3D perovskite solar cells (PSCs) have shown great promise for use in next-generation photovoltaic devices. However, some challenges need to be addressed before their commercial production, such as enormous defects formed on the surface, which result in severe SRH recombination, and inadequate material interplay between the composition, leading to thermal-, moisture-, and light-induced degradation. 2D perovskites, in which the organic layer functions as a protective barrier to block the erosion of moisture or ions, have recently emerged and attracted increasing attention because they exhibit significant robustness. Inspired by this, surface passivation by employing 2D perovskites deposited on the top of 3D counterparts has triggered a new wave of research to simultaneously achieve higher efficiency and stability. Herein, we exploited a vast amount of literature to comprehensively summarize the recent progress on 2D/3D heterostructure PSCs using surface passivation. The review begins with an introduction of the crystal structure, followed by the advantages of the combination of 2D and 3D perovskites. Then, the surface passivation strategies, optoelectronic properties, enhanced stability, and photovoltaic performance of 2D/3D PSCs are systematically discussed. Finally, the perspectives of passivation techniques using 2D perovskites to offer insight into further improved photovoltaic performance in the future are proposed. 

Surface Passivation Using 2D Perovskites toward Efficient and Stable Perovskite Solar Cells

Flexible perovskite solar cells (f‐PSCs) have attracted great attention because of their unique advantages in lightweight and portable electronics applications. However, their efficiencies are far inferior to those of their rigid counterparts. Herein, a novel histamine diiodate (HADI) is designed based on theoretical study to modify the SnO2/perovskite interface. Systematic experimental results reveal that the HADI serves effectively as a multifunctional agent mainly in three aspects: 1) surface modification to realign the SnO2 conduction band upward to improve interfacial charge extraction; 2) passivating the buried perovskite surface, and 3) bridging between the SnO2 and perovskite layers for effective charge transfer. Consequently, the rigid MA‐free PSCs based on the HADI‐SnO2 electron transport layer (ETL) display not only a high champion power conversion efficiency (PCE) of 24.79% and open‐circuit voltage (VOC) of 1.20 V but also outstanding stability as demonstrated by the PSCs preserving 91% of their initial efficiencies after being exposed to ambient atmosphere for 1200 h without any encapsulation. Furthermore, the solution‐processed HADI‐SnO2 ETL formed at low temperature (100 °C) is utilized in f‐PSCs that achieve a PCE as high as 22.44%, the highest reported PCE for f‐PSCs to date.

Record‐Efficiency Flexible Perovskite Solar Cells Enabled by Multifunctional Organic Ions Interface Passivation

All-inorganic CsPbI3 perovskite presents preeminent chemical stability and desirable band gap as the front absorber for perovskite/silicon tandem solar cells. Unfortunately, CsPbI3 perovskite solar cells (PSCs) still show low efficiency due to high density of defects in solution-prepared CsPbI3 films. Herein, three kinds of hydrazine derivatives (benzoyl hydrazine (BH), formohydrazide (FH) and benzamide (BA)) are designed to reduce the defect density and stabilize the phase of CsPbI3 . Calculation and characterization results corroborate that the carboxyl and hydrazine groups in BH form strong chemical bonds with Pb2+ ions, resulting in synergetic double coordination. In addition, the hydrazine group in the BH also forms a hydrogen bond with iodine to assist the coordination. Consequently, a high efficiency of 20.47% is achieved, which is the highest PCE among all pure CsPbI3-based PSCs reported to date. In addition, an unencapsulated device showed excellent stability in ambient air.

Hydrazide Derivatives for Defect Passivation in Pure CsPbI3 Perovskite Solar Cells.

Review on efficiency improvement effort of perovskite solar cell

Despite the swift development in perovskite solar cells (PSCs), suppressing the ion defects in the perovskite bulk and further extending the long‐lasting stability of the cells remain the concerned issues that are yet to be solved. Here, a symmetrical organic acceptor−donor−acceptor (A−D−A) molecule with the core architecture of indaceno[1,2‐b:5,6‐b']dithiophene (IDT) and bilateral arms of oxindole, named IDT‐OD, as a versatile defect passivation agent, is adopted to inactivate the nonradiative recombination sites in the perovskite absorber. The S element in the IDT unit and carbonyl group CO in the bilateral acceptor unit as the Lewis‐base contributes to the passivation sites that are the under‐coordinated Pb2+ cation defects and the N−H group in oxindole unit interacts with halide dangling bonds. The molecular structure with its symmetrical double arms assists the formation of a superior perovskite layer with enlarged grain size, smooth surface topography, hydrophobic property, and low density of defect state. Consequently, the corresponding PSCs with the proper IDT‐OD additive yield a remarkable increase in efficiency from 22.77% to 24.04%, along with excellent long‐term environmental and thermal stabilities. This study offers a propitious approach for ionic defect passivation engineering toward high‐performance PSCs.

Symmetrical Acceptor–Donor–Acceptor Molecule as a Versatile Defect Passivation Agent toward Efficient FA0.85MA0.15PbI3 Perovskite Solar Cells

Passivating undercoordinated ions is an effective way to reduce the defect densities at the surface and grain boundaries (GBs) of perovskite materials for enhanced photovoltaic performance and stability of perovskite solar cells (PSCs). Here, (BBF) complex is chosen as a multifunctional additive, which contains both C7H9N and BF3 groups working as Lewis base and Lewis acid, respectively, can bond with Pb2+/I- and FA+ on the surface and in the GBs in the perovskite film, affording passivation of both cation and anion defects. The synergistic effect of the C7H9N and BF3 complex slows the crystallization during the perovskite film deposition to improve the crystalline quality, which reduces the trap density and the recombination in the perovskite film to suppress nonradiative recombination loss and minimizes moisture permeation to improve the stability of the perovskite material. Meanwhile, such an additive improves the energy-level alignment between the valence band of the perovskite and the highest occupied molecular orbital of the hole-transporting material, Spiro-OMeTAD. Consequently, our work achieves power conversion efficiency of 23.24%, accompanied by enhanced stability under ambient conditions and light illumination and opens a new avenue for improving the performance of PSCs through the use of a multifunctional complex.

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Sheng Zhan

Papers

A Special Additive Enables All Cations and Anions Passivation for Stable Perovskite Solar Cells with Efficiency over 23

Graded 2D/3D (CF3-PEA)2FA0.85MA0.15Pb2I7/FA0.85MA0.15PbI3 heterojunction for stable perovskite solar cell with an efficiency over 23.0%