Advances and Challenges in Two-Dimensional Organic-Inorganic Hybrid Perovskites Toward High-Performance Light-Emitting Diodes

Organic Ligand Engineering for Tailoring Electron-Phonon Coupling in 2D Hybrid Perovskites

In the emerging two-dimensional organic-inorganic hybrid perovskites, the electronic structures and carrier behaviors are strongly impacted by intrinsic electron-phonon interactions, which have received inadequate attention. In this study, we report an intriguing phenomenon of negative carrier diffusion induced by electron-phonon coupling in (2T)2PbI4. Theoretical calculations reveal that the electron-phonon coupling drives the band alignment in (2T)2PbI4 to alternate between type I and type II heterostructures. As a consequence, photoexcited holes undergo transitions between the organic ligands and inorganic layers, resulting in abnormal carrier transport behavior compared to other two-dimensional hybrid perovskites. These findings provide valuable insights into the role of electron-phonon coupling in shaping the band alignments and carrier behaviors in two-dimensional hybrid perovskites. They also open up exciting avenues for designing and fabricating functional semiconductor heterostructures with tailored properties.

Can 2D Semiconductors Be Game-Changers for Nanoelectronics and Photonics?

2D semiconductors have interesting physical and chemical attributes that have led them to become one of the most intensely investigated semiconductor families in recent history. They may play a crucial role in the next technological revolution in electronics as well as optoelectronics or photonics. In this Perspective, we explore the fundamental principles and significant advancements in electronic and photonic devices comprising 2D semiconductors. We focus on strategies aimed at enhancing the performance of conventional devices and exploiting important properties of 2D semiconductors that allow fundamentally interesting device functionalities for future applications. Approaches for the realization of emerging logic transistors and memory devices as well as photovoltaics, photodetectors, electro-optical modulators, and nonlinear optics based on 2D semiconductors are discussed. We also provide a forward-looking perspective on critical remaining challenges and opportunities for basic science and technology level applications of 2D semiconductors.

Enhancing the Stability and Efficiency of Inverted Perovskite Solar Cells with a Mixed Ammonium Ligands Passivation Strategy

The perovskite solar cell (PSC), which has achieved efficiencies of more than 26%, is expected to be a promising technology that can alternate silicon-based solar cells. However, the performance of PSCs is still limited due to defects and ion migration that occur at the large number of grain boundaries present in perovskite thin films. In this study, the mixed ammonium ligands passivation strategy (MAPS) is demonstrated, which combines n-octylammonium iodide (OAI) and 1,3-diaminopropane (DAP) can effectively suppress the grain boundary defects and ion migration through grain boundaries by the synergistic effect of OAI and DAP, resulting in improved efficiency and stability of PSCs. It has also been revealed that MAPS not only enhances crystallinity and reduces grain boundaries but also improves charge transport while suppressing charge recombination. The MAPS-based opaque PSC shows the best power conversion efficiency (PCE) of 21.29% with improved open-circuit voltage (VOC ) and fill factor (FF), and retained 84% of its initial PCE after 1900 h at 65 °C in N2 atmosphere. Amazingly, the MAPS-based semi-transparent PSC (STP-PSC) retained 94% of their maximum power (21.00% at around 10% AVT) after 1000 h under 1 sun illumination and MAPS-based perovskite submodule (PSM) achieved a PCE of 19.59%, which is among the highest values reported recently.

Ultra-Broad-Band-Excitable Cu-Based Halide (C4H10N)4Cu4I8 with High Stability for LED Applications

Currently, organic-inorganic hybrid cuprous-based halides are receiving substantial attention for their eco-friendliness, distinctive structures, and outstanding photophysical properties. Nevertheless, most of the reported cuprous-based halides demand deep ultraviolet excitation with a narrow excitation range that can meet the commercial requirement. Herein, zero-dimensional (0D) cuprous-based halide (C4H10N)4Cu4I8 single crystals (SCs) were synthesized, with an ultrabroad band excitation ranging 260-450 nm and a greenish-yellow emission band peaking at 560 nm. Excitingly, (C4H10N)4Cu4I8 also features a large Stokes shift of 300 nm, a high photoluminescence quantum yield (PLQY) of up to 84.66%, and a long lifetime of 137 μs. Furthermore, density functional theory calculations were performed to explore the relationship between structure and photophysical properties, and the photoluminescence performance of (C4H10N)4Cu4I8 originates from the electron interactions in [Cu2I4]2- clusters. Taking advantage of broad band excitation and excellent photoluminescent performances, a high luminescence characteristic UV-pumped light-emitting diode (LED) device with remarkable color stability was fabricated by employing the as-synthesized (C4H10N)4Cu4I8 SCs, which present the promising applications of low-dimensional cuprous-based halides in solid-state lighting.

