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

Nucleation-Layer Assisted Quasi-2D Ruddlesden-Popper Tin Perovskite Solar Cells With High Oxygen Stability

Tin (Sn)-based perovskite solar cells (PSCs) are extremely vulnerable to oxygen. Nevertheless, mechanism understanding and fundamental strategies to achieve oxygen-stable Sn-based PSCs are lacking. Here a nucleation-layer assisted (NLA) strategy by forming nucleation layer at the interface of hole transport layer and perovskite to attain highly oxygen-stable quasi-2D Ruddlesden-Popper (RP) Sn-based PSCs is reported. The formation process of nucleation layer consists of washing off the prepared perovskite film and annealing the residue on the substrate, which produces a new substrate for perovskite film fabrication. Such nucleation layer can transform the subsequently deposited perovskite film from a small-n-value dominated wide phase distribution with random crystal orientation into an intermediate-n-value dominated narrow phase distribution with vertical crystal orientation. This nucleation layer also improves the perovskite film morphology with highly coadjacent flake-like grains, leading to reduced grain boundaries and pinholes. The resultant NLA perovskite film shows more efficient carrier transport capability, lower exciton-binding energy, weakened electron-phonon coupling, and significantly decreased oxygen diffusion rate upon oxygen exposure. Consequently, a quasi-2D RP Sn-based PSC with a champion efficiency of 11.18% is obtained. The unencapsulated device preserves 95% of its initial efficiency after a 2700-h oxygen aging test, creating a record oxygen stability for Sn-based PSCs.

Unraveling the Impact of a Cyclized Phenylethylamine-Derived Spacer Cation on the Structural, Electrical, and Photovoltaic Performance of Quasi-2D Ruddlesden-Popper Perovskites

The discovery of new ligand molecules is crucial for advancing the performance and stability of 2D perovskites in optoelectronic devices. In this study, dihydroindole (IDN) cation, a novel organic spacer derived from the cyclization of phenylethylamine (PEA), is employed to fabricate stable and efficient quasi-2D Ruddlesden-Popper (RP) perovskite solar cells (PSCs). The IDN-based perovskite, (IDN)2PbI4, exhibits an average Pb─I─Pb bond angle exceeding 170°, with minimal distortion in the inorganic layer. Furthermore, the IDN molecules possess a larger dipole moment, reducing exciton binding energy to 79.86 meV. The IDN-based perovskite films demonstrate exceptional quality, with significantly enlarged grain sizes. This is attributed to the interaction between IDN molecules and [PbI6]4- octahedra, which enhances crystallinity, decreases trap density, extends carrier diffusion length, and increases carrier lifetime. The optimized device achieves an efficiency of 17.60%, markedly surpassing that of PEA-based devices (11.46%). Unencapsulated IDN-based quasi-2D RP PSCs exhibit superior thermal and humidity stability, making them promising for practical applications. These findings offer an effective strategy for the development of novel spacer cations, paving the way for high-performance 2D RP PSCs.

Manipulating Exciton-Phonon Coupling to Optimize Carrier Properties for High-Performance 2D Perovskite Photodetectors

The charge transport properties of two-dimensional (2D) perovskites are crucial for high-performance optoelectronic devices, yet the relationship between their carrier dynamics and structural properties remains inadequately understood. This study demonstrates that the dielectric screening effect and structural rigidity of 2D perovskites, both governed by ligand properties, significantly modulate carrier transport by reducing exciton-phonon coupling. We synthesized 2D perovskites using linear, cyclic, and aromatic organic ligands. Among these, (3AMPY)PbI4 (3AMPY = 3-(aminomethyl)pyridinium) exhibited the highest structural rigidity (Young's modulus ∼30.4 GPa) and relative dielectric constant (6.66). Increased rigidity resulting from multiple hydrogen bonds suppressed the lattice vibrations. The large dipole moment of the asymmetric structure (3AMPY) enhanced the dielectric screening effect, weakening localized interactions between electrons, holes, and phonons, further diminishing the exciton-phonon coupling to 134.8 meV. Consequently, the 3AMPY-based photodetector exhibited a responsivity of 0.97 A W-1 and a high light on/off ratio exceeding 105 under 532 nm illumination.

