Akio Kunioka

Direct evidence of Cd diffusion into Cu(In, Ga)Se2 thin films during chemical-bath deposition process of CdS films

The diffusion behavior at the Cu(In, Ga)Se2 (CIGS)/CdS interface of high efficiency CIGS thin film solar cells has been investigated using energy dispersive x-ray spectroscopy (EDX) and transmission electron microscopy. CdS layers were deposited on CIGS thin films using the chemical bath deposition (CBD) process. EDX analysis revealed that Cd was present in the CIGS layer approximately 100 A from the interface boundary. In contrast to the diffusion of Cd, the Cu concentration decreased near the surface of the CIGS film, suggesting substitution of Cd for Cu atoms. These results are direct evidence of Cd diffusion into CIGS thin films during the CBD process.

High-efficiency Cu(In,Ga)Se2 thin-film solar cells with a CBD-ZnS buffer layer

High-efficiency cadmium-free Cu(In,Ga)Se2 (CIGS) thin-film solar cells have been fabricated using a chemical bath deposition (CBD)-ZnS buffer layer, a wider band gap material than of conventional CBD-CdS. Energy dispersive X-ray microanalysis (EDX) revealed Zn interdiffusion in the CIGS thin film at the CBD-ZnS/CIGS solar cell interface, implying formation of a buried np junction at the surface of the CIGS film. The best cell to date yielded an active area efficiency of 17.2% after light soaking. This result suggests that CIGS solar cells with efficiencies as high as those fabricated using CdS buffer can be achieved even if Cd is not utilized.

Effects of Sodium on Cu(In, Ga)Se2-Based Thin Films and Solar Cells

Cu(In, Ga)Se2 (CIGS) films were deposited with intentionally incorporated Na2Se by a multi-step process onto SiOx /soda-lime glass substrates at elevated temperatures. The effects of sodium on film properties were investigated using X-ray diffraction, X-ray photoelectron spectroscopy and Hall effect measurements. By the addition of Na2Se, the hole concentration of the CIGS-based films (Ga/(In+Ga) ratio=0.37–0.39) increased dramatically to the 1016–1017cm-3 range for a wide range of Cu/(In+Ga) ratios from 0.4 to 0.8. The increased hole concentration resulted in improved CIGS-based solar cells with efficiencies of 10–13.5% over an extremely wide Cu/(In+Ga) ratio range of 0.51–0.96. P-type Cu(In, Ga)3Se5 phase films with hole concentrations high enough to be used as absorber layers of photovoltaic devices were obtained for the first time by the Na control technique. The possibility of a new type of solar cell with a ZnO:Al/buffer/ p-Cu(In, Ga)3Se5 heterojunction structure is suggested.

High-efficiency cadmium-free Cu(In,Ga)Se/sub 2/ thin-film solar cells with chemically deposited ZnS buffer layers

Cadmium-free Cu(In,Ga)Se/sub 2/ (CICS) thin-film solar cells with a MgF/sub 2//ZnO:Al/CBD-ZnS/CIGS/Mo/SLG structure have been fabricated using chemical bath deposition (CBD)-ZnS buffer layers and high-quality CICS absorber layers grown using molecular beam epitaxy (MBE) system. The use of CBD-ZnS, which is a wider band gap material than CBD-CdS, improved the quantum efficiency of fabricated cells at short wavelengths, leading to an increase in the short-circuit current. The best cell at present yielded an active area efficiency of 16.9% which is the highest value reported previously for Cd-free CIGS thin-film solar cells. The as-fabricated solar cells exhibited a reversible light-soaking effect under AM 1.5, 100 mW/cm/sup 2/ illumination. This paper also presents a discussion of the issues relating to the use of the CBD-ZnS buffer material for improving device performance.