Sb3+-Doped Indium-Based Metal Halide (Gua)3InCl6 with Efficient Yellow Emission

In recent years, low-dimensional organic-inorganic hybrid metal halides (OIHMHs) have shown excellent photophysical properties due to their quantum structure, adjustable energy levels, and energy transfer between inorganic and organic components, which have attracted extensive attention from researchers. Herein, we synthesize a zero-dimensional (0D) OIHMH, Sb3+:(Gua)3InCl6, by introducing Sb3+ into (Gua)3InCl6, which undergoes a significant enhancement of the emission peak at 580 nm with the photoluminescence quantum yield (PLQY) boosted from 17.86 to 95.72% when excited at 340 nm. This boost in photoluminescence of the doped sample was studied by combining ultrafast femtosecond transient absorption, temperature-dependent photoluminescence (PL) spectra, and density functional theory (DFT) calculation, revealing the process of self-trapped exciton (STE) recombination to emit light at both Sb and In sites in this 0D structure simultaneously. This material with the lowest dark STE level at the In site for emission in the undoped sample can amazingly yield very strong emission in the doped sample, which has never been observed before. Finally, we tested its application in a photoelectric device. This work not only helps to gain a deeper understanding of the formation of STEs in In-based halides but also plays a certain guiding role in the design of new luminescent materials.

Investigation on broadband emission of two-dimensional melamine lead iodide perovskite (2D-C3H8N6PbI4): An experimental and theoretical approach

Novel two-dimensional melamine lead iodide perovskite (2D-C3H8N6PbI4) is synthesized to investigate its crystallinity, optical band gap and broadband emission properties and to make comparisons with 2D-C3H8N6PbCl4/2D-C3H8N6PbBr4 perovskites. Both experimental and density functional theory (DFT) interrogations on 2D-C3H8N6PbX4 (X = Cl, Br and I) are conducted. The crystal structure, morphology and percentile of Pb and halide elements are confirmed using scanning electron microscope (SEM), and energy dispersive spectrum (EDS), powder/single crystal X-ray diffraction (PXRD/SXRD), DFT and X-ray crystallography simulations. The optical band gaps of 2D-C3H8N6PbX4 perovskites are determined from the Tauc plot fitting of absorbance and DFT studies. Distinct broadband emission of 2D-C3H8N6PbX4 perovskites between 300 and 800 nm is observed, which can be fitted with multiple Gaussian distributions. The fittings of broad PL spectra from 2D-C3H8N6PbCl4/2D-C3H8N6PbBr4 perovskites confirm the involvement of both Dexter energy transfer from melamine cation and self-trapped excitons (STEs). However, the broadband emission of 2D-C3H8N6PbI4 is attributed only to the Dexter energy transfer from melamine cation and the absence of STEs is attributed to the larger lattice deformation of 2D-C3H8N6PbI4. Moreover, the involvement of spin-orbit coupling (SOC) in the energy transfer is clarified to attest that the broadband emission of 2D-C3H8N6PbI4 is distinct among its halide family.

Achieving Tunable Cold/Warm White-Light Emission in a Single Perovskite Material with Near-Unity Photoluminescence Quantum Yield

Single materials that exhibit efficient and stable white-light emission are highly desirable for lighting applications. This paper reports a novel zero-dimensional perovskite, Rb4CdCl6:Sn2+, Mn2+, which demonstrates exceptional white-light properties including adjustable correlated color temperature, high color rendering index of up to 85, and near-unity photoluminescence quantum yield of 99%. Using a co-doping strategy involving Sn2+ and Mn2+, cyan-orange dual-band emission with complementary spectral ranges is activated by the self-trapped excitons and d-d transitions of the Sn2+ and Mn2+ centers in the Rb4CdCl6 host, respectively. Intriguingly, although Mn2+ ions doped in Rb4CdCl6 are difficult to excite, efficient Mn2+ emission can be realized through an ultra-high-efficient energy transfer between Sn2+ and Mn2+ via the formation of adjacent exchange-coupled Sn-Mn pairs. Benefiting from this efficient Dexter energy transfer process, the dual emission shares the same optimal excitation wavelengths of the Sn2+ centers and suppresses the non-radiative vibration relaxation significantly. Moreover, the relative intensities of the dual-emission components can be modulated flexibly by adjusting the fraction of the Sn2+ ions to the Sn-Mn pairs. This co-doping approach involving short-range energy transfer represents a promising avenue for achieving high-quality white light within a single material.

