Jiro Nishinaga

Effects of long-term heat-light soaking on Cu(In,Ga)Se2 solar cells with KF postdeposition treatment

The effects of long-term heat-light soaking on Cu(In,Ga)Se2 (CIGS) solar cells with KF postdeposition treatment (KF-PDT) have been investigated. CIGS solar cells with KF-PDT frequently deteriorate after storage in the dark because of decreasing hole concentrations in the CIGS layers. Although light soaking improves hole concentrations, the resulting increase in conversion efficiency is not significant. In contrast, we found that long-term heat-light soaking effectively increases conversion efficiency, since the saturation current density and ideality factor are clearly improved by passivating the recombination centers at CdS/CIGS interfaces.

Nanoscale selective area epitaxy of C60 crystals on GaAs by molecular beam epitaxy

Fabrications of nanoscale structures of C60 crystals on GaAs substrates are successfully achieved by selective area molecular beam epitaxy. The grown structures are examined by scanning electron and atomic force microscopies. The {100} and {110} facets appear at the side boundaries of the C60 crystals. The surface morphology of C60 crystals grown on GaAs (111)B is much smoother than that grown on GaAs (001). Good crystalline properties of C60 films grown on GaAs (111)B are also confirmed by x-ray diffraction φ (phi) scan measurement. During C60 deposition, C60 molecules landing on SiO2 mask either evaporate or migrate to the open area enhancing the growth rate at the edges of the area. The mean diffusion length of C60 molecules on SiO2 mask at 200°C is evaluated to be 200–300nm. The C60 crystals grown on GaAs (001) with narrow open area (100–200nm) have a fourfold symmetry, indicating that the epitaxial orientation of these C60 crystals is [001]. This result should be compared with the [111] orientation o...

Effects of Mo surface oxidation on Cu(In,Ga)Se2 solar cells fabricated by three-stage process with KF postdeposition treatment

The surface oxidation condition of the Mo back contact on a soda lime glass (SLG) substrate was varied by air annealing and chemical etching. Then, the evolution of a photovoltaic property was studied for Cu(In,Ga)Se2 (CIGS) solar cells grown by a three stage process with KF postdeposition treatment. Upon the removal of the oxidized layer from the Mo surface by chemical etching, the c-axis orientation of MoSe2 tended to be random, whereas the c-axis was perpendicular when the Mo surface was oxidized. An enhancement of the diffusion of Na and K from SLG to CIGS was observed upon removing the molybdenum oxide, which functions as a barrier to alkali-metal diffusion. The varied orientation of MoSe2 can also affect the alkali-metal diffusion kinetics. The open-circuit voltage (VOC) markedly increased after removing the oxidized layer from the Mo surface, mainly as a result of an increase in carrier density in CIGS.

Investigation of C60 epitaxial growth mechanism on GaAs substrates

Intensity oscillations of reflection high-energy electron diffraction are observed during epitaxial growth of a C60 layer on GaAs (111)B, (111)A, and (001) substrates. The frequencies of the oscillations coincide well with the growth rates of C60 layers, suggesting that C60 layers grow by repeating nucleation and step-flow growth mode, as is the case with GaAs and other semiconductor materials. Anomalous oscillations are observed in the initial stage of C60 layer growth on a GaAs (111)B surface with (2×2) structure. These oscillations indicate that the growth of the first C60 layer is completed at the point of approximately 0.5 monolayer coverage by C60 molecules. This phenomenon is explained by a model in which C60 adsorption sites are limited by As trimers adsorbed on the GaAs (111)B surface. A similar relationship between C60 adsorption and surface reconstruction is observed on (001) GaAs substrates, i.e., the first C60 layer on the (2×4) surface is terminated at approximately 0.5 monolayer coverage, while full coverage is needed of the c(4×4) surface. These observed results are strongly supported by the reported results of scanning tunnel microscopy.

Effect of excitons in AlGaAs/GaAs superlattice solar cells

The effect of excitonic absorption on solar cell efficiency has been investigated using solar cells with AlGaAs/GaAs superlattice active regions. Numerical calculations reveal that excitonic absorption considerably enhances the overall absorption of bulk GaAs. Excitonic absorption shows strong and sharp peaks at the absorption edge and in the energy region above the band gap. Absorption enhancement is also achieved in the AlGaAs/GaAs superlattice. The measured quantum efficiency spectra of AlGaAs/GaAs solar cells are found to be quite similar to the calculated absorption spectra considering the excitonic effect. The miniband structures of the superlattice and the electric field of the p–i–n junction enhance the dissociation of excitons and the extraction of separated carriers. These results suggest that the enhanced absorption by excitons can increase the quantum efficiency of solar cells.