Suppressing the Bottom Small n Phases of Quasi-2D Perovskites for High-Performance Photovoltaic Applications

The bottom small n phases in quasi-two-dimensional (Q-2D) perovskite films significantly hinder their photovoltaic performance development due to their severely low conductivity and nonideal band alignment in the corresponding solar cells. In this study, we successfully suppressed the growth of small n phases in Q-2D Ruddlesden-Popper (RP) perovskite (BA2MA4Pb5I16, ⟨n⟩ = 5) films by introducing 2,7-bis(diphenylphosphoryl)-9,9'-spirobifluorene (SPPO13) as an additive into the perovskite precursor solution. It is interesting to find that the hole transport layer poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) in our p-i-n device can attract the SPPO13 due to the π-π stacking effect. As a result, the SPPO13 concentrates at the bottom, and the coordination between SPPO13 and PbI2 leads to more [PbI6]4- octahedra gathering at the downside of the Q-2D perovskite film. Thereby, more large n phases remain at the bottom, and the unwanted small n phases are suppressed. The optimized device achieves a remarkable power conversion efficiency of 18.41%, which, according to our knowledge, is the highest value for the BA-MA-based perovskite. Moreover, our device also demonstrates outstanding stability, maintaining 99.5% and 95.3% of the initial efficiency after being stored for over 3500 h and under maximum power point tracking operation for over 400 h, respectively. Unlike conventional methods that primarily address bulk or interface properties, this approach uniquely combines π-π stacking effects and defect passivation through phosphine oxide groups, leading to enhanced crystallinity, vertical orientation, and suppressed nonradiative recombination. This work provides a new approach to regulate n-phase growth and promote the photovoltaic behavior of Q-2D perovskite solar cells.

Dipolar Carbazole Ammonium for Broadened Electric Field Distribution in High-Performance Perovskite Solar Cells

Perovskite solar cells (PSCs) with ammonium passivation exhibit superior device performance and stability. Beyond typical chemical passivation, ammonium salts control the electronic structure of perovskite surfaces, yet the molecular structure-property relationship requires further understanding, especially the dipole effect. Here, we employed carbazole and its halogenated counterpart as the functional group of ammonium salts. 2-Chloro-carbazol-9-ethylammonium iodide (CzCl-EAI) with a rigid, conjugated molecular structure further provides chemical passivation and enhances the ambient stability of perovskites. In addition, we found that halogenation enhances the intramolecular charge transfer for a larger molecular dipole moment, leading to the depletion region of perovskite films threefold wider than that of the PDAI2 condition. The power conversion efficiency (PCE) of inverted PSCs based on mixed passivation reached 25.16% and certified 24.35% under the quasi-steady-state (QSS) measurement. Unencapsulated devices retained over 91% of initial PCE under ISOS-D-2 conditions over 1100 h and maintained 80% of their initial performance after 500 h of continuous light illumination in ambient air with a 50-60% relative humidity (RH).

Theoretical insights into spacer molecule design to tune stability, dielectric, and exciton properties in 2D perovskites

Two-dimensional organic-inorganic perovskites have garnered extensive interest owing to their unique structure and optoelectronic performance. However, their loose structures complicate the elucidation of mechanisms and tend to cause uncertainty and variations in experimental and calculated results. This can generally be rooted in dynamically swinging spacer molecules through two mechanisms: one is the intrinsic geometric steric effect, and the other is related to the electronic effect via orbital overlapping and electronic screening. Herein, we design three types of spacer molecules, phenyl methyl ammonium (PMA), thiophene methyl ammonium (THMA), and furan methyl ammonium (FUMA), that adopt different aromatic units. We examine the influence of different aromatic spacers on the structural properties of the inorganic layer of the perovskite based on first-principles calculations and find that a marginal change in the aromatic ending group in the spacer ligand would trigger significant changes in the octahedral inorganic layer. We predict that using THMA and FUMA can improve the stability and increase the size of crystal domains because of enhanced binding between the organic and inorganic layers. Compared to the prototype phenyl-based perovskite (PMA)2PbI4, the thiophene-based perovskite (THMA)2PbI4 exhibits states closer to the band edge, thus boosting carrier transport across inorganic and organic layers. Compared with the perovskite using PMA as a spacer cation, the THMA-based perovskite demonstrates a higher dielectric constant and smaller exciton binding energy, suggesting that THMA is more suitable as an organic spacer and a good passivation agent in 3D perovskites. The difference in the screening ability of the molecules induces varying interlayer excitonic binding energy. Our work provides theoretical grounds for the engineering of spacer molecules toward high-efficiency light conversion of mixed perovskites.

