Riding the New Wave of Perovskites
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(PSCO) meeting discussing new advances and challenges in the field (DOI: 10.1021/acsenergylett.7b00217). A virtual issue on lead-free perovskites jointly produced with Chemistry of Materials and The Journal of Physical Chemistry Letters is being introduced in the second EF article (Lead-Free Perovskite Solar Cells, DOI: 10.1021/acsenergylett.7b00246). Testifying to the impact that perovskites have had in PVs, Martin Green, who holds the world efficiency record for silicon solar cells, and colleague Anita Ho-Baillie introduce us to the perovskite world starting with the birth of the perovskite name in 1839, moving to the “perovskite fever” of 2011 and 2012, to nowadays’ latest developments and achievements (DOI: 10.1021/acsenergylett.7b00137). The rapid perovskite progress is likely the outcome of several years of research in dyesensitized solar cell and organic PVs. While efficiency over 22% was confirmed in early 2016 and perovskite/silicon tandem cells with confirmed efficiencies of 23.6% have been demonstrated, stability still remains an issue. Some progress has been made in this area, thanks to the development of robust mixed cation/mixed halides perovskite solid solutions. Encouragingly, with appropriate encapsulation, small-area perovskite cells have passed the standard module qualification damp heat and thermal cycling tests. Assuming that stability issues will be sorted out, we can be optimistic about the perovskite solar cells in light of the fast progress made in this area. However, the presence of lead in all high-efficiency perovskite cells may represent an obstacle to perovskite solar cell commercialization due to the possible need for containment of degradation products from perovskite modules. In this sense, there are two different routes that one could take: (i) using inherently more stable Pb-based compounds, such as low-dimensional-networked perovskites, possibly in combination with high-performing standard compounds or (ii) developing new Pb-free perovskites. Low-dimensional perovskites (see the perspective by Bakr and colleagues, DOI: 10.1021/acsenergylett.6b00705) are generally more thermodynamically stable against moisture than their 3D counterparts, with hydrophobic linkers protecting them against moisture. Being not restricted by Goldsmidt’s tolerance factor, low-dimensional perovskites may provide unlimited compositional and structural versatility. An attractive feature of such compounds is the possibility of incorporating functional organic molecules with which one can further modulate the optical and electrical properties of perovskites. Low-dimensional perovskites are materials with high exciton binding energies, raising huge interest in phosphor and LED applications. 2D perovskites in PVs have not (yet) led to device fabrication with efficiency comparable to that of their 3D counterparts. It is likely that 2D−3D composites with properly engineered interfaces will be a winning strategy to retain the best of both types of perovskites.
et us start this Editorial with a clarification: Organohalide perovskites are not organometallic compounds. As per the IUPAC definition (International Union of Pure and Applied Chemistry), organometallic compounds are defined as “compounds having bonds between one or more metal atoms and one or more carbon atoms of an organic group”.1 Thus, one should not refer to methylammonium lead halides (MAPbX3) as organometallic perovskites because no metal− carbon bond exists in these compounds. Considering also the fully inorganic perovskites, for example, CsPbX3, one should probably generically refer to the broader materials class simply as metal-halide perovskites. Despite being admittedly provocative, this ambiguity in naming metal-halide perovskites reflects the fast-moving landscape that has characterized the early stages of perovskite research. The early research popularized by solar cell performance has now diversified into many different subdisciplines. The emergence of perovskite solar cells as the contender for next-generation photovoltaics (PVs) has indeed encouraged researchers to explore new paradigms in the field; these include design of solar cells with new photoactive perovskite materials along with different electron/hole transport materials, excitedstate dynamics, low-dimension perovskite materials, and leadfree perovskites. While the first wave of the trial-and-error screening approach for designing solar cells is disappearing, a new wave of exciting science to explore new horizons in the field is emerging (Figure 1). What would the world be without perovskites? The answer is probably “a much less stimulating scientific world”, considering that in this ACS Energy Letters issue we feature two Energy Focus (EF) articles and six Perspective/Review papers devoted to perovskites. One of the EF articles summarizes the key highlights of the Perovskite Solar Cells and Optoelectronic
