Energy Transfer-Dominated Quasi-2D Blue Perovskite Light-Emitting Diodes

The bromide-chloride mixed quasi-two-dimensional (2D) perovskite, with a natural quantum well structure and tunable exciton binding energy, has gained significant attention for high-performance blue perovskite light-emitting diodes (PeLEDs). However, the relative importance of having a low trap state density or efficient exciton transfer for high-efficiency electroluminescence (EL) performance remains elusive. Here, two molecules with the benzoic acid group, sodium 4-fluorobenzoate (SFB) and 3,5-dibromobenzoic acid (DBA), are used to modulate the phase distribution and trap state to explore the effect between energy transfer and defect passivation. As a result, when the n = 1 phase is inhibited in both films, the DBA@SFB-modified perovskite films achieve a higher photoluminescence quantum yield (PLQY) than the SFB-modified perovskite films due to effective defect passivation. However, DBA@SFB-modified PeLEDs exhibit lower external quantum efficiency (EQE) compared to SFB-modified PeLEDs due to the poor exciton transfer between the low-dimensional phase. This demonstrates that passivation strategies may enhance photoluminescence through reducing nonradiative recombination, but the effect of phase distribution is pivotal for EL performance by efficient energy transfer in quasi-2D perovskites. Femtosecond time-resolved transient absorption measurements confirm the fastest carrier dynamics in SFB-modified perovskite films, further corroborating the above result. This work provides useful information about phase modulation and defect passivation for high-efficiency blue quasi-2D PeLEDs.

Precursor Engineering Induced High-Efficiency Electroluminescence of Quasi-Two-Dimensional Perovskites: A Synergistic Defect Inhibition and Passivation Approach

Despite light-emitting diodes (LEDs) based on quasi-two-dimensional (Q-2D) perovskites being inexpensive and exhibiting high performance, defects still limit the improvement of electroluminescence efficiency and stability by causing nonradiative recombination. Here, an organic molecule, 1-(o-tolyl) biguanide, is used to simultaneously inhibit and passivate defects of Q-2D perovskites via in situ synchronous crystallization. This molecule not only prevents surface bromine vacancies from forming through hydrogen bonding with the bromine of intermediaries but also passivates surface defects through its interaction with uncoordinated Pb. Via combination of defect inhibition and passivation, the trap density of Q-2D perovskite films can be significantly reduced, and the emission efficiency of the film can be improved. Consequently, the corresponding LED shows an external quantum efficiency of 24.3%, and its operational stability has been increased nearly 15 times.

Thermally Evaporated Blue Quasi-Two-Dimensional Perovskite Light-Emitting Diodes via Low-Dimensional Phase Distribution Arrangement

Perovskite light-emitting diodes (PeLEDs) have shown great potential in the display domain due to their wide color gamut, narrow emission, and low cost. In current PeLEDs manufacturing methods, thermal evaporation shows great competitiveness with its advantages of easy patterning, production line compatibility, and solvent-free processability. However, the development of thermally evaporated blue PeLEDs is limited by their low radiative recombination rate and high defect density. Herein, we report high-performance thermally evaporated blue PeLEDs by in situ introduction of ammonium cations. We confirm that phenethylammonium (PEA+) has lower adsorption energy, which significantly reduces the low-n phases in a quasi-2D perovskite film. The energy transfer rate is also promoted by the PEA+ addition. As a result, we fabricate blue PeLEDs with an external quantum efficiency of 1.56% by thermal evaporation. The strategy of arranging phase distribution could benefit the industrialization of full-color PeLEDs.

Colloidal CsBr Nanocrystals Triggered Inorganic Cation and Anion Exchange Enables High-Performance Perovskite Solar Cells

Achieving longitudinal doping of specific ions by surface treatment remains a challenge for perovskite solar cells, which are often limited by dopant and solvent compatibility. Here, with the flowing environment created by CsBr colloidal nanocrystals, ion exchange is induced on the surface of the perovskite film to enable the homogeneous distribution of Cs+ and gradient distribution of Br- simultaneously at whole depth of the film. Meanwhile, assisted by long-chain organic ligands, the excess PbI2 on the surface of perovskite film is converted to a more stable quasi-2D perovskite, which realizes effective passivation of defects on the surface. As a result, the unfavorable n-type doping on the top surface is suppressed, so that the energy level alignment between perovskite and hole transport layer is optimized. On the basis of co-modification of the surface and the bulk, the PCE of champion device reaches 23.22% with enhanced VOC of 1.12 V. Device maintains 97.12% of the initial PCE in dark ambient air at 1% RH after 1056 h without encapsulation, and 91.56% of the initial PCE under light illumination of 1 sun in N2 atmosphere for more than 200 h. The approach demonstrated here provides an effective strategy for the nondestructive introduction of inorganic ions in perovskite film.

