Takeshi Noda

Thermally Stable MAPbI3 Perovskite Solar Cells with Efficiency of 19.19% and Area over 1 cm2 achieved by Additive Engineering

Solution-processed perovskite (PSC) solar cells have achieved extremely high power conversion efficiencies (PCEs) over 20%, but practical application of this photovoltaic technology requires further advancements on both long-term stability and large-area device demonstration. Here, an additive-engineering strategy is developed to realize a facile and convenient fabrication method of large-area uniform perovskite films composed of large crystal size and low density of defects. The high crystalline quality of the perovskite is found to simultaneously enhance the PCE and the durability of PSCs. By using the simple and widely used methylammonium lead iodide (MAPbI3 ), a certified PCE of 19.19% is achieved for devices with an aperture area of 1.025 cm2 , and the high-performing devices can sustain over 80% of the initial PCE after 500 h of thermal aging at 85 °C, which are among the best results of MAPbI3 -based PSCs so far.

Growth of Shape- and Size-Selective Zinc Oxide Nanorods by a Microwave-Assisted Chemical Bath Deposition Method: Effect on Photocatalysis Properties

Herein, we demonstrate the shape- and size-selective growth of ZnO nanostructures on indium tin oxide-coated glass substrates by using a microwave-assisted chemical bath deposition method. By systematically controlling the deposition parameters, it is possible to produce shape- and size-selective nanostructures with high alignment and uniformity. Specifically, the pH of the bath can be used to control the shape of rods from bundled structures to tapered and flat tips. Furthermore, the deposition temperature can be used to control the size of the ZnO array from 770 to 125u2005nm. The prepared rods were active catalysts in the degradation of methylene blue under UV radiation, and exhibited size-dependent activity.

Optical properties of GaSb/GaAs type-ІІ quantum dots grown by droplet epitaxy

We study the optical properties of GaSb/GaAs type-ІІ quantum dots (QDs) on a GaAs substrate grown by droplet epitaxy. Ga droplets are formed on GaAs and then exposed to Sb flux to be clad by large granular crystals of Sb. Then the sample was annealed at 380u2009°C to enhance the reaction of Ga droplets with Sb and to evaporate the excess granular layer. In photoluminescence (PL) measurements, the peaks of the QDs and wetting layer (WL) are observed. The PL intensity of the QDs is much stronger than that of the WL, where the ratio IQD/IWL of the integral intensities is about 13.3. The PL peaks shift toward higher energies with increasing excitation energy, suggesting that the band lineups exhibit a type-ІІ staggered alignment. In addition, we investigate the temperature dependences of the PL peak energy and intensity.

GaAs∕AlGaAs quantum dot laser fabricated on GaAs (311)A substrate by droplet epitaxy

We have demonstrated photopumped laser action of self-assembled GaAs∕AlGaAs quantum dots (QDs) grown on GaAs (311)A substrate by droplet epitaxy. Due to the short migration distance of Ga adatoms across the (311)A surface, high-density QDs were created with high uniformity. The QDs exhibited a narrow spectral band of intense photoluminescence from the QD ensemble, reflecting their small size distribution and high quality. Using the QDs on the (311)A surface as an active laser medium, we observed multimodal stimulated emissions at temperatures of up to 300K.

Droplet epitaxial growth of highly symmetric quantum dots emitting at telecommunication wavelengths on InP(111)A

We demonstrate the formation of InAs quantum dots (QDs) on InAlAs/InP(111)A by means of droplet epitaxy. The C3v symmetry of the (111)A substrate enabled us to realize highly symmetric QDs that are free from lateral elongations. The QDs exhibit a disk-like truncated shape with an atomically flat top surface. Photoluminescence signals show broad-band spectra at telecommunication wavelengths of 1.3 and 1.5u2009μm. Strong luminescence signals are retained up to room temperature. Thus, our QDs are potentially useful for realizing an entangled photon-pair source that is compatible with current telecommunication fiber networks.