Fabrication and Characterization of 2D Layered Perovskites with a Gradient Band Gap

Vertical gradient band-gap heterostructures of two-dimensional (2D) layered perovskites have attracted considerable research interest due to their superior optoelectronic properties and demonstrated potential for use in optical devices. However, its fabrication has been challenging. In this investigation, 2D Ruddlesden-Popper mixed halide perovskite single crystals with a vertical gradient band gap were synthesized by using a solid-state halide diffusion process. X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements after diffusion confirm that the crystalline and morphology remain intact. The transmittance and photoluminescence (PL) spectra show the formation of a vertical gradient band gap that is ascribed to gradient halide distribution through halide intermixing. The mixed halide crystal exhibits high stability with completely suppressed phase segregation in the time-dependent PL measurement. The time-resolved photoluminescence (TRPL) spectra prove that the mixed halide sample has an enhanced carrier transport due to the Förster resonance energy transfer (FRET) effect. Besides, the halide diffusion behavior is found to be different from the previously proposed "layer-by-layer" diffusion model in exfoliated crystals. The gradient band-gap structure is critical for various applications in which vertical carrier transport is demanded.

Light Amplification in Fe-Doped CsPbBr3 Crystal Microwire Excited by Continuous-Wave Laser

Electrically pumped halide perovskite laser diodes remain unexplored, and it is widely acknowledged that continuous-wave (CW) lasing will be a crucial step. Here, we demonstrate room-temperature amplified spontaneous emission of Fe-doped CsPbBr₃ crystal microwire excited by a CW laser. Temperature-dependent photoluminescence spectra indicate that the Fe dopant forms a shallow level trap states near the band edge of the lightly doped CsPbBr₃ microcrystal. Pump intensity-dependent time-resolved PL spectra show that the introduced Fe dopant level makes the electron more stable in excited states, suitable for the population inversion. The emission peak intensity of the lightly Fe-doped microwire increases nonlinearly above a threshold of 12.3 kW/cm² under CW laser excitation, indicating a significant light amplification. Under high excitation, the uniform crystal structure and surface outcoupling in Fe-doped perovskite crystal microwires enhanced the spontaneous emission. These results reveal the considerable promise of Fe-doped perovskite crystal microwires toward low-cost, high-performance, room-temperature electrical pumping perovskite lasers.

Self-Generated Buried Submicrocavities for High-Performance Near-Infrared Perovskite Light-Emitting Diode

Embedding submicrocavities is an effective approach to improve the light out-coupling efficiency (LOCE) for planar perovskite light-emitting diodes (PeLEDs). In this work, we employ phenethylammonium iodide (PEAI) to trigger the Ostwald ripening for the downward recrystallization of perovskite, resulting in spontaneous formation of buried submicrocavities as light output coupler. The simulation suggests the buried submicrocavities can improve the LOCE from 26.8 to 36.2% for near-infrared light. Therefore, PeLED yields peak external quantum efficiency (EQE) increasing from 17.3% at current density of 114 mA cm-2 to 25.5% at current density of 109 mA cm-2 and a radiance increasing from 109 to 487 W sr-1 m-2 with low rolling-off. The turn-on voltage decreased from 1.25 to 1.15 V at 0.1 W sr-1 m-2. Besides, downward recrystallization process slightly reduces the trap density from 8.90 × 1015 to 7.27 × 1015 cm-3. This work provides a self-assembly method to integrate buried output coupler for boosting the performance of PeLEDs.

Rational adjustment to interfacial interaction with carbonized polymer dots enabling efficient large-area perovskite light-emitting diodes

Film uniformity of solution-processed layers is the cornerstone of large-area perovskite light-emitting diodes, which is often determined by the 'coffee-ring effect'. Here we demonstrate a second factor that cannot be ignored is the solid-liquid interface interaction between substrate and precursor and can be optimized to eliminate rings. A perovskite film with rings can be formed when cations dominate the solid-liquid interface interaction; whereas smooth and homogeneous perovskite emitting layers are generated when anions and anion groups dominate the interaction. This is due to the fact that the type of ions anchored to the substrate can determine how the subsequent film grows. This interfacial interaction is adjusted using carbonized polymer dots, who also orient the perovskite crystals and passivate their buried traps, enabling a 225 mm² large-area perovskite light-emitting diode with a high efficiency of 20.2%.

