We determined that the conduction band offset (CBO) and the valence band offset (VBO) at the CdS/ Cu2ZnSnSe4 (CZTSe) heterointerface are +0.56 and +0.89eV, respectively, by using X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) and inversed photoemission spectroscopy (IPES). A positive CBO value, so-called “spike” structure, means that the position of conduction band becomes higher than that of absorber layer. The evaluated CBO of +0.56 eV suggests that the conduction band alignment at CdS/CZTSe interface is enough to become an electron barrier. Despite such a large spike structure in the conduction band at the interface, a conversion efficiency of 8.7 % could be obtained for the CdS/CZTSe heterojunction solar cells.

This paper describes light trapping in superstrate-type amorphous Si solar cells incorporated with Ag nanostructures (nanodisks) fabricated by a transfer-printing approach. The changes in external quantum efficiency (EQE) and current-voltage characteristics were investigated by changing the position and size (thickness) of the Ag nanodisks in the cells fabricated on flat superstrates. It was confirmed that the optimized Ag nanodisk-configuration led to the enhanced EQE (20%) in the 600-800 nm wavelength range, and the enhanced EQE led to the improved overall conversion efficiency (7.5%) compared to the cell without Ag nanodisks (7.2%). However, the integration of the optimized Ag nanodisk-configuration with the cells fabricated on textured superstrates did not result in the enhanced EQE and conversion efficiency, suggesting further optical designs are necessary to exploit both texture- and plasmon-mediated light trapping effects.

This paper shows a new semiconductor bonding technology for mechanically stacked multi-junction solar cells. Our strategy is the combination of conductive nanoparticle alignments and the van der Waals bonding technique. With this method, reasonably low bonding resistances and minimal optical absorption losses were simultaneously attained for the use of mechanically stacked solar cells. We examined a GaInP(Eg-1.89 eV)/GaAs (Eg-1.42 eV)/InGaAsP (Eg-1.15 eV) three-junction solar cell fabricated with this bonding method. As a result, the total efficiency of 22.5% was achieved, which was in good agreement with the theoretically predicted value. These results suggested that our bonding method is highly useful to fabricate high-efficiency mechanically stacked multi-junction solar cells.

CIGS solar cells were fabricated on a hybrid back contact comprised of a TCO layer (ZnO:Ga (GZO)) and Mo layers. It was discovered that an additional Mo layer introduced underneath the TCO layer promotes sodium diffusion through the TCO back contact into the upper CIGS absorber layer. Improvement in VOC and JSC values relative to those of sodium-free solar cells was achieved with the Mo/GZO/Mo hybrid back contact as a result of the enhanced sodium diffusion.

We have grown device-grade Cu(In,Ga)Se2 (CIGS) thin films using a RF-cracked Se-radical beam source. A unique combination of film properties: smooth surface, large grain size and high photovoltaic performance are shown. A competitive energy conversion efficiency of 17 % has been demonstrated from a solar cell fabricated using a CIGS absorber grown with a Se-radical source. In addition to the unique combination of film properties and high photovoltaic performance, a significant improvement in the use of Se source material in comparison with the conventional Se-evaporative sources has been demonstrated.

Dependence of band alignments at interfaces between CdS by chemical bath deposition and Cu(In1-xGax)Se2 by conventional 3-stage co-evaporation on Ga substitution ratio x from 0.2 to 1.0 has been systematically studied by means of photoemission spectroscopy (PES) and inverse photoemission spectroscopy (IPES). For the specimens of the In-rich CIGS, conduction band minimum (CBM) by CIGS was lower than that of CdS. Conduction band offset of them was positive about +0.3 ~ +0.4 eV. Almost flat conduction band alignment was realized at x = 0.4 ~ 0.5. On the other hand, at the interfaces over the Ga-rich CIGS, CBM of CIGS was higher than that of CdS, and CBO became negative. The present study reveals that the decrease of CBO with a rise of x presents over the wide rage of x, which results in the sign change of CBO around 0.4 ~ 0.45. In the Ga-rich interfaces, the minimum of band gap energy, which corresponded to energy spacing between CBM of CdS and valence band maximum of CIGS, was almost identical against the change of band gap energy of CIGS. Additionally, local accumulation of oxygen related impurities was observed at the Ga-rich samples, which might cause the local rise of band edges in central region of the interface.

