Over 21% Efficiency Stable 2D Perovskite Solar Cells

Rational Strategies to Improve the Efficiency of 2D Perovskite Solar Cells

In the quest for durable photovoltaic devices, 2D halide perovskites have emerged as a focus of extensive research. However, the reduced dimension in structure is accompanied by inferior optical-electrical properties, such as widened band gap, enhanced exciton binding energy, and obstructed charge transport. As a result, the efficiency of 2D perovskite solar cells (PSCs) lags significantly behind their 3D counterparts. To overcome these constraints, extensive investigations into materials and processing techniques are pursued rigorously to augment the efficiency of 2D PSCs. Herein, The cutting-edge delve into developments in 2D PSCs, with a focus on chemical and material engineering, as well as their structure and photovoltaic properties. The review starts with an introduction of the crystal structure, followed by the key evaluation criteria of 2D PSCs. Then, the strategies around solution chemical engineering, processing technique, and interface optimization, to simultaneously boost efficiency and stability are systematically discussed. Finally, the challenges and perspectives associated with 2D perovskites to provide insights into potential improvements in photovoltaic performance will be outlined.

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%.

2D Hybrid Perovskites: From Static and Dynamic Structures to Potential Applications

2D perovskites have received great attention recently due to their structural tunability and environmental stability, making them highly promising candidates for various applications by breaking property bottlenecks that affect established materials. However, in 2D perovskites, the complicated interplay between organic spacers and inorganic slabs makes structural analysis challenging to interpret. A deeper understanding of the structure-property relationship in these systems is urgently needed to enable high-performance tunable optoelectronic devices. Herein, this study examines how structural changes, from constant lattice distortion and variable structural evolution, modeled with both static and dynamic structural descriptors, affect macroscopic properties and ultimately device performance. The effect of chemical composition, crystallographic inhomogeneity, and mechanical-stress-induced static structural changes and corresponding electronic band variations is reported. In addition, the structure dynamics are described from the viewpoint of anharmonic vibrations, which impact electron-phonon coupling and the carriers' dynamic processes. Correlated carrier-matter interactions, known as polarons and acting on fine electronic structures, are then discussed. Finally, reliable guidelines to facilitate design to exploit structural features and rationally achieve breakthroughs in 2D perovskite applications are proposed. This review provides a global structural landscape of 2D perovskites, expected to promote the prosperity of these materials in emerging device applications.

Compression of Organic Molecules Coupled with Hydrogen Bonding Extends the Charge Carrier Lifetime in BA2SnI4

Two-dimensional (2D) metal halide perovskites, such as BA2SnI4 (BA═CH3(CH2)3NH3), exhibit an enhanced charge carrier lifetime in experiments under strain. Experiments suggest that significant compression of the BA molecule, rather than of the inorganic lattice, contributes to this enhancement. To elucidate the underlying physical mechanism, we apply a moderate compressive strain to the entire system and subsequently introduce significant compression to the BA molecules. We then perform ab initio nonadiabatic molecular dynamics simulations of nonradiative electron-hole recombination. We observe that the overall lattice compression reduces atomic motions and decreases nonadiabatic coupling, thereby delaying electron-hole recombination. Additionally, compression of the BA molecules enhances hydrogen bonding between the BA molecules and iodine atoms, which lengthens the Sn-I bonds, distorts the [SnI6]4- octahedra, and suppresses atomic motions further, thus reducing nonadiabatic coupling. Also, the elongated Sn-I bonds and weakened antibonding interactions increase the band gap. Altogether, the compression delays the nonradiative electron-hole recombination by more than a factor of 3. Our simulations provide new and valuable physical insights into how compressive strain, accommodated primarily by the organic ligands, positively influences the optoelectronic properties of 2D layered halide perovskites, offering a promising pathway for further performance improvements.

Low-temperature strain-free encapsulation for perovskite solar cells and modules passing multifaceted accelerated ageing tests

Perovskite solar cells promise to be part of the future portfolio of photovoltaic technologies, but their instability is slow down their commercialization. Major stability assessments have been recently achieved but reliable accelerated ageing tests on beyond small-area cells are still poor. Here, we report an industrial encapsulation process based on the lamination of highly viscoelastic semi-solid/highly viscous liquid adhesive atop the perovskite solar cells and modules. Our encapsulant reduces the thermomechanical stresses at the encapsulant/rear electrode interface. The addition of thermally conductive two-dimensional hexagonal boron nitride into the polymeric matrix improves the barrier and thermal management properties of the encapsulant. Without any edge sealant, encapsulated devices withstood multifaceted accelerated ageing tests, retaining >80% of their initial efficiency. Our encapsulation is applicable to the most established cell configurations (direct/inverted, mesoscopic/planar), even with temperature-sensitive materials, and extended to semi-transparent cells for building-integrated photovoltaics and Internet of Things systems.

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.

Two-Dimensional Materials for Highly Efficient and Stable Perovskite Solar Cells

Perovskite solar cells (PSCs) offer low costs and high power conversion efficiency. However, the lack of long-term stability, primarily stemming from the interfacial defects and the susceptible metal electrodes, hinders their practical application. In the past few years, two-dimensional (2D) materials (e.g., graphene and its derivatives, transitional metal dichalcogenides, MXenes, and black phosphorus) have been identified as a promising solution to solving these problems because of their dangling bond-free surfaces, layer-dependent electronic band structures, tunable functional groups, and inherent compactness. Here, recent progress of 2D material toward efficient and stable PSCs is summarized, including its role as both interface materials and electrodes. We discuss their beneficial effects on perovskite growth, energy level alignment, defect passivation, as well as blocking external stimulus. In particular, the unique properties of 2D materials to form van der Waals heterojunction at the bottom interface are emphasized. Finally, perspectives on the further development of PSCs using 2D materials are provided, such as designing high-quality van der Waals heterojunction, enhancing the uniformity and coverage of 2D nanosheets, and developing new 2D materials-based electrodes.

