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• W2895549098 abstract "•2D-quasi-2D-3D hierarchy structure perovskite is fabricated for the first time•Removable pseudohalogen acts as a regulator to manipulate tin perovskite structure•The hierarchy structure effectively resists oxidation and increases carrier mobility•The hierarchy structure tin perovskite solar cells achieve a record PCE of 9.41% Recently, tin perovskite solar cells (PSCs) have attracted a great deal of research interest due to their low toxicity, ideal band gap, and earth-abundant elements. However, the poor stability and high density of defects severely limit the performance of tin PSCs. The incorporation of low-dimensional perovskite with molecule-protecting layers is vital to overcome the obstacles. Herein, we demonstrate a hierarchy structure tin perovskite induced by the removable pseudohalogen regulator NH4SCN. The hierarchy structure comprising highly parallel-orientation 2D-quasi-2D-3D FASnI3 significantly enhances stability and oxidation resistance. We then explored the hierarchy structure PSCs and achieved a PCE up to 9.41% with high reproducibility. This work suggests an effective strategy to build tin PSCs with high performance and long-term stability. The power conversion efficiency (PCE) of tin perovskite solar cells is impeded by the extremely poor resistance to oxidation and high density of intrinsic Sn vacancies. Herein, we grow a 2D-quasi-2D-3D Sn perovskite film using removable pseudohalogen NH4SCN as a structure regulator. This hierarchy structure remarkably enhances air stability resulting from the parallel growth of 2D PEA2SnI4 as the surface layer. We then explore the hierarchy structure perovskite films in planar structural solar cells, which generate a PCE up to 9.41%. The device retains 90% of its initial performance for almost 600 hr. Our results suggest that adding removable NH4SCN in a perovskite precursor can significantly improve the stability and photovoltaic performance of Sn perovskite. This finding provides a powerful strategy to manipulate the structure of low-dimensional perovskite in order to enhance the performance of perovskite solar cells. The power conversion efficiency (PCE) of tin perovskite solar cells is impeded by the extremely poor resistance to oxidation and high density of intrinsic Sn vacancies. Herein, we grow a 2D-quasi-2D-3D Sn perovskite film using removable pseudohalogen NH4SCN as a structure regulator. This hierarchy structure remarkably enhances air stability resulting from the parallel growth of 2D PEA2SnI4 as the surface layer. We then explore the hierarchy structure perovskite films in planar structural solar cells, which generate a PCE up to 9.41%. The device retains 90% of its initial performance for almost 600 hr. Our results suggest that adding removable NH4SCN in a perovskite precursor can significantly improve the stability and photovoltaic performance of Sn perovskite. This finding provides a powerful strategy to manipulate the structure of low-dimensional perovskite in order to enhance the performance of perovskite solar cells. Over the past few years, organic-inorganic hybrid halide perovskite solar cells (PSCs) have attracted tremendous attention and rapidly achieved a significantly high power conversion efficiency (PCE) of 23.3%.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 (15167) Google Scholar, 2Jeon N.J. Noh J.H. Kim Y.C. Yang W.S. Ryu S. Seok S.I. Solvent engineering for high-performance inorganic-organic hybrid perovskite solar cells.Nat. Mater. 2014; 13: 897-903Crossref PubMed Scopus (5280) Google Scholar, 3Yang W.S. Park B.W. Jung E.H. Jeon N.J. Kim Y.C. Lee D.U. Shin S.S. Seo J. Kim E.K. Noh J.H. Seok S.I. Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells.Science. 2017; 356: 1376-1379Crossref PubMed Scopus (4320) Google Scholar, 4Certified Best Cell Efficiency from NREL. https://www.nrel.gov/pv/assets/images/efficiency-chart-20180716.jpg.Google Scholar However, toxicity and instability of Pb-based PSCs remain to be issues that limit their large-scale application. A bunch of Pb-free PSCs based on elements such as tin (Sn), germanium (Ge), bismuth (Bi), stibium (Sb), and copper (Cu) have been investigated.5Liao Y. Liu H. Zhou W. Yang D. Shang Y. Shi Z. Li B. Jiang X. Zhang L. Quan L.N. et al.Highly oriented low-dimensional tin halide perovskites with enhanced stability and photovoltaic performance.J. Am. Chem. Soc. 2017; 139: 6693-6699Crossref PubMed Scopus (576) Google Scholar, 6Krishnamoorthy T. Ding H. Yan C. Leong W.L. Baikie T. Zhang Z. Sherburne M. Li S. Asta M. Mathews N. Mhaisalkar S.G. Lead-free germanium iodide perovskite materials for photovoltaic applications.J. Mater. Chem. A. 2015; 3: 23829-23832Crossref Google Scholar, 7Öz S. Hebig J.