High Efficiency Near-Infrared Perovskite Light Emitting Diodes With Reduced Rolling-Off by Surface Post-Treatment

The rolling-off phenomenon of device efficiency at high current density caused by quenching of luminescence in perovskite light-emitting diodes (PeLED) is challenging to be solved. Here, 2-amino-5-iodopyrazine (AIPZ) is dissolved in a mixed solvent of chlorobenzene (CB)/isopropanol (IPA) (7:3 volume ratio) for surface post-treatment of FAPbI₃ perovskite film. The interaction of AIPZ and perovskite surface not only balances the charge injection but also passivates defects to enhance radiative recombination in PeLED. Therefore, the PeLED champion yields peak external quantum efficiency reaching 23.2% at the current density of 45 mA cm^-2 with a radiance brightness of 290 W sr^-1 m^-2 . More importantly, the rolling-off of device efficiency is significantly reduced. The lowest rolling-off devices can maintain 80% of peak EQE (22.1%) at a high current density of 460 mA cm^-2 , whereas the control device only retains 25% of the peak EQE value. This work provides an effective strategy to improve performance and reduce the EQE rolling-off of PeLED for practical application.

Localized Bound Multiexcitons in Engineered Quasi-2D Perovskites Grains at Room Temperature for Efficient Lasers

Reducing the excitation threshold to minimize the Joule heating is critical for the realization of perovskite laser diodes. Although bound excitons are promising for low threshold laser, how to generate them at room temperature for laser applications is still unclear in quasi-2D perovskite-based devices. In this work, via engineering quasi-2D perovskite PEA₂ (CH₃ NH₃ )_{n -1} Pb_n Br_{3 n +1} microscopic grains by the anti-solvent method, room-temperature multiexciton radiative recombination is successfully demonstrated at a remarkably low pump density of 0.97 µJ cm^-2 , which is only one-fourth of that required in 2D CdSe nanosheets. In addition, the well-defined translational momentum in quasi-2D perovskite grains can restrict the Auger recombination which is detrimental to radiative emission. Furthermore, the quasi-2D perovskite grains are favorable for increasing binding energies of excitons and biexcitons and so as the related radiative recombination. Consequently, the prepared <n = 8> phase quasi-2D perovskite film renders a threshold of room-temperature stimulated emission as low as 13.7 µJ cm^-2 , reduced by 58.6% relative to the amorphous counterpart with larger grains. The findings in this work are expected to facilitate the development of solution-processable perovskite multiexcitonic laser diodes.

Phase Regulation and Defect Passivation Enabled by Phosphoryl Chloride Molecules for Efficient Quasi-2D Perovskite Light-Emitting Diodes

HIGHLIGHTS

The modification of perovskite precursor by a series of phosphoryl chloride molecules can indeed improve the performance of perovskite LEDs (Pero-LEDs). The bis(2-oxo-3-oxazolidinyl) phosphinic chloride can not only regulate the phase distribution by controlling the crystallization rate but also passivate the defects of the quasi-2D perovskite. Highly efficient and reproducible Pero-LEDs are achieved with an maximum external quantum efficiency (EQEmax) of 20.82% and an average EQE (EQEave) of around 20% on 50 devices. Quasi-2D perovskites have attracted tremendous interest for application as light-emission layers in light-emitting diodes (LEDs). However, the heterogeneous n phase and non-uniform distribution still severely limit the further development of quasi-2D perovskite LEDs (Pero-LEDs). Meanwhile, the increased defect density caused by the reduced dimension and grain size induces non-radiative recombination and further deteriorates the device performance. Here, we found that a series of molecules containing phosphoryl chloride functional groups have noticeable enhancement effects on the device performance of quasi-2D Pero-LEDs. Then, we studied the modification mechanism by focusing on the bis(2-oxo-3-oxazolidinyl) phosphinic chloride (BOPCl). It is concluded that the BOPCl can not only regulate the phase distribution by decreasing the crystallization rate but also remain in the grain boundaries and passivate the defects. As a result, the corresponding quasi-2D Pero-LEDs obtained a maximum external quantum efficiency (EQEmax) of 20.82% and an average EQE (EQEave) of around 20% on the optimal 50 devices, proving excellent reproducibility. Our work provides a new selection of molecular types for regulating the crystallization and passivating the defects of quasi-2D perovskite films.

