Two-dimensional (2D) halide perovskites have emerged as outstanding semiconducting materials thanks to their superior stability and structural diversity. However, the ever-growing field of optoelectronic device research using 2D perovskites requires systematic understanding of the effects of the spacer on the structure, properties, and device performance. So far, many studies are based on trial-and-error tests of random spacers with limited ability to predict the resulting structure of these synthetic experiments, hindering the discovery of novel 2D materials to be incorporated into high-performance devices. In this review, we provide guidelines on successfully choosing spacers and incorporating them into crystalline materials and optoelectronic devices. We first provide a summary of various synthetic methods to act as a tutorial for groups interested in pursuing synthesis of novel 2D perovskites. Second, we provide our insights on what kind of spacer cations can stabilize 2D perovskites followed by an extensive review of the spacer cations, which have been shown to stabilize 2D perovskites with an emphasis on the effects of the spacer on the structure and optical properties. Next, we provide a similar explanation for the methods used to fabricate films and their desired properties. Like the synthesis section, we will then focus on various spacers that have been used in devices and how they influence the film structure and device performance. With a comprehensive understanding of these effects, a rational selection of novel spacers can be made, accelerating this already exciting field.

The 2D Halide Perovskite Rulebook: How the Spacer Influences Everything from the Structure to Optoelectronic Device Efficiency.

Perovskite light-emitting diodes (PeLEDs) are considered as particularly attractive candidates for high-quality lighting and displays, due to possessing the features of wide gamut and real color expression However, most PeLEDs are made from polycrystalline perovskite films that contain a high concentration of defects, including point and extended imperfections Reducing and mitigating non-radiative recombination defects in perovskite materials are still crucial prerequisites for achieving high performance in light-emitting applications Here, ethoxylated trimethylolpropane triacrylate (ETPTA) is introduced as a functional additive dissolved in antisolvent to passivate surface and bulk defects during the spinning process The ETPTA can effectively decrease the charge trapping states by passivation and/or suppression of defects Eventually, the perovskite films that are sufficiently passivated by ETPTA make the devices achieve a maximum external quantum efficiency (EQE) of 2249% To our knowledge, these are the most efficient green PeLEDs up to now In addition, a threefold increase in the T50 operational time of the devices was observed, compared to control samples These findings provide a simple and effective strategy to make highly efficient perovskite polycrystalline films and their optoelectronics devices

Perovskite Light-Emitting Diodes with External Quantum Efficiency Exceeding 22% via Small-Molecule Passivation.

In the last decade, perovskite solar cells (PSCs) have undergone unprecedented rapid development and become a promising candidate for a new-generation solar cell. Among various PSCs, typical 3D halide perovskite-based PSCs deliver the highest efficiency but they suffer from severe instability, which restricts their practical applications. By contrast, the low-dimensional Ruddlesden-Popper (RP) perovskite-based PSCs have recently raised increasing attention due to their superior stability. Yet, the efficiency of RP perovskite-based PSCs is still far from that of the 3D counterparts owing to the difficulty in fabricating high-quality RP perovskite films. In pursuit of high-efficiency RP perovskite-based PSCs, it is critical to manipulate the film formation process to prepare high-quality RP perovskite films. This review aims to provide comprehensive understanding of the high-quality RP-type perovskite film formation by investigating the influential factors. On this basis, several strategies to improve the RP perovskite film quality are proposed via summarizing the recent progress and efforts on the preparation of high-quality RP perovskite film. This review will provide useful guidelines for a better understanding of the crystallization and phase kinetics during RP perovskite film formation process and the design and development of high-performance RP perovskite-based PSCs, promoting the commercialization of PSC technology.

High‐Quality Ruddlesden–Popper Perovskite Film Formation for High‐Performance Perovskite Solar Cells

Long-term durability is critically important for the commercialization of perovskite solar cells (PSCs). The ionic character of the perovskite and the hydrophilicity of commonly used additives for the hole-transporting layer (HTL), such as lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) and tert-butylpyridine (tBP), render PSCs prone to moisture attack, compromising their long-term stability. Here we introduce a trifluoromethylation strategy to overcome this drawback and to boost the PSC's solar to electric power conversion efficiency (PCE). We employ 4-(trifluoromethyl)benzylammonium iodide (TFMBAI) as an amphiphilic modifier for interfacial defect mitigation and 4-(trifluoromethyl)pyridine (TFP) as an additive to enhance the HTL's hydrophobicity. Surface treatment of the triple-cation perovskite with TFMBAI largely suppressed the nonradiative charge carrier recombination, boosting the PCE from 20.9% to 23.9% and suppressing hysteresis, while adding TFP to the HTL enhanced the PCS's resistance to moisture while maintaining its high PCE. Taking advantage of the synergistic effects resulting from the combination of both fluoromethylated modifiers, we realize TFMBAI/TFP-based highly efficient PSCs with excellent operational stability and resistance to moisture, retaining over 96% of their initial efficiency after 500 h maximum power point tracking (MPPT) under simulated 1 sun irradiation and 97% of their initial efficiency after 1100 h of exposure under ambient conditions to a relative humidity of 60-70%.

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Synergistic Effect of Fluorinated Passivator and Hole Transport Dopant Enables Stable Perovskite Solar Cells with an Efficiency Near 24.

The Future of Perovskite Photovoltaics—Thermal Evaporation or Solution Processing?