Tuning Hole Transport Properties and Perovskite Crystallization via Pyridine-Based Molecules for Quasi-2D Perovskite Solar Cells

Quasi-2D perovskites show great potential as photovoltaic devices with superior stability, but the power conversion efficiency (PCE) is limited by poor carrier transport. Here, it is simultaneously affected the hole transport layer (HTL) and the perovskite layer by incorporating pyridine-based materials into poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) to address the key problem above in 2D perovskites. With this approach, the enhanced optoelectronic performance of the novel PEDOT:PSS is due to electron transfer between the additives and PEDOT or PSS, as well as a dissociation between PEDOT and PSS based on experimental and theoretical studies, which facilitates the charge extraction and transfer. Concurrently, in-situ X-ray scattering studies reveal that the introduction of pyridine-based molecules alters the transformation process of the perovskite intermediate phase, which leads to a preferred orientation and ordered distribution caused by the Pb─N chemical bridge, achieving efficient charge transport. As a result, the pyridine-treated devices achieve an increased short-circuit current density (Jsc ) and PCE of over 17%.

Green Solvent Polishing Enables Highly Efficient Quasi-2D Perovskite Solar Cells

Preferred crystalline orientation at the surface of quasi-2D organic-inorganic halide perovskites is crucial to promote vertical carrier transport and interface carrier extraction, which further contribute to device efficiency and stability in photovoltaic applications. However, loose unoriented and defective surfaces are inevitably formed in the crystallization process, especially with the introduction of bulky organic cations into the quasi-2D perovskites. Here, a facile and effective surface polishing method using a natural-friendly green solvent, 2,2,2-trifluoroethanol, is proposed to reconstruct the surface. After solvent polishing, the randomly oriented phases containing trap sites on the surface are successfully removed, and the compact vertical-oriented phases underneath are revealed with less defectiveness and better smoothness, which greatly facilitates carrier transport and interfacial charge extraction. Consequently, the green solvent polished devices show a boosting efficiency of 18.38% with a high open-circuit voltage of 1.21 V. The devices also show improved storage and operational stability.

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.

Efficient Red-Emissive Circularly Polarized Electroluminescence Enabled by Quasi-2D Perovskite with Chiral Spacer Cation

Perovskites are promising environmentally sustainable materials for circularly polarized electroluminescence (CPEL). While another chiral nonemissive layer is required for the developed perovskite-based CPEL, we report herein a highly efficient circularly polarized electroluminescence based on a single layer of quasi-2D perovskite with achiral phenethylammonium iodide (PEAI) and chiral S/R-1-(1-naphthyl)ethylammonium iodide (S/R-NEAI) as dual spacer cations. The quasi-2D perovskite was further passivated by carbazole-functionalized phosphonium. The as-fabricated film exhibits not only a circular dichroism (CD) signal but also prominent circularly polarized luminescence (CPL) activity with a maximum photoluminescence dissymmetry factor (g_lum) of ∼2.1 × 10^-3. More importantly, a highly efficient, spin-polarized light-emitting diode (LED) was fabricated based on the in situ passivated quasi-2D perovskite with a peak external quantum efficiency of 3.7% and a maximum electroluminescence dissymmetry factor (g_EL) of ∼4.0 × 10^-3.