Recent Advances in Wide-Bandgap Organic-Inorganic Halide Perovskite Solar Cells and Tandem Application

Perovskite-based tandem solar cells have attracted increasing interest because of its great potential to surpass the Shockley-Queisser limit set for single-junction solar cells. In the tandem architectures, the wide-bandgap (WBG) perovskites act as the front absorber to offer higher open-circuit voltage (VOC) for reduced thermalization losses. Taking advantage of tunable bandgap of the perovskite materials, the WBG perovskites can be easily obtained by substituting halide iodine with bromine, and substituting organic ions FA and MA with Cs. To date, the most concerned issues for the WBG perovskite solar cells (PSCs) are huge VOC deficit and severe photo-induced phase separation. Reducing VOC loss and improving photostability of the WBG PSCs are crucial for further efficiency breakthrough. Recently, scientists have made great efforts to overcome these key issues with tremendous progresses. In this review, we first summarize the recent progress of WBG perovskites from the aspects of compositions, additives, charge transport layers, interfaces and preparation methods. The key factors affecting efficiency and stability are then carefully discussed, which would provide decent guidance to develop highly efficient and stable WBG PSCs for tandem application.

Water-Driven Synthesis of Deep-Blue Perovskite Colloidal Quantum Wells for Electroluminescent Devices

Perovskite colloidal quantum wells (QWs) are promising to realize narrow deep-blue emission, but the poor optical performance and stability suppress their practical application. Here, we creatively propose a water-driven synthesis strategy to obtain size-homogenized and strongly confined deep-blue CsPbBr₃ QWs, corresponding to three monolayers, which emit at the deep-blue wavelength of 456 nm. The water controls the orientation and distribution of the ligands on the surface of the nanocrystals, thus inducing orientated growth through the Ostwald ripening process by phagocytizing unstable nanocrystals to form well-crystallized QWs. These QWs present remarkable stability and high photoluminescence quantum yield of 94 %. Furthermore, we have prepared light-emitting diodes based on the QWs via the all-solution fabrication strategy, achieving an external quantum efficiency of 1 % and luminance of 2946 cd m^-2 , demonstrating state-of-the-art brightness for perovskite QW-based LEDs.

2D Organic Materials: Status and Challenges

In the past few decades, 2D layer materials have gradually become a central focus in materials science owing to their uniquely layered structural qualities and good optoelectronic properties. However, in the development of 2D materials, several disadvantages, such as limited types of materials and the inability to synthesize large-scale materials, severely confine their application. Therefore, further exploration of new materials and preparation methods is necessary to meet technological developmental needs. Organic molecular materials have the advantage of being customizable. Therefore, if organic molecular and 2D materials are combined, the resulting 2D organic materials would have excellent optical and electrical properties. In addition, through this combination, the free design and large-scale synthesis of 2D materials can be realized in principle. Furthermore, 2D organic materials exhibit excellent properties and unique functionalities along with great potential for developing sensors, biomedicine, and electronics. In this review, 2D organic materials are divided into five categories. The preparation methods and material properties of each class of materials are also described in detail. Notably, to comprehensively understand each material's advantages, the latest research applications for each material are presented in detail and summarized. Finally, the future development and application prospects of 2D organic materials are briefly discussed.

Temperature-Dependent Reversible Optical Properties of Mn-Based Organic-Inorganic Hybrid (C8H20N)2MnCl4 Metal Halides

Organic-inorganic metal halides (OIMHs) have abundant optical properties and potential applications, such as light-emitting diodes, displays, solar cells, and photodetectors. Herein, we report zero-dimensional Mn-based OIMH (C₈H₂₀N)₂MnCl₄ single crystals synthesized by a simple slow evaporation method, which exhibit intense green emission at 520 nm originating from ⁴T₁-⁶A₁ transition of Mn²⁺ ions. Large organic cations in the crystal structure result in the isolated [MnCl₄]^2- tetrahedrons, and the closest Mn-Mn distance reaches 9.07 Å, which effectively inhibits the migration of excitation energy between adjacent Mn²⁺ emission centers, thus achieving a high quantum yield (∼87%) and a long photoluminescence (PL) lifetime (3.42 ms). The different optical and structural properties at low and high temperatures are revealed by temperature-dependent PL and X-ray diffraction spectra. The PL spectra and lifetimes under the heating and cooling processes indicate that the optical property transitions are reversible at 220/240 K. Our work provides a promising strategy for building multifunctional optoelectronic materials and insights into the understanding convertible photophysical properties from isomers of metal halides.

