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

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.

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.

Aggregative Luminescence from CsPbBr3 Perovskite Precursors

Understanding the properties of the precursor can provide deeper insight into the crystallization and nucleation mechanisms of perovskites, which is vital for the solution-process device performance. Herein, we conducted a detailed investigation into the photophysics properties of CsPbBr3 precursors in a broad concentration and various solvents. The precursor transformed from the solution state into the colloidal state and exhibited aggregation-induced emission character as the concentration increased. The aggregative luminescence from the precursors originates from the polybromide plumbous that is formed through the coordination of solvent molecules to the lead metal center. Two adducts with monodentate (PbBr2 ⋅ solvent) and bidentate (PbBr2 ⋅ 2solvent) ligands can be obtained, accompanied by emission with photoluminescence at 610 and 565 nm, respectively. Furthermore, the aggregative luminescence intensity and color could be regulated by changing the solvent and precursor ratio. Besides, we discussed the difference between the molecular aggregate in the organic system and the ionic aggregate in the inorganic system: the ionic aggregate is composed of solvated ions rather than individual molecules as in organic systems, which could possess properties that ions do not have. The fluorescence that is sensitive to Pb2+ coordination reported here could be applied to screen perovskite additives and judge the precursor aging.

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.

Improvement of Photovoltaic Performance of Perovskite Solar Cells by Synergistic Modulation of SnO2 and Perovskite via Interfacial Modification

In the past decade, perovskite solar cell (PSC) photoelectric conversion efficiency has advanced significantly, and tin dioxide (SnO2) has been extensively used as the electron transport layer (ETL). Due to its high electron mobility, strong chemical stability, energy level matching with perovskite, and easy low-temperature fabrication, SnO2 is one of the most effective ETL materials. However, the SnO2 material as an ETL has its limitations. For example, SnO2 films prepared by low-temperature spin-coating contain a large number of oxygen vacancies, resulting in energy loss and high open-circuit voltage (VOC) loss. In addition, the crystal quality of perovskites is closely related to the substrate, and the disordered crystal orientation will lead to ion migration, resulting in a large number of uncoordinated Pb2+ defects. Therefore, interface optimization is essential to improve the efficiency and stability of the PSC. In this work, 2-(5-chloro-2-benzotriazolyl)-6-tert-butyl-p-cresol (CBTBC) was introduced for ETL modification. On the one hand, the hydroxyl group of CBTBC forms a Lewis mixture with the Sn atom, which reduces the oxygen vacancy defect and prevents nonradiative recombination. On the other hand, the SnO2/CBTBC interface can effectively improve the crystal orientation of perovskite by influencing the crystallization kinetics of perovskite, and the nitrogen element in CBTBC can effectively passivate the uncoordinated Pb2+ defects at the SnO2/perovskite interface. Finally, the prevailing PCE of PSC (1.68 eV) modified by CBTBC was 20.34% (VOC = 1.214 V, JSC = 20.49 mA/cm2, FF = 82.49%).