Stable Layered 2D Perovskite Solar Cells with an Efficiency of over 19% via Multifunctional Interfacial Engineering

What Matters for the Charge Transport of 2D Perovskites?

Compared to 3D perovskites, 2D perovskites exhibit excellent stability, structural diversity, and tunable bandgaps, making them highly promising for applications in solar cells, light-emitting diodes, and photodetectors. However, the trade-off for worse charge transport is a critical issue that needs to be addressed. This comprehensive review first discusses the structure of 3D and 2D metal halide perovskites, then summarizes the significant factors influencing charge transport in detail and provides a brief overview of the testing methods. Subsequently, various strategies to improve the charge transport are presented, including tuning A'-site organic spacer cations, A-site cations, B-site metal cations, and X-site halide ions. Finally, an outlook on the future development of improving the 2D perovskites' charge transport is discussed.

Phonon Properties and Lattice Dynamics of Two- and Tri-Layered Lead Iodide Perovskites Comprising Butylammonium and Methylammonium Cations-Temperature-Dependent Raman Studies

Hybrid lead iodide perovskites are promising photovoltaic and light-emitting materials. Extant literature data on the key optoelectronic and luminescent properties of hybrid perovskites indicate that these properties are affected by electron-phonon coupling, the dynamics of the organic cations, and the degree of lattice distortion. We report temperature-dependent Raman studies of BA2MAPb2I7 and BA2MA2Pb3I10 (BA = butylammonium; MA = methylammonium), which undergo two structural phase transitions. Raman data obtained in broad temperature (360-80 K) and wavenumber (1800-10 cm-1) ranges show that ordering of BA+ cations triggers the higher temperature phase transition, whereas freezing of MA+ dynamics occurs below 200 K, leading to the onset of the low-temperature phase transition. This ordering is associated with significant deformation of the inorganic sublattice, as evidenced by changes observed in the lattice mode region. Our results show, therefore, that Raman spectroscopy is a very valuable tool for monitoring the separate dynamics of different organic cations in perovskites, comprising "perovskitizer" and interlayer cations.

Improve the Charge Carrier Transporting in Two-Dimensional Ruddlesden-Popper Perovskite Solar Cells

Conventional 3D organic-inorganic halide perovskite materials have shown substantial potential in the field of optoelectronics, enabling the power conversation efficiency of solar cells beyond 26%. A key challenge limiting the further commercial application of 3D perovskite solar cells is their inherent instability over outer oxygen, humidity, light, and heat. By contrast, 2D Ruddlesden-Popper (2DRP) perovskites with bulky organic cations can effectively stabilize the inorganic slabs, yielding excellent environmental stability. However, the efficiencies of 2DRP perovskite solar cells are much lower than those of the 3D counterparts due to poor charge carrier transporting property of insulating bulky organic cations. Their inner structural, dielectric, optical, and excitonic properties remain to be primarily studied. In this review, the main reasons for the low efficiency of 2DRP perovskite solar cells are first analyzed. Next, a detailed description of various strategies for improving the charge carrier transporting of 2DRP perovskites is provided, such as bandgap regulation, perovskite crystal phase orientation and distribution, energy level matching, interfacial modification, etc. Finally, a summary is given, and the possible future research directions and methods to achieve high-efficiency and stable 2DRP perovskite solar cells are rationalized.

Manipulation of Crystal Orientation and Phase Distribution of Quasi-2D Perovskite through Synergistic Effect of Additive Doping and Spacer Engineering

The diammonium precursor 1,4-phenylenedimethanammonium (PDMA) was used as a large organic spacer for the preparation of Dion-Jacobson-type quasi-2D perovskites (PDMA)(MA)n-1PbnI3n+1 (MA = methylammonium). Films with composition ⟨n⟩ = 5 comprised randomly orientated grains and multiple microstructural domains with locally differing n values. However, by mixing the Dion-Jacobson-type spacer PDMA and the Ruddlesden-Popper-type spacer propylammonium (PA), the crystal orientation in both the vertical and the horizonal directions became regulated. High crystallinity owing to well-matched interlayer distances was observed. Combining this spacer-engineering approach with the addition of methylammonium chloride (MACl) led to full vertical alignment of the crystal orientation. Moreover, the microstructural domains at the substrate interface changed from low-n (n = 1, 2, 3) to high-n (n = 4, 5), which may be beneficial for hole extraction at the interface between perovskite and hole transport layer due to a more finely tuned band alignment. Our work sheds light on manipulating the crystallization behavior of quasi-2D perovskite and further paves the way for highly stable and efficient perovskite devices.

