光電所所訊

Crystalline Si Thin Foil of Completely Single Domain

Professor Ching-Fuh Lin

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所林清富教授

Producing ultra-thin Si wafer with low Si material waste is an important issue to achieve grid parity of Si solar cells. Although Si wafer with a thickness of 20μm can be fabricated by epitaxial techniques, these techniques need rigorous process environment, expensive equipment and high power consumption, limiting the potential of cost reduction. In this report, we explore a procedure involving very fast and low-cost process to fabricate crystalline Si thin foils of completely single domain from (100) Si bulk wafer.

The low-cost procedure is a multi-step metal-assisted etching (MAEtch). Figure 1 shows the procedure of forming Si thin foil using multi-step MAEtch. First of all, the Ag used as catalytic agent for MAEtch is deposited on the Si surface with photoresist (PR) pattern. The solution of electroless metal deposition consists of AgNO₃ and HF, which allows Ag⁺ to become silver dendrites on Si. Second, the multi-step MAEtch has two major steps including anisotropic and isotropic etching. The solution of the anisotropic etching (1^st step MAEtch) is composed of H₂O₂ and HF to form vertically-aligned Si micro-holes. In aqueous solution of H₂O₂ and HF, Ag dendrites will sink into Si substrate vertically. Then, at the other MAEtch step (2^nd step MAEtch), the Si micro-holes from the 1^st step MAEtch are undercut for lift-off of Si thin-foil. The solution of 2^nd step MAEtch contains a large H₂O₂/HF ratio to achieve isotropic etching, and thus Si micro-holes could be undercut from the bottom. Figures 2(a) and (b) are the SEM images of the micro-holes formed in 1^st step MAEtch, where vertical micro-holes can be observed. In Fig. 2(c) and (d), the micro-hole undercut with isotropic etching is also shown, and the thin foil can be lifted-off after 7-min of 2^nd step MAEtch. By multi-step metal-assisted etching, the entire procedure takes no more than 20 minutes to produce the Si thin foil with thickness of 15 μm.

The Si thin foil with micro-hole structure could induce strong light trapping effect, leading to high absorption. Figure 3 shows that the absorption of the Si thin foil is over 85% from 350 nm to 960 nm, exceeding the calculated absorption of planar Si thin foil with the same thickness. This is due to the fact that the back side of the thin foil provides excellent anti-reflection, and multiple internal light scattering is induced by the micro-hole structure. Finally, combined with solution-processed conducting polymer, the Si thin foil/PEDOT:PSS solar cell is demonstrated. The structure of the solar cell is depicted in inset of Fig.4. The interface between the PEDOT layer and n-type Si can form a good heterojunction for separation of electron-hole pairs, and it reveals rectifying characteristics as shown in Fig.4. In conclusion, the Si thin foil has potential for low-cost solar cell devices, and its light-weight and ultra-thin properties can enable wide applications.


Figure 1. (a) Deposition of silver on the PR -patterned Si surface. (b) 1^st step MAEtch to form vertically-aligned Si micro-holes. (c) 2^nd step MAEtch to undercut the Si micro-holes. (d) Lift-off of the Si thin foil from Si substrate.		Figure 2. (a) & (b) are side-view and top view of the Si micro-holes formed by 1^st step MAEtch, respectively. (c) Side view of Si micro-holes after 2^nd step MAEtch for 2min. (d) Side view of Si micro-holes after 2^nd step MAEtch for 5min.

Figure 3. Absorption spectra of the Si thin foil.		Figure 4. J-V characteristics of Si thin foil/PEDOT solar cell under dark and light conditions. Inset picture: Device structure of the Si thin foil/PEDOT solar cell.