The toxicity of lead perovskite hampers the commercialization of perovskite-based photovoltaics. While tin perovskite is a promising alternative, the facile oxidation of tin(II) to tin(IV) causes a high density of defects, resulting in lower solar cell efficiencies. Here, we show that tin(0) nanoparticles in the precursor solution can scavenge tin(IV) impurities, and demonstrate that this treatment leads to effectively tin(IV)-free perovskite films with strong photoluminescence and prolonged decay lifetimes. These nanoparticles are generated by the selective reaction of a dihydropyrazine derivative with the tin(II) fluoride additive already present in the precursor solution. Using this nanoparticle treatment, the power conversion efficiency of tin-based solar cells reaches 11.5%, with an open-circuit voltage of 0.76 V. Our nanoparticle treatment is a simple and broadly effective method that improves the purity and electrical performance of tin perovskite films. Tin based perovskites are easily oxidized, which generates large density of defects and compromised the solar cell efficiency. Here Nakamura et al. add metallic tin nanoparticles in the precursor solution to suppress tin (IV) impurities and enable high efficiency tin based perovskite solar cells.

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Sn(IV)-free tin perovskite films realized by in situ Sn(0) nanoparticle treatment of the precursor solution.

The cavity inside fullerenes provides a unique environment for the study of isolated atoms and molecules. We report encapsulation of hydrogen fluoride inside C60 using molecular surgery to give the endohedral fullerene HF@C60. The key synthetic step is the closure of the open fullerene cage while minimizing escape of HF. The encapsulated HF molecule moves freely inside the cage and exhibits quantization of its translational and rotational degrees of freedom, as revealed by inelastic neutron scattering and infrared spectroscopy. The rotational and vibrational constants of the encapsulated HF molecules were found to be redshifted relative to free HF. The NMR spectra display a large 1H-19F J coupling typical of an isolated species. The dipole moment of HF@C60 was estimated from the temperature-dependence of the dielectric constant at cryogenic temperatures and showed that the cage shields around 75% of the HF dipole.

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The dipolar endofullerene HF@C60

Developing dopant-free hole transporting layers (HTLs) is critical in achieving high-performance and robust state-of-the-art perovskite photovoltaics, especially for the air-sensitive tin-based perovskite systems. The commonly used HTLs require hygroscopic dopants and additives for optimal performance, which adds extra cost to manufacturing and limits long-term device stability. Here we demonstrate the use of a novel tetrakis-triphenylamine (TPE) small molecule prepared by a facile synthetic route as a superior dopant-free HTL for lead-free tin-based perovskite solar cells. The best-performing tin iodide perovskite cells employing the novel mixed-cation ethylenediammonium/formamidinium with the dopant-free TPE HTL achieve a power conversion efficiency as high as 7.23%, ascribed to the HTL’s suitable band alignment and excellent hole extraction/collection properties. This efficiency is one of the highest reported so far for tin halide perovskite systems, highlighting potential application of TPE HTL materi...

Dopant-Free Tetrakis-Triphenylamine Hole Transporting Material for Efficient Tin-Based Perovskite Solar Cells

This Perspective reviews the developments of tin-based perovskite solar cells (PSCs) during the period 2014–2019 based on two organic cations (methylammonium vs formamidinium) and two device archit...

Strategies to improve performance and stability for tin-based perovskite solar cells

Tin Halide Perovskites: Progress and Challenges

A series of solvent-coordinated tin halide complexes were prepared as impurity-free precursors for tin halide perovskites, and their structures were determined by single-crystal X-ray diffraction analysis. Using these precursors, the tin halide perovskites, MASnI3 and FASnI3, were prepared, and their electronic structures and photophysical properties were examined under inert conditions by means of photoelectron yield spectroscopy as well as absorption and fluorescence spectroscopies. Their valence bands (MASnI3: −5.02 eV; FASnI3: −5.16 eV) are significantly higher than those of MAPbI3 or the typical hole-transporting materials 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene and poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine). These results suggest that to develop the solar cells using these tin halide perovskites with efficient hole-collection properties, hole-transporting materials should be chosen that have the highest occupied molecular orbital levels higher than −5.0 eV.

Solvent-Coordinated Tin Halide Complexes as Purified Precursors for Tin-Based Perovskites.

The macroscopic-scale syntheses of the first endohedral aza[60]fullerenes X@C59N (X = H2O, H2) were achieved in two different ways: (1) synthesis from endohedral fullerene H2O@C60 as a starting material and (2) molecular surgical synthesis from a C59N precursor having a considerably small opening. In the neutral state of H2O@C59N, we expected the H-bonding interaction or repulsive N–O interaction between entrapped H2O and a nitrogen atom on the C59N cage. However, an attractive electrostatic N–O interaction was suggested from the results of variable temperature NMR, nuclear magnetic relaxation times (T1, T2), and density functional theory (DFT) calculations. Upon the reaction with acetone via cationic intermediate C59N+, we found a difference in reaction rates between H2O@C59N and H2@C59N dimers (observed reaction rates: k′(H2O)/k′(H2) = 1.74 ± 0.16). The DFT calculations showed thermal stabilization of C59N+ by entrapped H2O through the electrostatic interaction.

Synthesis and Properties of Endohedral Aza[60]fullerenes: H2O@C59N and H2@C59N as Their Dimers and Monomers

The palladium-catalyzed cyclization on the fullerene C60 cage has been achieved using several aryl halides and C60. This reaction was found to be accelerated by the addition of pivalic acid, which can be rationally explained by the computational study based on the concerted metalation–deprotonation mechanism. We also demonstrated the regioselective π-functionalization using prefunctionalized designed molecules possessing the same substructure on the C60 cage. The single crystal X-ray analysis and electrostatic potential map revealed that the orientation of entrapped H2O inside the naphthalene-fused open-cage C60 derivative is electrostatically demanded due to the naphthalene-fusion and construction of the opening.

Palladium-Catalyzed Cyclization: Regioselectivity and Structure of Arene-Fused C60 Derivatives.

Synthesis of Open-Cage Ketolactam Derivatives of Fullerene C60 Encapsulating a Hydrogen Molecule

The concept of the bond polarization is a useful tool to understand chemical reactions and fundamental properties of compounds. However, experimental considerations are limited owing to its difficulty of reliable description. We demonstrated that geometrically isolated H2O inside the cage of fullerene C60 is a possible probe to evaluate the polarization degree of covalent bonds C(C60)−X (X=heteroatom) on the C60 cage. The 1H NMR relaxation times of entrapped H2O have been systematically measured at variable temperatures for H2O@C60X (X=CR2, NR, O, and O2). The results followed in the order of electronegativities of C (2.55), N (3.04), and O (3.44), indicating that entrapped H2O can sensitively respond to the degree of the bond polarization.

Yoshifumi Hashikawa

Papers

Solvent-Coordinated Tin Halide Complexes as Purified Precursors for Tin-Based Perovskites.

Synthesis and Properties of Endohedral Aza[60]fullerenes: H2O@C59N and H2@C59N as Their Dimers and Monomers

Palladium-Catalyzed Cyclization: Regioselectivity and Structure of Arene-Fused C60 Derivatives.

Synthesis of Open-Cage Ketolactam Derivatives of Fullerene C60 Encapsulating a Hydrogen Molecule

Water Entrapped inside Fullerene Cages: A Potential Probe for Evaluation of Bond Polarization