Color-tunable stable quasi-2D hybrid metal halide perovskites: synthesis, characterization, and optical analysis

Metal halide perovskites show remarkable optical properties and useful applications in optoelectronic devices. However, the instability of three-dimensional (3D) metal halide perovskites limits their applications, leading to the emergence of more stable two-dimensional (2D) metal halide perovskites. Herein, we present a facile synthesis of the 2D hybrid metal halide perovskite (EDA)(MA)n-1PbnBr3n+1 (EDA: ethylene diammonium, MA: methylammonium), where n = 1-6, and MAPbBr3 perovskite layers using an anti-solvent co-precipitation technique. The synthesized materials exhibited tunable optical properties, and the color emissions of pure EDAPbBr4 and (EDA)(MA)2Pb3Br10 perovskites were successfully tailored by altering halide anion layers. The band gap decreases as the value of n in the (EDA)(MA)n-1PbnBr3n+1 compound increases from 1 to 6. The as-prepared materials were characterized using X-ray diffraction (XRD) technique, Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-Vis), photoluminescence spectroscopy (PL), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDX). Finally, the stability of the 2D hybrid metal halide perovskite structures was evaluated under ambient conditions over different periods. Their tunable color emission was investigated and robust fluorescence was observed after 55 days. Thus, this study provides valuable insights into the synthesis and characterization of 2D hybrid metal halide perovskites for tunable color emission, highlighting their potential for use in various optoelectronic applications.

The Synergistic Effect of Additives for Formamidinium-Based Inverted Dion-Jacobson 2D Perovskite Solar Cells with Enhanced Photovoltaic Performance

Two-dimensional (2D) perovskite solar cells (PSCs) have attracted rapid growing attention due to their excellent environmental and operational stability. As an important type of 2D perovskite, Dion-Jacobson (DJ) 2D perovskites exhibit better structural integrity and more stable optoelectronic properties than those of Ruddlesden-Popper (RP) ones because of the elimination of weak van der Waals interactions. Random phase distribution, phase impurity, and weak crystallinity, however, can lead to severe nonradiative recombination losses in 2D perovskites and inferior device stability. Herein, formamidinium chloride (FACl) and lead chloride (PbCl2) are selected as additives to fabricate efficient and stable DJ 2D PSCs. The synergistic effect of additives could efficiently induce crystallization and suppress the low-n phase perovskites. The obtained 2D perovskites exhibit extended charge lifetime and enhanced charge transfer. The corresponding PSC device delivers an efficiency of 16.63% with a significantly improved open-circuit voltage (VOC) of 1.18 V and a fill factor (FF) of 81.65% than the control one. This PCE ranks the highest for inverted FA-based 2D DJ PSCs. Moreover, this device has exhibited exceptional long-term stability, which retains more than 95% of the initial efficiencies at about 50% relative humidity for 600 h.

Layered Low-Dimensional Ruddlesden-Popper and Dion-Jacobson Perovskites: From Material Properties to Photovoltaic Device Performance

Layered low-dimensional halide perovskites (LDPs) with multiple quantum well structure have shown increasing research interest in photovoltaic solar cell applications owing to their intrinsic moisture stability and favorable photophysical properties in comparison with their three-dimensional (3D) counterparts. The most common LDPs are Ruddlesden-Popper (RP) phases and Dion-Jacobson (DJ) phases, both of which have made significant research advances in efficiency and stability. However, distinct interlayer cations between RP and DJ phase lead to disparate chemical bonds and different perovskite structures, which endow RP and DJ perovskite with distinctive chemical and physical properties. Plenty of reviews have reported the research progress of LDPs but no summary has elaborated from the perspective of the merits and drawbacks of the RP and DJ phases. Herein, in this review, we offer a comprehensive expound on the merits and promises of RP and DJ LDPs from their chemical structure, physicochemical properties, and photovoltaic performance research progress aiming to provide a new insight into the dominance of RP and DJ phases. Then, we reviewed the recent progress on the synthesis and implementation of RP and DJ LDPs thin films and devices, as well as their optoelectronic properties. Finally, we discussed the possible strategies to resolve existing toughs to realize the desired high-performance LDPs solar cells.