Enhanced Charge Transfer in Quasi-2D Perovskite by Formamidinium Cation Gradient Incorporation for Efficient and Stable Solar Cells

Quasi-2D perovskites have attracted much attention in perovskite photovoltaics due to their excellent stability. However, their photoelectric conversion efficiency (PCE) still lags 3D counterparts, particularly with high short-circuit current (JSC) loss. The quantum confinement effect is pointed out to be the sole reason, which introduces widened bandgap and poor exciton dissociation, and undermines the light capture and charge transport. Here, the gradient incorporation of formamidinium (FA) cations into quasi-2D perovskite is proposed to address this issue. It is observed that FA prefers to incorporate into the larger n value phases near the film surface compared to the smaller n value phases in the bulk, resulting in a narrow bandgap and gradient structure within the film. Through charge dynamic analysis using in situ light-dark Kelvin probe force microscopy and transient absorption spectroscopy, it is demonstrated that incorporating 10% FA significantly facilitates efficient charge transfer between low n-value phases in the bulk and high n-value nearby film surface, leading to reduced charge accumulation. Ultimately, the device based on (AA)2(MA0.9FA0.1)4Pb5I16, where AA represents n-amylamine renowned for its exceptional environmental stability as a bulky organic ligand, achieves an impressive power conversion efficiency (PCE) of 18.58% and demonstrates enhanced illumination and thermal stability.

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.

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.

Perovskite Smart Windows: The Light Manipulator in Energy-Efficient Buildings

Uncontrolled sunlight entering through windows contributes to substantial heating and cooling demands in buildings, which leads to high energy consumption from the buildings. Recently, perovskite smart windows have emerged as innovative energy-saving technologies, offering the potential to adaptively control indoor solar heat gain through their impressive sunlight modulation capabilities. Moreover, harnessing the high-efficiency photovoltaic properties of perovskite materials, these windows have the potential to generate power, thereby realizing more advanced windows with combined light modulation and energy harvesting capabilities. This review summarizes the recent advancements in various chromic perovskite materials for achieving light modulation, focusing on both perovskite structures and underlying switching mechanisms. The discussion also encompasses device engineering strategies for smart windows, including the improvement of their optical and transition performance, durability, combination with electricity generation, and the evaluation of their energy-saving performance in building applications. Furthermore, the challenges and opportunities associated with perovskite smart windows are explicated, aimed at stimulating more academic research and advancing their pragmatic implementation for building energy efficiency and sustainability.

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.

Improved Performance and Stability of Quasi-Two-Dimensional Perovskite Solar Cells by Post-treatment with Acetaminophen

Recently, perovskite solar cells (PSCs) based on quasi-two-dimensional (quasi-2D) perovskites have drawn more attention due to their excellent stability, although their efficiencies are still lower than those of 3D ones. Here we applied post-treatment of 2D perovskite GAMA5Pb5I16 (GA = guanidinium, MA = methylammonium) films with acetaminophen (AMP) to improve their performance. The efficiency of the solar cells with 2 mg/mL AMP post-treatment increased to 18.01% from 16.72% for those without post-treatment. The efficiency improvement results from the enlarged grain size, reduced trap state density, and better energy level matching after AMP post-treatment. In addition, the stability of the solar cells is improved. The solar cells with AMP post-treatment maintain 91% of the original power conversion efficiency value after aging for 30 days in the atmosphere. This work opens a new approach for the efficiency and stability enhancement of quasi-2D PSCs.

Comparative Analysis of Thiophene-Based Interlayer Cations for Enhanced Performance in 2D Ruddlesden-Popper Perovskite Solar Cells

2D Ruddlesden-Popper (RP) perovskites have appeared as a promising prospective material owing to their tunable optoelectronic peculiarities and structural stability. The choice of interlayer cations greatly influences the performance of the 2D RP perovskites. In this study, through theoretical calculations and experimental investigation, we demonstrate the intrinsic and device performance differences between two perovskites based on cations of thiophenemethylamine (TMA) and thiopheneethylamine (TEA). Using density functional theory (DFT) calculations, it exposes that as compared to (TMA)2PbI4, (TEA)2PbI4 exhibits more pronounced distortion of [PbI6]4- units and possesses a wider band gap and larger effective mass. The experimental results on the TMA- and TEA-based 2D perovskites further show that when TEA is used as the interlayer cation, the crystallization process tends to form more low-n phases, which hinder charge transfer and decrease light harvesting. On the other hand, when TMA is used as the interlayer cation, excessive low-n phases are not observed, and the thin film exhibits excellent quality with significantly improved electron mobility. The (TMA)2(FA)n-1PbnI3n+1 (n = 5) perovskite device shows a remarkable conversion efficiency of 16.56%, much higher than that of TEA-based devices (PCE = 2.58%). Moreover, the unencapsulated devices based on TMA were able to maintain 88% of their initial efficiency even after being exposed to the environment (RT, RH = 30 ± 5%) for a duration of 1080 h. These findings provide important insights into the differences between thiophene-based cations and the selection of organic interlayer cations for 2D RP perovskite solar cells.

Modulating the Dipole Moment of Secondary Ammonium Spacers for Efficient 2D Ruddlesden-Popper Perovskite Solar Cells

Layered two-dimensional (2D) perovskites are emerging as promising optoelectronic materials owing to their excellent environmental stability. Regulating the dipole moment of organic spacers has the potential to reduce the exciton binding energy (Eb ) of 2D perovskites and improve their photovoltaic performance. Here, we developed two azetidine-based secondary ammonium spacers with different electron-withdrawing groups, namely 3-hydroxyazatidine (3-OHAz) and 3,3-difluoroazetidine (3,3-DFAz) spacers, for 2D Ruddlesden-Popper (RP) perovskites. It was found that the large dipole moment of the fluorinated dipole spacer could effectively enhance the interaction between organic spacers and inorganic layers, leading to improved charge dissociation in 2D RP perovskite. In contrast to 3-OHAz spacer, the 2D perovskite using 3,3-DFAz as spacer also shows improved film quality, optimized energy level alignment, and reduced exciton binding energy. As a result, the 2D perovskite (n=4) device based on 3,3-DFAz yields an outstanding efficiency of 19.28 %, surpassing that of the 3-OHAz-Pb device (PCE=11.35 %). The efficiency was further improved to 19.85 % when using mixed A-site cation of MA0.95 FA0.05 . This work provides an effective strategy for modulating the energy level alignment and reducing the Eb by regulating the dipole moment of organic spacers, ultimately enabling the development of high-performance 2D perovskite solar cells.

