Shogo Ishizuka

Na-induced variations in the structural, optical, and electrical properties of Cu(In,Ga)Se2 thin films

The systematic variations in the structural, optical, and electrical properties of polycrystalline Cu(In,Ga)Se2 (CIGS) thin films with Na doping level were investigated. Precise control of the Na concentration in CIGS films was demonstrated using alkali-silicate glass thin layers of various thicknesses deposited on substrates prior to CIGS growth. The CIGS grain size was observed to decrease with increasing Na concentration, although the surface morphology became smoother and exhibited a stronger (112) texture, which has been demonstrated consequence of larger grain size. The Ga composition gradient in the CIGS films was found to become large due to the presence of Na during growth, which in turn led to a decrease in the nominal band gap energy. Variations in the photoluminescence spectra and electrical properties suggested that the formation of an acceptor energy state, which may originate from OSe point defects, was enhanced in the presence of Na. This result suggests that not only Na, but also the pres...

Nitrogen Doping into Cu2O Thin Films Deposited by Reactive Radio-Frequency Magnetron Sputtering

The effects of nitrogen doping into Cu2O thin films deposited by reactive radio-frequency magnetron sputtering were studied. It was found that nitrogen is an effective p-type dopant for Cu2O and the hole density can be controlled from 1×1015 cm-3 to approximately 1017 cm-3. The acceptor level of nitrogen was estimated to be about 0.14 eV by temperature-dependent Hall effect measurements and this value roughly agrees with that obtained by the effective mass theory. No significant degradation of structural and optical properties induced by nitrogen doping were observed. The resistivity of 15.2 Ωcm was obtained for a relatively high nitrogen flow rate, which is the lowest value reported to date for Cu2O thin films.

Thin-Film Deposition of Cu2O by Reactive Radio-Frequency Magnetron Sputtering

Deposition conditions of cuprous oxide (Cu2O) thin films on glass substrates by reactive radio-frequency (rf) magnetron sputtering method were studied. The substrate temperature was found to be important for obtaining high-quality films, and the optimum substrate temperature was about 500°C. The Cu2O deposited at 500°C shows a band-gap energy of about 2.0 eV and a typical hole concentration of the order of 1015 cm-3 with a Hall mobility of 60 cm2/Vs, which is the highest mobility reported thus far.

Alkali incorporation control in Cu(In,Ga)Se2 thin films using silicate thin layers and applications in enhancing flexible solar cell efficiency

Control of the alkali doping level in Cu(In,Ga)Se2 (CIGS) films was demonstrated using alkali-silicate thin layers of various thickness deposited on substrates prior to the sputtering of the Mo back contact layer. Not only the alkali density in the CIGS film, but also the Ga composition distribution in CIGS films, CIGS grain size, and consequent photovoltaic performance showed variations with the silicate layer thickness. Using alkali-silicate thin layers as an alkali source material, 17.4% and 17.7% efficiency flexible CIGS solar cells have been demonstrated on Ti and zirconia substrates, respectively.

Efficiency Enhancement of Cu(In,Ga)Se2 Solar Cells Fabricated on Flexible Polyimide Substrates using Alkali-Silicate Glass Thin Layers

Cu(In,Ga)Se2 (CIGS) absorber layers were grown at a maximum substrate temperature of 400 °C on polyimide (PI) films. The PI films were formed by spin-coating on glass substrates. Alkali doping into the CIGS layers was demonstrated using alkali-silicate glass thin layers (hereafter called ASTL) deposited on PI films prior to the sputtering of the Mo back contact layer. The quantum efficiency curves of CIGS solar cells fabricated with use of ASTL showed an enhanced absorption in the long wavelength region. Using the ASTL method, the cell efficiency of 14.7% has been demonstrated on PI substrate.

Flexible Cu(In,Ga)Se2 solar cells fabricated using alkali-silicate glass thin layers as an alkali source material

Flexible Cu(In,Ga)Se2 (CIGS) solar cells were fabricated using alkali-silicate glass thin layers (ASTL) as an alkali source material deposited on various flexible substrates prior to the Mo backcontact layer deposition. Control of univalent alkali metal incorporation into the CIGS layer with the use of ASTL was demonstrated regardless of the presence of other multivalent metal impurities in ASTL. Dramatic enhancement in cell efficiencies using ASTL was observed though the use of excessively thick ASTL led to a degradation in cell performance. The quantum efficiency curves of CIGS solar cells fabricated using ASTL showed enhanced absorption in the long wavelength region. The photovoltaic performance dependence on a variety of substrate materials is also discussed.

Growth of polycrystalline Cu(In,Ga)Se2 thin films using a radio frequency-cracked Se-radical beam source and application for photovoltaic devices

Cu(In,Ga)Se2 (CIGS) thin films were grown using a rf-cracked Se-radical beam source. A unique combination of film properties, a highly dense and smooth surface with large grain size, is shown. These features seem to have no significant influence on the photovoltaic performance. Defect control in bulk CIGS leading to corresponding variations in the electrical and photoluminescence properties was found to be possible by regulating the Se-radical source parameters. A competitive energy conversion efficiency of 17.5%, comparable to that of a Se-evaporative source grown CIGS device, has been demonstrated from a solar cell fabricated using a Se-radical source grown CIGS absorber.

Effect of band offset on the open circuit voltage of heterojunction CuIn1−xGaxSe2 solar cells

The reasons behind why the theoretically estimated open circuit voltage (Voc) of CuIn1−xGaxSe2 solar cells with large x values has not been realized are discussed. Typically, the reduction in Voc is estimated only on the basis of the conduction-band offset between the absorber and the window material. The importance of the electron affinity difference between the window and the transparent electrode must also be taken into account. Based upon both of these factors, a material selection guideline is reported for the window and the transparent electrode layers suitable for high-x CuIn1−xGaxSe2 absorber-based solar cells.

Excitation-Power Dependence of Free Exciton Photoluminescence of Semiconductors

We have derived an analytical formula for the excitation-power dependence of the free exciton photoluminescence (PL) intensity. It has been found that the PL intensity I depends on the power of the excitation laser L as I∝Lk, where k is the power index. We have deduced the analytical formula that describes the value of k for the free exciton PL emission under the above-band-gap excitation conditions. The results indicate that the value of k is in the region of 1<k<2 depending on both the value of L and material properties such as radiative and competitive nonradiative recombination probabilities.

Properties of Cu(In,Ga)Se2:Fe Thin Films For Solar Cells

We have fabricated CIGS:Fe polycrystalline thin films using a standard three-stage method, and investigated the effects of Fe doping on cell performances. The Ga / (In+Ga) ratio was varied between 0.3 ˜ 1.0 (= CGS), and the Fe concentration was varied between 0.0 ˜ 1.2 mol%. The films were characterized by various means, including the cell performance. Increment of the grain size with higher Fe content was observed. Redshift with higher Fe content was observed in the absorbance spectra. The spectral response of the fabricated solar cells deteriorated with higher Fe content, from the long wavelength side.