Two-dimensional (2D) perovskites are emerging photovoltaic materials because of their highly tunable photophysical properties and improved environmental stability in comparison with 3D perovskites. Here, a thiophene-based bulky dication spacer, namely, 2,5-thiophenedimethylammonium (ThDMA), was developed and applicated in 2D Dion-Jacobson (DJ) perovskite. High-quality 2D DJ perovskite, (ThDMA)(MA)n-1PbnI3n+1 (nominal n = 5), with improved crystallinity, preferred vertical orientation, and enlarged spatially resolved carrier lifetime could be achieved by a one-step method using a mixed solvent of DMF/DMSO (v/v, 9:1). The optimized device exhibits a high efficiency of 15.75%, which is a record for aromatic spacer-based 2D DJ perovskite solar cells (PSCs). Moreover, the unencapsulated 2D DJ perovskite devices sustained over 95% of their original efficiency after storage in N2 for 1655 h. Importantly, both the light-soaking stability and thermal stability (T = 80 °C) of the 2D DJ perovksite devices are dramatically improved in comparison with their 3D counterparts. These results indicate that highly efficient and stable 2D DJ PSCs could be achieved by developing thiophene-based aromatic spacers as well as device engineering.

Thiophene-Based Two-Dimensional Dion-Jacobson Perovskite Solar Cells with over 15% Efficiency.

Quasi-2D Ruddlesden-Popper (RP) perovskite solar cells (PSCs) have drawn significant attention due to their appealing environmental stability compared to their 3D counterparts. However, the relatively low power conversion efficiency (PCE) greatly limits their applications. Here, high photovoltaic performance is demonstrated for quasi-2D RP PSCs using 2-thiophenemethylammonium as spacer with nominal n-value of 5, which is based on the stoichiometry of the precursors. The incorporation of formamidinium (FA) in quasi-2D RP perovskites reduces the bandgap and improves the light absorption ability, resulting in enlarged photocurrent and an increased PCE of 16.18%, which is higher than that of reported analogous methylammonium (MA)-based quasi-2D PSC (≈15%). A record high PCE of 19.06% is further demonstrated by using an organic salt, namely, 4-(trifluoromethyl)benzylammonium iodide, assisted crystal growth (OACG) technique, which can induce the crystal growth and orientation, tune the surface energy levels, and suppress the charge recombination losses. More importantly, the devices based on OACG-processed quasi-2D RP perovskites show remarkable environmental stability and thermal stability, for example, the PCE retaining ≈96% of its initial value after storage at 80 °C for 576 h, while only ≈37% of the original efficiency left for FAPbI3 -based 3D PSCs.

Organic‐Salt‐Assisted Crystal Growth and Orientation of Quasi‐2D Ruddlesden–Popper Perovskites for Solar Cells with Efficiency over 19%

Two-dimensional (2D) perovskites with natural multi-quantum-well structure have been reported to offer better stability compared to 3D perovskites. However, the understanding of the exciton separation and transport mechanism in 2D perovskites and developing more efficient organic spacers remain considerable challenges, as the 2D perovskites exhibit large exciton binding energy due to quantum confinement. Here, a class of multiple-ring aromatic ammoniums, 1-naphthalenemethylammonium (NpMA) and 9-anthracenemethylammonium (AnMA), was developed as spacers for 2D Ruddlesden-Popper (RP) perovskite solar cells (PSCs). In addition to significantly enhanced stability, the device based on (NpMA)2(MA)n-1PbnI3n+1 (average n = 4) exhibits a champion efficiency of 17.25% and a high open-circuit voltage of 1.24 V. The outstanding photovoltaic performance could be ascribed to the ultrafast exciton migration (within 7 ps) from 2D phases to 3D-like phases, which were confirmed by charge carrier dynamics results, leading to efficient exciton separation, charge transportation, and collection. This work facilitates understanding the working mechanism of 2D PSCs in-depth and offers an efficient way to further boost their efficiency and stability by developing multiple-ring aromatic spacers.

Phase Distribution and Carrier Dynamics in Multiple-Ring Aromatic Spacer-Based Two-Dimensional Ruddlesden-Popper Perovskite Solar Cells.

2-Thiopheneformamidinium-Based 2D Ruddlesden–Popper Perovskite Solar Cells with Efficiency of 16.72% and Negligible Hysteresis

Replacing the dominated hole transport material (HTM), doped Spiro-OMeTAD, in state-of-the-art perovskite solar cells (PSCs) is a challenge but an urgent issue for commercialization of this technol...

Zhiyuan Xu

Papers

Thiophene-Based Two-Dimensional Dion-Jacobson Perovskite Solar Cells with over 15% Efficiency.

Organic‐Salt‐Assisted Crystal Growth and Orientation of Quasi‐2D Ruddlesden–Popper Perovskites for Solar Cells with Efficiency over 19%

Phase Distribution and Carrier Dynamics in Multiple-Ring Aromatic Spacer-Based Two-Dimensional Ruddlesden-Popper Perovskite Solar Cells.

2-Thiopheneformamidinium-Based 2D Ruddlesden–Popper Perovskite Solar Cells with Efficiency of 16.72% and Negligible Hysteresis

Multifunctional Two-Dimensional Conjugated Materials for Dopant-Free Perovskite Solar Cells with Efficiency Exceeding 22%