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Presents a study which investigated the local structure of CuInSe[sub2] thin films by extended x-ray absorption fine structure on the copper K and selenium K edges using synchroton radiation. Experimental details; Results and discussion; Conclusion.

Abstract: We have proposed a novel laser-assisted-deposition (LAD) process for improving the crystalline quality of CIGS thin films and cell performance. The influences of laser power, Ga content in CIGS, substrate temperature, and photon energy of laser on CIGS thin films and solar cells have been investigated. In the LAD process a pulsed excimer laser and a pulsed Nd:YAG laser were irradiated onto the substrate surface during CIGS deposition by the three-stage process. The crystalline quality of CIGS thin films and cell performance, particularly open-circuit-voltage, improved by LAD process for all ranges of Ga content and at substrate temperature ranges of 400–550°C. It was also found that the laser irradiation enhanced the diffusion of Ga into CIGS even at low substrate temperatures, which strongly affects the formation of double-graded bandgap. The PL decay time of LAD–CIGS solar cells was much longer than that of the fabricated by the three-stage process, which implies the reduced defects in CIGS absorber layer. The improved thin-film quality and cell performance became noticeable only when the laser wavelength was shorter than 266nm (4.66eV of photon energy). This result strongly suggests that the impacts of pulsed-laser irradiation are dominated by photon-energy rather than thermal-energy. [Copyright &y& Elsevier]

Abstract: Photoluminescence (PL) and PL decay characteristics of the near-band-edge (NBE) PL at room temperature have been studied on the Cu(In,Ga)Se2 (CIGS) solar cells. The carrier recombination process has been discussed with emphasis on the photovoltaic properties of the solar cell. It has been found that: (i) PL intensity of the CIGS solar cells is much stronger than that in the corresponding CIGS thin films, (ii) the PL decay time of the cell is longer than that of the CIGS film, and (iii) the PL decay time of the CIGS solar cell exhibits strong dependence on the PL excitation intensity. In the CIGS solar cell, intense PL is obtained under the open circuit condition (oc), in contrast to the very low PL yield under the short circuit (sc) condition. The PL decay time under the sc condition is much shorter than that under the oc condition. Excitation intensity dependence of PL intensity and the PL decay time have been studied, and they are discussed with relation to the photo-voltage due to the PL excitation light. PL and injection EL under the external DC bias have been studied. The mapping image of NBE-PL intensity has been compared with that of the laser beam induced current (LBIC), and the PL intensity image reflects the photovoltaic properties of the CIGS solar cells. We demonstrated that NBE-PL of the CIGS solar cell reflects the photovoltaic effect, and it can be utilized as a powerful characterization method. [Copyright &y& Elsevier]

Abstract: Room temperature time-resolved photoluminescence (TR-PL) measurements have been performed on Cu(In,Ga)Se2 (CIGS) thin films and solar cells to clarify the recombination process of the photo-generated minority carrier. Both films and solar cells exhibited PL decay curves composed of the dominant fast (0.7–2 ns) and weak slow (3–10 ns) exponential decay curves. PL lifetime of the cell is longer than that of the thin films, indicating the longer minority carrier lifetime for the hetero-structures than in thin films. The increase of PL lifetime is consistent with the enhancement of the PL intensity and the elimination of defect-related PL as a result of the solar cell formation. These results are discussed in terms of the recombination process of carriers in films and hetero-structures. The relationship between the PL lifetime of the CIGS solar cells and the cell conversion efficiency is described. [Copyright &y& Elsevier]

The Eu-doped GaN thin films (GaN:Eu) were deposited on the sapphire substrate by means of the rf magnetron sputtering method. The GaN:Eu films were characterized by x-ray diffraction, scanning electron microscope, and photoluminescence (PL). Although as-grown GaN:Eu films (2 mol % Eu in target) exhibited weak broad Eu-related red PL, the annealing in NH3 atmosphere (1000°C for 1 h) led to the remarkable enhancement of Eu-related PL lines. PL spectra of such annealed GaN:Eu films exhibited a series of sharp PL lines characteristic of substitutional Eu3+ ions at Ga sites in the GaN lattice (dominant PL line is at 622 nm for the 5D0-7F2 transition). On the other hand for GaN:Eu films (5 mol % Eu in target), as-grown films are amorphous and exhibited broad Eu3+-related PL lines. The annealing led to the crystallization of the films, the elimination of Eu3+-related PL lines, and the enhancement of the near-band-edge PL. The Eu-luminescent centers are considered to be removed during the crystallization. [ABSTRACT FROM AUTHOR]

An attempt is made to dope Zn impurity selectively at the In site in Cu(In,Ga)Se2 (CIGS) by supplying Zn at the first stage of a three‐stage method. Increase in carrier concentration in p‐type CIGS leads to increase in an open circuit voltage and a conversion efficiency (η). The Zn impurity loaded in (In,Ga)2Se3 a the 1st stage is expected to make a substitutional ZnIn acceptor in CIGS, which leads to increase in carrier concentration. The Zn doping profile at the 1st stage is expected to be reflected in the acceptor (ZnIn) depth profile. Acceptor in‐depth profile is expected to be changed by the Zn flux. The room‐temperature PL spectra of the CIGS:Zn exhibited peak shift to higher energy by 10–50 meV depending on the Zn‐doping condition due to the change of band gap energy. The carrier in‐depth profile is characterized using a capacitance‐voltage method for ZnO/CdS/CIGS:Zn solar cells. Carrier concentration of Zn‐loaded samples increase by one order of magnitude as compared with that in the intentionally undoped CIGS samples. The relationship between the Zn‐doping profile and solar cell characteristics is examined. Results are also discussed in terms of the back surface field created by the Zn‐doping profile. [ABSTRACT FROM AUTHOR]

Abstract: Structural, optical and electrical properties of polycrystalline Cu–In–Se films, such as CuInSe2 and ordered vacancy compounds (OVC), prepared by three-stage process of sequential chemical spray pyrolysis (CSP) of In–Se (first stage), Cu–Se (second stage) and In–Se (third stage) solutions have been studied in terms of substrate temperature at the second stage (T S2). The films grown at T S2⩽420°C exhibited larger grains in comparison with the Cu–In–Se films grown by the usual CSP method. Optical gap energy was approximately 1.06eV for 360°C⩽T S2⩽420°C, but increased dramatically from 1.06 to 1.35eV when the T S2 rose from 420 to 500°C. Conductivity type was p-type for T S2<420°C, but n-type for T S2>420°C. [Copyright &y& Elsevier]

Abstract: The Eu-doped GaN thin films (GaN:Eu) were deposited on the c plane of sapphire substrate by the radio frequency magnetron sputtering method. As a target, the GaN compound powder target was used, which contains small amount of EuN powder of 1, 2 and 5mol%. X-ray diffraction (XRD) and high resolution photoluminescence spectra (PL) of GaN:Eu films were studied with respect to the substrate temperature (300–550°C). PL spectra exhibited a series of sharp PL lines related to Eu3+ center with intense 622nm line (dominant PL line for the 5D0–7F2 transition). The PL intensity of the 622nm line tends to be maximum for 2mol% EuN in target. The PL intensity tends to be maximum at the substrate temperature of 400–450°C. This dependence indicated the existence of the optimum temperature at which the formation of the luminescent Eu3+ center is promoted. PL intensity was strongly enhanced by (i) deposition of GaN:Eu on the undoped buffer GaN layers grown on the sapphire substrate and (ii) increase of film thickness. [Copyright &y& Elsevier]

Abstract: Structural and optical properties of In-rich Cu–In–Se polycrystalline thin films (0.54