Selenophene-Based 2D Ruddlesden-Popper Perovskite Solar Cells with an Efficiency Exceeding 19

Two-dimensional (2D) Ruddlesden-Popper (RP) perovskites have emerged as attractive candidates for high-performance perovskite solar cells (PSCs) thanks to their superior environmental and structural stability. However, 2D RP PSCs exhibit larger exciton binding energy due to the dielectric mismatch between the organic and inorganic layers, resulting in poorer photovoltaic performance compared to their 3D analogs. Here, we developed a selenophene-based spacer, namely, 2-selenophenemethylammonium (SeMA), for stable and efficient 2D RP PSCs. The 2D perovskite film using methylammonium (MA) as the A-site cation (nominal n = 5) shows excellent film quality with large grain size and a preferred vertical orientation relative to the substrate. Furthermore, we have successfully demonstrated the effectiveness of a predeposition transport layer (PDTL) consisting of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in passivating surface defects of the perovskite film and inducing densification of the upper PCBM electron transport layer. This densification promotes efficient extraction and transport of electrons. The optimized PSCs based on 2D RP perovskite using MA as A-site cation (nominal n = 5) achieved a power conversion efficiency (PCE) of 17.25%, which was further boosted to 19.03% when using formamidinium (FA) as A-site cation. This represents a record PCE of 2D RP PSCs by using the selenophene-based spacer. Moreover, these 2D RP PSCs significantly improve thermal, moisture, and light stability. Our results provide significant implications for the synergistic strategy of developing selenophene-based spacers and device engineering methods for achieving highly efficient and stable 2D RP perovskite solar cells.

Mutually Tuned Dual Additive Engineering Synergistically Enhances the Photovoltaic Performance of Tin-Based Perovskite Solar Cells

Tin-based perovskite solar cells (T-PSCs) have become the star photovoltaic products in recent years due to their low environmental toxicity and superior photovoltaic performance. However, the easy oxidation of Sn2+ and the energy level mismatch between the perovskite film and charge transport layer limit its efficiency. In order to regulate the microstructure and photoelectric properties of tin-based perovskite films to enhance the efficiency and stability of T-PSCs, guanidinium bromide (GABr) and organic Lewis-based additive methylamine cyanate (MAOCN) are introduced into the FA0.9PEA0.1SnI3-based perovskite precursor. A series of characterizations show that the interactions between additive molecules and perovskite mutually reconcile to improve the photovoltaic performance of T-PSCs. The introduction of GABr can adjust the band gap of the perovskite film and energy level alignment of T-PSCs. They significantly increase the open-circuit voltage (Voc). The MAOCN material can form hydrogen bonds with SnI2 in the precursor, which can inhibit the oxidation of Sn2+ and significantly improve the short-circuit current density (Jsc). The synergistic modulation of the dual additives reduces the trap-state density and improves photovoltaic performance, resulting in an increased champion efficiency of 9.34 for 5.22% of the control PSCs. The unencapsulated T-PSCs with GABr and MAOCN dual additives prepared in the optimized process can retain more than 110% of their initial efficiency after aging for 1750 h in a nitrogen glovebox, but the control PSCs maintain only 50% of their initial efficiency kept in the same conditions. This work provides a new perspective to further improve the efficiency and stability of T-PSCs.

Synergy of 3D and 2D Perovskites for Durable, Efficient Solar Cells and Beyond

Three-dimensional (3D) organic-inorganic lead halide perovskites have emerged in the past few years as a promising material for low-cost, high-efficiency optoelectronic devices. Spurred by this recent interest, several subclasses of halide perovskites such as two-dimensional (2D) halide perovskites have begun to play a significant role in advancing the fundamental understanding of the structural, chemical, and physical properties of halide perovskites, which are technologically relevant. While the chemistry of these 2D materials is similar to that of the 3D halide perovskites, their layered structure with a hybrid organic-inorganic interface induces new emergent properties that can significantly or sometimes subtly be important. Synergistic properties can be realized in systems that combine different materials exhibiting different dimensionalities by exploiting their intrinsic compatibility. In many cases, the weaknesses of each material can be alleviated in heteroarchitectures. For example, 3D-2D halide perovskites can demonstrate novel behavior that neither material would be capable of separately. This review describes how the structural differences between 3D halide perovskites and 2D halide perovskites give rise to their disparate materials properties, discusses strategies for realizing mixed-dimensional systems of various architectures through solution-processing techniques, and presents a comprehensive outlook for the use of 3D-2D systems in solar cells. Finally, we investigate applications of 3D-2D systems beyond photovoltaics and offer our perspective on mixed-dimensional perovskite systems as semiconductor materials with unrivaled tunability, efficiency, and technologically relevant durability.