Investigation of the Role of K2SO4 Electrolyte in Hole Transport Layer for Efficient Quasi-2D Perovskite Light-Emitting Diodes

Quasi-two-dimensional (2D) perovskite light-emitting diodes are promising light sources for color display and lighting. However, poor carrier injection and transport between the bottom hole transport layer (HTL) and perovskite limit the device performance. Here we demonstrate a simple and effective way to modify the HTL for enhancing the performance of perovskite light-emitting diodes (PeLEDs). An electrolyte K2SO4 is used to mix with poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as the hole transport layer. The K+ doping helped the quasi-2D perovskite phases grow vertically along the interface of the PEDOT:PSS, fine-modulate the phase distribution, and simultaneously reduce the defect density of quasi-2D perovskites. It also significantly reduced the exciton quenching and injection barrier at PEDOT:PSS and quasi-2D perovskite interface. The optimized green PeLEDs with the K2SO4 doped PEDOT:PSS HTL showed a maximum luminance of 17185 cd/m2 which is almost 4.7 times brighter than the control one, with a maximum external quantum efficiency of 18.64%.

Impact of Alkyl Chain Length on the Formation of Regular- and Reverse-Graded Quasi-2D Perovskite Thin Films

Crystallization of low-dimensional perovskites is a complex process that leads to multidimensional films comprising two-dimensional (2D), quasi-2D, and three-dimensional (3D) phases. Most quasi-2D perovskite films possess a regular gradient with 2D phases located at the bottom of the film and 3D phases at the top. Recently, multiple studies have reported reverse-graded perovskite films, where the location of the 2D and 3D structures is inverted. The underlying reasons for such a peculiar phase distribution are unclear. While crystallization of regular-graded quasi-2D perovskites has been described as starting with 3D phases from the liquid-air interface, the film formation of reverse-graded films has not been investigated yet. Here, we examine the impact of the alkyl chain length on the formation of regular- and reverse-graded perovskites using n-alkylammonium ions. We find that long alkyl chains reverse the phase distribution gradient. By combining photoluminescence spectroscopy with in situ optical absorption measurements, we demonstrate that crystallization starts at the liquid-N2 interface, though as 3D phases for short-chain n-alkylammonium ions and as quasi-2D phases for long chains. We link this behavior to enhanced van der Waals interactions between long-chain n-alkylammonium ions in polar solvents and their tendency to accumulate at the liquid-N2 interface, creating a concentration gradient along the film thickness.

Rapid Spin Depolarization in the Layered 2D Ruddlesden-Popper Perovskite (BA)(MA)PbI

We report temperature-dependent spectroscopy on the layered (n = 4) two-dimensional (2D) Ruddlesden-Popper perovskite (BA)(MA)PbI. Helicity-resolved steady-state photoluminescence (PL) reveals no optical degree of polarization. Time-resolved PL shows a photocarrier lifetime on the order of nanoseconds. From simultaneously recorded time-resolved differential reflectivity (TRΔR) and time-resolved Kerr ellipticity (TRKE), a photocarrier lifetime of a few nanoseconds and a spin relaxation time on the order of picoseconds was found. This stark contrast in lifetimes clearly explains the lack of spin polarization in steady-state PL. While we observe clear temperature-dependent effects on the PL dynamics that can be related to structural dynamics, spin relaxation is nearly T-independent. Our results highlight that spin relaxation in 2D (BA)(MA)PbI occurs at time scales faster than the exciton recombination time, which poses a bottleneck for applications aiming to utilize this degree of freedom.

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.

Nucleation and Crystallization in 2D Ruddlesden-Popper Perovskites using Formamidinium-based Organic Semiconductor Spacers for Efficient Solar Cells

The conjugated organic semiconductor spacers have drawn wide attention in two-dimensional (2D) perovskites and formamidinium (FA) has been widely used as A-site cation in high-performance 3D perovskite solar cells (PSCs). However, the FA-based semiconductor spacers have rarely been investigated in 2D Ruddlesden-Popper (RP) perovskites. Here, we developed two FA-based spacers containing thieno[3,2-b]thiophene (TT) and 2,2'-bithiophene (BT) units, namely TTFA and BTFA, respectively, for 2D RP PSCs. The nucleation and crystallization kinetics of TTFA-Pb and BTFA-Pb from sol-gel to film were investigated using in situ optical microscopy and in situ grazing incidence wide-angle X-ray scattering (GIWAXS) measurements. It is found that the TTFA spacer could reduce the energy barrier of nucleation and induces crystal vertical orientation of 2D perovskite by forming larger clusters in precursor solution, resulting in much improved film quality. Benefiting from the enlarged crystal grains, reduced exciton binding energy, and decreased electron-phonon coupling coefficient, the photovoltaic device based on (TTFA)2 MAn-1 Pbn I3n+1 (n=5) achieved a champion efficiency of 19.41 %, which is a record for 2D RP PSCs with FA-based spacers. Our work provides deep understanding of the nucleation and crystallization process of 2D RP perovskite films and highlights the great potential of FA-based semiconductor spacers in highly efficient 2D PSCs.

Multiple Phase Regulation Enables Efficient and Bright Quasi-2D Perovskite Light-Emitting Diodes

Quasi-2D perovskites, multiquantum well materials with the energy cascade structure, exhibit impressive optoelectronic properties and a wide range of applications in various optoelectronic devices. However, the insufficient exciton energy transfer caused by the excess of small-n phases that induce nonradiative recombination and the spatially random phase distribution that impedes charge transport severely inhibit the device performance of light-emitting diodes (LEDs). Here, a faster energy transfer process and efficient carrier recombination are achieved by introducing the multifunctional additive 2-(methylsulfonyl)-4-(trifluoromethyl)benzoic acid (MTA) to manipulate the crystallization process of perovskites. The introduction of MTA not only constrains the PEA and restrains the formation of small-n phases to improve the energy transfer process but also optimizes the crystal orientation to promote charge transport. As a result, highly efficient pure green quasi-2D perovskite LEDs with a peak EQE of 25.9%, a peak current efficiency of 108.1 cd A-1, and a maximum luminance of 288798 cd m-2 are achieved.

