Takayuki Negami

Preparation of Zn1-xMgxO films by radio frequency magnetron sputtering

Abstract II–VI widegap semiconductors such as ZnO are widely utilized in various electronic and optical devices. To widen the band gap of ZnO, we conducted a systematic investigation of solid solution thin films of Zn 1− x Mg x O, a group of ternary compounds of the ZnMgO system. We prepared the thin films by radio frequency (RF) magnetron co-sputtering on fused silica substrates at room temperature. The thin films were a single phase of Zn 1− x Mg x O having the basic structure of ZnO at x ≤0.46 and the basic structure of MgO at x ≥0.62, with segregation of the ZnO and MgO phases at x =0.58. The band gap of Zn 1− x Mg x O having the basic structure of ZnO increased from 3.24 eV at x =0 (ZnO) to 4.20 eV at x =0.46. Transmittances of Zn 1− x Mg x O thin films were nearly equivalent to those of ZnO. Zn 1− x Mg x O has a wider band gap than ZnO and can be expected to provide a useful window layer of solar cells that improves the overall efficiency by decreasing the absorption loss.

Characterization of the Cu(In,Ga)Se2/Mo interface in CIGS solar cells

Abstract In a high efficiency CIGS solar cell, we observed a MoSe2 layer at the interface by SIMS, TEM and X-ray diffraction. MoSe2 had a layer structure and the layers were oriented perpendicular to the Mo layer. The MoSe2 contributes to the improvement of adhesion at the CIGS/Mo interface. The effects of the MoSe2 layer on the electrical and photovoltaic properties of CIGS solar cells were investigated. The CIGS/Mo heterocontact, including the MoSe2 layer, is not Schottky-type, but a favorable ohmic-type by the evaluation of dark I–V measurement at low temperature. A characteristic peak at 870 nm is observed in the differential quantum efficiency of a solar cell with a CIGS thickness of 500 nm. This peak relates to the absorption of the MoSe2 layer. The band gap of MoSe2 is calculated to be 1.41 eV from the absorption peak.

Cu(In,Ga)Se2 solar cells with controlled conduction band offset of window/Cu(In,Ga)Se2 layers

Our group studied the effects of conduction band offset of window/Cu(In,Ga)Se2 (CIGS) layers on CIGS-based solar cell performance. To control the conduction band offset, we considered the use of a window layer of Zn1−xMgxO thin film with a controllable band gap as an alternative to the conventional window layer using CdS film. From the measurement of valence band offset between Zn1−xMgxO/CIGS layers and the band gap of each layer, we confirmed that the conduction band offset of Zn1−xMgxO/CIGS layers could be controlled by changing the Mg content of the Zn1−xMgxO film. The CIGS-based solar cells prepared for this study consisted of an ITO/Zn1−xMgxO/CIGS/Mo/soda-lime glass structure. When the conduction band minimum of Zn1−xMgxO was higher than that of CIGS, the performance of CIGS-based solar cells with a Zn1−xMgxO window layer was equivalent to that of CIGS-based solar cells with CdS window layers. We confirmed that the control of the conduction band offset of the window/CIGS layers decreases the majority...

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.

Cu(In,Ga)Se2 thin-film solar cells with an efficiency of 18%

Abstract An efficiency of over 18% have been achieved in Cu(In,Ga)Se 2 (CIGS) thin-film solar cells. Solar cell parameters were estimated for the cells with efficiencies of more and less than 18%. A diode quality factor n and forward current (saturated current) J 0 of the cell with over 18% efficiency are lower than those with below 18% efficiency. This would be attributed to sufficient coverage of the CdS film with excellent uniformity as a buffer and/or window layer over the CIGS film because the process of CdS film formation was improved.

Chemical bath deposition of Cds buffer layer for GIGS solar cells

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. We investigated the chemical bath deposition of CdS thin flims on the Cu(In,Ga)Se 2 (CIGS) absorber layers and glasses. The process of the chemical bath deposition of CdS layer affected the performance of the CIGS solar cells. The CdS layers were deposited on the CIGS film from CdI 2 , thiourea (NH 2 CSNH 2 ) and ammonia solutions. The influence of pH on the chemical bath deposition process was studied. The surfaces of the CdS films were observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The compositions of the obtained CdS layers were analyzed by Auger electron spectroscopy (AES). The performance of the CIGS solar cells was discussed on the basis of the characteristics of the chemical bath deposited layer. We have successfully fabricated a high-efficiency CIGS solar cell with an efficiency of 17% using a CdS layer with stoichiometric composition.

Control of conduction band offset in wide-gap Cu(In,Ga)Se2 solar cells

The effects of conduction band offset of window/Cu(In,Ga)Se 2 (CIGS) layers in wide-gap CIGS based solar cells are investigated. In order to control the conduction band offset, a Zn 1 - x Mg x O film was utilized as the window layer. We fabricated CIGS solar cells consisting of an ITO/Zn 1 - x Mg x O/CdS/CIGS/Mo/glass structure with various CIGS band gaps (Eg 0.97-1.43 eV). The solar cells with CIGS band gaps wider than 1.15 eV showed higher open circuit voltages and fill factors than those of conventional ZnO/CdS/CIGS solar cells. The improvement is attributed to the reduction of the CdS/CIGS interface recombination, and it is also supported by the theoretical analysis using device simulation.

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.

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.