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BY 4.0 license Open Access Published by De Gruyter July 19, 2019

Perovskite nanocrystals for energy conversion and storage

  • Athanasia Kostopoulou

    Athanasia Kostopoulou received her BSc degree in Physics (2004) and her MSc degree (2006) in Materials Physics and Technology from the Physics Department at the Aristotle University of Thessaloniki. In 2012, she received her PhD from the Department of Chemistry at the University of Crete, and since then she was a Postdoctoral Fellow in the Institute of Electronic Structure and Laser at FORTH in Heraklion. Since 2016, she has been a part of the group of the Ultrafast Laser Micro and Nano Processing (ULMNP) Laboratory and she is working on the chemical synthesis and elucidation of the microscopic physical or photoinduced mechanisms involving nanocrystal systems. Recently she has been the coordinator of a project funded from the Hellenic Foundation for Research and Innovation (HFRI) and the General Secretariat for Research and Technology (GSRT) related to perovskite nanomaterials for photovoltaic applications.

     ORCID logo EMAIL logo     , Konstantinos Brintakis

    Konstantinos Brintakis is a postdoctoral researcher at IESL-FORTH. He received his BSc in Physics from the Aristotle University of Thessaloniki (AUTh, Greece). He then continued his studies in “Materials Physics and Technology” obtaining his MSc from the Physics Department, AUTh. In 2017, he graduated with a PhD from the same Department and University with “Excellence”. His PhD thesis was a collaboration with the Physics Department and the Institute of Electronic Structure and Laser, studying the growth and organization of hybrid nanocrystals and specifically their structural, electronic and magnetic properties. Joining the Ultrafast Laser Micro- and Nano-Processing Group, he is interested in the interaction of matter with lasers and the development of nanostructures in solutions or/and on substrates with physicochemical and laser-assisted approaches. He is currently working on the synthesis and characterization of perovskite nanostructures for energy and gas sensing applications. He is a highly skilled scientist on the characterization of structural and morphological properties with high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) microscopy of the developed materials. He is also interested in the exploitation of the produced nanostructures in useful applications.

         , Nektarios K. Nasikas

    Nektarios K. Nasikas is a physicist with a PhD in the field of Physical Chemistry of Materials. Over the years he has worked in prestigious research centers and universities, in Greece as well as in the USA, such as the Foundation for Research and Technology Hellas/Institute of Chemical Engineering Sciences (FORTH/ICEHT), the National Hellenic Research Foundation/Theoretical and Physical Chemistry Institute (NHRF/TPCI) and the Peter A. Rock Thermochemistry Laboratory/University of California at Davis. His research interests include novel materials synthesis, materials thermodynamics, novel experimental setup, and design with the use of strong lasers, as well as technology and applications of novel materials possessing special characteristics. He has authored several scientific papers in international scientific journals of high impact such as Chemistry of Materials, Advanced Functional Materials, Journal of the American Ceramic Society, etc. He currently serves as the Director of the Hellenic Foundation for Research and Innovation (HFRI), in Athens, Greece.

         and Emmanuel Stratakis

    Dr Emmanuel Stratakis is a Research Director at the Institute of Electronic structure and laser (IESL) (www.iesl.forth.gr) of the Foundation for Research and Technology–Hellas (FORTH) (www.forth.gr). He received his Ph.D. in Physics from the University of Crete in 2001 from the Physics Department, University of Crete. After graduating, he joined as a visiting Researcher the IESL-FORTH working on the ultrafast laser engineering of materials and as an Adjunct Professor at the Department of Materials Science and Technology, University of Crete. In the fall semesters of 2006 and 2008 he was appointed as a visiting Researcher at the Department of Mechanical Engineering of the University of California, Berkeley. In 2007 he was elected Researcher at IESL-FORTH where he is leading the “Ultrafast Laser Micro- and Nano-processing” laboratory (http://stratakislab.iesl.forth.gr; https://www.iesl.forth.gr/en/research/ULNMP-Group). His research interests are in the fields of ultrafast laser interactions with materials for (a) biomimetic micro- and nano-structuring (b) Advanced photonic processes for photovoltaics and energy storage, c) nanomaterials synthesis and diagnostics for optoelectronics and (c) biomaterials processing for tissue engineering. He has delivered more than 40 invited and keynote lectures and has been organizer and chair in major international scientific conferences. He has over 180 SCI publications and more than 6000 citations and he has coordinated many National and EU grants. Since 2015, he is the Director of the European Nanoscience Facility of FORTH, part of the NFFA-Europe EU Infrastructure, where he is a member of the General Assembly. He is a National Representative to the High-Level Group of EU on Nanosciences, Nanotechnology and Advanced Materials and a National Expert for the Horizon 2020 committee configurations on: Nanotechnologies, Advanced materials, Biotechnology, Advanced Manufacturing and Processing. He is a member of the Scientific Committee of COST, of the Physical Sciences sectoral scientific council of the National Council for Research & Innovation of Greece and national Delegate of the OECD Working Party on Bio-, Nano- and Converging Tech (BNCT).

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From the journal Nanophotonics
https://doi.org/10.1515/nanoph-2019-0119
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Abstract

The high demand for energy consumption in everyday life, and fears of climate change are driving the scientific community to explore prospective materials for efficient energy conversion and storage. Perovskites, a prominent category of materials, including metal halides and perovskite oxides have a significant role as energy materials, and can effectively replace conventional materials. The simultaneous need for new energy materials together with the increased interest for making new devices, and exploring new physics, thrust the research to control the structuring of the perovskite materials at the nanoscale. Nanostructuring of the perovskites offers unique features such as a large surface area, extensive porous structures, controlled transport and charge-carrier mobility, strong absorption and photoluminescence, and confinement effects. These features together with the unique tunability in their composition, shape, and functionalities make perovskite nanocrystals efficient for energy-related applications such as photovoltaics, catalysts, thermoelectrics, batteries, supercapacitor and hydrogen storage systems. The synthesis procedures of perovskite nanostructures in different morphologies is summarized and the energy-related properties and applications are extensively discussed in this paper.

Keywords: perovskite nanocrystals; energy conversion devices; energy storage devices; thermoelectrics

1 Introduction

The high demand for energy consumption in everyday life activities along with fears of the climate changes highlight the importance to develop efficient energy conversion and storage devices. Thus, sufficient energy conversion and storage together with low-cost energy materials are the most important requirements. In order to design such devices, it is crucial to study and understand the underlying principles and mechanisms of renewable energy conversion and storage. Each of these technologies has its own characteristics, requirements, and efficiency limits or constraints. Different mechanisms take place in each technology and this is the main reason for dealing them independently.

The design and engineering of novel materials with a suitable range of properties for the effective utilization for such applications is a basic requirement. The design of new energy-related materials is at the forefront of different sciences such as the material science, chemistry, physics, and engineering. It is important to reveal the relationship between the material structure and the device performance if we wish to propose new energy-related materials [1], [2], [3].

In the quest to find prospective energy materials for high performance energy devices, the perovskite compounds hold a prominent role due to their unique tunable properties [4], [5], [6], [7], [8]. Perovskites are a family of materials with the formula ABX3 and have a similar structure to the prototype CaTiO3 mineral. The cation “A” occupies the corner positions of the unit cell and the cation “B” is located at the center of the cell, while the anion “X” is on the unit cell faces [9]. This family comprises oxides and halide perovskite material. Some representative oxides are the ferroelectric BaTiO3 and PbTiO3, the dielectric (Ba,Sr)TiO3, the piezoelectric Pb(Zr,Ti)O3, the electro-strictive Pb(Mg,Nb)O3, the magneto-resistant (La,Ca)MnO3, and the multiferroic BiFeO3. In the case of metal-halide perovskites, M is a divalent metal from group 14 (Pb, Sn) or a rare earth element (Eu), and X is a halogen (X=F, Cl, Br, I, or a combination of them). According to the nature of the cation, the metal halides can be divided in two groups, the all-inorganic and the hybrid organic-inorganic metal halides. In the first category the cation A is a monovalent alkali metal (like Cs, K) while in the second it is a small organic cation (such as CH3NH3) [10], [11].

The exploitation of new synthesis methods for the fine control of the structural characteristics and improved stability is important in the design of perovskite energy-related materials. Furthermore, the progress on the synthesis strategies for nanoparticulate systems of high quality in terms of homogeneity and crystallinity, has led the research community to search whether these materials could replace conventional energy materials. Different morphologies and chemical structures have been introduced for both metal halide and perovskite oxide nanocrystals for such purposes [1], [12], [13].

Metal halide nanocrystals can be effectively used in energy conversion, due to their strong optical absorption, low non-radiative recombination rates, tunable band gaps, relatively high charge-carrier mobility, and long diffusion lengths coupled with solution processability [14]. These nanocrystals have been utilized as the absorbing material in perovskite solar cells [15], [16] or placed at the interface between the absorbing and the hole transport layer (HTL) in order to improve carrier transport and stability [17], [18]. They are also used as down-converters in silicon solar cells due to their excellent quantum-cutting properties giving efficiencies of 21.5% [19]. In contrast, perovskite oxide nanocrystals have been utilized as electron transport layers (ETLs) in perovskite solar cells, as these materials are characterized by high electron mobility, wide band-gap, and a well-aligned conduction band with the absorbing layer [20]. Furthermore, perovskite nanocrystals have been tested for catalytic carbon dioxide (CO2) reduction in solar fuel cells. By mimicking the natural photosynthesis in green plants, artificial conversion of CO2 into chemical fuels offers a promising approach to simultaneously mitigate the levels of greenhouse gas and produce renewable energy [21]. Artificial solar-driven CO2 reduction results in the partial reduction of the carbon monoxide (CO), methane (CH4), methanol (CH3OH), etc., which are common chemical fuels. Single-phase metal halide nanocrystals have shown promising results in CO2 reduction [22], [23], but enhanced performance when these are coupled with graphene oxide (GO) or palladium nanosheets [24], [25]. Besides, the perovskite materials are promising materials for thermoelectrics for the conversion of thermal energy to electricity [26], [27]. Compared to the traditional materials used for thermoelectric applications (metal chalcogenide materials like Bi2Te3 and PbTe), perovskite materials are less expensive and can be processed by low energy cost methods and can be used for flexible thermoelectric devices [27]. The fairly ionic, polar character with a large dielectric constant and the remarkable conduction band anisotropy of the metal halides convey robust thermopower and moderate room temperature electrical conductivity [28].

Perovskite nanocrystals have been utilized in energy storage in batteries or supercapacitors due to their excellent catalytic activity, electrical conductivity, and durability. Ion migration through perovskite lattices allows the use of such materials as electrodes for batteries. Electrochemical measurements of the nanoparticulate perovskite systems displayed superior catalytic activity for oxygen reduction, as well as a higher discharge plateau and specific capacity compared to the bulk materials of the same crystal structure [29]. Metal halide nanocrystal films have been formed for application as anodes, for stable Li-based batteries [30], [31], [32]. Furthermore, in the case of the perovskite oxides, the size and the morphology of the nanocrystals are two factors that affect their electrochemical performance. Factors such as the structural nanocrystal quality, the existence of defects in the lattice [33], the doping in of the A and/or B site of the perovskite lattice [34], [35], [36], [37], the nanocrystal porosity [38], [39], [40], [41], and the existence of synergetic effects in the bifunctional morphologies [42], [43], [44], [45], [46] play an important role in the final electrochemical behavior. In addition, in the case of supercapacitor storage, it was found that structuring perovskite oxides and forming nanocrystals lead to remarkably enhanced, specific capacitance, rate capability, and cycle stability compared to the corresponding bulk materials [47], [48], [49]. Finally, perovskite nanocrystals offer improved electrochemical performance, low cost production in hydrogen storage and energy sustainability for transportation, electricity generation, and heating. Perovskite oxide nanocrystals show a higher discharge capacity compared to the bulk counterpart of the same stoichiometry [50] and in some cases is comparable to that of common materials that have been used for hydrogen storage to date [51].