Simulation Design of Novel Non-Fluorine Polymers as Electron Transport Layer for Lead-Free Perovskite Solar Cells

Significant progress has been made in the advancement of perovskite solar cells, but their commercialization remains hindered by their lead-based toxicity. Many non-toxic perovskite-based solar cells have demonstrated potential, such as Cs2AgBi0.75Sb0.25Br6, but their power conversion efficiency is inadequate. To address this issue, some researchers are focusing on emerging acceptor-donor-acceptor'-donor-acceptor (A-DA'D-A)-type non-fullerene acceptors (NFAs) for Cs2AgBi0.75Sb0.25Br6 to find effective electron transport layers for high-performance photovoltaic responses with low voltage drops. In this comparative study, four novel A-DA'D-A-type NFAs, BT-LIC, BT-BIC, BT-L4F, and BT-BO-L4F, were used as electron transport layers (ETLs) for the proposed devices, FTO/PEDOT:PSS/Cs2AgBi0.75Sb0.25Br6/ETL/Au. Comprehensive simulations were conducted to optimize the devices. The simulations showed that all optimized devices exhibit photovoltaic responses, with the BT-BIC device having the highest power conversion efficiency (13.2%) and the BT-LIC device having the lowest (6.8%). The BT-BIC as an ETL provides fewer interfacial traps and better band alignment, enabling greater open-circuit voltage for efficient photovoltaic responses.

Mechanistic Understanding of Oxidation of Tin-based Perovskite Solar Cells and Mitigation Strategies

Tin (Sn)-based perovskites as the most promising absorber materials for lead-free perovskite solar cells (PSCs) have achieved the record efficiency of over 14 %. Although suppressing the oxidation of Sn-based perovskites is a frequently concerned topic for Sn-based PSCs, many studies have given vague explanations and the mechanisms are still under debate. This is in principal due to the lack of an in-depth understanding of various and complex intrinsic and extrinsic factors causing the oxidation process. In this context, we critically review the chemical mechanism of facile oxidation of Sn-based perovskites and differentiate its detrimental effects at material- and device-level. More importantly, we classify and introduce the intrinsic factors (raw materials and solvent of perovskite precursors) and extrinsic factors (exposure to neutral oxygen and superoxide) causing the oxidation with their corresponding anti-oxidation improvement methods. The presented comprehensive understanding and prospect of the oxidation provide insightful guidance for suppressing the oxidation in Sn-based PSCs "from the beginning to the end".

Trap or Triplet? Excited-State Interactions in 2D Perovskite Colloids with Chromophoric Cations

Movement of energy within light-harvesting assemblies is typically carried out with separately synthesized donor and acceptor species, which are then brought together to induce an interaction. Recently, two-dimensional (2D) lead halide perovskites have gained interest for their ability to accommodate and assemble chromophoric molecules within their lattice, creating hybrid organic-inorganic compositions. Using a combination of steady-state and time-resolved absorption and emission spectroscopy, we have now succeeded in establishing the competition between energy transfer and charge trapping in 2D halide perovskite colloids containing naphthalene-derived cations (i.e., NEA2PbX4, where NEA = naphthylethylamine). The presence of room-temperature triplet emission from the naphthalene moiety depends on the ratio of bromide to iodide in the lead halide sublattice (i.e., x in NEA2Pb(Br1-xIx)4), with only bromide-rich compositions showing sensitized emission. Photoluminescence lifetime measurements of the sensitized naphthalene reveal the formation of the naphthalene triplet excimer at room temperature. From transient absorption measurements, we find the rate constant of triplet energy transfer (kEnT) to be on the order of ∼109 s-1. At low temperatures (77 K) a new broad emission feature arising from trap states is observed in all samples ranging from pure bromide to pure iodide composition. These results reveal the interplay between sensitized triplet energy transfer and charge trapping in 2D lead halide perovskites, highlighting the need to carefully parse contributions from competing de-excitation pathways for optoelectronic applications.

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.

Fluorination and Conjugation Engineering Synergistically Enhance the Optoelectronic Properties of Two-Dimensional Hybrid Organic-Inorganic Perovskites

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) are expected to be a viable alternative to three-dimensional (3D) analogs in solar cells (SCs) and optoelectronic devices due to their high stability, diverse composition, and physical properties. However, unsuitable band alignment and large bandgaps limit the power conversion efficiency (PCE) improvement of SCs based on 2D HOIPs. Here, we report a molecular design strategy that combines fluorination and conjugation engineering to tune the electronic structure and optimize the PCE of 2D HOIPs. Our results show that type IIa band alignment and tunable bandgaps can be achieved in 2D Dion-Jacobson (DJ) HOIPs by H/F substitution of organic cations with different degrees of conjugation. In general, the bandgap of 2D DJ-HOIPs decreases monotonously with the increase of the number of F atoms, which is due to the gradual decrease of the lowest unoccupied molecular orbitals (LUMO) of organic cations. In addition, the enhanced interlayer charge transfer and higher dielectric constant suggest that the fluorination-induced dielectric limitations are weakened. The estimated PCE of 2D DJ-HOIPs is exponentially increased and positively correlated with the degree of conjugation and fluorination of organic cations, with a PCE approaching 29% under their synergistic effect. Our results not only provide promising candidates for photovoltaic device applications but also provide an effective method for PCE optimization.

Introduction to halide perovskite optoelectronics

An introduction to the Nanoscale themed collection on halide perovskite nanomaterials for optoelectronic applications, featuring a variety of articles that highlight the latest developments to address ongoing challenges in the field.

Electronic and optical properties of chemically modified 2D GaAs nanoribbons

We employed density functional theory calculations to investigate the electronic and optical characteristics of finite GaAs nanoribbons (NRs). Our study encompasses chemical alterations including doping, functionalization, and complete passivation, aimed at tailoring NR properties. The structural stability of these NRs was affirmed by detecting real vibrational frequencies in infrared spectra, indicating dynamical stability. Positive binding energies further corroborated the robust formation of NRs. Analysis of doped GaAs nanoribbons revealed a diverse range of energy gaps (approximately 2.672 to 5.132 eV). The introduction of F atoms through passivation extended the gap to 5.132 eV, while Cu atoms introduced via edge doping reduced it to 2.672 eV. A density of states analysis indicated that As atom orbitals primarily contributed to occupied molecular orbitals, while Ga atom orbitals significantly influenced unoccupied states. This suggested As atoms as electron donors and Ga atoms as electron acceptors in potential interactions. We investigated excited-state electron-hole interactions through various indices, including electron-hole overlap and charge-transfer length. These insights enriched our understanding of these interactions. Notably, UV-Vis absorption spectra exhibited intriguing phenomena. Doping with Te, Cu, W, and Mo induced redshifts, while functionalization induced red/blue shifts in GaAs-34NR spectra. Passivation, functionalization, and doping collectively enhanced electrical conductivity, highlighting the potential for improving material properties. Among the compounds studied, GaAs-34NR-edg-Cu demonstrated the highest electrical conductivity, while GaAs-34NR displayed the lowest. In summary, our comprehensive investigation offers valuable insights into customizing GaAs nanoribbon characteristics, with promising implications for nanoelectronics and optoelectronics applications.