Carriers, Quasi-particles, and Collective Excitations in Halide Perovskites

Halide perovskites (HPs) are potential game-changing materials for a broad spectrum of optoelectronic applications ranging from photovoltaics, light-emitting devices, lasers to radiation detectors, ferroelectrics, thermoelectrics, etc. Underpinning this spectacular expansion is their fascinating photophysics involving a complex interplay of carrier, lattice, and quasi-particle interactions spanning several temporal orders that give rise to their remarkable optical and electronic properties. Herein, we critically examine and distill their dynamical behavior, collective interactions, and underlying mechanisms in conjunction with the experimental approaches. This review aims to provide a unified photophysical picture fundamental to understanding the outstanding light-harvesting and light-emitting properties of HPs. The hotbed of carrier and quasi-particle interactions uncovered in HPs underscores the critical role of ultrafast spectroscopy and fundamental photophysics studies in advancing perovskite optoelectronics.

Performance Enhancement of Lead-Free 2D Tin Halide Perovskite Transistors by Surface Passivation and Its Impact on Non-Volatile Photomemory Characteristics

Two-dimensional (2D) tin (Sn)-based perovskites have recently received increasing research attention for perovskite transistor application. Although some progress is made, Sn-based perovskites have long suffered from easy oxidation from Sn²⁺ to Sn⁴⁺ , leading to undesirable p-doping and instability. In this study, it is demonstrated that surface passivation by phenethylammonium iodide (PEAI) and 4-fluorophenethylammonium iodide (FPEAI) effectively passivates surface defects in 2D phenethylammonium tin iodide (PEA₂ SnI₄ ) films, increases the grain size by surface recrystallization, and p-dopes the PEA₂ SnI₄ film to form a better energy-level alignment with the electrodes and promote charge transport properties. As a result, the passivated devices exhibit better ambient and gate bias stability, improved photo-response, and higher mobility, for example, 2.96 cm² V^-1 s^-1 for the FPEAI-passivated films-four times higher than the control film (0.76 cm² V^-1 s^-1 ). In addition, these perovskite transistors display non-volatile photomemory characteristics and are used as perovskite-transistor-based memories. Although the reduction of surface defects in perovskite films results in reduced charge retention time due to lower trap density, these passivated devices with better photoresponse and air stability show promise for future photomemory applications.

Seeding Agents in Metal Halide Perovskite Solar Cells: From Material to Mechanism

Metal halide perovskite solar cells (PSCs) have been showing up in the commercial field, with an inspiring power conversion efficiency (PCE) of over 26 % in the laboratory. The quality of perovskite films is still a bottleneck due to the random and fast crystallization of ionic perovskite materials. Seeding agent-mediated crystallization has consistently been recognized as an efficient method for preparing bulk single crystals and high-quality films. Herein, we summarized the seeding mechanism, characterization techniques, and seeding agents working in different locations during PSC device fabrication. This Review could further facilitate researchers with a deeper understanding of seeding agents and enhance more choices for seeding crystallization to improve the performance further and the device's large-scale fabrication toward commercialization.

Oxidation-resistant all-perovskite tandem solar cells in substrate configuration

The commonly-used superstrate configuration (depositing front subcell first and then depositing back subcell) in all-perovskite tandem solar cells is disadvantageous for long-term stability due to oxidizable narrow-bandgap perovskite assembled last and easily exposable to air. Here we reverse the processing order and demonstrate all-perovskite tandems in a substrate configuration (depositing back subcell first and then depositing front subcell) to bury oxidizable narrow-bandgap perovskite deep in the device stack. By using guanidinium tetrafluoroborate additive in wide-bandgap perovskite subcell, we achieve an efficiency of 25.3% for the substrate-configured all-perovskite tandem cells. The unencapsulated devices exhibit no performance degradation after storage in dry air for 1000 hours. The substrate configuration also widens the choice of flexible substrates: we achieve 24.1% and 20.3% efficient flexible all-perovskite tandem solar cells on copper-coated polyethylene naphthalene and copper metal foil, respectively. Substrate configuration offers a promising route to unleash the commercial potential of all-perovskite tandem solar cells.