-C. Jung E. Singh T. Lepcha A. Olthof S. Jan F. Gao Y. German R. van Loosdrecht P.H.M. et al.Zero-dimensional (CH3NH3)3Bi2I9 perovskite for optoelectronic applications.Sol. Energy Mater. Sol. Cells. 2016; 158: 195-201Crossref Scopus (127) Google Scholar, 8Saparov B. Hong F. Sun J.-P. Duan H.-S. Meng W. Cameron S. Hill I.G. Yan Y. Mitzi D.B. Thin-film preparation and characterization of Cs3Sb2I9: a lead-free layered perovskite semiconductor.Chem. Mater. 2015; 27: 5622-5632Crossref Scopus (517) Google Scholar, 9Cortecchia D. Dewi H.A. Yin J. Bruno A. Chen S. Baikie T. Boix P.P. Gratzel M. Mhaisalkar S. Soci C. Mathews N. Lead-free MA2CuClxBr4–x hybrid perovskites.Inorg. Chem. 2016; 55: 1044-1052Crossref PubMed Scopus (373) Google Scholar, 10Hao F. Stoumpos C.C. Cao D.H. Chang R.P.H. Kanatzidis M.G. Lead-free solid-state organic-inorganic halide perovskite solar cells.Nat. Photonics. 2014; 8: 489-494Crossref Scopus (2099) Google Scholar, 11Hao F. Stoumpos C.C. Guo P. Zhou N. Marks T.J. Chang R.P. Kanatzidis M.G. Solvent-mediated crystallization of CH3NH3SnI3 films for heterojunction depleted perovskite solar cells.J. Am. Chem. Soc. 2015; 137: 11445-11452Crossref PubMed Scopus (482) Google Scholar, 12Lee 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 (542) Google Scholar, 13Liao W. Zhao D. Yu Y. Grice C.R. Wang C. Cimaroli A.J. Schulz P. Meng W. Zhu K. Xiong R.G. Yan Y. Lead-free inverted planar formamidinium tin triiodide perovskite solar cells achieving power conversion efficiencies up to 6.22%.Adv. Mater. 2016; 28: 9333-9340Crossref PubMed Scopus (511) Google Scholar, 14Cao 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 (283) Google Scholar, 15Song T.-B. Yokoyama T. Aramaki S. Kanatzidis M.G. Performance enhancement of lead-free tin-based perovskite solar cells with reducing atmosphere-assisted dispersible additive.ACS Energy Lett. 2017; 2: 897-903Crossref Scopus (234) Google Scholar, 16Zhao Z. Gu F. Li Y. Sun W. Ye S. Rao H. Liu Z. Bian Z. Huang C. Mixed-organic-cation tin iodide for lead-free perovskite solar cells with an efficiency of 8.12%.Adv. Sci. 2017; 4: 1700204Crossref Scopus (325) Google Scholar, 17Shao S.Y. Liu J. Portale G. Fang H.H. Blake G.R. ten Brink G.H. Koster L.J.A. Loi M.A. Highly reproducible Sn-based hybrid perovskite solar cells with 9% efficiency.Adv. Energy Mater. 2018; 8: 1702019Crossref Scopus (589) Google Scholar Among these alternatives, Sn-based PSCs display promising device performance owing to their suitable band gap (1.2–1.4 eV), small exciton binding energy (18 meV), and high carrier mobility.18Stoumpos 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 (3935) Google Scholar, 19Chen Z. Yu C.L. Shum K. Wang J.J. Pfenninger W. Vockic N. Midgley J. Kenney J.T. Photoluminescence study of polycrystalline CsSnI3 thin films: determination of exciton binding energy.J. Lumin. 2012; 132: 345-349Crossref Scopus (161) Google Scholar, 20Huang L.Y. Lambrecht W.R.L. Electronic band structure, phonons, and exciton binding energies of halide perovskites CsSnCl3, CsSnBr3, and CsSnI3.Phys. Rev. B. 2013; 88: 165203Crossref Scopus (348) Google Scholar However, Sn-based perovskites suffer from self-doping due to the oxidation of Sn2+ to Sn4+ and Sn vacancies, leading to high intrinsic carrier density, instability, and poor reproducibility.21Noel N.K. Stranks S.D. Abate A. Wehrenfennig C. Guarnera S. Haghighirad A.A. Sadhanala A. Eperon G.E. Pathak S.K. Johnston M.B. et al.Lead-free organic-inorganic tin halide perovskites for photovoltaic applications.Energy Environ. Sci. 2014; 7: 3061-3068Crossref Google Scholar, 22Song T.B. Yokoyama T. Stoumpos C.C. Logsdon J. Cao D.H. Wasielewski M.R. Aramaki S. Kanatzidis M.G. Importance of reducing vapor atmosphere in the fabrication of tin-based perovskite solar cells.J. Am. Chem. Soc. 2017; 139: 836-842Crossref PubMed Scopus (371) Google Scholar, 23Yokoyama T. Cao D.H. Stoumpos C.C. Song T.B. Sato Y. Aramaki S. Kanatzidis M.G. Overcoming short-circuit in lead-free CH3NH3SnI3 perovskite solar cells via kinetically controlled gas-solid reaction film fabrication process.J. Phys. Chem. Lett. 2016; 7: 776-782Crossref PubMed Scopus (249) Google Scholar Composition engineering and structure modification are effective ways to inhibit the oxidation and enhance the PCE of Sn-based PSCs. SnF2 is a commonly used anti-oxidation compound to reduce carrier density and improve stability.12Lee 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 (542) Google Scholar, 13Liao W. Zhao D. Yu Y. Grice C.R. Wang C. Cimaroli A.J. Schulz P. Meng W. Zhu K. Xiong R.G. Yan Y. Lead-free inverted planar formamidinium tin triiodide perovskite solar cells achieving power conversion efficiencies up to 6.22%.Adv. Mater. 