Crystallinity and Phase Control in Formamidinium-Based Dion-Jacobson 2D Perovskite via Seed-Induced Growth for Efficient Photovoltaics

2D perovskites based on Formamidinium (FA) hold the potential for excellent stability and a broad absorption range, making them attractive materials for solar cells. However, FA-based 2D perovskites produced via one-step processing exhibit poor crystallinity and random quasi-quantum wells (QWs), leading to subpar photovoltaic performance. In this study, a seed-induced growth approach is introduced employing MAPbCl3 and BDAPbI4 in the deposition of FA-based Dion-Jacobson 2D perovskite films. This method yields high-quality perovskite films as the seeds preferentially precipitate and serve as templates for the epitaxial growth of FA-based counterparts, effectively suppressing the δ phase. Moreover, the epitaxial growth facilitated by uniformly dispersed seeds results in simultaneous crystallization from top to bottom, efficiently mitigating random phases (n = 2, 3, 4…) induced by the diffusion of organic cations and, in turn, minimizing energy loss. The impact of seed-induced growth on the crystallization and phase distribution of FA-based 2D perovskites is systematically investigated. As a result, the optimized FA-based 2D perovskite solar cell delivers an outstanding efficiency of 20.0%, accompanied by a remarkable fill factor of 0.823. Additionally, the unencapsulated device demonstrates exceptional stability, maintaining 98% of its initial efficiency after 1344 h of storage.

Can photoluminescence quenching be a predictor for perovskite solar cell efficiencies?

Bromide-based perovskites have large bandgaps, making them attractive for tandem solar cells developed to overcome the Shockley-Queisser limit. A perovskite solar cell architecture employs transporting layers to improve charge extraction and transport. Due to the wide variety of materials and preparation methods, it is critical to devise fast screening methods to rank transporting layers. Herein, we evaluate perovskite fluorescence quenching followed by time- and energy-resolved photoluminescence (TER-PL) and analyse the intensity dependence as a potential method to qualify charge-transporting layers rapidly. The capability of the technique was evaluated with TiO2/FAPbBr3 and SnO2/FAPbBr3, the most commonly used electron transporting layers, which were prepared using standard protocols to make best-performing devices. The results revealed that TiO2 is the most effective quencher due to the higher density of states in the conduction band, consistent with Marcus-Gerischer's theory. However, record-performance devices use SnO2 as the electron transport layer. This shows that the relationship between photoluminescence quenching and device performance is not bidirectional. Therefore, additional measurements like conductivity are also needed to provide reliable feedback for device performance.

Defect Regulation of Efficient Dion-Jacobson Quasi-2D Perovskite Solar Cells via a Polyaspartic Acid Interlayer

Interfacial modification is a promising strategy to fabricate highly efficient perovskite solar cells (PSCs). Nevertheless, research studies about optimization for the performance of Dion-Jacobson (DJ)-phase quasi-2D PSCs by underlying surface modification are rarely reported. The relevant influence of interfacial modification on defect regulation in the bulk and at the interface for PSCs is still unexplored. Herein, an interlayer of polyaspartic acid (PASP) was introduced at the interface of a hole transporting layer and a perovskite absorber to regulate both the film quality and interface property for BDA-based DJ quasi-2D PSCs (n = 5). The PASP interlayer suppressed the charge recombination, restricted the interfacial charge accumulation, and promoted the charge transport in devices and therefore improved the power conversion efficiency of PSCs from 15.03 to 17.34%. Moreover, through device simulation, it was concluded that the increase of open-circuit voltage (Voc) was mainly attributed to the suppression of interface defects, while the increase of short-circuit current (Jsc) was ascribed to the restriction of interface defects and perovskite bulk defects. The improvement of both Voc and Jsc originated from the passivation of shallow defect states. The present work provides a promising route for the fabrication of efficient quasi-2D PSCs and enriches the fundamental understanding of defect regulation on photovoltaic performance.

Recent Progress of Layered Perovskite Solar Cells Incorporating Aromatic Spacers

Layered two dimensional (2D) or quasi-2D perovskites are emerging photovoltaic materials due to their superior environment and structure stability in comparison with their 3D counterparts. The typical 2D perovskites can be obtained by cutting 3D perovskites along  < 100 >  orientation by incorporation of bulky organic spacers, which play a key role in the performance of 2D perovskite solar cells (PSCs). Compared with aliphatic spacers, aromatic spacers with high dielectric constant have the potential to decrease the dielectric and quantum confinement effect of 2D perovskites, promote efficient charge transport and reduce the exciton binding energy, all of which are beneficial for the photovoltaic performance of 2D PSCs. In this review, we aim to provide useful guidelines for the design of aromatic spacers for 2D perovskites. We systematically reviewed the recent progress of aromatic spacers used in 2D PSCs. Finally, we propose the possible design strategies for aromatic spacers that may lead to more efficient and stable 2D PSCs.

The Impact of Spacer Size on Charge Transfer Excitons in Dion-Jacobson and Ruddlesden-Popper Layered Hybrid Perovskites

Organic materials can tune the optical properties in layered (2D) hybrid perovskites, although their impact on photophysics is often overlooked. Here, we use transient absorption spectroscopy to probe the Dion-Jacobson (DJ) and Ruddlesden-Popper (RP) 2D perovskite phases. We show the formation of charge transfer excitons in DJ phases, resulting in a photoinduced Stark effect which is shown to be dependent on the spacer size. By using electroabsorption spectroscopy, we quantify the strength of the photoinduced electric field, while temperature-dependent measurements demonstrate new features in the transient spectra of RP phases at low temperatures resulting from the quantum-confined Stark effect. This study reveals the impact of spacer size and perovskite phase configuration on charge transfer excitons in 2D perovskites of interest to their advanced material design.