Secondary Anti-Solvent Treatment for Efficient 2D Dion-Jacobson Perovskite Solar Cells

Surface defects-mediated nonradiative recombination plays a critical role in the performance and stability of perovskite solar cells (PSCs) and surface post-treatment is widely used for efficient PSCs. However, the commonly used surface passivation strategies are one-off and the passivation defect ability is limited, which can only solve part of the defects in the topmost surface area. Here, a secondary anti-solvent strategy is proposed to further reduce surface defects based on conventional surface passivation for the first time. Based on this, the crystallization quality of 2D Dion-Jacobson perovskite is enhanced and the surface defects density is further reduced by nearly two orders. In addition, a gradient structure of perovskite with n = 2 phases located at the top of the film and 3D-like phases located at the bottom of the film can also be obtained. The modulated perovskite film boosts the efficiency of 2D perovskites (n = 5) up to 19.55%. This strategy is also very useful in other anti-solvent processed perovskite dipping systems, which paves a promising avenue for minimizing surface defects toward highly efficient perovskite devices.

Lasing in Two-Dimensional Tin Perovskites

Two-dimensional (2D) perovskites have been proposed as materials capable of improving the stability and surpassing the radiative recombination efficiency of three-dimensional perovskites. However, their luminescent properties have often fallen short of what has been expected. In fact, despite attracting considerable attention for photonic applications during the last two decades, lasing in 2D perovskites remains unclear and under debate. Here, we were able to improve the optical gain properties of 2D perovskite and achieve optically pumped lasing. We show that the choice of the spacer cation affects the defectivity and photostability of the perovskite, which in turn influences its optical gain. Based on our synthetic strategy, we obtain PEA₂SnI₄ films with high crystallinity and favorable optical properties, resulting in amplified spontaneous emission (ASE) with a low threshold (30 μJ/cm²), a high optical gain above 4000 cm^-1 at 77 K, and ASE operation up to room temperature.

Using the Diamagnetic Coefficients to Estimate the Reduced Effective Mass in 2D Layered Perovskites: New Insight from High Magnetic Field Spectroscopy

In this work, the current state of research concerning the determination of the effective mass in 2D layered perovskites is presented. The available experimental reports in which the reduced effective mass μ has been directly measured using magneto-absorption spectroscopy of interband Landau levels are reviewed. By comparing these results with DFT computational studies and various other methods, it is concluded that depending on the approach used, the μ found spans a broad range of values from as low as 0.05 up to 0.3 me. To facilitate quick and reliable estimation of μ, a model is proposed based solely on the available experimental data that bypass the complexity of interband Landau level spectroscopy. The model takes advantage of the μ value measured for (PEA)2PbI4 and approximates the reduced effective mass of the given 2D layered perovskites based on only two experimental parameters—the diamagnetic coefficient and the effective dielectric constant. The proposed model is tested on a broad range of 2D layered perovskites and captures well the main experimental and theoretical trends.

Dynamic Nuclear Polarization Enables NMR of Surface Passivating Agents on Hybrid Perovskite Thin Films

Surface and bulk molecular modulators are the key to improving the efficiency and stability of hybrid perovskite solar cells. However, due to their low concentration, heterogeneous environments, and low sample mass, it remains challenging to characterize their structure and dynamics at the atomic level, as required to establish structure–activity relationships. Nuclear magnetic resonance (NMR) spectroscopy has revealed a wealth of information on the atomic-level structure of hybrid perovskites, but the inherent insensitivity of NMR severely limits its utility to characterize thin-film samples. Dynamic nuclear polarization (DNP) can enhance NMR sensitivity by orders of magnitude, but DNP methods for perovskite materials have so far been limited. Here, we determined the factors that limit the efficiency of DNP NMR for perovskite samples by systematically studying layered hybrid perovskite analogues. We find that the fast-relaxing dynamic cation is the major impediment to higher DNP efficiency, while microwave absorption and particle morphology play a secondary role. We then show that the former can be mitigated by deuteration, enabling ¹H DNP enhancement factors of up to 100, which can be harnessed to enhance signals from dopants or additives present in very low concentrations. Specifically, using this new DNP methodology at a high magnetic field and with small sample volumes, we have recorded the NMR spectrum of the 20 nm (6 μg) passivating layer on a single perovskite thin film, revealing a two-dimensional (2D) layered perovskite structure at the surface that resembles the n = 1 homologue but which has greater disorder than in bulk layered perovskites.