2016; 28: 9333-9340Crossref PubMed Scopus (511) Google Scholar, 24Kumar M.H. Dharani S. Leong W.L. Boix P.P. Prabhakar R.R. Baikie T. Shi C. Ding H. Ramesh R. Asta M. et al.Lead-free halide perovskite solar cells with high photocurrents realized through vacancy modulation.Adv. Mater. 2014; 26: 7122-7127Crossref PubMed Scopus (802) Google Scholar Some other additives, such as pyrazine12Lee 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 (542) Google Scholar and hypophosphorous acid,25Li W.Z. Li J.W. Li J.L. Fan J.D. Mai Y.H. Wang L.D. Addictive-assisted construction of all-inorganic CsSnIBr2 mesoscopic perovskite solar cells with superior thermal stability up to 473 K.J. Mater. Chem. A. 2016; 4: 17104-17110Crossref Google Scholar were applied in Sn-based PSCs to also reduce oxidation. Low-dimensional perovskite, namely perovskite sandwiched by layers of large organic molecules can effectively boost stability of PSCs as a result of “nanoscale encapsulation.”26Tsai H. Nie W. Blancon J.C. Stoumpos C.C. Asadpour R. Harutyunyan B. Neukirch A.J. Verduzco R. Crochet J.J. Tretiak S. et al.High-efficiency two-dimensional Ruddlesden-Popper perovskite solar cells.Nature. 2016; 536: 312-316Crossref PubMed Scopus (2322) Google Scholar, 27Wang Z. Lin Q. Chmiel F.P. Sakai N. Herz L.M. Snaith H.J. Efficient ambient-air-stable solar cells with 2D-3D heterostructured butylammonium-caesium-formamidinium lead halide perovskites.Nat. Energy. 2017; 2: 17135Crossref Scopus (989) Google Scholar Organic molecules such as phenylethyl amine (PEA) and butyl amine (BA) were introduced to form low-dimensional Sn perovskite, achieving a PCE of 5.9% with improved stability.5Liao Y. Liu H. Zhou W. Yang D. Shang Y. Shi Z. Li B. Jiang X. Zhang L. Quan L.N. et al.Highly oriented low-dimensional tin halide perovskites with enhanced stability and photovoltaic performance.J. Am. Chem. Soc. 2017; 139: 6693-6699Crossref PubMed Scopus (576) Google Scholar, 14Cao 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 (283) Google Scholar However, the inclusion of 2D and quasi-2D perovskite quantum wells will impact carriers transport and cause the decrease of PCE.5Liao Y. Liu H. Zhou W. Yang D. Shang Y. Shi Z. Li B. Jiang X. Zhang L. Quan L.N. et al.Highly oriented low-dimensional tin halide perovskites with enhanced stability and photovoltaic performance.J. Am. Chem. Soc. 2017; 139: 6693-6699Crossref PubMed Scopus (576) Google Scholar, 26Tsai H. Nie W. Blancon J.C. Stoumpos C.C. Asadpour R. Harutyunyan B. Neukirch A.J. Verduzco R. Crochet J.J. Tretiak S. et al.High-efficiency two-dimensional Ruddlesden-Popper perovskite solar cells.Nature. 2016; 536: 312-316Crossref PubMed Scopus (2322) Google Scholar, 27Wang Z. Lin Q. Chmiel F.P. Sakai N. Herz L.M. Snaith H.J. Efficient ambient-air-stable solar cells with 2D-3D heterostructured butylammonium-caesium-formamidinium lead halide perovskites.Nat. Energy. 2017; 2: 17135Crossref Scopus (989) Google Scholar, 28Quan 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 (950) Google Scholar Great efforts have been made to manipulate the structure of low-dimensional perovskite to increase PCE and stability simultaneously, e.g., exploration of the Ruddlesden-Popper structure26Tsai H. Nie W. Blancon J.C. Stoumpos C.C. Asadpour R. Harutyunyan B. Neukirch A.J. Verduzco R. Crochet J.J. Tretiak S. et al.High-efficiency two-dimensional Ruddlesden-Popper perovskite solar cells.Nature. 2016; 536: 312-316Crossref PubMed Scopus (2322) Google Scholar, 27Wang Z. Lin Q. Chmiel F.P. Sakai N. Herz L.M. Snaith H.J. Efficient ambient-air-stable solar cells with 2D-3D heterostructured butylammonium-caesium-formamidinium lead halide perovskites.Nat. Energy. 2017; 2: 17135Crossref Scopus (989) Google Scholar, 29Stoumpos C.C. Cao D.H. Clark D.J. Young J. Rondinelli J.M. Jang J.I. Hupp J.T. Kanatzidis M.G. Ruddlesden-Popper hybrid lead iodide perovskite 2D homologous semiconductors.Chem. Mater. 2016; 28: 2852-2867Crossref Scopus (1272) Google Scholar, 30Stoumpos C.C. Soe C.M.M. Tsai H. Nie W. Blancon J.-C. Cao D.H. Liu F. Traoré B. Katan C. Even J. et al.High members of the 2D Ruddlesden-Popper halide perovskites: synthesis, optical properties, and solar cells of (CH3(CH2)3NH3)2(CH3NH3)4Pb5I16.Chem. 2017; 2: 427-440Abstract Full Text Full Text PDF Scopus (297) Google Scholar, 31Zhou N. Shen Y. Li L. Tan S. Liu N. Zheng G. Chen Q. Zhou H. Exploration of crystallization kinetics in quasi two-dimensional perovskite and high performance solar cells.J. Am. Chem. Soc. 2018; 140: 459-465Crossref PubMed Scopus (261) Google Scholar and reduction of the ratio of 2D and quasi-2D structures.17Shao S.Y. Liu J. Portale G. Fang H.H. Blake G.R. ten Brink G.H. Koster L.J.A. Loi M.A. Highly reproducible Sn-based hybrid perovskite solar cells with 9% efficiency.Adv. Energy Mater. 