Two-step deposition method for high-efficiency perovskite solar cells
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Introduction A perovskite with a chemical formula of ABX3 is a crystal structure having corner-sharing octahedral AX6 and facesharing cubo-octahedral BX12. The organometal halide RMX3 (R = methylammonium or foramidnium, M = Pb or Sn, X = Cl, Br, I, or mixed halide) is a key material for perovskite solar cells (see the Introductory article in this issue of MRS Bulletin). Three-dimensional (3D) lead and tin methylammonium halides were first reported in 1978,1,2 where their color changed from colorless to black as the halide would change from chloride to iodide due to a decrease in the bandgap. Layered 2D organotin halides were reported to show an insulator-metal transition when modulating the number of inorganic layers in the (C4H9NH3)2(CH3NH3)n–1SnnI3n+1 perovskite,3 which received attention because high-temperature superconducting properties were possible from organic–inorganic perovskite materials. Contrary to the 2D materials, little attention has been paid to the photovoltaic ability of 3D organic–inorganic halide perovskite materials due to their poor optoelectronic properties. In 2009, the Miyasaka group first used MAPbI3 (MA = CH3NH3) as an inorganic sensitizer in dye-sensitized liquid junction-type solar cells,4 yielding a power-conversion efficiency (PCE) of ∼4%. However, this paper was not cited for two years because it reported a much lower PCE as compared with ruthenium-based dyes, which was the most commonly
used molecular-type sensitizer in dye-sensitized liquid-type solar cells. In 2011, Park’s group reported a MAPbI3 perovskitesensitized solar cell with an improved efficiency of 6.5%.5 Although their report described more advanced technology for higher efficiency through the utilization of a spin-coating method using an optimized precursor solution, this result was not cited until the development of a long-term stable perovskite solar cell in 2012 by Park’s group,6 overcoming the instability of MAPbI3 in a polar liquid electrolyte. Research on perovskite solar cells has been triggered by the discovery of the long-term durable solid-state perovskite solar cell.6 The number of research papers related to perovskite solar cells has changed from 1 in 2009, 1 in 2011, 4 in 2012, and 56 in 2013, to 460 in 2014. As a result of subsequent intensive research, the National Renewable Energy Laboratory certified a PCE of 20.1%, fabricated by the Korea Research Institute of Chemical Technology, at the end of 2014.7 Such a high photovoltaic performance is mainly attributed to high-quality MAPbI3 material, whose quality depends significantly on the fabrication method used. MAPbI3 can be prepared by either a one-step spin-coating procedure or a sequential two-step deposition technique. The morphology and optoelectronic properties were found to be better controlled by a two-step method. In this article, two-step deposition technologies for high-efficiency perovskite solar cells are described in detail.
Jin-Wook Lee, Department of Energy Science, Sungkyunkwan University, South Korea; [email protected] Nam-Gyu Park, School of Che
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