Several review articles have been published on the application of nanocrystals in energy conversion and storage in the last couple of years [52], [53], [54], [55], [56], [57]. This review article seeks to summarize the colloidal methods of the perovskite nanocrystals both for metal halides and perovskite oxides but mainly focuses only on the applications of the nanoparticulate structures (Figure 1). This review is structured in three main sections: Section 2 deals with the synthesis strategies, morphology, and size control of the single-phase perovskite nanocrystals, Section 3 looks at perovskite nanocrystals for energy conversion, and Section 4 deals with perovskite nanocrystals for energy storage. In all these sections, we have summarized the literature for both metal halide and perovskite oxide nanocrystals and discuss the effect of structure, morphology, and size in the performance of these devices. This review article concludes with some open issues that require attention to succeed in designing efficient and low-cost devices.

Figure 1: Applications of perovskite (metal halides and perovskite oxides) nanocrystals for energy conversion and storage.
Figure 1:

Applications of perovskite (metal halides and perovskite oxides) nanocrystals for energy conversion and storage.

2 Synthesis strategies, morphology, and size control of the single-phase perovskite nanocrystals

Different methods have been introduced for the successful synthesis of perovskite nanocrystals. Metal halides have been synthesized by template-assisted methods and colloidal-based reactions, while perovskite oxides are created by solid-state or molten-salt reactions and colloidal processes. Due to the limited use in energy application of the metal halide nanocrystals synthesized by template-assisted methods we will focus only on the colloidal methods. In the case of the oxides, despite the fact that the solid state and molten-salt syntheses are more convenient compared to the colloidal ones, the latter have the advantage of achieving a better control of the characteristics of nanocrystals. Here, we focus on the colloidal methods that offer control on the morphology/structure and we think that they can be used to obtain energy devices of high performance with a reproducible and well-controlled manner despite whether they were used earlier or not.

Colloidal methods were used for the synthesis of both metal-halide and perovskite oxide nanocrystals of different morphology, isotropic or anisotropic one (Figure 2). In most cases, the metal halide nanocrystals were covered with organic molecules, usually acids and amines, while in the case of perovskite oxides, they are free of ligands (Table 1).

Figure 2: Summary of the solution-processed synthesis procedures of various perovskite nanocrystal morphologies.Metal halides: (a) Nanospheres synthesized by a hot method. (Reprinted with permission from [58] Copyright (2017), American Chemical Society.] (b) Nanocubes synthesized by ultrasound-irradiation. (Reproduced by permission of the Royal Society of Chemistry [59].) (c) Nanorods synthesized by solvothermal method. (Reprinted with permission from [60]. Copyright (2016), American Chemical Society.) (d) Nanowires synthesized by re-precipitation method. (Reproduced by permission of the Royal Society of Chemistry [61].) (e) Nanosheets synthesized by exfoliation. (Reproduced with permission from [62]. Copyright 2016, Wiley-VCH.) Perovskite oxides: (a) Irregular-shaped nanocrystals synthesized by sol-gel method. (Reproduced with permission from [63]. Copyright 2011, Elsevier.) (b) Spheres synthesized by the sol-gel method. (Reprinted with permission from [64]. Copyright (2001), American Chemical Society.) (c) Clusters synthesized by the sonochemical method. (Reproduced with permission from [65]. Copyright 2018, Elsevier.) (d) Cubes synthesized by solvothermal method. (Reproduced from [66] with permission from the Royal Society of Chemistry.) (e) Rods synthesized by hydrothermal methods. (Reproduced with permission from [67], Copyright 2005, Wiley-VCH.)
Figure 2:

Summary of the solution-processed synthesis procedures of various perovskite nanocrystal morphologies.

Metal halides: (a) Nanospheres synthesized by a hot method. (Reprinted with permission from [58] Copyright (2017), American Chemical Society.] (b) Nanocubes synthesized by ultrasound-irradiation. (Reproduced by permission of the Royal Society of Chemistry [59].) (c) Nanorods synthesized by solvothermal method. (Reprinted with permission from [60]. Copyright (2016), American Chemical Society.) (d) Nanowires synthesized by re-precipitation method. (Reproduced by permission of the Royal Society of Chemistry [61].) (e) Nanosheets synthesized by exfoliation. (Reproduced with permission from [62]. Copyright 2016, Wiley-VCH.) Perovskite oxides: (a) Irregular-shaped nanocrystals synthesized by sol-gel method. (Reproduced with permission from [63]. Copyright 2011, Elsevier.) (b) Spheres synthesized by the sol-gel method. (Reprinted with permission from [64]. Copyright (2001), American Chemical Society.) (c) Clusters synthesized by the sonochemical method. (Reproduced with permission from [65]. Copyright 2018, Elsevier.) (d) Cubes synthesized by solvothermal method. (Reproduced from [66] with permission from the Royal Society of Chemistry.) (e) Rods synthesized by hydrothermal methods. (Reproduced with permission from [67], Copyright 2005, Wiley-VCH.)

Table 1:

Synthesis strategies summary for the perovskite nanocrystals in liquid form.

PhaseMethodNanocrystal MorphologyLigand/surfactantReaction temperature (°C)References
Lead-halidesPrecipitation-based methodsQuantum dots, spheres, cubes platelets, sheets, rods, wires, hexagonsAcids, amines30 (only a few >30)[32], [61], [68], [69], [70], [71], [72], [73], [74], [75], [76], [77], [78], [79], [80], [81], [82]
Hot-injection methodsSpheres, cubes, rods, wires, platelets, sheetsAcids, amines, trioctylphosphineMost of them >140 (only a few <140°C)[58], [83], [84], [85], [86], [87], [88], [89], [90], [91], [92], [93], [94], [95], [96], [97], [98], [99], [100], [101], [102], [103]
SolvothermalCubes, wiresAcids, amines140–200[104]
Photo-inducedCubes, irregular-shapedNo ligands30[105], [106]
SonochemicalCubes, plateletsAmines30[59], [107]
ExfoliationPlateletsAmines30[62]
Microwave-assistedCubes, platelets, rodsTOPO, acids, amines30, 80–160[108], [109]
Lead-free halidesPrecipitation based methodsSpheresAmines, phosphine, thiol30[110], [111], [112]
Hot-injection methodCubes, spheresAcids, amines, no ligands80–220[113], [114], [115]
SolvothermalRodsAcids, amines180[60]
HydrothermalSpheresAmines180[116]
Perovskite oxidesSol-gelIrregular-shapes, spheres, polyhedral, cubes, honey-comb-likeFree of ligands (in few acids)400–1400 (only a few <400)[63], [64], [117], [118], [119], [120], [121], [122], [123], [124], [125], [126], [127], [128], [129], [130], [131], [132], [133], [134], [135]
Microwave sol-gelSpheresFree of ligands1100[136]
HydrothermalIrregular, spheres, cubes, tubes, wires, complex shapes (dentrites, stars, sea-weed, torus, worm-like), hollowFree of ligands, acids, amines, polyvinylpyrrolidone80–400[137], [138], [139], [140], [141], [142], [143], [144], [145], [146], [67], [147], [148], [149], [150], [151], [152], [153], [154]
Hydrothermal sol-gelIrregular, spheres, hollow, wiresFree of ligands80–220[36], [155], [156], [157], [158], [159], [160], [161]
Hydrothermal-microwaveCubesAcids200[162]
SolvothermalSpheres, worm-like, cubes, hollowFree of ligands, acids150–200[48], [66], [163], [164], [165], [166]
Solvothermal sol-gelSpheresFree of ligands60–240[167]
SonochemicalIrregular, spheres, clusters, rods, polygonsFree of ligands30–80[65], [168], [169], [170], [171], [172]

2.1 Colloidal synthesis for metal halide nanocrystals

Colloidal strategies at low or even at high temperature (>140°C) have been introduced for the morphological control of the metal halide nanocrystals through “top-down” or “bottom-up” approaches [12]. The first category comprises processes which start from molecules and ions and proceed with chemical reactions. In this type of reaction, the presence of capping ligands is important to control the size, morphology, and dispersity of the final nanocrystals. The second category includes the fragmentation of larger particles by an external stimulus such as irradiation or sonication in the presence of ligands or not.

Re-precipitation, hot-injection and solvothermal are the three main synthesis methods in the “bottom-up” approaches. The first one is a low-temperature process while the other two take place at high temperatures. All of them share common characteristics but have important differences [1]. For example, the re-precipitation methods are quick procedures, cost-effective, reproducible, they do not need complex apparatus, such as Schlenk line and inert gas flow, and are suitable for large-scale production. The hot-injection processes have a unique capability to finely control the shape and morphology of the nanocrystals, and also to produce complex structures with high homogeneity. This procedure is a time consuming procedure; it uses a Schlenk line coupled with a protective atmosphere and produces a small amount of the final product. Finally, the solvothermal process gives very good control of the nanocrystals by using a simple set-up, but the time duration of the reactions is a significant disadvantage of this procedure.

2.1.1 “Bottom-up” approaches

2.1.1.1 Room temperature re-precipitation methods

This solution-based process has been introduced to synthesize nanocrystals of different morphologies and chemical phases. The metal precursors are dissolved in a solvent usually in the presence of capping molecules. Then, this solution is added in a miscible co-solvent in which the solubility of the ions is low. Spontaneous crystallization and precipitation take place. This procedure has been proposed for both hybrid organic-inorganic or all inorganic metal halide nanocrystals and morphologies such as nanospheres [68], [69], [70], [71], [72], [73], nanocubes [71], [72], [74], [75], [76], nanohexagons [32], [76], nanorods [70], [71], [72], [77], nanowires [61], [70], [72], [78], nanoplatelets [70], [71], [72], [79], [80], [173], and nanosheets [70], [81]. In addition, such methods were reported last years for the synthesis of lead-free nanocrystals with quantum dot morphology [110], [111], [112].

The first report on the synthesis of metal halide nanocrystals with this was in 2014 by the group of Pérez-Prieto for the production of spherical CH3NH3PbBr3 nanocrystals with a 6 nm diameter [68]. The precursors in this reaction were CH3NH3Br and PbBr2 while the capping molecules were the oleic acid together with long chain alkyl ammonium bromide. The dispersive solvent was octadecene while the co-solvent was acetone. Later, in 2015, in order to simplify this procedure, commercially available precursors and capping ligands (n-octylamine and oleic acid) were used [69]. This modified procedure resulted in similar morphologies but smaller in size (3 nm). N-dimethylformamide (DMF) was used as dissolving solvent and toluene as co-solvent.

By combining organic molecules of a long and a short chain, nanoplatelet morphologies were formed [79]. Their lateral dimensions can be tuned by regulating the surfactant ratio while by adjusting the oleic acid amount one can obtain very thin platelets down to one layer. By changing the ratio between octylamonium bromide and oleic acid, the particles can be changed from spheres to anisotropic nanorods [77]. The amines found mainly affect the size of the nanocrystals by controlling the kinetics of crystallization while the acids suppress the aggregation effects and contribute to the stability of the colloids [69]. Furthermore, the way of adding the precursor solution can affect the final size of the nanocrystals [70], [174]. A longer duration of the addition of the precursor results in larger particles through an Ostwald ripening mechanism [72] or anisotropic morphologies [70].

The type of solvent and co-solvent in which the precursors are dissolved can also affect the morphology of the final nanocrystals. Elongated particles have been synthesized by using acetonitrile or γ-butyrolactone as the dissolving solvent [70]. While when the cosolvent was the ethyl acetate, the obtained morphology is varied from dots to nanoplates to nanobars by increasing the reaction time while by using toluene the nanocrystals transform from nanocubes to nanorods to nanowires [72].

2.1.1.2 Room temperature sonochemical methods

Metal halide nanocrystals have been synthesized via ultrasonication techniques. The solution of the reactants together with the organic ligands are positioned in a high density probe-type ultrasonicator in order to fabricate cubic or platelet-like crystals [59], [107].

