Breakthrough: Phase-Pure 2D Perovskite Films

Obtaining phase-pure 2D perovskite films will extend the understanding and applications in various optoelectronic fields. Recently in Nature Energy, Liang and coworkers first present phase-pure 2D perovskites by replacing BAI with BAAc, showing stronger ionic coordination with the perovskite framework. A PCE of 16.25% with enhanced stability was obtained. Obtaining phase-pure 2D perovskite films will extend the understanding and applications in various optoelectronic fields. Recently in Nature Energy, Liang and coworkers first present phase-pure 2D perovskites by replacing BAI with BAAc, showing stronger ionic coordination with the perovskite framework. A PCE of 16.25% with enhanced stability was obtained. The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has increased from 3.8% in 20091Kojima 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 (12872) Google Scholar to a certified 25.5%2NRELBest Research-Cell Efficiency Chart.Photovolt. Res. 2020; https://www.nrel.gov/pv/cell-efficiency.htmlGoogle Scholar in 2020, resulting in broad interest from academic and industrial photovoltaic (PV) fields. However, the long-term stability under various practical operating conditions are still short for the industrial application. Beside some usual strategies—such as defect passivation and interfacial modification3Zhang F. Zhu K. Additive Engineering for Efficient and Stable Perovskite Solar Cells.Adv. Energy Mater. 2020; 10: 1902579Crossref Scopus (199) Google Scholar,4Xue J. Wang R. Yang Y. The surface of halide perovskites from nano to bulk.Nat. Rev. Mater. 2020; 5: 809-827Crossref Scopus (66) Google Scholar—one important strategy is to use pure two-dimensional (2D) (n = 1) or quasi-2D perovskites in bulk or on the surface of three-dimensional (3D) perovskites (2D–3D).5Zhang F. Lu H. Tong J. Berry J.J. Beard M.C. Zhu K. Advances in two-dimensional organic–inorganic hybrid perovskites.Energy Environ. Sci. 2020; 13: 1154-1186Crossref Google Scholar The general formula of 2D perovskite structures is either (A’)2(A)n-1BnX3n+1, where A’ is a monovalent cation (Ruddlesden-Popper, RP) phase, or (A’)(A)n-1BnX3n+1, where A’ is a divalent cation (Dion-Jacobson, DJ) phase. The typical bulky cations include phenylethylammonium (PEA+) and butylammonium (BA+) for the RP phase and 3-(aminomethyl)piperidinium (3AMP2+) and 1,4-butane diammonium (BDA2+) for the DJ phase of 2D perovskites.5Zhang F. Lu H. Tong J. Berry J.J. Beard M.C. Zhu K. Advances in two-dimensional organic–inorganic hybrid perovskites.Energy Environ. Sci. 2020; 13: 1154-1186Crossref Google Scholar The n value is referred to as the thickness of the inorganic metal halide sheets layer and is often given based on the precursor components.5Zhang F. Lu H. Tong J. Berry J.J. Beard M.C. Zhu K. Advances in two-dimensional organic–inorganic hybrid perovskites.Energy Environ. Sci. 2020; 13: 1154-1186Crossref Google Scholar A pure 2D perovskite structure corresponds to n = 1, a quasi-2D perovskite structure often corresponds to 1 < n ≤ 5, and the general 3D perovskite structure corresponds to n approaching ∞. In practice, for 2D perovskites when n > 3, the resulting materials are often comprised of multiple quantum wells (MQWs) with different n values due to different formation energy. Thus, it is always challenging to prepare phase-pure high-n 2D perovskite films.6Zhang J. Qin J. Wang M. Bai Y. Zou H. Keum J.K. Tao R. Xu H. Yu H. Haacke S. Hu B. Uniform Permutation of Quasi-2D Perovskites by Vacuum Poling for Efficient, High-Fill-Factor Solar Cells.Joule. 2019; 3: 3061-3071Abstract Full Text Full Text PDF Scopus (81) Google Scholar,7Grancini G. Nazeeruddin M.K. Dimensional tailoring of hybrid perovskites for photovoltaics.Nat. Rev. Mater. 