Tomoko Aharen

Synthesis of a distinct water dimer inside fullerene C70

The water dimer is an ideal chemical species with which to study hydrogen bonds. Owing to the equilibrium between the monomer and oligomer structure, however, selective generation and separation of a genuine water dimer has not yet been achieved. Here, we report a synthetic strategy that leads to the successful encapsulation of one or two water molecules inside fullerene C70. These endohedral C70 compounds offer the opportunity to study the intrinsic properties of a single water molecule without any hydrogen bonding, as well as an isolated water dimer with a single hydrogen bond between the two molecules. The unambiguously determined off-centre position of water in (H2O)2@C70 by X-ray diffraction provides insights into the formation of (H2O)2@C70. Subsequently, the (1)H NMR spectroscopic measurements for (H2O)2@C70 confirmed the formation of a single hydrogen bond rapidly interchanging between the encapsulated water dimer. Our theoretical calculations revealed a peculiar cis-linear conformation of the dimer resulting from confinement effects inside C70.

Optical characterization of voltage-accelerated degradation in CH 3 NH 3 PbI 3 perovskite solar cells

We investigate the performance degradation mechanism of CH3NH3PbI3 perovskite solar cells under bias voltage in air and nitrogen atmospheres using photoluminescence and electroluminescence techniques. When applying forward bias, the power conversion efficiency of the solar cells decreased significantly in air, but showed no degradation in nitrogen atmosphere. Time-resolved photoluminescence measurements on these devices revealed that the application of forward bias in air accelerates the generation of non-radiative recombination centers in the perovskite layer buried in the device. We found a negative correlation between the electroluminescence intensity and the injected current intensity in air. The irreversible change of the perovskite grain surface in air initiates the degradation of the perovskite solar cells.

Photoelectronic properties of lead-free CH3NH3SnI3 perovskite solar cell materials and devices

Intensive research on lead halide perovskites clarified that these materials are indeed suitable candidates for photovoltaic applications due to their excellent photoelectronic properties. Yet, lead is considered a major issue for commercialization. Concurrently, in the last five years, increasing research efforts have been made to replace lead with tin. Although partially successful, the present conversion efficiencies of tin halide perovskite solar cells are limited. Further performance improvements should be possible, if the underlying energy loss mechanisms in these devices can be clarified. Here, we investigated the energy loss mechanisms in lead-free CH3NH3SnI3 (MASnI3) solar cells as well as intrinsic photoelectronic properties of MASnI3 to assess its potential for photovoltaics. Time-resolved photoluminescence (PL) measurements reveal that the short-circuit current (Jsc) in the MASnI3 solar cell deviates from an ideal value as a result of fast recombination of photogenerated carriers in the perovskite layer. Consequently, a larger Jsc should be possible with longer carrier lifetimes. Furthermore, resonantly excited PL and temperature-dependent PL data clearly reveal that the intrinsic electron–longitudinal optical phonon coupling governs the broadening of optical transitions at around 300 K. By performing a detailed comparison of the data of MASnI3 and MAPbI3, it is shown that the intrinsic optical properties of tin and lead perovskites are similar to each other. Our results suggest that solar cells based on tin halide perovskites can compete with lead halide perovskite solar cells, if the carrier lifetimes can be improved.

Carrier injection and recombination processes in perovskite CH3NH3PbI3 solar cells studied by electroluminescence spectroscopy

Organic-inorganic hybrid perovskite materials, CH3NH3PbX3 (X = I and Br), are considered as promising candidates for emerging thin-film photovoltaics. For practical implementation, the degradation mechanism and the carrier dynamics during operation have to be clarified. We investigated the degradation mechanism and the carrier injection and recombination processes in perovskite CH3NH3PbI3 solar cells using photoluminescence (PL) and electroluminescence (EL) imaging spectroscopies. By applying forward bias-voltage, an inhomogeneous distribution of the EL intensity was clearly observed from the CH3NH3PbI3 solar cells. By comparing the PL- and EL-images, we revealed that the spatial inhomogeneity of the EL intensity is a result of the inhomogeneous luminescence efficiency in the perovskite layer. An application of bias-voltage for several tens of minutes in air caused a decrease in the EL intensity and the conversion efficiency of the perovskite solar cells. The degradation mechanism of perovskite solar cells under bias-voltage in air is discussed.