Although known since the late 19th century, organic–inorganic perovskites have recently received extraordinary research community attention because of their unique physical properties, which make them promising candidates for application in photovoltaic (PV) and related optoelectronic devices. This review will explore beyond the current focus on three-dimensional (3-D) lead(II) halide perovskites, to highlight the great chemical flexibility and outstanding potential of the broader class of 3-D and lower dimensional organic-based perovskite family for electronic, optical, and energy-based applications as well as fundamental research. The concept of a multifunctional organic–inorganic hybrid, in which the organic and inorganic structural components provide intentional, unique, and hopefully synergistic features to the compound, represents an important contemporary target.

Organic–Inorganic Perovskites: Structural Versatility for Functional Materials Design

The photovoltaics of organic–inorganic lead halide perovskite materials have shown rapid improvements in solar cell performance, surpassing the top efficiency of semiconductor compounds such as CdTe and CIGS (copper indium gallium selenide) used in solar cells in just about a decade. Perovskite preparation via simple and inexpensive solution processes demonstrates the immense potential of this thin-film solar cell technology to become a low-cost alternative to the presently commercially available photovoltaic technologies. Significant developments in almost all aspects of perovskite solar cells and discoveries of some fascinating properties of such hybrid perovskites have been made recently. This Review describes the fundamentals, recent research progress, present status, and our views on future prospects of perovskite-based photovoltaics, with discussions focused on strategies to improve both intrinsic and extrinsic (environmental) stabilities of high-efficiency devices. Strategies and challenges regardi...

Halide Perovskite Photovoltaics: Background, Status, and Future Prospects

Metal halide perovskites of the general formula ABX3—where A is a monovalent cation such as caesium, methylammonium or formamidinium; B is divalent lead, tin or germanium; and X is a halide anion—have shown great potential as light harvesters for thin-film photovoltaics1–5. Among a large number of compositions investigated, the cubic α-phase of formamidinium lead triiodide (FAPbI3) has emerged as the most promising semiconductor for highly efficient and stable perovskite solar cells6–9, and maximizing the performance of this material in such devices is of vital importance for the perovskite research community. Here we introduce an anion engineering concept that uses the pseudo-halide anion formate (HCOO−) to suppress anion-vacancy defects that are present at grain boundaries and at the surface of the perovskite films and to augment the crystallinity of the films. The resulting solar cell devices attain a power conversion efficiency of 25.6 per cent (certified 25.2 per cent), have long-term operational stability (450 hours) and show intense electroluminescence with external quantum efficiencies of more than 10 per cent. Our findings provide a direct route to eliminate the most abundant and deleterious lattice defects present in metal halide perovskites, providing a facile access to solution-processable films with improved optoelectronic performance. Incorporation of the pseudo-halide anion formate during the fabrication of α-FAPbI3 perovskite films eliminates deleterious iodide vacancies, yielding solar cell devices with a certified power conversion efficiency of 25.21 per cent and long-term operational stability.

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Pseudo-halide anion engineering for α-FAPbI3 perovskite solar cells.

This article gives an overview of the application of nanomaterials in environmental remediation. In the area of environmental remediation, nanomaterials offer the potential for the efficient removal of pollutants and biological contaminants. Nanomaterials in various shapes/morphologies, such as nanoparticles, tubes, wires, fibres etc., function as adsorbents and catalysts and their composites with polymers are used for the detection and removal of gases (SO2, CO, NOx, etc.), contaminated chemicals (arsenic, iron, manganese, nitrate, heavy metals, etc.), organic pollutants (aliphatic and aromatic hydrocarbons) and biological substances, such as viruses, bacteria, parasites and antibiotics. Nanomaterials show a better performance in environmental remediation than other conventional techniques because of their high surface area (surface-to-volume ratio) and their associated high reactivity. Recent advances in the fabrication of novel nanoscale materials and processes for the treatment of drinking water and industrial waste water contaminated by toxic metal ions, radionuclides, organic and inorganic solutes, bacteria and viruses and the treatment of air are highlighted. In addition, recent advances in the application of polymer nanocomposite materials for the treatment of contaminants and the monitoring of pollutants are also discussed. Furthermore, the research trends and future prospects are briefly discussed.

