Tooru Tanaka

(Cd,Zn)S thin films prepared by chemical bath deposition for photovoltaic devices

Abstract The preparation and characterization of (Cd,Zn)S thin films were attempted using two kinds of chemical bath deposition process: all the reactants were mixed either at room temperature (process A) or after preheating the source solutions (process B). The band gaps and the lattice constants of the thin films prepared by process B changed from the values for CdS to those for ZnS, but those of the films prepared by process A were not changed. X-ray diffraction analyses showed that all the peaks from the thin films prepared by process B could be assigned to diffraction lines of wurtzite (Cd,Zn)S. The photoconductivity of the thin films was larger than the dark conductivity, in particular 5×10 3 times larger in the thin films prepared by process B with [Zn]/([Cd] + [Zn]) = 0.9 in the solution.

Preparation and characterization of (Cd,Zn)S thin films by chemical bath deposition for photovoltaic devices

Abstract Thin films of (Cd,Zn)S were prepared by the improved chemical bath deposition process which was optimized with the amounts of ammonia playing an important role, and their structural, electrical and optical properties were investigated for photovoltaic device applications. X-ray diffraction peaks for the deposited thin films were assigned to diffraction lines from wurtzite (Cd,Zn)S. The resistivity of (Cd,Zn)S thin films was in the range of 4–8 × 107 Ω cm. The [Zn]/([Cd] + [Zn]) ratio and the band gaps in the thin films could be controlled by varying the mixture ratio of CdI2 and ZnI2 solutions used as reactants. The variation of band gaps yielded the bowing parameter of 0.724 eV.

Fabrication and characterization of CuIn(SxSe1-x)2 thin films deposited by r.f. sputtering

Abstract Thin films of CuIn(SxSe1−x)2 were prepared by r.f. sputtering from powder targets which were previously mixed from powdered binary compounds. X-ray diffraction analyses showed the films had the single phase chalcopyrite structure, and the lattice parameters (a and c) varied linearly with the increase in sulphur content x from x=0 (CuInSe2) to x=1 (CuInS2). The orientation to the (112) plane of the CuIn(SxSe1−x)2 thin films increases as the sulphur content x increases. From scanning electron microscope images, the grain sizes in the films were found to decrease with the decrease in sulphur content. Optical transmittance measurements showed that bowing behaviour is not, observed in the films.

Characterization of CuInS2 thin films prepared by sputtering from binary compounds

For fabricating more inexpensive thin film solar cells, CuInS2 thin films were prepared by RF sputtering from binary compounds (Cu2S and In2S3). From EPMA analyses, composition of the thin films was varied with changing the mixing ratio ‘x’ (x = [Cu2S][In2S3]). X-ray diffraction studies showed that the thin film sputtered from the target with the mixing ratio of x = 1.5 had a single phase with chalcopyrite structure. In this case, CuInS2 thin films showed higher optical absorption coefficients and a band gap of 1.52 eV, suitable for absorbing incident solar spectrum.

PREPARATION OF CU(IN,GA)2SE3.5 THIN FILMS BY RADIO FREQUENCY SPUTTERING FROM STOICHIOMETRIC CU(IN,GA)SE2 AND NA2SE MIXTURE TARGET

Defect chalcopyrite thin films of Cu(In,Ga)2Se3.5 were prepared by rf sputtering from stoichiometric CuInxGa1−xSe2 (x=0.6) and Na mixture target. The composition of the thin films fabricated in the ratio of [Na]/[Cu(In,Ga)Se2] above 5% was changed from the stoichiometric composition of Cu(In,Ga)Se2 to Cu-poor one, and identified as Cu:(In+Ga):Se=1:2:3.5. From the results of x-ray diffraction, the lattice parameters of these thin films were slightly smaller than that of Cu(In,Ga)Se2 and, besides the peaks appearing for chalcopyrite structure Cu(In,Ga)Se2, the additional peak was observed. The optical band gap is increased from 1.24 to 1.36 eV with increasing the [Na]/[Cu(In,Ga)Se2] ratio from 0% to 10% in the target. These films showed n- or p-type conduction.

