Nanophotonics 2019; 8(10): 1607–1640 Review article Athanasia Kostopoulou*, Konstantinos Brintakis, Nektarios K. Nasikas and Emmanuel Stratakis* Perovskite nanocrystals for energy conversion and storage https://doi.org/10.1515/nanoph-2019-0119 perovskite nanostructures in different morphologies is Received April 20, 2019; revised June 18, 2019; accepted June 19, summarized and the energy-related properties and appli- 2019 cations are extensively discussed in this paper. Abstract: The high demand for energy consumption in Keywords: perovskite nanocrystals; energy conversion everyday life, and fears of climate change are driving devices; energy storage devices; thermoelectrics. the scientific community to explore prospective materi- als for efficient energy conversion and storage. Perovs- kites, a prominent category of materials, including metal 1 Introduction halides and perovskite oxides have a significant role as energy materials, and can effectively replace conventional The high demand for energy consumption in everyday life materials. The simultaneous need for new energy mate- activities along with fears of the climate changes highlight rials together with the increased interest for making new the importance to develop efficient energy conversion devices, and exploring new physics, thrust the research and storage devices. Thus, sufficient energy conversion to control the structuring of the perovskite materials at and storage together with low-cost energy materials are the nanoscale. Nanostructuring of the perovskites offers the most important requirements. In order to design such unique features such as a large surface area, extensive devices, it is crucial to study and understand the under- porous structures, controlled transport and charge-car- lying principles and mechanisms of renewable energy rier mobility, strong absorption and photoluminescence, conversion and storage. Each of these technologies has its and confinement effects. These features together with own characteristics, requirements, and efficiency limits the unique tunability in their composition, shape, and or constraints. Different mechanisms take place in each functionalities make perovskite nanocrystals efficient technology and this is the main reason for dealing them for energy-related applications such as photovoltaics, independently. catalysts, thermoelectrics, batteries, supercapacitor and The design and engineering of novel materials with hydrogen storage systems. The synthesis procedures of 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 *Corresponding authors: Athanasia Kostopoulou, Institute of sciences such as the material science, chemistry, physics, Electronic Structure and Laser (IESL), Foundation for Research and and engineering. It is important to reveal the relationship Technology Hellas (FORTH), P.O. Box 1385, Heraklion 70013, Crete, between the material structure and the device perfor- Greece, e-mail: [email protected]. https://orcid.org/0000- mance if we wish to propose new energy-related materials 0002-7231-9876; and Emmanuel Stratakis, Institute of Electronic [1–3]. Structure and Laser (IESL), Foundation for Research and Technology Hellas (FORTH), P.O. Box 1385, Heraklion 70013, Crete, Greece; and In the quest to find prospective energy materials for University of Crete, Department of Physics, 710 03 Heraklion, Crete, high performance energy devices, the perovskite com- Greece, e-mail: [email protected]. https://orcid.org/0000-0002- pounds hold a prominent role due to their unique tunable 1908-8618 properties [4–8]. Perovskites are a family of materials with Konstantinos Brintakis: Institute of Electronic Structure and Laser the formula ABX and have a similar structure to the pro- (IESL), Foundation for Research and Technology Hellas (FORTH), 3 totype CaTiO mineral. The cation “A” occupies the corner P.O. Box 1385, Heraklion 70013, Crete, Greece 3 Nektarios K. Nasikas: Hellenic Foundation for Research and positions of the unit cell and the cation “B” is located at Innovation (HFRI), 127 Vass. Sofias Ave., 11521, Athens, Greece the center of the cell, while the anion “X” is on the unit Open Access. © 2019 Athanasia Kostopoulou, Emmanuel Stratakis et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 Public License. 