Two-Dimensional Organic-Inorganic van der Waals Hybrids

Two-dimensional organic-inorganic (2DOI) van der Waals hybrids (vdWhs) have emerged as a groundbreaking subclass of layer-stacked (opto-)electronic materials. The development of 2DOI-vdWhs via systematically integrating inorganic 2D layers with organic 2D crystals at the molecular/atomic scale extends the capabilities of traditional 2D inorganic vdWhs, thanks to their high synthetic flexibility and structural tunability. Constructing an organic-inorganic hybrid interface with atomic precision will unlock new opportunities for generating unique interfacial (opto-)electronic transport properties by combining the strengths of organic and inorganic layers, thus allowing us to satisfy the growing demand for multifunctional applications. Here, this review provides a comprehensive overview of the latest advancements in the chemical synthesis, structural characterization, and numerous applications of 2DOI-vdWhs. Firstly, we introduce the chemistry and the physical properties of the recently rising organic 2D crystals (O2DCs), which feature crystalline 2D nanostructures comprising carbon-rich repeated units linked by covalent/noncovalent bonds and exhibit strong in-plane extended π-conjugation and weak interlayer vdWs interaction. Simultaneously, representative inorganic 2D crystals (I2DCs) are briefly summarized. After that, the synthetic strategies will be systematically summarized, including synthesizing single-component O2DCs with dimensional control and their vdWhs with I2DCs. With these synthetic approaches, the control in the dimension, the stacking modes, and the composition of the 2DOI-vdWhs will be highlighted. Subsequently, a special focus will be given on the discussion of the optical and electronic properties of the single-component 2D materials and their vdWhs, which will be closely relevant to their structures, so that we can establish a general structure-property relationship of 2DOI-vdWhs. In addition to these physical properties, the (opto-)electronic devices such as transistors, photodetectors, sensors, spintronics, and neuromorphic devices as well as energy devices will be discussed. Finally, we provide an outlook to discuss the key challenges for the 2DOI-vdWhs and their future development. This review aims to provide a foundational understanding and inspire further innovation in the development of next-generation 2DOI-vdWhs with transformative technological potential.

Uniform Phase Permutation of Efficient Ruddlesden-Popper Perovskite Solar Cells via Binary Spacers and Single Crystal Coordination

2D Ruddlesden-Popper perovskites (RPPs) have attracted extensive attention in recent years due to their excellent environmental stability. However, the power conversion efficiency (PCE) of RPP solar cells is much lower than that of 3D perovskite solar cells (PSCs), mainly attributed to their poor carrier transport performance and excessive heterogeneous phases. Herein, the binary spacers (n-butylammonium, BA and benzamidine, PFA) are introduced to regulate the crystallization kinetics and n-value phase distribution to form uniform phase permutation of RPP films. The study then incorporates n = 5 BA2MA4Pb5I16 memory single crystal to achieve ultrafast stepped-type carrier transport from the low n-value phases to the high n-value phases in the high-quality (BA0.75PFA0.25)2MA4Pb5I16 films. These binary spacers and single-crystal-assisted crystallization strategies produce high-quality films, leading to fast carrier extraction and significant nonradiative recombination suppression. The resulting PSC presents a champion PCE of 21.15% with an impressive open circuit voltage (VOC) of 1.26 V, which is the record high efficiency and VOC for low n-value RPP solar cells (n ≤ 5).

Molecular Structure Tailoring of Organic Spacers for High-Performance Ruddlesden-Popper Perovskite Solar Cells

Layer-structured Ruddlesden-Popper (RP) perovskites (RPPs) with decent stability have captured the imagination of the photovoltaic research community and bring hope for boosting the development of perovskite solar cell (PSC) technology. However, two-dimensional (2D) or quasi-2D RP PSCs are encountered with some challenges of the large exciton binding energy, blocked charge transport and poor film quality, which restrict their photovoltaic performance. Fortunately, these issues can be readily resolved by rationally designing spacer cations of RPPs. This review mainly focuses on how to design the molecular structures of organic spacers and aims to endow RPPs with outstanding photovoltaic applications. We firstly elucidated the important roles of organic spacers in impacting crystallization kinetics, charge transporting ability and stability of RPPs. Then we brought three aspects to attention for designing organic spacers. Finally, we presented the specific molecular structure design strategies for organic spacers of RPPs aiming to improve photovoltaic performance of RP PSCs. These proposed strategies in this review will provide new avenues to develop novel organic spacers for RPPs and advance the development of RPP photovoltaic technology for future applications.