Characterization of Cu(InxGa1−x)2Se3.5 thin films prepared by rf sputtering

Abstract Cu(InxGa1−x)2Se3.5 thin films were fabricated by rf sputtering from CuInxGa1−xSe2 and Na mixture target by controlling the mixture ratio. X-ray diffraction analyses show that the structure of Cu(InxGa1−x)2Se3.5, thin films is different from chalcopyrite structure: especially, CuIn2Se3.5 thin films have a defect chalcopyrite structure. The lattice parameters for Cu(InxGa1−x)2Se3.5 thin film are slightly smaller than those for CuInxGa1−xSe2 thin film and linearly decreased with increasing Ga content. The optical absorption coefficients for Cu(InxGa1−x)2Se3.5, thin films exceed 2 × 104 cm−1 in energy region above the fundamental band edge. The band gap for Cu(InxGa1−x)2Se3.5 thin films is larger than that for CuIn.Ga1−x2Se2 with the same Ga content and increased with increasing Ga content.

Effect of Mg ion implantation on electrical properties of CuInSe2 thin films

The effects of Mg ion implantation on the electrical properties of CuInSe2 epitaxial thin films have been investigated. The implantation was carried out using the multienergy implantation technique to obtain a constant profile of the Mg concentration along the depth direction. After implantation, the layer was annealed at 400u200a°C in N2 atmosphere for 60 min. From the results of reflection high-energy electron diffraction, it was confirmed that the damages due to ion implantation were removed by the thermal annealing. The conductivity type in all implanted films was n type, and the carrier concentration was increased with increasing Mg concentration in the films. Consequently, it is concluded that the Mg atom acts as a donor in CuInSe2.

Influence of annealing temperature on the properties of Cu(In,Ga)Se2 thin films by thermal crystallization in Se vapor

Abstract Thermal crystallization of amorphous - like Cu(In,Ga)Se 2 precursors was attempted in a Se atmosphere in the temperature range from 200°C to 600°C. Thin films crystallized over 400°C had a single phase Cu(In,Ga)Se 2 with chalcopyrite structure, while those crystallized at less than 300°C were composed of CuSe 2 and Cu 7 Se 4 . Grain size in Cu(In,Ga)Se 2 thin films increased with increasing annealing temperature, specifically 2–3 gm in Cu(In,Ga)Se 2 ([Ga]/([In] + [Ga]) = 0.66) thin films crystallized at 600°C.

Large Grain Growth in Cu(In, Ga)Se 2 Thin Films with Band Gap of around 1.4 eV by Thermal Crystallization in Saturated Se Vapors

Thin films of Cu(In, Ga)Se2 were prepared by thermal crystallization in saturated Se vapors from a precursor which was evaporated from Cu(In, Ga)Se2 compound without substrate heating. These thin films had a single phase with chalcopyrite structure. By scanning electron microscopy, large grain growth in Cu(In, Ga)Se2 thin films crystallized in saturated Se vapors was observed in comparison with that in samples prepared without Se shots. Grain sizes of more than 1 µm were obtained with uniform distribution in Cu(In, Ga)Se2 thin films crystallized in saturated Se vapors. The Hall mobility was increased and a band gap of around 1.4 eV was obtained for Cu(In, Ga)Se2 thin films crystallized in saturated Se vapors. This value of band gap lies in the optimum range for solar energy conversion theoretically.

Effect of substrate temperature on properties of thin films prepared by RF sputtering from CuInSe2 target with Na2Se

We fabricated the thin films by RF sputtering from Na-mixed CuInSe 2 target, and investigated the effect of the substrate temperature on the formation of CuIn 3 Se 5 thin films. From electron probe microanalysis (EPMA) measurements, the copper content was found to decrease with increasing the substrate temperature, and the band gap of the thin films became large, suggesting the formation of CuIn 3 Se 5 structure. The reduction of copper content was presumed to be due to the surface reaction during the growth. Based on the results, one of the growth models for CuIn 3 Se 5 is proposed.