1608 A. Kostopoulou et al.: Perovskite nanocrystals for energy conversion and storage cell faces [9]. This family comprises oxides and halide per- (CH3OH), etc., which are common chemical fuels. Single- ovskite material. Some representative oxides are the ferro- phase metal halide nanocrystals have shown promising electric BaTiO3 and PbTiO3, the dielectric (Ba,Sr)TiO3, the results in CO2 reduction [22, 23], but enhanced perfor- piezoelectric Pb(Zr,Ti)O3, the electro-strictive Pb(Mg,Nb) mance when these are coupled with graphene oxide (GO) O3, the magneto-resistant (La,Ca)MnO3, and the multifer- or palladium nanosheets [24, 25]. Besides, the perovskite roic BiFeO3. In the case of metal-halide perovskites, M is materials are promising materials for thermoelectrics for a divalent metal from group 14 (Pb, Sn) or a rare earth the conversion of thermal energy to electricity [26, 27]. element (Eu), and X is a halogen (X=F, Cl, Br, I, or a com- Compared to the traditional materials used for thermo- bination of them). According to the nature of the cation, electric applications (metal chalcogenide materials like the metal halides can be divided in two groups, the all- Bi2Te3 and PbTe), perovskite materials are less expensive inorganic and the hybrid organic-inorganic metal halides. and can be processed by low energy cost methods and can In the first category the cation A is a monovalent alkali be used for flexible thermoelectric devices [27]. The fairly metal (like Cs, K) while in the second it is a small organic ionic, polar character with a large dielectric constant and cation (such as CH3NH3) [10, 11]. the remarkable conduction band anisotropy of the metal The exploitation of new synthesis methods for the halides convey robust thermopower and moderate room fine control of the structural characteristics and improved temperature electrical conductivity [28]. stability is important in the design of perovskite energy- Perovskite nanocrystals have been utilized in energy related materials. Furthermore, the progress on the syn- storage in batteries or supercapacitors due to their excel- thesis strategies for nanoparticulate systems of high lent catalytic activity, electrical conductivity, and dura- quality in terms of homogeneity and crystallinity, has led bility. Ion migration through perovskite lattices allows the research community to search whether these materi- the use of such materials as electrodes for batteries. Elec- als could replace conventional energy materials. Differ- trochemical measurements of the nanoparticulate per- ent morphologies and chemical structures have been ovskite systems displayed superior catalytic activity for introduced for both metal halide and perovskite oxide oxygen reduction, as well as a higher discharge plateau nanocrystals for such purposes [1, 12, 13]. and specific capacity compared to the bulk materials of Metal halide nanocrystals can be effectively used in the same crystal structure [29]. Metal halide nanocrys- energy conversion, due to their strong optical absorption, tal films have been formed for application as anodes, low non-radiative recombination rates, tunable band for stable Li-based batteries [30–32]. Furthermore, in gaps, relatively high charge-carrier mobility, and long dif- the case of the perovskite oxides, the size and the mor- fusion lengths coupled with solution processability [14]. phology of the nanocrystals are two factors that affect These nanocrystals have been utilized as the absorbing their electrochemical performance. Factors such as the material in perovskite solar cells [15, 16] or placed at the structural nanocrystal quality, the existence of defects interface between the absorbing and the hole transport in the lattice [33], the doping in of the A and/or B site layer (HTL) in order to improve carrier transport and sta- of the perovskite lattice [34–37], the nanocrystal poros- bility [17, 18]. They are also used as down-converters in ity [38–41], and the existence of synergetic effects in the silicon solar cells due to their excellent quantum-cutting bifunctional morphologies [42–46] play an important role properties giving efficiencies of 21.5% [19]. In contrast, in the final electrochemical behavior. In addition, in the perovskite oxide nanocrystals have been utilized as elec- case of supercapacitor storage, it was found that struc- tron transport layers (ETLs) in perovskite solar cells, turing perovskite oxides and forming nanocrystals lead as these materials are characterized by high electron to remarkably enhanced, specific capacitance, rate capa- mobility, wide band-gap, and a well-aligned conduction bility, and cycle stability compared to the corresponding band with the absorbing layer [20]. Furthermore, perovs- bulk materials [47–49]. Finally, perovskite nanocrystals kite nanocrystals have been tested for catalytic carbon offer improved electrochemical performance, low cost dioxide (CO2) reduction in solar fuel cells. By mimick- production in hydrogen storage and energy sustainabil- ing the natural photosynthesis in green plants, artificial ity for transportation, electricity