In-situ characterization of composition, electronic structure and their depth profiles of surface of Cu(In1-xGax)Se2 (CIGS) film grown by three stage co-evaporation has been carried out by means of photoemission and inverse photoemission spectroscopy (PES/IPES), for the purpose of investigating the formation mechanism of the CIGS-side wide band-gap region adjacent to CBD-CdS/CIGS interface in cell structure. Sample-transportation in vacuum below 1 x 10-8 Torr yielded almost contamination-free feature of the CIGS surface. The as-transferred surface of Cu0.93(In0.65Ga0.35)Se2 grown at the identical condition for the high performance solar cell exhibited seriously Cu and Ga deficient composition. Chemical formula of this region was inbetween Cu : (In+Ga): Se = 1 : 3 : 5 and 1 : 5 : 8. In-situ UPS/IPES measurements CIGS showed that the as-grown surface region of the CIGS already had expanded band gap energy up to 1.4 eV and n-type character. A gradual decrease of band energy and a rise of valence band maximum as a function of depth from the original surface were observed. These results have revealed that the surface of CIGS by the three stage method already has the wide band gap, which might originate in so-called Cu-vacancy ordered phase.

For understanding the origin of the improvements of properties in the CIGS-based cells, of which the CIGS absorber has been fabricated by H2O-introduced co-evaporation [CIGS-H2O], band alignment at the interfaces between chemical bath deposited CdS and CIGS-H2O with Ga substitution ratio ~ 40 % has been studied by photoemission and inverse photoemission spectroscopy. The CdS layer over the CIGS-H2O showed an identical electronic structure with that of CdS on the conventionally grown CIGS; band gap energy of 2.3 ~ 2.4 and a location of conduction band minimum (CBM) and valence band maximum (VBM) relative to Fermi level were + 0.75 eV and -1.6 ~ -1.7 eV, respectively. In the interface region, decreases of CBM and a rise of VBM were observed. Total amount of the decrease of CBM over the interface was 0.2 ~ 0.3 eV. XPS measurements of the core-level signals over the interface showed a small upward bend bending of 0.1 ~ 0.2 eV. Consequently, the conduction band offset (CBO) and valence bad offset (VBO) at the CBD-interface over the CIGS-H2O (Ga~40%) are about +0.1, and 0.9 ~ 1.1 eV, respectively. This positive CBO is contrast with a slightly negative CBO at the interface between CBD-CdS/conventionally grown CIGS with Ga ~ 40 % measured previously. These results indicate that the H2O introduction is effective to extend the upper limit of the Ga substitution ratio where the Type-I conduction band alignment is maintained. The observed band alignments are consistent with the rise of Voc and efficiency in the CIGS-H2O based cells.

The reasons why the open circuit voltage (Voc) of high-x CuIn1-xGaxSe2 (CIGS)/ZnO solar cells remain low are discussed. Here it is shown that the Voc ceiling can be interpreted simply on the basis of a model that the valence-band energy (Ev) of CIGS is almost immovable irrespective of x. When the conduction-band energy (Ec) of ZnO is lower than that of high-x CIGS (DEc<0), the built-in potential (Vbi) of a CIGS/ZnO junction is equivalent to the flat-band potential (Vbi) that arises from the separation between the Fermi energies of the two materials. If the Ev (and therefore the Fermi energy) of p-type CIGS is constant with increasing x, the Vbi and Voc that follows the Vbi remain unchanged since the Fermi energy of ZnO is constant. This unchangeable Voc reduces the conversion efficiency of high-x CIGS cells in cooperation with reduced photocurrents due to a larger bandgap. A positive offset, ΔEc>o gives rise to a photoelectrons barrier in the conduction-band that partially cancels Voc, thus the Voc of a low-x CIGS cell is governed by the Ec of CIGS. Based upon this concept, a material selection guideline is given for the windows and transparent electrodes appropriate for high-x CIGS absorbers-based solar cells.

Using EBIC and EDX measurements, CIGS solar cells prepared under several different conditions were observed and characterized. The results of EBIC and EDX measurements suggest that Cd plays an important role in the forming of a buried pn-junction in the CIGS layer via diffusion, and de-emphasize the possibility of the formation of the hetero pn-junction at the CdS/CIGS heterointerface. The correlation of the extent of the space charge region and the observed shift in the pn-junction location with the diffusion of the constituent elements in CIGS was investigated.

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.

Epitaxial CuGaSe2 films were grown on GaAs substrates under Cu-excess conditions to obtain stoichiometric compositions. The films were annealed in Ar, Sex or O2 ambients with or without a Cu or Cu-Se cap layer with the intention of changing the intrinsic defect concentrations. Samples were evaluated using low-temperature photoluminescence (PL) measurements. Annealing of the samples dramatically changed the PL spectra indicating that not only interdiffusion had occurred, but defect species and populations were changed. Comprehensive consideration of the changes led to the conclusion that the emissions at 1.62 eV, 1.66 eV and in the range from 1.2 to 1.4 eV are related to specific defects of Se vacancies, Cu vacancy-Se vacancy complexes and interstitial Cu, respectively.

