Youhei Numata

Chemistry of fullerene epoxides: synthesis, structure, and nucleophilic substitution-addition reactivity.

Fullerene epoxides, C60On, having epoxide groups directly attached to the fullerene cage, constitute an interesting class of fullerene derivatives. In particular, the chemical transformations of fullerene epoxides are expected to play an important role in the development of functionalized fullerenes. This is because such transformations can readily afford a variety of mono- or polyfunctionalized fullerene derivatives while conserving the epoxy ring arrangement on the fullerene surface, as seen in representative regioisomeric fullerene polyepoxides. The first part of this review addresses the synthesis and structural characterization of fullerene epoxides. The formation of fullerene epoxides through different oxidation reactions is then explored. Adequate characterization of the isolated fullerene epoxides was achieved by concerted use of NMR and LC-MS techniques. The second part of this review addresses the substitution of fullerene epoxides in the presence of a Lewis acid catalyst. Most major substitution products have been isolated as pure compounds and their structures established through spectroscopic methods. The correlation between the structure of the substitution product and the oxygenation pattern of the starting materials allows elucidation of the mechanistic features of this transformation. This approach promises to lead to rigorous regioselective production of various fullerene derivatives for a wide range of applications.

Impacts of Heterogeneous TiO2 and Al2O3 Composite Mesoporous Scaffold on Formamidinium Lead Trihalide Perovskite Solar Cells

Heterogeneous TiO2 and Al2O3 composites were employed as a mesoporous scaffold in formamidinium lead trihalide (FAPbI3-xClx)-based perovskite solar cells to modify surface properties of a mesoporous layer. It was found that the quality and morphology of the perovskite film were strongly affected by the TiO2/Al2O3 ratio in the mesoporous film. The conversion efficiency of the perovskite solar cell was improved by using a composite of TiO2 and Al2O3 in comparison with TiO2- and Al2O3-based cells, yielding 11.0% for a cell with a 7:3 TiO2/Al2O3 composite. Our investigation shows a change of electron transport path depending on a composition ratio of insulating Al2O3 to n-type semiconducting TiO2 in a mesoporous layer.

Controlled Crystal Grain Growth in Mixed Cation–Halide Perovskite by Evaporated Solvent Vapor Recycling Method for High Efficiency Solar Cells

We developed a new and simple solvent vapor-assisted thermal annealing (VA) procedure which can reduce grain boundaries in a perovskite film for fabricating highly efficient perovskite solar cells (PSCs). By recycling of solvent molecules evaporated from an as-prepared perovskite film as a VA vapor source, named the pot-roast VA (PR-VA) method, finely controlled and reproducible device fabrication was achieved for formamidinium (FA) and methylammonium (MA) mixed cation-halide perovskite (FAPbI3)0.85(MAPbBr3)0.15. The mixed perovskite was crystallized on a low-temperature prepared brookite TiO2 mesoporous scaffold. When exposed to very dilute solvent vapor, small grains in the perovskite film gradually unified into large grains, resulting in grain boundaries which were highly reduced and improvement of photovoltaic performance in PSC. PR-VA-treated large grain perovskite absorbers exhibited stable photocurrent-voltage performance with high fill factor and suppressed hysteresis, achieving the best conversion efficiency of 18.5% for a 5 × 5 mm2 device and 15.2% for a 1.0 × 1.0 cm2 device.

Surface Free Energy and Wettability Determination of Various Fullerene Derivative Films on Amorphous Carbon Wafer

The surface energy of a polished amorphous carbon (a-C) wafer spin-coated with fullerene derivatives is evaluated by the contact angle measurements of test liquids. The contact angles of the epoxy prepolymer droplets are also measured to investigate the effects of fullerene derivatives on the adhesiveness between the epoxy resin and the a-C wafer surface. A marked increase in surface energy is observed when the wafer is spin-coated with the fullerene derivatives having a polar group. The three surface energy components, dispersive, polar, and hydrogen bonding forces, are determined from the contact angles by an expanded Fowkes method. The total surface energy thus obtained varies from 36.2 to 50.4 mN/m, depending on the nature of fullerene derivatives. The major factor, which increases the surface total energy, is found to be the polar component of the fullerene derivatives. Microscopic structures of the thin films of fullerenes on the wafer have no essential effects on the surface energy and the wettability between the a-C wafer and the epoxy resin. In the present study, we demonstrate that the fullerene derivative with a polar group acts as an adhesive in the fixation between the epoxy resin and the a-C wafer: a polar group of the fullerene derivative has large affinity to the epoxy resin, and the fullerene core, to the a-C surface.