Recognizing the Importance of Fast Nonisothermal Crystallization for High-Performance Two-Dimensional Dion-Jacobson Perovskite Solar Cells with High Fill Factors: A Comprehensive Mechanistic Study

Two-dimensional (2D) Dion-Jacobson (DJ) perovskite solar cells (PSCs), despite their advantage in versatility of n-layer variation, are subject to poor photovoltaic efficiency, particularly in the fill factor (FF), compared to their three-dimensional counterparts. To enhance the performance of DJ PSCs, the process of growing crystals and hence the corresponding morphology of DJ perovskites are of prime importance. Herein, we report the fast nonisothermal (NIT) crystallization protocol that is previously unrecognized for 2D perovskites to significantly improve the morphology, orientation, and charge transport of the DJ perovskite films. Comprehensive mechanistic studies reveal that the NIT effect leads to the secondary crystallization stage, forming network-like channels that play a vital role in the FF's leap-forward improvement and hence the DJ PSC's performance. As a whole, the NIT crystallized PSCs demonstrate a high power conversion efficiency and an FF of up to 19.87 and 86.16%, respectively. This research thus provides new perspectives to achieve highly efficient DJ PSCs.

Highly Efficient and Stable 2D Dion Jacobson/3D Perovskite Heterojunction Solar Cells

Heterostructures involving two-dimensional/three-dimensional (2D/3D) perovskites have recently attracted increased attention due to their ability to combine the high photovoltaic performance of 3D perovskites with the increased stability of 2D perovskites. Here we report ammonium thiocyanate (NH₄SCN) passivated 3D methylammonium lead triiodide (MAPbI₃) perovskite active layer and deposition of 2D perovskite capping layer using xylylene diammonium iodide (XDAI) organic cation. The 2D/3D perovskite heterojunction formation is probed by using FESEM and UPS spectroscopy. The NH₄SCN passivated MAPbI₃ perovskite has shown 19.6% PCE compared to the 17.18% PCE of pristine MAPbI₃ perovskite solar cells (PSCs). Finally, the champion 2D/3D perovskite heterojunction based solar cells have achieved the remarkable PCE of 20.74%. The increased PCE in 2D/3D PSCs is mainly attributed to the reduced defect density and suppressed nonradiative recombination losses. Moreover, the hydrophobic 2D capping layer endows the 2D/3D heterojunction perovskites with exceptional moisture, thermal and UV stability, highlighting the promise of highly stable and efficient 2D/3D PSCs.

Modulation of Perovskite Grain Boundaries by Electron Donor-Acceptor Zwitterions R,R-Diphenylamino-phenyl-pyridinium-(CH2) n -sulfonates: All-Round Improvement on the Solar Cell Performance

Inverted perovskite solar cells (PSCs) have attracted intense attention because of their insignificant hysteresis and low-temperature fabrication process. However, the efficiencies of inverted PSCs are still inferior to those of commercialized silicon solar cells. Also, the poor stability of PSCs is one of the major impedances to commercialization. Herein, we rationally designed and synthesized a new series of electron donor (R,R-diphenylamino) and acceptor (pyridimium-(CH₂) _n -sulfonates) zwitterions as a boundary modulator and systematically investigated their associated interface properties. Comprehensive physical and optoelectronic studies verify that these zwitterions provide a four-in-one functionality: balancing charge carrier transport, suppressing less-coordinated Pb²⁺ defects, enhancing moisture resistance, and reducing ion migration. Although each functionality may have been reported by specific passivating molecules, a strategy that simultaneously regulates the charge-transfer balance and three other functionalities has not yet been developed. The results are to make an omnidirectional improvement of PSCs. Among all zwitterions, 4-(4-(4-(di-(4-methoxylphenyl)amino)phenyl)propane-1-ium-1-yl)butane-1-sulfonate (OMeZC3) optimizes the balance hole/electron mobility ratio of perovskite to 0.91, and the corresponding PSCs demonstrate a high power conversion efficiency (PCE) of up to 23.15% free from hysteresis, standing out as one of the champion PSCs with an inverted structure. Importantly, the OMeZC3-modified PSC exhibits excellent long-term stability, maintaining almost its initial PCE after being stored at 80% relative humidity for 35 days.

Origin of Contrasting Emission Spectrum of Bromide versus Iodide Layered Perovskite Semiconductors

The origin of broadband emission is studied using temperature-dependent time-resolved photoluminescence (PL) spectra for two-dimensional (2D) layered halide perovskites (i.e., (PEA)₂PbBr₄ = phenylethylammonium lead bromide and (PEA)₂PbI₄ = phenylethylammonium lead iodide) semiconductors. Both perovskite systems show only a single peak exciton emission at room temperature, which becomes multipeak exciton emissions at low temperatures. For temperatures below 100 K, the (PEA)₂PbBr₄ film gives broad PL emission, Stokes shifted by 750 meV from narrow exciton emission peaks, whereas the (PEA)₂PbI₄ film does not show any broad emission. Kinetics of various peaks could provide useful insight to propose a consistent energy level scheme associated with a barrier (PEA) and well (PbX₆^4-) material system's electronic states. This broad emission in (PEA)₂PbBr₄ perovskite is observed due to coupling of triplet states in the inorganic well (PbBr₆^4-) and organic barrier (PEA) layer, which is in contrast to a proposed model based on self-trapped exciton.

