Enhanced efficiency of crystalline Si solar cells based on kerfless-thin wafers with nanohole arrays

Abstract

Several techniques have been proposed for kerfless wafering of thin Si wafers, which is one of the most essential techniques for reducing Si material loss in conventional wafering methods to lower cell cost. Proton induced exfoliation is one of promising kerfless techniques due to the simplicity of the process of implantation and cleaving. However, for application to high efficiency solar cells, it is necessary to cope with some problems such as implantation damage removal and texturing of (111) oriented wafers. This study analyzes the end-of-range defects at both kerfless and donor wafers and ion cutting sites. Thermal treatment and isotropic etching processes allow nearly complete removal of implantation damages in the cleaved-thin wafers. Combining laser interference lithography and a reactive ion etch process, a facile nanoscale texturing process for the kerfless thin wafers of a (111) crystal orientation has been developed. We demonstrate that the introduction of nanohole array textures with an optimal design and complete damage removal lead to an improved efficiency of 15.2% based on the kerfless wafer of a 48 μm thickness using the standard architecture of the Al back surface field.

Random nanohole arrays and its application to crystalline Si thin foils produced by proton induced exfoliation for solar cells

“…8(c). In our previous report, we demonstrated the solar cells of a standard cell architecture or Al back surface field with the same Si thin foils used in this study and an efficiency of 15.2% was achieved 10 . The enhancement of the LFC PERC cell over the previous Al BSF cell is due to suppressed recombination and increased internal reflectance at the rear side, which are the main benefits we expect from the PERC solar cell architecture.…”

“…Subsequently, the donor wafer is annealed for hydrogen micro bubble formation and crack propagation for the kerfless wafer cleavage 3 . The donor wafer can be reused in multiple times for the kerfless wafering 10 . The proton beams are implanted by raster scan to have a uniform dose of 1 × 10 17 cm −2 .…”

“…The thickness of the thin Si foils was measured to be 48 µm by scanning electron microscopy (SEM) after the damage removal etching. Our fabrication procedure of the kerfless Si thin foils by the PIE technique and the damage removal process were more detailed in our previous publication 10 .

Figure 1( a ) Crystalline Si thin foils of a 58 µm thickness as-cleaved by the proton induced exfoliation (PIE) technique. ( b ) Demonstration of the flexible Si thin foil.

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“…However, the efficiency of the solar cells based on the kerfless thin wafer fabricated by proton induced exfoliation has been reported to lag behind the counter part technology based solar cells. The epitaxial lift-off solar cells have reached an efficiency of 21.2% 8 , and the metallic stressor induced spalling solar cells showed an efficiency of 14.9% 9 whereas the thin Si wafers produced by the PIE process only led to 13.2% with a standard cell architecture of Al back surface field and recently reached 15.2% 3,10 .…”

“…The conventional pyramid texturing with alkaline solution is not applicable to the Si wafers of a (111) orientation because the etch rate of the (111) surface is extremely slower compared with the (100) one 13 . In our previous report, we combined laser interference lithography and a reactive ion etch process for nano-scale texturing of the kerfless-thin wafers with a (111) crystal orientation 10 . However, the laser interference lithography has a limitation in the large area process 14 .…”