2.1.1.3 Hot-injection methods

This synthesis procedure is utilized for both lead-containing or lead-free metal halide nanocrystals (Table 1). This process includes the injection of a precursor solution in a hot liquid of the surfactants. A high-boiling point solvent is needed for these reactions. When the hot solution is injected, an instantaneous formation of nuclei takes place. The nuclei grow slowly, due to the considerable amount of precursors, by increasing the temperature and the surfactant molecules coordinate on the surface of the nanocrystals. The relatively high temperatures of the reactions allow the annealing of the nanocrystals and the formation of well-crystallined and defect-free nanocrystals capped with the organic molecules. This method gives nanocrystals of small size and narrow size distribution. The size and the morphology found can be regulated by three crucial parameters during the synthesis: (i) the injection temperature of the precursor solution into the solution of the surfactants, (ii) the time of the reaction and (iii) the ratio of the precursors to the surfactants and the polarity of the reaction medium.

The first synthesis of metal halide nanocrystals using a hot-injection approach was reported by Kovalenko’s group for cubic-shaped CsPbBr3 nanocrystals of 4–15 nm edge length [83]. The reaction temperature is the main parameter that controls the size of the nanocubes in this case. Then, various morphologies such as nanospheres [58], [84], [85], [86], [113], nanocubes [83], [84], [87], [88], [89], [90], [91], [92], [93], [113], nanorods [60], [93], nanowires [94], [95], [96], [97], [98], nanoplatelets [85], [91], [92], [93], [99], [100] and nanosheets [98], [99], [100], [101] have been synthesized. This wet chemistry method is also capable for fabricating more complex structures because the nucleation and the growth stages can easily be separated and controlled independently, that is not possible with the re-precipitation methods.

The role of the reaction medium polarity to the final morphology of the formed nanocrystals was described by the Zhang’s group [84]. When the polarity of the solvent is high (diethylene glycol dibutyl ether and tetraethylene glycol dibutyl ether), spherical nanocrystals of 3–4 nm diameter are obtained. Cubes of 10 nm have been formed in a solvent with lower polarity (ethylene glycol dibutyl ether) with all the other parameters kept constant. The lateral dimensions of the nanocrystals can be tuned by lowering the temperature down to 90–130°C [91], [92], [93], [99], [100], [101]. The thickness of these large structures can be controlled by the reaction temperature as well as the type of ligands which are used for their capping [91]. Elongated nanocrystals have been synthesized at higher temperatures (150–250°C) but with longer reaction times [94], [96], [97].

2.1.1.4 High temperature solvothermal methods

In this method, the precursors and the surfactants are dissolved in a high boiling point solvent and closed in a sealed reaction container (stainless steel autoclave). The temperature is increased above the boiling point of the solvent and maintained at this temperature for a desired period. This method has been used for III–IV semiconductors and recently also for perovskite nanocrystals. This type of synthesis is based on the same starting materials with the previous methods, but it is capable for large-scale production. The first synthesis of perovskite nanocrystals with this approach was reported in 2016 from Chen’s group [60]. Lead-free tin halide nanorods were fabricated at a temperature of 180°C and reaction time of 6 h.

Such a method has also been utilized for the synthesis of all-inorganic nanocubes or nanowires at 160°C [104]. Without pre-dissolving of the precursors, the final morphology of the nanocrystals are cubes while when this step takes place the final results are nanowires due to the higher concentration of the precursors.

2.1.2 “Top-down” approaches

2.1.2.1 Room temperature exfoliation

Hybrid organic-inorganic metal halide nanoplatelets of varied thickness have been obtained through the exfoliation of bulkier particles/microcrystals synthesized previously via a solid-state reaction [62]. These microcrystals are dispersed in an organic solvent together with a capping ligand and this solution is placed on a tip sonication. Different steps of centrifugation lead to nanoplatelets of different thickness. This method can be used for very thin nanoplatelets down to that of single unit cell.

2.1.2.2 Room temperature photo-induced methods

Two reports exist about the use a laser-induced procedure for the synthesis of metal halide nanocrystals. In the first, the nanocrystals were obtained through a photo-fragmentation process from larger particles. Bulk crystals grown by inverse temperature crystallization grinded into smaller structures in an organic solvent [105]. Ten minutes of irradiation with 532 nm laser pulses (9 ns, 10 Hz, 0.8 J/cm2) of a Nd-YAG laser was enough to obtain cubic nanocrystals of around 60 nm. The ligand protects from the continuous growth of the crystals. In the second, a laser-ablation of metal-halide bulk material was used to obtain nanocrystals of the same phase [106]. This material was placed in the bottom of a vial containing an organic solvent together with an organic capping ligand. Nanocrystals of around 30–70 nm were formed after irradiation for 70 min with a 532 nm laser.

2.1.2.3 High temperature and room temperature microwave-assisted methods

In the first process, all the reactants are mixed together in a microwave tube in air atmosphere in contrast to the protective atmosphere of the hot-injection method [108]. Then the tube is placed in a microwave reactor and the temperature is increased gradually. The shape of the all-inorganic metal halide nanocrystals synthesized by this method at high temperature are small cubes while they are plate-like for lower temperatures. Ultra-thin nanowires are obtained when the precursors are pre-dissolved before increasing the temperature. The role of the trioctylphosphine oxide (TOPO) ligand is important in this reaction, it favors the dissolution of the precursors and thus helps to obtain high-quality nanocrystals.

Besides, this reaction can take place at room temperature [109]. In this case the ligand is bis(2,4,4-trimethylpentyl) phosphinic acid (TMPPA) instead of oleic acid and the precursor is cesium acetate instead of cesium carbonate, and cubic nanocrystals of 19 nm in size are formed. The type of the precursor plays an important role in the luminescence properties. The luminescence is higher when CsOAc is used as the precursor compared to that of using Cs2CO3.

2.2 Colloidal synthesis for perovskite oxide nanocrystals

Solid-state reaction or molten-salt methods have been extensively used for the synthesis of perovskite oxide nanocrystals. These two processes are easy and use simple equipment. The nanocrystals synthesized by such methods are well-crystallined but they have irregular shapes and wide size distribution. The solid-state reactions take place at high temperatures while the molten-salt method at moderate ones (600–800°C) due to the existence of the inorganic molten salt which serves as a medium to enhance the reaction rate and reduce the temperature of the reactant oxides [13]. The solid-state process has been used to synthesize simple shapes such as irregular-shaped or spherical nanoparticles [175], [176], [177], and only a few reports exist for nanocubes or nanowires [178]. The starting materials are mixed together, a milling process is followed and then calcination at high temperature. In contrast, the molten-salt method has been proposed for various structures including irregular shapes [179], [180], [181], [182], [183], [184], morphologies of high-aspect ratio [185], [186], [187], [188] and platelets [189]. In order to have a better control over the morphology than the previous methods, bottom-up solution-processed approaches have been realized including sol-gel, hydrothermal, solvothermal, sonochemical, or microwave-assisted reactions (Table 1, Figure 2). Lower temperatures and in some cases organic ligands are utilized in such approaches.

2.2.1 Sol-gel methods

In these methods, a sol is formed when metal alkoxide, metal-organic, or metal-inorganic salt precursors are dissolved in an appropriate solvent, it is then dried and sintered at high temperatures. The morphologies obtained by this approach are irregular-shaped [63], [117], [118], [119], [120], [121], [122], [123], [124], [125], [126], [136], [190] and spherical [64], [127], [128], [129], [130]. Only a few reports exist about this method for different structures such as cubic [64], [131] or honeycomb-like [132] structures. Reaction parameters which play important role on the morphology and the size of the synthesized nanocrystals are the temperature, time of the reaction, and heating rate [133], as well as the usage or not of an organic ligand [64], [119], [123]. In order to save energy and to be cost effective, a sol-gel approach combined with a microwave-assisted sintering has been proposed for perovskite oxide nanocrystals [136].

2.2.2 Hydrothermal methods

An aqueous suspension of insoluble salts is positioned in an autoclave and the temperature is increased. Precipitation from the solution of the crystalline material occurs at temperatures between the boiling point and the critical point of water. Various and more complex morphologies including randomly-shaped [137], [138], [139], spheres [140], [141], [142], [143], [144], cubes [141], [145], [146], [147], [163], nanowires [67], nanotubes [148], as well as more complex dendrite [149] or star-like [150] structures have been synthesized by this method compared to the simple structures synthesized by the sol-gel method. The combination of hydrothermal with sol-gel method has been used for the synthesis of rounded/randomly shaped [36], [155], [156], [157], [158], [159], hollow [160], or elongated [161] nanocrystals. A microwave-hydrothermal process has also been utilized for the synthesis of cubic perovskite oxide nanocrystals [162].

2.2.3 Solvothermal methods

The solvothermal method is a general procedure for perovskite oxide nanocrystals that are free of ligands. The first synthesis was performed by Antonietti’s group for the synthesis of irregular shaped BaTiO3, BaZrO3, and LiNbO3 nanocrystals [164]. Lithium or barium metal was dissolved in benzyl alcohol at slightly elevated temperature. Then a metal alkoxide was added to this solution, placed in an autoclave and heated in a furnace at temperatures between 200 and 220°C for more than 2 days. Benzyl alcohol has been proved to be a versatile solvent and reactant for controlled crystallization and stabilization of oxidic nanocrystals. The solvent polarity is crucial for the morphology control of the nanocrystals. The nanocrystals can be spheres or cubes by tuning this parameter [66]. The size can also be tuned by changing the precursors concentration and the temperature of the reaction. This method has been used for the efficaciously synthesis of spherical [66], [164], [165], [166], cubic [66], [166] or even hollow [48] morphologies.

2.2.4 Sonochemical methods

These processes take place at room temperature, where all the reactants are dissolved in a solvent under ultra-sound irradiation. With this method, irregular-shaped [65], [168], spherical [169], [170], rods [171], and polygons [172] are fabricated. All of these nanocrystals are free of ligands.

2.3 Perovskite nanocrystals for energy conversion

2.3.1 Perovskite nanocrystals in solar cells

2.3.1.1 Metal halide nanocrystals in perovskite solar cells

The metal halide nanocrystals have been used in perovskite solar cells by forming the active layer and/or placing them at the interface.

  1. Perovskite nanocrystals as active layer

    Metal halide nanocrystals of various morphologies and chemical phases have been used as absorber material in perovskite solar cells. Hybrid organic-inorganic lead halides of spheres [191], [192], nanosheets [193] and nanowires [103], [194] have been used for active layer with the nanowires to show the higher efficiency to date (18.7%) [103]. Although, all-inorganic lead halides nanocrystals with spherical [16], [195], [196], [197], [198], cubic [15], [199], [200] and elongated [201] morphologies have been used reaching an efficiency of 13.43% [202]. The Νanocrystals which showed this efficiency have a cubic morphology and they are capped with oleic acid [202].

    The first hybrid organic-inorganic nanocrystal-based solar cell was reported by Park’s group in 2011 [191]. In the case of the hybrid organic-inorganic solar cells, the efficiency was increased as the perovskite nanocrystal morphology changes from the spheres (2.4–6.54%) [191], [192] to nanosheets (10%) [193] and nanowires (14.71–18.7%) [103], [194]. The use of nanowires in the photoactive layer is an effective way for enhancing light trapping and improving charge transport efficiency. For this reason, the charge separation and conductivity were higher in the case of the nanowires compared to the bulk film [103], [196]. Very recently nanowires synthesized from the same chemical phase synthesized by a two-step spin coating process have reached the value of the 16.8% (Figure 3) [103]. Partially developed perovskite nanowires in the photoactive layer contribute more to photocurrent generation than in compact films (Figure 3E, F). These nanowires’ solar cell efficiency improved by using a PC60PB additive and as a result raised the power conversion efficiency (PCE) to 18.7%.

