Mikihiko Nishitani

Preparation of Device-Quality Cu(In, Ga)Se2 Thin Films Deposited by Coevaporation with Composition Monitor

The chemical composition of Cu(In, Ga)Se2 (CIGS) thin film was monitored in real time during the physical vapor deposition. The temperature of growing CIGS film was found to depend on the composition ratio of Cu/(In+Ga) when the film was deposited under constant heating power. The composition monitoring system can be easily applied to a 3-stage deposition process of the CIGS films. The solar cells (active area: 1 cm2) fabricated by using the obtained CIGS absorber layer showed an efficiency of 15.4% under standard AM 1.5 illumination.

Preparation and characterization of Cu(In1−xGax)3Se5 thin films

Polycrystalline Cu(In1−xGax)3Se5 thin films were prepared by four source evaporation with controlling and shielding of the molecular beams from elemental sources. Ga content x, can be controlled by deposition times of CuIn3Se5 and CuGa3Se5 layers, which form Cu(In1−xGax)3Se5 films through the interdiffusion. X‐ray diffraction analyses showed that the films with x≲0.5 have an ordered vacancy chalcopyrite and the films with x≳0.5 have a zinc blende structure. The optical band gap of the films linearly increased from 1.23 eV (x=0) to 1.85 eV (x=1) with increasing Ga content. The conductivity of the films was about 10−6/Ω cm and about 10−7/Ω cm under and above x=0.3, respectively.

ENHANCED CONDUCTIVITY OF ZINC OXIDE THIN FILMS BY ION IMPLANTATION OF HYDROGEN ATOMS

Enhancement of the conductivity of zinc oxide through doping with hydrogen atoms was examined by using ion implantation of highly resistive thin films deposited by rf magnetron sputtering at room temperature. With a doping of 1×1017 atoms cm−2, the conductivity after annealing at 200 °C in an N2 atmosphere at 1 atm rose from the initial 1×10−7 Ω−1 cm−1 to 5.5×102 Ω−1 cm−1.

Chemical and Structural Characterization of Cu(In,Ga)Se2/Mo Interface in Cu(In,Ga)Se2 Solar Cells

Interfaces between the Cu(In,Ga)Se2 (CIGS) absorber and the Mo back contact in CIGS solar cells were investigated by secondary ion mass spectroscopy (SIMS) and analytical transmission electron microscopy (TEM). The solar cell with MgF2/ITO/ZnO/CdS/CIGS/Mo/glass structure exhibited an efficiency of over 15%. In the SIMS depth profile, the Se intensity had a peak at the CIGS/Mo interface. Cross-sectional TEM observation showed that there were two layers at the interface. One was a MoSe2 layer and the other was an amorphous layer. The thickness of the interface layers depends on the deposition conditions of the Mo layers.

Enhanced electrical conductivity of zinc oxide thin films by ion implantation of gallium, aluminum, and boron atoms

Effect of ion implantation on the conductivity of zinc oxide was examined by using highly resistive zinc oxide thin films deposited by rf magnetron sputtering at room temperature to reduce the effect of oxygen vacancies. With the doping by 1×1017 atoms/cm2 gallium the conductivity is 1.0×103/Ω cm for as‐implanted film and it increases up to 3.7×103/Ω cm, the highest conductivity reported for zinc oxide films, with raising the annealing temperature in either a nitrogen or oxygen atmosphere. The conductivity of aluminum‐doped films is slightly lower than those of gallium‐doped films. Among the elements gallium, aluminum, and boron, gallium is the most effective in enhancing the conductivity and boron is the least. The order of the effectiveness is explained by the electronegativity of the dopants.

Surface Characterization of Chemically Treated Cu(In, Ga)Se2 Thin Films

The surfaces of Cu-rich and stoichiometric (slightly [In, Ga]-rich) Cu(In, Ga)Se2 (CIGS) thin films were investigated by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). At the surface of the Cu-rich and stoichiometric CIGS films, Cu2-x Se and Cu(In, Ga)3Se5 exist, respectively. The films were treated using KCN and NH3 aqueous solutions. In the Cu-rich film, the treatment in the KCN solution completely eliminated the Cu2-x Se impurity and the treatment in the NH3 solution removed Cu2-x Se only at the front surface. In the stoichiometric CIGS film, the NH3 treatment removed Cu(In, Ga)3Se5 from the surface. The recombination of the carriers occurs more in the heterojunction of the CdS/NH3-treated CIGS system than in that of the CdS/as-deposited CIGS system.

Composition monitoring method in CuInSe2 thin film preparation

Abstract The real-time monitoring method of the Cu/In ratio in the bilayer deposition process of CuInSe2 thin film was originally established and demonstrated by applying the bilayer process. The principles of this method were based on the idea that the hole concentration of the film was abruptly changed at the stoichiometric composition while the film composition gradually changed from Cu rich to In rich through the stoichiometric composition during the deposition process. This method was very useful for the composition control of the CuInSe2 film.

Preparation of Ordered Vacancy Chalcopyrite-Type CuIn3Se5 Thin Films

Polycrystalline CuIn3Se5 films were successfully prepared by three-source coevaporation with controlling and shielding of the molecular beams from elemental sources. The CuIn3Se5 film exhibited good chemical homogeneity and an ordered vacancy chalcopyrite-type structure with lattice constants of a=5.742 A and c=11.486 A. High-resolution transmission electron microscopy showed that the CuIn3Se5 film had a columnar microstructure and each grain contained a high density of twins in {112} planes. The band-gap energy of the film was determined to be 1.23 eV from optical transmission measurements. The film showed n-type conduction and low conductivity of 3.7×10-7/ Ωcm. These characteristics of the CuIn3Se5 film are compared with those of the chalcopyrite-type CuInSe2 film.

Preparation of CuInS2 films by sulfurization of Cu‐In‐O films

CuInS2 thin films are prepared by using a newly developed two‐stage process comprised of a first process by which Cu‐In‐O films are prepared from a Cu2In2O5 target by a pulsed laser deposition, and a second process by which the prepared Cu‐In‐O films are transformed into CuInS2 films by applying an annealing in a H2S gas. The characteristics of thus obtained CuInS2 films are determined by using an x‐ray diffractometer, an energy dispersive x‐ray spectrometer, and a scanning electron microscope, in addition to a spectrophotometer. The CuInS2 film with chalcopyrite‐type structure is obtained when it is annealed at a temperature higher than 400 °C. The effect of annealing temperature on its structural and optical properties is being analyzed.

Preparation and Characterization of CuInSe2 Thin Films by Molecular-Beam Deposition Method

Polycrystalline CuInSe2 films were prepared by coevaporation of the elements under an ultrahigh vacuum by a molecular-beam deposition method. The composition of the film was controlled by changing the In molecular-beam flux intensity while the other elements remained at a constant value. It is shown, at the substrate temperature of 500°C, that there is a critical In molecular-beam flux intensity for the fabrication of stoichiometric films. At the In molecular-beam intensities higher than the critical value, single-phase CuInSe2 films with nearly constant compositions are obtained as a result of the removal effects of excess In. It is shown that the present coevaporation process is suitable for the fabrication of stoichiometric or slightly In-rich composition films. Furthermore, the structural and electrical properties of the films were investigated and discussed in relation to film composition.