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The behavior of oxide precipitates during solar cell fabrication processes and the resulting effect on device performance have been investigated by transmission electron microscopy (TEM) observation. Samples were prepared with different carbon concentration and under different crystal growth conditions, namely using the conventional and an advanced process. The density of oxide precipitates increased monotonically with the carbon concentration, while the cell efficiency improved with decreasing oxygen precipitate density. When the carbon concentration was reduced to below 1016 cm−3, the oxide precipitates grew largely and dislocations were introduced. TEM observations confirmed that the morphology of the oxide precipitates clearly differed depending on the crystal growth conditions. Precipitates grown in platelet form introduced high density dislocations in their surroundings, while the dislocation density was relatively lower around polyhedral-type precipitates. These results thus reveal that oxygen precipitation can be controlled by varying the crystal growth conditions, possibly contributing to the production of high-efficiency solar cells.

The cost of solar cell production can be reduced by wafer thinning. A thinner wafer provides flexibility, and crystalline silicon solar cells are promising as flexible solar cells due to their flexibility. However, as wafers become thinner, production yield decreases due to wafer breakage caused by sawing damage; thus, to further reduce wafer thickness, it is necessary to suppress sawing damage. We investigated the flexibility of wafers under various slice conditions by conducting biaxial bending tests and clarified the dominant factor causing sawing damage to further reduce the wafer thickness for crystalline silicon solar cells. The results of damage observation by scanning electron microscopy and evaluation of the crystal structure by Raman spectroscopy confirm that the damage structure changes significantly depending on wire specifications. The results from the biaxial bending tests indicate that the three-dimensional flexibility of a wafer is determined by wire specifications.

In this study, we evaluated sawing damage of thin silicon wafers. Ultra-thin silicon wafers with great advantages of bendability and lightweight are suitable for needs such as to realize PV-powered vehicles. For thin wafers, it is important to realize high precision slicing to prevent breakage. We evaluated the distribution of sawing damage and crystallinity in order to clarify the cause of wafer breakage and establish low damage slice conditions. We confirmed phase transformation of silicon and crystalline defects on wire exit side on the conventional slicing surface, while the advanced slicing surface shows almost only single-crystalline Si phase.

In pursuit for the improvement of heterojunction Si solar cell performance, we evaluated Sn-doped Indium oxide (ITO)/a-Si structure by using conventional and hard X-ray photoelectron spectroscopy (XPS, HAXPES) to identify the cause of solar cell performance degradation. HAXPES allows us to evaluate the SiOx layer at the ITO/a-Si interface non-destructively. The SiOx formation at the ITO/a-Si interface leads to an increase the contact resistance, which can be reduced by post deposition annealing (PDA). In addition, PDA promoted the evaluation of Fermi level, the precipitation of ITO component in a-Si layer, and the increase of interface roughness. Before PDA, the diffusion of Sn atoms in a-Si was observed. Furthermore, it was also confirmed by PDA that Si atoms were diffused to ITO. These reaction at the ITO/a-Si interface may be part of the degradation factor of the Si solar cells.

We investigated the degradation factor of the conversion efficiency of the silicon heterojunction solar cells. In particular, we clarified the effect of the transparent conductive oxide film deposition conditions on the film quality and interface states, and the relationship between these factors and the conversion efficiency. Heat process causes a decrease in carrier mobility due to a decrease in the grain size of the TCO film, but has an effect of improving the TCO/a-Si:H(p) interface states. In particular, the silicon oxide layer between the TCO film and a-Si:H(p) increases the series resistance and moderates the band bending at the TCO/a-Si:H(p) interface, leading to FF degradation. Since In2O3:H film easily forms a silicon oxide layer at the TCO/a-Si:H(p) interface, the interface treatment is important factor in the cell manufacturing process.

We evaluated the ITO/p-type a-Si interface for Si heterojunction solar cells using XPS and TEM. It was found that ITO/a-Si interface which is 20 nm from the surface is oxidized by using non-destructive and non-contact HAXPES. The ITO/a-Si interface was oxidized during ITO film deposition by RPD technique, which leads to an increase in contact resistance. The chemical bonding states at the ITO/a-Si interface was modified by post deposition annealing (PDA). The TEM and the EDX mapping images revealed the interdiffusion of Si and Sn resulting in the interface roughness enhancement and the possible In precipitation in the a-Si layer.

Komatsu NTC developed ultra-thin wafer slicing with low kerf-loss by multi diamond-wire saw. We modified slicing conditions and diamond-wire specifications to keep the straightness of wire for the fine pitch slicing and established 150 μm pitch slicing technology. The as-cut wafer thickness is 90 μm, and the kerf-loss is 60 μm. As a result, the highly flexible ultrathin wafer can be obtained with well-controlled surface crystallinity. The PERT cell fabrication trial using ultra-thin wafers showed that the relative deference of cell efficiency between 166 μm thick and 84 μm thick solar cells was just only 1.2%.

We evaluated the damage to crystalline silicon caused by sputtering deposition of transparent conductive oxide films (TCO). We confirmed that direct deposition of TCO on the crystalline silicon deteriorates the carrier lifetime. Also, the difference in discharge voltage during the TCO sputtering has an influence on plasma damage and damage penetration depth to the crystalline silicon. We consider that the carrier lifetime is reduced because of the surface recombination posed by the damage. Furthermore, photoluminescence (PL) spectroscopy was performed, and the so-called “irradiation-induced defects,” defects that are formed typically after electron beam and ion irradiation, were observed at deep levels of 0.789 eV, 0.767 eV, and 0.714 eV. These irradiation-induced defects act as recombination centers and may cause the deterioration of the conversion efficiency in crystalline silicon solar cells. We believe understanding and controlling these defects are important for improving the conversion efficiency of solar cells.

This research investigates the cause of lifetime reduction properties of a crystalline defect layer introduced by the plasma process such as reactive plasma deposition (RPD). The plasma irradiation damage to silicon substrate with the different oxygen and carbon concentrations were evaluated. Minority carrier lifetime of the silicon substrate after the RPD process has been significantly reduced by plasma irradiation. Furthermore, photoluminescence (PL) spectroscopy revealed that the cause of the lifetime degradation on the silicon substrate is Ci–Oi defect generation originated in the plasma irradiation during the RPD process.

We investigated the damage using hard X-ray photoemission spectroscopy (HAXPES), and found that the a-Si surface was oxidized due to the energetic oxygen ambient during the deposition. For the conventional and advanced sputter, the SiO peak intensity at the ITO/a-Si interface did not change by post deposition annealing, but for RPD the peak intensity decreased. From the O 1s and In 3d spectra, it is considered the oxygen at the ITO/a-Si interface moved to the oxygen deficient in ITO. We also verified the damaged oxide was partially recovered by the post deposition annealing process.