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

www.nature.com/scientificreports OPEN Random nanohole arrays and its application to crystalline Si thin foils produced by proton induced exfoliation for solar cells Hyeon-Seung Lee1,2,6, Jae Myeong Choi1,2, Beomsic Jung1,2, Joonkon Kim3, Jonghan Song3, Doo Seok Jeong 4, Jong-Keuk Park1, Won Mok Kim1, Doh-Kwon Lee5, Taek Sung Lee1, Wook Seong Lee1, Kyeong-Seok Lee1, Byeong-Kwon Ju2 & Inho Kim1* We report high efficiency cell processing technologies for the ultra-thin Si solar cells based on crystalline Si thin foils (below a 50 µm thickness) produced by the proton implant exfoliation (PIE) technique. Shallow textures of submicrometer scale is essential for effective light trapping in crystalline Si thin foil based solar cells. In this study, we report the fabrication process of random Si nanohole arrays of ellipsoids by a facile way using low melting point metal nanoparticles of indium which were vacuumdeposited and dewetted spontaneously at room temperature. Combination of dry and wet etch processes with indium nanoparticles as etch masks enables the fabrication of random Si nanohole arrays of an ellipsoidal shape. The optimized etching processes led to effective light trapping nanostructures comparable to conventional micro-pyramids. We also developed the laser fired contact (LFC) process especially suitable for crystalline Si thin foil based PERC solar cells. The laser processing parameters were optimized to obtain a shallow LFC contact in conjunction with a low contact resistance. Lastly, we applied the random Si nanohole arrays and the LFC process to the crystalline Si thin foils (a 48 µm thickness) produced by the PIE technique and achieved the best efficiency of 17.1% while the planar PERC solar cell without the Si nanohole arrays exhibit 15.6%. Also, we demonstrate the ultra-thin wafer is bendable to have a 16 mm critical bending radius. The use of thinner wafers is one of the most straightforward methods to lower the module price of the crystalline Si solar cells because the cost of the Si material account for more than 30% of the module1. The incessant research efforts have been made to develop the fabrication techniques to produce the thinner Si wafers. Currently, a multi-wire sawing has been adopted for Si wafer fabrication by the photovoltaics industry; however, this technique will face the wafer thickness limitation in the near term future due to the finite wire size making it difficult to produce the Si wafers thinner than 80 μm2. Several techniques such as proton induced exfoliation 3 , metallic stressor induced spalling 4,5 , electrodeposit-assisted stripping (EAS)6 and epitaxial lift-off7 have been proposed for kerfless wafering of thin Si wafers or thin foils (<50 µm) to reduce a Si material loss in the conventional wafering method to lower the module cost. Proton induced exfoliation (PIE) which we adopted in this study is one of the promising kerfless techniques due to the process simplicity of implantation and cleaving. In this technique, protons are implanted into Si donor wafers with MeV acceleration energy. In the subsequent thermal treatment, the implanted protons aggregate and turn into hydrogen gas, which induces the crack propagation resulting in the cleavage of the thin Si wafers. 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 1 Center for Electronic Materials, Korea Institute of Science and Technology, Seongbuk-gu, Seoul, 02792, Republic of Korea. 2School of Electrical Engineering, Korea University, Seoul, 02841, Republic of Korea. 3Advanced Analysis Center, Korea Institute of Science and Technology, Seongbuk-gu, Seoul, 02792, Republic of Korea. 4Division of Materials Science and Engineering, Hanyang University, Seoul, 04763, Republic of Korea. 5Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology, Seongbuk-gu, Seoul, 02792, Republic of Korea. 6Hanwha Q CELLS Korea Corporation, Chungcheongbuk-do, 27816, Republic of Korea. *email: Scientific Reports | (2019) 9:19736 | https://doi.org/10.1038/s41598-019-56210-7 1 www.nature.com/scientificreports/ www.nature.com/scientificreports 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. 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. One of the main reasons for the lower efficiency of the PIE solar cells arises from a difficulty in texturing. The critical proton dose for the exfoliation of the Si kerfless inherently relies on the Si crystal orientation. The (111) orientation known as a cleavage plane has the lowest the threshold proton dose for exfoliation11,12. However, for the application of the (111) thin wafers to high efficiency solar cells, it is necessary to cope with texturing of the (111) wafers for effective light trapping. 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) one13. 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 orientation10. However, the laser interference lithography has a limitation in the large area process14. In this study, we developed an isotropic nano-texturing process with a low melting point metal as etch mask which can be processed in the large area. We demonstrate that our nano-texturing provides high light trapping performances comparable to conventional micro-pyramid textures. Many interesting approaches to fabricate the semiconductor nanostructures of various shapes have been reported and demonstrated to show performance boost up of the optoelectronic devices such as solar cells, photodetectors and light emitting diodes15–19. Further improvements of optical performances would be expected by introducing the novel three dimensional nanostructures in our ultrathin Si solar cells. For the successful adoption of the Si thin foils in the photovoltaic industry, the cell processing technology of metallization especially designed for the thin foils need to be developed. The conventional metallization process based on screen printing using thick metal pastes is hardly applicable to the thin Si foils because of the severe wafer bowing induced by the thermal expansion coefficient differences between metal electrodes and Si wafers especially at high temperature in the range of 700 °C20. This can be avoided by metallization a (...truncated)