Figure 1. A new wave of innovative hybrid perovskites is bringing exciting science to explore. © 2017 American Chemical Society
Published: April 14, 2017 922
DOI: 10.1021/acsenergylett.7b00256 ACS Energy Lett. 2017, 2, 922−923
Editorial
http://pubs.acs.org/journal/aelccp
ACS Energy Letters
Editorial
In the search for new perovskites, high-throughput computational material screening offers a substantial aid to chemical or physical intuition in designing new materials. In their Perspective, Mhaisalkar and colleagues discuss the rational design and combinatorial screening of new compounds (DOI: 10.1021/acsenergylett.7b00035). For AMX3 perovskite halides, full and partial substitutions with 6 monovalent cations (A-site), 27 divalent metal cations (M-site), and 3 halides could yield a number of >104 material combinations. Extending the range to metal-deficient, double perovskites, and multidimensional perovskites will further increase the combinations to >106. This Perspective, focusing on perovskite materials design and selection, outlines a rational design methodology. It combines combinatorial computational high-throughput screening with experimental validation to down-select and highlight the candidates that have the highest propensity for the synthesis of lead-free perovskites. An important aspect in perovskites research is the proper characterization of materials obtained by different synthetic techniques. This is particularly a challenge in the case of the aforementioned mixed perovskites, which may consist of multiple stoichiometries and phases. In their Perspective, Vela and co-workers discuss the opportunity offered by 207Pb solidstate nuclear magnetic resonance (DOI: 10.1021/acsenergylett.6b00674). Advanced techniques, such as dynamic nuclear polarization, are proposed, which drastically diminish the long collection times (usually of several hours) of typical 207Pb experiments, allowing for faster and more precise determination of the local lead coordination sphere. High-quality perovskite single crystals can show extremely different optical and electronic properties at the surface compared to that at the bulk. Mohammed and co-workers discuss in their Perspective the relation between optoelectronic properties and the surface chemistry of single crystals in ambient air (DOI: 10.1021/acsenergylett.6b00680). Various methods are employed to compare the carrier mobilities of the bulk and surface, with an eye at surface recombination losses. A comprehensive picture of surface disordering, change in the charge carrier dynamics, surface hydration, and the effect of ion migration on the surface behavior are discussed. Surface passivation methods to possibly alleviate the challenges for device integration are also presented. Finally, Huang and co-workers discuss in their Perspective the important function of fullerene and, in general, of carbonbased materials in perovskite solar cells (DOI: 10.1021/ acsenergylett.6b00657). Carbon materials have been intensively applied in perovskite solar cells for multiple purposes, ranging from acting as an electrode and encapsulation layer to effective electron- and hole-transporting materials. Fullerenes, in particular, are widely employed as electron acceptors in inverted perovskite solar cells, achieving over 20% efficiency in such devices. The dual role of fullerenes allows efficient electron extraction from the perovskite layer and simultaneous passivation of defects at the surfaces and grain boundaries of polycrystalline thin films. The authors propose that ion/defect diffusion, typical of polycrystalline perovskite films, may be suppressed due to filling of the grain boundaries by fullerenes. The topics discussed in this issue provide important new areas of metal halide perovskite-based materials to explore. Fundamental understanding of optical and electronic properties of newly designed materials remains a key to the design of stable devices.
Computational Laboratory for Hybrid/Organic Photovoltaics, CNR-ISTM, Perugia I-06123, Italy
Prashant V. Kamat, Editor-in-Chief
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University of Notre Dame, Notre Dame, Indiana 46556, United States
AUTHOR INFORMATION
ORCID
Filippo De Angelis: 0000-0003-3833-1975 Prashant V. Kamat: 0000-0002-2465-6819 Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS.
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REFERENCES
(1) Moss, G. P.; Smith, P. A. S.; Tavernier, D. Glossary of Class Names of Organic-Compounds and Reactive Intermediates Based on Structure. Pure Appl. Chem. 1995, 67, 1307−1375.
Filippo De Angelis, Senior Editor
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DOI: 10.1021/acsenergylett.7b00256 ACS Energy Lett. 2017, 2, 922−923