2018; 8: 1702019Crossref Scopus (589) Google Scholar However, for low-dimensional perovskite, especially Sn-based materials, a key issue to obtain long-term stability and high efficiency simultaneously demands prompt solution. Here we introduced removable pseudohalogen ammonium thiocyanate (NH4SCN, hereafter described as SCN) to manipulate the crystal growth process of perovskite film. We demonstrate that the addition of SCN separates the nucleation and crystallization growth processes, leading to the formation of 2D-quasi-2D-3D hierarchy structure perovskite (HSP). The final HSP shows great resistance to oxidation, giving rise to reduced carrier density and enhanced carrier mobility. As a result, the PSCs based on this HSP achieved a record PCE up to 9.41% for lead-free perovskite, and retains 90% of its initial performance for as long as 600 hr without encapsulation. Typically, the perovskite film was prepared based on a spin coating process using a precursor of PEA0.15FA0.85SnI3 and SnF2 in a mixture of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) with different amounts of SCN. An annealing process was subsequently performed to remove the solvent residue and turn it into perovskite phase. Scanning electron microscope (SEM) images indicate that both the control film without SCN and the film with 5% SCN show smooth morphology with small pinholes (Figures 1A and 1B). When the amount of SCN increases, the diameter of the pinholes are remarkably enlarged (Figures S1A and S1D). To investigate the structure of the perovskite films, X-ray diffraction (XRD) was conducted for films with and without the addition of SCN (Figures 1C and 1D). Three dominant diffraction peaks at angles of 14.0°, 28.3°, and 42.9° are ascribed to the (100), (200), and (300) lattice planes for 3D orthorhombic (Amm2) FASnI3.10Hao F. Stoumpos C.C. Cao D.H. Chang R.P.H. Kanatzidis M.G. Lead-free solid-state organic-inorganic halide perovskite solar cells.Nat. Photonics. 2014; 8: 489-494Crossref Scopus (2099) Google Scholar, 17Shao S.Y. Liu J. Portale G. Fang H.H. Blake G.R. ten Brink G.H. Koster L.J.A. Loi M.A. Highly reproducible Sn-based hybrid perovskite solar cells with 9% efficiency.Adv. Energy Mater. 2018; 8: 1702019Crossref Scopus (589) Google Scholar HSP with the inclusion of 5% SCN shows two additional diffraction peaks at angles of 5.5° and 27.4°, assigned to the crystallographic planes (200) and (1000) of 2D PEA2SnI4.5Liao Y. Liu H. Zhou W. Yang D. Shang Y. Shi Z. Li B. Jiang X. Zhang L. Quan L.N. et al.Highly oriented low-dimensional tin halide perovskites with enhanced stability and photovoltaic performance.J. Am. Chem. Soc. 2017; 139: 6693-6699Crossref PubMed Scopus (576) Google Scholar, 17Shao S.Y. Liu J. Portale G. Fang H.H. Blake G.R. ten Brink G.H. Koster L.J.A. Loi M.A. Highly reproducible Sn-based hybrid perovskite solar cells with 9% efficiency.Adv. Energy Mater. 2018; 8: 1702019Crossref Scopus (589) Google Scholar, 32Papavassiliou G.C. Koutselas I.B. Terzis A. Whangbo M.-H. Structural and electronic properties of the natural quantum-well system (C6H5CH2CH2NH3)2Snl4.Solid State Commun. 1994; 91: 695-698Crossref Scopus (120) Google Scholar We carried out energy-dispersive X-ray spectroscopy (EDS) analysis and X-ray photoelectron spectroscopy (XPS) to analyze the composition of the perovskite film (Table S1; Figure S3). EDS results indicate that the atomic ratio of sulfur in the sample is zero, implying the complete removal of the SCN additive. Such a conclusion is further supported by the absence of an S element signal in the XPS measurements. This indicates that the diffraction peaks related to 2D perovskite derive from PEA2SnI4 rather than structures containing SCN−. We exploited grazing-incidence wide-angle X-ray scattering (GIWAXS) to characterize the structure of HSP and the control (Figure 2). Scattering spectra at different incident angles of 0.2°, 0.5°, 1.0°, 1.5°, and 2° were measured to track the perovskite structure evolution from the shallow surface to the bottom side.17Shao S.Y. Liu J. Portale G. Fang H.H. Blake G.R. ten Brink G.H. Koster L.J.A. Loi M.A. Highly reproducible Sn-based hybrid perovskite solar cells with 9% efficiency.Adv. Energy Mater. 2018; 8: 1702019Crossref Scopus (589) Google Scholar Bragg spots and Debye-Scherrer rings belonging to low-dimensional and 3D perovskite polycrystalline films are indexed.17Shao S.Y. Liu J. Portale G. Fang H.H. Blake G.R. ten Brink G.H. Koster L.J.A. Loi M.A. Highly reproducible Sn-based hybrid perovskite solar cells with 9% efficiency.Adv. Energy Mater. 2018; 8: 1702019Crossref Scopus (589) Google Scholar, 32Papavassiliou G.C. Koutselas I.B. Terzis A. Whangbo M.-H. Structural and electronic properties of the natural quantum-well system (C6H5CH2CH2NH3)2Snl4.Solid State Commun. 