Fabrication of GaNAs/AlGaAs Heterostructures with Large Band Offset Using Periodic Growth Interruption

We studied the growth of GaNAs/AlGaAs heterostructures on GaAs (100) substrates by plasma-assisted molecular beam epitaxy. By introducing periodic growth interruption and nitrogen (N) supply to the interrupted surfaces during the growth of GaNAs, we achieved high controllability of the average N concentration in GaNAs layers. We observed three-dimensional island growth of GaNAs on the N-rich surfaces. The GaNAs island structures exhibit narrow photoluminescence emission at around 1 µm at low temperature, indicating the formation of a large band offset between GaNAs and AlGaAs.

Selective Deposition of Insulating Metal Oxide in Perovskite Solar Cells with Enhanced Device Performance

We report a simple methodology for the selective deposition of an insulating layer on the nanoparticulate TiO2 (np-TiO2) mesoporous layer of perovskite solar cells. The deposited MgO insulating layer mainly covered the bottom part of the np-TiO2 layer with less coverage at the top. The so-called quasi-top-open structure is introduced to act as an efficient hole-blocking layer to prevent charge recombination at the physical contact of the transparent conducting oxide with the perovskite. This leads to an open-circuit voltage higher than that of the reference cell with a compact TiO2 hole-blocking layer. Moreover, such a quasi-top-open structure can facilitate the electron injection from perovskite into the np-TiO2 mesoporous layer and improve the spectral response at longer wavelength because of the less covered insulating layer at the top. This work provides an alternative way to fabricate perovskite solar cells without the need to use a conventional compact TiO2 layer.

Growth and optical properties of GaSb/GaAs type-II quantum dots with and without wetting layer

We investigated the effect of a wetting layer (WL) on the optical properties of GaSb type-II quantum dots (QDs) in GaAs. GaSb QDs were grown by droplet epitaxy, where Ga droplets are first formed on GaAs and then exposed to Sb flux, followed by an annealing step. By adjusting the annealing temperature, we fabricated GaSb QDs with and without a well-defined WL-like structure. Photoluminescence (PL) measurements showed that the high-temperature-annealed samples exhibit a strong PL of the WL, whereas the WL luminescence is quite weak for the samples annealed at low temperatures. As the measurement temperature T increases, the PL energy decreases for the GaSb QDs without the WL. In contrast, the PL energy of the GaSb QDs with the WL has little dependence on T. These PL energy shifts are explained by considering the effects of the hole population and energy-dependent absorption.

Growth of GaSb and AlSb quantum dots on high-index GaAs substrates

We investigate the growth of GaSb and AlSb quantum dots (QDs) on high-index GaAs substrates at various substrate temperatures (Ts). The GaSb QD density on GaAs(311)A is about 1.5?2 times larger than that on GaAs(100) substrates. Small and dense AlSb QDs are formed by AlSb deposition on GaAs(311)A substrates at relatively high temperatures (Ts ? 430 ?C) with QD densities as high as 29 ? 1010 cm?2. In contrast, no QDs are observed for low-temperature AlSb deposition (Ts ? 410 ?C). In contrast to InAs QDs, GaSb and AlSb QDs are easily formed even on GaAs(111)A surfaces. We use a rate equation to analyze the experimental data and discuss QD density and size and their Ts dependences.

Open-Circuit Voltage in AlGaAs Solar Cells With Embedded GaNAs Quantum Wells of Varying Confinement Depth

We investigate the photovoltaic properties of AlGaAs solar cells with embedded GaNAs quantum wells (QWs) with N concentrations in the range of 0-3.1%, for which the QW confinement energy can be tuned by adjusting the N concentration. We systematically study the dependence of open-circuit voltage VOC in relation to the lowest band-to-band transition energy. In samples with low N concentrations (shallow QW confinement), VOC degrades and is limited by the lowest transition energy in the solar cell, i.e., the QW transition. With increasing N concentration, N > 0.5% (deep QW confinement), VOC does not degrade further and is no longer limited by the QW transition energy. The highest N sample exhibits a remarkably small offset between the lowest transition energy and the achieved VOC of 0.23 V, which is beyond the detailed balance limit of standard solar cells. VOC dependence is explained by analyzing the current-voltage (I-V) characteristics under different illumination conditions, from which information about the balance of escape and recombination rates of carriers from the QWs is extracted. In the deeply confined QWs, tunneling and thermal carrier escape is completely suppressed, allowing the recovery of VOC.