Tin Perovskite Solar Cells Are Back in the Game

Abstract

This issue of Joule features an article by Wang et al. that demonstrates that high-efficiency tin-based solar cells can be realized by the formation of a heterostructure consisting of two-dimensional (2D) and three-dimensional (3D) perovskites based on phenylethylammonium tin iodide (PEA2SnI4) and formamidinium tin iodide (FASnI3), respectively, forming from the study of the PEA2(FA)n-1SnnI3n+1 system. The authors convincingly show that these self-assembled heterostructures can be manipulated and ordered sequentially by chemical means, thus providing a new avenue for further improving the perovskite solar cell technology. This issue of Joule features an article by Wang et al. that demonstrates that high-efficiency tin-based solar cells can be realized by the formation of a heterostructure consisting of two-dimensional (2D) and three-dimensional (3D) perovskites based on phenylethylammonium tin iodide (PEA2SnI4) and formamidinium tin iodide (FASnI3), respectively, forming from the study of the PEA2(FA)n-1SnnI3n+1 system. The authors convincingly show that these self-assembled heterostructures can be manipulated and ordered sequentially by chemical means, thus providing a new avenue for further improving the perovskite solar cell technology. Halide perovskites have changed the scenery of the photovoltaics landscape, since less than a decade after their original demonstration for methylammonium lead iodide (MAPbI3) in 2009,1Kojima A. Teshima K. Shirai Y. Miyasaka T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells.J. Am. Chem. Soc. 2009; 131: 6050-6051Crossref PubMed Scopus (15275) Google Scholar and following further developments, they are already bound to produce consistent conversion efficiencies >20%, thus setting firm candidacy for exploitation in practical applications. To make things even more attractive, their processability in solution provides plenty of scope for manipulating the chemical composition and also makes them compatible with most semiconductor technologies toward fabrication of tandem heterostructures. A key element in this path lies in the fabrication of efficient perovskite solar cells based on Sn, since this subclass of perovskites possesses the narrowest band gap, thereby allowing for a most effective absorption of solar radiation.2Stoumpos C.C. Malliakas C.D. Kanatzidis M.G. Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties.Inorg. Chem. 2013; 52: 9019-9038Crossref PubMed Scopus (3952) Google Scholar Nevertheless, the fabrication of Sn-based devices has been problematic because of the inherent tendency of Sn2+ in the material to oxidize to Sn4+, leading to the formation of heavily doped semiconductors, where the excess charge carriers lead to the deterioration of the photoconductive response.3Ke W. Stoumpos C.C. Kanatzidis M.G. “Unleaded” perovskites: status quo and future prospects of tin-based perovskite solar cells.Adv. Mater. 2018; : e1803230Crossref PubMed Scopus (225) Google Scholar Since there is, in theory, no obvious justification for the Sn-based perovskites being inferior to their high-performance Pb congeners, researchers have painstakingly tried to prove that high-performance Sn-based perovskites can also be realized, focusing universally on remedying the doping effect by using suitable additives. From the initial use of SnF2 as an additive to maintain the Sn2+ stoichiometry,4Lee S.J. Shin S.S. Kim Y.C. Kim D. Ahn T.K. Noh J.H. Seo J. Seok S.I. Fabrication of efficient formamidinium tin iodide perovskite solar cells through SnF2–Pyrazine complex.J. Am. Chem. Soc. 2016; 138: 3974-3977Crossref PubMed Scopus (543) Google Scholar to the employment of reducing agents that can reverse the self-oxidation of the metal,5Cao D.H. Stoumpos C.C. Yokoyama T. Logsdon J.L. Song T.