Intercalated Architecture of the Ca2A2Z5 Monolayer with High Electron Mobilities and High Power Conversion Efficiencies

The exploration of novel two-dimensional (2D) materials with a direct band gap and high mobility has attracted huge attention due to their potential application in electronic and optoelectronic devices. Here, we propose a feasible way to construct multiatomic monolayer Ca2A2Z5 (A = Al and Ga and Z = S, Se, and Te) by first-principles calculations. Our results indicated that the energies of α1-phase Ca2A2Z5 are slightly lower than those of experimentally synthesized α3-phase-like Ca2A2Z5 monolayers with excellent structural stability. Moreover, the α1- and α3-phase Ca2A2Z5 monolayers possess not only direct band gaps but also high electron mobilities (up to ∼103 cm2 V-1 s-1), demonstrating an intriguing range of visible light absorption. Importantly, α1- and α3-phase Ca2Ga2Se5 monolayers are good donor materials, and the corresponding Ca2Ga2Se5/ZrSe2 type-II heterostructures exhibit desirable power conversion efficiencies of 22.4% and 22.9%, respectively. Our findings provide a feasible way to explore new 2D materials and offer several Ca2A2Z5 candidate monolayers for the application of high-performance solar cells.

Halogen-Functionalized Hole Transport Materials with Strong Passivation Effects for Stable and Highly Efficient Quasi-2D Perovskite Solar Cells

The performance of quasi-two-dimensional (Q-2D) perovskite solar cells (PSCs) strongly depends on the interface characteristics between the hole transport material (HTM) and the perovskite layer. In this work, we designed and synthesized a series of HTMs with triphenylamine-carbazole as the core structure and modified end groups with chlorine and bromine atoms. These HTMs show deeper highest occupied molecular orbital energy levels than commercial HTMs. This reduced energy band mismatch between the HTM and perovskite layer facilitates efficient charge extraction at the interface. Moreover, these HTMs containing halogen atoms on the end groups could form halogen bonding with the Pb2+ ions at the buried interface of the perovskite layer, effectively passivating defects to suppress nonradiative recombination. Additionally, halogen bonding also contributes to the formation of vertically oriented perovskite crystals with a high quality. By incorporation of chlorohexane-substituted HTMs, the resultant Q-2D PSCs exhibited the highest power conversion efficiency of 21.07%. Furthermore, the devices show improved stability, retaining 97.2% of their initial efficiency after 1100 h of continuous illumination.

Multifunctional Buffer Layer Engineering for Efficient and Stable Wide-Bandgap Perovskite and Perovskite/Silicon Tandem Solar Cells

Inverted perovskite solar cells (PSCs) are preferred for tandem applications due to their superior compatibility with diverse bottom solar cells. However, the solution processing and low formation energy of perovskites inevitably lead to numerous defects at both the bulk and interfaces. We report a facile and effective strategy for precisely modulating the perovskite by incorporating AlOx deposited by atomic layer deposition (ALD) on the top interface. We find that Al3+ can not only infiltrate the bulk phase and interact with halide ions to suppress ion migration and phase separation but also regulate the arrangement of energy levels and passivate defects on the perovskite surface and grain boundaries. Additionally, ALD-AlOx exhibits an encapsulation effect through a dense interlayer. Consequently, the ALD-AlOx treatment can significantly improve the power conversion efficiency (PCE) to 21.80 % for 1.66 electron volt (eV) PSCs. A monolithic perovskite-silicon TSCs using AlOx-modified perovskite achieved a PCE of 28.5 % with excellent photothermal stability. More importantly, the resulting 1.55 eV PSC and module achieved a PCE of 25.08 % (0.04 cm2) and 21.01 % (aperture area of 15.5 cm2), respectively. Our study provides an effective way to efficient and stable wide-band gap perovskite for perovskite-silicon TSCs and paves the way for large-area inverted PSCs.

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.

Is Dielectric Mismatch Actually Important in 2D Perovskites?