1994; 91: 695-698Crossref Scopus (120) Google Scholar, 33Koh T.M. Krishnamoorthy T. Yantara N. Shi C. Leong W.L. Boix P.P. Grimsdale A.C. Mhaisalkar S.G. Mathews N. Formamidinium tin-based perovskite with low Eg for photovoltaic applications.J. Mater. Chem. A. 2015; 3: 14996-15000Crossref Google Scholar The Bragg spots indicate strongly preferential orientation for low-dimensional polycrystalline films in the shallow surface where the c-planes of perovskite extend parallel to the substrate (Figures 2A and 2D). As the incident angle increases to 2°, three Debye-Scherrer rings emerge (Figures 2B and 2E), indicating the existence of 3D perovskite grains with random orientation in the depths of the film. The (001) and (004) Bragg spots (Figure 2A) located on the qz axis can be ascribed to quasi-2D perovskite PEA2FASn2I7 containing double-layer SnI6 octahedra.17Shao S.Y. Liu J. Portale G. Fang H.H. Blake G.R. ten Brink G.H. Koster L.J.A. Loi M.A. Highly reproducible Sn-based hybrid perovskite solar cells with 9% efficiency.Adv. Energy Mater. 2018; 8: 1702019Crossref Scopus (589) Google Scholar The (002)2D spot above (001) in Figure 2D can be ascribed to a single-layer 2D perovskite of PEA2SnI4.17Shao S.Y. Liu J. Portale G. Fang H.H. Blake G.R. ten Brink G.H. Koster L.J.A. Loi M.A. Highly reproducible Sn-based hybrid perovskite solar cells with 9% efficiency.Adv. Energy Mater. 2018; 8: 1702019Crossref Scopus (589) Google Scholar, 32Papavassiliou G.C. Koutselas I.B. Terzis A. Whangbo M.-H. Structural and electronic properties of the natural quantum-well system (C6H5CH2CH2NH3)2Snl4.Solid State Commun. 1994; 91: 695-698Crossref Scopus (120) Google Scholar As the incident angle increases to 0.5°, 1.0°, and 1.5° (Figure S4), the Debye-Scherrer rings are enlarged, indicating a low-dimensional perovskite with fewer layers of SnI6 octahedra, which tends to be distributed to the surface of the films, and 3D perovskite is concentrated at the bottom (Figures 2C, 2F, and S5). We infer that the film forms a 2D-quasi-2D-3D structure with parallel orientation and layered distribution, and hence name it “hierarchy structure perovskite” (HSP) to show the structure evolution trend with the increase of film depth. The GIWAXS spectra of the control film without the addition of SCN are quite different. The lack of a (002)2D spot (Figure 2A) indicates the absence of single-layer perovskite (PEA2SnI4). Quasi-2D perovskite (PEA2FASn2I7) dominates the surface, demonstrated by the presence of (001) and (004) characteristic Bragg spots. The addition of SCN is therefore critical to the formation of hierarchy structure. The existence of 2D perovskite in a HSP film is further confirmed by luminescence spectra and transient absorption spectra (TAS). An additional photoluminescence (PL) peak at 622 nm was observed for the HSP film, which originates from a 2D PEA2SnI4 film (Figures 3A, 3C, and S2).5Liao Y. Liu H. Zhou W. Yang D. Shang Y. Shi Z. Li B. Jiang X. Zhang L. Quan L.N. et al.Highly oriented low-dimensional tin halide perovskites with enhanced stability and photovoltaic performance.J. Am. Chem. Soc. 2017; 139: 6693-6699Crossref PubMed Scopus (576) Google Scholar In TAS (Figures 3B and 3D), several bleaching peaks were observed related to the transient luminescence. The negative band from 700 to 780 nm can be ascribed to transient bleaching for 3D perovskite.34Liu J.X. Leng J. Wu K.F. Zhang J. Jin S.Y. Observation of internal photoinduced electron and hole separation in hybrid two-dimensional perovskite films.J. Am. Chem. Soc. 2017; 139: 1432-1435Crossref PubMed Scopus (388) Google Scholar For the HSP film, two negative bands at 580–625 and 625–700 nm are presented, corresponding to PEA2SnI4 and PEA2FASn2I7 perovskite, respectively.34Liu J.X. Leng J. Wu K.F. Zhang J. Jin S.Y. Observation of internal photoinduced electron and hole separation in hybrid two-dimensional perovskite films.J. Am. Chem. Soc. 2017; 139: 1432-1435Crossref PubMed Scopus (388) Google Scholar, 35Shang Y.Q. Li G. Liu W.M. Ning Z.J. Quasi-2D inorganic CsPbBr3 perovskite for efficient and stable light-emitting diodes.Adv. Funct. Mater. 2018; 28: 1801193Crossref Scopus (90) Google Scholar By contrast, the control perovskite film shows only a band of 600–700 nm, corresponding to the bleaching of PEA2FASn2I7 perovskite. This further indicates the absence of 2D perovskite for the control sample. Note that all the bleaching bands show a red shift with time-delay increased, which may result from the exciton-exciton interaction.36Dar M.I. Franckevičius M. Arora N. Redeckas K. Vengris M. Gulbinas V. Zakeeruddin S.M. Grätzel M. High photovoltage in perovskite solar cells: new physical insights from the ultrafast transient absorption spectroscopy.Chem. Phys. Lett. 2017; 683: 211-215Crossref Scopus (24) Google Scholar To study the formation mechanism for the HSP structure, we explored in situ PL spectra