Progress on lead-free metal halide perovskites for photovoltaic applications: a review

Abstract

Abstract: Metal halide perovskites have revolutionized the field of solution-processable photovoltaics. Within just a few years, the power conversion efficiencies of perovskite-based solar cells have been improved significantly to over 20%, which makes them now already comparably efficient to silicon-based photovoltaics. This breakthrough in solution-based photovoltaics, however, has the drawback that these high efficiencies can only be obtained with lead-based perovskites and this will arguably be a substantial hurdle for various applications of perovskite-based photovoltaics and their acceptance in society, even though the amounts of lead in the solar cells are low. This fact opened up a new research field on lead-free metal halide perovskites, which is currently remarkably vivid. We took this as incentive to review this emerging research field and discuss possible alternative elements to replace lead in metal halide perovskites and the properties of the corresponding perovskite materials based on recent theoretical and experimental studies. Up to now, tin-based perovskites turned out to be most promising in terms of power conversion efficiency; however, also the toxicity of these tin-based perovskites is argued. In the focus of the research community are other elements as well including germanium, copper, antimony, or bismuth, and the corresponding perovskite compounds are already showing promising properties.

Keywords: Hybrid organic–inorganic materials; Material science; Semiconductor; Solar cell; Transition metals compounds.

Fig. 1

Crystal structure of ABX₃-type metal halide perovskites

Fig. 2

Schematic representation of the stacking of inorganic octahedral layers (n) in the 〈100〉-oriented two-dimensional perovskite structure. A three-dimensional perovskite is formed, when n is ∞. Reprinted with permission from [56]. Copyright (2001) Royal Society of Chemistry

Fig. 3

Lead replacement candidates in perovskite-type compounds from the periodic table of elements with the focus on homovalent substitution with group-14 elements (Ge, Sn), alkaline-earth metals (Mg, Ca, Sr, Ba), transition metals (Cu, Fe, Pd), lanthanides and actinides (Eu, Tm, Yb), heterovalent substitution with Tl, Au, Sb, Bi, and Te, and metal chalcogenide perovskites (Ti, Zf, Hf)

Fig. 4

a Cross-sectional SEM image of a CH₃NH₃SnI₃-based perovskite solar cell in meso-structured configuration. b J–V curves of tin- (CH₃NH₃SnI₃) and lead-based (CH₃NH₃PbI_3−xCl_x) perovskite solar cells under illuminated and dark conditions. Adapted with permission from [27]. Copyright (2014) Royal Society of Chemistry

Fig. 5

a Absorption properties, b energy level diagram and c J–V curves of CH₃NH₃Sn(I,Br)₃-based perovskite solar cells. Adapted with permission from [75]. Copyright (2014) Macmillan Publishers Limited

Fig. 6

a UV–Vis absorption data of CsSnI₃, CsGeI₃, CH₃NH₃GeI₃, and CH(NH₂)₂GeI₃ and b J–V curves of CsGeI₃- and CH₃NH₃GeI₃-based solar cells. Adapted with permission from [33]. Copyright (2015) Royal Society of Chemistry

Fig. 7

Schematic representation of 〈100〉-oriented perovskites with organic monoammonium ((R-NH₃)₂MX₄, left) and diammonium ((NH₃-R-NH₃)MX₄, right) cations. Reprinted with permission from [56]. Copyright (2001) Royal Society of Chemistry

Fig. 8

a J–V curves under illuminated and dark conditions and b IPCE (incident photon-to-electron conversion efficiency) spectra of copper halide perovskite-based solar cells using (p-F-C₆H₅C₂H₄NH₃)₂CuBr₄ (P1) and (CH₃(CH₂)₃NH₃)₂CuBr₄) (P2) as absorber materials. Adapted with permission from [34]. Copyright (2015) Elsevier

Fig. 9

Anionic sublattices present in antimony halide perovskites in polyhedral representation: a zero-dimensional dimers of face-sharing octahedra, b one-dimensional double chains of corner-connected octahedra, and c two-dimensional double-layered structures of corner-sharing octahedra. Reproduced with permission of the International Union of Crystallography [159]. Copyright (1996) International Union of Crystallography

Fig. 10

a J–V curves of (CH₃NH₃)₃Sb₂I₉-based perovskite solar cells scanned in forward and reverse direction, and b corresponding EQE spectra including a reference device without absorber material. Adapted with permission from [55]. Copyright (2016) American Chemical Society

Fig. 11

a Schematic representation of the influence of the ionic radius of the A-site cation on the structure and dimensionality of A₃Sb₂I₉-type perovskite compounds, and b J–V curves of Rb₃Sb₂I₉-based solar cells under illuminated and dark conditions in forward and reverse scan direction (inset energy level diagram). Reprinted with permission from [35]. Copyright (2016) American Chemical Society

Fig. 12

a Energy level diagram and b J–V curves under illumination of a photovoltaic device with a (CH₃NH₃)₃Bi₂I₉-based absorber material (blue) and a reference solar cell without absorber (black). Adapted with permission from [182]. Copyright (2016) Elsevier

Fig. 13

a Cross-sectional SEM image of a (CH₃NH₃)₃Bi₂I₉-based perovskite solar cell in meso-structured configuration (ITO/c-TiO₂/mp-TiO₂/(CH₃NH₃)₃Bi₂I₉/Spiro-OMeTAD/MoO₃/Ag, scale bar 1 µm), b J–V curve under illumination (100 mW/cm²). Adapted with permission from [183]. Copyright (2016) Springer

Fig. 14

a J–V curves and b IPCE spectra of perovskite solar cells in meso-structured configuration using (CH₃NH₃)₃Bi₂I_9−xCl_x, (CH₃NH₃)₃Bi₂I₉, and Cs₃Bi₂I₉ absorber materials, respectively. Adapted with permission from [36]. Copyright (2015) WILEY–VCH Verlag GmbH & Co. KGaA, Weinheim

Fig. 15

a Crystal structure of rock-salt ordered double halide perovskites (turquoise: monovalent A-site cation, gray monovalent B^I cation, orange trivalent B^II cation, brown halide counterion). b Face-centered cubic sublattice in double halide perovskites comprising edge-sharing tetrahedral positions. Adapted with permission from [64]. Copyright (2016) American Chemical Society

Fig. 16

a Atomic structures of CH₃NH₃PbI₃ and CH₃NH₃BiSeI₂, and schematic representation of the split-anion approach for the replacement of Pb in CH₃NH₃PbI₃; b Calculated band gaps of CH₃NH₃BiXY₂ (X = S, Se, Te; Y = Cl, Br, I) using HSE functional with spin–orbit coupling. The dashed line indicates the optimal band gap for single-junction solar cells according to the Shockley–Queisser theory. Adapted with permission from [197]. Copyright (2016) Royal Society of Chemistry

Fig. 17

Calculated band gaps of 18 ABX₃ compounds in the distorted, hexagonal, and needle-like phase using HSE06 functional. The optimal band gap region for solar cells is highlighted in green, while an extended region is highlighted in light red. Adapted with permission from [201]. Copyright (2015) American Chemical Society