    Later in 2016, all-inorganic metal halide nanocrystals were used in perovskite solar cells instead of hybrid organic-inorganic materials to improve their stability [15], [16]. α-CsPbI3 nanocubes of 9 nm edge length were introduced and the perovskite solar cells showed the high efficiency of 10.77%. This high efficiency has been attributed to the stable cubic phase (and not to the orthorhombic unstable phase) in which the nanocubes are crystalline [15]. It is known that the CsPbI3 chemical phase which exhibits the smallest band gap is not structurally stable in the bulk form. Direct deposition of the CsPbI3 nanocubes by spin casting, followed by stabilization of the perovskite structure via post deposition chemical treatment or annealing, contributed positively to the high quality of the active layer. The efficiency can be improved more and reach a value of 13.43% by tuning the surface chemistry via an A-site cation halide salt (AX) treatment (Figure 4A) [202]. The AX treatment provides a method for tuning the coupling among the nanocubes and improving the charge transport. The mobility of the treated film doubles, enabling an increased photocurrent and improved efficiency. Furthermore, higher stability has been observed in similar nanocrystals capped with TOP synthesized using a hot-injection method and a PbI2/GeI2 dual iodine source [200]. These nanocubes showed a near unity photoluminescence (PL) quantum yield and improved chemical stability compared to the previous systems. The ensuing nanocubes solar cells deliver PCE of 12.15% and retain 85% of its peak performance after storage over 90 days.

    In a different approach, all-inorganic metal halides have been introduced into the absorber MAPbI3 layer to reduce charge recombination and improve the charge transfer [203]. This process was used to improve the quality of the absorber layer in terms of film structure, morphology, and crystallinity as the nanocrystals behave as nucleation centers in the growth of perovskite films. The high quality of the films leads to improved charge transport and solar cell PCE. At the same time, a protecting passivation layer of Cs1-yMAyPbI3-xBrx is formed on the top of the perovskite absorber layer and this contributes to the final stability of the solar cell. A champion PCE of 20.46% is obtained from the perovskite solar cells based on high quality perovskite film.

    Anion exchange at ambient conditions verified that this process could be an effective and simple way to obtain mixed halide nanocrystals and showed really promising results in perovskite solar cells [198]. These perovskite solar cells displayed a photoconversion efficiency of 5.3% and open circuit up to 1.31 V. In addition, cation exchange approaches for tunable A-site alloys of cesium (Cs+) and formamidium (FA+) lead triiodide perovskite nanocrystals (Cs1−xFAxPbI3) lead to quantum dot solar cells with high open circuit voltage (VOC) with a lower loss than the thin-film perovskite devices of similar compositions [204]. These solar cells showed an efficiency of 10%.

    Furthermore, CsPbBr3 was developed and applied as “inks” to fabricate fully air-processed, electrical stable solar cells exhibiting a PCE exceeding 5%. This method provides a new pathway for single-step, large-scale fabrication of inorganic perovskite solar cells. Inks of CsPbBr3 nanocrystals of 15–20 nm synthesized with a fast room temperature synthesis using short, low boiling point ligands and environmentally friendly solvents, have been proposed by Manna’s group [16]. These inks can be used directly to fabricate films of high optoelectronic quality. An active layer of 550 nm prepared by nine sequential depositions, exhibited a PCE of 5.4% and a VOC= 1.5 V.

    The quality of the absorber layers of CsPbBr3 can be improved further by treating them using an NH4SCN ethyl acetate solution to quickly transform the CsPbBr3 nanocrystals film into CsPbBr3-CsPb2Br5 composite film (Figure 4B) [196]. The treated film is uniform and compact after a surface dissolution-recrystallization process, with large grain size and low defect density. The recorded PCE by using this composite was 6.81% in this case.

    Finally, lead-free metal halide nanocrystals, free of toxic elements, were also introduced in perovskite solar cells. Tin-based metal halide nanocrystals have been synthesized in the form of nanospheres [205] or nanorods [60]. The efficiency of the devices using hybrid organic-inorganic tin halide nanospheres was 8.79% [205], while the efficiency of the devices including all inorganic nanorods can be ranged from 9.66 to 12.96% depending on the metal halide composition. The highest solar cell performance was recorded for the device using the phase CsSnI3 [60]. These nanorods exhibit colloidal stability in air for more than 2 months and a decomposition temperature significantly higher than that of MAPbI3. The photovoltaic parameters recorded for such devices were a short-circuit current density (Jsc)=23.21 mA/cm2, open-circuit voltage (Voc)=0.86 V, fill factor (FF)=0.65, and PCE of 12.96%. In addition, recently reported all inorganic bismuth-based cesium halide nanocrystals in the form of nanosheets of 4.3 nm in thickness, synthesized by a dissolution-recrystallization process and were used for such applications [206]. The efficiency of this device was 3.2% but it is the highest reported to date in bi-based solar cells.

  2. Perovskite nanocrystals at interfaces

    Metal halide nanocrystals have been used at the interface between the perovskite absorbing layer and the HTL [17], [18]. The interface engineering is an effective way for obtaining high efficiency and improved stability in the perovskite solar cells through interfacial charge transfer control. A layer of hybrid organic-inorganic FAPbX3 quantum dots is placed between the absorbing MAPbI3 layer and the C60 HTL [17]. In this way, an increased short-circuit current and an improved solar cell efficiency by 43.7% were observed. Moreover, all inorganic α-CsPbI3 quantum dots have been placed at the same position and the PCE increased from 15.17 to 18.56% in solar cells using MAPbI3 as the absorbing material [18]. The charge-transfer efficiency at the interface of the perovskite/HTL is enhanced by CsPbI3 quantum dots due to their intermediate valence band position between the perovskite and the HTL. In addition, the enhancement of the stability of perovskite solar cells can be attributed to the coating of the perovskite layer with the all inorganic CsPbI3, which has a high moisture stability and results in long-term stability of the perovskite solar cells in the air.

    In order to make stable the hybrid organic-inorganic quantum dots, they are covered with a shell of C18 [207]. These core-shell quantum dots – C18 were also used at the interface with the HTL. The efficiency of these solar cells reaches over 10%. The presence of long chain ligands bound to the quantum dots did not appear to damage hole extraction.

Figure 3: CH3NH3PbI3 nanowires in perovskite solar cells.Scanning electron microscopy (SEM) images of the c-perovskite (compact): (A), c-perovskite: PC60BM (B), perovskite nanowires (nw), (C), perovskite nanowires: PC60BM (D) films. J-V characteristics measured under sun and dark conditions (E), and EQE spectra for the highest-performing films (F). Reproduced with permission from [103]. Copyright 2018, Elsevier.
Figure 3:

CH3NH3PbI3 nanowires in perovskite solar cells.

Scanning electron microscopy (SEM) images of the c-perovskite (compact): (A), c-perovskite: PC60BM (B), perovskite nanowires (nw), (C), perovskite nanowires: PC60BM (D) films. J-V characteristics measured under sun and dark conditions (E), and EQE spectra for the highest-performing films (F). Reproduced with permission from [103]. Copyright 2018, Elsevier.

Figure 4: (A) Schematic representation of the AX-coated CsPbI3 quantum dot sensitized solar cell, IV, and external quantum efficiency results. Reproduced with permission from [202]. Copyright 2017, AAAS. (B) Schematic structure of the device using CsPbBr3-CsPb2Br5 composite as an absorbed layer, fabrication process and comparison with fabricated layers at higher temperatures in the literature. (Reprinted with permission from [196]. Copyright (2018), American Chemical Society).
Figure 4:

(A) Schematic representation of the AX-coated CsPbI3 quantum dot sensitized solar cell, IV, and external quantum efficiency results. Reproduced with permission from [202]. Copyright 2017, AAAS. (B) Schematic structure of the device using CsPbBr3-CsPb2Br5 composite as an absorbed layer, fabrication process and comparison with fabricated layers at higher temperatures in the literature. (Reprinted with permission from [196]. Copyright (2018), American Chemical Society).

2.3.1.2 Metal halide nanocrystals in dichalcogenide quantum dot sensitized solar cells

All-inorganic metal halide nanocrystals have been employed as carrier blocking layers between the absorber layer of PbSe nanocrystals and the metal contact in dichalcogenide quantum dots sensitized solar cells [208]. The relatively large Eg (2.4 to 2.7 eV) of the CsPbBr3 nanocrystals compared to that of PbSe nanocrystals (1.2 to 1.5 eV) was the crucial factor to use them as a blocking layer. On the one hand, these nanocrystals have been chosen for their good air-stability, their high photoluminescence quantum yield and their ability to be synthesized independently and on the other hand, the quality of the perovskite layer seems not to be affected by the PbSe quantum dot layer. The PCE of this solar cell configuration is 7.2%. An open-circuit voltage (VOC) of 482 mV, a current density (JSC) of 23.9 mA/cm2, and a fill factor (FF) of 62.4% under one-sun conditions were recorded.

In a different approach, the metal halides were utilized as a passivation layer on the surface of the dichalcogenide quantum dots (PbS) forming a core-shell structure [209], [210], [211]. A shell of hybrid organic-inorganic MAPbI3 [209], [210] or all-inorganic CsPbI3 [211] metal halide was introduced for quantum dot passivation. In the first case, the film of the core-shell nanocrystals was incorporated in a photovoltaic device with graded band structure and recorded a PCE of 8.95% for this solar cell [209]. Two years later, a funtionalized quantum dot HTL was introduced in such structures to block the back flow of the photo-generated electrons, leading to enhanced photocurrent and fill factor compared to undoped devices [210]. The ligand of the quantum dots was 1,2-ethanedithiol (EDT) and the solar cell performance reached the value of 9.5% due to the enhanced bending at HTL-absorber junction. The utilization of an all-inorganic shell around the PbS quantum dots led to a performance of 10.5% [211]. In this case the shell was epitaxially grown on the core surface. The improved passivation significantly diminished the sub-bandgap trap-state-assisted recombination, leading to improved charge collection and therefore higher photovoltaic performance.

2.3.1.3 Metal halide nanocrystals in dye-sensitized solar cells

Irregular-shaped, free of ligands, hybrid organic-inorganic perovskite nanocrystals were used to enhance the light absorption of dye-sensitized solar cells employing liquid electrolytes [212]. This incorporation resulted in a photovoltaic efficiency of 3.8% and photovoltage of 1.0 V. In this device, nanocrystalline particles of CH3NH3PbX3 (X=Br, I) were deposited onto the TiO2 surface by a self-organization process, starting with the coating of a precursor DMF-based solution. This CH3NH3PbX3-deposited TiO2 electrode was used as photoelectrode (anode) together with a Pt-coated FTO glass as counter electrode (cathode) and a 50 μm-thick separator layer. The gap between the electrodes was filled with an organic electrolyte solution containing lithium halide and halogen as a redox couple. A higher PCE was measured for the CH3NH3PbI3 perovskite sensitizer (3.8%) among the different stoichiometries tested.

Furthermore, spherical amine-capped CH3NH3PbI3 nanocrystals synthesized by a re-precipitation method have been introduced in quasi-solid-state sensitized solar cells with a configuration FTO/m-TiO2/perovskite quantum dots/dye/long persistence phosphor/gel electrolyte/Pt/FTO [213]. The perovskite nanocrystals were employed as charge-transfer bridge between the TiO2 and the N719 dye to extract photo-induced charges from a light-harvester. From the metal halides tested, the CH3NH3PbBr1.5I1.5 nanocrystals have been proved as the most preferred co-sensitizers for the efficiency enhancement due to their optimal energy level compared to that of a dye molecule, in which the photogenerated electrons from the dye molecule can transfer to nanocrystals and then to TiO2. Furthermore, the long persistence phosphor (LPP) which has light-storing and light-emitting ability covered the m-TiO2 layers aiming to harvest the longer wavelength light which permeated across the FTO/m-TiO2/PQDs/dye and which subsequently emits monochromatic green photofluorescence to re-excite the sensitizers. This device showed the impressive power conversion efficiency of 7.91%, well above the previous report in 2009.

2.3.1.4 Metal halide nanocrystals in silicon solar cells

Metal halide nanocrystals have been utilized for the improvement of the c-Si solar cell efficiency. Spherical organic-inorganic metal halide nanocrystals [214] and all-inorganic nanocubes [19] have been used for such purposes with the second showing the higher performance.