Cu(In,Ga)Se2 Solar Cells with Amorphous In2O3-Based Front Contact Layers

Amorphous (a-) In2O3-based front contact layers composed of transparent conducting oxide (TCO) and transparent oxide semiconductor (TOS) layers were proved to be effective in enhancing the short-circuit current density (Jsc) of Cu(In,Ga)Se2 (CIGS) solar cells with a glass/Mo/CIGS/CdS/TOS/TCO structure, while maintaining high fill factor (FF) and open-circuit voltage (Voc). An n-type a-In-Ga-Zn-O layer was introduced between the CdS and TCO layers. Unlike unintentionally doped ZnO broadly used as TOS layers in CIGS solar cells, the grain-boundary(GB)-free amorphous structure of the a-In-Ga-Zn-O layers allowed high electron mobility with superior control over the carrier density (N). High FF and Voc values were achieved in solar cells containing a-In-Ga-Zn-O layers with N values broadly ranging from 2 × 1015 to 3 × 1018 cm-3. The decrease in FF and Voc produced by the electronic inhomogeneity of solar cells was mitigated by controlling the series resistance within the TOS layer of CIGS solar cells. In addition, a-In2O3:H and a-In-Zn-O layers exhibited higher electron mobilities than the ZnO:Al layers conventionally used as TCO layers in CIGS solar cells. The In2O3-based layers exhibited lower free carrier absorption while maintaining similar sheet resistance than ZnO:Al. The TCO and TOS materials and their combinations did not significantly change the Voc of the CIGS solar cells and the mini-modules.

High absorption efficiency of AlGaAs/GaAs superlattice solar cells

The effects of excitonic absorption on the solar cell efficiency have been investigated in solar cells with AlGaAs/GaAs superlattice absorption layers. Numerical calculations reveal that excitonic absorption considerably enhances the overall absorption coefficient. The excitonic absorption shows strong peaks at the absorption edge and in the energy region above the band gap. Absorption enhancement is also achieved in the AlGaAs/GaAs superlattice. The measured quantum efficiency spectra of superlattice solar cells at room temperature are reasonably well reproduced by simulations taking excitonic effects into account. The superlattice solar cells are confirmed to have a high absorbance and good temperature stability. The theoretical analysis of the experimental results confirms that the enhanced excitonic absorption in the superlattice absorption layers survives even at 100 °C, which is considered as the actual device temperature under realistic device operations.

High-Absorption-Efficiency Superlattice Solar Cells by Excitons

The effect of excitonic absorption on solar cell efficiency has been investigated using solar cells with AlGaAs/GaAs superlattice structures. Numerical calculations reveal that excitonic absorption considerably enhances the overall absorption of bulk GaAs. Excitonic absorption shows strong and sharp peaks at the absorption edge and in the energy region above the band gap. Absorption enhancement is also achieved in the AlGaAs/GaAs superlattice. The measured quantum efficiency spectra of the superlattice solar cells are quite similar to the calculated absorption spectra considering the excitonic effect. The superlattice solar cells are confirmed to have high absorption coefficient compared with the GaAs and AlGaAs bulk solar cells. These results suggest that the enhanced absorption by excitons can increase the quantum efficiency of solar cells. This effect is more prominent for the solar cells with small absorption layer thicknesses.

Growth of CuGaSe2 Layers on Closely Lattice-Matched GaAs Substrates by Migration-Enhanced Epitaxy

CuGaSe2 single-crystal films are grown on the As-stabilized (2×4) surface of (001) GaAs by migration-enhanced epitaxy (MEE), where Cu+Ga and Se are alternately deposited. The growth process is monitored by refraction high-energy electron diffraction (RHEED) in the [110] azimuth. Under the Cu-enriched growth condition, a deformed 4-fold pattern is observed in both Cu+Ga and Se deposition periods. The deformed 4-fold pattern is found to be related to the segregation of Cu2Se on the CuGaSe2 surface as confirmed by the results of X-ray diffraction (XRD) measurement. By reducing the beam equivalent pressure of Cu (Cu-BEP), clear 4-fold patterns appear in both Cu+Ga and Se deposition periods instead of deformed 4-fold patterns. Further reduction of Cu-BEP results in clear 4- and 2-fold patterns for Cu+Ga and Se deposition periods. Under these growth conditions, Cu2Se-segregation-free CGS growth is achieved. Thus, the CuGaSe2 single-crystal layers without Cu2Se-segregation are successfully grown on GaAs(001) substrates by optimizing the Cu-BEP.

Structural properties of C60-multivalent metal composite layers grown by molecular beam epitaxy

C60-multivalent metal composite layers (aluminum, gallium, and germanium) are grown on GaAs and quartz glass substrates by molecular beam epitaxy. The structural properties of the C60-metal composite layers are investigated by reflection high-energy electron diffraction and transmission electron microscopy measurements, and it is confirmed that these layers have an amorphous structure. Mechanical properties of the layers are investigated by Vickers hardness test, and the values of the C60-metal composite layers are confirmed to be dramatically increased. The structural change and the hardness enhancement are induced by the bonding between C60 molecules and multivalent metal atoms. Optical properties of the layers are measured by the absorption coefficient spectra. The absorption peaks in C60–Ge composite layers become less pronounced with increasing Ge concentration and the intensity in visible light spectrum is increased. Pure C60, C60–Al, and C60–Ga composite layers are confirmed to be insulators in air...