Compact-Type Quasi-2D Perovskite Based on Two Conventional 3D Perovskites

Quasi-2D perovskites are natural quantum well (QW) structures composed of insulating organic layers inserted between conducting [A_n-1Pb_nX_3n+1]^2- slabs. The presence of the bulky organic layer improves the stability but meanwhile sacrifices carrier transport performance. By utilizing two A-site cations of formamidinium (FA⁺) and cesium (Cs⁺), we synthesize unique compact-type quasi-2D perovskites CsPbBr₃@FABr. Instead of the bulky organic cations, the FA⁺ cation was employed to work as interlayer "spacer", while the smaller Cs⁺ cation was chosen to occupy perovskite cages. Transient absorption reveals an energy transfer from small-n-value QWs to large-n-value QWs, enabling a photoluminescence quantum yield (PLQY) of 36.1%. After further promoting the formation of middle-n-value QWs, the homogeneous QW distribution provides a complete energy cascade to access more efficient energy transfer, leading to significant PLQY raise to 70.1%. We break the shackles to report the first case of compact-type quasi-2D perovskites, providing new guidelines for designing high-performance perovskite materials for optoelectronic devices.

Unravelling Alkali-Metal-Assisted Domain Distribution of Quasi-2D Perovskites for Cascade Energy Transfer toward Efficient Blue Light-Emitting Diodes

Solution processable quasi-2D (Q-2D) perovskite materials are emerging as a promising candidate for blue light source in full-color display applications due to their good color saturation property, high brightness, and spectral tunability. Herein, an efficient energy cascade channel is developed by introducing sodium bromide (NaBr) in phenyl-butylammonium (PBA)-containing mixed-halide Q-2D perovskites for a blue perovskite light-emitting diode (PeLED). The incorporation of alkali metal contributes to the nucleation and growth of Q-2D perovskites into graded distribution of domains with different layer number <n>. The study of excitation dynamics by transient absorption (TA) spectroscopy confirms that NaBr induces more Q-2D perovskite phases with small n number, providing a graded energy cascade pathway to facilitate more efficient energy transfer processes. In addition, the nonradiative recombination within the Q-2D perovskites is significantly suppressed upon Na⁺ incorporation, as validated by the trap density estimation. Consequently, the optimized blue PeLEDs manifest a peak external quantum efficiency (EQE) of 7.0% emitting at 486 nm with a maximum luminance of 1699 cd m^-2 . It is anticipated that these findings will improve the understanding of alkali-metal-assisted optimization of Q-2D perovskites and pave the way toward high-performance blue PeLEDs.

Enhancing the Performance of Quasi-2D Perovskite Light-Emitting Diodes Using Natural Cyclic Molecules with Distinct Phase Regulation Behaviors

In this study, two natural small molecules, α-cyclodextrin (α-CD) and β-cyclodextrin (β-CD), are used as additives to improve the performance of quasi-2D PEA₂Cs_n-1Pb_nBr_3n+1 (n = 3, herein) PeLEDs. Both of them are shown to efficiently passivate the quasi-2D perovskite films to afford improved film quality and morphology, but they exhibit distinct phase regulation behaviors possibly due to their different pore sizes. It reveals that α-CD effectively suppresses the formation of the low-n phases (n ≤ 2), while β-CD better regulates the phase with a medium-n value (n = 3). Because of effectively suppressing the formation of low-n phases, the CD-assisted quasi-2D perovskite films possess facilitated exciton energy transfer and reduced nonradiative recombination. Consequently, the optimized α-CD-derived PeLED shows the highest luminance (L_max) of 37,825 cd/m² with an external quantum efficiency (EQE) of 3.81%, while the β-CD-derived PeLED delivers a lower L_max of 24,793 cd/m² with an EQE of 3.09%. Compared to the pristine device, L_max is enhanced by 6.3 and 3.8 times for α-CD- and β-CD-based PeLEDs, respectively, and EQE is enhanced by ∼4.8 times for both devices; besides, both CD-assisted devices also exhibit improved color purity and a lower bias dependency of electroluminescent intensity.

Plasmonic and Graphene-Functionalized High-Performance Broadband Quasi-Two-Dimensional Perovskite Hybrid Photodetectors

