Fabricating 43-μm-Thick and 12% Efficient Heterojunction Silicon Solar Cells by Using Kerfless Si(111) Substrates

Abstract

A 43-μm-thick Si(111) substrate was obtained using the stress-induced lift-off method with a screen-printed metal paste layer as the stress-generation layer. The reflection of the metal-removed side of the Si(111) substrate was lower than that of the exfoliated side because of high surface roughness resulting from the reaction between the metal paste and the silicon substrate at 700 • C. After aggressive etching with an alkaline solution for the peeled silicon substrate, the efficiency of the heterojunction silicon solar cell was improved from 0.87% to 12%. For decades, silicon (Si)-based solar cells have been the conventionally used products in the photovoltaic (PV) industry. The Si solar cell module price is lower than 0.75 USD/W, which could be mainly attributed to the extensive research on and development of these cells and support from local governments.1 Although the price of poly-Si decreased markedly from 450 USD/kg in 2008 to 20 USD/kg in 2015, the price of poly-Si accounts for more than 50% of the cost of a Si solar cell.2 Thus, reducing the use of Si is the most effective method for reducing the cost of a Si solar cell. Therefore, the thickness of a Si substrate will be reduced from 200 μm in 2010 to 120 μm in 2020.2 In addition, reducing the thickness can further increase the cell efficiency, which is generally achieved by enhancing the open circuit voltage (Voc).3,4 Recently, a 98-μm-thick and 24.7% efficient heterojunction Si (HJS) solar cell, with a Voc of 750 mV, was developed by Panasonic. 5Using a thin Si substrate obtained by applying a kerfless process, which can potentially reduce the cost of solar cells, has become a major topic recently.6-11 Kuzma-Filipek et al. used a 30-μm-thick epitaxial Si film grown on a highly doped P + -Si substrate with a porous multilayer structure to obtain a 16.2% efficient Si solar cell. 6 Petermann et al. used a 43-μm-thick and freestanding epitaxial Si film by using the layer transfer technique to obtain a 19% efficient Si solar cell.7 High-efficiency solar cells can be fabricated using these thin epitaxial Si substrates; however, the high-temperature fabrication process (>1000• C) and expensive epitaxy system increase the cost. The stress-induced lift-off method, which is based on the thermal expansion difference between a deposited metal film and a Si substrate, could be an alternative because it entails a simple, low-temperature (<900• C) process for obtaining a thin Si substrate. 8,12,13 Dross et al. compared the differences between Si(111) and Si(100) substrates after exfoliation and obtained a 50-μm-thick and 10% efficient Si solar cell.9 Saha et al. further alleviated the difficulty of handling thin Si substrates and obtained a 25-μm-thick and 14.9% efficient Si solar cell. 10 We recently reported the thermal stress-induced pattern transfer technique, which is a combination of the thin-film deposition process and the layer transfer technique, and obtained a 3-μm-thick Si film with a textured surface. The short-circuit current of the HJS solar cell fabr...

Controlled Cracking for Large-Area Thin Film Exfoliation: Working Principles, Status, and Prospects

“…For photovoltaic applications, stressor layer deposition was investigated on Si target substrates with Al and Ag paste by a screen-printing method. , The screen printing method provides an advantage of compatibility with conventional photovoltaic device fabrication. However, a high temperature (>700 °C) annealing process was needed to generate sufficient thermal misfit stress for crack growth in the Si substrate.…”

Fabricating 40 µm-thin silicon solar cells with different orientations by using SLiM-cut method

“…This method makes particularly efficient use of bulk material by reducing kerf losses and producing wafers with a thickness of approximately 50 mm, thus reducing the silicon cost. Du et al used SLiM-cut silicon foil and created a heterojunction technology (HJT) solar cell with an energy conversion efficiency of 12% on a 43 µm-thick silicon foil [14].…”