Reducing Energy Disorder for Efficient and Stable Sn−Pb Alloyed Perovskite Solar Cells

An organic small molecule, 1-bromo-4-(methylsulfinyl)benzene (BBMS), was utilized to reduce the energy disorder of a Sn-Pb alloyed perovskite film via hydrogen bonding and coordination bonding interactions, and the resultant BBMS-treated device showed a high efficiency of over 22 % as well as outstanding long-term stability.

Emerging doping strategies in two-dimensional hybrid perovskite semiconductors for cutting edge optoelectronics applications

The past decade has witnessed tremendous progress in metal halide perovskites, particularly in lead (Pb) halide perovskites, because of their extraordinary performance in cutting-edge optoelectronic devices. However, the toxicity of Pb and the environmental stability of the perovskites are two major issues that this field is currently facing. In recent years, 2D layered perovskites have emerged as a promising alternative to the traditional 3D perovskites due to their structural flexibility and higher environmental stability, though they lack the desired level of device efficiency. Doping with target ions can drastically tune the crystal structure, optical properties, charge recombination dynamics, and electronic properties of the 2D perovskite. Although the field of doping in 2D perovskites has seen substantial growth in recent times, no comprehensive review is available on the recent advances in doping of 2D perovskites and its effect on the optoelectronic properties. In this review, we summarize the progress in doping in 2D perovskites based on different doping sites including progress in different synthesis strategies and their impact on crystal structures and various optoelectronic properties. We then highlight the recent achievements in doped 2D perovskites for photovoltaic, LED and other emerging applications. Finally, we conclude with the challenges and the future scope in the doping studies of 2D layered perovskites, which need to be addressed for further developments of next-generation 2D perovskite-based optoelectronic devices.

Charge-Carrier Transport in Quasi-2D Ruddlesden-Popper Perovskite Solar Cells

In recent years, 2D Ruddlesden-Popper (2DRP) perovskite materials have been explored as emerging semiconductor materials in solar cells owing to their excellent stability and structural diversity. Although 2DRP perovskites have achieved photovoltaic efficiencies exceeding 19%, their widespread use is hindered by their inferior charge-carrier transport properties in the presence of diverse organic spacer cations, compared to that of traditional 3D perovskites. Hence, a systematic understanding of the carrier transport mechanism in 2D perovskites is critical for the development of high-performance 2D perovskite solar cells (PSCs). Here, the recent advances in the carrier behavior of 2DRP PSCs are summarized, and guidelines for successfully enhancing carrier transport are provided. First, the composition and crystal structure of 2DRP perovskite materials that affect carrier transport are discussed. Then, the features of 2DRP perovskite films (phase separation, grain orientation, crystallinity kinetics, etc.), which are closely related to carrier transport, are evaluated. Next, the principal direction of carrier transport guiding the selection of the transport layer is revealed. Finally, an outlook is proposed and strategies for enhancing carrier transport in high-performance PSCs are rationalized.

Tuning Precursor-Amine Interactions for Light-Emitting Lead Bromide Perovskites

Organic additives with amino moieties are effective in improving the properties of archetypical formamidinium (FA)-based hybrid perovskites for photovoltaic and light-emitting applications. However, a detailed understanding of how amino additives affect the perovskite materials is lacking, impeding developments in this area. Here, by investigating the interactions of lead bromide perovskite precursors with phenethylamine (PEA) and its derivatives with small variations in chemical structure, we reveal that only the secondary amine (N-methyl-2-phenylethylamine (N-PEA)) results in strengthened hydrogen bonds with FABr in precursor solutions, allowing the formation of high-quality perovskite films. The photoluminescence quantum efficiencies (PLQEs) of the resultant perovskite samples on widely used charge-transport substrates are retained to 82% of their original values, indicating reduced sensitivity to interfacial nonradiative traps critical to device applications. Using a standard device structure, green perovskite light-emitting diodes with peak external quantum efficiencies of 12.7% at ∼500 cd m^-2 and operational lifetimes (T₅₀) exceeding 10 h (at 100 cd m^-2) are obtained.

