include an optimal band gap energy of ≈1.55 eV, high absorption coefficient (10 4 -10 5 cm −1 ) due to the direct band gap transition, reduced exciton binding energy (10-30 meV) enabling free carrier generation, high charge-carrier mobilities due to low charge effective masses (m h * or m e * ≈ 0.1m 0 ) derived from the large band dispersion, good defect tolerance that avoids the formation of deep trap states and long charge-carrier diffusion lengths. [11][12][13][14][15][16][17][18][19] Additionally, 3D MAPbI 3 is a solutionprocessable material constituted by earthabundant raw materials, which minimizes fabrication costs. [20][21][22][23][24][25] However, there are two important limitations for the further industrial application of these solar devices: the high toxicity of Pb, which poses a risk to humans and causes environmental contamination, and the low stabilities of perovskite materials, which is derived from degradation to its initial precursors due to thermal, moisture and operational effects. [26][27][28][29][30][31][32][33][34][35] Conventional perovskites utilized in photovoltaics exhibit a stoichiometric formula of ABX 3 , where A is a monovalent cation, B is a divalent cation, and X is a halide anion. The perovskite structure is comprised of corner-sharing BX 6 octahedra with in between voids occupied by A cations to achieve charge neutrality (see Figure 1). The perovskite crystal lattice is formed by a 3D network of inorganic octahedra that are primarily responsible for the outstanding optoelectronic properties of this material, including the low binding energy and isotropic charge transport. [12][13][14]36] The size of the A cation suitable for the octahedral voids is experimentally defined by the Goldschmidt tolerance factor t =and R x are the ionic radii for each component. Only A cations with tolerance factors between 0.8 < t < 1.0 give rise to valuable 3D perovskite materials. [37][38][39][40] The main architecture of solar devices fabricated with perovskites is a mesoporous scaffold configuration; however, a planar arrangement offers rivaling PCE values. [36,41] The mesoporous layer is predominately formed by TiO 2 acting as an electrontransport layer (ETL), while the typical solid-state hole-transport layer (HTL) is the organic compound, 2,2′,7,7′-tetrakis(N,N′-di-pmethoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD). [2,3] In the planar configuration, the ETL layer is usually formed by compact, thin layers of TiO 2 or SnO 2 deposited through different methods. The deposition of the perovskite layer is usually performed by a spin-coating technique from a solution containing the precursors (PbI 2 + MAI for the MAPbI 3 perovskite) and further annealing at ≈120 °C. [5,42] The use of the so-called "antisolvent" method consisting of the addition of a nonpolar solvent during the spin-coating process has been found to provide optimum solar efficiencies. [43,44] The typical layer configuration is The discovery of unique optoelectronic properties of 3D ABX 3 perovskites has produced a great impact ...
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