CuGaSe2 (CGS) is a promising material for high efficiency thin film solar cells though predicted device performance has not been realized. Understanding the difference in the chemical nature between CuInSe2 (CIS) and CGS is critical for improving Cu (In, Ga) Se2 solar cells with high Ga concentrations. In this work, we have investigated the effects of oxygen-annealing on Ga-rich CGS epitaxial films focusing on compositional changes and secondary phase formations. The photoluminescence (PL) spectrum of Ga-rich films after oxygen-annealing was observed to always change into a spectrum characteristic of CGS grown under Cu-excess conditions. Electron probe micro-analysis (EPMA) measurements indicate the formation of Ga-O after oxygen-annealing. Selective etching of the Ga-O phase showed the composition of the CGS phase became close to stoichiometric. The oxygen-annealed films showed multiple pits ∼ 100 nm in depth and ∼ 2.5 μm in width. The Ga-O phase is founded in a layer formed on the surface of the CGS phase and in a columnar form rising from the bottom of the pits to the film/substrate interface. The above results suggest that excess Ga in Ga-rich CGS tends to react with oxygen to form Ga-O, thus the composition of the remaining CGS approaches stoichiometry consistent with the changes observed in PL.

The electronic structure and thermoelectric properties of Yb partially filled CoSb3 skutterudite compounds have been investigated by x-ray photoelectron spectroscopy and band calculation in terms of an itinerant f electron model. In these materials, the significant effect of Yb filling is the large reduction of lattice thermal conductivity, remaining relatively high electron mobility and Seebeck coefficient, resulting in high thermoelectric figure of merit. We discuss the effects of the valence fluctuation between Yb2+ and Yb3+ and the strong hybridization of Yb 4f states with the valence band states on the electronic properties and their relation to thermoelectric properties for Yb partially filled CoSb3 compounds.

ZnO films were grown by radical-source molecular beam epitaxy (RS-MBE) on sapphire and glass substrates, and they were characterized in terms of Hall mobility and optical transmission. Undoped ZnO films exhibit a low intrinsic defect density and optical properties close to bulk ZnO. By Ga doping, a resistance ρ as low as 2×10−4 Ωcm could be achieved. Balancing high conductivity and low transmission losses due to free carrier absorption in the infrared, the optimum was obtained for ρ=3.4×10−4Ωcm, electron mobility μe=37 cm2/Vs and an average transmission T of 96% in the wavelength range 400-1100 nm. Polycrystalline growth on glass yields slightly reduced but still good film quality (μe=30 cm2/Vs, T=90%). By the incorporation of Mg, conducting Mg0.3Zn0.7O films with an increased band gap up to ∼ 4eV were realized.

ZnO films were grown directly on epitaxial CuInSe2 (001) (CIS) by radical-source molecular beam epitaxy (RS-MBE). The substrate-film interdiffusion was investigated dependent on the ZnO growth temperature. Secondary Ion Mass Spectroscopy (SIMS) profiles indicate the mutual temperature-activated diffusion of Zn and In at a growth temperature of 440°C which is absent at 250°C. Zn indiffusion into the CIS substrate leads to characteristic changes in the photoluminescence (PL) properties, whereas the In outdiffusion into the growing ZnO film causes an increased carrier concentration.

The technologies utilizing Fluorinated Silicon Oxide (FSG, k=3.6) and Hydrogen Silsesquioxane (HSQ, k=3.0) have been established for 0.25-μm and 0.18-μm generation ULSIs. However, low-k materials for the next generation ULSIs, which have a dielectric constant of less than 3.0, have not become mature yet. In this paper, we review process integration issues in applying FSG and HSQ, and describe integration results and device performance using Fluorinated Amorphous Carbon (a-C:F, k=2.5) as one of the promising low-k materials for the next generation ULSIs.

The technologies utilizing Fluorinated Silicon Oxide (FSG, k=3.6) and Hydrogen Silsesquioxane (HSQ, k=3.0) have been established for 0.25-µm and 0.1 8-µm generation ULSIs. However, low-k materials for the next generation ULSIs, which have a dielectric constant of less than 3.0, have not become mature yet. In this paper, we review process integration issues in applying FSG and HSQ, and describe integration results and device performance using Fluorinated Amorphous Carbon (a-C:F, k=2.5) as one of the promising low-k materials for the next generation ULSIs.