-B. Farha O.K. Wasielewski M.R. Hupp J.T. Kanatzidis M.G. Thin films and solar cells based on semiconducting two-dimensional ruddlesden–popper (CH3(CH2)3NH3)2(CH3NH3)n-1SnnI3n+1 perovskites.ACS Energy Lett. 2017; 2: 982-990Crossref Scopus (285) Google Scholar which only offer partial improvements, to the breakthrough of employing two-dimensional (2D) perovskites6Quan L.N. Yuan M. Comin R. Voznyy O. Beauregard E.M. Hoogland S. Buin A. Kirmani A.R. Zhao K. Amassian A. et al.Ligand-Stabilized reduced-dimensionality perovskites.J. Am. Chem. Soc. 2016; 138: 2649-2655Crossref PubMed Scopus (963) Google Scholar to make use of the fact that the tendency of self-doping decreases with decreased dimensionality,7Mitzi D.B. Feild C.A. Harrison W.T.A. Guloy A.M. Conducting tin halides with a layered organic-based perovskite structure.Nature. 1994; 369: 467Crossref Scopus (835) Google Scholar the last few years have seen an enormous progress in the development of Sn-based perovskites. In this perspective, the work by Wang et al.8Wang F. Jiang X. Chen H. Shang Y. Liu H. Wei J. Zhou W. He H. Liu W. Ning Z. 2D-Quasi-2D-3D hierarchy structure for tin perovskite solar cells with enhanced efficiency and stability.Joule. 2018; 2 (this issue): 2732-2743Abstract Full Text Full Text PDF Scopus (257) Google Scholar proceeds one step further to show that these reduced-dimensionality Sn-based perovskite structures can be manipulated to produce hierarchically assembled heterostructures where 2D and 3D perovskites can coexist, forming compositional gradients across the perovskite layer. In their study, the authors use NH4SCN as a crystallization moderator to disturb the perovskite crystallization process so that the formation of the 2D (as phenylethylammonium tin iodide, PEA2SnI4) and 3D (as formamidinium tin iodide, FASnI3) perovskites occurs at discrete times during the film formation in the PEA2(FA)n-1SnnI3n+1 system. Even though the role of NH4SCN is still an open question, the beneficial net effect of slowing down the perovskite crystallization is evident. At the moment, it is uncertain whether it is SCN− that coordinates to the metal, since Sn2+ is a softer Lewis acid than Pb2+ to favor a similar coordination to that of MA2PbI2(SCN)2,9Daub M. Hillebrecht H. Synthesis, single-crystal structure and characterization of (CH3 NH3 )2 Pb(SCN)2 I2.Angew. Chem. Int. Ed. 2015; 54: 11016-11017Crossref PubMed Scopus (123) Google Scholar or if it is NH4+ that performs the role of a hydrogen bond scavenger, acting as a sacrificial hydrogen donor to the polar solvents, thereby allowing PEA+ and FA+ cations to assemble the perovskite unhindered. The answer to this question will very likely be given in future studies, yet it is clear from the work of Wang et al.8Wang F. Jiang X. Chen H. Shang Y. Liu H. Wei J. Zhou W. He H. Liu W. Ning Z. 2D-Quasi-2D-3D hierarchy structure for tin perovskite solar cells with enhanced efficiency and stability.Joule. 2018; 2 (this issue): 2732-2743Abstract Full Text Full Text PDF Scopus (257) Google Scholar that it is possible to influence the hierarchy of the perovskite heterostructure by using simple additives that can reversibly interact with the perovskite precursors and alter the thermodynamics of the reaction, in addition to the solvent engineering effect, which can direct the orientation of the 2D perovskite layers relative to the substrate.5Cao D.H. Stoumpos C.C. Yokoyama T. Logsdon J.L. Song T.-B. Farha O.K. Wasielewski M.R. Hupp J.T. Kanatzidis M.G. Thin films and solar cells based on semiconducting two-dimensional ruddlesden–popper (CH3(CH2)3NH3)2(CH3NH3)n-1SnnI3n+1 perovskites.ACS Energy Lett. 2017; 2: 982-990Crossref Scopus (285) Google Scholar These fine nuances in the film deposition of 2D perovskites are always important to consider because they will ultimately define the core property of the solar cell: the sequencing of the perovskite heterostructure. Wang et al.8Wang F. Jiang X. Chen H. Shang Y. Liu H. Wei J. Zhou W. He H. Liu W. Ning Z. 2D-Quasi-2D-3D hierarchy structure for tin perovskite solar cells with enhanced efficiency and stability.Joule. 