measurement to track the kinetic process of perovskite growth. With the addition of 5% SCN, a PL peak around 800 nm appears during the anti-solvent dropping process (Figure 4), and the PL intensity grows immediately once annealed. On the contrary, the control film shows no PL peak during the anti-solvent dropping process and at the beginning of annealing (Figures 4A and 4C). We hypothesize that the SCN can manipulate perovskite growth by the following two reactions:2FAI+2SnI2+xNH4SCN→FASnI3+FASnI3−xSCNx+xNH4I(Reaction 1) 2PEAI+FASnI3−xSCNx+xNH4I→PEA2SnI4+xHSCN↑+ xNH3↑+FAI(Reaction 2) The π-conjugated Lewis base SCN− interacts strongly with Lewis acid SnI2 and forms FASnI3–xSCNx,37Jiang Q. Rebollar D. Gong J. Piacentino E.L. Zheng C. Xu T. Pseudohalide-induced moisture tolerance in perovskite CH3NH3Pb(SCN)2I thin films.Angew. Chem. Int. Ed. 2015; 54: 7617-7620Crossref PubMed Scopus (276) Google Scholar which accelerates the crystal nucleation as well as the formation of FASnI3 right after dropping anti-solvent toluene (Reaction 1). During the annealing process of the HSP film, with the removal of HSCN (Reaction 2), PEA2SnI4 starts to form, and the linear shape of SCN− favors the formation of 2D nanocrystal plates.37Jiang Q. Rebollar D. Gong J. Piacentino E.L. Zheng C. Xu T. Pseudohalide-induced moisture tolerance in perovskite CH3NH3Pb(SCN)2I thin films.Angew. Chem. Int. Ed. 2015; 54: 7617-7620Crossref PubMed Scopus (276) Google Scholar, 38Ahn N. Son D.Y. Jang I.H. Kang S.M. Choi M. Park N.G. Highly reproducible perovskite solar cells with average efficiency of 18.3% and best efficiency of 19.7% fabricated via Lewis base adduct of lead(II) iodide.J. Am. Chem. Soc. 2015; 137: 8696-8699Crossref PubMed Scopus (1843) Google Scholar Since crystallization generally starts at the bottom where solvent escapes first, it can be concluded that more 3D structure will be distributed at the bottom, while the 2D structure formed later will be concentrated at the surface. This explains how the hierarchy structure shown above is formed. The crystal growth process (Figures 4B and 4D) is remarkably different for the control film, for which the nucleation and crystal growth occur synchronously in the annealing process. This facilitates the growth of quasi-2D and 3D structures at the same time. Moreover, the absence of SCN− anion results in the disappearance of a nucleation center, which induces the formation of 2D structure. As a result, no 2D structure with single-layer SnI6 octahedra is observed. Note that, compared with the normalized PL spectra with a dominant peak at 890 nm, the in situ PL spectra show a blue shift to 850 nm for perovskite on a hot plate at 80°C. This could be attributed to electron-photon coupling with temperature increasing.39Wu K. Bera A. Ma C. Du Y. Yang Y. Li L. Wu T. Temperature-dependent excitonic photoluminescence of hybrid organometal halide perovskite films.Phys. Chem. Chem. Phys. 2014; 16: 22476-22481Crossref PubMed Google Scholar, 40Wright A.D. Verdi C. Milot R.L. Eperon G.E. Perez-Osorio M.A. Snaith H.J. Giustino F. Johnston M.B. Herz L.M. Electron-phonon coupling in hybrid lead halide perovskites.Nat. Commun. 2016; 7: 11755Crossref Scopus (750) Google Scholar, 41Yang B. Ming W. Du M.H. Keum J.K. Puretzky A.A. Rouleau C.M. Huang J. Geohegan D.B. Wang X. Xiao K. Real-time observation of order-disorder transformation of organic cations induced phase transition and anomalous photoluminescence in hybrid perovskites.Adv. Mater. 2018; 30: 1705801Crossref Scopus (46) Google Scholar 2D tin perovskite exhibits higher formation energy than the 3D structure, which enables it to be a protecting layer to retard oxidation.5Liao Y. Liu H. Zhou W. Yang D. Shang Y. Shi Z. Li B. Jiang X. Zhang L. Quan L.N. et al.Highly oriented low-dimensional tin halide perovskites with enhanced stability and photovoltaic performance.J. Am. Chem. Soc. 2017; 139: 6693-6699Crossref PubMed Scopus (576) Google Scholar, 28Quan 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 (950) Google Scholar We tested the oxidation degree of an HSP film and the control film in air for 30 s and 3 min using XPS. For the control film, the binding energy shift from 485.6 to 486.8 eV shows that Sn2+ was quickly oxidized to Sn4+ after exposure to air for 3 min (Figure S6).5Liao Y. Liu H. Zhou W. Yang D. Shang Y. Shi Z. Li B. Jiang X. Zhang L. Quan L.N. et al.Highly oriented low-dimensional tin halide perovskites with enhanced stability and photovoltaic performance.J. Am. Chem. Soc. 2017; 139: 6693-6699Crossref PubMed Scopus (576) Google Scholar, 25Li W.Z. Li J.W. Li J.L. Fan J.D. Mai Y.H. Wang L.D. Addictive-assisted construction of all-inorganic CsSnIBr2 mesoscopic perovskite solar cells with superior thermal stability up to 473 K.J. Mater. Chem. A. 2016; 4: 17104-17110Crossref Google Scholar, 42Zhang J. Xiong Z. Zhao X.S. Graphene-metal-oxide composites for the degradation of dyes under visible light irradiation.J. Mater. Chem. 2011; 21: 3634-3640Crossref Scopus (560) Google Scholar, 43Jiang Q. Zhang L. Wang H. Yang X. Meng J. Liu H. Yin Z. Wu J. Zhang X. You J. Enhanced electron extraction using SnO2 for high-efficiency planar-structure HC(NH2)2PbI3-based perovskite solar cells.Nat. Energy. 