In the first case, a c-Si solar cell architecture based on a heterojunction between n-type c-Si and a conducting polymer (PEDOT:PSS) is demonstrated as an alternative concept of the field effect solar cell (Figure 5A). Usually, these solar cells consume extra electric energy originated from an external bias. In this case by introducing the perovskite nanocrystals, the extra potential is generated by the light [214]. The organometal trihalide nanocrystals synthesized by a low-temperature precipitation method are deposited on the top of the PEDOT:PSS top electrode and act as potential generation layer (Figure 5A). The device operates as a Schottky heterojunction solar cell with the light-induced electric polarization in the perovskite nanocrystals enhancing the electric field in the c-Si depletion region. The light harvested by organometal trihalide perovskite nanocrystals induces molecular alignment on a conducting polymer, which generates a positive electrical surface field. Photoinduced carriers generated in c-Si are directed to the respective contacts by the electric field in the depletion region at the interface between n-Si and MoO3/PEDOT:PSS. The successful combination of the light-driving polarization perovskite nanocrystals with n-type c-Si leads to the fabrication of a “field-effect solar cell” with a PCE of 14.3%. This PCE showed a 12% enhancement compared to the one without using such nanocrystal coverage (12.7%). This device displayed a Jsc of 30.84 mA/cm2, an FF of 73%, a Voc of 635 mV, which results in a PCE of 14.3%. While the device without the perovskite nanocrystals exhibits a short circuit current (Jsc) of 30.42 mA/cm2, an FF of 70%, an open circuit voltage (Voc) of 594 mV, yielding a PCE of 12.7%.

Figure 5: (A) “Field-effect solar cell” by using spherical hybrid organic-inorganic metal halide nanocrystals. Reproduced with permission from [214]. Copyright 2017, Wiley-VCH. (B) CsPbCl1.5Br1.5:Yb (7.1 %), Ce (2 %) perovskite nanocubes film, used as a downconverter in commercial silicon solar cells. Reproduced with permission from [19]. Copyright 2017, Wiley-VCH.
Figure 5:

(A) “Field-effect solar cell” by using spherical hybrid organic-inorganic metal halide nanocrystals. Reproduced with permission from [214]. Copyright 2017, Wiley-VCH. (B) CsPbCl1.5Br1.5:Yb (7.1 %), Ce (2 %) perovskite nanocubes film, used as a downconverter in commercial silicon solar cells. Reproduced with permission from [19]. Copyright 2017, Wiley-VCH.

The second report on using metal halide nanocrystals demonstrated a cheap, convenient, and effective way to enhance the PCE of the commercial silicon solar cells (Figure 5B) [19]. Doped all-inorganic metal halide nanocubes synthesized by a hot-injection method have been used as a downconverter of these solar cells due to their excellent quantum-cutting properties. The PCE in this case is improved from 18.1 to 21.5%. They were self-assembled on the surface of the commercial single crystal silicon solar cell via liquid-phase deposition and the thickness was controlled ranging from 60 to 770 nm. Compared to 18.1% PCE of the uncoated solar cells, the performance for the 110, 210, and 350 nm perovskite nanocrystals coated samples increases to different contents, while the further increasing thickness of the nanocrystals induces the decrease of solar cell performance. The best thickness of the nanocrystal layer is 230 nm. The PCE of this device reaches to 21.5%, with an open-circuit voltage 0.65 V and a short-circuit current 39.8 mA/cm2 with good reproducibility. In the dark, the luminescent intensity of this layer is decreased less than 5% and the PCE decreased only about 5% after being placed in the air for 700 h, indicating a high stability. Irradiated by simulated AM 1.5 sunlight for 50 h continuously, the PCE is rarely changed, implying its excellent photostability. Finally, by testing various photoluminescent converters, the optimum enhancement of PCE was observed for the Yb3+, Ce3+ co-doped perovskite nanocrystals. The PCE is comparable to the perovskite/silicon tandem solar cells but in this case the fabrication is easier and simpler to repeat.

2.3.1.5 Perovskite oxide nanocrystals in perovskite solar cells

Perovskite oxide nanocrystals have been used as electron transporting materials. Films of sol-gel synthesized nanocrystals of Zn2SnO4 have been utilized as ETLs for highly efficient perovskite solar cells [215]. There is a dual role of these materials in perovskite solar cells. On the one hand, the introduction of such films significantly improves the transmittance of flexible polyethylene naphthalate/indium-doped tin oxide (PEN/ITO)-coated substrate from ~75 to ~90% over the entire range of wavelengths and, on the other hand, due to its antireflection properties and low refractive index lead to the improvement of the PCE. The PCE in such perovskite solar cells leads the value of 14.85 % under AM 1.5G-100 mW/cm2 illumination (Figure 6). Furthermore, by replacing them with hydrothermally synthesized Zn2SnO4 nanocrystals the performance has been further increased to a PCE of 17.7% [20]. These materials were characterized by high electron mobility of 10–25 cm2/Vs, a wide optical bandgap (3.8 eV), and a well-aligned conduction band edge (~−4.1 eV) with that of the absorbing layer (CH3NH3PbI3).

Figure 6: Structure and performance of flexible perovskite solar cells including Zn2SnO4 nanocrystals as hole transporting layer.Cross-sectional SEM image and photograph of the ZSO-based flexible perovskite solar cell (scale bar, 500nm) (A). Energy levels of the materials (B). Photocurrent density–voltage (J–V) curve measured by reverse scan with 10mV voltage steps and 40 ms delay times under AM 1.5 G illumination (C). EQE spectrum of the ZSO-based flexible perovskite solar cell (D). Transmittance and reflectance spectra of PEN/ITO/ZSO, PEN/ITO/TiO2 and PEN/ITO substrate (E). Reproduced with permission from [215]. Copyright 2015, Nature Publishing Group.
Figure 6:

Structure and performance of flexible perovskite solar cells including Zn2SnO4 nanocrystals as hole transporting layer.

Cross-sectional SEM image and photograph of the ZSO-based flexible perovskite solar cell (scale bar, 500nm) (A). Energy levels of the materials (B). Photocurrent density–voltage (J–V) curve measured by reverse scan with 10mV voltage steps and 40 ms delay times under AM 1.5 G illumination (C). EQE spectrum of the ZSO-based flexible perovskite solar cell (D). Transmittance and reflectance spectra of PEN/ITO/ZSO, PEN/ITO/TiO2 and PEN/ITO substrate (E). Reproduced with permission from [215]. Copyright 2015, Nature Publishing Group.

2.3.1.6 Perovskite oxide nanocrystals in dye-sensitized solar cells

Quite spherical perovskite nanocrystals have been used in ETL in dye sensitized solar cells due to their superior electron collection property. The energy conversion reported for such solar cells is 4.7% for using Zn2SnO4 [152] and 5.2% for BaSnO3 [216] nanocrystals. The electron capture in the perovskite oxide films was higher than in TiO2 and the electron in the conduction band can diffuse rapidly resulting in greater photovoltaic performance.

2.3.2 Perovskite nanocrystals for photocatalytic CO2 reduction in solar fuel cells

The emission of CO2 by human activities is an important factor for the dramatic change of the environment and phenomena such as the climate change and global warming. Photocatalytic reduction of CO2 using solar energy into renewable hydrocarbon fuels has gained much attention in the effort to conserve energy [21]. By mimicking the natural photosynthesis in green plants, artificial conversion of CO2 into chemical fuels such as carbon monoxide [CO], methane [CH4], methanol [CH3OH], offers a promising approach to simultaneously mitigate the levels of greenhouse gas and produce renewable energy. Nanocrystals of metal halides or perovskite oxides have been introduced as efficient photocatalysts for such purposes.

2.3.2.1 Metal halide nanocrystals for photocatalytic reduction of the CO2

Metal halides have not been applied for photochemical conversion (water splitting or CO2 reduction) due to their instability in the presence of moisture or polar solvents. But there are some recent reports on novel photocatalysis to convert CO2 into solar fuels in non-aqueous media. Single-phase lead-containing or lead-free metal halide nanocrystals have been proposed as novel catalysts for solar cell CO2 reduction. CsPbBr3 quantum dots with diameters 3–12 nm showed a highly selectivity over 99% and achieve an efficient yield rate of 20.9 μmol/g towards solar CO2 reduction (Figure 7A). [22] While lead-free nanocrystals of Cs2AgBiBr6 showed an impressive stability against moisture, light, and temperature and under AM 1.5G illumination for 6 h achieved a total electron consumption of 105 μmol/g [23].

Figure 7: Metal halide nanocrystals for photocatalytic CO2 reduction.(A) Single phase CsPbBr3 nanocatalysts. Reproduced with permission from [22]. Copyright 2017, Wiley-VCH. (B) Composite CsPbBr3/GO catalysts. Reprinted with permission from ref [24]. Copyright (2017), American Chemical Society. (C) Composite CsPbBr3/Pd catalysts. Reprinted with permission from [217]. Copyright (2018), American Chemical Society.
Figure 7:

Metal halide nanocrystals for photocatalytic CO2 reduction.

(A) Single phase CsPbBr3 nanocatalysts. Reproduced with permission from [22]. Copyright 2017, Wiley-VCH. (B) Composite CsPbBr3/GO catalysts. Reprinted with permission from ref [24]. Copyright (2017), American Chemical Society. (C) Composite CsPbBr3/Pd catalysts. Reprinted with permission from [217]. Copyright (2018), American Chemical Society.

Enhanced photocatalytic reduction of CO2 to ethyl acetate observed when CsPbBr3 quantum dots are coupled with GO (Figure 7B) [24]. Under AM 1.5G simulated illumination, the primary CsPbBr3 nanocrystals of 6 nm in diameter steadily generated and injected electrons into CO2, catalyzing CO2 reduction at a rate of 23.7 μmol/g h with the important selectivity over 99.3%. The growth of the perovskite on GO results in the increase of the electron rate to 25.5% due to the improved electron extraction and transport. These rates are superior compared to the common CdS quantum dots photocatalysts. These photocatalysts are stable after 12 h of photocatalytic reaction and no phase transformation or degradation are observed. The effective CO2 reduction capacity (1.05 μmol/cm2 h) and selectivity (84%) are much smaller in the case of nanocomposites including hybrid organic-inorganic metal halide nanocrystals and GO [25]. The photocatalytic performance of the previous nanocomposite is improved when the metal halide nanocrystals were coupled with palladium nanosheets instead of GO (Figure 7C) [217]. Their optimized performance in this case was 33.79 μmol/g h, corresponding to 2.43-fold enhancement compared to the pristine metal halide nanocrystals.

Furthermore, the encapsulation of the CsPbBr3 nanocrystals in amorphous-TiO2 showed a marvelous 6.5-fold improvement on the consumption of photoelectrons in photocatalytic CO2 reduction reactions compared to that of individual CsPbBr3 nanocrystals [218]. Despite the poor photocatalytic behavior of the amorphous TiO2, its good chemical stability makes it good candidate as a protection layer for the lead halides. The amorphous TiO2 coverage has been witnessed as a pivotal driving force for preeminent photocatalytic performance by enhancing the extraction and separation of the photoinduced charges, and increasing the adsorption of the CO2 simultaneously. Such combined effects finally boost the photoelectron consumption from 25.72 to 193.36 μmol/g during the 3 h photocatalytic reaction. Photocatalytic reduction of CO2 to CH4 is more thermodynamically favorable than the formation of CO and H2, which however, is kinetically challenging since eight electrons were involved.

2.3.2.2 Perovskite oxide nanocrystals for photocatalytic reduction of the CO2

NaNbO3 and NaTaO3 nanocrystals of similar size and synthesized by the same method have been tested as photocatalysts for the reduction of the CO2 [219]. Both perovskites give rise to the similar conversions in the CO2 reduction reaction with a slightly higher carbon product evolution for the nanocrystals of NaTaO3. Furthermore, the crystal structure of the nanocrystals seems to be a crucial factor for the photoreduction performance of the NaNbO3 nanocrystals [220]. The photocatalytic H2 evolution and CO2 reduction activities over cubic NaNbO3 were nearly twice of those over orthorhombic NaNbO3 structure. The electronic structure of the cubic phase is beneficial for electron excitation and transfer. Furthermore, nanowires of the same chemical structure covered with the polymer g-C3N4 showed an enhanced photocatalytic performance (8 times higher) compared to the single-phase g-C3N4 or the NaNbO3 nanowires [221]. The remarkable enhancement of photocatalytic activity was mainly ascribed to the improved separation and transfer of photogenerated electron-hole pairs at the intimate interface of g-C3N4/NaNbO3 heterojunctions, which originated from the well-aligned overlapping band structures of C3N4 and NaNbO3. Finally, KNbO3 (where A=Na, K) nanocrystals showed a higher photocatalytic activity compared to the NaNbO3 due to its narrower band gap and higher mobile charge carriers [222].