2D PEA2SnI4 Inkjet-Printed Halide Perovskite LEDs on Rigid and Flexible Substrates

Lead-free PEA₂SnI₄-based perovskite LEDs are successfully inkjet-printed on rigid and flexible substrates. Red-emitting devices (λ_max = 633 nm) exhibit, under ambient conditions, a maximum external quantum efficiency (EQE_max) of 1% with a related brightness of 30 cd/m² at 10 mA/cm².

Competing Energy Transfer in Two-Dimensional Mn2+-Doped BDACdBr4 Hybrid Layered Perovskites with Near-Unity Photoluminescence Quantum Yield

Two-dimensional (2D) hybrid layered perovskites (HLPs) have attracted extensive attention due to their excellent optoelectronic properties. Herein, we successfully prepared high-quality Mn-doped BDACdBr₄ (BDA = NH₂(CH₂)₄NH₂, butylene diammonium) HLP single crystals (SCs). The incorporation of Mn²⁺ ions modulates the electronic band structure of BDACdBr₄ perovskites and tailors the energy transfer process of excited states. A near-unity photoluminescence (PL) quantum yield of 96% from the Mn²⁺ emission at 608 nm is achieved. Excitation wavelength-dependent spectroscopic characterizations help to clarify the energy transfer mechanism of Mn-doped BDACdBr₄, in which competing PL from the ³E_g → ¹A_1g transition of Cd²⁺ and the ⁴T₁(G) → ⁶A₁(S) transition of Mn²⁺ dopants is observed. Temperature-dependent PL spectroscopic characterizations indicate that the efficient energy transfer from BDACdBr₄ perovskite host to Mn²⁺ dopants requires thermal activation to overcome a potential barrier. This work provides new insight into the photophysics and optical properties of 2D HLPs, especially the influence of Mn²⁺ doping on competing energy transfer in hybrid luminescent materials.

Green Triplet Self-Trapped Exciton Emission in Layered Rb3Cd2Cl7:Sb3+ Perovskite: Comparison with RbCdCl3:Sb3

Metal halide materials have recently sparked intense research because of their excellent photophysical properties and chemical stability. For example, RbCdCl₃:Sb³⁺ exhibits broad emission at about 600 nm with a high photoluminescence quantum yield (PLQY) over 91% and double emission bands with bright white color. Herein, we obtained a novel Rb and Cd layered perovskite Rb₃Cd₂Cl₇ doped with Sb³⁺, which gives luminescence at 525 nm with a large Stokes shift of 200 nm, originating from a self-trapped exciton (STE). Its PLQY is 57.47%, but its low-temperature PLQY becomes much higher at the same wavelength. When Rb₃Cd₂Cl₇:Sb³⁺ and RbCdCl₃:Sb³⁺ were compared, the two classes of quantum confinement effects by Rb and Cd ions in the lattice were identified to describe their electronic states and different optical properties. These results suggest that properties of Sb-doped cadmium halides could be modified by the structure type and local atomic confinement to find applications as promising luminescent materials for optoelectronic devices.

Competing Energy Transfer-Modulated Dual Emission in Mn2+-Doped Cs2NaTbCl6 Rare-Earth Double Perovskites

A₂B^IB^IIIX₆ double perovskites are promising materials due to their outstanding photoelectronic properties and excellent stability in the environment. Herein, we synthesized Mn²⁺:Cs₂NaTbCl₆ with dual emission through a solvothermal method for the first time. Mn²⁺:Cs₂NaTbCl₆ double perovskites exhibit excellent environmental stability and high photoluminescence quantum yields (PLQYs). The Cs₂NaTbCl₆ was successfully doped with Mn²⁺ in two modes: at Mn-feeding concentrations below 1%, Mn²⁺ first tend to insert into the interstitial void, but if the Mn-feeding concentration exceeds 1%, Mn²⁺ will further substitute Na⁺ site of the Cs₂NaTbCl₆ lattice and thus both two doping modes coexist. After Mn²⁺ doping, efficient energy transfer from the ⁵D₄ level of Tb³⁺ ions to the ⁴T₁ level of Mn²⁺ ions occurs, resulting in tunable dual emission from the Tb³⁺⁵D₄ → ⁷F_J=6,5,4,3 transition and Mn²⁺⁴T₁ → ⁶A₁ transition. Further, LED based on the Mn²⁺:Cs₂NaTbCl₆ double perovskites exhibits excellent performance and stability. This work demonstrates a strategy to achieve novel lanthanide-based double perovskites with potential applications in photonics.