A review on nanomaterials for environmental remediation

2.3. Ionic Liquids (ILs) 5374 2.4. Complexation Reactions on Oxidized CNTs 5375 2.5. Halogenation 5376 2.6. Cycloaddition Reactions 5377 2.7. Radical Additions 5379 2.8. Nucleophilic Additions 5381 2.9. Electrophilic Additions 5381 2.10. Electrochemical Modifications 5381 2.11. Plasma-Activation 5381 2.12. Mechanochemical Functionalizations 5382 3. Noncovalent Interactions 5382 3.1. Polynuclear Aromatic Compounds 5382 3.2. Interactions with Other Substances 5384 3.3. Interactions with Biomolecules 5385 4. Endohedral Filling 5386 4.1. Encapsulation of Fullerenes 5386 4.2. Encapsulation of Organic Substances 5387 4.3. Encapsulation of Inorganic Substances 5387 5. Decoration of CNTs with Metal Nanoparticles 5388 5.1. Covalent Linkage 5388 5.2. Direct Formation on Defect Sites 5388 5.3. Electroless Deposition 5388 5.4. Electrodeposition 5389 5.5. Chemical Decoration 5390 5.6. Deposition of Nanoparticles onto CNTs 5391 5.7. π-π Stacking and Electrostatic Interactions 5391 6. Concluding Remarks 5392 7. Acknowledgments 5392 8. References 5392

Current progress on the chemical modification of carbon nanotubes.

Morphology control is critical to achieve high efficiency CH3NH3PbI3 perovskite solar cells (PSC). The surface properties of the substrates on which crystalline perovskite thin films form are expected to affect greatly the crystallization and, thus, the resulting morphology. However, this topic is seldom examined in PSC. Here we developed a facile but efficient method of modifying the ZnO-coated substrates with 3-aminopropanioc acid (C3-SAM) to direct the crystalline evolution and achieve the optimal morphology of CH3NH3PbI3 perovskite film. With incorporation of the C3-SAM, highly crystalline CH3NH3PbI3 films were formed with reduced pin-holes and trap states density. In addition, the work function of the cathode was better aligned with the conduction band minimum of perovskite for efficient charge extraction and electronic coupling. As a result, the PSC performance remarkably increased from 9.81(±0.99)% (best 11.96%) to 14.25(±0.61)% (best 15.67%). We stress the importance of morphology control through ...

Enhanced photovoltaic performance of CH3NH3PbI3 perovskite solar cells through interfacial engineering using self-assembling monolayer.

DOI: 10.1002/aenm.201702498 with chemical formula of A2Bn−1MnX3n+1 (n ≥ 2), wherein M is Pb2+ or Sn2+, B is CH3NH3 (MA+), HC(NH2)2 (FA+), or Cs+, A is the spacer cation including CH3(CH2)2NH3 (BA+), C6H5(CH2)2NH3 (PEA+), etc., have been developed and applied as the light-harvesting layer in PVSCs.[18–24] The usage of the spacer cation favors the formation of stable 2D perovskite, but the related device efficiency decreases rapidly. The partial replacement of MA+ with the spacer cations leads to the anisotropy in crystal structure and subsquently anisotropic charge carriers transportation. As a result, the movement of the charge carriers in 2D perovskite is blocked by the insulating spacer cation layer.[20] It is considered as one of the main reasons why 2D perovskite-based PVSCs show a much lower PCE than 3D-based PVSCs, for example, 4.73% for (PEA)2(MA)n−1PbnI3n+1 and 4.02% for (BA)2(MA)n−1PbnI3n+1 with n = 3. To improve the PCE of the 2D-based PVSCs, researchers tried to reduce the insulating effects of the organic layer by increasing the n value. Mora-Seró and coworkers[27] fabricated (C6H5NH3I)2(CH3NH3I)n−1(PbI2)n film on compact TiO2/mesoporous TiO2 substrate and obtained PCEs from 0.66% (n = 1) to 5.94% (n = 5). Sargent and co-workers[28] prepared (PEA)2(MA)n−1PbnI3n+1 perovskite on dense compact TiO2 substrate and achieved the highest PCE of 15.3% in the case of n = 60. But the PCE reduced to 9.22% with decreasing n to 6 due to the wide bandgap and poor carrier transport. Etgar and co-workers[29] fabricated (PEA)2(MA)n−1PbnBr3n+1 based Increasing the power conversion efficiency (PCE) of the two-dimensional (2D) perovskite-based solar cells (PVSCs) is really a challenge. Vertical orientation of the 2D perovskite film is an efficient strategy to elevate the PCE. In this work, vertically orientated highly crystalline 2D (PEA)2(MA)n–1PbnI3n+1 (PEA= phenylethylammonium, MA = methylammonium, n = 3, 4, 5) films are fabricated with the assistance of an ammonium thiocyanate (NH4SCN) additive by a one-step spin-coating method. Planar-structured PVSCs with the device structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/ (PEA)2(MA)n–1PbnI3n+1/[6,6]-phenyl-C61-butyric acid methyl ester /bahocuproine/ Ag are fabricated. The PCE of the PVSCs is boosted from the original 0.56% (without NH4SCN) to 11.01% with the optimized NH4SCN addition at n = 5, which is among the highest PCE values for the low-n (n < 10) 2D perovskitebased PVSCs. The improved performance is attributed to the vertically orientated highly crystalline 2D perovskite thin films as well as the balanced electron/ hole transportation. The humidity stability of this oriented 2D perovskite thin film is also confirmed by the almost unchanged X-ray diffraction patterns after 28 d exposed to the moisture in a humidity-controlled cabinet (Hr = 55 ± 5%). The unsealed device retains 78.5% of its original PCE after 160 h storage in air atmosphere with humidity of 55 ± 5%. The results provide an effective approach toward a highly efficient and stable PVSC for future commercialization. Perovskite Solar Cells

