Reflectance Studies in Silicon Nanowires Grown By Electroless Etching

One of the major energy loss mechanisms of solar cells is optical reflection.  Utilization of nanostructures in solar cells may eliminate the need for antireflective coating [1]. The antireflective nature of Silicon nanowires (SiNW’s) revealed by their black color and dull appearances have drawn the attention of researchers [2]. Highly oriented SiNWs arrays prepared by electroless etching method can significantly suppress light reflection across a broad spectrum. Reflectivity studies on these show a significantly decrease in optical reflectivity down to 1.3% for 10 µm long SiNWs arrays [3]. Electroless etching is a top-down technique for growing SiNWs and does not need costly and expensive equipment to grow SiNWs as bottom-up methods usually do.

The electroless process begins with the cleaning of silicon substrates. They were cleaned in acetone, methanol and deionized water, then blow-dried with N₂. They are dipped in hydrofluoric acid (HF) 49% solution for 1 min to remove any remaining oxide.

The silicon (Si) wafers are dipped for 45 minutes at room temperature in an electroless solution prepared in a petri dish containing silver nitrate (AgNO₃) and hydrofluoric acid (HF) [4].

The substrates are dipped and galvanic reactions disperse silver ions on their surface, losing silicon as SiF₆. As a consequence of this etching process, the removal of Si via Ag ions results in vertically aligned arrays of SiNWs covered with silver dendrites. Varying the heights of the SiNWs(fig. 1). changes their optical reflectance.  A target optical reflection would be 0.8.

Fig 1. Cross sectional SEM images of the vertically standing Si NW arrays obtained by electroless etching.

REFERENCES

[1] Jin-Young Jung, Han-DonUm, Sang-WonJee, Kwang-TaePark, JinHoBang, Jung-HoLee, “Optimal design for antireflective Si nanowire solar cells,”   Solar Energy Materials & Solar Cells, 2013.

[2] Baris Ozdemir, Mustafa Kulakci, Rasit Turan and Husnu Emrah Unalan, “Effect of electroless etching parameters on

the growth and reflection properties of

silicon nanowires,” IOP Science, 2011.

[3] Hua Bao and Xiulin Ruan, "Optical absorption enhancement in disordered

vertical silicon nanowire

arrays for photovoltaic applications," OPTICS LETTERS, 2010.

[4] R. G. Mertens and K. B. Sundaram, “Recession and Characterization of Patterned Nanowires Grown by Electroless Etching of Silicon”. ECS Journal of Solid state Science and Technology, 2012.