2018; 4: 4-22Crossref Scopus (355) Google Scholar In 2014, Smith et al. first reported (PEA)2(MA)2Pb3I10 as absorbers in PSCs and obtained a PCE of 4.73%.8Smith I.C. Hoke E.T. Solis-Ibarra D. McGehee M.D. Karunadasa H.I. A layered hybrid perovskite solar-cell absorber with enhanced moisture stability.Angew. Chem. Int. Ed. 2014; 53: 11232-11235Crossref PubMed Scopus (1219) Google Scholar The best PCE of 2D PSCs (n ≤ 5) has so far reached above 19%9Lai H. Lu D. Xu Z. Zheng N. Xie Z. Liu Y. Organic-Salt-Assisted Crystal Growth and Orientation of Quasi-2D Ruddlesden-Popper Perovskites for Solar Cells with Efficiency over 19%.Adv. Mater. 2020; 32: 2001470Crossref Scopus (59) Google Scholar with a short-circuit current density (Jsc) comparable to the normal 3D PSC, which is likely resulting from an increased fraction of 3D perovskite in the 2D MQWs films. The as-prepared 2D films with mixed n values could restrict their effectiveness for various optoelectronic applications, especially for obtaining the wavelength-tunable light-emitting diodes (LEDs) with highly pure emission colors. It also complicates the scientific understanding of the various physical and chemical properties of the 2D perovskite structures. These will in turn induce another challenge of tailoring materials and structures design. Thus, developing synthetic tools and strategies to obtain pure-phase 2D perovskite structures or test probing percentages of different n values will promote the development of 2D-3D perovskites and extend the understanding of their optoelectronic properties and application in other optoelectronic fields. Recently in Nature Energy, C. Liang and coworkers first present the phase-pure quantum wells (QW) width films by replacing traditional n-butylamine iodide (BAI) with n-butylamine acetate (BAAc).10Liang C. Gu H. Xia Y. Wang Z. Liu X. Xia J. Zuo S. Hu Y. Gao X. Hui W. et al.Two-dimensional Ruddlesden–Popper layered perovskite solar cells based on phase-pure thin films.Nat. Energy. 2020; https://doi.org/10.1038/s41560-020-00721-5Crossref Scopus (78) Google Scholar Due to the stronger ionic coordination between Ac− and Pb2+, the particles in precursor solution with BAAc present a very narrow size distribution (Figure 1), rather than randomly distributed particles in solution with BAI, due to the suppressed aggregation of colloids with more than one unit cell. The stronger interaction between the carbonyl groups of acetate and Pb2+ was proven by Fourier-transform infrared spectra, X-ray absorption fine structure spectroscopy, and 1H nuclear magnetic resonance (NMR) spectra. During the initial stage of spin coating, an intermediate phase (BA)2(MA)3Pb4I13-xAcx⋅(MAI)2 (x ≤ 2) with uniformly distributed, near-monodisperse unit cell particles could be gelled. Subsequently, the unstable Ac- in the intermediate phase will escape and coordinate with MA+, forming the easily decomposed methylammonium acetate (MAAc), and I- will occupy the left vacancy of Ac-, forming the phase-pure QW film. The mechanisms process is also illustrated by the lowest reaction formation enthalpies after introducing BAAc. A reasonable PCE of 16.25% and an open-circuit voltage of 1.31 V were obtained along with a good stability by maintaining above 90% of the initial value after 1,100 h continuous light illumination. This report opens a new direction of how to make 2D films with pure phase, which will extend the understanding of their optoelectronic properties and application in other optoelectronic fields. However, the mechanism is still not very clear for the perovskite formation process involving the intermediate phase. In the near future, studies should focus on more bulky cations and other acidic salts to extend its general use for a wider range of 2D perovskites as well as cheaper and easy-to-obtain raw materials (formamidine acetate salt, FAAc; etc) for scalable applications. With these advancements, various pure-phase 2D perovskite films with certain n values will be easily produced for different optoelectronic applications.