but robust route for fabricating sub-30 nm 1D nanopatterns of 2D perovskites over a large area is presented. This method is based on nanoimprinting a thin precursor film of a 2D perovskite with a topographically pre-patterned hard poly(dimethylsiloxane) mold replicated from a block copolymer nanopattern consisting of guided self-assembled monolayered in-plane cylinders. 1D nanopatterns of various 2D perovskites (A'2 MAn -1 Pbn X3 n +1 ,A' = BA, PEA, X = Br, I) are developed; their enhanced photoluminescence (PL) quantum yields are approximately four times greater than those of the corresponding control flat films. Anisotropic photocurrent is observed because 2D perovskite nanocrystals are embedded in a topological 1D nanopattern. Furthermore, this 1D metal-coated nanopattern of a 2D perovskite is employed as a color conversion optical polarizer, in which polarized PL is developed. This is due to its capability of polarization of an incident light arising from the sub-30 nm line pattern, as well as the PL of the confined 2D perovskite nanocrystals in the pattern.

Enhanced optical absorption in two-dimensional Ruddlesden-Popper (C6H5CH2NH3)2PbI4 perovskites via biaxial strain and surface doping

Two-dimensional Ruddlesden-Popper (2D RP) perovskites can form layered protective materials using long organic cations as "barrier" caps, which is expected to solve the problem of instability of perovskites in the working environment. In this work, we systematically studied the 2D Ruddlesden-Popper (C6H5CH2NH3)2PbI4 hybrid perovskites using density functional theory. The results reveal that the 2D (C6H5CH2NH3)2PbI4 perovskites are semiconductors with band gaps of 2.22 eV. The optical absorption peak of the 2D (C6H5CH2NH3)2PbI4 perovskite structure is located at 532 nm in the visible region. Interestingly, the optical absorption spectrum of the 2D (C6H5CH2NH3)2PbI4 perovskite structure enhanced under suitable strains. The highest optical absorption peak appears in 2D (C6H5CH2NH3)2PbI4 under a -2% strain, and its theoretical photoelectric conversion efficiency is 28.5%. More interestingly, the replacement of surface I atoms with Br is another ways to enhance the optical absorption spectrum of the 2D (C6H5CH2NH3)2PbI4 perovskite structure. The optical absorption peak blue-shifts to the high energy region, which has higher solar energy flux density than the low energy region. The good stability, tuneable band gap and excellent theoretical photoelectric conversion efficiency of the 2D (C6H5CH2NH3)2PbI4 perovskite structure make it a promising candidate for novel 2D hybrid perovskite based photoelectronic devices and solar cells.

Patterning of Metal Halide Perovskite Thin Films and Functional Layers for Optoelectronic Applications

In recent years, metal halide perovskites have received significant attention as materials for next-generation optoelectronic devices owing to their excellent optoelectronic properties. The unprecedented rapid evolution in the device performance has been achieved by gaining an advanced understanding of the composition, crystal growth, and defect engineering of perovskites. As device performances approach their theoretical limits, effective optical management becomes essential for achieving higher efficiency. In this review, we discuss the status and perspectives of nano to micron-scale patterning methods for the optical management of perovskite optoelectronic devices. We initially discuss the importance of effective light harvesting and light outcoupling via optical management. Subsequently, the recent progress in various patterning/texturing techniques applied to perovskite optoelectronic devices is summarized by categorizing them into top-down and bottom-up methods. Finally, we discuss the perspectives of advanced patterning/texturing technologies for the development and commercialization of perovskite optoelectronic devices.

Lead-free 2D MASnBr3 and Ruddlesden-Popper BA2MASn2Br7 as light harvesting materials

