Nobuyuki Matsuki

Photovoltaic Action in Polyaniline/n-GaN Schottky Diodes

Schottky diodes were fabricated on n-GaN films by coating them with an organic polyaniline layer as transparent conducting electrodes. These diodes have a high Schottky barrier height (1.28 eV) and a low reverse leakage current (2.7×10-9 A/cm2 at an applied bias of -1 V). The photovoltaic action of these diodes (VOC = 0.67 V and external quantum efficiency ~30%) was studied under the illumination of an Air Mass 1.5 solar simulator. The polyaniline/n-GaN Schottky contacts were found to be sensitive to shorter wavelengths, indicating their potential for use as solar cells.

Deep-Level Characterization of n-GaN Epitaxial Layers Using Transparent Conductive Polyaniline Schottky Contacts

We have successfully investigated surface-related deep levels in n-GaN epilayers with high carrier concentrations by using transparent conductive polyaniline Schottky contacts. High quality Schottky barrier diodes fabricated showed a typical capacitance dispersion phenomenon at ~10 kHz, which is characteristic of conductive polyaniline films with polarization capacitance and resistance components. Steady-state photocapacitance spectroscopy measurements at over this cutoff frequency revealed five photoemission states with their onsets at ~1.40, ~1.70, ~2.08, ~2.64, and ~2.90 eV below the conduction band, being identical with the deep levels commonly observed in GaN and AlGaN/GaN. Particularly, the concentrations of the ~1.70 and ~2.90 eV levels were found to increase significantly with decreasing their probing depth range to the near-surface region of the n-GaN layers. Therefore, these levels are probably subject to the surface conditions of the n-GaN layers.

Nondestructive characterization of textured a-Si:H/c-Si heterojunction solar cell structures with nanometer-scale a-Si:H and In2O3:Sn layers by spectroscopic ellipsometry

Nanometer-scale hydrogenated amorphous silicon (a-Si:H) layers formed on crystalline silicon (c-Si) with pyramid-shaped textures have been characterized by spectroscopic ellipsometry (SE) using a tilt angle measurement configuration, in an attempt to establish a nondestructive method for the structural characterization of the a-Si:H/c-Si heterojunction solar cells. By applying an a-Si:H dielectric function model developed recently, the thickness and SiH2 content of the a-Si:H layer have been determined even on the textured substrates. Furthermore, from the SE analysis incorporating the Drude model, the carrier properties of the In2O3:Sn layers in the textured solar-cell structure have been characterized.

Combinatorial screening of halide perovskite thin films and solar cells by mask-defined IR laser molecular beam epitaxy

Abstract As an extension of combinatorial molecular layer epitaxy via ablation of perovskite oxides by a pulsed excimer laser, we have developed a laser molecular beam epitaxy (MBE) system for parallel integration of nano-scaled thin films of organic–inorganic hybrid materials. A pulsed infrared (IR) semiconductor laser was adopted for thermal evaporation of organic halide (A-site: CH3NH3I) and inorganic halide (B-site: PbI2) powder targets to deposit repeated A/B bilayer films where the thickness of each layer was controlled on molecular layer scale by programming the evaporation IR laser pulse number, length, or power. The layer thickness was monitored with an in situ quartz crystal microbalance and calibrated against ex situ stylus profilometer measurements. A computer-controlled movable mask system enabled the deposition of combinatorial thin film libraries, where each library contains a vertically homogeneous film with spatially programmable A- and B-layer thicknesses. On the composition gradient film, a hole transport Spiro-OMeTAD layer was spin-coated and dried followed by the vacuum evaporation of Ag electrodes to form the solar cell. The preliminary cell performance was evaluated by measuring I-V characteristics at seven different positions on the 12.5 mm × 12.5 mm combinatorial library sample with seven 2 mm × 4 mm slits under a solar simulator irradiation. The combinatorial solar cell library clearly demonstrated that the energy conversion efficiency sharply changes from nearly zero to 10.2% as a function of the illumination area in the library. The exploration of deposition parameters for obtaining optimum performance could thus be greatly accelerated. Since the thickness ratio of PbI2 and CH3NH3I can be freely chosen along the shadow mask movement, these experiments show the potential of this system for high-throughput screening of optimum chemical composition in the binary film library and application to halide perovskite solar cell.

HETEROINTERFACE PROPERTIES OF NOVEL HYBRID SOLAR CELLS CONSISTING OF TRANSPARENT CONDUCTIVE POLYMERS AND III-NITRIDE SEMICONDUCTOR

We have investigated the heterointerface properties of recently developed hybrid solar cells comprising a Schottky contact made of transparent conductive polymer (TCP) and an underlying GaN semiconductor layer. The heterointerface capacitance induced by the depletion layer under the TCP Schottky contact showed a rapid drop at a specific frequency. An intrinsic capacitance component that was derived from the capacitance–frequency (C–f) characteristics of the heterointerface showed clear correlation with the open circuit voltage. Hence, the C–f characterization using TCP Schottky contacts is indicative of the quality of the heterointerface.

Impact of sputter-induced ion bombardment at the heterointerfaces of a-Si:H/c-Si solar cells with double-layered In2O3:Sn structures

The effect of ion bombardment on photovoltaic characteristics, induced during the sputtering deposition of In2O3:Sn (ITO) layers, has been investigated using hydrogenated amorphous silicon/crystalline silicon (a-Si:H/c-Si) heterojunction solar cells with double-layered ITO structures. The experimental results indicate that the significant decrease in the fill factor (FF) induced at a sputtering rf power higher than 30 W is attributed to the increase in series resistance (Rs), which is caused by property degradation in the ITO/a-Si:H interface region. Furthermore, it has been found that the impact of the sputter-induced ion bombardment on the property degradation reaches the ITO/a-Si:H interface region through the deposited ITO layer even up to a thickness of 20 nm.