Additive Engineering for Stable and Efficient Dion-Jacobson Phase Perovskite Solar Cells

Because of their better chemical stability and fascinating anisotropic characteristics, Dion-Jacobson (DJ)-layered halide perovskites, which owe crystallographic two-dimensional structures, have fascinated growing attention for solar devices. DJ-layered halide perovskites have special structural and photoelectronic features that allow the van der Waals gap to be eliminated or reduced. DJ-layered halide perovskites have improved photophysical characteristics, resulting in improved photovoltaic performance. Nevertheless, owing to the nature of the solution procedure and the fast crystal development of DJ perovskite thin layers, the precursor compositions and processing circumstances can cause a variety of defects to occur. The application of additives can impact DJ perovskite crystallization and film generation, trap passivation in the bulk and/or at the surface, interface structure, and energetic tuning. This study discusses recent developments in additive engineering for DJ multilayer halide perovskite film production. Several additive-assisted bulk and interface optimization methodologies are summarized. Lastly, an overview of research developments in additive engineering in the production of DJ-layered halide perovskite solar cells is offered.

Pinhole-free 2D Ruddlesden-Popper perovskite layer with close packed large crystalline grains, suitable for optoelectronic applications

Here, we achieved pinhole-free 2D Ruddlesden-Popper Perovskite (RPP) BA₂PbI₄ layers with close packed crystalline grains with dimension of about 30 × 30 µm², which have been demonstrated to be favorable for optoelectronic applications, such as fast response RPP-based metal/semiconductor/metal photodetectors. We explored affecting parameters in hot casting of BA₂PbI₄ layers, and proved that oxygen plasma treatment prior to hot casting plays a significant role to achieve high quality close packed polycrystalline RPP layers at lower hot cast temperatures. Moreover, we demonstrate that crystal growth of 2D BA₂PbI₄ can be dominantly controlled by the rate of solvent evaporation through substrate temperature or rotational speed, while molarity of the prepared RPP/DMF precursor is the dominant factor that determines the RPP layer thickness, and can affect the spectral response of the realized photodetector. Benefiting from the high light absorption and inherent chemical stability of 2D RPP layers, we achieved high responsivity and stability, and fast response photodetection from perovskite active layer. We achieved a fast photoresponse with rise and fall times of 189 µs and 300 µs, and the maximum responsivity of 119 mA/W and detectivity of 2.15 × 10⁸ Jones in response to illumination wavelength of 450 nm. The presented polycrystalline RPP-based photodetector benefits from a simple and low-cost fabrication process, suitable for large area production on glass substrate, a good stability and responsivity, and a promising fast photoresponse, even around that of exfoliated single crystal RPP-based counterparts. However, it is well known that exfoliation methods suffer from poor repeatability and scalability, which make them incompatible with mass production and large area applications.

The Adsorption Effect of Methane Gas Molecules on Monolayer PbSe with and without Vacancy Defects: A First-Principles Study

In this paper, the adsorption effect of methane (CH₄) gas molecular on monolayer PbSe with and without vacancy defects is studied based on first-principles calculations. The effects of the adsorption of methane molecular on monolayer PbSe and on the Se vacancy (V_Se) and Pb vacancy (V_Pb) of monolayer PbSe are also explored. Our results show that methane molecules exhibit a good physical adsorption effect on monolayer PbSe with and without vacancy defects. Moreover, our simulations indicate that the adsorption capacity of CH₄ molecules on monolayer PbSe can be enhanced by applying strain. However, for the monolayer PbSe with Vse, the adsorption capacity of CH₄ molecules on the strained system decreases sharply. This indicates that applying strain can promote the dissociation of CH₄ from V_Se. Our results show that the strain can be used as an effective means to regulate the interaction between the substrate material and the methane gas molecules.

Lead-Free FACsSnI3 Based Perovskite Solar Cell: Designing Hole and Electron Transport Layer

In recent years, lead-based perovskites solar cells have demonstrated excellent power-conversion efficiency. Despite their remarkable progress, the commercialization of lead-based perovskites is hampered by lead toxicity concerns. The recently discovered non-toxic FACsSnI₃ perovskite has the potential to replace lead-based perovskites in solar cell applications. Since the perovskite material FACsSnI₃ (FA_0.85Cs_0.15SnI₃) is relatively new, there is a lack of information, particularly regarding the design features required for electron and hole-transport layers for efficient photovoltaic responses. The important variables, such as electron affinity, energy band gap, film thickness, and doping density of both electron and hole-transport layers, were simulated and modeled separately and iteratively in this study to achieve the most efficient photovoltaic response. Finally, the absorber layer thickness of FACsSnI₃ perovskite is tuned to achieve a maximum power-conversion efficiency of slightly more than 24%. We hope that the findings of this study will serve as a strong guideline for future research and the design of lead-free perovskite solar cells for efficient photovoltaic responses.

Bridging the Buried Interface with Piperazine Dihydriodide Layer for High Performance Inverted Solar Cells

Given that it is closely related to perovskite crystallization and interfacial trap densities, buried interfacial engineering is crucial for creating effective and stable perovskite solar cells. Compared with the in-depth studies on the defect at the top perovskite interface, exploring the defect of the buried side of perovskite film is relatively complicated and scanty owing to the non-exposed feature. Herein, the degradation process is probed from the buried side of perovskite films with continuous illumination and its effects on morphology and photoelectronic characteristics with a facile lift-off method. Additionally, a buffer layer of Piperazine Dihydriodide (PDI₂ ) is inserted into the imbedded bottom interface. The PDI₂ buffer layer is able to lubricate the mismatched thermal expansion between perovskite and substrate, resulting in the release of lattice strain and thus a void-free buried interface. With the PDI₂ buffer layer, the degradation originates from the growing voids and increasing non-radiative recombination at the imbedded bottom interfaces are suppressed effectively, leading to prolonged operation lifetime of the perovskite solar cells. As a result, the power conversion efficiency of an optimized p-i-n inverted photovoltaic device reaches 23.47% (with certified 23.42%) and the unencapsulated devices maintain 90.27% of initial efficiency after 800 h continuous light soaking.