We have explored the structural, electronic, charge transport, and optical properties of lead-free 2D hybrid halide perovskites, MASnBr₃ and Ruddlesden-Popper perovskites, BAMASn₂Br₇ monolayers. Under density functional theory (DFT) calculation, we applied mechanical strain, i.e., tensile and compressive strain up to 10% in both cases. The mechanical strain engineering technique is useful for a tuned bandgap of 2D MASnBr₃ and 2D BAMASn₂Br₇. The calculated carrier mobility for the electron is 404 cm² V^-1 s^-1 and for the hole is up to 800 cm² V^-1 s^-1 for MASnBr₃. For BAMASn₂Br₇ the highest carrier mobility is up to 557 cm² V^-1 s^-1 for electrons and up to 779 cm² V^-1 s^-1 for the hole, which is 14% and 24% higher than the reported lead-iodide based perovskites, respectively. The calculated solar cell efficiency of 2D MASnBr₃ is 23.46%, which is 18% higher than the reported lead-based perovskites. Furthermore, the optical activity of the 2D MASnBr₃ and 2D BAMASn₂Br₇ shows a high static dielectric constant of 2.48 and 2.14, respectively. This is useful to show nanodevice performance. Also, 2D MASNBr₃ shows a high absorption coefficient of 15.25 × 10⁵ cm^-1 and 2D BAMASn₂Br₇ shows an absorption coefficient of up to 13.38 × 10⁵ cm^-1. Therefore our theoretical results suggest that the systems are under mechanical strain engineering. This is convenient for experimentalists to improve the performance of the 2D perovskites. The study supports these materials as good candidates for photovoltaic and optoelectronic device applications.

Self-Assembly of CsPbBr3 Perovskites in Micropatterned Polymeric Surfaces: Toward Luminescent Materials with Self-Cleaning Properties

In this work, we present a series of porous, honeycomb-patterned polymer films containing CsPbBr₃ perovskite nanocrystals as light emitters prepared by the breath figure approach. Microscopy analysis of the topography and composition of the material evidence that the CsPbBr₃ nanocrystals are homogeneously distributed within the polymer matrix but preferably confined inside the pores due to the fabrication process. The optical properties of the CsPbBr₃ nanocrystals remain unaltered after the film formation, proving that they are stable inside the polystyrene matrix, which protects them from degradation by environmental factors. Moreover, these surfaces present highly hydrophobic behavior due to their high porosity and defined micropatterning, which is in agreement with the Cassie-Baxter model. This is evidenced by performing a proof-of-concept coating on top of 3D-printed LED lenses, conferring the material with self-cleaning properties, while the CsPbBr₃ nanocrystals embedded inside the polymeric matrix maintain their luminescent behavior.

Micro-to-Nanometer Scale Patterning of Perovskite Inks via Controlled Self-Assemblies

In the past decade, perovskite materials have gained intensive interest due to their remarkable material properties in optoelectronics and photodetectors. This review highlights recent advances in micro-to-nanometer scale patterning of perovskite inks, placing an undue emphasis on recently developed approaches to harness spatially ordered and crystallographically oriented structures with unprecedented regularity via controlled self-assemblies, including blade coating, inkjet printing, and nanoimprinting. Patterning of the perovskite elements at the micro- or nanometer scale might be a key parameter for their integration in a real system. Nowadays, unconventional approaches based on irreversible solution evaporation hold an important position in the structuring and integration of perovskite materials. Herein, easier type patterning techniques based on evaporations of polymer solutions and the coffee ring effect are systematically reviewed. The recent progress in the potential applications of the patterned perovskite inks is also introduced.

Structural and compositional properties of 2D CH3NH3PbI3 hybrid halide perovskite: a DFT study

Two-dimensional (2D) hybrid halide perovskites have been scrutinized as candidate materials for solar cells because of their tunable structural and compositional properties. Results based on density functional theory demonstrate its thickness-dependent stability. We have observed that the bandgap decreases from the mono- to quad-layer because of the transformation from 2D towards 3D. Due to the transformation, the carrier mobility is lowered with the corresponding smaller effective mass. On the other hand, the multilayer structures have good optical properties with an absorption coefficient of about 10⁵ cm⁻¹. The calculated absorption spectra lie between 248 nm and 496 nm, leading to optical activity of the 2D multilayer CH₃NH₃PbI₃ systems in the visible and ultraviolet regions. The strength of the optical absorption increases with an increase in thickness. Overall results from this theoretical study suggest that this 2D multilayer CH₃NH₃PbI₃ is a good candidate for photovoltaic and optoelectronic device applications.

We have studied 2D CH₃NH₃PbI₃ and its multilayer halide perovskites. These systems have a high formation energy. The optical properties absorption spectra lie between 248 nm to 496 nm with an absorption coefficient of about 10⁵ cm⁻¹.