Role of spiro-OMeTAD in performance deterioration of perovskite solar cells at high temperature and reuse of the perovskite films to avoid Pb-waste

As the long-term stability and toxicity of Pb are two profound concerns for the commercialization of Pb-based perovskite solar cells, we have undertaken this study to understand the performance degradation of perovskite solar cells at high temperature (60, 80, 100 and 120 °C) and under a humid (30–50% relative humidity) environment, and then tried to reuse the perovskite films from the degraded cells to recover the cell efficiency so as to avoid dumping of Pb-waste into the environment. As found in this study, the performance of MAPbI3 (MA = CH3NH3) cells using spiro-OMeTAD as the hole transport material (HTM) deteriorated mainly not due to the degradation of the perovskite but because of the modification of the interface between the perovskite and spiro-OMeTAD at high temperature. In addition, the spiro-OMeTAD layer underwent a severe morphological deformation at high temperature, showing large voids in it, which reduced the cell performance further. However, despite the complete reconversion of PbI2 to perovskite and the replacement of the degraded spiro-OMeTAD film with a fresh layer in the MAPbI3 cells, the cell performance was not recovered to the initial value because the modified perovskite/spiro-OMeTAD interface became worse after recycling. The thermal stability and performance recovery upon recycling were found to depend on the composition of the perovskite; faster degradation of MAPbI3 cells with a slight excess of MAI and better stability of cells not containing MA (FA0.85Cs0.15PbI3, FA = CH(NH2)2) indicated certain involvement of MA+ ions in the alteration of the perovskite/spiro-OMeTAD interface. Unlike the heat-treatment case, MAPbI3 cells, when exposed to a humid environment, degraded significantly to PbI2 and the reconversion of this PbI2 to perovskite followed by deposition of a fresh spiro-OMeTAD layer increased the cell performance but it did not recover to the initial value. This poor recovery in these cells was due to inefficient carrier transport that resulted in a lower photocurrent in the recycled devices.

Influences of Electron-Withdrawing Groups of Organic Dyes on Spectral Property and Photovoltaic Performance in Dye-sensitized Solar Cells Application

We synthesized five donor–π-spacer–acceptor type organic photosensitizers bearing different types of electron-withdrawing groups (EWGs); trifluoromethyl, o-nitrophenyl, p-nitrophenyl, cyano, and carboxyl groups, on their acceptor part in the aim of observing an influence of the EWGs on spectral and photovoltaic properties from viewpoints of steric structure and π-conjugation. The EWG possessing smaller dihedral angle between the EWG and dye skeleton exhibited larger bathochromic shift in absorptions. Highly planer cyano group presented the most red-shifted absorption at 464 nm, and the highest conversion efficiency of 5.69% was obtained. In contrast, highly distorted o- and p-nitrophenyl groups exhibited blue-shifted absorption at 416 and 422, respectively; however, despite of resemble spectral properties, o- and p-nitrophenyl gave second best and the worst conversion efficiencies of 4.05 and 2.51%, respectively. By combination with computational chemistry, it was indicated that the configuration of the EWG and distance between TiO2 surface and the EWG dominated electron injection efficiency.

Lead-free perovskite solar cells using Sb and Bi-based A 3 B 2 X 9 and A 3 BX 6 crystals with normal and inverse cell structures

Research of CH3NH3PbI3 perovskite solar cells had significant attention as the candidate of new future energy. Due to the toxicity, however, lead (Pb) free photon harvesting layer should be discovered to replace the present CH3NH3PbI3 perovskite. In place of lead, we have tried antimony (Sb) and bismuth (Bi) with organic and metal monovalent cations (CH3NH3+, Ag+ and Cu+). Therefore, in this work, lead-free photo-absorber layers of (CH3NH3)3Bi2I9, (CH3NH3)3Sb2I9, (CH3NH3)3SbBiI9, Ag3BiI6, Ag3BiI3(SCN)3 and Cu3BiI6 were processed by solution deposition way to be solar cells. About the structure of solar cells, we have compared the normal (n-i-p: TiO2-perovskite-spiro OMeTAD) and inverted (p-i-n: NiO-perovskite-PCBM) structures. The normal (n-i-p)-structured solar cells performed better conversion efficiencies, basically. But, these environmental friendly photon absorber layers showed the uneven surface morphology with a particular grow pattern depend on the substrate (TiO2 or NiO). We have considered that the unevenness of surface morphology can deteriorate the photovoltaic performance and can hinder future prospect of these lead-free photon harvesting layers. However, we found new interesting finding about the progress of devices by the interface of NiO/Sb3+ and TiO2/Cu3BiI6, which should be addressed in the future study.

Amorphous Metal Oxide Blocking Layers for Highly Efficient Low-Temperature Brookite TiO2-Based Perovskite Solar Cells

A fully low-temperature-processed perovskite solar cell was fabricated with an ultrathin amorphous TiOx hole-blocking layer in combination with brookite TiO2 prepared at temperature <150 °C. Structured with TiOx/brookite TiO2 bilayer electron collector, the perovskite solar cells exhibit high efficiency up to 21.6% being supported by high open-circuit voltage and fill factor up to 1.18 V and 0.83, respectively. Compared to SnOx hole-blocking layer, TiOx has better electron band alignment with brookite TiO2 and hence, results in higher efficiency.

Solar Water Splitting Utilizing a SiC Photocathode, a BiVO4 Photoanode, and a Perovskite Solar Cell

We have successfully demonstrated solar water splitting using a newly fabricated photoelectrochemical system with a Pt-loaded SiC photocathode, a CoOx -loaded BiVO4 photoanode, and a perovskite solar cell. Detection of the evolved H2 and O2 with a 100 % Faradaic efficiency indicates that the observed photocurrent was used for water splitting. The solar-to-hydrogen (STH) efficiency was 0.55 % under no additional bias conditions.