2016; 2: 16177Crossref Scopus (1296) Google Scholar By contrast, the HSP film shows no obvious shift of the peak position, indicating that the oxidation process is inhibited. Similarly, the O1s peak intensity for the HSP film is much weaker than that of the control film, providing further evidence of improved oxidation resistance (Figure S7). Carrier mobility is crucial for high-performance solar cells. We utilized the space charge limited current (SCLC) method to investigate the carrier transport characteristics.43Jiang Q. Zhang L. Wang H. Yang X. Meng J. Liu H. Yin Z. Wu J. Zhang X. You J. Enhanced electron extraction using SnO2 for high-efficiency planar-structure HC(NH2)2PbI3-based perovskite solar cells.Nat. Energy. 2016; 2: 16177Crossref Scopus (1296) Google Scholar, 44Yin X. Guan L. Yu J. Zhao D. Wang C. Shrestha N. Han Y. An Q. Zhou J. Zhou B. et al.One-step facile synthesis of a simple carbazole-cored hole transport material for high-performance perovskite solar cells.Nano Energy. 2017; 40: 163-169Crossref Scopus (80) Google Scholar, 45Ji C.M. Wang P. Wu Z.Y. Sun Z.H. Li L.N. Zhang J. Hu W.D. Hong M.C. Luo J.H. Inch-size single crystal of a lead-free organic-inorganic hybrid perovskite for high-performance photodetector.Adv. Funct. Mater. 2018; 28: 1705467Crossref Scopus (112) Google Scholar, 46Tsai H. Nie W. Blancon J.C. Stoumpos C.C. Soe C.M.M. Yoo J. Crochet J. Tretiak S. Even J. Sadhanala A. et al.Stable light-emitting diodes using phase-pure Ruddlesden-Popper layered perovskites.Adv. Mater. 2018; 30: 1704217Crossref Scopus (217) Google Scholar The measured mobility of electron and hole for the HSP film are 0.054 and 0.005 cm2/Vs, respectively; higher than the carrier mobility 0.013 and 0.003 cm2/Vs of the control (Figure S8). The improved carrier mobility derives from the structure modification. On the one hand, the presence of single-layer 2D perovskite effectively prohibited the contact of oxygen to the inner structure and, in return, suppressed the increase of the defect concentration. This favors the improvement of the current and voltage of the device. On the other hand, since Sn2+ in PEA2SnI4 would “consume” more PEA than in PEA2FASn2I7, the increase in the amount of PEA2SnI4 could reduce the amount of quasi-2D perovskite and increase the ratio of 3D FASnI3. The increase of the ratio of FASnI3 at the bottom enhances the carrier transportation capability of the film simultaneously. We performed capacitance-voltage (C-V) characterization to determine the carrier density (Figure S8) using the Mott-Schottky equation.5Liao Y. Liu H. Zhou W. Yang D. Shang Y. Shi Z. Li B. Jiang X. Zhang L. Quan L.N. et al.Highly oriented low-dimensional tin halide perovskites with enhanced stability and photovoltaic performance.J. Am. Chem. Soc. 2017; 139: 6693-6699Crossref PubMed Scopus (576) Google Scholar, 47Heo S. Seo G. Lee Y. Lee D. Seol M. Lee J. Park J.B. Kim K. Yun D.J. Kim Y.S. et al.Deep level trapped defect analysis in CH3NH3PbI3 perovskite solar cells by deep level transient spectroscopy.Energy Environ. Sci. 2017; 10: 1128-1133Crossref Google Scholar The carrier density (6.28 × 1016 cm−3) of HSP films is around half that of control films (1.04 × 1017 cm−3). The lower carrier density illustrates the suppressed Sn2+ oxidation and reduced defect concentration in HSP films.17Shao S.Y. Liu J. Portale G. Fang H.H. Blake G.R. ten Brink G.H. Koster L.J.A. Loi M.A. Highly reproducible Sn-based hybrid perovskite solar cells with 9% efficiency.Adv. Energy Mater. 2018; 8: 1702019Crossref Scopus (589) Google Scholar Carrier lifetime was estimated by time-resolved PL spectroscopy for perovskite films (Figure S9). The carrier lifetimes of HSP and the control film on NiOx substrates are 1.77 and 1.75 ns, respectively, and the lifetime decreases to 1.03 ns for both of them when sandwiched by NiOx and PCBM. The reduced carrier lifetime indicates efficient carrier transport and carrier injection into carrier-transporting layers.5Liao Y. Liu H. Zhou W. Yang D. Shang Y. Shi Z. Li B. Jiang X. Zhang L. Quan L.N. et al.Highly oriented low-dimensional tin halide perovskites with enhanced stability and photovoltaic performance.J. Am. Chem. Soc. 2017; 139: 6693-6699Crossref PubMed Scopus (576) Google Scholar We then estimated the band structure of perovskite films with different ratios of SCN additive using UV photoelectron spectroscopy (UPS) (Figure S11).48Shan Q. Li J. Song J. Zou Y. Xu L. Xue J. Dong Y. Huo C. Chen J. Han B. Zeng H. All-inorganic quantum-dot light-emitting diodes based on perovskite emitters with low turn-on voltage and high humidity stability.J. Mater. Chem. C. 2017; 5: 4565-4570Crossref Google Scholar The valence band maximum (VBM) is calculated to be around 5.2 eV. In combination with the bandgap calculated from the onset position of absorption spectra (Figure S10), the conduction band minimum (CBM) is determined to be around 3.8 eV. The Fermi energy level, and the positions of VBM and CBM, are quite close for perovskite films with different amounts of SCN (Table S2). We fabricated solar cells based on an inverted structure, with NiOx as a hole-transporting layer and PCBM as an electron-transporting layer (Figure S12). The CBM of perovskite is higher than PCBM, and the VBM matches well with NiOx, which enables effective carrier transport into transporting layers. The PSCs based on HSP induced by 5% SCN show the highest solar conversion efficiency of 9.41% under AM1.5G illumination (Figure 5). The current density reaches 22.0 mA/cm2, remarkably higher than the device with a 20% PEA molecule (Figure 5A; Table 1).5Liao Y. Liu H. Zhou W. Yang D. Shang Y. Shi Z. Li B. Jiang X. Zhang L. Quan L.N. et al.Highly oriented low-dimensional tin halide perovskites with enhanced stability and photovoltaic performance.J. Am. Chem. Soc. 2017; 139: 6693-6699Crossref PubMed Scopus (576) Google Scholar The integrated JSC value (20.8 mA/cm2) from the external quantum efficiency (EQE) curve (Figure 5B) agrees well with JSC. In comparison with the device based on the control film, the device based on HSP shows improved FF and VOC (Table 1; Figure S14). This can be attributed to the increase of carrier mobility and the reduction of defects arising from the reduction of Sn oxidation. When more than 5% SCN additive is incorporated, JSC decreases from 22.0 to 17.3 mA/cm2 (Table 1; Figure S13). This can be ascribed to the reduction of carrier mobility with the increase of the 2D structure. To evaluate the real performance of the HSP device, steady-state PCE measurement was carried out, and the HSP device showed a quite stable power output (Figure S15). Moreover, a large-area device of ∼0.09 cm2 achieved a high PCE of 8.82% (Figure S16).Table 1Device Parameters of Different Perovskite FilmsDeviceVOC (V)JSC (mA/cm2)FF (%)PCE (%)SCN 0%champion0.5822.165.28.34average0.55 ± 0.0219.5 ± 1.668.4 ± 4.07.27 ± 0.17SCN 5%champion0.6122.070.19.41average0.60 ± 0.0121.6 ± 1.267.7 ± 3.88.75 ± 0.47SCN 10%champion0.6018.767.37.53average0.57 ± 0.0318.2 ± 1.664.1 ± 6.06.28 ± 0.59SCN 15%champion0.6117.367.77.15average0.57 ± 0.0115.0 ± 0.967.3 ± 4.75.72 ± 0.36 Open table in a new tab Devices based on HSP show improved reproducibility and stability compared with the control films. We prepared and measured several batches of devices: 45 in total. The PCE histogram of HSP devices (Figure 5C) shows excellent reproducibility. We tracked the stability of device performance in the N2-filled glovebox without encapsulation. The HSP device is rather stable in duration as long as ∼600 hr (Figure 5D). The efficiency of the control device dropped to 90% of its initial efficiency after 300 hr (Figure S17). Compared with the control device, the HSP device also shows improved air stability, thermal stability, and illumination stability (Figures S18–S20). The improved device stability is due to the suppressed oxidation induced by the 2D perovskite on the top of HSP films. In this work, we fabricated hierarchy structure Sn-based perovskite thin films using removable pseudohalogen NH4SCN as a “catalyst” in the perovskite growth process. The addition of a small amount of NH4SCN separates the nucleation and growth processes of perovskite polycrystalline films and yields a 2D-quasi-2D-3D hierarchy structure. The 2D perovskite with parallel orientation on the film surface significantly enhances stability and oxidation resistance. Together with the increase of carrier mobility and reduced background carrier density, the photocurrent and fill factor of the device are much improved. As a result, a record efficiency of 9.41% is achieved for lead-free PSCs. The device based on HSP shows much enhanced stability, and sustained 90% of its initial PCE for near 600 hr. The work provides an effective way to develop Sn-based PSCs with both high performance and long-term stability, and it sheds some light on the structure manipulation of perovskite film by removable regulator." @default.
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• W2895549098 title "2D-Quasi-2D-3D Hierarchy Structure for Tin Perovskite Solar Cells with Enhanced Efficiency and Stability" @default.
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