Oriented Halide Perovskite Nanostructures and Thin Films for Optoelectronics

Oriented semiconductor nanostructures and thin films exhibit many advantageous properties, such as directional exciton transport, efficient charge transfer and separation, and optical anisotropy, and hence these nanostructures are highly promising for use in optoelectronics and photonics. The controlled growth of these structures can facilitate device integration to improve optoelectronic performance and benefit in-depth fundamental studies of the physical properties of these materials. Halide perovskites have emerged as a new family of promising and cost-effective semiconductor materials for next-generation high-power conversion efficiency photovoltaics and for versatile high-performance optoelectronics, such as light-emitting diodes, lasers, photodetectors, and high-energy radiation imaging and detectors. In this Review, we summarize the advances in the fabrication of halide perovskite nanostructures and thin films with controlled dimensionality and crystallographic orientation, along with their applications and performance characteristics in optoelectronics. We examine the growth methods, mechanisms, and fabrication strategies for several technologically relevant structures, including nanowires, nanoplates, nanostructure arrays, single-crystal thin films, and highly oriented thin films. We highlight and discuss the advantageous photophysical properties and remarkable performance characteristics of oriented nanostructures and thin films for optoelectronics. Finally, we survey the remaining challenges and provide a perspective regarding the opportunities for further progress in this field.

Balanced Charge Carrier Transport Mediated by Quantum Dot Film Post-organization for Light-Emitting Diode Applications

In light-emitting diodes (LEDs), balanced electron and hole transport is of particular importance to achieve high rates of radiative recombination. Most quantum dot (QD)-based LEDs, however, employ infinitesimal core-shell QDs which inherently have different electron and hole mobilities. As QDs are the core building blocks of QD-LEDs, the inherent mobility difference in the core-shell QDs causes significantly unbalanced charge carrier transport, resulting in detrimental effects on performances of QD-LEDs. Herein, we introduce a post-chemical treatment to reconstruct the QD films through the solvent-mediated self-organization process. The treatment using various poly-alkyl alcohol groups enables QD ensembles to transform from disordered solid dispersion into an ordered superlattice and effectively modulate electron and hole mobilities, which leads to the balanced charge carrier transport. In particular, ethanol-treated QD films exhibit enhanced charge carrier lifetime and reduced hysteresis due to the balanced charge carrier transport, which is attributed to the preferential-facet-oriented QD post-organization. As a result, 63, 78, and 54% enhancements in the external quantum efficiency were observed in red, green, and blue QD-LEDs, respectively. These results are of fundamental importance to understand both solvent-mediated QD film reconstruction and the effect of balanced electron and hole transport in QD-LEDs.

Halide Perovskites: A New Era of Solution-Processed Electronics

Organic-inorganic mixed halide perovskites have emerged as an excellent class of materials with a unique combination of optoelectronic properties, suitable for a plethora of applications ranging from solar cells to light-emitting diodes and photoelectrochemical devices. Recent works have showcased hybrid perovskites for electronic applications through improvements in materials design, processing, and device stability. Herein, a comprehensive up-to-date review is presented on hybrid perovskite electronics with a focus on transistors and memories. These applications are supported by the fundamental material properties of hybrid perovskite semiconductors such as tunable bandgap, ambipolar charge transport, reasonable mobility, defect characteristics, and solution processability, which are highlighted first. Then, recent progresses on perovskite-based transistors are reviewed, covering aspects of fabrication process, patterning techniques, contact engineering, 2D versus 3D material selection, and device performance. Furthermore, applications of perovskites in nonvolatile memories and artificial synaptic devices are presented. The ambient instability of hybrid perovskites and the strategies to tackle this bottleneck are also discussed. Finally, an outlook and opportunities to develop perovskite-based electronics as a competitive and feasible technology are highlighted.

Ion Exchange Lithography: Localized Ion Exchange Reactions for Spatial Patterning of Perovskite Semiconductors and Insulators

Patterning materials with different properties in a single film is a fundamental challenge and essential for the development of next-generation (opto)electronic functional components. This work introduces the concept of ion exchange lithography and demonstrates spatially controlled patterning of electrically insulating films and semiconductors with tunable optoelectronic properties. In ion exchange lithography, a reactive nanoparticle "canvas" is locally converted by printing ion exchange "inks." To demonstrate the proof of principle, a canvas of insulating nanoporous lead carbonate is spatioselectively converted into semiconducting lead halide perovskites by contact printing an ion exchange precursor ink of methylammonium and formamidinium halides. By selecting the composition of the ink, the photoluminescence wavelength of the perovskite semiconductors is tunable over the entire visible spectrum. A broad palette of conversion inks can be applied on the reactive film by printing with customizable stamp designs, spray-painting with stencils, and painting with a brush to inscribe well-defined patterns with tunable optoelectronic properties in the same canvas. Moreover, the optoelectronic properties of the converted canvas are exploited to fabricate a green light-emitting diode (LED), demonstrating the functionality potential of ion exchange lithography.