Efficient Orange Emission in Mn2+-Doped Cs3Cd2Cl7 Perovskites with Excellent Stability

Low-dimensional metal halides are attractive for applications in photodetectors, solid-state lighting, and solar cells, but poor stability is an obstacle that must be overcome in commercial applications. Herein, we successfully synthesized a Ruddlesden-Popper (RP)-phased perovskite Mn²⁺:Cs₃Cd₂Cl₇ with high photoluminescence quantum yield (PLQY) and outstanding thermal and environmental stability by a solvothermal method. The pristine sample Cs₃Cd₂Cl₇ exhibits a weak cyan broad emission centered at 510 nm with a low PLQY of ∼4%. Once Mn²⁺ ions are introduced into the host lattice, a bright orange emission peaking at 580 nm with a high PLQY of ∼74% was achieved, which is attributed to the efficient energy transfer from the host to Mn²⁺ ions and thus results in the ⁴T₁ → ⁶A₁ radiation transition of Mn²⁺ ions. The photoluminescence (PL) intensity and environmental stability of Mn²⁺:Cs₃Cd₂Cl₇ can be further improved through A-site Rb alloying. Finally, an orange LED with outstanding color stability was fabricated on the basis of the Mn²⁺:Cs₃Cd₂Cl₇. Our work successfully elucidates that dopant plays an integral role in tailoring optical properties.

H2O-NH4+-Induced Emission Modulation in Sb3+-Doped (NH4)2InCl5·H2O

Lead-based metal halide perovskites have received widespread attention for their promising application prospects in the field of lighting and display due to their excellent optical properties. However, the toxicity of lead may hinder their further commercial application. Herein, a zero-dimensional (0D) metal halide (NH₄)₂InCl₅·H₂O with an orthorhombic structure and the Pnma space group was produced. With doping with Sb³⁺, these products exhibit one highly efficient and wide yellow emission band (∼450-850 nm) in their photoluminescence (PL) spectra, which covers almost the entire visible spectral range at room temperature; however, they give two emission bands with long decay lifetimes (microseconds) at low temperature. Temperature-dependent steady-state PL, transient PL spectroscopy, temperature-dependent Raman spectra characterization, and theoretical band structure calculations confirm that the dual-band emission at low temperature originates from the dual vibronic levels of the self-trapped exciton (STE) in the hole-vibration state, whose vibration energy is related to the H₂O-NH₄⁺ connection in the valence band. This result proves that the vibronic state in STE formation involves both electrons and holes in the excited states, the opposite of this happens in the electron-vibration band in most perovskite halides. These results provide new insight into the luminescent mechanism of Sb³⁺ in halide perovskites, especially used for emission color modulation by the temperature-dependent electron- or hole-vibration processes.

Universal Bifacial Stamping Approach Enabling Reverse-Graded Ruddlesden-Popper 2D Perovskite Solar Cells

Quasi 2D perovskite solar cells (PSCs) are promising light absorbers that overcome the inherent instabilities of 3D perovskites. High-performance and stable 2D PSCs require careful control over the crystallographic orientation and phase distribution. This study introduces a simple and universal bifacial stamping method to obtain highly oriented perovskite crystals with a reverse-graded structure, where the low-n-value 2D perovskite phases are located mainly at the film surfaces. Bifacial stamping of 3D perovskite films atop the 2D films enables incorporation of 2D spacer cations into the 3D film surfaces, forming reverse-graded quasi-2D perovskite films. During stamping, suppressed evaporation of the precursor solvent induces heterogeneous nucleation from the contact interface between the 2D and 3D films, resulting in well-crystallized perovskite films having out-of-plane alignments with respect to the substrate. Thus, a highly oriented and reverse-graded quasi-2D perovskite with an average n value of 18 is obtained with power conversion efficiency exceeding 17% and high open-circuit voltage of 1.11 V for iso-butylammonium (iso-BA)-based (iso-BA₂ MA_{n -1} Pb_n I_{3 n +1} ) PSCs. The unencapsulated device retains 92% of its initial efficiency after aging at 40 ± 5% relative humidity for 1200 h. This work provides a new strategy for fabricating highly oriented and phase-controlled quasi-2D PSCs.