Zwitterion-Assisted Crystal Growth of 2D Perovskites with Unfavorable Phase Suppression for High-Performance Solar Cells

Despite two-dimensional (2D) Ruddlesden-Popper-phase layered perovskites (RPLPs) exhibiting excellent environmental stability, most solar cells based on 2D RPLP films are fabricated in a controlled inert atmosphere. Meanwhile, the poor charge transport of 2D RPLP films owing to the unfavorable phase arrangement and defects limits the efficiency of 2D RPLP solar cells. Here, we fabricate high-efficiency 2D RPLP solar cells in ambient air assisted by a zwitterion (ZW) additive. We show that the ZW additive suppresses the formation of the bottom 2D phases (n ≤ 2) and the top 3D-like phases in 2D RPLP films. These 2D phases usually grow parallel to the substrate and act as trap sites that inhibit charge transport in the vertical direction. The 3D-like phases, on the other hand, aggravate the long-term stability due to the intrinsic instability of MA⁺ cations. With improved phase distribution, crystal orientation, and reduced trap states in 2D RPLP films, efficient charge transport is obtained. Finally, a record-high open-circuit voltage (V_oc) of 1.19 V and a power conversion efficiency of 17.04% with an enhanced stability are achieved for (BA_0.9PEA_0.1)₂MA₃Pb₄I₁₃-based (n = 4) solar cells fabricated under high humidity (∼65% RH).

Shedding light on the energy applications of emerging 2D hybrid organic-inorganic halide perovskites

Unique performance of the hybrid organic-inorganic halide perovskites (HOIPs) has attracted great attention because of their continuous exploration and breakthrough in a multitude of energy-related applications. However, the instability and lead-induced toxicity that arise in bulk perovskites are the two major challenges that impede their future commercialization process. To find a solution, a series of two-dimensional HOIPs (2D HOIPs) are investigated to prolong the device lifetime with highly efficient photoelectric conversion and energy storage. Herein, the recent advances of 2D HOIPs and their structural derivatives for the energy realms are summarized and discussed. The basic understanding of crystal structures, physicochemical properties, and growth mechanisms is presented. In addition, the current challenges and future directions to provide a roadmap for the development of next generation 2D HOIPs are prospected

Tailoring Interlayer Spacers for Efficient and Stable Formamidinium-Based Low-Dimensional Perovskite Solar Cells

2D Dion-Jacobson (DJ) perovskite solar cells generally show mediocre device performances as they are restrained by their defective film quality. The rigid diammonium organic interlayer spacers are intolerant to lattice mismatches, which induces defects and distortions and ultimately deteriorates the optoelectronic properties. Herein, a secondary interlayer spacer is introduced into formamidinium (FA)-based low-dimensional perovskite, which substantially improves the film quality. The flexible monovalent spacer cations effectively alleviate lattice distortions and reduce crystal defects, providing perovskite films with desirable microscopic morphology, preferable crystal orientation, reduced defect states, and improved charge transport capability. As a result, the optimized perovskite solar cell based on the (PDA_0.9 PA_0.2 )(FA)₃ Pb₄ I₁₃ (PDA = propane-1,3-diammonium, PA = propylammonium) film exhibits the exceptional power conversion efficiency of 16.0%, the highest reported value in its class. In addition, the device demonstrates the enhanced thermal stability, retaining 90% of its initial efficiency after aging at 85 °C for 800 h.

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.

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

Two-dimensional (2D) perovskites are known as one of the most promising luminescent materials due to their structural diversity and outstanding optoelectronic properties. Compared with 3D perovskites, 2D perovskites have natural quantum well structures, large exciton binding energy (E_b) and outstanding thermal stability, which shows great potential in the next-generation displays and solid-state lighting. In this review, the fundamental structure, photophysical and electrical properties of 2D perovskite films were illustrated systematically. Based on the advantages of 2D perovskites, such as special energy funnel process, ultra-fast energy transfer, dense film and low efficiency roll-off, the remarkable achievements of 2D perovskite light-emitting diodes (PeLEDs) are summarized, and exciting challenges of 2D perovskite are also discussed. An outlook on further improving the efficiency of pure-blue PeLEDs, enhancing the operational stability of PeLEDs and reducing the toxicity to push this field forward was also provided. This review provides an overview of the recent developments of 2D perovskite materials and LED applications, and outlining challenges for achieving the high-performance devices.