Understanding and controlling exciton properties are important for the design of 2D semiconductors, such as monolayer transition metal dichalcogenides (TMDCs) and 2D halide perovskites (HPs). This paper demonstrates that the widespread strategy used for the exciton engineering of 2D HPs, based on dielectric mismatch, is flawed since dielectric mismatch has very little correlation with exciton properties. For monolayer TMDCs, however, the dielectric mismatch is shown to be more important.

Efficient and Stable Inverted Perovskite Solar Modules Enabled by Solid-Liquid Two-Step Film Formation

A considerable efficiency gap exists between large-area perovskite solar modules and small-area perovskite solar cells. The control of forming uniform and large-area film and perovskite crystallization is still the main obstacle restricting the efficiency of PSMs. In this work, we adopted a solid-liquid two-step film formation technique, which involved the evaporation of a lead iodide film and blade coating of an organic ammonium halide solution to prepare perovskite films. This method possesses the advantages of integrating vapor deposition and solution methods, which could apply to substrates with different roughness and avoid using toxic solvents to achieve a more uniform, large-area perovskite film. Furthermore, modification of the NiOx/perovskite buried interface and introduction of Urea additives were utilized to reduce interface recombination and regulate perovskite crystallization. As a result, a large-area perovskite film possessing larger grains, fewer pinholes, and reduced defects could be achieved. The inverted PSM with an active area of 61.56 cm2 (10 × 10 cm2 substrate) achieved a champion power conversion efficiency of 20.56% and significantly improved stability. This method suggests an innovative approach to resolving the uniformity issue associated with large-area film fabrication.

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.

Dimensional Regulation from 1D/3D to 2D/3D of Perovskite Interfaces for Stable Inverted Perovskite Solar Cells

Constructing low-dimensional/three-dimensional (LD/3D) perovskite solar cells can improve efficiency and stability. However, the design and selection of LD perovskite capping materials are incredibly scarce for inverted perovskite solar cells (PSCs) because LD perovskite capping layers often favor hole extraction and impede electron extraction. Here, we develop a facile and effective strategy to modify the perovskite surface by passivating the surface defects and modulating surface electrical properties by incorporating morpholine hydriodide (MORI) and thiomorpholine hydriodide (SMORI) on the perovskite surface. Compared with the PI treatment that we previously developed, the one-dimensional (1D) perovskite capping layer derived from PI is transformed into a two-dimensional (2D) perovskite capping layer (with MORI or SMORI), achieving dimension regulation. It is shown that the 2D SMORI perovskite capping layer induces more robust surface passivation and stronger n-N homotype 2D/3D heterojunctions, achieving a p-i-n inverted solar cell with an efficiency of 24.55%, which retains 87.6% of its initial efficiency after 1500 h of operation at the maximum power point (MPP). Furthermore, 5 × 5 cm2 perovskite mini-modules are presented, achieving an active-area efficiency of 22.28%. In addition, the quantum well structure in the 2D perovskite capping layer increases the moisture resistance, suppresses ion migration, and improves PSCs' structural and environmental stability.

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.

Crystal Growth Regulation of Ruddlesden-Popper Perovskites via Self-Assembly of Semiconductor Spacers for Efficient Solar Cells

The crystal growth and orientation of two-dimensional (2D) perovskite films significantly impact solar cell performance. Here, we incorporated robust quadrupole-quadrupole interactions to govern the crystal growth of 2D Ruddlesden-Popper (RP) perovskites. This was achieved through the development of two unique semiconductor spacers, namely PTMA and 5FPTMA, with different dipole moments. The ((5FPTMA)0.1 (PTMA)0.9 )2 MAn-1 Pbn I3n+1 (nominal n=5, 5F/PTMA-Pb) film shows a preferred vertical orientation, reduced grain boundaries, and released residual strain compared to (PTMA)2 MAn-1 Pbn I3n+1 (nominal n=5, PTMA-Pb), resulting in a decreased exciton binding energy and reduced electron-phonon coupling coefficients. In contrast to PTMA-Pb device with an efficiency of 15.66 %, the 5F/PTMA-Pb device achieved a champion efficiency of 18.56 %, making it among the best efficiency for 2D RP perovskite solar cells employing an MA-based semiconductor spacer. This work offers significant insights into comprehending the crystal growth process of 2D RP perovskite films through the utilization of quadrupole-quadrupole interactions between semiconductor spacers.