2018; 2 (this issue): 2732-2743Abstract Full Text Full Text PDF Scopus (257) Google Scholar brilliantly succeed in this respect as their in situ monitoring of the crystallization process reveals valuable information on how these structures are formed, showing how the 3D perovskite deposits first to allow the 2D perovskite to terminate the film’s interface. This stacking sequence not only provides a band gap gradient that guides the charge carriers to the electrodes through the heterostructure but it also forms a protective layer that shields the sensitive components from the atmosphere. In fact, similar heterostructures have been known to possess some of the longest device stabilities reported to date, for the case of Pb-based perovskites,10Grancini G. Roldán-Carmona C. Zimmermann I. Mosconi E. Lee X. Martineau D. Narbey S. Oswald F. De Angelis F. Graetzel M. Nazeeruddin M.K. One-year stable perovskite solar cells by 2D/3D interface engineering.Nat. Commun. 2017; 8: 15684Crossref PubMed Scopus (1312) Google Scholar suggesting the universality and the usefulness of the heterostructure approach. As promising as the heterostructure approach can be, a word of caution needs to be heeded: hierarchical heterostructures are not synonymous to 2D perovskites. From the early days of 2D halide perovskites, there were already hints for the disproportionation of the 2D perovskites toward thicker and thinner 2D members within a given homologous series. This susceptibility toward disproportionation has caused great confusion in the perovskite literature since the intended composition and the actual composition of the materials in the films differ. In many cases, there is a serious mismatch between the nominal compound and the actual composition of the film, which usually consists of a composite of multiple 2D phases (heterostructured or otherwise) based on the presented experimental evidence. Therefore, it would be useful if a common language could be developed, in order to distinguish between the “2D perovskites” (registered 2D compounds that can be crystallographically and/or spectroscopically defined), the “quasi-2D perovskites” (unregistered thick 2D perovskites, approaching the 3D boundary, that cannot be crystallographically or spectroscopically identified), and “3D perovskites” (crystallographically and spectroscopically defined 3D perovskites). In this manner, unlikely compositions that are deduced from arbitrary nominal stoichiometries rather than from the detailed characterization of the material will be avoided and, in this direction, the manuscript by Wang et al.8Wang F. Jiang X. Chen H. Shang Y. Liu H. Wei J. Zhou W. He H. Liu W. Ning Z. 2D-Quasi-2D-3D hierarchy structure for tin perovskite solar cells with enhanced efficiency and stability.Joule. 2018; 2 (this issue): 2732-2743Abstract Full Text Full Text PDF Scopus (257) Google Scholar makes an excellent start toward categorizing the various possibilities in perovskite films deriving from 2D perovskites. As far as Sn-based perovskites are concerned, the work by Wang et al.8Wang F. Jiang X. Chen H. Shang Y. Liu H. Wei J. Zhou W. He H. Liu W. Ning Z. 2D-Quasi-2D-3D hierarchy structure for tin perovskite solar cells with enhanced efficiency and stability.Joule. 2018; 2 (this issue): 2732-2743Abstract Full Text Full Text PDF Scopus (257) Google Scholar provides new scope in manipulating and understanding film formation in the reduced-dimensionality perovskites and provides new tools for the manipulation of the materials toward more stable and efficient solar cells, on their way to catching up and competing on equal terms with the Pb-based pacesetters of the field. 2D-Quasi-2D-3D Hierarchy Structure for Tin Perovskite Solar Cells with Enhanced Efficiency and StabilityWang et al.JouleOctober 5, 2018In BriefDevelopment of tin halide perovskites is limited by the extremely poor stability and high background carrier density. Here, based on a pseudohalogen “catalyst,” we fabricated a Sn-based hierarchy structure perovskite in a one-step process, comprising highly parallel-orientation 2D PEA2SnI4 on the surface of 3D FASnI3. The hierarchy structure delivers significantly enhanced stability and oxidation resistance in air atmosphere. We then explored hierarchy structure perovskite films in planar structure solar cells and achieved a PCE up to 9.41%. Full-Text PDF Open Archive