Quasi-two-dimensional (2D) layered organic-inorganic hybrid perovskites have attracted extensive attention, owing to their excellent optoelectronic tunability and moisture stability compared with three-dimensional perovskite counterparts and show great potential for application in photodetectors (PDs). However, owing to the unavoidable grain boundary defects of perovskite polycrystalline films, the photocurrent is limited by poor light absorption and charge mobility. Therefore, the preparation of quasi-2D perovskite films with strong light trapping and high charge mobility has been challenging. In this study, novel broadband quasi-2D perovskite (BA)₂(FA)_n-1Pb_nI_3n+1 hybrid-structure PDs with good stability were fabricated by combining both monolayer graphene and Au square nanoarrays. The hybrid system using both graphene and Au square nanoarrays effectively improved the carrier mobility and light absorption and simultaneously maximized light trapping and light-induced carrier extraction, which resulted in PDs with greatly enhanced photocurrent in the visible and near-infrared range. The graphene-Au array-perovskite-based PDs had a low dark current of 10^-10 A, large on/off ratio of 10⁴, high responsivity of 18.71 A W^-1, and detectivity of 2.21 × 10¹³ Jones. The responsivity and detectivity were two orders of magnitude higher than those of PDs based only on perovskites. This work demonstrates a promising and feasible device based on the coupling of a gold array, layered graphene, and quasi-2D perovskites, which shows great potential for the development of high-performance broadband perovskite PDs.

Ionic Liquid Passivation Eliminates Low-n Quantum Well Domains in Blue Quasi-2D Perovskite Films

In quasi-two-dimensional (quasi-2D) perovskite films, carriers transport in the cascade structural systems involving various quantum wells (QWs) n, but their efficiency is limited by the severe nonradiative recombination within plentiful n = 1, 2, 3 domains induced by traditional ammonium bromide passivation. Here, we fabricate the quasi-2D films with the elimination of n = 1, 2, 3 domains by introducing the ionic liquid n-butylamine acetate (BAAc) instead of n-butylamine hydrobromide (BABr), which increases the photoluminescence quantum yield (PLQY) and lowers the surface roughness of films. Due to the anion exchange between BAAc and methylamine hydrobromide (MABr), BAAc exhibits a sole passivation effect on methylamine-based perovskites. As a result, the ionic liquid-derived perovskite light-emitting diodes (PeLEDs) display blue emission at 479 nm and show significantly improved performance on external quantum efficiency (EQE) and luminance. Our finding provides insights into the passivating effect of ionic liquid on quasi-2D perovskites and will benefit fabricating PeLEDs with enhanced performance.

Highly Efficient Quasi-2D Perovskite Light-Emitting Diodes Incorporating a TADF Dendrimer as an Exciton-Retrieving Additive

Although small organics or polymer additives have been introduced to enhance film formation and radiative recombination of perovskite light-emitting diodes (PeLEDs), the exciton utilization and quantum efficiency need further optimization. Here, we introduce a thermal-activated delayed fluorescence (TADF) dendrimer as an additive to enhance the surface coverage and reduce the trap state of the grain boundary. More importantly, the TADF nature of such an additive can retrieve the exciton dissociated from perovskite or trapped by the grain boundary and then transfer the energy back to emissive perovskite through the Förster energy transfer process. Since the triplets can be reused by reverse intersystem crossing in such a TADF additive, the theoretical exciton utilization is 100%. As a result, the optimized PeLEDs cooperating with a TADF additive achieved a high current efficiency of 39.0 cd A^-1 and an ultrabright luminescence of 18,000 cd m^-2, which are almost 5 times higher than those of the control device without an additive. Moreover, the device stability monitored by half-lifetime at 1000 cd m^-2 enhanced 2 times after introducing the TADF dendrimer as an additive. The parent dendrimer without a TADF feature was also synthesized as an additive to explore the mechanism action, which found that 54% enhancement of device efficiency can be attributed to defect passivating, while 46% was assigned to retrieved energy. This research first demonstrates that the TADF dendrimer is a promising exciton-retrieving additive for enhancing the performance of PeLEDs by passivating defect, filling up grain boundary, and retrieving leakage exciton.

Inkjet-Printed Full-Color Matrix Quasi-Two-Dimensional Perovskite Light-Emitting Diodes

Full-color matrix devices based on perovskite light-emitting diodes (PeLEDs) formed via inkjet printing are increasingly attractive due to their tunable emission, high color purity, and low cost. A key challenge for realizing PeLED matrix devices is achieving high-quality perovskite films with a favorable emission structure via inkjet printing techniques. In this work, a narrow phase distribution, high-quality quasi-two-dimensional (quasi-2D) perovskite film without a "coffee ring" was obtained via the introduction of a phenylbutylammonium cation into the perovskite and the use of a vacuum-assisted quick-drying process. Relatively efficient emissions of red, green, and blue (RGB) uniform quasi-2D perovskite films with high photoluminescence quantum yields were cast by the inkjet printing technique. The RGB monochrome perovskite matrix devices with 120 pixel-per-inch resolution exhibited electroluminescence, with maximum external quantum efficiencies of 3.5, 3.4, and 1.0% (for red, green, and blue light emissions, respectively). Furthermore, a full-color perovskite matrix device with a color gamut of 102% (NTSC 1931) was realized. To the best of our knowledge, this is the first report of a full-color perovskite matrix device formed by inkjet printing.