BiWO6 nanocrystals of different morphologies have been synthesized for photo-induced CO2 reduction. Square BiWO6 nanoplateles of 9.5 nm in thickness and capped with oleylamine were used for such purposes [223]. Their enhanced catalytic activity is due to the preferentially exposed {001} surface. The ultrathin geometry of these nanocrystals also promotes charge carriers to move rapidly from the interior to the surface to participate in the photoreduction reaction and should also favor an improved separation of the photogenerated electron and hole and the lower electron-hole recombination rate. More complex nanosheet-based nanocrystals have been also designed to improve the catalytic performance. Ball-flower-like nanostructures composed by nanoplatelets [224] or nanoplatelets decorated with core-shell Au-CdS [225] also synthesized for such purposes.

2.3.3 Perovskite nanocrystals for thermoelectrics

Generating energy through thermoelectric materials is becoming increasingly important as the challenges faced nowadays in terms of energy production and efficiency are more intense than ever. Much work has been carried out during the past decades in an effort to enhance the production of energy through novel materials and processes [226], [227], [228].

Thermoelectric generators (TEG) constitute a new technology in order to recover heat which is based on the Seebeck effect and is broadly used for power generation. The Seebeck effect can be described as the connection of two different type (p-type and n-type) of conductors or semiconductors. This connection is formed by a parallel thermal connection along with an electrically connection in series which in turn causes a difference in voltage between the two materials [229], [230]. When connecting the two different components with a heated junction one can observe on the n-type component the transport of electrons from the hot junction to a heat sink whereas the p-type component transports holes which are positively charged, following the same direction as the temperature gradient.

The efficiency that a specific material can possess in the conversion process of heat to electricity can be given by the formula below.

(1)ZT=α2σTκ

where ZT is the dimensionless figure of merit, σ is the electrical conductivity of the material, κ is the thermal conductivity of the material and α the Seebeck coefficient [231], [232].

In order for thermoelectric materials to be competitive with ordinary power generators the figure of merit for TEG must be larger than 3 [233], [234]. Generally, finding materials with a ZT value above 2 is a challenging task but recent advancements [235], [236] in the effort to increase the figure of merit to around 3 has been made possible with the use of nanocomposites.

In this direction, lead and tin halide perovskites namely CH3NH3PbI3 and CH3NH3SnI3 have been regarded as very promising photovoltaic materials mainly because of their relatively large absorption coefficient, high charge carrier mobility, and diffusion length properties [212], [237] also possessing a large Seebeck coefficient [238], [239]. Recent first principle studies of these materials have confirmed this and have provided detail insides especially when results are combined with the Rashba effect.

Recently Nafradi’s group has also shown that the ZT of the Sn component of the hybrid halide perovskite mentioned can be augmented by three orders of magnitude at room temperature by appropriate chemical doping [240]. This finding suggests that CH3NH3SnI3 can constitute a very promising candidate for low cost and mass production processes.

Similar improvement in the thermoelectric properties of perovskites was observed with W doping for CaMnO3 by Tan’s group [241]. They reported a two-fold increase in the figure of merit which in turn is attributed to structural characteristics involving the existence of MnO6 distorted octahedra. This case is also interesting as two mechanisms are reported to occur simultaneously although their effect is canceling one another. The W doping seems to increase carrier concentration which ultimately leads to enhanced electrical conductivity and a decreased Seebeck coefficient. The enhanced electrical conductivity is a positive effect that outweighs the negative impact of the decreased Seebeck coefficient thus leading to an increased power factor. This increase in ZT is also temperature depended and seems to increase almost linearly with increasing temperature.

Overall the structural dependence of the ZT still remains a big challenge and is a promising field for more intensive research in order to elucidate the structural dependence of the above-mentioned phenomena.

2.4 Perovskite nanocrystals for energy storage

2.4.1 Perovskite nanocrystals for batteries

Emerging autonomous electronic devices require compaction and miniaturization of energy storage devices. Perovskite materials have received considerable attention for energy storage applications due to their excellent catalytic activity, electrical conductivity, and durability. Ion migration through perovskite lattices allowing the use of such materials as electrodes for batteries. Electrochemical measurements on nanoparticulate perovskite systems showed that they displayed superior catalytic activity for oxygen reduction, as well as a higher discharge plateau and specific capacity compared to the bulk materials of the same crystal structure [29]. Perovskite oxide nanocrystals have been investigated for such application but in recent years metal halides have also shown high specific capacitance and promising stability upon cycling. This section summarizes all the reports on such applications focused on nanoparticulate systems of both metal halides and perovskite oxides and tries to correlate and understand the role of the size, the morphology and the intrinsic properties of the nanocrystals to the final performance of the batteries.

2.4.1.1 Metal halide nanocrystals in batteries

The first report of using metal halide materials for Li-ion batteries was in 2015 by Peng’s group [30]. Hydrothermally grown organic-inorganic metal halide microcrystals were used as the active material in Li-ion storage devices presenting a discharge capacity of 331.8 mA h/g (at current density of 200 mA/g) [30]. In this system, the capacity decreased rapidly in the first 30 cycles, it subsequently decayed slowly, showing a relative capacity retention of 76.9 % in the next 170 cycles. A comparison between different halides indicated that the batteries with the CH3NH3PbBr3 showed a larger discharge capacity compared to that with CH3NH3PbI3. The discharge capacity for the first system was 331.8 mA h/g while for the second only 43.6 mA h/g. Two years later, CH3NH3PbBr3 nanocrystals of 65 nm in size, synthesized by a precipitation method combined with a heating process, showed similar electrochemical response (Figure 8A) [31]. Long-term specific capacity attained significant values, approaching 200 mA h/g. Very recently, the electrochemical performance of metal halide nanoparticulate electrodes by using aqueous electrolyte were evaluated by our group [32]. This is the first report of using metal halide nanocrystals in batteries using aqueous electrolytes. In particular, the electrodes comprised a layer of 100 nm-size Cs4PbBr6 nanohexagons deposited on ITO substrates and coated with a thin TiOx film (Figure 8B) [32]. The nanocrystals were prepared at room temperature, by a fast, solution-processed co-precipitation method. The electrodes were subjected to successive annealing cycles to optimize their electrochemical stability. The electrodes of five annealing cycles showed the best performance. It was observed that these electrodes showed the best performance among the nanoparticulate anodes using metal halides presented previously in terms of stability (high stability for 40 scans), specific capacity (377 mA h/g) and coulombic efficiency (dropped to 98% after 100 scans). On the contrary, the non-annealed and/or uncoated nanohexagon layers displayed poor stability, immediately after the first scan. A water-triggered transformation of the metal halide material occurred in the aqueous medium from Cs4PbBr6 to CsPb2Br5. Then, the Li-ion intercalation/deintercalation mechanism is a reversible process for the 40 consecutive scans, as the crystal structure of the CsPb2Br5 remains unaltered. After this period, an irreversible conversion reaction of CsPb2Br5 to CsBr and PbBr2 occurs up to 100 scans.

Figure 8: Metal halide perovskite nanocrystals for Li-air batteries. Electrochemical performance of the anodes consisted of (A) hybrid organic-inorganic CH3NH3PbBr3 nanocrystals of 65 nm in size, synthesized by a precipitation method combined with a heating process and (B) all-inorganic metal halide, Cs4PbBr6 nanohexagons of 100 nm in size deposited on ITO electrodes and subjected to three and five cycles of thermal annealing. These layers are coated with a few-nanometer thin TiOx layer. (A) Reprinted with permission from [31]. Copyright (2017), American Chemical Society. (B) Reproduced from [32] with permission from the Royal Society of Chemistry.
Figure 8:

Metal halide perovskite nanocrystals for Li-air batteries. Electrochemical performance of the anodes consisted of (A) hybrid organic-inorganic CH3NH3PbBr3 nanocrystals of 65 nm in size, synthesized by a precipitation method combined with a heating process and (B) all-inorganic metal halide, Cs4PbBr6 nanohexagons of 100 nm in size deposited on ITO electrodes and subjected to three and five cycles of thermal annealing. These layers are coated with a few-nanometer thin TiOx layer. (A) Reprinted with permission from [31]. Copyright (2017), American Chemical Society. (B) Reproduced from [32] with permission from the Royal Society of Chemistry.

The overall capacity of the batteries is strongly dependent on the accessibility of the host material interior to the ions [242]. Different mechanisms Li+ intake/release have been proposed for the metal halides in order to explain the different final performance of the devices. Topotactic insertion into organic-inorganic metal halide host (CH3NH3PbBr3) has been proposed by Garcia-Belmonte’s group [31]. Non-drastic structural alterations or rearrangements in the crystal lattice have been observed in this case. A different mechanism was proposed by Islam’s group for the same anode material. Using a combination of density functional theory and results by means of electrochemical characterization and diffraction techniques [243], Li intercalation and conversion reactions in the CH3NH3PbX3 (where X: Br, Cl, I) take place. The conversion process with the production of CH3NH3X, lithium halides (LiX), and Pb metal was found to be energetically more favorable than Li intercalation. Furthermore, it was also found that the specific capacity is dependent on the crystal structure of the perovskite material [244]. This could be improved by changing the dimensionality of the halide perovskites from three-dimensional (3D) to a one-dimensional (1D) lattice [245]. Indeed, experiments on organic-inorganic hybrid lead halide perovskites showed that the Li intercalation in the two-dimensional (2D) tetragonal structure is enhanced compared to the 3D orthorhombic one, due to the larger cell volume [243]. Finally, the type of the anion plays role in the charging performance [243]. It is observed that the Li intercalation is more favorable in the case of the iodides than in the chlorides or bromides [245].

2.4.1.2 Perovskite oxide nanocrystals in batteries

The perovskite oxide nanocrystals started to be used in batteries from 2014. The first nanocrystals were from LaNi1-xMnxO3, La0.6Sr0.4CoO3 and Ba0.9Co0.5Fe0.4Nb0.1O3-δ chemical phases and succeed to be stable for less than 50 battery cycles [132], [246], [247]. Later in 2016, LaNiO3 nanoparticles showed an improved cycling ability up to 155 cycles [248]. Single phase nanocrystals such as spherical or randomly-shaped [33], [34], [36], [132], [246], [247], [248], [249], [250], [251], [252], [253], nanocubes [147], and anisotropic ones [38], [39], [40], [41], [254], [255] or bifunctional structures such as core-shell morphologies [42], decorated structures with a second material (metal, carbon, or oxides) [43], [44], [45], or composites [46] have been tested in order to improve the catalytic performance in batteries. Except the size and the morphology, there are other factors that affect the electrochemical performance in the case of the perovskite oxides such as the structural nanocrystal quality and the existence of defects in the lattice [33], the doping in the A and/or B site of the perovskite lattice [34], [35], [36], [37], the nanocrystal porosity [38], [39], [40], [41] and the existence of synergetic effects in the bifunctional morphologies [42], [43], [44], [45], [46].

For example, nanocrystals synthesized by a ball-milling process showed superior catalytic activities compared to the nanocrystals without this process due to the structural change and defects in the crystal structure [33]. B-site doping in the manganite perovskite oxides (La0.8Sr0.2Mn1-xNixO3) showed enhanced performance compared to the undoped one due to the introduction of more oxygen vacancies at the surface (Figure 9A) [252]. The capacity can be tuned by doping in the A site of the perovskite and decreased from 7211 to 6205, 6760, and 5925 mA h/g for La0.65Pb0.35MnO3, La0.65Ba0.35MnO3, La0.65Sr0.35MnO3 nanocrystals respectively [34]. The effect of the doping in the B-site has been studied in the La-based perovskite oxides and showed that the performance is improved in the order of LaCrO3, LaFeO3, LaNiO3, LaMnO3, and LaCoO3 [37].

