Methylammonium lead triiodide perovskite solar cells: A new paradigm in photovoltaics
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Introduction Organic–inorganic hybrid perovskite materials pioneered by Mitzi in 2001 have been recognized for their use in optoelectronic applications.1 Group 14 iodometalates, such as ABX3, adopt perovskite structures that are flexible for compositional variations. A hybrid perovskite results when “A” is a small organic cation, “B” is a Group 14 metal lead (Pb), and “X” is a halogen that forms a three-dimensional inorganic framework with the organic cation. The ease with which these hybrid perovskite materials can be prepared and processed from solution have made them attractive for photovoltaic (PV) applications following the pioneering work of Miyasaka and co-workers in 2009, where they replaced dye in dye-sensitized solar cells and obtained a power-conversion efficiency of 3.8% using iodine/triiodide-based liquid electrolyte.2 After an initial slow following, perovskite solar cells have created a tsunami effect in the PV community because of their highpower-conversion efficiencies reaching over 20%, enabled by their unexpected optoelectronic properties.3
Initially, perovskite solar-cell device architecture was based on dye-sensitized solar cells, as shown in Figure 1.4 The device is composed of three major components: an absorber to absorb light, a semiconductor to accept electrons from photoexcited perovskite, and a hole-transporting p-type semiconductor sandwiched between two conducting electrodes. The working principle of a perovskite solar cell involves promotion of electrons from the ground state of the perovskite to the excited state upon irradiation with photons and injection of the excited electrons to the conductance band of the semiconductor. The positive charge on the perovskite is transferred to the hole-transporting material and then to the gold contact electrode. The perovskite plays a critical role in this process for determining the amount of light absorbed both in intensity and breadth (wavelength range). The key breakthrough, which sparked broad excitement in the PV community, was the realization of 10% efficient solid-state perovskite solar cells in 2012.4,5 Specifically, the unexpectedly high open-circuit voltages generated by these
Mohammad Khaja Nazeeruddin, Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, Switzerland; mdkhaja.nazeeruddin@epfl.ch Henry Snaith, Department of Physics and Clarendon Laboratory, University of Oxford, UK; [email protected] DOI: 10.1557/mrs.2015.169
© 2015 Materials Research Society
MRS BULLETIN • VOLUME 40 • AUGUST 2015 • www.mrs.org/bulletin
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METHYLAMMONIUM LEAD TRIIODIDE PEROVSKITE SOLAR CELLS: A NEW PARADIGM IN PHOTOVOLTAICS
layer, a 100–200-nm-thick mesoporous TiO2 layer, which acts as an electron transport material and scaffold infilled with perovskite, a solid perovskite overlayer of 200–300 nm thickness, an additional light-absorbing layer, a hole-transporting layer, and a metal back electrode.10 Often, mesoporous Al2O3 is used in place of TiO2,
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