Micro- and Nanopatterning of Halide Perovskites Where Crystal Engineering for Emerging Photoelectronics Meets Integrated Device Array Technology

Tremendous efforts have been devoted to developing thin film halide perovskites (HPs) for use in high-performance photoelectronic devices, including solar cells, displays, and photodetectors. Furthermore, structured HPs with periodic micro- or nanopatterns have recently attracted significant interest due to their potential to not only improve the efficiency of an individual device via the controlled arrangement of HP crystals into a confined geometry, but also to technologically pixelate the device into arrays suitable for future commercialization. However, micro- or nanopatterning of HPs is not usually compatible with conventional photolithography, which is detrimental to ionic HPs and requires special techniques. Herein, a comprehensive overview of the state-of-the-art technologies used to develop micro- and nanometer-scale HP patterns, with an emphasis on their controlled microstructures based on top-down and bottom-up approaches, and their potential for future applications, is provided. Top-down approaches include modified conventional lithographic techniques and soft-lithographic methods, while bottom-up approaches include template-assisted patterning of HPs based on lithographically defined prepatterns and self-assembly. HP patterning is shown here to not only improve device performance, but also to reveal the unprecedented functionality of HPs, leading to new research areas that utilize their novel photophysical properties.

Perovskite Single-Crystal Microwire-Array Photodetectors with Performance Stability beyond 1 Year

Compared with thin-film morphology, 1D perovskite structures such as micro/nanowires with fewer grain boundaries and lower defect density are very suitable for high-performance photodetectors with higher stability. Although the stability of perovskite microwire-based photodetectors has been substantially enhanced in comparison with that of photodetectors based on thin-film morphology, practical applications require further improvements to the stability before implementation. In this study, a template-assisted method is developed to prepare methylammonium lead bromide (MAPbBr₃ ) micro/nanowire structures, which are encapsulated in situ by a protective hydrophobic molecular layer. The combination of the protective layer, high crystalline quality, and highly ordered microstructures significantly improve the stability of the MAPbBr₃ single-crystal microwire arrays. Consequently, these MAPbBr₃ single-crystal microwire-array-based photodetectors exhibit significant long-term stability, maintaining 96% of the initial photocurrent after 1 year without further encapsulation. The lifetime of such photodetectors is hence approximately four times longer than that of the most stable previously reported perovskite micro/nanowire-based photodetector; this is thought to be the most stable perovskite photodetector reported thus far. Furthermore, this work should contribute further toward the realization of perovskite 1D structures with long-term stability.

Ring-Patterned Perovskite Single Crystals Fabricated by the Combination of Rigid and Flexible Templates

Regular microstructures can improve the electrical and optical characteristics of perovskite single crystals because of the removal of defects and grain boundaries. Microstructured single crystals are commonly fabricated by either rigid or flexible templates. However, rigid templates usually need surface treatment before crystal fabrication to create an antiadhesion layer, while flexible templates encounter difficulties in achieving a large area of uniform single crystals without any deformation. In this work, we present a facile and robust method to fabricate perovskite single crystals using rigid silicon pillars coated with flexible polymer solutions, in which surface treatment is avoided in the preparation process, and deformation is absent in the formed crystals. The method realized the fabrication of colorful concentric-ring patterns composed of nanoscale single crystals for the first time. In order to concisely control the preparation of the template, the Newton's ring phenomenon was used to value the droplet height because the number of rings changed with the optical path difference. A related digital simulation was performed to find the correlation of the Newton's ring pattern with the shape of the droplets. The simulated results were consistent with the experimental observations generally, indicating that the pattern could be controlled mechanically. Concomitantly, the resulting perovskite nanoscale single crystals formed a regular colorful concentric-ring pattern. By changing the design of the rigid templates, the parameters of the fabrication process, or the selection of the coating polymer solution, different ring-patterned single crystals were successfully prepared without surface treatment and deformation. The crystals have potential applications in lasers or photodetectors.

