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

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. GRAPHICAL ABSTRACT: [Image: see text

Iodobismuthates Containing One-Dimensional BiI₄^– Anions as Prospective Light-Harvesting Materials: Synthesis, Crystal and Electronic Structure, and Optical Properties

Four iodobismuthates, LiBiI₄·5H₂O (<b>1</b>), MgBi₂I₈·8H₂O (<b>2</b>), MnBi₂I₈·8H₂O (<b>3</b>), and KBiI₄·H₂O (<b>4</b>), were prepared by a facile solution route and revealed thermal stability in air up to 120 °C. Crystal structures of compounds <b>1</b>–<b>4</b> were solved by a single crystal X-ray diffraction method. <b>1</b>: space group <i>C</i>2/<i>c</i>, <i>a</i> = 12.535(2), <i>b</i> = 16.0294(18), <i>c</i> = 7.6214(9) Å, β = 107.189(11)°, <i>Z</i> = 4, <i>R</i> = 0.029. <b>2</b>: space group <i>P</i>2₁/<i>c</i>, <i>a</i> = 7.559(2), <i>b</i> = 13.1225(15), <i>c</i> = 13.927(4) Å, β = 97.14(3)°, <i>Z</i> = 2, <i>R</i> = 0.031. <b>3</b>: space group <i>P</i>2₁/<i>c</i>, <i>a</i> = 7.606(3), <i>b</i> = 13.137(3), <i>c</i> = 14.026(5) Å, β = 97.14(3)°, <i>Z</i> = 2, <i>R</i> = 0.056. <b>4</b>: space group <i>P</i>2₁/<i>n</i>, <i>a</i> = 7.9050(16), <i>b</i> = 7.7718(16), <i>c</i> = 18.233(4) Å, β = 97.45(3)°, <i>Z</i> = 4, <i>R</i> = 0.043. All solid state structures feature one-dimensional (BiI₄)⁻ anionic chains built of [BiI₆] octahedra that share two opposite edges in such a fashion that two iodine atoms in <i>cis</i>-positions remain terminal. The calculated electronic structures and observed optical properties confirmed that compounds <b>1</b>–<b>4</b> are semiconductors with direct band gaps of 1.70–1.76 eV, which correspond to their intense red color. It was shown that the cations do not affect the optical properties, and the optical absorption is primarily associated with the charge transfer from the I 5p orbitals at the top of the valence band to the Bi 6p orbitals at the bottom of the conduction band. Based on their properties and facile synthesis, the title compounds are proposed as promising light-harvesting materials for all-solid solar cells

Black hybrid iodobismuthate containing linear anionic chains

Three hybrid 1,1′-(1,n-alkanediyl)bis(4-methylpyridinium) iodobismuthates 1–3 were prepared by a facile solution route and showed thermal stability in air up to 230 °C. The structures of solids 1 and 3 contain zero-dimensional anions, and the structure of 2 contains one-dimensional linear anionic chains [BiI5]n2n−. Photoluminescence (PL) in the spectral range between 600 and 750 nm was observed for 1 and 2. DFT calculations and optical studies confirmed that compounds 1–3 are semiconductors with band gaps of 1.73–2.10 eV, which correspond with their intense black (for 2) or red (for 1 and 3) colors. The optical absorption of 2 in the red spectral range is primarily due to charge transfer from the I5p orbitals at the top of the valence band to the Bi6p orbitals at the bottom of the conduction band