Figure 9: Electrochemical performance of Li-air batteries including perovskite oxide nanocrystals. Three factors that affect this performance are: Ni-doping in manganite perovskite oxide (A), porosity of the nanocrystals (LaNiO3 nanocubes) (B), and synergetic effects in bifunctional nanocrystals (La0.6Sr0.4Co0.8Mn0.2O3 nanofibers loaded with RuO2) (C).(A) Reprinted with permission from [252]. Copyright (2016), American Chemical Society. (B) Reproduced with permission from [147]. Copyright 2014, Springer. (C) Reprinted with permission from [45]. Copyright (2017), American Chemical Society.
Figure 9:

Electrochemical performance of Li-air batteries including perovskite oxide nanocrystals. Three factors that affect this performance are: Ni-doping in manganite perovskite oxide (A), porosity of the nanocrystals (LaNiO3 nanocubes) (B), and synergetic effects in bifunctional nanocrystals (La0.6Sr0.4Co0.8Mn0.2O3 nanofibers loaded with RuO2) (C).

(A) Reprinted with permission from [252]. Copyright (2016), American Chemical Society. (B) Reproduced with permission from [147]. Copyright 2014, Springer. (C) Reprinted with permission from [45]. Copyright (2017), American Chemical Society.

The morphology and also the porosity of the nanostructures affect the electrochemical performance, including the first discharge specific capacity, the overpotential, the rate capability, and the cycle stability. The enhanced performance is due to the formation of direct current pathways that facilitate electron transport, short O22−/Li+ diffusion lengths that can improve ion transferring rates to oxygen electrode, the existence of a more efficient electrolyte-electrode contact, the increase of active sites for ORR/OER during battery operation, and the existence of more space to store discharged products. [38]. Porous nanocubes (Figure 9B) [147] or elongated nanocrystals (nanorods or nanotubes or nanofibers) [39], [40], [41], [256] of perovskite oxides have been introduced for such purposes. Furthermore, bifunctional nanocrystals have been utilized to enhance the performance in metal-air batteries. Synergetic effect have been utilized to improve the catalytic activity by covering the La0.8Sr0.2MnO3 nanorods with a layer of NiCo2O4 [42]. The ORR takes place mainly at the core, while the OER takes place at the nanoscale shell and their synergetic effect leads to the enhanced catalytic performance. Synergetic effects have also been observed in different bifunctional morphologies such as perovskite oxide nanocrystals decorated with nitrogen-doped carbon nanotubes [43], perovskite nanorods/graphene composite decorated with Ag nanocrystals [46], perovskite nanofibers functionalized with RuO2 nanoparticles and non-oxidized graphene nanoflakes [44], perovskite porous nanofibers loaded with RuO2 nanosheets (Figure 9C) [45].

2.4.2 Perovskite nanocrystals for supercapacitors

Perovskites have found also use as electrode materials in supercapacitors for energy storage. A simple design of a supercapacitor is based on two electrodes separated by an ion-permeable membrane and an electrolyte ionically connecting to both electrodes. During the polarization of the electrodes the ions are moving to oppose the electrode’s charges, forming electric double layers of opposite polarity. Supercapacitors are divided into three categories, the double-layer capacitors where the charge storage is electrostatically, the pseudocapacitors with electrochemically charge storage, and the hybrid ones which combine electrostatically and electrochemically charge storage [257]. The nanodimensional perovskites that are used for such purposes are some metal oxides, a few nanocomposites and even more limited halides in contrast with the many reports for using all-inorganic or hybrid halides in photovoltaic applications. Many studies have been reported in lanthanum-based perovskite nanocrystals due to their structural stability at high temperatures and inherent nature to contain oxygen vacancies. Additionally, the structure of lanthanum-based perovskites allows the substitution of ions by other ions of varying oxidation states changing on demand the electronic and physical properties [258]. Specifically, in LaMnO3 nanocrystals by tuning the oxygen content, capacitance of 586.7–609.8 F/g has been achieved [258]. Introducing a secondary phase of La2O3 attached on the nanocrystals could result in the capacitance of 520 F/g [259]. Important also is the substitution of the B site (ABO3) in the perovskite crystal structure with elements such Mn, Fe, Cr, and Ni which leads to capacitances of 56.78, 16.43, 24.4, and 106.58 F/g, respectively, after 500 charge-discharge cycles in a 3 m LiOH solution [260]. Furthermore, among the lanthanum-based candidates for supercapacitors the perovskites with Ni in the B site hold a prominent role. The incorporation of Ni offers excellent electrical conductivity and presents capacitances of a few hundred F. Specifically, LaNiO3 nanosheets [49], hollow nanospheres [48], and randomly-shaped nanocrystals [261] exhibit capacitance of 139.2 mA h/g (at 1.0 A/g), 422 F/g (at 1.0 A/g), and 478.7 F/g (at 0.1 mV/s), respectively, while their cycling stability reaches the 10000, 5000, and 15000 cycles, respectively. Figure 10 shows their morphology and the cyclic voltammetry curves at different scan rates.

Figure 10: LaNiO3 perovskite nanocrystals for electrodes in supercapacitors with morphologies; (A) nanosheets, (B) hollow nanospheres, (C) irregular-shaped nanocrystals (above figures) and their cyclic voltammetry measurements at different scan rates (below figures).(A) Reproduced with permission from ref [49]. Copyright 2017, Elsevier. (B) Reproduced with permission from ref [48]. Copyright 2017, Elsevier. (C) Reproduced with permission from ref [261]. Copyright 2018, Elsevier.
Figure 10:

LaNiO3 perovskite nanocrystals for electrodes in supercapacitors with morphologies; (A) nanosheets, (B) hollow nanospheres, (C) irregular-shaped nanocrystals (above figures) and their cyclic voltammetry measurements at different scan rates (below figures).

(A) Reproduced with permission from ref [49]. Copyright 2017, Elsevier. (B) Reproduced with permission from ref [48]. Copyright 2017, Elsevier. (C) Reproduced with permission from ref [261]. Copyright 2018, Elsevier.

A different type of lanthanum-based supercapacitor is that of incorporating composite materials. In this direction, a promising nanocomposite is the CeO2 mixed LaMnO3 which has been assessed as a negative electrode material [262]. The advantage of such a mixture is the high surface to volume ratio of the CeO2 nanocrystals which increases the active sites of the electrode. During the three-electrode measurement, the supercapacitor displayed 262 F/g for 1 A/g, retaining the 98% of capacitance after 2000 cycles. Another interesting nanocomposite system is that which combines the LaMnO3 with nitrogen-doped reduced graphene oxide (N-rGO). Doping the rGO with an amount of 25% of N-rGO, the active material exhibits 687 F/g at 5 mV/s and retains stability of 79% after 2000 cycles [263]. While the nanocomposite LaMnO3@Mn3O4 demonstrates remarkable stability of the specific capacitance even in 50,000 cycles (135 F/g at 1 A/g) with a maximum energy density of 75 W h/kg.

In addition, the substitution of La atoms in the crystal structure of the perovskite with Sr gives very high specific capacitances. According to this, La0.7Sr0.3CoO3-δ nanofibers [47] or La0.85Sr0.15MnO3@NiCo2O4 (LSM15@NC) nanoflowers [264] have shown 747 F/g (in Na2SO4 electrolyte at 2 A/g current density) and 1341 F/g (in 6 m KOH at 0.5 A/g current density), respectively. These values are among the highest reported for perovskites (Figure 11). In the case of the nanofibers, the substitution of La with Sr2+ into LaCoO3 lattice induces more oxygen vacancies which are active sites for storage in pseudocapacitive applications. Similar mechanism takes place in the LSM15@NC perovskites. Remarkably, the LSM15@NC nanocomposite in an asymmetric supercapacitor delivers energy density of 63.5 W h/kg at the power density of 900 W/kg. This energy density is higher than those recorded for Ni, Co, and/or Mn-based asymmetric capacitors [264]. The specific capacitance also increases slowly for the first 3000 cycles, becomes double above the 3000 cycles up to 10000 cycles. This indicates the efficiency of this material for high performance supercapacitors.

Figure 11: SEM images (above figures) and cyclic voltammograms (below figures) for (A) La0.7Sr0.3CoO3-δ nanofibers, (B) La0.85Sr0.15MnO3@NiCo2O4 nanoflowers.(A) Reproduced with permission from [47]. Copyright 2015, Elsevier. (B) Reproduced with permission from ref [264]. Copyright 2018, Elsevier.
Figure 11:

SEM images (above figures) and cyclic voltammograms (below figures) for (A) La0.7Sr0.3CoO3-δ nanofibers, (B) La0.85Sr0.15MnO3@NiCo2O4 nanoflowers.

(A) Reproduced with permission from [47]. Copyright 2015, Elsevier. (B) Reproduced with permission from ref [264]. Copyright 2018, Elsevier.

Bimetallic Co-Mn and Ni-Co perovskite fluorides are also promising electrode materials for supercapacitors. KCoxMn1-xF3 with Co:Mn 6:1, governed by strong synergistic effect of Co/Mn redox species exhibited a specific capacity of 226–192 F/g at 1–16 A/g coupled with a cycling stability of 5000 cycles (at 8 A/g). In an asymmetric capacitor design, it delivers 8–2.4 W h/kg at 0.14–8.7 kW/kg retaining the 90% of capacity in 10,000 cycles at 5 A/g [265]. While the similar structure with Ni, KNi0.8Co0.2F3 nanocrystals showed an energy density of 42.7–13.8 W h/kg at 0.242–18.8 kW/g power density [266].

Various oxide nanocrystals different than the previous have also been introduced for supercapacitor applications. Among these, BiFeO3 nanocrystalline porous film showed a capacitance of 81 F/g [267], BiFeO3 nanoplates 254.6 F/g [268], Y2NiMnO6 nanowires 77.76 F/g [269], LaFeO3 nanocrystals with Na and Mn substitutions 56.4 F/g [270], doped SrMnO3 nanofibers 321.7–446.8 F/g depending the doping of Ba/Ca on Sr and Co/Fe/Ni on Mn [271], SrTiO3 nanocubes with Co doping 75.28 F/g [272] and nano-Ru-based perovskites on rGO 564-316 F/g [273].

2.4.3 Perovskite nanocrystals for hydrogen storage

Hydrogen, the most sustainable fuel offering higher efficiencies compared to diesel and gasoline, is compatible with fuel cells and produces renewal waste (i.e. water). There are various hydrogen storage methods such as gas compression or liquefaction, however, they face safety issues. The most safe approach is the storage in solid-state materials such as metal alloys, metal oxides, hydroxides, carbon, chalcogenides, and recently in perovskites [51]. The perovskite powders were reported in 2004 by Esaka’s group as new anode materials for a hydrogen battery [274]. Later, in 2010, LaFeO3 and LaCrO3 powders were proposed as negative electrodes for Ni/MH batteries by Chen’s group [275], [276].

Three years later, Chen’s group succeeded in improving the electrochemical kinetics of the LaFeO3 perovskites by nanostructuring. These nanocrystals formed aggregates with sizes ranging from 50 to 100 nm [50]. The nanocrystals showed a higher discharge capacity than the bulk counterparts of the same stoichiometry. The discharge capacity is a value which characterizes the hydrogen storage efficiency of a material and it is estimated by the galvanostatic behavior of charge and discharge. In this method the investigated material was deposited on the electrode and circles of charges and discharges were followed [51]. In the case of nanostructured LaFeO3, the discharge capacity reaches the value of 531.5 mA h/g for 333 K for the first cycle (Figure 12A), degraded and stabilized above 350 mA/g for 20 cycles (a bit higher than the bulk). However, the LaFeO3 nanostructures showed higher current densities and hydrogen diffusion coefficients. Decreasing the size of the nanostructures is expected to increase the discharge capacity due to the larger surface area. A different perovskite system which tested for its H2 storage capability was the DyFeO3 nanocrystals reported by Salavati-Niasari’s group [277]. These nanocrystals have a size around 16–18 nm and showed a discharge capacity of 2100 mA/g after 15 cycles (Figure 12B), which is very high not only among the perovskites but also between other common materials for hydrogen storage [51]. Furthermore, Ba2Co9O14 nanocrystals of 10–30 nm were studied for the same applications by the same group but the recorded discharge capacity was lower (850 mA/g after 15 cycles) (Figure 12C) than the DyFeO3 nanocrystals. [278]. The proposed mechanism of hydrogen storage lies in a two-step reaction. The first step takes place on the surface of the material over a few atomic layers, while the second occurs inside as the H-atoms are diffused [277]. Although, these studies mentioned that the structural defects may play role in the storage capability, their exact role on the final performance are not yet clear. Also, it would be interesting to study newly solution-processed metal halide nanocrystals in such storage applications.