Ionic Liquid Assisted Imprint for Efficient and Stable Quasi-2D Perovskite Solar Cells with Controlled Phase Distribution

Although two-dimensional perovskite devices are highly stable, they also lead to a number of challenges. For instance, the introduction of large organic amines makes crystallization process complicated, causing problems such as generally small grain size and blocked charge transfer. In this work, imprint assisted with methylamine acetate were used to improve the morphology of the film, optimize the internal phase distribution, and enhance the charge transfer of the perovskite film. Specifically, imprint promoted the dispersion of spacer cations in the recrystallization process with the assistance of methylamine acetate, thus inhibited the formation of low-n phase induced by the aggregation of spacer cations and facilitated the formation of 3D-like phase. In this case, the corresponding quasi-2D perovskite solar cells delivered improved efficiency and exhibited superior stability. Our work provides an effective strategy to obtain uniform phase distribution for quasi-2D perovskite.

Mechanistic Study of the Transition from Antimony Oxide to Antimony Sulfide in the Hydrothermal Process to Obtain Highly Efficient Solar Cells

Obtaining high-quality absorber layers is a major task for constructing efficient thin-film solar cells. Hydrothermal deposition is considered a promising method for preparing high-quality antimony sulfide (Sb₂ S₃ ) films for solar cell applications. In the hydrothermal process, the precursor reactants play an important role in controlling the film formation process and thus the film quality. In this study, Sb₂ O₃ is applied as a new Sb source to replace the traditional antimony potassium tartrate to modulate the growth process of the Sb₂ S₃ film. The reaction mechanism of the transition from Sb₂ O₃ to Sb₂ S₃ in the hydrothermal process is revealed. Through controlling the nucleation and deposition processes, high-quality Sb₂ S₃ films are prepared with longer carrier lifetimes and lower deep-level defect densities than those prepared from the traditional Sb source of antimony potassium tartrate. Consequently, a solar cell device based on this improved Sb₂ S₃ delivers a high power conversion efficiency of 6.51 %, which is in the top tier for Sb₂ S₃ -based solar devices using hydrothermal methods. This research provides a new and competitive Sb source for hydrothermal growth of high-quality antimony chalcogenide films for solar cell applications.

Highly Efficient and Stable FA-Based Quasi-2D Ruddlesden-Popper Perovskite Solar Cells by the Incorporation of β-Fluorophenylethanamine Cations

2D Ruddlesden-Popper (2D RP) perovskite, with attractive environmental and structural stability, has shown great application in perovskite solar cells (PSCs). However, the relatively inferior photovoltaic efficiencies of 2D PSCs limit their further application. To address this issue, β-​fluorophenylethanamine (β-​FPEA) as a novel spacer cation is designed and employed to develop stable and efficient quasi-2D RP PSCs. The strong dipole moment of the β-​FPEA enhances the interactions between the cations and [PbI₆ ]^4- octahedra, thus improving the charge dissociation of quasi-2D RP perovskite. Additionally, the introduction of the β-​FPEA cation optimizes the energy level alignment, improves the crystallinity, stabilizes both the mixed phase and a-FAPbI₃ phase of the quasi-2D RP perovskite film, prolongs the carrier diffusion length, increases the carrier lifetime and decreases the trap density. By incorporating the β-​FPEA, the quasi-2D RP PSCs exhibit a power conversion efficiency (PCE) of 16.77% (vs phenylethylammonium (PEA)-based quasi-2D RP PSCs of 12.81%) on PEDOT:PSS substrate and achieve a champion PCE of 19.11% on the PTAA substrate. It is worth noting that the unencapsulated β-​FPEA-based quasi-2D RP PSCs exhibit considerably improved thermal and moisture stability. These findings provide an effective strategy for developing novel spacer cations for high-performance 2D RP PSCs.

Highly Efficient 2D/3D Mixed-Dimensional Cs2PbI2Cl2/CsPbI2.5Br0.5 Perovskite Solar Cells Prepared by Methanol/Isopropanol Treatment

All-inorganic perovskite solar cells are attractive photovoltaic devices because of their excellent optoelectronic performance and thermal stability. Unfortunately, the currently used efficient inorganic perovskite materials can spontaneously transform into undesirable phases without light-absorption properties. Studies have been carried out to stabilize all-inorganic perovskite by mixing low-dimensional perovskite. Compared with organic two-dimensional (2D) perovskite, inorganic 2D Cs₂PbI₂Cl₂ shows superior thermal stability. Our group has successfully fabricated 2D/3D mixed-dimensional Cs₂PbI₂Cl₂/CsPbI_2.5Br_0.5 films with increasing phase stability. The high boiling point of dimethyl sulfoxide (DMSO) makes it a preferred solvent in the preparation of Cs₂PbI₂Cl₂/CsPbI_2.5Br_0.5 inorganic perovskite. When the perovskite films are prepared by the one-step solution method, it is difficult to evaporate the residual solvent molecules from the prefabricated films, resulting in films with rough surface morphology and high defect density. This study used the rapid precipitation method to control the formation of perovskite by treating it with methanol/isopropanol (MT/IPA) mixed solvent to produce densely packed, smooth, and high-crystallized perovskite films. The bulk defects and the carrier transport barrier of the interface were effectively reduced, which decreased the recombination of the carriers in the device. As a result, this effectively improved photoelectric performance. Through treatment with MT/IPA, the photoelectric conversion efficiency (PCE) of solar cells prepared in the N₂ atmosphere increased from 13.44% to 14.10%, and the PCE of the device prepared in the air increased from 3.52% to 8.91%.

