Nano-Scale Smoothing of Double Layer Porous Si Substrates for Detaching and Fabricating Low Cost, High Efficiency Monocrystalline Thin Film Si Solar Cell By Zone Scanning Annealing
Authors: Takazawa Chiaki , Hasegawa Kei , Lukianov Anatolii , Zhang Xiaomei , Ihara Manabu 
Published: February 2020

1. Introduction

Monocrystalline Si solar cell (c-Si SC) prepared from Si wafer which cut and sliced from Si ingot exhibit kerf loss and also has extra thickness of Si wafer for power generation given absorbance. In this study, we therefore focused on “thin film c-Si SC”. Thin Si is expected to reduce cost and improve open circuit voltage. Fig.1 outlines the preparation of a thin film c-Si SC. A seed layer which also works as a detach layer is fabricated on monocrystalline Si wafer, and then a thin film Si is deposited by epitaxial growth on it and is then detached. However defects of the seed layer are influenced to the crystallinity of epitaxial growth film, which is a key factor of photoelectric conversion efficiency as c-Si SC.
A previously proposed low cost fabrication method for c-Si SC uses double layer porous Si (DLPS) as the seed layer for epitaxial growth.1) DLPS consists of a low porosity layer (LPL) on top and a high porosity layer (HPL) at the bottom as a detach layer. Thin film Si is prepared by epitaxial growth on DLPS as a seed layer, and it is possible to prepare thin film c-Si SC and to reuse of the Si wafer by mechanical detachment. Thick and high porosity HPL is needed to improve the detaching process. However, the nonuniform structure of LPL, including surface roughness (Rms) and crystalline structure, which is the base of epitaxial growth affects the quality of the thin Si film. So, a treatment after the formation of DLPS is one approach to achieving a balance between improved efficiency of thin film c-Si SC and the yield of the detaching process.
To achieve this balance, we previously proposed the “zone-melting crystallization (ZMC) method2)” (Fig.2) in which amorphous Si is melted and crystallized at the solid-liquid interface by melting Si surface in a line (using lamp line heater) and then scanned in one direction. Previously, SiO2/Si/SiO2 tri-laminar structure films with smooth amorphous Si annealed using high-speed ZMC (7.0 mm/s) and monocrystalline Si film oriented (100) were successfully prepared 3).
In this study, the seed layer of DLPS have less crystal defects and smoother surface were fabricated by applying ZMC to retain the structure of HPL while changing only the surface of LPL by using a scanning lamp heater. The dependence of nano-order surface structure on the amount of heat produced by the scanning lamp heater was investigated.

2. Results and Discussion

2-1. DLPS preparation

Fig.3 shows cross-section FE-SEM image of DLPS, revealing successful preparation by two-step anodization of monocrystalline Si. Fig.4 shows the dependence of Rms of DLPS on the HPL thickness, revealing that Rms increased increasing HPL thickness. Because thicker HPL makes it easier to detach the epitaxial Si thin film, there is a trade-off between Rms and improving the yield of the detaching process.

2-2. Effect of using ZMC method for DLPS

The optimal conditions for ZMC method (i.e., the amount of heat and scan speed) were evaluated by calculating the total amount of heat based on the heat, time and area.
Fig.5 shows the dependence of Rms on the total amount of heat, revealing that Rms was minimum when scan speed was 5 mm/s and the total amount of heat was 20 × 103 kJm-2. Rms, Peak-to-valley and pore size on the surface decreased to 0.631 (17%), 2.248 (16%), 27.5 (2.3%) nm, respectively. And then, Rms increased with increasing amount of heat higher than 20 × 103 kJm-2. Rms also depended on scan speed, and tended to increase Rms with too slow scanning. This dependence explains why the porous Si structure was changed by heating; the surface energy decreased by heating, thus promoting surface oxidation by hydrogen desorption. Moreover, although the percentage of upper lamp heating in the total amount of heat when scan speed of it is as small as 1, 5, 15, 26 mm/s is 24, 6.2, 2.1, 1.3%, respectively, Rms is largely influenced by the amount of heat from the upper lamp heater. In summary, Rms can be successfully decreased by adjusting the lamp heater and minimizing the effect of LPL of DLPS.
In conclusion, heating the DLPS by the ZMC method is crucial for controlling nano surface structure. Controlling surface oxidization and hydrogen desorption is also crucial.

3. References

1) M. Karim, etal., Nanoscale Research Letters, 9, 348 (2014).
2) M. Ihara, etal., Applied Physics Letters
79, 3809 (2001).
3) S. Yokoyama, etal., IEEE, 312-315 (2002).

Figure 1