Rearranging Low-Dimensional Phase Distribution of Quasi-2D Perovskites for Efficient Sky-Blue Perovskite Light-Emitting Diodes

Metal halide perovskites have received much attention for their application in light-emitting diodes (LEDs) in the past several years. Rapid progress has been made in efficient green, red, and near-infrared perovskite LEDs. However, the development of blue perovskite LEDs is still lagging far behind. Here, we report efficient sky-blue perovskite LEDs by rearranging low-dimensional phase distribution in quasi-2D perovskites. We incorporated sodium ions into the mixed-Cl/Br quasi-2D perovskites with phenylethylammonium as the organic spacer and cesium lead halide as the inorganic framework. The inclusion of the sodium ion was found to significantly reduce the formation of the n = 1 phase, which was dominated by nonradiative transition, and increase the formation of other small-n phases for efficient exciton energy transfer. By managing the phase distribution, a maximum external quantum efficiency (EQE) of 11.7% was achieved in the sky-blue perovskite LED, with a stable emission peak at 488 nm. Further optimizing the phase distribution and film morphology with Pb content, we demonstrated the sky-blue devices with the average EQE approaching 10%. This strategy of engineering phase distribution of quasi-2D perovskites with a sodium ion could provide a useful way for the fabrication of high-performance blue perovskite LEDs.

Nonconfinement Structure Revealed in Dion-Jacobson Type Quasi-2D Perovskite Expedites Interlayer Charge Transport

Dion-Jacobson (DJ) type 2D perovskites with a single organic cation layer exhibit a narrower distance between two adjacent inorganic layers compared to the corresponding Ruddlesden-Popper perovskites, which facilitates interlayer charge transport. However, the internal crystal structures in 2D DJ perovskites remain elusive. Herein, in a p-xylylenediamine (PDMA)-based DJ perovskite bearing bifunctional NH₃ ⁺ spacer, the compression from confinement structure (inorganic layer number, n = 1, 2) to nonconfinement structure (n > 3) with the decrease of PDMA molar ratio is unraveled. Remarkably, the nonconfined perovskite displays shorter spacing between 2D quantum wells, which results in a lower exciton binding energy and hence promotes exciton dissociation. The significantly diminishing quantum confinement promotes interlayer charge transport leading to a maximum photovoltaic efficiency of ≈11%. Additionally, the tighter interlayer packing arising from the squeezing of inorganic octahedra gives rise to enhanced ambient stability.

Optimization of Low-Dimensional Components of Quasi-2D Perovskite Films for Deep-Blue Light-Emitting Diodes

Compared to efficient green and near-infrared light-emitting diodes (LEDs), less progress has been made on deep-blue perovskite LEDs. They suffer from inefficient domain [various number of PbX₆ ^- layers (n)] control, resulting in a series of unfavorable issues such as unstable color, multipeak profile, and poor fluorescence yield. Here, a strategy involving a delicate spacer modulation for quasi-2D perovskite films via an introduction of aromatic polyamine molecules into the perovskite precursor is reported. With low-dimensional component engineering, the n₁ domain, which shows nonradiative recombination and retarded exciton transfer, is significantly suppressed. Also, the n₃ domain, which represents the population of emission species, is remarkably increased. The optimized quasi-2D perovskite film presents blue emission from the n₃ domain (peak at 465 nm) with a photoluminescence quantum yield (PLQY) as high as 77%. It enables the corresponding perovskite LEDs to deliver stable deep-blue emission (CIE (0.145, 0.05)) with an external quantum efficiency (EQE) of 2.6%. The findings in this work provide further understanding on the structural and emission properties of quasi-2D perovskites, which pave a new route to design deep-blue-emissive perovskite materials.