Exploring Solar Cells Based on Lead- and Iodide-Deficient Halide Perovskite (d-HP) Thin Films

Perovskite solar cells have become more and more attractive and competitive. However, their toxicity induced by the presence of lead and their rather low stability hinders their potential and future commercialization. Reducing lead content while improving stability then appears as a major axis of development. In the last years, we have reported a new family of perovskite presenting PbI⁺ unit vacancies inside the lattice caused by the insertion of big organic cations that do not respect the Goldschmidt tolerance factor: hydroxyethylammonium HO-(CH₂)₂-NH₃⁺ (HEA⁺) and thioethylammonium HS-(CH₂)₂-NH₃⁺ (TEA⁺). These perovskites, named d-HPs for lead and halide-deficient perovskites, present a 3D perovskite corner-shared Pb_1-xI_3-x network that can be assimilated to a lead-iodide-deficient MAPbI₃ or FAPbI₃ network. Here, we propose the chemical engineering of both systems for solar cell optimization. For d-MAPbI₃-HEA, the power conversion efficiency (PCE) reached 11.47% while displaying enhanced stability and reduced lead content of 13% compared to MAPbI₃. On the other hand, d-FAPbI₃-TEA delivered a PCE of 8.33% with astounding perovskite film stability compared to classic α-FAPI. The presence of TEA⁺ within the lattice impedes α-FAPI degradation into yellow δ-FAPbI₃ by direct degradation into inactive Pb(OH)I, thus dramatically slowing the aging of d-FAPbI₃-TEA perovskite.

Dredging the Charge-Carrier Transfer Pathway for Efficient Low-Dimensional Ruddlesden-Popper Perovskite Solar Cells

Low-dimensional Ruddlesden-Popper (LDRP) perovskites still suffer from inferior carrier transport properties. Here, we demonstrate that efficient exciton dissociation and charge transfer can be achieved in LDRP perovskite by introducing γ-aminobutyric acid (GABA) as a spacer. The hydrogen bonding links adjacent spacing sheets in (GABA)₂ MA₃ Pb₄ I₁₃ (MA=CH₃ NH₃ ⁺ ), leading to the charges localized in the van der Waals gap, thereby constructing "charged-bridge" for charge transfer through the spacing region. Additionally, the polarized GABA weakens dielectric confinement, decreasing the (GABA)₂ MA₃ Pb₄ I₁₃ exciton binding energy as low as ≈73 meV. Benefiting from these merits, the resultant GABA-based solar cell yields a champion power conversion efficiency (PCE) of 18.73 % with enhanced carrier transport properties. Furthermore, the unencapsulated device maintains 92.8 % of its initial PCE under continuous illumination after 1000 h and only lost 3 % of its initial PCE under 65 °C for 500 h.

Critical Influence of Organic A'-Site Ligand Structure on 2D Perovskite Crystallization

Organic A'-site ligand structure plays a crucial role in the crystal growth of 2D perovskites, but the underlying mechanism has not been adequately understood. This problem is tackled by studying the influence of two isomeric A'-site ligands, linear-shaped n-butylammonium (n-BA⁺ ) and branched iso-butylammonium (iso-BA⁺ ), on 2D perovskites from precursor to device, with a combination of in situ grazing-incidence wide-angle X-ray scattering and density functional theory. It is found that branched iso-BA⁺ , due to the lower aggregation enthalpies, tends to form large-size clusters in the precursor solution, which can act as pre-nucleation sites to expedite the crystallization of vertically oriented 2D perovskites. Furthermore, iso-BA⁺ is less likely to be incorporated into the MAPbI₃ lattice than n-BA⁺ , suppressing the formation of unwanted multi-oriented perovskites. These findings well explain the better device performance of 2D perovskite solar cells based on iso-BA⁺ and elucidate the fundamental mechanism of ligand structural impact on 2D perovskite crystallization.

Photoluminescence Modulation of Ruddlesden-Popper Perovskite via Phase Distribution Regulation

The intrinsic chaotic phase distribution in Ruddlesden-Popper Perovskite (RPP) hinders its further improvement of photoluminescence (PL) emission and limits its application in optical devices. In this work, we achieve the phase distribution regulation of RPP by varying the composition ratio of organic bulky spacer cations 1-naphthylmethylamine (NMA) and phenylethyl-ammonium (PEA), which is controllable and nondestructive for structures of RPP. By suppressing the small n-phase, the PL intensity emission of RPP is further improved. Through the time-resolved PL (TRPL) measurements, we find the PL lifetime of the sample with 66% PEA concentration increases with the temperature initially and possesses the highest values of τ1 and τ2 at ~255 K, indicating the immediate state assisting exciton radiative recombination, and it can be modulated by phase manipulation in RPP. The immediate state may outcompete other non-radiative decay channels for excited carriers, leading to the PL enhancement in RPP, and broadening its further application.

Non-preheating fabricated semitransparent quasi-2D perovskite solar cells

Although the hot-casting (HC) method can obtain efficient quasi-2D perovskite solar cells, this method cannot effectively control the uniformity of the thin film, and the high preheating substrate temperature will also form low n perovskite phases (n = 2) at the interface, which is not conducive to the transport of carriers. Semitransparent solar cells have great application prospects in building-integrated photovoltaic and tandem devices. Herein, a non-preheating (NP) film-casting method is proposed to realize a highly uniform and phase controllable quasi-2D perovskite film (BA₂MA₃Pb₄I₁₃, BA⁺:C₄H₁₁NH₃⁺, MA⁺:CH₃NH₃⁺). As a result, the NP-processed film gets the highest light utilization efficiency (LUE = 4.01%) for semitransparent quasi-2D perovskite solar cells (ST-Quasi-2D-PSCs) with power conversion efficiency (PCE) of 9.60%, average visible transmittance (AVT) of 41.73%, good bifaciality factor, high LUE in low light intensity and good stability.

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.

Deciphering the Morphology Change and Performance Enhancement for Perovskite Solar Cells Induced by Surface Modification

Organic-inorganic perovskite solar cells (PSCs) have achieved great attention due to their expressive power conversion efficiency (PCE) up to 25.7%. To improve the photovoltaic performance of PSCs, interface engineering between the perovskite and hole transport layer (HTL) is a widely used strategy. Following this concept, benzyl trimethyl ammonium chlorides (BTACls) are used to modify the wet chemical processed perovskite film in this work. The BTACl-induced low dimensional perovskite is found to have a bilayer structure, which efficiently decreases the trap density and improves the energy level alignment at the perovskite/HTL interface. As a result, the BTACl-modified PSCs show an improved PCE compared to the control devices. From device modeling, the reduced charge carrier recombination and promoted charge carrier transfer at the perovskite/HTL interface are the cause of the open-circuit (V_oc ) and fill factor (FF) improvement, respectively. This study gives a deep understanding for surface modification of perovskite films from a perspective of the morphology and the function of enhancing photovoltaic performance.