Photolithographic Patterning of Perovskite Thin Films for Multicolor Display Applications

Metal halide perovskites are emerging as attractive materials for light-emitting diode (LED) applications. The external quantum efficiency (EQE) has experienced a rapid progress and reached over 21%, comparable to the state of the art organic and quantum dot LEDs. For metal halide perovskites, their simple solution-processing preparation, facile band gap tunability, and narrow emission line width provide another attractive route to harness their superior optoelectronic properties for multicolor display applications. In this work, we demonstrate a high-resolution, large-scale photolithographic method to pattern multicolor perovskite films. This approach is based on a dry lift-off process which involves the use of parylene as an intermediary and the easy mechanical peeling-off of parylene films on various substrates. Using this approach, we successfully fabricated multicolor patterns with red and green perovskite pixels on a single substrate, which could be further applied in liquid crystal displays (LCDs) with blue backlight. Besides, a prototype green perovskite micro-LED display under current driving has been demonstrated.

Bright magnetic dipole radiation from two-dimensional lead-halide perovskites

Light-matter interactions in semiconductors are uniformly treated within the electric dipole approximation; multipolar interactions are considered "forbidden." We experimentally demonstrate that this approximation inadequately describes light emission in two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs), solution processable semiconductors with promising optoelectronic properties. By exploiting the highly oriented crystal structure, we use energy-momentum spectroscopies to demonstrate that an exciton-like sideband in 2D HOIPs exhibits a multipolar radiation pattern with highly directed emission. Electromagnetic and quantum-mechanical analyses indicate that this emission originates from an out-of-plane magnetic dipole transition arising from the 2D character of electronic states. Symmetry arguments and temperature-dependent measurements suggest a dynamic symmetry-breaking mechanism that is active over a broad temperature range. These results challenge the paradigm of electric dipole-dominated light-matter interactions in optoelectronic materials, provide new perspectives on the origins of unexpected sideband emission in HOIPs, and tease the possibility of metamaterial-like scattering phenomena at the quantum-mechanical level.

Ruddlesden-Popper Perovskites: Synthesis and Optical Properties for Optoelectronic Applications

Ruddlesden-Popper perovskites with a formula of (A')2(A) n -1B n X3 n +1 have recently gained widespread interest as candidates for the next generation of optoelectronic devices. The variations of organic cation, metal halide, and the number of layers in the structure lead to the change of crystal structures and properties for different optoelectronic applications. Herein, the different synthetic methods for 2D perovskite crystals and thin films are summarized and compared. The optoelectronic properties and the charge transfer process in the devices are also delved, in particular, for light-emitting diodes and solar cells.

Enhanced Photoresponsivity of All-Inorganic (CsPbBr3) Perovskite Nanosheets Photodetector with Carbon Nanodots (CDs)

A hybrid composite photodetector based on cesium lead bromine perovskite (CsPbBr3) nanosheets and carbon nanodots (CDs) was fabricated on a quartz substrate by a one-step method of spin-coating and hot-plate annealing. The responsivity of the CsPbBr3/CD hybrid composite photodetector was 608 mAW−1 (under a 520-nm laser diode source applied at 0.2 mWcm−2), almost three times higher than that of a CsPbBr3-based photodetector (221 mAW−1). The enhanced performance of the CsPbBr3/CD photodetector is attributable to the high band alignment of the CDs and CsPbBr3, which significantly improves the charge extraction at the CsPbBr3/CD interface. Moreover, the hybrid CsPbBr3/CD photodetector exhibited a fast response time with a rise and decay time of 1.55 and 1.77 ms, which was faster than that of a pure CsPbBr3 based photodetector, indicating that the CDs accelerate the extraction of electrons and holes trapped in the CsPbBr3 film.

Patterned films of a hybrid lead halide perovskite grown using space-confined conversion of metallic lead by reactive polyiodide melts

A unique technique for preparation of thin patterned perovskite films is suggested based on an interaction of reactive polyiodide melts with metallic lead coatings using a patterned die with a given relief. The growth of perovskite in confined space results in pin-hole free textured films.

A unique technique for preparation of thin patterned perovskite films is suggested based on an interaction of reactive polyiodide melts with metallic lead coatings using a patterned die with a given relief.