Recent advances in lead-free double perovskites for x-ray and photodetection

Over the last decade, lead halide perovskites have attracted significant research attention in the field of photovoltaics, light-emitting devices, photodetection, ionizing radiation detection, etc, owing to their outstanding optoelectrical properties. However, the commercial applications of lead-based perovskite devices are restricted due to the poor ambient stability and toxicity of lead. The encapsulation of lead-based devices can reduce the possible leakage of lead. However, it is hard to ensure safety during large-scale production and long-term storage. Recently, considerable efforts have been made to design lead-free perovskites for different optoelectronic applications. Metal halide double perovskites with the general formula of A₂M^IM^IIIX₆or A₂M^IVX₆could be potentially considered as green and stable alternatives for different optoelectronic applications. In this review article, we focus on the recent progress and findings on lead-free halide double perovskites for x-ray and UV-vis photodetection applications. Lead-free halide double perovskite has recently drawn a great deal of attention for superior x-ray detection due to its high absorption coefficient, large carrier mobility-lifetime product, and large bulk resistance. In addition, these materials exhibit good performance in photodetection in the UV-vis region due to high photocarrier generation and efficient carrier separation. In this review, first, we define the characteristics of lead-free double perovskite materials. The fundamental characteristics and beneficial properties of halide perovskites for direct and indirect x-ray detection are then discussed. We comprehensively review recent developments and efforts on lead-free double perovskite for x-ray detection and UV-vis photodetection. We bring out the current challenges and opportunities in the field and finally present the future outlook for developing lead-free double perovskite-based x-ray and UV-vis photodetectors for practical applications.

Phase-Selective Solution Synthesis of Cd-Based Perovskite Derivatives and Their Structure/Emission Modulation

The rich phase structures of perovskite derivatives have attracted extensive attention and can be applied in the fields of optoelectronics due to their high emission efficiency and tunable emission. Herein, we explored a phase-selective solution synthetic route to obtain different Cd-based perovskite derivatives. First, the pristine tetragonal Cs₇Cd₃Br₁₃ was obtained by a solvothermal method, and its photoluminescence quantum yield (PLQY) was boosted from 8.28% to 57.62% after appropriate Sb³⁺ doping. Furthermore, halogen substitution was adopted to modify Sb:Cs₇Cd₃Br₁₃ and produced a series of Cd-based perovskite derivatives with different crystal structures and tunable emission from cyan to orange (517-625 nm). The mechanisms behind such experimental phenomena were further investigated and discussed on the basis of material characterization and theoretical computation. This study presented an effective strategy to synthesize bright Cd-based perovskite derivatives with different structures and modulated emission, and it also provided insights to understand the structure/emission modulation via halogen substitution.

Mixed-halide copper-based perovskite R2Cu(Cl/Br)4 with different organic cations for reversible thermochromism

Mechanically exfoliated flakes of mixed-halide Cu-based perovskite crystals, R2Cu(Cl/Br)4, with three alkyl chains exhibit reversible thermochromic behavior with differences in crystal lattice behavior depending on the organic spacer used.

Spectral Stable Blue-Light-Emitting Diodes via Asymmetric Organic Diamine Based Dion-Jacobson Perovskites

The spectral instability issue is a challenge in blue perovskite light-emitting diodes (PeLEDs). Dion–Jacobson (DJ) phase perovskites are promising alternatives to achieve high-quality blue PeLEDs. However, the current exploration of DJ phase perovskites is focused on symmetric divalent cations, and the corresponding efficiency of blue PeLEDs is still inferior to that of green and red ones. In this work, we report a new type of DJ phase CsPb(Br/Cl)₃ perovskite via introduction of an asymmetric molecular configuration as the organic spacer cation in perovskites. The primary and tertiary ammonium groups on the asymmetric cations bridge with the lead halide octahedra forming the DJ phase structures. Stable photoluminescence spectra were demonstrated in perovskite films owing to the suppressed halide segregation. Meanwhile, the radiative recombination efficiency of charges is improved significantly as a result of the confinement effects and passivation of charge traps. Finally, we achieved an external quantum efficiency of 2.65% in blue PeLEDs with stable spectra emission under applied bias voltages. To our best knowledge, this is the first report of asymmetric cations used in PeLEDs, which provides a facile solution to the halide segregation issue in PeLEDs.