We carried out theoretical calculation for Cu(In,Ga)Se2 (CIGS) solar cells with energy bandgap of 1.4 eV assuming formation of a Cu-poor layer on the surface of CIGS films. This calculation result revealed that formation of a thinner Cu-poor layer such as a few nanometers leads to improvement of the solar cells performance. This is because interfacial recombination was suppressed due to repelling holes from the interface by valence band offset (ΔEV). Next, we investigated composition distribution in the cross section of CIGS solar cells with Ga contents of 30% and 70% by transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDX). It was revealed that the Cu-poor layer was formed on the surface and at the grain boundary (GB) in the case of conversion efficiency (η) of 17.3%, although it was not formed in the case of lower η of 13.8% for a Ga content of 30%. These results indicate that formation of the Cu-poor layer contributed to improvement of cell performance by suppression of carrier recombination. Moreover, it was also confirmed that although the Cu-poor layer was observed on the surface, it was not observed at the GB in the case of CIGS solar cells with a Ga content of 70% which had η of 12.7%. It is thought that the effect of repelling holes by ΔEV is not obtained at the GB and the solar cell performance in the Ga content of 70% is lower than that in the Ga content of 30%. Thus, we suggest importance of the Cu-poor layer at the GB for high efficiency of CIGS solar cells with high Ga contents.

Al2O3 was deposited on silicon nanowire (SiNW) arrays by atomic layer deposition (ALD) as a passivation layer to reduce surface recombination velocity. As a result, effective minority carrier lifetime was improved from 1.82 to 26.2 μs. From this result, the relative low-surface recombination rate of 2.73 cm/s was obtained from a calculation using one-dimensional device simulation (PC1D). The performance of SiNW solar cells was also simulated by considering the surface recombination velocity on the side of SiNWs using two-dimensional device simulation. It was found that Al2O3 deposited by ALD can improve open-circuit voltage of SiNW solar cells even if the structure has a high-aspect ratio and large surface area. Therefore, improvement in the performance of SiNW solar cells can be expected.

The electrical characteristics of silicon nanowire (SiNW) solar cells with p-type hydrogenated amorphous silicon oxide (Eg=1.9 eV)/n-type SiNWs embedded in SiO2/n-type hydrogenated amorphous silicon (Eg=1.7 eV) structure have been investigated using a two-dimensional device simulator with taking the quantum size effects into account. The average bandgap of a SiNW embedded in SiO2 increased from 1.15 eV to 2.71 eV with decreasing the diameter from 10 nm to 1 nm due to the quantum size effect. It should be noted that under the sunlight with AM1.5G the open-circuit voltage (Voc) of SiNW solar cells also increased to 1.54 V with decreasing the diameter of the SiNWs to 1 nm. This result suggests that it is possible to enhance the Voc by the quantum size effect and a SiNW is a promising material for the all silicon tandem solar cells.

We prepared size-controlled silicon quantum dots superlattices (Si-QDSLs) by thermal annealing of stoichiometric hydrogenated amorphous silicon carbide (a-SiC:H)/silicon rich hydrogenated amorphous silicon carbide (a-Si1+xC:H) multilayers for thin-film solar cell applications. Transmission electron microscope (TEM) observation revealed that the size of silicon quantum dots can be controlled by the thickness of the a-Si1+xC:H layers. It was found that hydrogen plasma treatment (HPT) significantly enhanced the photoluminescence (PL) of the Si-QDSLs. From the results of the PL measurement, the bandgap of the Si-QDSLs can be controlled from 1.1 eV to 1.6 eV by varying the diameter of silicon quantum dots. ESR measurement indicated that HPT reduced the defect density in a Si-QDSL from 1.83 ×1019 to 1.67 sup1018 cm-3.

A high-concentration ozone generator operating at atmospheric pressure has been developed for fabrication of ultra thin silicon oxide films. The generator can supply atmospheric pressure of ozone jet with ozone concentration up to 80 vol%. The ozone jet is generated by desorbing ozone at nearbyn room temperature, which has been condensed on silica-gel by passing ozone/oxygen mixture gas from a commercial ozonizer at a low temperature (<-50°C); at the temperature ozone is more selectively adsorbed on silica-gel than oxygen. The high purity ozone jet with a concentration of 25 vol% at a pressure of 1 × 105 Pa had so large oxidation power that SiO2 film as thick as 3.3 nm grew on a Si surface after 60 min exposure at 375°C. The density of the film was equivalent to that of the film formed by a conventional thermal oxidation process, judging from etching rate with dilute HF solution.

The oxidation of H/Si(100) and H/Si(111) with high concentration ozone gas was investigated with X-ray photoelectron spectroscopy(XPS). The ozone oxidation of partially hydride-covered surface was observed. The hydrogen termination reduced the rate of oxygen insertion into silicon backbond. The reduction of oxygen insertion rate by the H-termination for H/Si(100) was larger than that for HSi(111). The dissociation rate of ozone molecule on H/Si was estimated to be ≃0.2 with a directional mass analyzer.