Orientation Regulation of Phenylethylammonium Cation Based 2D Perovskite Solar Cell with Efficiency Higher Than 11

Vertically oriented highly crystalline 2D layered (BA)2 (MA)n-1 Pbn I3n+1 (BA = CH3 (CH2 )3 NH3 , MA = CH3 NH3 , n = 3, 4) perovskite thin-films are fabricated with the aid of ammonium thiocyanate (NH4 SCN) additive through one-step spin-coating process. The humidity-stability of the film is certified by the almost unchanged X-ray diffraction patterns after exposed to humid atmosphere (Hr = 55 ± 5%) for 40 d. The photovoltaic devices with the structure of indium tin oxide(ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate)/(BA)2 (MA)n-1 Pbn I3n+1 (n = 3,4)/[6,6]-phenyl-C61 -butyric acid methyl ester/Bathocuproine/Ag are fabricated. The devices based on (BA)2 (MA)2 Pb3 I10 perovskite (n = 3) with the precursor composition of BAI:methylammonium iodide:PbI2 :NH4 SCN = 2:2:3:1 (by molar ratio) show an averaged power conversion efficiency (PCE) of 6.82%. In the case of (BA)2 (MA)3 Pb4 I13 (n = 4), a higher PCE of 8.79% is achieved. Both of the unsealed devices perform unique stability with almost unchanged PCE during the period of storage in purified N2 glove box. This work provides a simple and effective method to enhance the efficiency of the 2D perovskite solar cell.

Vertically Oriented 2D Layered Perovskite Solar Cells with Enhanced Efficiency and Good Stability

As an important category of perovskite materials, two-dimensional (2D) perovskites are attracting increasing research attention these days. Their possibility of combining high performance and stability for perovskite based optoelectronic devices has triggered a new wave of research. This review mainly focuses on the application of 2D perovskite materials for solar cells. We start with a brief introduction of 2D perovskite structures and their unique properties. The recent progress in 2D perovskite solar cells is summarized in three aspects according to the existing forms of the perovskite materials in the devices. In the end, a short outlook with our opinion is given to indicate the possible development trend for this kind of perovskite material.

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Recent progress in 2D/quasi-2D layered metal halide perovskites for solar cells

Thiocyanate ammonium (NH4SCN) is introduced into the fabrication of formamidinium lead triiodide (FAPbI3) films through one-step spin-coating. The promoted formation of black trigonal phase α-FAPbI3 with better crystallinity has been observed after the addition of NH4SCN, together with the supression of the formation of yellow hexagonal phase δ-FAPbI3. Planar perovskite solar cells (PVSCs) based on NH4SCN-assisted formed α-FAPbI3 films with high quality present a highest power conversion efficiency of 11.44% when 30 mol% NH4SCN is applied. Notably, the addition of NH4SCN is found to enhance the moisture stability of the perovskite. As a result, the planar PVSCs with 30 mol% NH4SCN additive show improved stability under ambient conditions (RH: 30–40%) over those based on pristine FAPbI3. NH4SCN simultaneously enhances the efficiency and moisture stability of FAPbI3 based PVSCs through a single one-step solution method, facilitating their commercial fabrication and application.

Gang Wu

Papers

Enhanced photovoltaic performance of CH3NH3PbI3 perovskite solar cells through interfacial engineering using self-assembling monolayer.

Orientation Regulation of Phenylethylammonium Cation Based 2D Perovskite Solar Cell with Efficiency Higher Than 11

Vertically Oriented 2D Layered Perovskite Solar Cells with Enhanced Efficiency and Good Stability

Recent progress in 2D/quasi-2D layered metal halide perovskites for solar cells

Thiocyanate assisted performance enhancement of formamidinium based planar perovskite solar cells through a single one-step solution process