Masaru Endo

Photocarrier recombination dynamics in perovskite CH3NH3PbI3 for solar cell applications.

Using time-resolved photoluminescence and transient absorption measurements at room temperature, we report excitation-intensity-dependent photocarrier recombination processes in thin films made from the organo-metal halide perovskite semiconductor CH3NH3PbI3 for solar-cell applications. The photocarrier dynamics are well described by a simple rate equation including single-carrier trapping and electron-hole radiative recombination. This result provides clear evidence that the free-carrier model is better than the exciton model for interpreting the optical properties of CH3NH3PbI3. The observed large two-carrier recombination rate suggests the promising potential of perovskite semiconductors for optoelectronic device applications. Our findings provide the information about the dynamical behaviors of photoexcited carriers that is needed for developing high-efficiency perovskite solar cells.

Near-band-edge optical responses of solution-processed organic?inorganic hybrid perovskite CH3NH3PbI3 on mesoporous TiO2 electrodes

We studied the near-band-edge optical responses of solution-processed CH3NH3PbI3 on mesoporous TiO2 electrodes, which is utilized in mesoscopic heterojunction solar cells. Photoluminescence (PL) and PL excitation spectra peaks appear at 1.60 and 1.64 eV, respectively. The transient absorption spectrum shows a negative peak at 1.61 eV owing to photobleaching at the band-gap energy, indicating a direct band-gap semiconductor. On the basis of the temperature-dependent PL and diffuse reflectance spectra, we clarified that the absorption tail at room temperature is explained in terms of an Urbach tail and consistently determined the band-gap energy to be ~1.61 eV at room temperature.

Photoelectronic Responses in Solution-Processed Perovskite CH

Photoelectronic responses of organic-inorganic hybrid perovskite CH3NH3PbI3 on mesoporous TiO2 electrodes are investigated. On the basis of near-band-edge optical absorption and photoluminescence spectra, the bandgap energy and exciton binding energy as a function of temperature are obtained. The exciton binding energy is much smaller than thermal energy at room temperature, which means that most excitons are thermally dissociated, and optical processes are determined by the photoexcited electrons and holes. We determined the temperature dependence of exciton binding energy, which changes from ~30 meV at 13 K to 6 meV at 300 K. In addition, the bandgap energy and the exciton binding energy show abrupt changes at 150 K due to structural phase transition. Our fundamental optical studies provide essential information for improving the device performance of solar cells based on halide perovskite semiconductors.

_{\bf 3}

We utilized time-resolved photoluminescence (PL) spectroscopy to investigate the photocarrier recombination dynamics in CH3NH3PbI3 thin films as a function of the time elapsed from the films fabrication. We found that the PL lifetime gradually increased and began to level out once the age of the film reached ∼30 h. Even under weak excitation, the PL dynamics depended on the excitation intensity in the fresh sample, while the mature sample displayed no excitation-intensity dependence associated with the PL dynamics. We submit that this can be explained by the fact that a significant number of defects are initially formed in CH3NH3PbI3 thin films fabricated by the sequential method and are spontaneously reduced by room-temperature annealing. Our results provide important insights for reducing the nonradiative recombination centers, which improves the power conversion efficiency of perovskite solar cells.

NH

We investigate the degradation behavior of non-sealed perovskite CH3NH3PbI3 diode devices in air at room temperature by means of electroluminescence (EL) and photoluminescence (PL) imaging techniques. From the comparison of these images, we determine that the spatial fluctuation of the EL intensity is mainly due to fluctuations in the luminescence efficiency of the perovskite layer itself. By applying a constant voltage for tens of minutes, the EL intensity decreases gradually. It is observed that the temporal evolution of the EL intensity is governed by the degradation of the perovskite layer and the carrier injection at the interface.

_{\bf 3}

Herein, the open-circuit voltage (VOC) loss in both polymer solar cells and perovskite solar cells is quantitatively analyzed by measuring the temperature dependence of VOC to discuss the difference in the primary loss mechanism of VOC between them. As a result, the photon energy loss for polymer solar cells is in the range of about 0.7-1.4 eV, which is ascribed to temperature-independent and -dependent loss mechanisms, while that for perovskite solar cells is as small as about 0.5 eV, which is ascribed to a temperature-dependent loss mechanism. This difference is attributed to the different charge generation and recombination mechanisms between the two devices. The potential strategies for the improvement of VOC in both solar cells are further discussed on the basis of the experimental data.

PbI

In this paper, we try to fabricate the planer heterojunction (PHJ) perovskite solar cell based on low temperature TiOx compact layer by chemical bath deposition (CBD) method. We compared solar cell properties between compact-TiO_x layer and compact-TiO₂ layer, where CBD deposited films were heated at 250 °C in order to convert into TiO_x amorphous state and 450 °C intoTiO₂ anatase state, respectively. Power conversion efficiency of PHJ perovskite solar cell based on our compact-TiO_x shows almost same as that of TiO₂ one. In addition, we discuss the origin of hysteresis of PHJ perovskite solar cell based on compact-TiO_x and TiO₂. We conclude that hysteresis of PHJ perovskite solar cell is heavily influenced by surface coverage and roughness of compact-TiO_x or TiO₂ layers on transparent conducting oxide.