Effect of water vapor on the growth of textured ZnO-based films for solar cells by DC-magnetron sputtering

Large-grain zinc oxide (ZnO) films have been deposited on glass substrates by DC magnetron sputtering in H2O-Ar mixtures at elevated substrate temperatures. The (002)-oriented columnar structure grown in pure Ar changed into the granular structure as the partial pressure of water vapor increased. Low-resistivity pyramidal textured ZnO:Al films were sputter-deposited in a mixture of H2O-Ar(1:1) at 300-400°C. The 2-µm-thick films possessed a sheet resistance of 9 Ω/ and optical transmission of 80% at 550 nm after annealing in a vacuum. These films have considerable potential as window materials for solar cells.

Improved Cu(In,Ga)(S,Se)2 thin film solar cells by surface sulfurization

Surface sulfurization was developed as a technique for fabricating efficient ZnO : Al/CdS/graded Cu(In,Ga)(S,Se)2/ Mo/glass solar cells. Prior to the sulfurization, single-graded Cu(In,Ga)Se2 (CIGS) films were deposited by a multi-stage process. The sulfurization of CIGS films was carried out using a H2SAr mixture at elevated temperatures. The crystallographic and compositional properties of the absorber layers were investigated by XRD, SEM and AES analyses. After sulfurization, sulfur atoms were substituted for selenium atoms at the surface layer of CIGS films to form a Cu(In,Ga)(S,Se)2 absorber layer. The diffusion of sulfur depends strongly on the grain structure of CIGS film. The cell efficiency of the 8–11% range before sulfurization was improved dramatically to 14.3% with Voc = 528 mV, Jsc = 39.9 mA/cm2 and FF = 0.68 after the sulfurization process.

Electrical and Optical Properties of Sputtered CdO Films

CdO films were prepared by a D–C reactive sputtering method with various sputtering voltages. The characters of the prepared films are compared with the reflection electron diffraction and X-ray diffraction patterns. Especially, the effects of the sputtering voltage and partial pressure of oxygen were investigated concerning the electron densities and the Hall mobilities. The observations on lattice parameters lead to the conclusion that CdO films with high carrier concentrations contain some amount of interstitial excess Cd atoms. The absorption edge variation of annealed films prepared by high voltage sputtering agrees with the theoretical curve which is accounted from the Fermi level shift caused by the change in carrier concentrations.

Metal‐insulator semiconductor Schottky‐barrier solar cells fabricated on InP

High‐efficiency Schottky‐barrier solar cells with a metal‐insulator semiconductor structure have been fabricated on p‐type InP. The open‐circuit voltage and the energy conversion efficiency can be increased by introducing a thin oxide layer between the metal and the semiconductor. The open‐circuit voltage and the energy conversion efficiency are 0.78 V and 14.5% under AM2 conditions.

Improved compositional flexibility of Cu(In,Ga)Se2-based thin film solar cells by sodium control technique

The effects of sodium on off-stoichiometric Cu(In,Ga)Se2 (CIGS)-based thin films and solar cells were investigated. The CIGS-based films were deposited with intentionally incorporated Na2Se on Mo-coated SiOx/soda-lime glass substrates by a multi-step process. By sodium control technique high-efficiency ZnO : Al/CdS/CIGS solar cells with efficiencies of 10–13.5% range were obtained over an extremely wide Cu(In + Ga) ratio range of 0.51–0.96, which has great merit for the large-area manufacturing process. The improved efficiency in the off-stoichiometric regions is mainly attributed to the increased acceptor concentration and the formation of the Cu(In,Ga)3Se5 phase films with p-type conductvity. A new type of solar cell with p-type Cu(In,Ga)3Se5 phase absorber materials is also suggested.

InP metal‐insulated‐semiconductor Schottky contacts using surface oxide layers prepared with bromine water

Oxide layers prepared with bromine water are found to increase the barrier height for Au‐InP Schottky diodes. Electrical characteristics are measured and the relationships between the preparation conditions and the diode parameters are examined. The typical value of the barrier height and the ideality factor are 0.83 eV and 1.1, respectively. Some instabilities are observed in the diode characteristics.