Figure 12: Perovskite nanostructures for H2 storage and their discharge capacities.(A) LaFeO3 nanocrystals. Reproduced with permission from [50]. Copyright 2013, American Institute of Physics. (B) DyFeO3 nanocrystals. Reproduced with permission from [277]. Copyright 2018, Elsevier. (C) Ba2Co9O14 nanocrystals. Reproduced with permission from [278]. Copyright 2019, Elsevier.
Figure 12:

Perovskite nanostructures for H2 storage and their discharge capacities.

(A) LaFeO3 nanocrystals. Reproduced with permission from [50]. Copyright 2013, American Institute of Physics. (B) DyFeO3 nanocrystals. Reproduced with permission from [277]. Copyright 2018, Elsevier. (C) Ba2Co9O14 nanocrystals. Reproduced with permission from [278]. Copyright 2019, Elsevier.

3 Conclusions and open issues

In recent years the perovskite nanocrystals have been introduced to effectively replace conventional energy materials. The simultaneous need for new energy materials together with the increasing interest for the development of new devices and even exploring new physics, have pushed the research to manipulate the structuring of the perovskite materials at the nanoscale level. The nanostructuring of the perovskites due to their reduced dimensions is advantageous in offering a large surface area, extensive porous structures, controlled transport, and high charge-carrier mobility, strong absorption, and photoluminescence, and confinement effects. In recent year there is a lot of work incorporating them into photovoltaics as active materials or covering the active layer to improve its stability but there has been limited effort to use them as thermoelectric materials or photocatalysts for the CO2 reduction in solar fuel cells. The utilization of them in CO2 reduction is a completely new scientific field which has gained increased interest very recently. In addition, perovskite nanocrystals have received considerable attention for energy storage applications due to their excellent catalytic activity, electrical conductivity, and durability. Ion migration through perovskite lattices allows the use of such materials as electrodes for batteries or supercapacitors. Perovskite oxide nanostructures are more investigated for such applications but very recently the metal halides have also shown high specific capacitance and promising stability upon cycling. Finally, the utilization of such nanocrystals in hydrogen storage could be really interesting as hydrogen is the most abundant element on the planet, with the highest energy content amongst all the existing energy sources, but the number of the perovskite nanocrystals used for such purposes is still limited.

This review article has covered many aspects of the synthesis of nanocrystals made of metal halides or perovskite oxides, but also their applications in energy conversion and storage. Despite the important evolution in the synthesis procedures, there are some open issues which require attention when we use these materials in these applications. Some of these open issues are:

3.1 Synthesis strategies

3.1.1 Surface chemistry and role of the ligands

Despite the huge evolution of synthesis strategies for the fabrication of nanocrystals of different morphologies and chemical phases, there is a poor understanding of the role of the ligands on the nanocrystal quality concerning their stability, carrier transport, but also on the energy device performance in which are included [279]. It is not clear if the ligands passivate structure trap states or introduce new ones and how the crystal defects play a role in the whole reactivity and electronic properties of the passivated nanocrystals.

3.1.2 Stability

Long-term stability issues at ambient conditions or more harsh environments such as high temperature, direct irradiation, light, and humidity have to be carefully addressed when we are interested in industrial applications. The careful choice of a protective ligand has been proposed as an effective way to improve the stability of the nanocrystals but the effect on device performance is something that has to be studied. The encapsulation of the nanocrystals in a matrix or a different material could be another way, but it is still unknown if such shelling could really prevent the nanocrystals from oxygen and moisture [12]. Finally, all-inorganic metal halides or lead-free compounds could effectively improve the stability of the devices, but still the performance of these devices remains very low.

3.1.3 Lead-free compounds

The synthesis of lead-free and environmentally friendly nanocrystals is a demand. Tin- or bismuth-based compounds have been introduced as possible stoichiometries and more recently double perovskites with an elpasolite structure [115]. The synthesis approaches for these perovskite nanocrystals remain limited. Only a few reports exist for elpasolite nanocrystals and all these nanocrystals are of spherical morphology and capped with organic ligands. The performance of bismuth based solar cells remains is very low.

3.1.4 Scale-up synthesis

One of the drawbacks of the synthesis procedures reported in this review is the small quantity of the final product. While there is a huge variety of synthesis procedures for nanocrystals of different morphologies/chemical phases and homogeneous in size and shape, there is still the difficulty of modifying them for large scale production if the purpose is to use these nanocrystals for industrial application. The development of large-scale synthesis procedures which will be cheap and easy is still a real challenge.

3.2 Deposition of nanocrystals in layer form, free of cracks and defects

Perovskite nanocrystals have been used in energy applications due to their large surface area, efficient carrier transport, high absorption coefficient, long-term stability, and tunable bandgap. The morphology and crystallinity are some of the important intrinsic features that affect the final performance of the devices. But in most of the applications these nanocrystals are assembled in films. The shape and size of the nanocrystals and the existence or not of ligands on the surface determine the final structure of the film. The formation of compact and smooth films is a real challenge for such applications. Many methods for the fabrication of films of high quality free of pinholes and cracks have been proposed, but many parameters remain unexplored and have to be controlled. The removal of the capping ligands is a necessity in order to fabricate such films with enhanced electrical properties. For such purposes, various methods for this removal have been proposed, but many times they are insufficient which result in the release of nanocrystals from the surface or cause their undesired growth of the nanocrystals. These affect also the stability of the devices in which are utilized such nanocrystals. The development of new efficient strategies for the effective removal of the capping ligands without affecting their primary structural or morphological features is a requirement.

Accordingly, it is important here to comment on attempts such as the encapsulation of perovskite nanocrystals in perovskite matrices [196] or the incorporation of nanocrystals between the active layer and the hole transporting layer [17], [18]. Interface engineering is an effective way for obtaining high efficiency and improved stability in the perovskite solar cells through interfacial charge transfer control. In addition, perovskite nanocrystals have been introduced into the absorber layer to reduce charge recombination and improve the charge transfer [203]. This process used to improve the quality of the absorber layer in terms of film structure, morphology, and crystallinity as the nanocrystals behave as nucleation centers in the growth of perovskite films. The high quality of the films leads to improved charge transport and solar cell power conversion efficiency.

About the authors

Athanasia Kostopoulou

Athanasia Kostopoulou received her BSc degree in Physics (2004) and her MSc degree (2006) in Materials Physics and Technology from the Physics Department at the Aristotle University of Thessaloniki. In 2012, she received her PhD from the Department of Chemistry at the University of Crete, and since then she was a Postdoctoral Fellow in the Institute of Electronic Structure and Laser at FORTH in Heraklion. Since 2016, she has been a part of the group of the Ultrafast Laser Micro and Nano Processing (ULMNP) Laboratory and she is working on the chemical synthesis and elucidation of the microscopic physical or photoinduced mechanisms involving nanocrystal systems. Recently she has been the coordinator of a project funded from the Hellenic Foundation for Research and Innovation (HFRI) and the General Secretariat for Research and Technology (GSRT) related to perovskite nanomaterials for photovoltaic applications.

Konstantinos Brintakis

Konstantinos Brintakis is a postdoctoral researcher at IESL-FORTH. He received his BSc in Physics from the Aristotle University of Thessaloniki (AUTh, Greece). He then continued his studies in “Materials Physics and Technology” obtaining his MSc from the Physics Department, AUTh. In 2017, he graduated with a PhD from the same Department and University with “Excellence”. His PhD thesis was a collaboration with the Physics Department and the Institute of Electronic Structure and Laser, studying the growth and organization of hybrid nanocrystals and specifically their structural, electronic and magnetic properties. Joining the Ultrafast Laser Micro- and Nano-Processing Group, he is interested in the interaction of matter with lasers and the development of nanostructures in solutions or/and on substrates with physicochemical and laser-assisted approaches. He is currently working on the synthesis and characterization of perovskite nanostructures for energy and gas sensing applications. He is a highly skilled scientist on the characterization of structural and morphological properties with high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) microscopy of the developed materials. He is also interested in the exploitation of the produced nanostructures in useful applications.

Nektarios K. Nasikas

Nektarios K. Nasikas is a physicist with a PhD in the field of Physical Chemistry of Materials. Over the years he has worked in prestigious research centers and universities, in Greece as well as in the USA, such as the Foundation for Research and Technology Hellas/Institute of Chemical Engineering Sciences (FORTH/ICEHT), the National Hellenic Research Foundation/Theoretical and Physical Chemistry Institute (NHRF/TPCI) and the Peter A. Rock Thermochemistry Laboratory/University of California at Davis. His research interests include novel materials synthesis, materials thermodynamics, novel experimental setup, and design with the use of strong lasers, as well as technology and applications of novel materials possessing special characteristics. He has authored several scientific papers in international scientific journals of high impact such as Chemistry of Materials, Advanced Functional Materials, Journal of the American Ceramic Society, etc. He currently serves as the Director of the Hellenic Foundation for Research and Innovation (HFRI), in Athens, Greece.

Emmanuel Stratakis

Dr Emmanuel Stratakis is a Research Director at the Institute of Electronic structure and laser (IESL) (www.iesl.forth.gr) of the Foundation for Research and Technology–Hellas (FORTH) (www.forth.gr). He received his Ph.D. in Physics from the University of Crete in 2001 from the Physics Department, University of Crete. After graduating, he joined as a visiting Researcher the IESL-FORTH working on the ultrafast laser engineering of materials and as an Adjunct Professor at the Department of Materials Science and Technology, University of Crete. In the fall semesters of 2006 and 2008 he was appointed as a visiting Researcher at the Department of Mechanical Engineering of the University of California, Berkeley. In 2007 he was elected Researcher at IESL-FORTH where he is leading the “Ultrafast Laser Micro- and Nano-processing” laboratory (http://stratakislab.iesl.forth.gr; https://www.iesl.forth.gr/en/research/ULNMP-Group). His research interests are in the fields of ultrafast laser interactions with materials for (a) biomimetic micro- and nano-structuring (b) Advanced photonic processes for photovoltaics and energy storage, c) nanomaterials synthesis and diagnostics for optoelectronics and (c) biomaterials processing for tissue engineering. He has delivered more than 40 invited and keynote lectures and has been organizer and chair in major international scientific conferences. He has over 180 SCI publications and more than 6000 citations and he has coordinated many National and EU grants. Since 2015, he is the Director of the European Nanoscience Facility of FORTH, part of the NFFA-Europe EU Infrastructure, where he is a member of the General Assembly. He is a National Representative to the High-Level Group of EU on Nanosciences, Nanotechnology and Advanced Materials and a National Expert for the Horizon 2020 committee configurations on: Nanotechnologies, Advanced materials, Biotechnology, Advanced Manufacturing and Processing. He is a member of the Scientific Committee of COST, of the Physical Sciences sectoral scientific council of the National Council for Research & Innovation of Greece and national Delegate of the OECD Working Party on Bio-, Nano- and Converging Tech (BNCT).

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Received: 2019-04-20
Revised: 2019-06-18
Accepted: 2019-06-19
Published Online: 2019-07-19

© 2019 Athanasia Kostopoulou, Emmanuel Stratakis et al., published by De Gruyter, Berlin/Boston

This work is licensed under the Creative Commons Attribution 4.0 Public License.

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