How Diamond Wire and Black Silicon are Driving the Photovoltaic Industry

By Cyril Robinson Azariah JJul 27 2018

The use of diamond wire and saws has increased in the photovoltaic industry, thanks to its faster production and eco-friendly credentials. Black silicon offers another way to achieve mass production more easily and at a lower cost.

HSBortecin / Shutterstock

Diamond Wire

The method of wafering, cropping and slicing of silicon wafers can be achieved using fixed diamond wire. The diamond wire is able to cut 75% quicker than wire slurry saws. It reduces the production time of silicon wafers, and as the production time decreases, mass production increases; eventually reducing the costs.

Diamond wire cuts more precisely than existing conventional methods, meaning wafers can be made thinner than previously. Thus, it increases the material yield and decreases the time needed for lapping or grinding. The process of using wire impregnated with diamond nano/micro dust particles of various sizes, is known as diamond wire cutting (DWC). Due to the hardness of diamonds, this cutting technology offers a way of cutting through almost any material.

Additionally, as the diamond wire saws use water and sometimes a coolant to lubricate the cut, little recycling is needed. Whereas with a slurry saw, it is not possible, due to the slurry mixture.

Italian researcher S. Turchetta and his team recently carried out analysis of the cutting process, cutting natural stones using diamond wire, and focussing on the cutting forces and optimization of diamond bead wear, to allow for a better design of cutting systems.

As a result, the use of diamond wire and saws has increased in PV industry due to its faster production and promising solution for producing excellent photovoltaic cells. This leads the path of the eco-friendly green energy source to be used worldwide in the upcoming years.

Black Silicon

The other promising way to achieve mass production using polycrystalline diamond wire for cutting, is through black silicon. As soon as the conductive silver paste overcomes some of the existing problems, such as clear patterning on black silicon textured surfaces, mass production can be quickly reached.

Gianmario Scotti defined black silicon as "the surface morphology of silicon etched in reactive plasma under passivating conditions of high oxygen to SF 6 ratio". It is a quasi-regular array of narrow pyramidal pillars. His team studied the effects of coupling black silicon with carbon cloth for increasing the galvanic contact for the usage of micro fuel cells. They found that the micro fuel cells with the current collector covered with black silicon performed better than the otherwise identical counterparts without this material.

SEM Image of Black Silicon - Wikimedia Commons

Taiwan scientist C-H Hsu and his team found that by continuous change in the refractive index of blakc silicon, very low reflectivities with the surfaces are observed (>0.9%). In his review, he narrated the most recent and considerable progresses of black silicon for use in solar cells and explains the list of benefits and difficulties of the different methods of fabrication.

Despite what is often said or assumed about black silicon, the low reflectance from the surface is not triggered by light trapping. The structures of such a surface - whether dry etched pyramids or wet etched pores - is smaller than the wavelength of light incident on the surface. The typical dimension is 10 nm for black silicon surface features, compared with the micro size ranging 0.1-1µm for the wavelengths of light absorbed.

An explanation can be found in the effective medium approximation (EMA) first put forward by Bruggemann in 1935. Using the EMA, it is possible to say that the index of refraction of a silicon surfaces, with both types of nanoscale structures (either pillars or pores), can vary fundamentally with the dimensions of the depth for the black silicon surface. The removal of any abrupt interface means that no reflection can arise.

Researchers from Aalto University in Finland have obtained the improved efficiency of 22.1% efficiency on nanostructured silicon solar cells. A 3-4% absolute increase on their previous record was achieved by applying a thin passivating film on the nanostructures and by integrating all metal contacts on the back side of the cell. As they have verified, black cells generate considerably more electricity than traditional cells even though both cells have identical efficiency values. Harvard and Rice University also claimed that the black silicon makes it possible to use less silicon for versatility of light sensors, making the devices cheaper, simpler and reliable.

References:

1. Turchetta, S., L. Sorrentino, and C. Bellini. "A method to optimize the diamond wire cutting process." Diamond and Related Materials 71 (2017): 90-97.
2. Lu, Yen-Tien. "Synthesis of black silicon anti-reflection layers for silicon solar cells." (2015) Diss., Rice University.
3. Bruggeman, Von DAG. "Berechnung verschiedener physikalischer Konstanten von heterogenen Substanzen. I. Dielektrizitätskonstanten und Leitfähigkeiten der Mischkörper aus isotropen Substanzen." Annalen der physik 416.7 (1935): 636-664.
4. Hsu, Chih-Hung. et. al. "Fabrication and characteristics of black silicon for solar cell applications: An overview" Materials Science in Semiconductor Processing 25 (2014): 2-17
5. Savin, H. et. al. "Black Silicon Solar Cells with Interdigitated Back-Contacts Achieve 22.1% Efficiency." Nature Nanotechnology 10 (2015): 624-628
6. Scotti, G. et. al. "Velcro-Type Attachment of Black Silicon and Carbon Cloth for Improved Galvanic Contact in Micro Fuel Cells." (2011)

Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.